Synthesis of Nanostructured Materials

Using Metal Cluster Ion Beams

Muhammad Hanif

Ph.D. Thesis

Supervisor:

Associate Prof. Vladimir N. Popok

Thesis submitted: January 06, 2016

Ph.D. Supervisor: Associate Prof. Vladimir N. Popok, Aalborg University

Ph.D. committee: Prof. Karl-Heinz Meiwes-Broer, University of Rostock

Associate Prof. Jonas Beerman, University of Southern

Denmark

Associate Prof. Leonid Gurevich, Aalborg University

Ph.D. Series: Department of Physics and Nanotechnology

Aalborg University

ISSN: 2246-1248

ISBN: 978-87-7112-461-3

Published and distributed by:

Department of Physics and Nanotechnology, Aalborg University.

Skjernvej 4A, DK – 9220 Aalborg Øst, Denmark

Phone: +45 99409215

Fax: +45 99409235

www.nano.aau.dk

© Copyright 2016 by Muhammad Hanif and Department of Physics and

Nanotechnology, Aalborg University, Denmark

Printed in Denmark by Aalborg University Press, Aalborg 2016

All rights reserved. No part of the publication may be reproduced, transmitted or

translated in any form or by any means, electronic, mechanical, including

photocopying, recording, or any information storage and retrieval system, without

the prior written permission of the author and the publisher.

What would the properties of materials be if we could arrange the atoms the way we

want them? They would be very interesting to investigate theoretically. I can't see

what would happen, but I can hardly doubt that when we have some control of the

arrangement of things on a small scale, we will get an enormously greater range of

possible properties that substances can have, and of different things that we can do.

Richard Feynman, 1959

Synthesis of Nanostructured Materials using Metal Cluster Ion Beams

Author: Supervisor:

Muhammad Hanif Assoc. Prof. Vladimir N. Popok

List of Published Papers:

A. M. Hanif, V. N. Popok, “Magnetron sputtering cluster apparatus for

formation and deposition of size-selected metal nanoparticles”,

Proceedings of International Conference Nanomeeting-2015, May 26-29,

2015, Minsk; in Physics, Chemistry, and Applications of Nanostructures,

Eds. V.E. Borisenko, S.V. Gaponenko, V.S. Gurin and C.H. Kam, World

Sci. Publ., Singapore, 2015, p. 416-419.

B. M. Hanif and V. N. Popok, “Low-energy interaction of metal cluster ions

with surfaces”, Proceedings of XXII International Conference. Ion-Surface

Interactions, Eds. E.Y. Zykova, P.A. Karaseov, A.I. Titov, and V.E.

Yurasova, Aug. 20-24, 2015, Moscow, V. 1, p. 47-52.

C. M. Hanif, R. R. Juluri, M. Chirumamilla, and V. N. Popok, “Poly

(methacrylate) composites with size-selected silver nanoparticles

fabricated using cluster beam technique”, Journal of Polymer Science B:

Polymer Physics, submitted.

D. V. N. Popok, M. Hanif, A. Mackova, R. Miksova, “Structure and

plasmonic properties of thin PMMA layers with ion-synthesized Ag

nanoparticles”, Journal of Polymer Science B: Polymer Physics (2015), 53,

664-672.

E. P. Fojan, M. Hanif, S. Bartling, H. Hartmann, I. Barke, and V. N.

Popok, “Supported silver clusters as nanoplasmonic transducers for

protein sensing”, Sensors and Actuators B: Chemical (2015), 212, 377-381.

This thesis has been submitted for assessment in partial fulfillment of the Ph.D.

degree. The thesis is based on the submitted or published scientific papers that are

listed above. Parts of the papers are used directly or indirectly in the extended

summary of the thesis. As part of the assessment, co‐author statements have been

made available to the assessment committee and are also available at the Faculty.

The thesis is not in its present form acceptable for open publication but only in

limited and closed circulation as copyright may not be ensured.

III

Abstract

Modern research and industry require sophisticated and versatile tools for

surface engineering and synthesis of the nanoscale object. Among a range of

various methods, cluster beam technique attracts considerable attention for

material modification on the nanoscale. The work presented in this thesis

involves experimental studies related to development and optimization of

magnetron sputtering cluster apparatus (MaSCA), production of metal

clusters of variable sizes and formation of nanostructures that can be used as

optical transducers for sensing.

MaSCA is built, and experimental conditions are optimized for the formation

and size selection of metal clusters, primarily made of copper and silver.

Copper and silver clusters are deposited at low (thermal) kinetic energies

onto different substrates to investigate cluster shape, structure and

arrangements on the surfaces. It is observed that clusters are tend to diffuse

on the atomically flat surfaces (graphite and mica), but the diffusion is rather

limited on a bit rougher surfaces (silicon and quartz) at room temperature.

The clusters are tend to preserve almost spherical shape with a slight

tendency to oblate on the interaction with the surface. Size (mass) selection

of clusters is performed utilizing electrostatic quadrupole mass selector

(EQMS). Atomic force microscopy studies of deposited silver clusters

allows to obtain the dependence between EQMS voltages and cluster sizes

(diameters) and demonstrate good size-selection capabilities with 5-7% of

standard deviation in diameters.

The second experimental block describes the research on the deposition of

size-selected silver clusters on poly (methyl methacrylate) (PMMA).

Nanosized silver particles are widely used as plasmonic structures.

Utilization of localized surface plasmon resonance of nanoparticles located

on the surface or embedded in PMMA paves a way for the formation of

metal-polymer nanocomposites with controllable non-linear optical

properties. It is found that immersion of silver particles into the polymer can

be controlled through tuning of its viscosity or hardness utilizing simple

IV

thermal annealing of the samples. This method allows to fabricate thin

PMMA layers with embedded size-controlled silver clusters demonstrating

remarkable plasmonic properties.

Finally, the thesis describes the experiments on the formation of optical

transducers with silver nanoparticles on quartz and PMMA demonstrating a

possibility of detection of small protein molecules (albumin) utilizing

standard antibody-antigen scheme of protein incubation.

V

Resumé

Moderne forskning og industri kræver sofistikerede og alsidige værktøjer til

at syntetisere og designe overflader af objekter på nanoskala. Blandt en

række af forskellige metoder tiltrækker cluster beam technique sig

betragtelig opmærksomhed som en metode til at modificere på nanoskala.

Arbejdet der præsenteres i denne tese involverer eksperimentelle studier der

relaterer til udvikling og optimisering af magnetron sputtering cluster

apparatus (MaSCA), produktion af metalklynger af forskellige størrelser og

dannelse af nanostrukturer som kan bruges som sensorer i optiske

transducere.

Et MaSCA er bygget, og eksperimentelle betingelser er optimeret til

dannelse og størrelsesudvælgelse af metalklynger som primært er lavet af

kobber of sølv. Kobber- og sølvklynger deponeres ved lav (termisk) kinetisk

energi på forskellige substrater for at undersøge klyngeform, -struktur og -

arrangering på overfladerne. Det observeres at klynger har tendens til at

diffundere på den atomart flade overflade (grafit og mica), men diffusionen

er ret begrænset på mere ru overflader (silicium og kvarts) ved

stuetemperatur. Klyngerne har tendens til at bevare deres næsten sfæriske

form med en tilbøjelighed til at blive fladtrykte når de interagerer med

overfladen. Størrelsesudvælgelse (masseudvælgelse) af klyngerne udføres

ved at benytte electrostatic quadrupole mass selector (EQMS). Studier med

atomar kraftmikroskopi af deponerede sølvklynger giver sammenhængen

mellem EQMS spændinger og klyngestørrelser (diameter) og demonstrerer

gode størrelsesudvælgelsesevner med 5-7% standardafvigelse i diameteren.

Den anden eksperimentelle del beskriver undersøgelsen af deponering af

størrelsesudvalgte sølvklynger på poly (methyl metacrylate) (PMMA).

Sølvpartikler på nanostørrelse bliver i vid udstrækning brugt som

plasmoniske strukturer. Anvendelsen af lokaliserede overfladeplasmon-

resonanser fra nanopartikler deponeret på overfladen eller indlejret i PMMA

baner vejen for dannelsen af nanokompositter af metal og polymer med

kontrollerbare ikke-lineære optiske egenskaber. Indlejringen af sølvpartikler

VI

i polymeren kan kontrolleres ved justering af dens viskositet eller hårdhed

ved at benytte simpel termisk hærdning af prøverne. Denne metode tillader

fabrikation af tynde PMMA lag med indlejrede størrelsesudvalgte

sølvklynger, som udviser bemærkelsesværdige plasmoniske egenskaber.

Slutteligt beskriver tesen eksperimenter vedrørende dannelsen af optiske

transducere med sølvnanopartikler på kvarts og PMMA, som er brugt til at

demonstrere muligheden for detektion af små proteinmolekyler (albumin)

ved at bruge den almindelige antibody-antigen metode til proteininkubation.

VII

Preface

There is a considerable interest in the study of the cluster (aggregates of

atoms or molecules) production, detection and deposition onto a surface for

a number of research and application-oriented reasons. From a fundamental

point of view, it is important to control the cluster size atom by atom that

helps to pave a way for understanding how and when bulk properties

develop in a matter as a function of increasing dimensions. There are also

many practical applications utilizing clusters that are often called

nanoparticles when deposited on or embedded into the material. For

example, coinage metal clusters are of particular interest for their exciting

plasmonic properties upon interaction with light and it has been a pleasure to

manipulate them to develop optical transducers for biomolecule sensing

during my work as a Ph.D. student.

This thesis is, primarily, consists of introduction, seven chapters,

bibilography and Appendixes A to E representing the published and

submitted papers. The first chapter sheds light on the processes involved in

formation and production of clusters from various sources, particularly

magnetron sputtering. Metal clusters structure and size-dependent cluster

properties are described in chapter 2. Cluster-surface interaction regimes and

state of the art in supported metal clusters are briefly presented in chapter 3.

Chapter 4 describes the MaSCA facility at Aalborg Univeristy, sample

preparation and very briefly sample characterization techniques. The results

of cluster deposition on different substrates, size-selection experiments and

formation of metal-polymer composites are discussed in chapter 5. Chapter 6

focuses on the study of supported silver clusters as optical transducer on two

different substrates quartz and polymer films. Finally, a summary of the

obtained key results and future perspectives are presented in chapter 7.

The main work presented in this thesis has been conducted at the

Department of Physics and Nanotechnology of Aalborg University. TEM

study was performed at Interdisciplinary Nanoscience Center (iNANO) of

VIII

Aarhus University. The work made could not have been achieved without

the help from many people and I would like to acknowledge them here.

This one goes to the greatest person in my life: Mum “Maa G” Thank you

dad, for your unbelievable support from my first day of school till today.

From a professional point of view first and foremost, I would like to express

my deepest gratitude towards my supervisor Vladimir Popok for guidance,

support and giving me the freedom to explore the world of cluster physics. I

really enjoyed working with him.

I would probably end up with a too long list of colleagues and friends during

the three years of my PhD, but I would like to mention a few names: Peter

Fojan and Leonid Gurevich for introducing me to the field of biophysics and

scientific discussions, Ane Kold Di Gennaro for teaching me chemical

synthesis of NPs, Deyong Wang and Peter Kjær Kristensen for introducing

me to the Nanolab, Manohar Chirumamilla for TEM sample preparation,

Raghavendra Rao Juluri for TEM studies at iNANO Aarhus Univeristy, Kim

Houtved Jensen, Hans Nilsson and Christian Jensen for their significant

technical help in building the cluster lab.

I am also very grateful towards Dr. Ingo Barke and his group for giving me

an opportunity to participate in ongoing projects at the University of Rostock

and also for the help in making some parts of our MaSCA at their

mechanical workshop.

Thanks to Morten Slyngborg, Paw Simesen and Mariia Rozhavskaia for the

good times we’ve had together. Big shouts go to all my friends out there –

without you, this adventure would not have been the same. Finally, I would

like to offer my warm regards and blessings to my family. I appreciate and

thank anyone who has contributed in one way or the other to the successful

completion of this work. I also appreciate the financial support for my work

by the Spar Nord and The Obel Family Foundation.

Muhammad Hanif Aalborg, January 2016

IX

Contents

Introduction .............................................................................................................. 1

1 Formation and Growth of Clusters ................................................................ 5

1.1 Nucleation and cluster formation ............................................................... 5

1.2 Practical methods ....................................................................................... 9

1.2.1 Supersonic expansion (jets) .............................................................. 10

1.2.2 Gas aggregation ................................................................................ 11

1.2.3 Erosion sources ................................................................................ 11

1.2.4 Liquid metal ion sources .................................................................. 13

1.3 Magnetron sputtering ............................................................................... 13

2 Structure and Properties of Clusters ............................................................ 19

2.1 Geometric and electronic structure........................................................... 19

2.1.1 Geometric structure .......................................................................... 19

2.1.2 Electronic versus geometric structure .............................................. 21

2.2 Models for metal clusters ......................................................................... 24

2.2.1 Liquid drop model ............................................................................ 24

2.2.2 Jellium model ................................................................................... 25

2.3 Transition to the bulk ............................................................................... 27

2.4 Properties of clusters ................................................................................ 27

2.5 Optical properties of clusters .................................................................... 30

2.5.1 Dipole plasmon resonance ............................................................... 31

2.5.2 Plasmonic coupling .......................................................................... 34

3 Cluster-Surface Interaction and Applications of Clusters on Surfaces ..... 35

3.1 Interaction regimes ................................................................................... 35

3.2 Cluster deposition..................................................................................... 37

3.3 Metal nanoparticles for biosensing........................................................... 38

4 Experimental Techniques and Methods ....................................................... 43

4.1 Magnetron sputtering cluster apparatus ................................................... 43

4.2 Electrostatic quadrupole mass selector ..................................................... 47

4.3 Materials and sample preparation............................................................. 48

4.4 Investigation of deposited clusters ........................................................... 50

X

4.4.1 Atomic force microscopy ................................................................. 50

4.4.2 Transmission electron microscopy ................................................... 51

4.4.3 Optical spectroscopy ........................................................................ 52

4.4.4 Ellipsometry ..................................................................................... 52

5 Deposition and Characterization of Metal Clusters .................................... 55

5.1 Calibration and optimization of cluster beam production ........................ 55

5.2 Copper clusters ......................................................................................... 56

5.2.1 Cun clusters deposition on different substrates ................................. 56

5.3 Silver clusters ........................................................................................... 59

5.3.1 Deposition on different substrates .................................................... 59

5.3.2 Shape and structure .......................................................................... 61

5.3.3 Clusters size selection ...................................................................... 62

5.4 Metal-polymer nanocomposites ............................................................... 64

6 Nanoparticle-Based Optical Transducers .................................................... 69

6.1 Silver clusters on quartz – Method I ........................................................ 69

6.1.1 Sample preparation ........................................................................... 69

6.1.2 Surface morphology: AFM .............................................................. 71

6.1.3 Protein sensing ................................................................................. 72

6.2 Metal-polymer composites for protein sensing – Method II .................... 74

6.2.1 Sample preparation ........................................................................... 74

6.2.2 AFM characterization ....................................................................... 75

6.2.3 Optical measurements: protein sensing ............................................ 77

7 Conclusions and Perspectives ........................................................................ 81

Bibliography ........................................................................................................... 83

Appendix A ............................................................................................................. 99

Appendix B ........................................................................................................... 107

Appendix C ........................................................................................................... 115

Appendix D ........................................................................................................... 131

Appendix E ........................................................................................................... 141

XI

List of Figures

Figure 1.1. The mechanisms of cluster growth after initial nucleation process. ........................ 6 Figure 1.2. Coagulation process: kinetic and diffusion regimes ................................................ 8 Figure 1.3. Schematics of a planar magnetron......................................................................... 14 Figure 1.4. Schematic representation of plasma confinement ................................................. 17 Figure 2.1. Geometries of Agn clusters, n=4-20 atoms ............................................................ 20 Figure 2.2. Mass spectrum of sodium clusters, n=2-75. .......................................................... 22 Figure 2.3. LDM prediction for the variation of ionization potential ...................................... 24 Figure 2.4. Energy level occupations and magic numbers ...................................................... 26 Figure 2.5. Top: Icosahedral clusters growth .......................................................................... 28 Figure 2.6. Melting temperature of gold clusters as a function of radius R. ............................ 29 Figure 2.7. Schematic of plasmon oscillation for a metal NP .................................................. 30 Figure 2.8. Calculated extinction efficiency of Ag NPs .......................................................... 33 Figure 3.1. Illustration of cluster-surface interaction............................................................... 36 Figure 3.2. Schematic illustration of nanoplasmonic sensing. ................................................. 39 Figure 4.1. Schematic view of MaSCA ................................................................................... 44 Figure 4.2. Schematic of gas flow through a nozzle-skimmer configuration. ......................... 45 Figure 4.3. Schematic picture of an Einzel lens is ................................................................... 46 Figure 4.4. A cross-sectional view of the EQMS .................................................................... 47 Figure 4.5. Schematic of sample preparation procedure .......................................................... 49 Figure 4.6. Force vs. distance curve. ....................................................................................... 51 Figure 4.7.The hardware features of the PerkinElmer lambda-1050 spectrometer. ................. 52 Figure 4.8. Schematic setup for thin film measurement. ......................................................... 53 Figure 5.1. Deposited material on the shutter plate ................................................................. 55 Figure 5.2. AFM images and corresponding height histogram of Cun clusters ....................... 57 Figure 5.3. AFM images and corresponding height histograms of Agn clusters ...................... 60 Figure 5.4. TEM image and size histogram fitted by Gaussian distribution. ........................... 61 Figure 5.5. HR-TEM image of two overlapping Agn clusters ................................................. 62 Figure 5.6. Cluster mean height for different quadrupole voltages ......................................... 63 Figure 5.7. AFM images and corresponding height histograms for viscous PMMA ............... 65 Figure 5.8. TEM cross-sectional image of viscous PMMA film ............................................. 65 Figure 5.9. AFM images and corresponding height histograms for soft PMMA..................... 66 Figure 5.10. TEM image of soft PMMA sample after annealing ............................................ 67 Figure 5.11. Normalized absorption spectra of PMMA implanted .......................................... 68 Figure 6.1. Silanization of quartz with APTMS ...................................................................... 70 Figure 6.2. AFM images of ..................................................................................................... 71 Figure 6.3. Optical transmission spectra of pristine quartz and ............................................... 73 Figure 6.4. AFM images for viscous PMMA sample .............................................................. 75 Figure 6.5. AFM images of hard PMMA sample .................................................................... 76 Figure 6.6. AFM images of soft PMMA sample ..................................................................... 77 Figure 6.7. AFM images of soft PMMA sample ..................................................................... 77 Figure 6.8. Normalized extinction spectra of viscous and hard PMMA .................................. 78 Figure 6.9. Normalized extinction spectra for soft PMMA with silver clusters ...................... 79

XIII

List of Tables

Table 5.1. The experimental parameter for cluster production. ............................................... 56 Table 5.2. Mean heights and standard deviations for clusters ................................................. 63

1

Introduction

Clusters, aggregates of atoms or molecules, possess an intermediate position

between atoms and molecules, on the one hand, and bulk condensed matter,

on the other.1 One can consider the beginning of cluster physics as a branch

of science with first experiments carried out at the very end of the 50’s and

beginning of the 60's.2-3

After that, clusters representing an embryonic form

of matter4 have become a rapidly developing subject of research, especially

in last two decades. Besides the research interests of fundamental nature,

clusters have started to be used as a new kind of material and tool with many

potential technological applications.4-6

Nanoparticle (NP) is one of the

commonly used terms nowadays referring, to a large extent, to clusters of

atoms and molecules deposited on surface or embedded in matter.

Two discoveries have added strong impetus in the development of

modern cluster physics research. The first one is the “magic numbers”

discovered first in the packaging of atoms in a cluster by O. Echt and co-

workers7 and then in the electronic structure of clusters by W.D. Knight and

co-workers.8 These magic numbers represent clusters of certain sizes with

extra stability either due to filled facets or shells of geometric structure or

filled electronic shells. Both discoveries uncovered the principals of the

structure of the nanoscale objects that were found to be rather different from

those of molecules and bulk matter. The second key work, reported by Kroto

et al. in 1985, is the discovery of the C60 fullerene and its unusual cage

structure,9 which greatly facilitated a better understanding of matter on the

nanoscale. It has also been suggested that in the creation of the universe,

very stable clusters such as C60 may have been formed.10

The study of the evolution of geometric and electronic structures of

clusters as a function of their size is one of the most interesting topics in

fundamental cluster physics research. Small size and tunable composition

allow clusters to have unusual combinations of physical and chemical

2

properties such as metallic clusters can become insulating if they are small

enough,11

chemical elements which unmixable in macroscopic quantities can

make alloys on the nanoscale12

etc. Thus, the ability to synthesize and

characterize clusters has led to the foundation to a new field that bridges not

only between atoms and bulk matter but also puts together research interests

from different disciplines of physics, chemistry, biology, material science,

medicine, and environmental science.13

For example, metal NPs can be used

as plasmonic systems for optics and electronics14

, transducers for protein

sensing15-17

, antimicrobial agents18

, anti-odour in textiles industry19

, agents

for catalysis20

and many other fascinating applications. Varying the cluster

size is also important capability to make tools on the appropriate scale

especially in biology and medicine where the clusters can be adjusted to size

of enzymes or proteins or made sufficiently small to penetrate into cells or

interact with bacteria.21-23

Practical applications require reliable methods capable of producing

NPs of desired composition and size as well as well-developed technologies

to position NPs at the places of interest or embedding them in a controllable

manner. There are two different concepts to produce nanoscale objects:

bottom-up, to aggregates individual atoms or molecules, and top-down, to

break macroscopic objects by cleaving, grinding, etching and similar.24

Despite the many years of research and development, there are still

challenges in respect to the approaches mentioned above.

In the current work, the first approach is used. The clusters are

produced by the aggregation of material sputtered from a target by plasma

ignited with the help of magnetron. Magnetron sputtering method has been

used for thin film deposition since the early 1970s.25-26

In the beginning, of

the 90’s first cluster sources based on this principle were tested. Since then

the magnetron sputtering approach has been developed into a reliable and

efficient method for formation of NPs from different precursors, mostly

metals.

Therefore, for the current project magnetron sputtering cluster

apparatus (MaSCA) has been constructed and optimized for the production

of metal clusters, in particular, from silver and copper. Significant efforts

were put on the development of an efficient system for size (mass) selection

3

of the clusters. This choice of metals and necessity of size-selection are

closely related to the main goal of the work on the formation of optical

transducers for biosensing.27

NPs of coinage metals are well-known object

providing the phenomenon of localized surface plasmon resonance (LSPR)

and the control of size allows efficient tuning the plasmonic properties. The

work includes careful study of the cluster deposition mechanisms on

different substrate materials to develop appropriate conditions for stability of

transducers as well as the development of approaches for efficient incubation

of biomolecules (proteins) on the clusters.

5

1 Formation and Growth of Clusters

Formation of clusters in the gaseous phase is a very popular route, as it

provides a high control over the chemical composition of the clusters and

gives a possibility for clusters size selection using some well-defined

methods. However, cluster nucleation is a challenging process, which

requires special conditions. Similar to all methods is that they require a

target material, from which atoms may be transmitted to the gas phase

through some energy exchange between the surface of the target and an

exterior source.28

In some cases, clusters can be formed directly from gas

phase precursors.29

1.1 Nucleation and cluster formation

When clusters are grown in the gas phase, they start from individual

atoms or molecules of the desired material. Upon rapid cooling of the gas,

supersaturation will occur, making possible the formation and growth of

clusters from the excess vapor. The cluster formation can be divided into two

steps: nucleation and growth. During nucleation process clusters of a stable

phase are formed from the metastable one. A metastable state for a gas

aggregation source is a supersaturated vapor of material of interest. The

nucleation process starts from the collision of atoms which leads to the

formation of a small cluster known as a nucleus. For example in the case of

metal species (M) in the buffer gas (Ar), this process can be described as a

three-body collision:

𝑀 + 𝑀 + 𝐴𝑟 → 𝑀2 + 𝐴𝑟. 1.1

Minimum three atoms can trigger the nucleation process since the

conservation of energy and momentum must be fulfilled at the same time.

An argon atom is needed to carry away the excess of energy resulting from

the binding of the metal atoms, and to stabilize the nascent dimer. According

to the phase transition theory, only the clusters (or nuclei) with a size of

6

certain values are thermodynamically stable30

and can serve for further

cluster growth. This value is called critical size31

and depends on the

experimental conditions such as temperature, pressure, etc. According to

theoretical calculations, dimers can be considered a stable nucleus for cluster

growth.32-33

A detailed review of the various nucleation theories and their

comparison can be found elsewhere.34-35

After the cluster nucleation step, further cluster growth is controlled by

four processes, shown in Figure 1.1: atom attachment, coagulation (kinetic

and diffusion modes), coalescence and aggregation.32, 36

As a general rule,

the more time the clusters spend in the aggregation region, the larger they

will grow on average as these processes then have a longer time to occur.

Figure 1.1. The mechanisms of cluster growth after initial nucleation process.

The first panel in Figure 1.1 represents the growth of clusters of single

atoms addition to the clusters Mn consisting of n atoms:

𝑀𝑛 + 𝑀 → 𝑀𝑛+1. 1.2

It is assumed that additional energy from the attachment of atoms is transferred into

the buffer gas.The rate constant kn for the attachment process given by (1.2) is

proportional to the cluster size:32

7

𝑘𝑛 = 𝑘𝑜𝑛2

3⁄ , 1.3

where ko is a reduced rate constant depending on the material and

experimental conditions. Rate constants for cluster growth are much higher

than for dimer formation, leading to large cluster sizes.32, 37

The character of

the nucleation process is also determined by the value of the dimensionless

parameter G:

𝐺 =𝑘𝑜

𝐾𝑁𝑏𝑢𝑓 , 1.4

where Nbuf is the density of the buffer gas and K is the rate constant of the

process 1.1. Usually, G≫1, so that the initial nucleation process, dimer

formation, lasts long compared to the attachment of atoms to dimers and

clusters. Hence, this requirement leads to larger clusters at any instant of

time.37

The solutions of set of balance equations provide the following

expressions for the average cluster size ��, maximum cluster size nmax at a

given instant of time, density of clusters Ncl and cluster size distribution

function fn:37

�� = 0.31𝐺3

4⁄ , 1.5

𝑛𝑚𝑎𝑥 = 1.2𝐺3

4⁄ , 1.6

𝑁𝑐𝑙 = 3.2𝑁𝐺−3

4⁄ , 1.7

𝑓𝑛 =𝐶

𝑛2

3⁄, 𝑛 < 𝑛𝑚𝑎𝑥 ; 𝑓𝑛 = 0, 𝑛 > 𝑛𝑚𝑎𝑥, 1.8

where C is normalization constant and N is the density of free clusters.

The second step of cluster growth is coagulation (see Figure 1.1).

Coagulation is the formation of a single cluster from two individual clusters

of smaller sizes:

𝑀𝑛−𝑚 + 𝑀𝑚 → 𝑀𝑛, 1.9

where n and m are the numbers of atoms in each cluster. Coagulation process

can happen in two regimes; kinetic and diffusion. The difference between

these two regimes is illustrated in Figure 1.2. For the kinetic regime of

8

cluster interaction with a buffer gas, only single atom or molecule may

strongly interact with the cluster under consideration, while in diffusion

regime many atoms interact simultaneously with the cluster. These inelastic

collisions occur due to the Brownian motion of the gas atoms and clusters.

The size distributions of growth in this way depend on the regime of

atom(s)-cluster interaction.32, 38

Figure 1.2. Coagulation process: kinetic and diffusion regimes of cluster interaction

with surrounding atoms in the course of cluster motion in a buffer gas.

The process of coalescence results from the interaction of clusters

with a parent vapor and it is also known as Ostwald ripening. This process

happens for the very low degree of supersaturation.32, 36 For smaller clusters,

the rate of atom evaporation is higher than the rate of atom attachment while

for larger clusters the inverse relation is fulfilled. As a result, large clusters

grow, while small clusters evaporate. Coalescence depends on the critical

cluster size or radius for equilibrium with a parent gas or vapor.32

Aggregation is a process where the individual clusters are joined due to their

contacts; while still retain much of their original shape. This process results

in the fractal growth of clusters. It should be noted that sometimes in the

literature these terms may be grouped together, or their meaning can be

ambiguous (e.g. coagulation is sometimes referred to as coalescence),

leading to some confusion.36

The size distribution of the clusters is also dependent on the charge of

the clusters, and the ratio of positive to negatively charged particles.

Theoretical studies of the charging of clusters in different plasma

environments show that the charge of clusters is non-trivial.32

However, a

good approximation for experimental work is to consider that the vast

9

majority of the clusters leaving the aggregation chamber will be either singly

charged or neutral. As an example, one can refer to the comparison of

theoretical treatment by Blazek et al.39

with the results presented by Ganeva

et al.40

The charge distribution is highly dependent on cluster production

source and experimental parameters.

1.2 Practical methods

Typically, a cluster apparatus consists of three main parts: production, mass

selection (if required) and detection. When designing an experimental setup,

there are fundamental choices to be made, depending on whether experiment

involves free or deposited clusters. Also, there is a requirement for cluster

material, size and energy distribution as well as whether the clusters should

be charged or neutral and so on. A number of known sources used today for

an efficient production of clusters belong to a few main categories described

below.

Supersonic nozzle (free jet) sources: a gas is expanded from a high-pressure

region to low pressure region through a nozzle of specific configuration

leading to velocities with Mach numbers above unity.41

Clusters are formed

in result of adiabatic expansion and corresponding cooling of gas atoms or

molecules.

Gas aggregation sources: a liquid or solid is evaporated into a colder inert

gas, which condense the vapor and facilitates the formation of clusters. Such

sources are particularly efficient in the production of large clusters

(N>1000).42

Fog, smoke and cloud formation in nature are good examples of

this process. Aggregation approach can also be combined with supersonic

expansion.

Surface erosion sources: clusters are formed by sputtering of material from a

solid surface by intense laser radiation, heavy particle impact, arc discharge

or by plasma.38

There are also thermospray and electrospray sources for making clusters

from liquids and solutions.43-46

The latter approach can also be applied to

metals in the liquid phase, opening a way for cluster formation by liquid-

metal ion sources.

10

Large varieties of techniques for the production of clusters have

grown up in line with the different experimental requirements.28

The

following section briefly reviews the most common methods of producing a

cluster beams in a vacuum.

1.2.1 Supersonic expansion (jets)

The supersonic jet is one of the well-understood sources for the production

of intense cluster beams of low-boiling-point materials or directly from gas

phase precursors.38, 47

These sources can produce both pulsed as well as

continuous beams with relatively wide size distribution, from a few up to

several thousands of atoms per cluster. Gas is expanded from a high-pressure

reservoir (or stagnation vessel) through the nozzle of small diameter into the

vacuum. Since the value of the stagnation pressure is usually on the order of

bars, the mean free path of atoms or molecules is much smaller than a typical

nozzle diameter. Clusters are formed due to adiabatic cooling of the gas. In

this isentropic expansion, the density and translational temperature decrease

with distance from the nozzle.48

Clusters are collimated into a beam by

passing through the skimmer, which should be located close to the nozzle

and enter so-called silent zone of the gas expanding under supersonic

conditions. Therefore, geometry parameters and the distance between the

nozzle and skimmer are very important for production and transformation of

clusters into a beam. The skimmer’s edge should be sharp and polished to

avoid any disturbance for the cluster beam.49

In the case of metals, seeded supersonic nozzle sources are often used,

in which an inert gas such as argon at a stagnation pressure of several

atmospheres is mixed with the metal vapor.50

Thus, the source combines the

aggregation approach (see details in next section) with the advantage of

supersonic expansion. Metal clusters formed by this method are typically

composed of a few hundred or thousand atoms.51

Seeded supersonic sources

are suitable for metals with low-boiling temperature, as it helps to maintain

the good vacuum during operation and easy to heat the crucible to vaporize

the metal. The first seeded supersonic expansion source was constructed by

Kappes et al.52

Pulsed supersonic sources have various advantages e.g. less

gas consumption, smaller pumps, distribution of cluster sizes and

temperature depend on the gas pulse.49

11

1.2.2 Gas aggregation

Gas aggregation is a simple method to produce efficiently large clusters.

These sources are used for low- to medium-boiling point materials (<2000

K).38

In such source, a target material is simply heated until the material

reaches the gas phase. Evaporated atoms interact with a stream of inert gas,

at very low pressure and temperature, which cools the evaporated material

forcing cluster formation. This condensation process triggers three body

collisions for clusters nucleation and growth as described in the previous

section. Cluster growth continues in the condensation chamber until

expanded through a nozzle. If only the evaporated material is of interest for

cluster formation, the cooling gas should not be able to react with the

evaporated species, and as such an inert gas should be used. The properties

of cluster growth will thus depend on many parameters involved in the

experiment, which makes it more difficult to control. Especially

agglomeration can be difficult to avoid, when the evaporated material

releases heat to its surroundings.42, 53

The distribution of cluster sizes is

controlled by the temperature and the gas flow rate. These parameters can be

adjusted to produce clusters of up to 105 atoms in size. Smoke sources are

widely used for the production of carbon clusters, for example, C60, C70 and

larger ones.49

1.2.3 Erosion sources

Laser vaporization: The idea of laser vaporization cluster source was first

applied to the production of clusters by Smalley and co-workers.9 In this

method a high power laser produces a plume of material in a short time

interval from a localized target area. A flow of a cold inert gas is introduced

into the ablation chamber to facilitate cluster formation. Typically, high

power pulsed (order of tens of nanosecond) excimer or Nd: YAG lasers are

used. In principle, these sources can be applied for production of neutral as

well as positively and negatively charged clusters. The kinetics of the laser

ablated plasma and the properties of clusters depend on the number of

factors such as the type and amount of vaporized material, the plasma-inert

gas interaction, the plasma source wall interaction etc.38

Laser vaporization

is one of the common techniques for producing clusters due to its simplicity

and applicability to a wide range of target material including refractory

12

metals with high melting points.54

However, it also has some limitations in

large-scale cluster production due to localized vaporization, low cluster

beam intensity, and the target needs to be moved to provide fresh areas for

every pulse.

Arc discharge: The production of clusters from an arc discharge was

proposed by Meiwes-Broer and co-workers in 1990.55-56

. The pulsed arc

cluster ion source (PACIS) is based on the ignition of electric discharge that

is used to vaporize the material. In this method plasma is generated by

applying a large electric current between two electrodes for a short time,

which causes an electric discharge between the cathode and anode leading to

evaporation of material. About 10% of the clusters formed are ionized and

there is no need for separate cluster ionization step.55

Several parameters

influence the PACIS performance regarding produced species, stability, and

intensity.38

Several sub-methods exist, e.g. either where the cathode is hot

(hot thermionic cathode arcs) or cold (cold cathode arcs).38, 42

Milani et al.

have introduced several modifications as compared to original arc discharge

source.56

Apparatus consists of three differentially evacuated chambers and

target rods are placed in a small cavity inside the ceramic body of the source.

A pulsed valve injects He in the cavity to produce electric discharge between

the rods.57-58

By confining the ablation plasma to a small region on the target,

this source provides unusual stability and intensity that overcomes the

limitations of PACIS and laser sources.

Ion sputtering sources: Sputtering or ion bombardment, involves the

interaction of high-energy heavy particles with the target materials, in which

exchange of energy with surface atoms of a target chip off some material to

produce clusters. For cluster production, typically 10-20 keV ion beams of

either heavy gasses (Kr, Xe or Cs) are used.59

Typically, sputtering sources

are used to produce intense continuous beams of singly charged clusters. In

theory, any particles can be used in this process, however, to reach sufficient

sputtering intensities certain methods are more convenient than others.38, 42

Ion beams can be substituted by plasma discharges. The combination of

plasma sputtering with gas condensation was reported in 1986 and then

developed by Haberland and co-workers through the use of magnetron

sputtering.60-61

Advantages of such sources are easy to operate, high beam

intensity, wide size distribution and possibility to produce clusters of

13

different metals. More details about magnetron sputtering process are

presented in Section1.3.

1.2.4 Liquid metal ion sources

Liquid-metal-ion sources (LMIS) are used for the generation of clusters of

low-melting-point metals. These sources can produce singly and multiply

charged clusters and during cluster flight evaporation and fission process is

observed. In LMIS, a liquid metal is filled into a capillary tube and high

electric potential is applied between the capillary tube and extractor plate. At

the tip of the tube, known as Taylor cone62

, the field is so intense that

clusters ions may undergo fragmentation. Coulomb explosion of clusters

may also occur in the presence of an intense electric field. This method was

used to produce gold clusters of different sizes (5,7,27 and 33 atoms).63

More detail of LMIS can be found elsewhere.64

Like other sources, this

method also has some limitations, for example, wide cluster energy

distribution, fission, etc.

1.3 Magnetron sputtering

Magnetron sputtering method belongs to the family of the surface erosion

sources that are briefly overviewed above. Since this method constitutes the

basis of cluster formation for the current research project, it will be described

in detail.

In 1939, Penning presented a good way of electrons confinement near

a surface by applying both electric and magnetic fields, resulting in

increased ionization of the plasma in the vicinity of the target surface.65

Plasmas are moderately conductive and, therefore, are only weakly perturbed

by the external electric fields, but the external magnetic field can have very

significant effects. Many variations of the Penning’s invention were

demonstrated such as configuration of electric and magnetic fields, multiple

sputtering targets, balanced and unbalanced magnetrons, direct current (DC)

and the radio frequency (RF) magnetron etc.66

The reason for strong electron confinement is the magnetic field

configuration combined with an applied electric field. In a simple

configuration, magnets are positioned with one pole at the central axis of the

14

circular magnetron, and the second pole placed in a ring configuration

around the first pole. Magnetron sputtering utilizes the ionization of a low-

pressure inert gas (typically argon), thus forming a highly reactive plasma.

The gas can be ionized by applying an electric potential between two

electrodes. Ionization starts when free electrons are generated in the gas. The

electric field accelerates and drags the electron towards the anode. The

electrons ionize neutral argon atoms during collisions (assuming the electron

has gained enough energy), this can be described as:

𝑒− + 𝐴𝑟 → 𝐴𝑟+ + 2𝑒−. 1.10

Thus, argon ionization releases more electrons, which may be used for

further ionization. Figure 1.3 illustrates a typical planar magnetron and

direction of electric and magnetic fields.

Figure 1.3. Schematics of a planar magnetron. (a) 2D view of the typical planar

magnetron, directions of E- and B-fields are shown. (b) eroded copper target (c)

cycloidal motion of electrons.

The magnetron consists of a cathode (target) biased with a negative

voltage and a grounded anode. Such configuration accelerates the inert gas

ions towards the cathode (or target) resulting sputtering of target atoms by an

energetic ion impact. Secondary electrons are also emitted from the target

surface due to ion bombardment. These electrons are highly energetic and

they increase the probability of ionizing collisions leading to an increase in

15

the sputtering yield, the number of atoms ejected from a target surface per

incident ion. A magnetic field is used to trap secondary electrons in a torus

above the so-called target race track. The race track is radially located where

electric and magnetic fields are perpendicular to each other, which is

somewhere between the inner magnetic and the outer ring magnet. In this

configuration, the secondary electrons cannot escape from the surface of the

target (or cathode). This results in non-uniform target sputtering i.e. the

formation of a narrow and deep toroidal erosion trench in the target. The

Lorentz force equation can be used to describe the secondary electron

trapping in a magnetic and electric field:

𝐹 = 𝑞(𝐸 + 𝑣 × 𝐵), 1.11

where F is the force acting on a particle of charge q and velocity v with E

and B to be the electric and magnetic fields, respectively. The resulting total

motion (or drift velocity) of the secondary electrons will be directed

perpendicular to both fields, i.e. circles a guiding center or inner magnet.

Figure 1.3 illustrates the planar magnetron and direction of movements

of ions and electrons. Secondary electrons will be trapped in the area, where

the magnetic field is perpendicular to the electric field. The Lorentz force

yields a circular path around the surface of the cathode, which is illustrated

2-dimensionally in Figure 1.3(a). Furthermore, the trapped electrons will

move in a cycloid curve as illustrated and explained in Figure 1.3(c). Here

the electron takes on a cycloidal motion because its initial velocity is

generated by the acceleration of the applied electric field. The electric and

magnetic components of the Lorentz force are given as FE and FB

respectively. The magnetic field increases sputtering yield in two ways; one

by trapping secondary electrons and second by increasing their path length

on the distance from the cathode to the anode, which increases the

probability of argon ionization.

A planar magnetron-based cluster sources have a number advantages

over other material vaporization techniques: efficient sputtering25

, broad

cluster sizes, high deposition rate, a higher percentage of charged clusters,

etc.60-61

The characteristic of such sources depend on many parameters that

lead to challenges in the production of desired clusters composition and size.

16

The main disadvantage of this technique issues related to beam stability and

reliability.67

The continuous target erosion affects the cluster size

distribution67-69

, the ion distribution function in the near-cathode region70

and

deposition rate.71

There is a number of solutions have been proposed to

overcome inefficient target utilization by changing the scheme and shape of

magnetrons.72-73

For example, in 1986 Windows and co-workers presented

an idea of unbalanced magnetron sputtering.74

In an unbalanced magnetron,

the outer ring of magnets is strengthened about the central pole magnet or

vice versa. In this case, some magnetic field lines are directed towards the

substrate. Thus, some electrons can escape from the confining E×B field and

follow these field lines resulting in better transport of plasma to the

substrate.75

An additional feature of the unbalance magnetron is that when

the escaping magnetic field is linked correctly (north to south poles) to

another unbalanced magnetron, the plasma generation area can be

significantly increased (known as closed-field unbalanced magnetron

sputtering).75

Figure 1.4 shows the plasma confinement in different

magnetron modes.

The magnetron configuration influences not only the plasma

confinement but also the target profile, sputter and deposition rate. Mostly

unbalanced magnetron sources are used for thin film deposition. However, in

a cluster production source, which is our case, conventional balanced

magnetron scheme is applied, as far as I am aware, nowadays the wide

majority of magnetron cluster sources are based on the conventional

magnetron.

17

Figure 1.4. Schematic representation of plasma confinement in conventional and

unbalanced magnetrons (top) closed-field magnetron configurations (bottom).75

ICD

stands for ion current density. Reprinted with permission from [75].

19

2 Structure and Properties of Clusters

A quantitative understanding of the lattice structure, stability and electronic

properties of crystals has been possible due to a number of important

developments in solid-state physics both experimentally and theoretically.

For example, Bloch’s theorem deals with the long-range periodicity of a

perfect crystal and density functional theory (DFT) have permitted accurate

determination of lattice structure, cohesive energy and the relative structure

stability, of various crystal phases. In this chapter, the interplay between the

geometric and electronic structure of cluster and how the properties of

clusters differ from bulk are presented. Models for metal clusters and the

effect of dielectric environment on optical properties of metal clusters are

described in detail.

2.1 Geometric and electronic structure

Clusters, as nanoscale objects, show distinct physical and chemical

properties that can significantly vary with size and composition. Clusters can

consist of either identical atoms (homoclusters) or two or more different

species (heteroclusters). Therefore, knowledge of the geometric and

electronic structure has a key importance in the understanding of cluster

properties and possible applications.

2.1.1 Geometric structure

An ideal crystal is constructed by the infinite repetition of identical structural

units in space. Crystals can exhibit 14 different lattice symmetries. Among

these, face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal

close-packed (hcp) structures are most common.76

For example, coinage

metals follow the fcc structure, alkali metals prefer the bcc structure and

alkaline-earth elements can form the hcp structure. Several structures are

possible for clusters. It has been a fundamental question: how these

20

structures evolve and how many atoms or molecules are needed for a

particular structure.

It is difficult to determine cluster structure unambiguously using the

same experimental techniques that are applied to bulk materials.

Nevertheless, there is some experimental techniques that are frequently used

for this purpose such as photoelectron spectroscopy, trapped ion electron

diffraction, ion mobility and infrared spectroscopy, reflection high energy

electron diffraction, X-ray scattering, etc. The experimental findings are

often compared with modelings and simulations and a good agreement about

cluster geometries is developed. As an example, the predicted structures of

small silver clusters are shown in Figure 2.1. The geometries of Ag clusters

show an evolution from planar to the three-dimensional pattern. Different

structural arrangements are possible for a fixed metal cluster size, which are

called isomers.77

78

Figure 2.1. Geometries of Agn clusters, n=4-20 atoms.78

Reprinted with permission

from [78].

21

In general, clusters of size n can favor structures that should minimize

the binding energy (Eb). This can be written in the form:79

𝐸𝑏 = 𝑎𝑛 + 𝑏𝑛2

3⁄ + 𝑐𝑛1

3⁄ + 𝑑, 2.1

where the first term represents volume contribution and the others represent

surface contributions from facets, edges and vertices, respectively. a, b, c

and d are constants as described by Northby et al.80

Volume and surface

contributions are in competition. To minimize the surface energy, clusters

require quasispherical shapes and close-packed facets. A volume

contribution to the structure comes from the internal strain, which build up

clusters of spherical shape.79

Heteroatomic clusters, consisting of more than

one kind of atom, provide a route to study the interaction of different atoms.

In general, compound clusters of any composition are possible even though

the corresponding elements cannot be alloyed in the bulk form. For example,

Al and K elements do not form an alloy in the bulk phase, but a single K

atom can be attached strongly to an Al13 cluster, another example is Al12C.4,

81-82 Numerous research has been performed to study geometric, electronic

structure and properties of heteroatomic clusters but these details are out of

focus for the current work and, therefore, are not presented here.

2.1.2 Electronic versus geometric structure

Electrons in atoms occupy discrete energy levels and the energy gap between

the highest occupied and lowest unoccupied orbital (HOMO-LUMO)

determines to a large extent their reactivity, stability and electronic

properties. Energy levels overlap when atoms form clusters. Thus, the

HOMO-LUMO gap position and size become very much dependent on a

number of atoms constituting the cluster and cluster species. In bulk metals,

electrons are delocalized and there is no energy gap at the Fermi level.

However, in the case of small clusters, there is always an energy gap

between the HOMO and LUMO irrespective of clusters material type. Thus,

clusters provide a playground to study transition in material properties at the

atomic level. For instance, small metal clusters do not exhibit metallic

bonding.83

Knight et al. observed distinct regularities in the mass spectra of

alkali metal (Na) clusters containing n=2-100 atoms per cluster.8 These

pronounced peaks in mass spectra of sodium clusters correspond to n=2, 8,

22

20, 40, 58 and 92. Points at which these peaks occur have been termed

magic number. See Figure 2.2.

Figure 2.2. (a) Mass spectrum of sodium clusters, n=2-75. (b) The calculated change

in the electronic energy difference versus n. The labels of the peaks correspond to

the closed-shell orbitals.8 Reprinted with permission from [8].

The closed atomic shells of delocalized valence electrons led to

increased stability of specific cluster sizes and magic numbers can be

modeled by applying a nearly free-electron model or the jellium model. In

this model the ionic charge is spread uniformly over a sphere of radius R and

Schrödinger equation is solved numerically for each value of n revealed

discrete states with decreased energies.84

Jellium model has been used

extensively to study the electronic structure of simple metal clusters.

For nonmetallic clusters, the occurrence of magic numbers can be

dominated by geometric arrangement of atoms in a cluster. Particularly in

rare gas clusters, the most stable ones are those for which the icosahedral

shells are completely filled.85-86

There is, typically, one central-positioned

atom and the others make shells around it. The magic numbers (N*) as a

function of complete icosahedral shells K are given as:

𝑛∗(𝐾) =1

3(10𝐾3 + 15𝐾2 + 11𝐾 + 3). 2.2

The peaks are at n* =13, 55, 147, 309, 561… for K=1, 2, 3, 4, 5…

respectively.84, 87

Clusters can also follow other geometric packings like

decahedron, octahedron, cuboctahedron etc.88

In the case of transition-metal clusters and clusters of semiconductor

elements, electronic shell-closure rule cannot be applied for stable clusters.

23

However, 18-electron rule has been used to illustrate the stability of these

clusters.4, 89-90

Moreover, this rule is applied to design stable clusters that are

useful for hydrogen storage.91

Another characteristic electronic behavior of metals is that the spacing

between energy levels depends on the number of atoms constituting the

particle. An HOMO-LUMO gap of a small cluster is larger compared to that

of the cluster with increased number of atoms. This situation directly affects

conductivity of nanoparticle which can be explained by the size of the Kubo

gap (𝛿) near the Fermi energy (𝐸𝑓):92

𝑘𝐵𝑇 = 𝛿 =4𝐸𝑓

3𝑁, 2.3

where N is the number of valence electrons and kB is the Boltzmann

constant. If the Kubo gap is smaller than the thermal energy the particle will

be metallic and if the gap is higher than the thermal energy the particle will

behave like an insulator.93

Hence, the smaller particles (clusters) having

larger gap require higher temperatures T for metallic conduction. The density

of states at EF is always finite for a cluster which means conductivity is

bound to be thermally activated, unlike in the case of bulk metals. However,

a sharp metal-insulator transition in monovalent and bivalent metals is not

strictly defined at some definite size or size range except for mercury

clusters.92

Different models, including Jellium model, have been developed

to describe the measured various properties of metal clusters. Further details

about these models are given in next section.

Clusters magic numbers related to geometric structure depend on a

number of factors, such as the atomic electron configuration, the electronic

density of states, cluster melting temperature (Tm) and the temperature of the

cluster (T). These factors also affect the size-dependent metal-insulator

transition in metallic clusters. The both temperature parameters are of critical

importance in deciding whether electronic or geometric effects prevail. For

T>Tm, geometric shell structure vanishes and cluster behaves like a spherical

liquid drop. In general, electronic shell structure is shown by clusters at

T>Tm and geometric shell structure is appeared in cold clusters, T<Tm. Thus,

transition from electronic to geometric shell structures can be achieved by

decreasing the T or increasing the cluster nuclearity.84

24

2.2 Models for metal clusters

A fascinating aspect of clusters is a change of properties as a function of

size. Theoretical models have been developed to describe this size

dependence of properties. Some of the models are overviewed below about

metal clusters.

2.2.1 Liquid drop model

One of the simplest approaches to describe properties of small particles is a

liquid drop model (LDM). LDM is an electrostatic model, in which the metal

cluster is considered as a uniform conducting sphere of radius R and its main

idea is to predict how some physical and chemical properties of a cluster

vary with cluster size.84

According to the LDM, the energy required to

remove an electron, ionization potential (IP), from a cluster should decrease

with increasing cluster size and electron affinity (EA) of a cluster should

increase as the cluster size increases.84

Accordingly, the following

expression has been developed for linear dependence of the IP and EA on the

reciprocal of the cluster radius, R:

𝐼𝑃(𝑅) = 𝑊 +3

8×

1

4𝜋𝜀𝑜𝑅 ,

𝐸𝐴(𝑅) = 𝑊 −5

8×

1

4𝜋𝜀𝑜𝑅 ,

2.4

with W being the work-function of the bulk metal and ɛo is the permittivity

of vacuum. Both equations show that the IP and EA of the cluster approach

the bulk property, i.e. bulk work function, as the size of the cluster increases

(i.e. n→∞). This is illustrated in Figure 2.3.

Figure 2.3. LDM prediction for the

variation of ionization potential

(IP) and electron affinity (EA) of

metal cluster versus the inverse of

cluster radius.With increase in

cluster size, the IP and EA moves

towards the bulk work function,

W.84

25

Thus, the LDM predicts the formation of an energy gap between

HOMO and LUMO, which increases linearly with decreasing cluster size.

However, for small clusters, the LDM breaks down due to quantum size

effects (QSEs). It is not able to explain, for example, magic numbers.

Intuitively, it makes sense that the clusters with filled geometric or electronic

shells are more stable, and thus it becomes harder to ionize such clusters.

Whereas if an extra atom is added to a cluster corresponding to a magic

number, it would be easier to ionize as the additional atom would have a

lower binding energy to the surface compared to those in the closed shell.

This also explains the immediate drop in ionization potential when moving

away from a magic number.84

The QSEs are related to the electronic shell closure of a cluster and

are ignored by the classical LDM. This model fails to explain the intense

peaks−magic numbers in the mass spectra of metals. These failures show

that a new model is required, which explicitly takes into account QSEs in

small clusters. A quantum mechanical treatment of small clusters is

presented in jellium model.8

2.2.2 Jellium model

In contrast to the LDM, the jellium model is a quantum mechanical model

giving rise to electronic shell structure for clusters with up to several

thousands of atoms (particularly large alkali metal clusters).94

In the

spherical jellium model, a metal cluster is seen as a uniform positively

charged sphere filled with an electron gas. The Schrödinger equation is

solved for an electron constrained by the potential created by the positively

charged background. The position of the ionic cores can be neglected when

valence electrons are weakly bound, and when the ionic background

responds very easily to perturbations, which is most likely to be satisfied for

delocalized valence electrons have s-wave character. These conditions are

fulfilled in both the noble and alkali metals.28, 84

Furthermore, the jellium model (as implied by its name) can only be

applied when the clusters are molten. To apply the jellium model, a spherical

jellium potential must be defined. Within this potential the interactions

between electrons are constructed (e.g. using DFT), and it is thus important

26

to find the most suited potential on experimental data. The results yield a

specific ordering (depending on the chosen radial potential) of jellium

orbitals according to the principal quantum number. The exact energy

ordering of the jellium levels, however, depends on the radial form of the

assumed effective jellium potential. For example, 3-D harmonic potential, 3-

D square well potential and Woods-Saxon potential.84, 94

Figure 2.4 shows

the comparison of energy levels and prediction of magic numbers using three

different jellium potentials. It can be seen that changing the good shape not

only changes the relative level spacings but may even alter their ordering.

Figure 2.4. Energy level

occupations and magic

numbers for three spherically

symmetric potential wells. The

numbers above the energy

levels indicate cumulative

totals of elections.28

Reprinted

with permission from [28].

The first difference in orbital ordering between the Woods-Saxon and square

well potentials occurs above the 2d orbitals; i.e. for more than 68 electrons.

As the cluster size increases, a long order oscillation in the abundance

spectrum becomes apparent which can be explained as the merging of

electronic shells into a band like supershells.

The spherical jellium model offers improvements over the liquid drop

model. However, there are certain observables that cannot be explained

adequately using the jellium model, for instance, IP, EA, magnetism and

polarizabilities in simple metal clusters. The spherical jellium model can

thus be expanded to include other non-spherical cluster geometries such as

oblate and prolate spheroid clusters.28, 84

27

2.3 Transition to the bulk

Small metallic clusters often have non-crystalline icosahedral or decahedral

geometries, while bulk metals are crystalline and normally possess fcc, bcc

or hcp structures. Thus, the structures having five-fold symmetry axes

involve non-crystalline packing of atoms. This means that at some cluster

size structural phase transition take place as a function of cluster size. This

leads to the important question: How many atoms does it take in the cluster

to mimic bulk behavior? It is still not clear to define critical nuclearity, at

which transition from non-crystalline to crystalline structure take place. For

example, the studies have shown that silver clusters of a 4 nm in diameter

have a decahedron structure while cluster with a diameter of 20 nm has a

multi-twinned fcc structure.95

For small size clusters, the ratio of surface to

volume atoms is high, resulting in minimum surface energy corresponding to

packing according to geometrical polyhedral. As the cluster gets larger, the

elastic strain increases and the cluster will be more inclined to adopt bulk

crystalline structure. The transition depends on the surface to bulk ratio of

atoms, which varies as N-1/3

.84

However, recent studied have shown that this

is not always the case. Large (on the scale of 100 nm) silver clusters can

follow regular shape, including species with fivefold symmetry.96

2.4 Properties of clusters

Physical properties of clusters are altered with size, including the binding

energy of surface atoms, ionization potential, electron affinities, melting

temperature, chemical reactivity, etc. An example of melting temperature,

latent heat of fusion and entropy dependence on sodium cluster size is given

in Figure 2.5.

28

Figure 2.5. Top: Icosahedral clusters growth with 2nd

, 3rd

and 4th

layers are colored

in yellow, green and red respectively. Bottom: size dependence of melting

temperature (black), latent heat of fusion (red) and entropy change upon melting per

atom (blue). The solid black lines are calculated entropy change upon melting.97

Reprinted with permission from [97].

These properties show pronounced maxima for magic numbers. The

ionization potential, for instance, is at its maxima for magic numbers, and

then it decreases to a lower value for the next cluster size, from which it

increase again towards the next magic number and so on. The cluster size

equation attempts to provide a quantitative understanding of the size

dependence of a cluster properties, which is given as98

𝜒(𝑛) = 𝜒(∞) + 𝐴𝑛−𝛽, 2.5

where A is a numerical constant and the exponent β is in the range 0 ≤ 𝛽 ≥

1.There are two types of cluster size effects. Specific size effects which

appear in small clusters due to closure of electronic or geometric

shells−magic numbers. The other one is smooth cluster size effects, which

are observed in large clusters and can be interpolate linearly to the bulk

properties. As most of the clusters properties depend on the ratio of surface

to volume atoms. Therefore, applying spherical cluster approximation, the

fraction Fs of atoms which lie on the cluster surface can be computed as84

𝐹𝑠 =𝑛𝑠

𝑛= 4𝑛

−13⁄ . 2.6

29

The scaling laws for multiple cluster properties can be fitted to a high degree

of accuracy for large clusters while there are deviations for medium and

smaller clusters. An example of such a relation is given by Müller et al. for

the melting temperature of gold clusters as a function of radius1 and

ionization potential of potassium clusters as a function of clusters size n.84

See Figure 2.6.

Figure 2.6. (a) Melting temperature

of gold clusters as a function of

radius R. (b) Ionization potential of

potassium clusters as a function of

cluster size n. The dashed line

indicates the bulk corresponding

bulk property. The dots are

experimental data and interpolation

of data; red line.84

Clusters can exhibit a range of unique and fascinating reactive,

optical, electronic and magnetic properties. This unique behavior is due to a

variety of factors such as the high ratio of surface-to-volume atoms,

electronic or geometric shell closings, etc. Control over the cluster size on

atomic level has proven to be a valuable method for increasing activity and

tuning the selectivity in a catalytic process. For example bulk, gold is

unreactive, small gold clusters (Au8) are active for the oxidation of CO and

Pdn (n≤25) clusters deposited on rutile for the oxidation of CO.20

There is a

close relation between the topology of a cluster and its magnetic properties.

For example, it has been observed in the study of Li4 cluster, in which planar

structure shows nonmagnetic behavior (spin singlet state) and when Li4

forms a tetrahedron, the preferred spin is a triplet and magnetic moment is

activated.99

The optical properties of elements are determined by their

electronic structure and energy band gap. However, in the case of clusters,

the HOMO-LUMO energy gap varies with clusters size and composition. It

30

is also possible to manipulate cluster optical properties for desire practical

applications e.g. coating the clusters with different ligands or surfactants.

The coinage metal nanoparticles change color when illuminated by white

light, with the change in their sizes. This phenomenon has been known for

centuries and successfully used in stained glasses. The optical biosensors

based on the principle of LSPR have been developed.15, 100

101

The sensitivity

and reliability of LSPR transducers can be improved by changing the size,

shape, spatial positioning and composition of the plasmonic nanostructures.

More detail about LSPR in metal NPs and effect of surrounding medium on

plasmon resonance is given in the next section.

2.5 Optical properties of clusters

Optical properties of metal NPs have been in use for over thousands of

years, for example, stained glass windows in churches and the Lycurgus Cup

that dates back to the 14th century. However, a scientific understanding of

the phenomena was not reached until 1908, when the German physicist

Gustav Mie found exact solutions to Maxwell’s equations for spherical

particles of a small size.102

When a metal NP is exposed to an

electromagnetic wave with a wavelength much greater than the radius of the

NP, the oscillating electric field will cause the conductive electrons to

oscillate coherently which will lead to a displacement of the electron cloud

about the particle cores as seen in Figure 2.7.

Figure 2.7. Schematic of plasmon oscillation for a metal NP, showing the

displacement of the conduction electron cloud about the core or nuclei.

31

The Coulomb attraction between the core and the electron cloud will give

rise to a restoring force that induces the electron cloud to oscillate relatively

to the core.103-104

This phenomenon is known as LSPR.

Optical properties of NPs depend on a range of parameters such as

particle size, shape, type of material and the dielectric environment around

them. Therefore, it is of utmost interest to investigate simple theoretical

solutions, which give an approximate form of the extinction cross-section of

an NP with respect to Mie’s solutions.103-104

The collective oscillation of the

electrons, seen in Figure 2.7 is often called dipole plasmon resonance. For

large particles higher modes, such as quadrupole, can appear. In this case,

half of the electron cloud will oscillate in the parallel direction of the applied

electric field while the other half will oscillate in the anti-parallel direction to

the electric field.

2.5.1 Dipole plasmon resonance

To describe the LSPR phenomenon, a model is required to link some of the

characteristics of the metal NP to the extinction spectrum. For calculations,

the key equations of Mie solution are used while a full analysis of the Mie

theory is beyond the scope of this work.

Assuming a spherical particle with a diameter much smaller than the

wavelength of the irradiated light, the electric field can be approximated to

be constant over the entire particle that is called the quasi-static

approximation. It is then possible to relate the dielectric constant εnp, which

is measurable as a function of the incident wavelength for bulk material, to

the dipole plasmon resonance. Hence, electric field can be denoted �� 0 = 𝐸0𝑥

with 𝑥 being the unit vector when the electric field is travelling in the x-

direction. It is known that the electric field is related to the electric potential

(𝜑) by �� = −∇𝜑, so by solving the Laplace’s equation ∇2𝜑 = 0 for

electrostatics it is possible to calculate the electric field outside the

sphere.103-104

The solutions of Laplace’s equation are radial. The radial

solutions are of the form 𝑟𝑙 and 𝑟−(𝑙+1), where Ɩ is the angular momentum (Ɩ

= 0, 1, 2, …) of atomic orbitals.104

32

For Ɩ = 1 and a being the radius of the sphere, the potential inside and

outside the sphere is

𝜑𝑖𝑛𝑠𝑖𝑑𝑒 = 𝐴𝑟 sin 𝜃 cos𝜙 , 𝑟 < 𝑎 2.7

and

𝜑𝑜𝑢𝑡𝑠𝑖𝑑𝑒 = (−𝐸0𝑟 +𝐵

𝑟2) sin 𝜃 cos 𝜙 , 𝑟 > 𝑎 2.8

where A and B are constants that can be obtained by applying the boundary

conditions stating that both the electrical potential (𝜑) and the normal

component of the electric displacement �� = 𝜖�� are continuous at the

particle surface.104

For a metal sphere with a complex dielectric function of the form

εnp = εr + iεi and radius a, the polarizability is described as104

𝛼 = 𝑔𝑑𝑎3, 2.9

with

𝑔𝑑 =𝜀𝑛𝑝−𝜀0

𝜀𝑛𝑝+2𝜀0, 2.10

where 휀0 is the dielectric constant of the surrounding medium. From the

above equation it can be seen that the polarizability will be at its maxima

when the denominator in Eq. (2.10) is at a minimum.

Applying the Mie theory,102

the following expression for the

extinction efficiency 𝑄𝑒𝑥𝑡 for a spherical metal NP of radius a irradiated by

an electromagnetic field is obtained.104

𝑄𝑒𝑥𝑡 =8𝜋𝑎√𝜀0

𝜆𝐼𝑚[𝑔𝑑], 2.11

where λ is the wavelength of the incident electromagnetic wave and the

imaginary part of 𝑔𝑑 is then given as

𝐼𝑚[𝑔𝑑] =3𝜀𝑖𝜀0

(𝜀𝑟+2𝜀0)2+𝜀𝑖2. 2.12

33

It is worth noting that Eq. (2.11) is developed for the case when a particle is

so small that the absorption makes the major contribution to the extinction

while the scattering is neglected due to the small cross-section. At this

conditions 𝑄𝑒𝑥𝑡 reaches a maximum for 휀𝑟 = −2휀0, i.e. plasmon resonance

will occur. For noble metals such as silver, copper and gold, the LSPR will

then give rise to an absorption band in the visible range, which makes these

NPs suitable for several applications.15

The optical signature of metal NPs depends on the shape, size,

interparticle distance and the dielectric constant of the environment

surrounding the NP.103-105

While considering the shape and interparticle

distance requires the development of more complex theories, using Eq.

(2.11) one can make a very simple prediction for extinction efficiency of

spherical particles in different dielectric environments. One of such simple

models is made for Ag NPs in air and PMMA, see Figure 2.8. It is worth

noting that these models are of practical interest for the goals of the current

project and they are used for comparison with experiments presented in

publication D of the appendix. The real part of the dielectric functions of

PMMA and air are considered to be constant in the visible spectral range

having values 2.60 and 1.00, respectively. Dielectric function of silver,

which is wavelength dependent, is obtained from Johnson et al.106

Figure 2.8. (a) Calculated extinction efficiency of Ag NPs of different radii. (b)

Extinction cross section of Ag NP of radius 5nm for different environments: air,

PMMA, a-carbon, mixtures of PMMA, a-carbon and air.

As shown in Figure 2.8(a), the intensity of extinction signal is

increased with particle size and there is no change in peak position and

width. However, this is not in agreement with numerous experimental

34

studies, in which it is found that a decrease in nanoparticle size leads to an

increase in resonance bandwidth and the peak is “blue” shifted.103

In reality

dielectric function varies with cluster size as well. Therefore, the Mie theory

must be extended to include that the dielectric function is also size

dependence. In Figure 2.8(b), extinction efficiency of Ag cluster, the

diameter of 10 nm, is plotted for different dielectric environments such as

air, PMMA, amorphous carbon (a-C), 0.5PMMA/0.5air, 0.2a-

C/0.6PMMA/0.2air and 0.3PMMA/0.7air, demonstrating how the position

and intensity of the plasmon band depend on the dielectric environment.

2.5.2 Plasmonic coupling

So far in the above descriptions, the particles have been implicitly assumed

to be non-interacting. However, if the particles are sufficiently close to each

other, it is important to consider what effects that might have on their optical

properties. Moreover, the manner in which the particles are also assembled

affects their extinction efficiency, which has been studied by Amendola et

al.107

The most common effect is that a spectrum will experience a red-shift

and peak broadening upon decreasing the interparticle distance.108

Gong et

al. show that as the interparticle distance becomes very small, a particle

plasmon coupling effect is observed. Furthermore, they also noted that even

for particles of small sizes, quadrupole and higher terms need to be included

in the calculations of the extinction spectra.105

Atay et al. showed that for a

pair of gold NPs in proximity, the second peak at longer wavelength appears

and this long wavelength resonance eventually dominates as the particles

merge to nearly ellipsoidal shape due to dipole-dipole interactions of two

particles.109

This phenomenon will be considered in more detail about

experimental data of this work.

35

3 Cluster-Surface Interaction and Applications of Clusters on Surfaces

3.1 Interaction regimes

The use of interaction of cluster beam with a substrate represents a new way

of preparing nanostructured surfaces. One can bombard the surfaces by

energetic clusters to modify them on the nanoscale or deposit NPs onto a

substrate and utilize their specific properties. Thus, to understand and use

clusters in modern applications, knowledge of the cluster-surface interaction

is required. When describing the cluster-surface interaction, the kinetic

energy, Ekin, of the cluster as well as the binding energy of cluster

constituents are of particular importance. Therefore, the cluster-surface

interaction processes are often divided into two groups: low- and high-

energy ones.110-113

These clusters interaction regimes are discussed in detail

below.

Low-energy interaction is often called soft-landing or deposition,

where the main goal is to preserve unique cluster properties for practical

applications. In the low-energy regime, the kinetic energy per atom, Eat, of

the cluster is lower than the binding energy or cohesive energy, Ecoh, of the

cluster constituents. Typically, the cohesive energy is below the level of

1eV/atom but also depends on the size and material of the cluster. One of the

advantages of soft landing is that almost no deformations or fragmentation of

the clusters are expected to impact with the target surface. However, as the

difference between kinetic and cohesive energy is lowered, the more

deformed the cluster will be. This is illustrated in Figure 3.1(a), where the

cluster shape is slightly distorted upon landing. It is equally important to

consider what happens to the clusters after deposition. The cluster can

diffuse on the surface and originate two types of processes. If the clusters are

small, they can coalesce, which means that the clusters merge and form a

new larger cluster. The other possibility is agglomeration that likely to

happen for large clusters or at low temperatures. Coalescence of larger

clusters is also possible, which may form clusters of different shape as well.

36

The chance of a cluster to diffuse more than a diameter during a soft-landing

is negligible during the time another cluster would impact the neighboring

position. If the clusters do not coalesce or agglomerate, the morphology of

the surface is then a random paving of clusters on the substrate, but if the

clusters are aggregating and no coalescence occurs, large branching islands

will appear on the surface.110

The degree of diffusion will depend on the

surface-cluster bonding, which is, for example, low for graphite surface

where the diffusion was found to be prominent.53, 110, 114

Figure 3.1. Illustration of cluster-surface interaction. (a) Soft-landing regime and (b)

high-energy impact.

If the kinetic energy per atom in a cluster exceeds Ecoh, the impact is

considered to be high-energy and it is often called implantation or hard

landing. It is illustrated in Figure 3.1 (b). If Ekin is only slight above the Ecoh,

the clusters will undergo significant deformation, where it fragments

partially, but most of the cluster atoms will remain intact. If Ekin is further

increased and larger than the threshold implantation energy of the substrate,

it will result in implantation.110

Furthermore, high-energy impacts can be

used to generate films of the cluster material on a surface.111

The Ekin of the cluster becomes more important as the cluster size

increases and thus soft or hard landing must be considered differently. For

smaller clusters the cluster-surface interaction is thus highly influenced by

the difference between Eat and Ecoh, whereas, for larger clusters it is mainly

influenced by Ekin and the properties of the substrate. One should also

mention a boundary case between soft and hard landing which is called

pinning regime.112, 115-116

In this interaction, the cluster displaces some

surface atoms and become bound to the formed radiation defects. Pinning of

37

the incident clusters prevents the diffusion on the surface. These immobilize

pinned clusters can be used as preferential binding sites for proteins.117

3.2 Cluster deposition

The state of the art of cluster-surface interaction shows that the field is very

promising regarding practical applications and both low- and high-energy

cluster beams are used in industry.47, 118

Metallic clusters have been of great

interest to researchers during the last couple of decades for a number of

reasons. Deposition of clusters has been shown to be an effective way to

grow high-quality thin films at low temperatures and high deposition rate.119

Surface scientists have been involved in investigating behavior of deposited

clusters, in particular, diffusion, aggregation or agglomeration the role of

surface defects and roughness.120-122

For example, Stefano and Richard found that the size-selected gold

clusters on different optical substrates (glass, quartz, mica and poly (methyl

methacrylate (PMMA)) remain monodispersed even months after deposition.

They have found cluster flattening after the impact on soft PMMA and

quartz surface.121

The roughness of glass and PMMA surfaces limits cluster

diffusion. On the other hand, clusters mobility is favored on weakly

interacting surfaces, such as graphite or amorphous carbon. This

phenomenon was extensively studied for different metals.114, 120, 123-124

Cluster deposited on graphite substrate diffuses towards step edges and form

nanofractal aggregates on the smooth graphite surface. Surface coverage of

clusters on terraces, areas between steps, can be controlled by either

reducing terrace width or deposition at higher temperatures.114

In this work

we have also seen a similar pattern on graphite surface which will be

discussed in chapter 5.

A lot of practical work is going on to use metal clusters for electronic

and plasmonic applications, sensing and catalysis. For many of them,

polymer substrates with deposited or embedded metal NPs become attractive

systems for practical use. However, metals and polymers are materials with

very contrast properties and, therefore, the appropriate methodology should

be used for the formation of metal-polymer nanocomposites. Kovacs et al.125-

126 showed that complete embedding of NP into a polymer is expected due to

38

the higher surface tension of metal NP compared to the polymer and NP-

polymer interfacial tension. However, this is not always the case for particles

sitting on the polymer surface. It is further found that long-range polymer

chain mobility is required for particles immersion and it is temperature

dependent. A new embedding mechanism is proposed in which gold NPs

deposited onto polystyrene (PS), where the NP becomes covered by a thin

PS wetting layer (1.3-1.8 nm) upon annealing above the glass transition

temperature. This thin layer creates capillary pressure on a NP and facilitates

the embedment.127

Other methods have been employed to produce metal-

polymer composites such as solvent annealing128

and metal ion

implantation.129-130

It is worth noting that all above-presented methods

provide very limited control over particles size, coverage and distribution

within polymer layer.

Nobel metal clusters or NPs can serve as optical probes for

biomolecules due to their remarkable optical properties.131

Two steps are

important to convert NPs into a useful sensor: immobilization of NPs on a

surface and functionalization of metal NPs surface for attachment of

biological species. In this work both steps were optimized to achieve stable

transducers for optical sensing of particular proteins,15

which is presented in

Chapter 6.

3.3 Metal nanoparticles for biosensing

In the modern world, we are surrounded by sensors, constantly transferring

to our information about our environment. In general, a sensor is defined as a

device that detects a change of given physical or chemical parameters. For

example, canary birds in coal mines have been used to detect the presence of

carbon monoxide, a toxic gas, if the bird passed out, they knew it was time

to get out of the mine. Today, this “cruel” method has been replaced with

modern gas sensors that have better sensitivity compared to the canary bird.

The performance and accuracy of these sensors have been improved over the

past years, but there is still a demand for faster and more reliable sensing

devices with improved sensitivity and selectivity. Most common daily life

examples of biosensors are glucose sensors and home pregnancy test. A

significant progress in fabrication of nanostructures has made it possible to

39

achieve high level of performance especially in the field of biomolecule

sensing.132

The work included in this thesis focuses exclusively on optical

biosensors: a type of sensor that is designed to recognize a biological reagent

by the change of optical properties of the detecting system that is called a

transducer. Optical transducers offer a broad range of signal parameter such

as a change in wavelength, phase, polarizability and intensity of light.

Biosensors are composed of three main components: a biorecognition

species that interacts (binds or recognizes) the analyte under study, a

transducer that converts biological interactions into a measurable physical

signal and a device that registers this signal.

Figure 3.2 is a schematic illustration of a biosensor based on LSPR of

nanoparticles. Enzymes or proteins of interest become bound to antibodies

incubated on NPs serving as transducers. This binding changes optical

properties of the transducers that are registered by the optical spectrometer.

Various types of molecules have been used as biorecognition objects

including antibodies, receptors, enzymes, nucleic acids, etc.

Figure 3.2. Schematic illustration of nanoplasmonic sensing. A shift of the plasmon

peak position is observed in the extinction spectra when proteins interact with Ag

clusters.

Biosensors can be divided into two classes: label-free and labeling.

Label-free techniques such as surface plasmon resonance (SPR) provide

real-time and quantitative analysis of molecular binding events while

40

labeling method involves tagging of the biomolecules that can alter the

functionality of the molecule and induced molecular aggregation. However,

in label-free method an appropriate surface chemistry has a vital role in

device performance. The surface chemistry should provide a binding site to

the biorecognition elements without affecting the functionality. Therefore, a

lot of work has been performed to develop appropriate surface chemistry

methods.133-134

SPR-based sensing has been extensively used to monitor and study

biomolecular interactions in real-time.135

It was presented by Liedberg et al.

in 1983, where they studied the interaction between an antibody and antigen

on a silver surface.136

These sensors measured changes in real-time in the

reflected light due to change in refractive index close to the metal film.

When it comes to sensitivity and local molecular interaction on a metal

surface, the idea of LSPR biosensors was presented. As described in section

2.5, the phenomenon of LSPR in metal nanostructures depends on the

dielectric properties of the surrounding medium. This helps us to study the

molecular interactions that occur in the local proximity of the metal

nanostructures. Attachment of molecules to metal nanostructure changes the

dielectric environment; the resonance condition will be changed resulting in

shift in the plasmon peak, see Figure 3.2. The concentration of attached

molecules can be estimated by measuring the plasmon peak shift.

Englebienne in 1998, first time demonstrated experimentally that LSPR in

gold NPs could be utilized for biosensing.137

However, there is a huge gap between the demonstration of a principle

and real reliable sensors that can be applied in industry. One need to

consider a number of aspects: (i) formation of a transducer, for instance,

nanoparticles of given composition, size, shape deposited or embedded on/in

appropriate environment; (ii) good quality signal generation (possibility of

tuning the signal parameters and high signal-to-noise ratio) and repeatability;

(iii) design of detection scheme (sample injection and drainage, reduction in

detection time, binding approaches, etc.); (iv) surface immobilization

chemistry providing selectivity of sensing (capture efficiency, elimination of

false detection); (v) data analysis (signal registration and evaluation

procedure). Therefore, there is a continues research on LSPR-based sensor

systems which involves studies on detection of a large variety of

41

biomolecular interactions such as such as antibody-antigen,15, 138

DNA-

DNA,139

biotin-steptavidin140

as well as various aspects of design, the

selectivity of sensing and approaches for data analysis.

43

4 Experimental Techniques and Methods

In this chapter, it is presented how clusters are formed, collimated into a

beam and mass-selected before landing onto a substrate. After that, some

properties of target materials and substrate preparation method are given in

detail. Finally, the methods used for investigation of deposited metal

clusters are briefly overviewed.

4.1 Magnetron sputtering cluster apparatus

The MaSCA for the production and deposition of size-selected metal clusters

is shown in Figure 4.1. The description that is presented below goes along

with the publications presented in Appendixes A and B. The experimental

setup consists of four main chambers: source chamber, ion optics chamber,

electrostatic quadrupole mass selector (EQMS) chamber and deposition

chamber. The setup is equipped with three turbomolecular pumps (from 230-

1250 l/s) backed by fore-vacuum pumps, all from Pfeiffer Vacuum. Using

differential pumping a background pressure of 1×10-8

mbar can be achieved

in the deposition chamber. Two ion gauges are installed to measure the

pressure inside the chambers. Two pneumatic UHV gate valves (leak rate <

1×10-10

Torr l/s), that are controlled by an electronic module, are used to

separate vacuum chambers. In particular, they isolate EQMS chamber from

ion optics chamber and the cluster implantation and deposition apparatus

(CIDA).29

As CIDA has not been used in this work, therefore it is not

described further in detail.

Source: One of the most important parts of MaSCA is cluster source (see

Figure 4.1) which is commercially available gas aggregation source based on

magnetron sputtering, NC-200, from Oxford Applied Research. The target

material, for instance, Cu or Ag, is sputtered into aggregation chamber with

walls cooled by liquid nitrogen. Aggregation region is a cylinder of 100 mm

in diameter. The aggregation length can be varied by changing the distance

between the target and nozzle. Ar and He are used as a sputter and carrier

44

gas, respectively. Magnetron power, gasses flows, and length of the

aggregation region determine the cluster sizes and beam intensity. The

clusters are expanded through a nozzle at the end of the aggregation tube

into the source chamber, where they are collimated into a beam of clusters

by passing through a skimmer.

Figure 4.1. Schematic view of MaSCA and its lab picture at Aalborg University.

Nozzle-Skimmer: Nozzle and skimmer play an important role in clusters

beam formation. Therefore distance between them and their geometric

shapes are very important.38

In the current setup, the distance between nozzle

and skimmer can be varied but for all the experiments it is kept to be 3 cm.

The main purpose of the nozzle is to provide a pressure difference between

45

the aggregation region (p1 in Figure 4.2) and source chamber ((p2 in Figure

4.2) and to accelerate expanding clusters. This change in velocity can be

explained by applying the equation of continuity for two regions of different

cross-sectional areas ((A1 and A2 in Figure 4.2). To form a cluster beam, the

streamlines of expanded gas should be collimated. For beam formation, a

conical shape skimmer is placed in-line with the main axis of the nozzle. A

schematic of fluid flow through nozzle-skimmer is shown in Figure 4.2.

Figure 4.2. Schematic of gas flow through a nozzle-skimmer configuration.

Ideally, the skimmer should collimate the beam in a sense, such that it

will revert from turbulent flow, which can be achieved by applying

appropriate geometry of the skimmer. One of the easy solutions is a conical

skimmer of slant angle ~30o to avoid any disturbance in central beam line of

flow. It also requires that cone edges should be as sharp as possible. The

alignment of nozzle-skimmer main axes is very important and this was done

for the current setup using a laser.

Ion optics: After the skimmer the cluster beam enters the ion optics

chamber, see Figure 4.1. This chamber consists of an Einzel lens and two

pairs of deflector plates, which enable focusing the beam of charged clusters

and steering with respect to the central axis of the chambers. The motion of

charged particle can be manipulated by electric or magnetic fields. In this

work, an electrostatic lens is used because the focusing power of the

magnetic lenses is inversely proportional to the mass of the particle.

Therefore, magnetic lenses are typically used for focusing of electrons.141

For massive clusters a very strong magnetic field would be required. An

Einzel lens is typically made by aligning three cylindrical apertures along the

main axis. The symmetric electric field is created by applying positive or

negative potential at the middle aperture, which keeping two others to be

46

grounded. A schematic illustration of an Einzel lens is shown in Figure

4.3(a). The focal point is determined by the applied potential. The kinetic

energy of the charged particle is preserved during the focusing process, but

the direction of motion is changed. Two pairs of electrostatic deflectors (see

Figure 4.3(b)) are used to move the beam up/down or left/right on the main

axis by applying an appropriate voltage between the plates of the defectors.

Figure 4.3. (a) Schematic picture of an Einzel lens is focusing positively charged

clusters. The red line shows the main axis. (b) Inset is CAD drawing of the ion

optics chamber.

As the kinetic energy is conserved, the particle velocity is also

unchanged. This means that the outermost particles will arrive later at the

focal point than the central line particles because they will travel a longer

distance to the focal point, resulting in aberrations. In the specific case of

particles of different masses moving through an Einzel lens with the same

velocity, their kinetic energy will only depend on the mass of the particle. As

the Einzel lens is made to preserve the kinetic energy of particles moving

through it, this means that the focusing power will be different for particles

of different mass. Consequently, charged particles of different mass (but

same charge), will have different focal points. This leads to a broadening of

the cluster beam, but it also opens the possibility of moving heavier particles

away from the focal point, which can be advantageous for the mass

separation process.142

47

4.2 Electrostatic quadrupole mass selector

Clusters produced using a source usually have a wide size (mass)

distribution. For many practical applications, however, the clusters must be

mass separated. Techniques used for the size-selection depend on the charge

state and precision of the mass selection. A significant fraction of clusters

produced using surface erosion typically is ionized. For instance in the case

of magnetron sputtering, over 50% of ejected material can be charged

(positively and negatively). However, post-ionisation can be employed to

increase the fraction of the charged clusters or charge the neutral clusters. A

range of mass selecting methods is available for the charged clusters such as

Wien filter, linear TOF, and quadrupole spectrometers. More details can be

found elsewhere. 28, 49, 143

In this work, an EQMS similar to that described by Hartmann et al. is

used.144

EQMS is illustrated in Figure 4.4. It consists of four hyperbolic

shaped electrodes. The electrodes are divided into two pairs that are biased

with the same absolute voltages but opposite polarities. These hyperbolic

electrodes are surrounded by a grounded shield with circular orifices for the

cluster beam. The orifices are covered by nickel grids to minimize field

penetration. Electric potential applied to the electrodes (UQP) produces

hyperbolical equipotential field deflecting the clusters in opposite directions

Figure 4.4. (a) A cross-sectional view of the EQMS with particles trajectories. (b)

Photo of the EQMS.

depending on their charge, see Figure 4.4(a). The deflection depends on the

balance between the kinetic energy of particles and energy of the

48

electrostatic field inside the mass selector. Thus, by tuning the potential, the

clusters of desired mass can be selected.

The energy (E) needed for a cluster to be deflected for 90o at given

UQP is

𝐸 = 𝑞𝑈𝑄𝑃 , 4.1

where q is the particle charge. This energy should balance the kinetic energy

of the particle, yielding

𝑚

𝑞=

2𝑈𝑄𝑃

𝑣2. 4.2

This equation shows how the mass-to-charge ratio (m/q) is determined by the

UQP. It also assumes that the particles have the same velocity, as they enter

the EQMS, and thus justifies the use of an Einzel lens for cluster beam

focusing. EQMS is useful for high throughput but does not provide very high

mass resolution.

However, above description is based on ideal conditions in which particles

are assumed to have the same charge, velocity and beam adjusted exactly

along the main axes. Experimentally, one has to deal with many practical

issues such as beam divergence, velocity dispersion, beam angle, and multi-

charged clusters. Also, the shape of the electrodes, if differs from the ideal

hyperbolic geometry, requires a geometric correction factor on the right-

hand side of Eq. 4.2.144

However, it will be shown in the next chapter that

assumptions on the same cluster velocity and single charged particles are

rather good approximation allowing in practice to reach good size selection

without overcomplicating Eq. 4.2.

4.3 Materials and sample preparation

Copper (99.99%) and silver (99.99%), from Goodfellow Cambridge Ltd and

China Rare Metal Material Co. Ltd, were used as sputtering target materials

for the cluster production.

A few series of samples have been produced by cluster deposition on

different types of substrates such as n-type silicon (100), highly ordered

49

pyrolytic graphite (HOPG), mica, quartz and quartz covered by PMMA thin

film prepared by spin coating. Silicon and quartz substrates were thoroughly

sonicated in three step using acetone, ethanol and isopropanol before

transferring to the deposition chamber. Fresh surfaces of HOPG and mica

were prepared by cleavage. PMMA films (20-100 nm in thickness) were

produced by standard spin coating from 1% polymer (950 PMMA C 9,

MicroChem, America) solution onto quartz and Si (100).145

PMMA on Si

samples were primarily used to measure the film thicknesses in a standard

ellipsometer. A schematic of sample preparation technique is shown in

Figure 4.5. For cross-sectional TEM analysis, the additional SiO2 (40 nm

thick) layer was deposited on PMMA with clusters using electron beam

evaporation technique as shown in the bottom row in Figure 4.5.

Figure 4.5. Schematic of sample preparation procedure for cluster deposition on bare

substrates and those covered by PMMA and SiO2.

In the present work, copper and silver clusters are deposited at thermal

velocities not exceeding 200-250 m/s, corresponding to a few meV/atom,

which is in the soft-landing regime. The prepared samples can be split into

three main experimental series. In the first series, the clusters of copper and

silver were soft-landed at different EQMS voltages to find a correlation

between the applied potential and real sizes of the deposited clusters. It was

also studied how the different substrates affect the surface cluster

arrangement. In the second series, silver clusters of a given size (ca. 13 nm

in diameter) were deposited on PMMA of different hardness and subjected

to post-deposition thermal annealing to study immersion of particles into the

50

polymer. This case was compared to the cluster formation in PMMA using

ion implantation. In the third series, silver clusters deposited on

functionalized quartz and PMMA were used as optical transducers to study

the possibility of protein detection.

4.4 Investigation of deposited clusters

The clusters deposited on different substrates were studied using atomic

force microscopy (AFM), scanning electron microscopy (SEM) and

transmission electron microscopy (TEM). Ellipsometry was employed to

measure the thickness of spin-coated PMMA films. Optical transmission

spectroscopy was used to study the LSPR phenomenon. A brief description

and modes of operation of these techniques are given below.

4.4.1 Atomic force microscopy

AFM was invented in 1985 by Binnig, Quate and Gerber.146

The basic

principle of AFM is to measure the interactive forces (van der Waals,

electrostatic, magnetic, etc.) between a tip and a sample surface using an

elastic cantilever with a sharp tip located at its end. The most common

cantilevers are made of silicon with 5-20 nm tip radius. The forces on the tip

result in a bend of the cantilever, which can be registered by the deflection of

the laser beam and translated into software enabling generation of

topography or other images. There are several AFM modes of operation, but

the most common modes of obtaining the surface morphology are: contact,

tapping (semicontact) and noncontact ones, see Figure 4.6 (a). In the current

work, the only tapping mode was in use. In this mode, the cantilever is

driven near its resonance frequency using a piezo-oscillator.147

The

cantilever is placed very close to the surface and the tip experience attractive

van der Waals forces most of the time except the bottom swing where it can

go into a repulsive regime with the surface atoms. Furthermore, when the tip

interacts with the sample, forces may cause a phase change in the

oscillations made by the cantilever. This phase change can give additional

information about the sample.

51

Figure 4.6. (a) Force vs. distance curve. Illustrating different tip-surface interaction

regimes (b) Picture of Ntegra-Aura system used for the measurements.

The AFM measurements in the present work were performed using

the NT-MDT NTEGRA-Aura nanolaboratory. The samples were studied

using tapping mode under ambient conditions. Commercial silicon

cantilevers having tips with a nominal radius of curvature ~10nm and the

resonance frequency of 90-400 kHz were used.

4.4.2 Transmission electron microscopy

A transmission electron microscope produces a monochromatic electron

beam and it operates on the same basic principle as the light microscope but

uses electrons instead of photons.148

The optimal resolution attained for TEM

images is thousand times better than that for a light microscope because of

the lower wavelength of electrons. Therefore, TEM can reveal the finest

details of internal structure – in some cases as small as individual atoms. A

TEM typically has three essential systems: (1) electron gun and a condenser

lens for focusing the beam onto the object, (2) image-producing system,

composed of an objective lens, specimen stage, intermediate and projector

lenses and (3) the image-recording system, that converts the electron image

into black-white 2D-image. To determine the cluster size, shape and

immersion into PMMA layer, a focused ion beam milling utilizing FEI-

Versa was applied and TEM analysis was performed using FEI-Talos

microscope operating at 200 kV. TEM measurements were carried out at

Interdisciplinary Nano Center of Aarhus University by Dr. Raghavendra

Juluri in the frameworks of collaboration.

52

4.4.3 Optical spectroscopy

Optical spectroscopy is concerned with the transmittance, absorption and

reflection of UV-, IR- and visible light. PerkinElmer Lambda-1050

UV/VIS/NIR dual-beam spectrometer was used to obtain transmission

spectra of the samples. The study was carried out on the samples with

clusters deposited onto quartz and PMMA covered quartz substrates. The

spectrometer uses two identical beams of light to determine the absorbance

of a given sample; a test beam and a reference beam. The test beam passes

through the sample and photodetector measures the irradiance of the beam at

specific wavelengths. The irradiance of the reference beam is measured in

the same way, however, in this case, beam travels unobstructed to the

photodetector. Thus, difference in irradiance of the two beams gives

transmission of the sample for specific wavelengths. In this work, the

interest was mostly in registering LSPR of silver clusters. Therefore, the

measurements were carried out for a wavelength range 300 -750 nm with an

internal of 1nm. As described in section 2.5, the change of transmittance in

the case of LSPR is caused by combined phenomena of absorption and

scattering that is called extinction. Thus, the obtained transmission spectra

were converted into extinction ones. In Figure 4.7 the hardware features of

the spectrometer are shown.

Figure 4.7.The hardware features of the PerkinElmer lambda-1050 spectrometer.

4.4.4 Ellipsometry

Ellipsometry measures a change in polarization as light reflects from a

material. The change in polarization depends on optical properties and

53

thickness of individual materials. Therefore, ellipsometry is primarily used

to determine film thickness and optical constants. Two parameters become

relevant: the amplitude ratio and the phase change between s- and p-

polarized electromagnetic waves. The film thickness is determined by

interference between light reflecting from the surface and light traveled

through the film but been reflected from the interface with the substrate. The

phase information is very sensitive to film thickness; therefore, ellipsometry

is typically used to measure film thickness in the ranges from sub-

nanometers to microns. A simplified model of light beam refraction and

reflection is shown in Figure 4.8. In this work, film thickness measurements

were performed on silicon spin coated by PMMA films (20-100 nm thick). A

model is constructed for different layers of the sample, such as

Si/SiO2/PMMA/air. Then experimental data are compared with the model

and the simulated values can be adjusted to fit with experimental data at

every wavelength. The ellipsometer used in this work is model SE850 from

Sentech Instruments GmbH. The main components of an ellipsometer are a

light source, polarization generator, polarization analyzer, sample stage and

detector. The software used for the thickness estimation.

Figure 4.8. Schematic setup for thin film measurement.

55

5 Deposition and Characterization of Metal Clusters

In this chapter, the steps taken to optimize and calibrate the MaSCA, to

study the efficiency of EQMS as well as the experiments on the deposition

of copper and silver clusters on different substrates are described.

5.1 Calibration and optimization of cluster beam production

The first thing that was attempted to achieve with the MaSCA was to obtain

cluster beam alignment with the main axis of the setup. At first place, laser

alignment technique was used and copper cluster beam was deposited on a

round shutter, placed at main axis after the ion optics chamber. Figure 5.1

shows the accumulated material before and after laser alignment procedure.

The spot shows that the beam is almost spherical in shape and traveling

along the main axis. With such a good alignment the deflectors were not

required and they were used in the deposition experiments very rarely.

Figure 5.1. Deposited material on the shutter plate (a) before and (b) after laser

alignment.

Cluster production by magnetron sputtering is determined by many

interrelated parameters such as magnetron power, gasses flows, aggregation

length, aggregation tube temperature and nozzle-skimmer shape. For

example, it is well-known that production of small clusters is facilitated at

shorter aggregation lengths due to the fewer number of collisions while to

grow larger clusters the aggregation length must be increased. In this work,

parameters used for the production of copper and silver clusters were varied

56

within certain ranges to achieve higher beam intensities. These ranges are

given in Table 5.1.

Table 5.1. The experimental parameter for cluster production.

Aggregation length 90-100 mm

Nozzle-skimmer distance 23 mm

Ar flow 50-70 sccm

He flow 03-18 sccm

Magnetron power 54-60 W

5.2 Copper clusters

5.2.1 Cun clusters deposition on different substrates

Copper clusters were deposited on various substrates to study clusters

diffusion, aggregation and agglomeration phenomenon and also to find out

which substrate would be used in the size-selection experiments. We tried n-

type (100) silicon, HOPG, mica and quartz. All samples were prepared at the

same UQP = ±400 V to deposit clusters of the same sizes for comparison.

Figure 5.2 shows a series of AFM images and corresponding height

histograms of Cun clusters deposited on the different substrates. Cluster

coverage varies from sample to sample due to the difference in beam

intensities and deposition times.

Panel (a) in Figure 5.2 shows the monodispersity of Cun clusters

indicating that diffusion and agglomeration are limited on Si surface. In

Figure 5.2 (c) a quartz sample with deposited Cu clusters is also shown

demonstrating randomly located clusters with no evidence for considerable

agglomeration. Clusters deposited on atomically flat HOPG and mica has

higher tendency to diffuse and become caught on surface defects (see

Figures 5.2 (e) and (g)) which is a very well-known tendency.120, 122

HOPG

provide very low diffusion barrier (0.020 eV) meaning that lateral diffusion

of clusters is rapid and can be further than the typical terrace width of 1-2

μm.149

In the assumption that our Cun clusters have crystalline structure very

close to that of bulk Cu,144

the large lattice mismatch between Cu (3.6 Å)

and mica (5.20 Å) would promote cluster diffusion on mica. Figure 5(g)

shows that the Cun clusters are tended to agglomerate forming a kind of

57

Figure 5.2. AFM images and corresponding height histogram of Cun clusters

deposited at UQP = ±400 V on (a-b) Si, (c-d) quartz, (e-f) HOPG and (g-h) mica

substrates.

58

chains. It was also observed that HOPG and mica provide poor adhesion of

the deposited clusters. They can be easily put in motion by AFM tip in

tapping mode if the value of set-point is just a bit too low.

Analysis of height distributions demonstrates good size selection with

standard deviations in the height of 6-9 % for all samples. It can be expected

that the clusters are slightly flattened (become oblate) on impact, but this

issue will be addressed later. Comparing the mean height shows that they are

consistent with Si, HOPG and mica (between 13.0-13.9 nm). However, the

mean cluster height on quartz is considerably lower, 10.5 nm. One of the

reasons could be much higher roughness of quartz samples compared to

other substrates. Even short-distance diffusion can lead to cluster allocation

in a pit or small surface valley that would reduce cluster height.

Another probable mechanism can be suggested from comparison of

surface free energy (γ) of copper, 1650 mJ/m2,150

with corresponding values

of the materials forming the substrates. In the case of (100) silicon, γ is

higher, 2130 mJ/m2,151

than that of copper. Therefore, the deposited copper

clusters should not have any strong tendencies to minimize the interfacial

tension meaning that particle shapes will not be distorted. For graphite, the

literature data are a bit controversial; γ can vary in a very wide range of

values. For basal planes, surface free energies can be as low as (100-160

mJ/m2), on the other hand, γ 3600 mJ/m

2 was reported for cleaved

pyrolytic graphite.152

Such a difference can be related to the presence of

different C-face surfaces, for example, zig-zag or armchair faces. Latter one

can theoretically provide surface enthalpy as high as 5500 mJ/m2. Thus, the

clusters deposited on atomically flat planes should have very strong

tendency to minimize the interfacial tension leading to diffusion towards

sites with comparable or higher γ, which are defects or faces mentioned

above. For the case of quartz, values of surface free energy are low (typically

< 100 mJ/m2).

153 However, contrary to graphite the surface roughness is

higher preventing extensive diffusion. Thus, the cluster can minimize

interfacial tension by increasing the contact area that would lead to

flattening, i.e. in a significant decrease of the cluster height.

59

5.3 Silver clusters

In this section, the results of the experiments on Agn cluster deposition on

various substrates, size-selection and TEM investigations of cluster diameter

and structure are described. These investigations correspond to the data

given in publications presented in Appendixes A and B as well as partly to

the paper in Appendix C.

5.3.1 Deposition on different substrates

Agn clusters were deposited at the same UQP = ± 300 V on HOPG, Si and

quartz and investigated for comparison of surface arrangements and heights.

Figure 5.3 shows the AFM images and corresponding height histograms of

Agn clusters on the above-mentioned substrates.

Similar substrate effects have been observed for Agn clusters as for

Cun ones. Clusters are found to be randomly distributed on Si and quartz

surfaces while they have a tendency to collect at point defects and especially

linear defects (steps) on HOPG. Figure 5.3 (a) shows a silicon sample with

deposited Agn clusters. The histogram in panel (b) gives a mean cluster

height of 12.8 nm with a standard deviation of 1.1 nm corresponding to 9 %,

thus, demonstrating good size selection.154

For HOPG sample (see Figure 5.3

(e) and (f)), the mean measured height is about the same, 12.6 nm, but the

standard deviation is 1.8 nm corresponding to 14 % that is a bit higher. The

Larger standard deviation can be related to the fact that majority of measured

clusters are located not at flat terraces but along the steps that might increase

uncertainty in AFM measurements. As mentioned above, clusters are not

located randomly on the HOPG surface but prefer collection along the steps

or some other point defects. This is caused by post-deposition lateral

diffusion of the clusters and trapping them on surface imperfections. This is

a well-known phenomenon that is already discussed in the section about the

deposition of Cun clusters. With the increase of cluster coverage, the

particles caught at the steps can become nucleation points for fractal growth

of clusters on terraces that was, for example, observed in work of Schmidt et

al.120

It is worth mentioning that presence of a defect is not always a negative

phenomenon. Creation of pre-deposition surface defects will help to

immobilize the cluster.155

60

Figure 5.3. AFM images and corresponding height histograms of Agn clusters on (a-

b) Si, (c-d) quartz and (e-f) HOPG substrates. Inset in panel (e) is zoom in an area

around a step at the surface.

In Figure 5.3 (c) and (d), a quartz sample with deposited Agn clusters

and corresponding histograms are shown. The histogram gives a mean

cluster height of 9.1 nm with a standard deviation of 1.6 nm corresponding

to 18%. The mean height is smaller compared to HOPG and Si, which

mimics the situation with deposition of Cun clusters and the similar

arguments, which are presented in section 5.2.1, can be applied. In

particular, it should be taken into account that γ 1200 mJ/m2 156

for silver is

much higher compared to that of quartz. It is worth mentioning that in the

experiments on size-selection, which will be described below, the mean

61

heights of Agn clusters deposited at UQP = ±900 V on quartz (15.2 nm) are

also found lower compared to Si (18.7 nm), thus, proving the tendency of

flattening for larger clusters.

5.3.2 Shape and structure

Silver clusters deposited at UQP = ± 300 V on the grid with amorphous layer

were investigated by TEM to get deeper knowledge about cluster shape and

structure. In Figure 5.4 one of TEM images and size (diameter) distribution

histogram obtained by analysis of a few images are presented. As can be

seen neighboring Agn clusters can diffuse towards each other making small

agglomerates that are excluded from the size analysis. The mean lateral

diameter of 13.6 nm was calculated which is larger than the mean height

found by AFM measurements on both Si (12.8 nm) and HOPG (12.6 nm).

The aspect ratio (diameter/height) of 1.06-1.08 allows to conclude that the

clusters are slightly flattened upon impact with the surface.

Figure 5.4. (a) TEM image and (b) size histogram fitted by Gaussian distribution.

High-resolution TEM (HR-TEM) images show that Agn clusters have

crystalline face-centered cubic (fcc) structure. It follows from the measured

interplanar distances on the TEM images and Fast Fourier Transform (FFT)

plots. One of the examples can be seen in Figure 5.5. The interplanar

distances are 2.36 Å, 1.51Å and 1.15Å associated with the planes (111),

(220) and (222), respectively, corresponding to the fcc structure of a silver

crystal. Figure 5.5 (c) shows a pattern corresponding to the (111) atomic

planes of fcc crystal lattice with a lattice constant a = 4.11 ± 0.02 Å. This

concludes that Agn clusters have fcc structure and a lattice constant equal to

that of the bulk silver, 4.09Å.

62

Figure 5.5. (a) HR-TEM image of two overlapping Agn clusters with (b) its

corresponding FFT plot and (c) HR-TEM image showing (111) planes of fcc

structure.

5.3.3 Clusters size selection

Magnetron sputtering source is capable of producing a wide range of cluster

sizes, from tens to many thousands of atoms, and a large fraction of them is

charged. As described earlier, control of clusters size in MaSCA can be

achieved using EQMS144

by varying the applied voltages.154, 157

to find a

practical relation between the applied voltages and cluster sizes, a series of

experiments was performed covering a relatively wide range of EQMS

voltages: UQP = ±100, ±300, ±500, ±900, ±1200, ±1400, ±1600, ±1800, and

±2000 V. According to Eq. 4.2, low voltages allow the selection of smaller

in size (light mass) clusters while higher voltages result in the selection of

large clusters.157

To perform size-selection at various UQP, silicon was

chosen as the substrate due to a number of reasons such as the absence of

large scale defects and negligible clusters lateral diffusion allowing to avoid

agglomeration of deposited clusters, relatively good cluster adhesion to the

surface and low effect of cluster flattening. The experiments described in the

previous section showed that shape of the deposited metal clusters on Si is

only slightly oblate (flattened). Therefore, height, which can be measured by

AFM, is assumed to be almost equal to cluster diameter. Table 5.2 shows the

mean heights and standard deviations for the clusters deposited at different

UQP.

63

Table 5.2. Mean heights and standard deviations for clusters deposited on Si at

different UQP.

UQP (V) 100 300 500 900 1200 1400 1600 1800 2000

Cluster height, nm 6.5 12.6 15.0 18.7 20.3 21.4 22.7 24.3 25.7

St. deviation, nm 0.3 0.6 0.8 0.8 0.9 1.1 1.0 1.1 1.3

Standard deviations of heights are found to be within 6% indicating

the good size selection of clusters in the range of diameters between ca. 6

and 26 nm. This range is not the working limit of MaSCA. Further

optimization of the sputtering conditions can allow production of smaller

clusters or very large ones depending on requirements of particular

applications. Plotting the UQP vs. mean cluster height presented in Figure 5.6

allows to fit the experimental data with the curve that demonstrates

proportionality of cluster size to √𝑈𝑄𝑃3 and that well corresponds to the

theory.144

From this dependence on can easily find voltage to get clusters of

required size.

Figure 5.6. Cluster mean height for different quadrupole voltages with the fitting

curve (solid line). Error bars show standard deviations.

The cluster sizes selected at given UQP follow a Gaussian distribution;

see for example histograms in Figure 5.3. The EQMS, however, should, in

theory, give a much sharper size distribution (in the assumption that there are

no multi-charged clusters), but this requires that all clusters move with the

64

same velocity on the same main axis through the system. However, such an

ideal conditions are difficult to achieve due to beam divergence and velocity

dispersion. Another reason for the broadening of the size distribution can be

due to the diffusion of clusters after soft landing that can lead to additional

corrugation of the clusters shape and, therefore, size. These factors result in a

broadening of the size distribution. It is worth mentioning that the precisions

in size selection obtained for Agn clusters resemble those found for Cun

clusters produced by very similar cluster source,144

thus, demonstrating very

good capabilities of the apparatus.

5.4 Metal-polymer nanocomposites

As described in section 3.2, polymer composites with metal nanoparticles

have important practical applications and formation of nanocomposite

materials with required properties is a very actual task. Therefore, one of the

goals of the current work was to study deposition of size-selected silver

clusters on polymer (PMMA) films and to find ways for control of particle

distribution and incorporation into PMMA. These results, that go along with

the paper presented in Appendix C, are compared with another commonly

used method where the nanoparticles are synthesized in thin PMMA layer by

Ag+ ion implantation with high fluences. Latter is based on the paper

presented in Appendix D.129

PMMA films were spin-coated on Si and quartz substrate. Films on Si

substrates were used to study particles embedding into the polymer by TEM

and optical spectroscopy was performed on quartz-based samples. Two types

of PMMA films were prepared: viscous PMMA and soft PMMA. See

Appendix C for details about sample preparation. For all the samples, the

clusters were size-selected at UQP = ± 300 V, which correspond to mean size

of ca. 13 nm.157

AFM images of viscous PMMA with as deposited clusters

and after post-deposition annealing at 100 oC are shown in Figure 5.7.

65

Figure 5.7. AFM images and corresponding height histograms for viscous PMMA

samples (a, b) with as-deposited Agn clusters and (c, d) after annealing for 10 min at

100 oC.

The height histograms demonstrate that NPs are partially embedded in

PMMA upon landing and penetrate deeper into polymer after annealing. The

immersion of silver clusters is also confirmed by the TEM cross-sectional

studies and optical measurements as shown in Figure 5.8. See Appendix C

for details about the evolution of the optical spectra.

Figure 5.8. (a) TEM cross-sectional image of viscous PMMA film with embedded

Ag NPs after annealing (b) normalized extinction spectra of viscous PMMA with

deposited Agn clusters. PMMA layer is marked by dashed lines for better

visualization in panel (a).

66

Figure 5.9 shows the AFM images of soft PMMA samples and

corresponding height histograms before and after annealing. The height

histogram reveals that the Agn clusters are located on the PMMA surface

and do not immerse. Annealing at or below glass transition temperature of

PMMA does not make any considerable change in the sample topography

(therefore, AFM images are not shown here). However, after annealing at

125 °C, which is higher than glass transition temperature Tg, topography

changes significantly which can be seen in Figure 5.9 (c) and (d). The

particles height decreases from 13.2 nm to 5.9 nm and the surface coverage

becomes much lower inferring embedding process of Ag NPs into the

PMMA, which is also confirmed by TEM measurements and extinction

spectra of the samples, see Figure 5.10.

Penetration of NPs certainly requires long range polymer chain

mobility. Therefore, no or small partial embedment can be expected at T<Tg.

However, for T >Tg NP can start immersed in the polymer film that is in

agreement with our findings and also with earlier experiments on the

embedment kinetics of Ag and Au NPs at annealing of PMMA at T > Tg.158

Figure 5.9. AFM images and corresponding height histograms for soft PMMA (a, b)

with as-deposited Agn clusters and (c, d) after annealing for 10 min at 125 oC.

67

Figure 5.10. (a) TEM image of soft PMMA sample after annealing and (b)

normalized extinction spectra before and after annealing.

Extinction spectra of both viscous and soft PMMA samples with as-

deposited Agn clusters show two overlapping bands with maxima at

wavelengths of 378 (or 375) nm and 465 (or 467) nm. The first band at a

short wavelength is assigned to LSPR on individual Ag NPs while the

presence of the second maximum at the higher wavelength is not completely

clear. One of the possible explanations is plasmon coupling109, 159

due to the

formation of self-assembled dimers.160

Such dimers can be seen in TEM

images (see Figure 5.4). After annealing, the intensity of the first plasmon

band is increased a few times and the band is “red” shifted. This behavior

well correlates with the microscopy studies showing that the absolute

majority of NPs become embedded into PMMA that changes the dielectric

environment (PMMA has higher dielectric constant compared to air, 2.6 and

1.0, respectively) causing the observed spectral changes. The second band

almost vanishes most probably because the NPs penetration into PMMA

breaks the self-assembled pairs.

It is interesting to compare the results on the embedding of NPs

deposited using cluster beams with those obtained on PMMA samples where

the NPs were synthesized by high fluence ion implantation of Ag+ ions (see

paper in Appendix D). In the latter case, the NPs are formed in a very wide

range of sizes and ion implantation also leads to radiation damage of the

polymer causing significant carbonization. Size dispersion of silver NPs

leads to relatively wide plasmon bands. The carbonization converts dielectric

PMMA into a medium with increased conductance. This effect leads to

“red” shift of the plasmon band as can be seen from the simulations

presented in Figure 5.11 (b). However, in the experiments the band is not

only “red” shifted but also broadened and dumped (see Figure 5.11(a)) due

68

to wide size dispersion of NPs and change of material towards high-

conductive one. The presence of conduction electrons leads to additional

absorption in the visible light interval that causes blurring of the specific

LSPR band of silver NPs.

Figure 5.11. (a) Normalized absorption spectra of PMMA implanted with different

fluences indicated on the panel. (b) Calculated extinction efficiency of Ag NP of

diameter 20 nm in different dielectric environments: air, PMMA, amorphous carbon

(α-C), a mixture of 0.8air+0.06α-C+0.14PMMA and mixture of 0.4α-C+0.6PMMA.

Hence, the study shows that samples prepared in MaSCA provide

better control over particle size, surface coverage, and penetration depth into

PMMA compared to some other methods, in particular, ion implantation. By

carefully adjusting the annealing temperature and time one can achieve

partially or fully embedded into polymer size-selected NPs. The narrow

width and strong intensity of the plasmon band is an advantage that is

favorable for practical applications. One of them is the formation of NP-

based optical transducers for sensing that is discussed in the following

chapter.

69

6 Nanoparticle-Based Optical Transducers

The excitation of localized surface plasmon in metal NPs by interaction with

light makes nanoplasmonic materials especially attractive for optical sensing

applications. Among nanosensors, biorecognition systems are of significant

importance for environmental, bioprocess, medical and pharmaceutical

applications.161-162

LSPR biosensors offer real-time, label-free molecular

detection15

and show a huge potential towards device miniaturization and

multiplexing. There are some issues related to metal NPs based biosensors

such as weak adhesion to surface and functionalization of NPs surfaces for

antibody coupling. Therefore, in the current work these processes have been

optimized to develop a sensing system that provides better stability to NPs

against wet chemistry and surface functionalization schemes.

In this chapter fabrication and testing of optical transducers based on

silver NPs are presented. Two different approaches have been employed to

immobilize clusters on substrates for sensing of proteins. First, an approach

utilizing “silanization” of quartz surface before Agn cluster deposition

(presented data go along with paper in Appendix E) and second, partial

embedment of silver clusters in the polymer layer.

6.1 Silver clusters on quartz – Method I

6.1.1 Sample preparation

Silver clusters were produced using the experimental setup based on

magnetron sputtering and described in Ref. 144.144

For this study clusters

were deposited without size-selection on quartz substrates. Heights of the

particles were estimated from AFM images and found to be between 8 and

12 nm.15

All the samples were prepared at room temperature in high vacuum

chamber at a background pressure of ca. 1 × 10−8 𝑚𝑏𝑎𝑟. Ag NPs were

deposited at thermal energies and preserve almost spherical shape upon

impact with the surface.

70

Silanization: Silver clusters deposited on bare quartz were found to have low

adhesion and, thus, low resistance against wet chemistry procedures used for

incubation of proteins. Therefore, functionalisation of quartz surface was

required to improve the adhesion. Despite many other surface

functionalisation methods, silanization is a straightforward approach to

introducing chemical functional groups onto silica-based substrates. It was

carried out using aminopropyltrimethoxysilane (APTMS). A schematic

picture of silanization process is shown in Figure 6.1(a), and more details

can be found in Appendix E.

Figure 6.1. (a) Silanization of quartz with APTMS and (b) functionalisation of Agn

cluster with 11-MUA.

To activate Ag NPs surface, i.e. to make possible attachment of

biomolecules, substrates with as deposited clusters were incubated with 11-

mercaptoundecanoic acid (11-MUA) solution. 11-MUA becomes selectively

attached to Ag NP by the sulphur end and provides the carboxyl group

(COOH) at the other end, see Figure 6.1(b). To activate COOH-group the

samples were further incubated with a 1-ethyl-3-[3-

dimethlaminoproply]carbodiimide hydrochloride/N-Hydroxysuccinimide

(EDC/NHS) mix.15

Three series of samples were prepared: the first one follows classical

antibody-antigen scheme (with anti-chicken egg albumin antibody and

chicken egg albumin as antigen), the second one is of inversed sequence of

protein deposition (first chicken egg albumin, then the corresponding

71

antibody) and the third one is of antibody-antigen scheme but with lysozyme

as the antigen, which should not be recognised by the anti-chicken egg

albumin antibody. The proteins used in this work are chosen only to test the

applicability of the developing detection approach and they are not of high

practical importance.

6.1.2 Surface morphology: AFM

Silver clusters deposited on APTMS-functionalized substrates show

significantly improved adhesion against wet chemistry compared to clusters

deposited on bare quartz. The bonding mechanism is most probably the

polarization interaction between the amine group of APTMS and Ag NPs.

Figure 6.2 shows the AFM images of the samples with as-deposited clusters,

after antibody-antigen incubation (classical scheme) and after antigen-

antibody incubation (inversed scheme). AFM images reveal that the Ag NPs

are stable against every step of functionalization and proteins deposition.

Figure 6.2. AFM images of (a) as-deposited clusters, (b) after antibody-antigen

incubation on clusters and (c) after antigen-antibody incubation on clusters.

In classical scheme, the proteins (antibodies) are expected to be

incubated selectively on the functionalized Agn clusters. Indeed, it is the case

as follows from AFM image in Figure 6.2 (b) showing an increase in the

height and lateral diameter of the clusters. Taking into account average size

of antibodies of about 4 nm, one can suggest that one or two antibodies are

72

incubated on a NP, which will later provide the coupling of albumin

(antigen) molecules.15

However, in the case of inversed scheme, albumin (ca.

1-2 nm in size) becomes spread all over the sample surface and provides a

non-selective coating of the substrate, as follows from Figure 6.2 (c). The

clusters are hardly recognized and this is, thus, related to the fact that

albumin covers the clusters and fills the gap between them.15

Further

investigation of the samples is performed using optical spectroscopy.

6.1.3 Protein sensing

Figure 6.3 shows the optical spectra of the samples for two different proteins

deposition schemes. The position of the LSPR peak for as- deposited clusters

varies slightly from sample to sample due to small variation in mean particle

sizes between the series of samples. There is also a second peak (shoulder) at

around 500 nm that could be due to plasmon coupling between two particles

which is increased with decreasing interparticle distance.109

This

phenomenon is discussed in Section 2.5.2.

After the deposition of proteins the LSPR peak changes significantly

and the difference between the two schemes is presented in Figure 6.3 (b-d).

For classical scheme (antibody-antigen) we used two different

concentrations of albumin: the antigen-antibody ratio of 2:1 (high) and 1:3

(low), see Figure 6.3 (b) and (c), respectively. One can clearly see a larger

increase in the plasmon intensity for higher albumin concentration, thus,

being indicative of albumin detection. 11-MUA molecules are attached to

NPs via the sulfhydryl group and its carboxyl group is used to form a

covalent amide bond to the antibody. Thus, strong chemical bonding of the

antibody to the NP changes the dipole characteristics leading to an

enhancement of the SPR absorption. A small shift in plasmon peak is

observed after incubation of albumin to the antibody. This is explained by

the albumin size which is smaller compared to the size of the antibody and

its subsequent attachment to antibody changes the NP-antibody interaction

only a little. Therefore, a shift in plasmon peak is small. However, an

increase in absorption intensity is found to be dependent on albumin

concentration, thus, demonstrating the detection of albumin by the

transducers. This incubation of proteins was also observed in AFM image,

73

where particle size changes after protein deposition compared to as-

deposited Ag NPs; see Figure 6.2(b).

Figure 6.3. (a) Optical transmission spectra of pristine quartz and after APTMS

functionalisation. (b) and (c) Normalized optical absorption spectra for the samples

with classical incubation scheme and high and low concentration of albumin,

respectively.(d) Normalized optical absorption spectra for the samples with inversed

scheme, antigen-antibody incubation.

For the inversed scheme of protein deposition, the morphology of the

sample reveals that small albumin molecules (1-2 nm) are not selectively

attached to NPs. It means that albumin is not only coupled to NPs but also

filling the gaps between the NPs. As one can see the clusters are hardly

recognized in the topography (see Figure 6.2(c)). Figure 6.3(d) shows that

the intensity of the LSPR band after albumin incubation is decreased; the

band broadens and shifted toward longer wavelength due to change in the

dielectric environment of the NPs. Thus, albumin detection is not possible

for the reverse scheme.15

74

Thus, it can be concluded that silver NPs deposited on functionalized

quartz surface can serve as optical transducers for recognition of protein of

interest through incubation using classical antibody-antigen scheme. Change

of intensity and spectral position of LSPR band can be used for the

detection. It is also demonstrated that the appropriate preparation stages and

immobilization schemes are the key issues for the formation of plasmonic

transducers. Despite the overall convincing results on the utilization of Ag

NPs for sensing, one need to mention that adhesion of NPs to functionalized

quartz is not perfect.

6.2 Metal-polymer composites for protein sensing – Method II

To improve NPs adhesion to the substrate, the step on functionalisation of

quartz surface with APTMS is substituted by spin-coating of a thin layer of

PMMA on which Agn NPs were deposited. In this experiments size selected

clusters were used to get improvement is the plasmon band parameters.

6.2.1 Sample preparation

PMMA films were spin coated from 1% polymer (950 PMMA C 9,

MicroChem, America) solution in toluene onto quartz and Si substrates.

Latter was used to measure film thicknesses that are found to be ~50 nm

using ellipsometry. In order to develop an optical transducers three different

types of PMMA films were prepared on quartz: (i) viscous PMMA, spin

coated and dried overnight at room temperature, (ii) soft PMMA, annealed at

100 oC for 10 min after the spin-coating and (iii) hard PMMA, annealed at

180 oC for 2 min after the spin-coating. Silver clusters of the mean diameter

of 13 nm were soft-landed on these different types of samples. For same

particle coverages, cluster deposition parameters and deposition time were

adjusted. Samples of type (i) were annealed at 100 oC for 10 min after the

cluster deposition to harden the polymer with embedded NPs improving

their adhesion. Samples of type (ii) were annealed at 180 oC for 5 min after

the cluster deposition for the same reason. Deposited NPs were further

functionalized in the same manner as described in section 6.1.3 to activate

them for antibody (AB) coupling and following antigen (AG) incubation.

Same AB and AG proteins were used.

75

In this study, only classical “antibody-antigen” scheme was

performed on all three types of PMMA samples. The proteins used in this

work are chosen only to test the applicability of the developing detection

approach that can further be developed for other important biomolecules.

Samples morphology and optical measurements were performed from very

beginning to the last step of proteins deposition.

6.2.2 AFM characterization

Figure 6.4 shows embedded Agn NPs into viscous PMMA and incubation of

AB and AG on the functionalized particles. As described earlier, the

particles deposited onto viscous PMMA become partly embedded in order to

minimize the surface tension because silver has much higher γ 1200

mJ/m2156

compared to PMMA with γ30-40 mJ/m2.158

Annealing the sample

Figure 6.4. AFM images for viscous PMMA sample (a) with as-deposited Agn

clusters, (b) after subsequent annealing at 100 oC, (c) after following AB deposition

and (d) after AG incubation.

even at 100 oC, which is lower than Tg, after particles deposition

significantly changed the morphology and particles move deeper into

PMMA layer, see Figure 6.4 (b) because the chains are flexible allowing

76

easy embedding. Particles are randomly distributed on PMMA surface and

due to partial embedment have improved adhesion compared to

functionalized quartz on wet chemistry (11-MUA deposition). AFM images

corresponding to the AB-AG incubation (classical scheme) are shown in

Figure 6.4 (c) and (d), respectively. AFM images reveal that the silver NPs

are stable against every step of functionalization and proteins deposition.

One can also see that AB molecules are predominantly incubated on the NPs

with only small amounts located around and after following AG deposition

these proteins become mainly bound to ABs located on the NPs.

Morphology of the clusters deposited on hard PMMA and after AB

incubation is shown in Figure 6.5. The mean height of the particles is found

to be ~12 nm meaning that they do not immerse into PMMA, which is

annealed at a temperature much higher than Tg. It is observed that particles

are weakly attached to the PMMA surface, as some of the clusters can be

easily moved by the cantilever tip during AFM scans. Protein incubation

changes the sample morphology: silver NPs easily agglomerate on the

PMMA surface (see Figure 6.5 (b)). It is observed that size of the particle is

increased after AB incubation proving that these proteins are predominantly

localized in the functionalized silver clusters creating sites for AG

attachments.

Figure 6.5. AFM images of hard PMMA sample (a) with as-deposited Agn clusters

and (b) after AB deposition.

Soft PMMA samples with embedded Ag NPs are chosen to test

detection efficiency of the transducers. For that AG (albumin) is deposited in

two different concentrations with respect to the concentration of AB. They

are called high, in which AB-AG ratio is kept 1:1, and low, in which the

77

ratio is reduced to 1:5, i.e. too few antibody molecules compared to antigens.

AFM images corresponding to these two cases as presented in Figure 6.6 and

Figure 6.7, respectively. For high ratio, one can see that the ABs and

attached to them AGs are predominantly located on the NPs increasing their

sizes while for low ratio there are too many albumin molecules to become

accommodated on the antibodies, thus, leading to the situation that AGs are

located everywhere on the sample surface.

Figure 6.6. AFM images of soft PMMA sample (a) with as-deposited and annealed

at 180 oC clusters and (b) after antibody-antigen incubation (high ratio).

Figure 6.7. AFM images of soft PMMA sample (a) with as-deposited and annealed

at 180 oC clusters and (b) after antibody-antigen incubation (low ratio).

6.2.3 Optical measurements: protein sensing

Optical spectra of the samples with as-deposited clusters clearly demonstrate

the presence of a LSPR absorption band at 410-413 nm as can be seen in

Figure 6.8 for viscous and hard PMMA samples. This band is narrow and

intensive for the viscous PMMA, i.e. with the high quality factor. The

second peak (shoulder) at a higher wavelength (around 540 nm) is most

probably related to particle-particle interaction and “plasmon-coupling.”

78

This shoulder is almost negligible for viscous PMMA due to the immersion

of clusters that breaks the self-agglomeration of pairs as discussed in section

5.4. All extinction spectra are obtained from transmission measurements. To

eliminate the substrate contribution, a reference spectrum of quartz covered

with PMMA is subtracted from those with clusters and proteins.

Figure 6.8. Normalized extinction spectra of (a) viscous and (b) hard PMMA with

clusters followed by incubation of proteins.

As can be seen in Figure 6.8, AB incubation on NPs leads to the shift of

the plasmon peak towards higher wavelength and decrease in intensity.

Moreover, the width of the plasmon band is increased. Comparing the

optical results with AFM images (see Figure 6.4 and Figure 6.5), one can

suggest that decrease in intensity is related to drop in particle coverage after

each step of wet chemistry. Obviously, this effect is very small for the

viscous PMMA, while quite many clusters are gone for hard PMMA due to

relatively low adhesion. Agglomeration of some particles can contribute to

the band broadening that makes the shoulder at a high wavelength much less

pronounced. Carboxyl group end of the 11-MUA molecules form strong

bonding with antibody and the observed “red” shift of the plasmon band is

caused by the change in the dielectric environment of the NP. Further

attachment of albumin to AB changes the NP-AB interaction only a little.

Therefore, the band shift is small but observable enough to ensure the

albumin detection. It is worth mentioning that there is also a small increase

in the band intensity for the viscous PMMA on albumin attachment similar

to the case observed for Agn clusters on functionalized quartz (see Figure

6.3).

79

Figure 6.9 shows the normalized extinction spectra of classical AB-AG

deposition scheme for different concentrations of antigen and the case of soft

PMMA. Peak positions and band intensities are very similar for both

samples with as deposited clusters. One can see that 11-MUA

functionalisation of clusters results in “red”-shift of the plasmon band and

it’s widening with the formation of a shoulder. This shoulder is related to the

presence of ethanol in the solution penetrating into PMMA and changing the

environment of silver NPs. The most probably presence of ethanol also

reduces the band intensity.

Figure 6.9. Normalized extinction spectra for soft PMMA with silver clusters and

deposition of AB-AG in two different ratios (a) high (1:1) and (b) low (1:5).

Following incubation with AB proteins leads to further “red” shift and

some increase of the band intensity. The last step corresponding to albumin

deposition is the most interesting one. In panel (a) there is a considerable

increase in the band intensity, which is almost reaching the initial one,

followed by tiny (about 2 nm) “red” shift. In panel (b), there is also the same

band shift, but the increase in the intensity is small taking into account that

albumin concentration for this sample is five times higher. Comparison of

these two cases allows to conclude that the NP-based transducers are

sensitive only to antigens that are coupled to the antibodies incubated on

NPs. If antigen molecule is not able to bind to the antibody but located aside,

it is not “seen” by NP, i.e. there is no change in plasmonic parameters. This

finding creates good perspective for selective sensing of proteins.

Thus, it can be concluded that one can achieve detection of proteins of

interest by incubation on supported silver NPs utilizing classical antibody-

80

antigen scheme. However, the practical realization of this approach requires

appropriate functionalization of the substrate carrying the clusters as well as

cluster surface to provide coupling of proteins. The advantage of such

approach is in selective detection of antigens, only those which are bound to

the antibodies located on NPs. It is experimentally found that transducers

based on size-selected silver NPs partially embedded into thin PMMA layer

provide a simple and reliable way to the detection. However, further work is

required for optimization of solutions and excluding agents affecting the

detection as well as considering other polymers.

81

7 Conclusions and Perspectives

The purpose of this project was to develop magnetron sputtering cluster

apparatus, optimize the setup for the production of metal clusters and carry

out a series of experiments on the synthesis of nanomaterials and their

application for biosensing. These goals were successfully reached.

The first step included construction work, development and manufacturing

of individual parts with an especial focus on capability for adjustment of

certain parameters of the experimental setup. Significant efforts were made

to the development of efficient systems for steering the cluster beam, size-

selection of the clusters and design of sample holders for cluster deposition.

The developed setup – MaSCA - was successfully tested and the parameters

of magnetron sputtering (gasses flows, magnetron power, aggregation length

and temperature of aggregation tube) were optimized for formation of

copper clusters. The found parameters allowed the easy switch to another

metal, for example, silver. Both copper and silver clusters were found to be

monocrystalline with lattice parameter very close to those of bulk metals.

A series of experiments was carried out to find the exact relation between the

voltages applied to EQMS and sizes (diameters) of the clusters. The

efficiency of size-selection was found to be very good giving the capability

to get clusters of required size within a standard deviation of 7%. The

apparatus was developed providing good conditions for a soft landing of

metal clusters preserving almost spherical shapes after the deposition. The

aspect ratio (lateral diameter to height) was found to be 1.06 for silver

nanoparticles deposited on amorphous carbon. Similar conditions were

observed in soft-landing on HOPG, mica and silicon. Silver clusters

deposited on quartz were found to oblate most probably due to the very high

difference in surface tension of the metal clusters and the substrate. Thus, the

tendency to minimize the difference in free surface energy at the interface

might lead to cluster flattening.

82

A series of experiments on the deposition of size-selected silver clusters on

PMMA can be considered to be of special interest and importance. By

controlling the hardness of polymer layers prior the deposition and varying

conditions of thermal annealing after cluster deposition, the regimes for the

formation of metal-polymer composites were developed allowing to tune the

nanoparticle immersion into PPMA. The composites with partly or fully

immersed silver nanoparticles showed remarkable plasmonic properties,

much better compared to those formed by other methods, for example, ion

beam synthesis of metal nanoparticles in polymers.

In the final series of experiments, the supported silver clusters were tested

for detection of proteins utilizing the phenomenon of localized surface

plasmon resonance. A special protocol was developed for the formation of

optical transducers based on silver clusters deposited on quartz and these

transducers were proved for the detection of some proteins. To improve the

reliability and stability of detection the transducers utilizing the above-

mentioned metal-polymer composites were tested showing promising results

towards practical applications.

Future Perspectives

Metal nanoparticles are indeed a promising candidate for a number of

applications. Magnetron sputtering was demonstrated to be a very powerful

technique to produce metal clusters with controlled structure, composition

and sizes. However, there is plenty of room for technical improvements

regarding cluster beam stability and intensity, which are quite interesting and

practically important research tasks.

The first steps towards the application of silver clusters in biosensing, which

are made by this work, show high perspective. However, there is still a

number of topics to study mostly related to optimization of the procedures of

cluster deposition and immersion into polymer film (role of cluster size, role

of type of polymer and thickness of the film) as well as stability of

transducers against wet chemistry (testing of different solutions and

reagents). Selectivity of sensing and avoiding false detection is still an open

question for the developed transducers. Another interesting approach is the

use of lithographic patterns and formation of ordered arrays of clusters for

sensing.

83

Bibliography

1. Haberland, H., Clusters of Atoms and Molecules: Theory, Experiment, and

Clusters of Atoms; Springer Science & Business Media, 2013; Vol. 52.

2. Becker, E. W.; Bier, K.; Henkes, W., Z. Phys. 1956, 146, 333.

3. Henkes, W., Z. Naturforschg 1961, 16a, 842.

4. Jena, P.; Castleman, A. W., Nanoclusters: A Bridge across Disciplines;

Elsevier, 2010.

5. Schmid, G.; Baumle, M.; Geerkens, M.; Helm, I.; Osemann, C.; Sawitowski, T.,

Current and Future Applications of Nanoclusters. Chem Soc Rev 1999, 28, 179-185.

6. Tan, X. H.; Jin, R. C., Ultrasmall Metal Nanoclusters for Bio-Related

Applications. Wires Nanomed Nanobi 2013, 5, 569-581.

7. Echt, O.; Kreisle, D.; Knapp, M.; Recknagel, E., Evolution of Magic Numbers

in Mass-Spectra of Water Clusters. Chem Phys Lett 1984, 108, 401-407.

8. Knight, W. D.; Clemenger, K.; Deheer, W. A.; Saunders, W. A.; Chou, M. Y.;

Cohen, M. L., Electronic Shell Structure and Abundances of Sodium Clusters. Phys

Rev Lett 1984, 52, 2141-2143.

9. Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E., C60:

Buckminsterfullerene. Nature 1985, 318, 162-163.

10. Curl, R. F.; Smalley, R. E., Fullerenes. Sci Am 1991, 265, 54-&.

11. Edwards, P. P.; Johnston, R. L.; Rao, C. N. R., On the Size-Induced Metal-

Insulator Transition in Clusters and Small Particles. In Metal Clusters in Chemistry,

Wiley-VCH Verlag GmbH: 2008; pp 1454-1481.

12. Kappes, M. M.; Schär, M.; Schumacher, E., Are Cfuster Abundances

Thermodynamic Properties? Observation of Lithium. Enrichment in Lix Nan-X, N <

42. J. Phys Chem. 1987, 91, 658-663.

13. Castleman, A. W.; Jena, P., Clusters: A Bridge between Disciplines. P Natl

Acad Sci USA 2006, 103, 10552-10553.

84

14. Ozbay, E., Plasmonics: Merging Photonics and Electronics at Nanoscale

Dimensions. Science 2006, 311, 189-193.

15. Fojan, P.; Hanif, M.; Bartling, S.; Hartmann, H.; Barke, I.; Popok, V. N.,

Supported Silver Clusters as Nanoplasmonic Transducers for Protein Sensing.

Sensors and Actuators B: Chemical 2015, 212, 377-381.

16. Haes, A. J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P., Nanoscale Optical

Biosensor:  Short Range Distance Dependence of the Localized Surface Plasmon

Resonance of Noble Metal Nanoparticles. The Journal of Physical Chemistry B

2004, 108, 6961-6968.

17. McFarland, A. D.; Van Duyne, R. P., Single Silver Nanoparticles as Real-Time

Optical Sensors with Zeptomole Sensitivity. Nano Letters 2003, 3, 1057-1062.

18. Link, S.; Wang, Z. L.; El-Sayed, M. A., Alloy Formation of Gold−Silver

Nanoparticles and the Dependence of the Plasmon Absorption on Their

Composition. The Journal of Physical Chemistry B 1999, 103, 3529-3533.

19. Walser, T.; Demou, E.; Lang, D. J.; Hellweg, S., Prospective Environmental

Life Cycle Assessment of Nanosilver T-Shirts. Environmental Science &

Technology 2011, 45, 4570-4578.

20. Tyo, E. C.; Vajda, S., Catalysis by Clusters with Precise Numbers of Atoms.

Nat Nano 2015, 10, 577-588.

21. Franci, G.; Falanga, A.; Galdiero, S.; Palomba, L.; Rai, M.; Morelli, G.;

Galdiero, M., Silver Nanoparticles as Potential Antibacterial Agents. Molecules

2015, 20, 8856.

22. Ivask, A., et al., Size-Dependent Toxicity of Silver Nanoparticles to Bacteria,

Yeast, Algae, Crustaceans and Mammalian Cells <Italic>in Vitro</Italic>. PloS one

2014, 9, e102108.

23. Lara, H.; Ayala-Núñez, N.; Ixtepan Turrent, L.; Rodríguez Padilla, C.,

Bactericidal Effect of Silver Nanoparticles against Multidrug-Resistant Bacteria.

World Journal of Microbiology and Biotechnology 2010, 26, 615-621.

24. Philip, M., Nanostructured Materials. Reports on Progress in Physics 2001, 64,

297.

25. Robert, K. W., Chapter Planar Magnetron Sputtering. In Thin Film Processes,

Vossen, J. L.; Kern, W., Eds. Academic Press, Inc., 24/28 Oval Road, London NW1

7DX.: 1978; pp 134-173.

85

26. John, A. T.; Alan, S. P., Chapter Cylindrical Magnetron Sputtering. In Thin

Film Processes, Vossen, J. L.; Kern, W., Eds. Academic Press, Inc., 24{28 Oval

Road, London NW1 7DX: 1978; pp 75-113.

27. Fan, X.; White, I. M.; Shopova, S. I.; Zhu, H.; Suter, J. D.; Sun, Y., Sensitive

Optical Biosensors for Unlabeled Targets: A Review. Analytica chimica acta 2008,

620, 8-26.

28. de Heer, W. A., The Physics of Simple Metal Clusters: Experimental Aspects

and Simple Models. Reviews of Modern Physics 1993, 65, 611-676.

29. Popok, V. N.; Prasalovich, S. V.; Samuelsson, M.; Campbell, E. E. B., Design

and Capabilities of a Cluster Implantation and Deposition Apparatus: First Results

on Hillock Formation under Energetic Cluster Ion Bombardment. Review of

Scientific Instruments 2002, 73, 4283-4287.

30. Granqvist, C. G.; Buhrman, R. A., Ultrafine Metal Particles. Journal of Applied

Physics 1976, 47, 2200-2219.

31. Landau, L. D.; Lifshit s, E. M., Physical Kinetics; Pergamon Press: Oxford,

England, 1981; Vol. 10 Course of theoretical physics.

32. Smirnov, B. M., Processes Involving Clusters and Small Particles in a Buffer

Gas. Phys-Usp+ 2011, 54, 691-721.

33. Kashtanov, P. V.; Smirnov, B. M., Nanoclusters: Properties and Processes. High

Temp+ 2010, 48, 846-859.

34. Kalikmanov, V. I., Classical Nucleation Theory. In Nucleation Theory, Springer

Netherlands: 2013; Vol. 860, pp 17-41.

35. Gránásy, L., Comparison of Modern Theories of Vapor Condensation. AIP

Conference Proceedings 2000, 534, 209-212.

36. Smirnov, B. M., Nanoclusters and Microparticles in Gases and Vapors; De

Gruyter, 2012.

37. Kashtanov, P. V.; Smirnov, B. M.; Hippler, R., Magnetron Plasma and

Nanotechnology. Phys-Usp+ 2007, 50, 455-488.

38. Wegner, K.; Piseri, P.; Tafreshi, H. V.; Milani, P., Cluster Beam Deposition: A

Tool for Nanoscale Science and Technology. J Phys D Appl Phys 2006, 39, R439-

R459.

86

39. Blazek, J.; Bartos, P.; Basner, R.; Kersten, H. In Distribution of Charge on

Clusters in a Magnetron Discharge, ICPIG, Belfast, Northern Ireland, UK, Belfast,

Northern Ireland, UK, 2011; p In Proc. 30th ICPIG.

40. Ganeva, M.; Pipa, A. V.; Smirnov, B. M.; Kashtanov, P. V.; Hippler, R.,

Velocity Distribution of Mass-Selected Nano-Size Cluster Ions. Plasma Sources

Science and Technology 2013, 22, 045011.

41. Hagena, O. F.; Obert, W., Cluster Formation in Expanding Supersonic Jets:

Effect of Pressure, Temperature, Nozzle Size, and Test Gas. The Journal of

Chemical Physics 1972, 56, 1793-1802.

42. Milani, P.; Iannotta, S., Cluster Beam Synthesis of Nanostructured Materials;

Springer Berlin Heidelberg, 1999.

43. Yamashita, M.; Fenn, J. B., Electrospray Ion Source. Another Variation on the

Free-Jet Theme. The Journal of Physical Chemistry 1984, 88, 4451-4459.

44. Dole, M.; Mack, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. D.; Alice, M.

B., Molecular Beams of Macroions. The Journal of Chemical Physics 1968, 49,

2240-2249.

45. Clampitt, R.; Aitken, K. L.; Jefferies, D. K., Abstract: Intense Field‐Emission

Ion Source of Liquid Metals. Journal of Vacuum Science & Technology 1975, 12,

1208-1208.

46. Helm, H.; Möller, R., Field Emission Ion Source of Molecular Cesium Ions.

Review of Scientific Instruments 1983, 54, 837-840.

47. Popok, V. N.; Barke, I.; Campbell, E. E. B.; Meiwes-Broer, K. H., Cluster-

Surface Interaction: From Soft Landing to Implantation. Surf. Sci. Rep. 2011, 66,

347-377.

48. Miller, R., Free Jet Sources. In Atomic and Molecular Beam Methods, Scoles,

G., Ed. Oxford University Press: 1988; Vol. 1, p 745.

49. Haberland, H.; Buck, U.; Scoles, G., Experimental Methods. In Clusters of

Atoms and Molecules, Haberland, H., Ed. Springer Berlin Heidelberg: 1994; Vol.

52, pp 207-252.

50. Hagena, O. F., Cluster Ion Sources (Invited). Review of Scientific Instruments

1992, 63, 2374-2379.

87

51. Seki, T.; Matsuo, J.; Takaoka, G. H.; Yamada, I., Generation of the Large

Current Cluster Ion Beam. Nuclear Instruments and Methods in Physics Research

Section B: Beam Interactions with Materials and Atoms 2003, 206, 902-906.

52. Kappes, M. M.; Kunz, R. W.; Schumacher, E., Production of Large Sodium

Clusters (axX<) by Seeded Beam Expansions. Chem Phys Lett 1982, 91, 413-

418.

53. Popok, V.; Campbell, E. E., Beams of Atomic Clusters: Effects on Impact with

Solids. Rev. Adv. Mater. Sci 2006, 11, 19-45.

54. Vučković, S.; Svanqvist, M.; Popok, V. N., Laser Ablation Source for

Formation and Deposition of Size-Selected Metal Clusters. Review of Scientific

Instruments 2008, 79, 073303.

55. Siekmann, H. R.; Lüder, C.; Faehrmann, J.; Lutz, H. O.; Meiwes-Broer, K. H.,

The Pulsed Arc Cluster Ion Source (Pacis). Z Phys D - Atoms, Molecules and

Clusters 1991, 20, 417-420.

56. Ganteför, G.; Siekmann, H. R.; Lutz, H. O.; Meiwes-Broer, K. H., Pure Metal

and Metal-Doped Rare-Gas Clusters Grown in a Pulsed Arc Cluster Ion Source.

Chem Phys Lett 1990, 165, 293-296.

57. Barborini, E.; Piseri, P.; Li Bassi, A.; Ferrari, A. C.; Bottani, C. E.; Milani, P.,

Synthesis of Carbon Films with Controlled Nanostructure by Separation of Neutral

Clusters in Supersonic Beams. Chem Phys Lett 1999, 300, 633-638.

58. Barborini, E.; Piseri, P.; Milani, P., A Pulsed Microplasma Source of High

Intensity Supersonic Carbon Cluster Beams. Journal of Physics D: Applied Physics

1999, 32, L105.

59. Begemann, W.; Meiwes-Broer, K. H.; Lutz, H. O., Unimolecular

Decomposition of Sputtered Aln+ , Cun

+ , and Sin

+ Clusters. Phys Rev Lett 1986, 56,

2248-2251.

60. Haberland, H.; Mall, M.; Moseler, M.; Qiang, Y.; Reiners, T.; Thurner, Y.,

Filling of Micron‐Sized Contact Holes with Copper by Energetic Cluster Impact.

Journal of Vacuum Science &amp; Technology A 1994, 12, 2925-2930.

61. Haberland, H.; Karrais, M.; Mall, M.; Thurner, Y., Thin Films from Energetic

Cluster Impact: A Feasibility Study. Journal of Vacuum Science &amp; Technology

A 1992, 10, 3266-3271.

88

62. Taylor, G. I.; McEwan, A. D., The Stability of a Horizontal Fluid Interface in a

Vertical Electric Field. Journal of Fluid Mechanics 1965, 22, 1-15.

63. DiCenzo, S. B.; Berry, S. D.; Hartford, E. H., Photoelectron Spectroscopy of

Single-Size Au Clusters Collected on a Substrate. Physical Review B 1988, 38,

8465-8468.

64. Berry, S. D., Continuous Mass Selected Cluster Ion Production Using a Liquid

Metal Ion Source. In Physics and Chemistry of Small Clusters, Jena, P.; Rao, B. K.;

Khanna, S. N., Eds. Springer US: 1987; pp 49-54.

65. Frans, M. P., Coating by Cathode Disintegration. Google Patents: 1939.

66. Rossnagel, S. M., Ii-1 - Glow Discharge Plasmas and Sources for Etching and

Deposition. In Thin Film Processes, Kern, J. L. V., Ed. Academic Press: San Diego,

1991; pp 11-77.

67. Pratontep, S.; Carroll, S. J.; Xirouchaki, C.; Streun, M.; Palmer, R. E., Size-

Selected Cluster Beam Source Based on Radio Frequency Magnetron Plasma

Sputtering and Gas Condensation. Review of Scientific Instruments 2005, 76,

045103.

68. Ganeva, M.; Pipa, A. V.; Hippler, R., The Influence of Target Erosion on the

Mass Spectra of Clusters Formed in the Planar Dc Magnetron Sputtering Source.

Surface and Coatings Technology 2012, 213, 41-47.

69. Ganeva, M.; Peter, T.; Bornholdt, S.; Kersten, H.; Strunskus, T.;

Zaporojtchenko, V.; Faupel, F.; Hippler, R., Mass Spectrometric Investigations of

Nano-Size Cluster Ions Produced by High Pressure Magnetron Sputtering.

Contributions to Plasma Physics 2012, 52, 881-889.

70. Welzel, T.; Ellmer, K., The Influence of the Target Age on Laterally Resolved

Ion Distributions in Reactive Planar Magnetron Sputtering. Surface and Coatings

Technology 2011, 205, Supplement 2, S294-S298.

71. Chapman, B., Glow Discharge Processes: Sputtering and Plasma Etching;

Wiley, 1980.

72. De Gryse, R.; Haemers, J.; Leroy, W. P.; Depla, D., Thirty Years of Rotatable

Magnetrons. Thin Solid Films 2012, 520, 5833-5845.

73. Gryse, R. D.; Depla, D.; Mahieu, S.; Haemers, J., Rotatable Magnetron

Sputtering: Downscaling for Better Understanding. Materials Technology 2011, 26,

3-9.

89

74. Window, B.; Savvides, N., Charged Particle Fluxes from Planar Magnetron

Sputtering Sources. Journal of Vacuum Science &amp; Technology A 1986, 4, 196-

202.

75. Kelly, P. J.; Arnell, R. D., Magnetron Sputtering: A Review of Recent

Developments and Applications. Vacuum 2000, 56, 159-172.

76. Kittel, C., Introduction to Solid State Physics; Wiley, 2004.

77. Sieber, C.; Buttet, J.; Harbich, W.; Félix, C.; Mitrić, R.; Bonačić-Koutecký, V.,

Isomer-Specific Spectroscopy of Metal Clusters Trapped in a Matrix: Ag9. Physical

Review A 2004, 70, 041201.

78. Dhillon, H.; Fournier, R., Geometric Structure of Silver Clusters with and

without Adsorbed Cl and Hg. Computational and Theoretical Chemistry 2013,

1021, 26-34.

79. Baletto, F.; Ferrando, R., Structural Properties of Nanoclusters: Energetic,

Thermodynamic, and Kinetic Effects. Reviews of Modern Physics 2005, 77, 371-

423.

80. Northby, J. A.; Xie, J.; Freeman, D.; Doll, J. D., Binding Energy of Large

Icosahedral and Cuboctahedral Lennard-Jones Clusters. In Small Particles and

Inorganic Clusters, Chapon, C.; Gillet, M.; Henry, C., Eds. Springer Berlin

Heidelberg: 1989; pp 69-71.

81. Jena, P.; Behera, S. N., Clusters and Nanostructured Materials; Nova Science

Publishers, 1996.

82. Bernstein, J.; Armon, E.; Zemel, E.; Kolodney, E., Formation of Indium Carbide

Cluster Ions: Experimental and Computational Study. The Journal of Physical

Chemistry A 2013, 117, 11856-11865.

83. Vijayakrishnan, V.; Chainani, A.; Sarma, D. D.; Rao, C. N. R., Metal-Insulator

Transitions in Metal Clusters: A High-Energy Spectroscopy Study of Palladium and

Silver Clusters. The Journal of Physical Chemistry 1992, 96, 8679-8682.

84. Johnston, R. L., Atomic and Molecular Clusters; CRC Press, 2002.

85. Beck, T. L.; Jellinek, J.; Berry, R. S., Rare Gas Clusters: Solids, Liquids, Slush,

and Magic Numbers. The Journal of Chemical Physics 1987, 87, 545-554.

86. Soler, J. M.; Saenz, J. J.; Garcia, N.; Echt, O., The Effect of Ionization on Magic

Numbers of Rare-Gas Clusters. Chem Phys Lett 1984, 109, 71-75.

90

87. Prasalovich, S.; Hansen, K.; Kjellberg, M.; Popok, V. N.; Campbell, E. E. B.,

Surface Entropy of Rare-Gas Clusters. J Chem Phys 2005, 123.

88. Martin, T. P., Shells of Atoms. Phys Rep 1996, 273, 199-241.

89. Abreu, M. B.; Reber, A. C.; Khanna, S. N., Does the 18-Electron Rule Apply to

Crsi12? The Journal of Physical Chemistry Letters 2014, 5, 3492-3496.

90. Pyykkö, P.; Runeberg, N., Icosahedral Wau12: A Predicted Closed-Shell

Species, Stabilized by Aurophilic Attraction and Relativity and in Accord with the

18-Electron Rule. Angewandte Chemie 2002, 114, 2278-2280.

91. Kiran, B.; Kandalam, A. K.; Jena, P., Hydrogen Storage and the 18-Electron

Rule. The Journal of Chemical Physics 2006, 124, 224703.

92. von Issendorff, B.; Cheshnovsky, O., Metal to Insulator Transitions in Clusters.

Annu Rev Phys Chem 2005, 56, 549-580.

93. Morton, S. M.; Silverstein, D. W.; Jensen, L., Theoretical Studies of Plasmonics

Using Electronic Structure Methods. Chem Rev 2011, 111, 3962-3994.

94. Brack, M., The Physics of Simple Metal Clusters: Self-Consistent Jellium

Model and Semiclassical Approaches. Reviews of Modern Physics 1993, 65, 677-

732.

95. Johnston, R. L., Atomic and Molecular Clusters; Taylor & Francis: London ;

New York, 2002, p xviii, 236 p.

96. Barke, I., et al., The 3d-Architecture of Individual Free Silver Nanoparticles

Captured by X-Ray Scattering. Nature Communications 2015, 6.

97. Haberland, H.; Hippler, T.; Donges, J.; Kostko, O.; Schmidt, M.; von Issendorff,

B., Melting of Sodium Clusters: Where Do the Magic Numbers Come From? Phys

Rev Lett 2005, 94.

98. Jortner, J., Cluster Size Effects. Z Phys D - Atoms, Molecules and Clusters

1992, 24, 247-275.

99. Jena, P.; Castleman Jr, A. W., Chapter 1 - Introduction to Atomic Clusters. In

Science and Technology of Atomic, Molecular, Condensed Matter & Biological

Systems, Purusottam, J.; Castleman, A. W., Eds. Elsevier: 2010; Vol. Volume 1, pp

1-36.

91

100. Sepúlveda, B.; Angelomé, P. C.; Lechuga, L. M.; Liz-Marzán, L. M., Lspr-

Based Nanobiosensors. Nano Today 2009, 4, 244-251.

101. Schalkhammer, T., Metal Nano Clusters as Transducers for Bioaffinity

Interactions. Monatshefte für Chemie - Chemical Monthly 1998, 129, 1067-1092.

102. Mie, G., Beiträge Zur Optik Trüber Medien, Speziell Kolloidaler

Metallösungen. Annalen der Physik 1908, 330, 377-445.

103. Moores, A.; Goettmann, F., The Plasmon Band in Noble Metal

Nanoparticles: An Introduction to Theory and Applications. New Journal of

Chemistry 2006, 30, 1121-1132.

104. Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C., The Optical

Properties of Metal Nanoparticles:  The Influence of Size, Shape, and Dielectric

Environment. The Journal of Physical Chemistry B 2003, 107, 668-677.

105. Gong, Y.; Zhou, Y.; He, L.; Xie, B.; Song, F.; Han, M.; Wang, G.,

Systemically Tuning the Surface Plasmon Resonance of High-Density Silver

Nanoparticle Films. The European Physical Journal D 2013, 67, 1-6.

106. Johnson, P. B.; Christy, R. W., Optical Constants of the Noble Metals.

Physical Review B 1972, 6, 4370-4379.

107. Amendola, V.; Bakr, O.; Stellacci, F., A Study of the Surface Plasmon

Resonance of Silver Nanoparticles by the Discrete Dipole Approximation Method:

Effect of Shape, Size, Structure, and Assembly. Plasmonics 2010, 5, 85-97.

108. Ung, T.; Liz-Marzán, L. M.; Mulvaney, P., Optical Properties of Thin Films

of Au@Sio2 Particles. The Journal of Physical Chemistry B 2001, 105, 3441-3452.

109. Atay, T.; Song, J.-H.; Nurmikko, A. V., Strongly Interacting Plasmon

Nanoparticle Pairs:  From Dipole−Dipole Interaction to Conductively Coupled

Regime. Nano Lett. 2004, 4, 1627-1631.

110. Popok, V. N.; Barke, I.; Campbell, E. E. B.; Meiwes-Broer, K. H., Cluster-

Surface Interaction: From Soft Landing to Implantation. Surf Sci Rep 2011, 66, 347-

377.

111. Haberland, H.; Insepov, Z.; Moseler, M., Molecular-Dynamics Simulation

of Thin-Film Growth by Energetic Cluster-Impact. Physical Review B 1995, 51,

11061-11067.

92

112. Xirouchaki, C.; Palmer, R. E., Pinning and Implantation of Size-Selected

Metal Clusters: A Topical Review. Vacuum 2002, 66, 167-173.

113. Carroll, S. J.; Nellist, P. D.; Palmer, R. E.; Hobday, S.; Smith, R., Shallow

Implantation of "Size-Selected" Ag Clusters into Graphite. Phys Rev Lett 2000, 84,

2654-2657.

114. Francis, G. M.; Goldby, I. M.; Kuipers, L.; vonIssendorff, B.; Palmer, R. E.,

Deposition and Growth of Noble Metal Clusters on Graphite. J Chem Soc Dalton

1996, 665-671.

115. Smith, R.; Nock, C.; Kenny, S. D.; Belbruno, J. J.; Di Vece, M.; Palomba,

S.; Palmer, R. E., Modeling the Pinning of Au and Ni Clusters on Graphite. Physical

Review B 2006, 73.

116. Di Vece, M.; Palomba, S.; Palmer, R. E., Pinning of Size-Selected Gold and

Nickel Nanoclusters on Graphite. Physical Review B 2005, 72.

117. Palmer, R. E.; Pratontep, S.; Boyen, H. G., Nanostructured Surfaces from

Size-Selected Clusters. Nat Mater 2003, 2, 443-448.

118. Popok, V. N., Energetic Cluster Ion Beams: Modification of Surfaces and

Shallow Layers. Materials Science and Engineering: R: Reports 2011, 72, 137-157.

119. Haberland, H.; Karrais, M.; Mall, M.; Thurner, Y., Thin Films from

Energetic Cluster Impact: A Feasibility Study. Journal of Vacuum Science &

Technology A 1992, 10, 3266-3271.

120. Schmidt, M.; Kébaïli, N.; Lando, A.; Benrezzak, S.; Baraton, L.; Cahuzac,

P.; Masson, A.; Bréchignac, C., Bent Graphite Surfaces as Guides for Cluster

Diffusion and Anisotropic Growth. Physical Review B 2008, 77, 205420.

121. Palomba, S.; Palmer, R. E., Patterned Films of Size-Selected Au Clusters on

Optical Substrates. Journal of Applied Physics 2007, 101, 044304.

122. Francis, G. M.; Goldby, I. M.; Kuipers, L.; von Issendorff, B.; Palmer, R.

E., Deposition and Growth of Noble Metal Clusters on Graphite. Journal of the

Chemical Society, Dalton Transactions 1996, 665-671.

123. Yoon, B.; Akulin, V. M.; Cahuzac, P.; Carlier, F.; de Frutos, M.; Masson,

A.; Mory, C.; Colliex, C.; Bréchignac, C., Morphology Control of the Supported

Islands Grown from Soft-Landed Clusters. Surf Sci 1999, 443, 76-88.

93

124. Lando, A.; Kébaïli, N.; Cahuzac, P.; Masson, A.; Bréchignac, C.,

Coarsening and Pearling Instabilities in Silver Nanofractal Aggregates. Phys Rev

Lett 2006, 97, 133402.

125. Kovacs, G. J.; Vincett, P. S., Subsurface Particle Monolayer and Film

Formation in Softenable Substrates: Techniques and Thermodynamic Criteria. Thin

Solid Films 1984, 111, 65-81.

126. Kovacs, G. J.; Vincett, P. S.; Tremblay, C.; Pundsack, A. L., Vacuum

Deposition onto Softenable Substrates: Formation of Novel Subsurface Structures.

Thin Solid Films 1983, 101, 21-40.

127. Deshmukh, R. D.; Composto, R. J., Direct Observation of Nanoparticle

Embedding into the Surface of a Polymer Melt. Langmuir 2007, 23, 13169-13173.

128. Amarandei, G.; Clancy, I.; O’Dwyer, C.; Arshak, A.; Corcoran, D., Stability

of Ultrathin Nanocomposite Polymer Films Controlled by the Embedding of Gold

Nanoparticles. ACS Appl. Mater. Interfaces 2014, 6, 20758-20767.

129. Popok, V. N.; Hanif, M.; Mackova, A.; Miksova, R., Structure and

Plasmonic Properties of Thin Pmma Layers with Ion-Synthesized Ag Nanoparticles.

J. Polym. Sci. B: Polym. Phys. 2015, 53, 664-672.

130. Maggioni, G.; Vomiero, A.; Carturan, S.; Scian, C.; Mattei, G.; Bazzan, M.;

Fernandez, G. D.; Mazzoldi, P.; Quaranta, A.; Della Mea, G., Structure and Optical

Properties of Au-Polyimide Nanocomposite Films Prepared by Ion Implantation.

Appl. Phys. Lett. 2004, 85, 5712-5714.

131. Hammond, J.; Bhalla, N.; Rafiee, S.; Estrela, P., Localized Surface Plasmon

Resonance as a Biosensing Platform for Developing Countries. Biosensors 2014, 4,

172.

132. Brolo, A. G., Plasmonics for Future Biosensors. Nat Photon 2012, 6, 709-

713.

133. Hosseini, S.; Ibrahim, F.; Djordjevic, I.; Koole, L. H., Recent Advances in

Surface Functionalization Techniques on Polymethacrylate Materials for Optical

Biosensor Applications. Analyst 2014, 139, 2933-2943.

134. Wijaya, E.; Lenaerts, C.; Maricot, S.; Hastanin, J.; Habraken, S.; Vilcot, J.-

P.; Boukherroub, R.; Szunerits, S., Surface Plasmon Resonance-Based Biosensors:

From the Development of Different Spr Structures to Novel Surface

Functionalization Strategies. Current Opinion in Solid State and Materials Science

2011, 15, 208-224.

94

135. Homola, J., Surface Plasmon Resonance Sensors for Detection of Chemical

and Biological Species. Chem Rev 2008, 108, 462-493.

136. Liedberg, B.; Nylander, C.; Lunström, I., Surface Plasmon Resonance for

Gas Detection and Biosensing. Sensors and Actuators 1983, 4, 299-304.

137. Englebienne, P., Use of Colloid Al Gold Surface Plasmon Resonance Peak

Shift to Infer Affinity Constants from the Interactions between Protein Antigens and

Antibodies Specific for Single or Multiple Epitopes. Analyst 1998, 123, 1599-1603.

138. Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P., A

Localized Surface Plasmon Resonance Biosensor:  First Steps toward an Assay for

Alzheimer's Disease. Nano Letters 2004, 4, 1029-1034.

139. Otte, M. A.; Estévez, M. C.; Carrascosa, L. G.; González-Guerrero, A. B.;

Lechuga, L. M.; Sepúlveda, B., Improved Biosensing Capability with Novel

Suspended Nanodisks. The Journal of Physical Chemistry C 2011, 115, 5344-5351.

140. Nath, N.; Chilkoti, A., Label-Free Biosensing by Surface Plasmon

Resonance of Nanoparticles on Glass:  Optimization of Nanoparticle Size.

Analytical Chemistry 2004, 76, 5370-5378.

141. Cui, Z., Nanofabrication: Principles, Capabilities and Limits; Springer US,

2009.

142. Liebl, H., Applied Charged Particle Optics; Springer Berlin Heidelberg,

2008.

143. Milani, P.; Iannotta, S., Cluster Beam Synthesis of Nanostructured

Materials; Springer Berlin Heidelberg, 2012.

144. Hartmann, H.; Popok, V. N.; Barke, I.; von Oeynhausen, V.; Meiwes-Broer,

K.-H., Design and Capabilities of an Experimental Setup Based on Magnetron

Sputtering for Formation and Deposition of Size-Selected Metal Clusters on Ultra-

Clean Surfaces. Rev. Sci. Instrum. 2012, 83, 073304.

145. Hall, D. B.; Underhill, P.; Torkelson, J. M., Spin Coating of Thin and

Ultrathin Polymer Films. Polymer Engineering & Science 1998, 38, 2039-2045.

146. Binnig, G.; Quate, C. F.; Gerber, C., Atomic Force Microscope. Phys Rev

Lett 1986, 56, 930-933.

147. Mironov, L. V., Fundamentals of Scanning Probe Microscope The Russion

Academy of Science, 2004.

95

148. Williams, D.; Carter, C. B., The Transmission Electron Microscope. In

Transmission Electron Microscopy, Springer US: 1996; pp 3-17.

149. Appy, D.; Lei, H.; Han, Y.; Wang, C.-Z.; Tringides, M. C.; Shao, D.;

Kwolek, E. J.; Evans, J. W.; Thiel, P. A., Determining Whether Metals Nucleate

Homogeneously on Graphite: A Case Study with Copper. Physical Review B 2014,

90, 195406.

150. Sundquist, B. E., A Direct Determination of the Anisotropy of the Surface

Free Energy of Solid Gold, Silver, Copper, Nickel, and Alpha and Gamma Iron.

Acta Metallurgica 1964, 12, 67-86.

151. Jaccodine, R., Surface Energy of Germanium and Silicon. Journal of The

Electrochemical Society 1963, 110, 524-527.

152. Abrahamson, J., The Surface Energies of Graphite. Carbon 1973, 11, 337-

362.

153. Janczuk, B.; Zdziennicka, A., A Study on the Components of Surface Free

Energy of Quartz from Contact Angle Measurements. Journal of materials science

1994, 29, 3559-3564.

154. Hanif, M.; Popok, N. V. In Low-Energy Interaction of Metal Cluster Ions

with Surfaces, International Conference Ion-Surface Interactions, Moscow, Zykova,

Y. E.; Karaseov, A. P.; Titov, I. A.; Yurasova, E. V., Eds. XYZ: Moscow, 2015; pp

47-52.

155. Claeyssens, F.; Pratontep, S.; Xirouchaki, C.; Palmer, R. E., Immobilization

of Large Size-Selected Silver Clusters on Graphite. Nanotechnology 2006, 17, 805.

156. Aqra, F.; Ayyad, A., Surface Free Energy of Alkali and Transition Metal

Nanoparticles. Applied Surface Science 2014, 314, 308-313.

157. Hanif, M.; Popok, V. N., Magnetron Sputtering Cluster Apparatus for

Formation and Deposition of Size-Selected Metal Nanoparticles. In Physics,

Chemistry and Applications of Nanostructures, World Scientific 2015; pp 416-419.

158. Ruffino, F.; Torrisi, V.; Marletta, G.; Grimaldi, M. G., Effects of the

Embedding Kinetics on the Surface Nano-Morphology of Nano-Grained Au and Ag

Films on Ps and Pmma Layers Annealed above the Glass Transition Temperature.

Appl. Phys. A 2012, 107, 669-683.

96

159. Chau, Y. F.; Chen, M. W.; Yeh, H. H.; Wu, F. L.; Li, H. Y.; Tsai, D. P.,

Highly Enhanced Surface Plasmon Resonance in a Coupled Silver Nanodumbbell.

Appl. Phys. A: Mater. Sci. Process. 2011, 104, 801-805.

160. Mao, P.; Chen, J.; Xu, R. Q.; Xie, G. Z.; Liu, Y. J.; Gao, G. H.; Wu, S.,

Self-Assembled Silver Nanoparticles: Correlation between Structural and Surface

Plasmon Resonance Properties. Appl. Phys. A: Mater. Sci. Process. 2014, 117,

1067-1073.

161. Mary, A. A.; Aleksandr, S., Novel Trends in Affinity Biosensors: Current

Challenges and Perspectives. Measurement Science and Technology 2014, 25,

032001.

162. Van Dorst, B.; Mehta, J.; Bekaert, K.; Rouah-Martin, E.; De Coen, W.;

Dubruel, P.; Blust, R.; Robbens, J., Recent Advances in Recognition Elements of

Food and Environmental Biosensors: A Review. Biosensors and Bioelectronics

2010, 26, 1178-1194.

97

Appendixes A to E Reprints of Publications

99

Appendix A

Magnetron Sputtering Cluster Apparatus for Formation and

Deposition of Size-Selected Metal Nanoparticles M. Hanif and V. N. Popok

416

PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES, 2015

MAGNETRON SPUTTERING CLUSTER APPARATUS FOR

FORMATION AND DEPOSITION OF SIZE-SELECTED METAL

NANOPARTICLES

M. HANIF AND V. N. POPOK

Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A,

DK-9220 Aalborg East, Denmark

The experimental setup utilizing a DC magnetron sputtering source for production of metal

clusters, their size (mass) selection and following deposition in high vacuum is described. The

source is capable to form clusters of various metals, for example, copper, silver, gold etc.

Cluster size selection is achieved using an electrostatic quadrupole mass selector. The

deposited silver clusters are studied using atomic force microscopy. The height distributions

show typical relative standard size deviation of 9-13% for given sizes in the range between 5-

23 nm. Thus, the apparatus demonstrates good capability in formation of supported size-

selected metal nanoparticles with controllable coverage for various practical applications.

1. Introduction

In the past two decades there has been an increasing interest in the deposition of

size-selected metal clusters on various surfaces. Properties of clusters depend on the

number of constituent atoms or molecules. Therefore, controlling the size and

structure of a cluster allows to tune electronic, optical, magnetic and chemical

parameters of the synthesized nanostructures. Cluster ion beam technique provides a

number of advantages such as a precise control of the composition and size of

clusters, surface coverage as well as kinetic energy which defines the cluster-surface

interaction regimes [1,2]. Clusters (nanoparticles) on surfaces define a new class of

systems highly relevant for practical applications such as biosensing,

nanoelectronics, non-linear optics, catalysis and formation of nanocomposites for

various applications [1-5].

Clusters can be produced by different methods [2]. Sources based on plasma

sputtering using a magnetron provide vaporization of metal atoms from a target and

their efficient agglomeration into clusters of various sizes [6]. Advantage of such

sources is in relatively high intensity of cluster beams as well as in a possibility to

produce nanoparticles of different metals. In the current paper, the design and

capabilities of a newly build cluster deposition apparatus based on the DC

magnetron sputtering source is described. Results of first test experiments on the

formation and deposition of size-selected silver clusters are presented.

417

2. Experimental

A schematic drawing of the magnetron sputtering cluster apparatus (MASCA) is

shown in Fig. 1. The setup consists of several vacuum chambers. For cluster

production a commercial source, NC200U from Oxford Applied Research, is

connected to the source chamber. In the source, target material is sputtered into an

aggregation region where clusters are formed and then expanded into the source

chamber. More details about the process of magnetron sputtering can be found

elsewhere [7,8]. Thereafter, the clusters are collimated into a beam by the conical in

shape skimmer. After the skimmer the cluster beam enters the ion optics section. By

Einzel lens and two pairs of deflectors the beam parameters are adjusted to enter the

electrostatic quadrupole mass selector (EQMS) where clusters are size selected.

EQMS consists of four equally distance hyperbolic electrodes surrounded by a

grounded shield. Four electrodes are divided into two pairs which can be biased

(UQP) with opposite polarity, thus, bending the beam of charged clusters of desire

masses for 90o into the deposition chamber. For details of the procedure see [6]. To

measure intensity of the beam, Faraday cups are used. All chambers are evacuated

by turbomolecular pumps (from 230 to 1250 l/s) backed by rotary vane pumps.

Using differential pumping a background pressure of 1.0×10-7

mbar is reached in the

deposition chamber.

Figure 1. Schematic drawing of MASCA.

A silver target of 99.99% purity (from Goodfellow Ltd) is used for cluster

production. Size-selected Agn cluster were deposited on clean Si(100) substrates at

room temperature. Cluster kinetic energy is kept in so-called thermal regime

providing good conditions for soft-landing without significant distortion of cluster

shape which is assumed to be close to spherical in the gas phase prior the deposition.

418

Supported nanoparticles are studied by atomic force microscopy (AFM) in

tapping mode using Ntegra Aura nanolaboratory from NT-MDT.

3. Results and Discussion

The clusters inside the source are formed in different sizes from a few up to 1000’s

of atoms; significant fraction of them is ionized. The cluster sizes can be tuned by

varying the discharge power, flows of sputtering (Ar) and aggregation (He) gases as

well as aggregation length in the source. All these parameters were optimized in

order to maximize beam intensity for the required cluster sizes.

Size-selection is carried out using following EQMS voltages UQP = ±100, ±300,

±500, ±900, ±1400, and ±2000 V. Clusters are deposited for 15-30 min to achieve

considerable surface coverage.

Figure 2. AFM image of silver clusters on silicon deposited at UQP = ±300 V and histogram of height distribution.

Heights of the deposited nanoparticles are analyzed using AFM. Earlier

experiments [6] showed that shape of the deposited clusters is only slightly deviate

from the spherical; the particles became a bit flattened. Thus, the height is almost

equal to the diameter. Typical AFM image of supported silver nanoparticles with

mean height h = 12.81.5 nm is presented in Fig. 2. Histogram of height distribution

for this sample is also shown, demonstrating relative standard deviation of ca. 12%.

By varying the voltage we achieved deposition of the clusters with good size

selection in the range between ca. 5 and 23 nm (see Table 1). By changing the

deposition time the cluster surface coverage can be easily controlled. Further

optimization of the sputtering conditions can allow to produce smaller cluster or to

go to very large ones depending on requirements of particular applications.

419

Table 1. Mean heights and standard deviations for the clusters deposited at

different EQMS voltages.

Voltage, V 100 300 500 900 1400 2000

Height, nm 5.21.1 12.81.5 13.51.7 18.21.6 19.92.3 23.01.3

MASCA is also tested for the formation of copper clusters showing the

capabilities very similar to those for silver ones.

4. Conclusions

Operation parameters of MASCA are optimized for the production of intense beams

of silver and copper clusters with required sizes. The performance and efficiency of

the electrostatic quadrupole mass selector is tested. Silver clusters of sizes between

5-23 nm are deposited on clean Si (100) substrates in high vacuum. Analysis using

AFM shows typical relative standard deviations of cluster sizes within ca. 9-13%.

Thus, the apparatus demonstrates good capability in formation of supported size-

selected metal nanoparticles with controllable coverage that can be utilized for

various practical applications, for example, for production of nanostructures with

well-reproduced parameters of surface plasmon resonance which are attractive for

optical sensing.

Acknowledgments

We acknowledge financial support from Spar Nord and Obel Family Foundations.

References

1. V.N. Popok, I. Barke, E.E.B. Campbell and K.-H. Meiwes-Broer, Surf. Sci. Rep.

66, 347 (2011).

2. V.N. Popok and E.E.B. Campbell, Rev. Adv. Mater. Sci. 11, 19 (2006).

3. C. Ghisleri, F. Borghi, L. Ravagnan, A. Podesta, C. Melis, L. Colombo and P.

Milani, J. Phys. D: Appl. Phys. 47, 015301 (2014)

4. K. Wegner, P. Piseri, H.V. Tafreshi and P. Milani, J. Phys. D: Appl. Phys. 39,

R439 (2006).

5. R.E. Palmer, S. Pratontep and H.-G. Boyen, Nature Mater. 2, 443 (2003).

6. H. Hartmann, V.N. Popok, I. Barke, V. von Oeynhausen and K.-H. Meiwes-

Broer, Rev. Sci. Instrum. 83, 073304 (2012).

7. K. Elmer, J. Phys. D: Appl. Phys. 33, R17 (2000).

8. J. A. Tornton, J. Vac. Sci. Technol. 15, 171 (1997).

107

Appendix B

Low-Energy Interaction of Metal Cluster Ions with Surfaces

M. Hanif and V. N. Popok

Russian Academy of Sciences National Research Nuclear University «MEPhI»

Moscow State University St. Petersburg State Polytechnic University

Moscow Aviation Institute Yaroslavl branch of Institute of Physics and Technology RAS

Institute of Microelectronics Technology and High Purity Materials RAS

Ion-Surface Interactions

ISI–2015

Proceedings of the XXII International Conference

August 20–24, 2015 Moscow, Russia

Editors E.Yu. Zykova Moscow State University P.A. Karaseov St. Petersburg State Polytechnic University A.I. Titov St. Petersburg State Polytechnic University V.E. Yurasova Moscow State University

Volume 1

LOW-ENERGY INTERACTION OF METAL CLUSTER IONS WITH SURFACES

M. Hanif and V.N. Popok

Department of Physics and Nanotechnology, Aalborg University, 9220 Aalborg, Denmark e-mail:vp@nano.aau.dk

Ion-beam treatment of materials is one of the widely applied methods for a number of

research and industrial purposes. Along with traditional monatomic species, atomic or

molecular clusters (aggregates of atoms or molecules) have attracted significant attention

during the last two decades [1-3]. One of advantages of cluster beam technique is in

possibility to control cluster sizes from a few up to many thousands of constituents. This

paves a way for investigation of fundamental physical aspects of the transition from the

atomic scale to bulk material [4]. On the other hand, finite size effects in supported

(deposited) clusters, which are often called nanoparticles (NP), lead to specific properties

providing a significant impact over a range of fields such as electronics and optics, biology

and medicine, catalysis and other nanotechnology-related branches [1, 5-8]. Increase of

cluster kinetic energy provides a possibility to apply this technique for shallow doping,

sputtering and polishing [2, 3, 8, 9]. In the recent years, clusters of inert gases and fullerenes

have become a powerful tool in secondary ion mass spectrometry [10].

In this paper, however, we would like to focus on the low-energy deposition of metal

clusters, so-called soft landing. In this regime, the kinetic energy per cluster atom or molecule

should be much below the binding (cohesive) energy of the cluster constituents. The soft

landing does not lead to cluster fragmentation, i.e. the cluster preserves its composition.

However, the shape can be slightly distorted compared to that in the gas phase due to the

physisorption and formation of van der Waals bonds with surface atoms (see schematic

picture in Fig. 1). Study of soft landed clusters is driven by the efforts to utilise the above-

mentioned unique properties of supported metal NPs for applications. A number of studies

were carried out and it was shown that surface diffusion of clusters must be considered [11].

Since the activation barrier strongly depends on the cluster-surface bonding, cluster mobility

is favored on weakly interacting materials, such as graphite or amorphous carbon. If the

lattice mismatch between the cluster and the substrate is significant, vibrational coupling

between these can overcome the small energy barrier of the surface potential, thus, leading to

Brownian-like motion of the cluster [12]. Particles moving on the surface can meet and

interact yielding two main possible scenarios: coalescence and agglomeration. Coalescence

leads to merging of two (or more) clusters and the formation of a single larger particle with

potentially different shape. This is preferable regime for small clusters. In the case of metal

clusters consisting of hundreds or thousands of atoms the tendency is to agglomerate and form

islands [12, 13]. Clusters are also tended to be immobilized at natural defect on the surfaces.

One of the very well-known phenomena is collection of NPs at step edges of graphite [14].

Fig. 1. Schematic picture of cluster deposition.

Despite of a significant number of publications on cluster soft-landing many fundamental

physical questions still have to be answered. In the current work we study soft-landing of

size-selected silver clusters on a few different substrates to resolve some issues related to

surface arrangement of clusters which are of practical importance for a number of

applications.

In the experiments, magnetron sputtering cluster apparatus (MaSCA) shown in Fig. 2 is

used. The setup consists of several vacuum chambers. For cluster production a commercial

source, NC200U from Oxford Applied Research, is connected to the source chamber. In the

source, target material is sputtered into an aggregation region where clusters are formed and

then expanded into the source chamber. More details about the process of magnetron

sputtering can be found elsewhere [15]. Thereafter, the clusters are collimated into a beam by

a skimmer. After the skimmer, the cluster beam enters the ion optics. By Einzel lens and two

pairs of deflectors the beam parameters are adjusted to enter the electrostatic quadrupole mass

selector (EQMS) where clusters are size selected. EQMS consists of four equally distance

hyperbolic electrodes surrounded by a grounded shield. These electrodes are divided into two

pairs which can be biased (UQP) with opposite polarity, thus, bending the beam of charged

clusters of desire masses for 90o into the deposition chamber. For details of the procedure see

[15]. To measure intensity of the beam, Faraday cups are used. All chambers are evacuated by

turbomolecular pumps (from 230 to 1250 l/s) backed by rotary vane pumps. Using differential

pumping a background pressure of 1.0×10-7 mbar is reached in the deposition chamber.

Fig. 2. Schematic drawing of MaSCA.

A silver target of 99.99% purity (from Goodfellow Ltd) is used for cluster production.

Size-selected Agn clusters are deposited on clean Si(100), highly ordered pyrolytic graphite

(HOPG), quartz and quartz spin-coated by 50 nm thick films of polymethylmethacrilate

(PMMA) at room temperature. Deposition time is typically varied between 15-30 min to get

considerable surface coverage. Cluster kinetic energy is kept in so-called thermal regime

providing good conditions for soft-landing from the gas phase where the clusters are assumed

to be close to spherical shape. Supported NPs are studied by atomic force microscopy (AFM)

in tapping mode using Ntegra Aura nanolaboratory from NT-MDT. Samples with clusters

deposited on quartz and PMMA/quartz substrates are also investigated by optical

transmittance spectroscopy using Perkin Elmer High Performance Lambda 1050 spectrometer

in the interval of wavelengths λ = 300-750 nm.

The clusters inside the source are formed in different sizes from a few up to many

thousands of atoms; significant fraction of clusters is ionized. The cluster sizes can be tuned

by varying the discharge power, flows of sputtering (Ar) and aggregation (He) gases as well

as aggregation length in the source. The size (mass) selection is defined by the geometry of

EQMS and to the large extent by voltages applied to the electrodes. Relatively low voltages

allow selection of smaller in size (light mass) clusters while with the voltage increase larger

(heavier) particles can be selected. Size of the particles is estimated from AFM measurements.

Earlier experiments [15] showed that shape of the deposited metal clusters is only slightly

deviate from the spherical; the particles became a bit flattened on Si. Thus, the measured

height is assumed to be almost equal to NP diameter. In the series of depositions the

following EQMS voltages UQP = ±100, ±300, ±500, ±900, ±1400, and ±2000 V are applied.

Fig. 3. AFM image of silver clusters on silicon deposited at UQP = ±300 V and histogram of height distribution.

The AFM study shows that the clusters are located on the surface randomly and they do not

agglomerate even for coverages close to a monolayer of clusters. This indicates low diffusion

mobility of silver clusters on Si at room temperature. An example of AFM image and

corresponding height histogram for UQP = ±300 V are shown in Fig. 3. Mean heights and

standard deviations for silver clusters deposited in this series of experiments are presented in

Fig. 4. The fit curve demonstrates proportionality of cluster size to which is expected

from the theory behind mass selection in EQMS (see for details [15]).

AFM image of clusters deposited on HOPG surface is shown in Fig. 5. Comparison of

mean cluster height on HOPG, which is found to be 12.7±1.7 nm, with that on Si, 12.8±1.5

nm, for the same UQP shows very good agreement. This allows concluding that the clusters

are soft landed on HOPG without significant shape corrugation. However, one can clearly see

Fig. 4. Mean cluster heights (with standard deviations) for different voltages at EQMS.

Fig. 5. AFM image of silver clusters deposited on HOPG at UQP = ±300 V.

Fig. 6. Normalized optical absorption spectra for bare quartz and quartz coated by PMMA

with deposited size-selected (at UQP = ±300 V) silver clusters.

from the AFM image a tendency for clusters to collect at surface defects especially at the

steps that is indicated by rows of clusters. In the case of deposition on quartz substrates at UQP

= ±300 V mean cluster height is measured to be 12.0±1.9 nm which can be an indication that

the clusters are more flattened on quartz compared to Si or HOPG but this issue requires

further investigation. The surface location of NPs is random similar to the case of silicon. For

the clusters deposited on PMMA mean height is found to be very similar to the case of quartz.

The clusters are randomly spread and not making any agglomerates.

The samples with clusters deposited on bare quartz and quartz coated by PMMA (i.e.

transparent in visible interval substrates) are also studied using optical spectroscopy. The

spectra are shown in Fig. 6. Presence of an absorption band at wavelength λ ≈ 400 nm, which

is related to localized surface plasmon resonance (LSPR) on silver NPs [16], can be seen. The

other less pronounced band at around 500 nm is most probably related to LSPR of interacting

NPs similar to the case described in [17]. Intensity of this band is found to be strongly

dependent on cluster surface coverage. With coverage decrease, i.e. increase of interparticle

distance, the band has tendency to disappear, thus, supporting our suggestion on the band

nature.

Good size selection and possibility to control cluster coverage by tuning the deposition

time make the cluster beam technique very attractive towards practical applications. One of

the directions can be formation of cluster-based plasmonic systems which are used as optical

transducers for biosensors. This possibility has been recently demonstrated for protein

detection [18].

1. K. Wegner, P. Piseri, H.V. Tafreshi, P. Milani, J. Phys. D 39 (2006) R439.

2. N. Toyoda, I. Yamada, IEEE Trans. Plasma Sci. 36 (2008) 1471.

3. V.N. Popok, Mater. Sci. Eng. R: Rep. 72 (2011) 137.

4. P. Jena, A.W. Castkeman Jr., Proc. Nation. Acad. Sci. 103 (2006) 10560.

5. C. Binns, Surf. Sci. Rep. 44 (2001) 1.

6. U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Berlin: Springer, 1995.

7. R.E. Palmer, S. Pratontep, H.-G. Boyen, Nature Mater. 2 (2003) 443.

8. V. Popok, In: Handbook of Nanophysics: Clusters and Fullerenes, ed. by K.D. Sattler, Boca

Raton: CRC Press, 2010, p. 19-1.

9. I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R: Rep. 34 (2001) 231.

10. M.P. Seah, I.S. Gilmore, Surf. Interface Anal. 43 (2011) 228.

11. A. Perez, P. Melinon, V. Dupius et al. J. Phys. D: Appl. Phys. 30 (1997) 709.

12. N. Vandamme, E. Janssens, F. Vanhoutte, P. Lievens, C. Van Haesendonck, J. Phys.:

Condens. Mater. 15 (2003) S2983.

13. I.M. Goldby, L. Kuipers, B. von Issendorf, R.E. Palmer, Appl. Phys. Lett. 69 (1996) 2819.

14. B. Klipp, M. Grass, J. Müller, D. Stolcic, U. Lutz, G. Ganteför, T. Schlenker, J. Boneberg,

P. Leiderer, Appl. Phys. A 73 (2001) 547.

15. H. Hartmann, V.N. Popok, I. Barke, V. von Oeynhausen, K.-H. Meiwes-Broer, Rev. Sci.

Instrum. 83 (2012) 073304.

16. V.N. Popok, M. Hanif, A. Mackova, R. Miksova, J. Polymer Sci. B: Polymer Phys. (2015)

in press. DOI: 10.1002/polb.23682.

17. P. Mao, J. Chen, R. Xu, G. Xie, Y. Liu, G. Gao, S. Wu, Appl. Phys. A 117 (2014) 1067.

18. P. Fojan, M. Hanif, S. Bartling, H. Hartmann, I. Barke, V.N. Popok, Sens. Actuators B:

Chem. 212 (2015) 377.

115

Appendix C

Poly (methyl methacrylate) Composites with Size-Selected

Silver Nanoparticles Fabricated using Cluster Beam Technique M. Hanif, R. R. Juluri, M. Chirumamilla, V. N. Popok

1

Poly (methyl methacrylate) Composites with Size-Selected Silver

Nanoparticles Fabricated using Cluster Beam Technique

Muhammad Hanif †, Raghavendra R. Juluri

‡, Manohar Chirumamilla

†, Vladimir N. Popok

†

†Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, 9220 Aalborg,

Denmark

‡Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14,

8000 Aarhus C, Denmark

Correspondence to: Muhammad Hanif (E-mail: mh@nano.aau.dk)

((Additional Supporting Information may be found in the online version of this article.))

INTRODUCTION

The physical and chemical properties of nanocomposites, consisting of organic and inorganic building blocks, are of considerable interest both from a fundamental point of view

1-2 and for

potential applications.3 Metal

nanoparticles (NPs) deposited on

polymer surfaces or embedded in polymer films can modify the polymer structure and lead to unique electrical, mechanical, optical, catalytic and antibacterial properties that can be tuned by varying the NP species, size, shape and surface functionality.

4-7 This

passive tuning requires the preparation of ingredients with predetermined

ABSTRACT

An embedment of metal nanoparticles of well-defined sizes in thin polymer films is of

significant interest for a number of practical applications, in particular, for preparing

materials with tunable plasmonic properties. In this paper, we present a fabrication

route for metal-polymer composites based on cluster beam technique allowing the

formation of monocrystalline size-selected silver nanoparticles with a 5-7% precision

of diameter and controllable embedment into poly (methyl methacrylate). It is shown

that the soft-landed silver clusters preserve almost spherical shape with a slight

tendency to flattening upon impact. By controlling the polymer hardness (from viscous

to soft state) prior the cluster deposition and annealing conditions after the deposition

the degree of immersion of the nanoparticles into polymer can be tuned, thus, making

it possible to create composites with either particles partly or fully embedded into the

film. Good size-selection and rather homogeneous dispersion of nanoparticles in the

thin polymer film lead to excellent plasmonic properties characterized by the narrow

band and high quality factor of localized surface plasmon resonance.

KEYWORDS: Cluster beam deposition, Size-selected clusters, Metal-polymer

nanocomposites, Optics, Localised surface plasmon resonance

2

parameter prior to fabrication of composites. However, metals and polymers are materials with very contrast properties and, therefore, appropriate methodology should be used for the formation of metal-polymer nanocomposites.

One of the very widely used techniques to fill a polymer with metal is evaporation.

8-10 The cohesive energy of

metals exceeds the cohesive energy of polymers by typically two orders of magnitude and generally it is expected that metals exhibit a strong tendency to aggregate and form clusters when deposited on the surface of an organic substrate.

11 However, in case of low

reactivity metals like Cu, Ag, Au and Pd, deposited atoms can diffuse towards bulk of a substrate causing subsequent formation of clusters below the surface under certain evaporation conditions. Kovacs et al.

12-14 showed that complete

embedding of NP into a polymer is expected if ϒM > ϒP + ϒMP , where ϒM and ϒP are the surface tensions (equivalent of surface free energy per unit area) of metal NP and polymer, respectively, and ϒMP is the NP-polymer interfacial tension. If this condition is satisfied NPs embedding is expected. In practice, however, this is not always the case resulting in NPs resided on polymer surfaces.

1, 15-16 Penetration of NPs

certainly requires long-range polymer “chain mobility”, thus, no or partial immersion can be expected at temperatures below glass transition (< Tg), whereas for temperature above Tg complete NP embedding in the polymer film can be observed.

8, 10, 17

Metal NPs have also been deposited from solutions directly onto a polymer surface, where NP immersion is driven by the formation of a thin wetting layer. For example, it was shown that Au NPs diffusion below the polystyrene surface follows a preliminary step in which very thin (1.3-1.8 nm) wetting layer covers the NP and creates a capillary pressure responsible for pushing the NP into the soft substrate.

6

NP embedding can also be achieved by exposing a polymer film to a saturated solvent vapor, commonly known as solvent annealing.

14 George et al.

1

showed that introducing gold NPs helps to reduce spinodal instability of ultrathin polymer layers. Since penetration of metal NPs needs long-range chain mobility of a polymer, embedding process can be used to probe the glass transition temperature (Tg) near the surface of polymer. Such experiments showed that surface Tg is lower than that of the bulk.

8, 18-20

Another feasible approach to form metal NPs inside a polymer matrix is ion implantation with high fluences.

21-22

Metal ions embedded at high concentrations are tended to form NPs.

23-24 An advantage of this method

is in possibility to use different metal species and synthesize particles in practically any polymer as well as to control the depth at which the particles are nucleated by tuning the ion energy. However, a high-fluence implantation leads to significant radiation damage of polymer structure and evolution of composition which becomes a disadvantage for practical use of such composites.

21, 25-26

3

It is worth noting that the all above-presented methods provide very limited possibility to control the size of NPs which is a disadvantage for many applications.

Recently, it has been demonstrated that metal NPs can be embedded into polymers using cluster beams.

27-28 This

is a very powerful technique allowing to produce the particles with well-controlled composition in vacuum and in many cases providing the possibilities for cluster size-selection along with kinetic energy tuning.

29

Thus, clusters can be implanted, pinned or soft landed.

29-32 Deposited on

polymers with relatively low kinetic energy Pdn and Aun clusters of a few nm in diameter were found to become implanted up to the depth of few tens of nm.

7, 27-28 These composite materials

were suggested for applications such as nanostructured elastomeric electrodes and plasmonic structures with stable tuning under cyclic strain conditions.

7,

28 However, the mechanism of such

implantation is not fully understood. On the one hand, it can be facilitated by the above-described difference in surface energies of metal NPs and polymer. On the other hand, the simulations showed that even clusters with rather low kinetic energies largely affect polymer structure and morphology around the impact spot favoring the penetration of the impinging NPs into the polymer.

33

In the current work, we focus on the soft landing of size-selected silver NPs formed by magnetron sputtering cluster apparatus (MaSCA)

34 and

development of a methodology for the

cluster embedment into thin poly (methyl methacrylate) (PMMA) films by controlling the polymer hardness and applying annealing. Investigations of structure and properties of the nanocomposites give important insights into the understanding of the mechanisms governing NP penetration and following distribution in the polymer substrates as well as demonstrate remarkable plasmonic properties of the synthesized materials.

EXPERIMENTAL

MaSCA utilizing a commercial source, NC200U from Oxford Applied Research, was used for cluster production from silver targets of 99.99% purity (from Goodfellow Ltd).

34 In the source, the

target material is sputtered by Ar plasma and condensed in the aggregation region with the help of buffer He gas. The clusters inside the source are formed in different sizes from a few up to 1000’s of atoms; a significant fraction of them is ionized. The cluster sizes and beam intensity can be tuned to some extent by varying the discharge power, flows of sputtering (Ar) and aggregation (He) gasses as well as aggregation length in the source.

35 The electrostatic

quadrupole mass selector (EQMS) similar to that described in ref. 36

36

was used for size separation of the clusters prior the deposition. Only charged particles were selected and steered towards deposition that was carried out in the vacuum with a background pressure of approximately 5x10

-8 mbar. For the experiments on

optimization of size-selection, Ag NPs with diameters (sizes) ranging between

4

ca. 5-25 nm were deposited. Kinetic energies defined from the cluster expansion conditions were estimated to vary on the scale of a few tens of meV/atom depending on the cluster size (mass). Thus, the depositions were carried out in soft landing regime which should not lead to any significant distortion of cluster shape or size.

To test the efficiency of EQMS and find a correlation between the voltages (UQP) used for the selection and actual cluster sizes a series of depositions was carried out on clean and smooth (roughness below 0.5 nm) Si(100) substrates. For high-resolution transmission electron microscopy (TEM) analysis, clusters were also deposited on standard copper TEM grids with 40 nm thick amorphous carbon layer.

PMMA films were prepared by standard spin coating procedure from 1% solution (molecular weight 950K PMMA C 9, MicroChem, America) in toluene onto Si and quartz substrates. The films were made at exactly same conditions for both types of substrates. Ellipsometry was used to measure the film thicknesses which varied between 25 – 50 nm for different samples. In order to investigate the particle embedment in PMMA, two strategies were followed. Type one PMMA samples were made by spin coating without annealing. They were only dried for 12 hours at room temperature, i.e. PMMA was kept in the viscous state prior the cluster deposition. These samples are called viscous PMMA throughout the paper. Type two samples were annealed at

100 oC for 10 min. after the spin

coating to solidify PMMA but avoid glass transition that occurs at Tg = 105 oC. This type of samples will be called

soft PMMA. Silver clusters used for the deposition on PMMA were selected at EQMS voltages (UQP) of 300 V that corresponds to mean diameter of approximately 13 nm, as will be shown below, and were soft-landed on both types of samples. Five samples of every type were prepared and analyzed.

Surface morphology of the samples with deposited clusters was studied by atomic force microscopy (AFM) operating in tapping mode using Ntegra Aura nanolaboratory (from NT-MDT). Commercial cantilevers with sharp silicon tips (radius of curvature < 10 nm and force constant k = 1.45-15.1 N/m) were used. To obtain cross-sectional images of PMMA with deposited clusters, focused ion beam milling utilizing FEI-Versa was applied and TEM measurements were carried out using FEI-Talos microscope operating at 200 kV. To avoid damage introduced by Ga ion beam, an additional SiO2 layer of 50 nm thicknesses was deposited on the PMMA with clusters by electron beam evaporation method. Optical transmission measurements were performed on the samples prepared on quartz by double beam Perkin Elmer High-Performance Lambda 1050 Spectrometer in a standard configuration.

5

RESULTS AND DISCUSION

Clusters Size-Selection

Parameters including magnetron power, gas pressure, gas flow and aggregation length were varied to optimize formation of silver clusters and beam intensity. In the first series of experiments, clusters were soft-landed onto Si substrates at different voltages applied to EQMS in order to vary the cluster size. The samples with deposited Agn clusters were studied using AFM and the mean cluster heights were found for every voltage

regime. Figure 1 shows an AFM image and corresponding height histogram for the sample obtained under UQP = ±300 V demonstrating the precision of the size-selection. The mean cluster height values <h> for other size-selecting UQP are presented in Figure 2. The dependence follows (UQP)

1/3 law

that well corresponds to the theory36

and allows choosing appropriate EQMS voltage in order to get required NP size. Standard deviations of heights are found to be within 7 % indicating the good size selection of clusters for all used voltage regimes.

FIGURE 1 (a) AFM image and (b) corresponding height histogram fitted by Gaussian

distribution for clusters deposited at UQP = ±300 V.

Figure 2 Cluster mean height for different

UQP. Error bars show standard deviations.

Cluster Shape and Structure

The clusters deposited at UQP = ±300 V on the grid with amorphous carbon layer were investigated by TEM. Figure 3 shows one of the typical images and size (diameter) distribution histogram obtained by analysis of a few images. One can see a number of individual NPs with round (spherical) shapes. However, due to relatively low interparticle distances cluster can easily diffuse and make larger agglomerates which are excluded from the size analysis. The histogram yields

6

Figure 3 (a) TEM image and (b) corresponding size histogram of Agn clusters deposited at UQP = ±300 V on TEM grid with a thin amorphous carbon layer. (c) High-resolution TEM image of middle part of individual cluster showing (111) atomic planes.

mean lateral NP diameter (<d>) of 13.6 nm. Comparison with the mean height obtained by AFM gives the aspect ratio (<d>/<h>) of 1.06. Similar aspect ratios were earlier found for copper clusters suggesting that the particles become only slightly oblate (flattened) under the soft landing.36 Thus, measurements of height by and efficient way for obtaining sizes of soft landed clusters.

High-resolution TEM images of individual silver NPs give a possibility to conclude about face-centered cubic (fcc) lattice structure with parameters very close to those of bulk silver. As an example, in Figure 3(c), one of such images shows atomic planes with an interplane distance of 2.36 Å that well corresponds to (111) crystal face of fcc lattice of bulk silver.

Silver-PMMA Composites

For all experiments with PMMA the clusters were size-selected at UQP = ±300 V, i.e. the expected size is ≈ 13 nm. AFM image of viscous PMMA with as deposited clusters and corresponding height histogram are shown in Figure 4. The histogram demonstrates relatively wide size

distribution with <h> = 7.6 nm and long tail towards smaller particle heights. According to the literature data presented in the introduction, it can be suggested that the NPs partly immerse into the viscous substrate and only tops of particles can be observed. As a next stage, the samples were annealed in two steps at 95 and 100 °C for 10 min each time. Corresponding AFM images and histograms are shown in Figure 5 and Figure 6. One can see that after the first annealing the cluster surface coverage is significantly decreased and <h> becomes much smaller, 4.1 nm. Next annealing further decreases both the surface coverage and height. The most probable scenario is that the NPs penetrate deeper into PMMA under thermal annealing and a large fraction of them becomes fully embedded. This suggestion is confirmed by the TEM image shown in Figure 7. The main driving force for cluster immersion is the minimization of surface free energy of NP-polymer system

11-12 and the

annealing process greatly facilitates NPs embedment. Since the polymer is viscous, it is not necessary to go with annealing above the glass transition temperature to initiate the particle immersion.

7

Figure 4 AFM image and corresponding height histogram of as-deposited Agn clusters onto

viscous PMMA.

Figure 5 AFM image and corresponding height histogram for viscous PMMA with deposited

Agn clusters after annealing for 10 min at 95 °C.

Figure 6 AFM image and height histogram for viscous PMMA with deposited Agn clusters

after second annealing for 10 min at 100 °C.

8

Figure 7 TEM cross-sectional image of

viscous PMMA layer with Ag clusters

embedded in the result of thermal

annealing as described in the text. PMMA

layer is marked by dashed lines for better

visualization.

Small metal particles are known to demonstrate a strong absorption peak in the UV-visible region due to localized surface plasmon resonance (LSPR).

37

Spectral position and intensity of the plasmon band are strongly dependent on the particle species, size, shape, filling factor and dielectric properties of the surrounding medium. An extinction spectrum of viscous PMMA with as-deposited silver clusters is shown in Figure 8 demonstrating two overlapping bands with maxima at wavelengths of 378 and 465 nm. The first band at 378 nm is assigned to LSPR on individual Ag NPs.

38 Nature of the

second band at the higher wavelength is not completely clear. It is found that its extinction efficiency gradually decreases with lowering the cluster surface coverage. One of the possible explanations is that the band is related to interaction of two neighboring NPs casing a well-known phenomenon of plasmon coupling.

39-41 Formation of NP

dimers can easily happen on cluster deposition due to self-assembly.

42-43

Such dimers can, for example, be seen

Figure 8 Normalized extinction spectra of

viscous PMMA samples with deposited Ag

clusters. The spectra are obtained by

subtraction of spectrum for pristine viscous

PMMA.

in Figure 3. This explains why the band

vanished with a decrease of cluster

coverage: interparticle distances

increase and probability for pair

interaction lowers.

The annealing changes the optical spectrum dramatically. As can be seen in Figure 8, the intensity of the first plasmon band is increased a few times and the band is “red” shifted having the maximum at 413 nm. This behavior well correlates with the microscopy studies showing that the absolute majority of NPs become embedded into PMMA that changes the dielectric environment (PMMA has higher dielectric constant compared to air, 2.6 and 1.0, respectively) causing the observed spectral changes. The second band is almost gone most probably because the NPs penetration into PMMA breaks the self-assembled pairs.

Figure 9 shows the AFM images of soft PMMA with as-deposited clusters and corresponding height histogram. The mean height of the particles, 13.2 nm,

9

Figure 9 AFM image and corresponding height histogram of Agn clusters as-deposited onto

soft PMMA.

Figure 10 AFM image and corresponding height histogram of Agn clusters deposited onto

soft PMMA after annealing at 125 oC.

obtained from AFM images, reveals that the silver clusters stays on the PMMA surface and do not immerse. Annealing at 95 and then at 100 °C do not indicate any considerable change in the height distribution and cluster coverage (therefore, AFM images are not shown here). Thus, contrary to viscous PMMA, temperatures below Tg are not sufficient to initiate immersion. However, after annealing at 125 °C, which is higher than Tg, topography undergoes significant changes that can be seen in Figure 10. The particles height decreases from 13.2 nm to 5.9 nm and the surface coverage becomes much lower inferring embedding

process of Ag NPs into the PMMA which is confirmed by TEM measurements (see Fig. 11). As mentioned in the introduction, embedment of NPs greatly depends on the polymer chain mobility. Thus, our observations support this statement demonstrating NPs immersion into the soft polymer at temperatures above the glass-transition point. It can also be concluded that the Gibbs free energy minimization of the system is the driving force for the immersion of Ag NPs in PMMA. Our findings are in good agreement with earlier experiments on the embedment kinetics of Ag and Au NPs prepared by metal sputtering and

10

annealing of PMMA at T > Tg.17

Figure 11 TEM cross-sectional image of soft

PMMA layer (bright stripe) with Ag clusters

embedded in the result of thermal

annealing as described in the text.

Further characterization of soft PMMA with Ag NPs has been done using optical spectroscopy. The extinction spectra are shown in Figure 12. Two maxima at 375 and 467 nm can be seen for the spectrum corresponding to as-deposited clusters likewise to the case of viscous PMMA. After the annealing at 125 °C the spectrum reveals a strong band with a maximum at 413 nm corresponding to Ag NPs immersed into PMMA. The second band almost vanishes for the same reason as in the case of viscous PMMA.

Figure 12 Normalized extinction spectra of

soft PMMA samples with deposited Ag

clusters. The spectra are obtained by

subtraction of spectrum for pristine soft

PMMA.

CONCLUSIONS

It has been shown that the developed cluster apparatus has a very good capability in formation of nanocrystalline silver clusters and size-

selection with precision within 7% of particle diameter. The soft-landed clusters are found to preserve almost spherical shape evidencing only slight oblation. Thus, the cluster technique provides excellent possibilities for the preparation of a nanofiller, which are metal nanoparticles, with very-well predefined parameters for the formation of nanocomposite materials, in particular metal-polymer systems.

Experiments on silver cluster deposition have shown that one can control immersion of nanoparticles into PMMA by varying the polymer state (viscous or soft) prior the deposition and by the following thermal annealing of the samples with as-deposited clusters. These technically simple treatments open great capabilities for the formation of polymer films of desired thickness, which can be easily controlled in the spin-coating process, with either partly or fully embedded size-selected particles. The prepared Ag-PMMA composites demonstrate excellent plasmonic properties characterized by the narrow band and a high quality factor of resonance. Such composites have high potential applications. One of them can be the use of the PMMA films with silver clusters as plasmonic transducers for biosensing. With methodology on the attachment of proteins to individual clusters, which is developed elsewhere,

38 and partial immersion of

11

Ag NPs into PMMA securing high adhesion, very stable and resistant against wet chemistry transducers can be developed.

ACKNOWLEDGEMENTS

The authors acknowledge partial financial support from the Spar Nord and Obel Family Foundations.

REFERENCES

1. Amarandei, G.; Clancy, I.; O’Dwyer, C.; Arshak, A.; Corcoran, D., Stability of Ultrathin Nanocomposite Polymer Films Controlled by the Embedding of Gold Nanoparticles. ACS Appl. Mater. Interfaces 2014, 6, 20758-20767. 2. Yang, J.; Han, C.-R.; Duan, J.-F.; Xu, F.; Sun, R.-C., Interaction of Silica Nanoparticle/Polymer Nanocomposite Cluster Network Structure: Revisiting the Reinforcement Mechanism. J. Phys. Chem. C 2013, 117, 8223-8230. 3. Lu, Z.; Yin, Y., Colloidal Nanoparticle Clusters: Functional Materials by Design. Chem. Soc. Rev. 2012, 41, 6874-6887. 4. Schwartzkopf, M., et al., From Atoms to Layers: In Situ Gold Cluster Growth Kinetics During Sputter Deposition. Nanoscale 2013, 5, 5053-5062. 5. Faupel, F.; Zaporojtchenko, V.; Strunskus, T.; Elbahri, M., Metal-Polymer Nanocomposites for Functional Applications. Adv Eng Mater 2010, 12, 1177-1190. 6. Deshmukh, R. D.; Composto, R. J., Direct Observation of Nanoparticle Embedding into the Surface of a Polymer Melt. Langmuir 2007, 23, 13169-13173.

7. Minnai, C.; Milani, P., Metal-Polymer Nanocomposite with Stable Plasmonic Tuning under Cyclic Strain Conditions. Appl. Phys. Lett. 2015, 107, 073106. 8. Erichsen, J.; Kanzow, J.; Schürmann, U.; Dolgner, K.; Günther-Schade, K.; Strunskus, T.; Zaporojtchenko, V.; Faupel, F., Investigation of the Surface Glass Transition Temperature by Embedding of Noble Metal Nanoclusters into Monodisperse Polystyrenes. Macromolecules 2004, 37, 1831-1838. 9. Zaporojtchenko, V.; Strunskus, T.; Behnke, K.; Bechtolsheim, C. v.; Thran, A.; Faupel, F., Formation of Metal–Polymer Interfaces by Metal Evaporation: Influence of Deposition Parameters and Defects. Microelectron. Eng. 2000, 50, 465-471. 10. Bechtolsheim, C. V.; Zaporojtchenko, V.; Faupel, F., Influence of Thermal Treatment on the Morphology and Adhesion of Gold Films on Trimethylcyclohexane-Polycarbonate. Appl. Surf. Sci. 1999, 151, 119-128. 11. Rosenthal, L.; Strunskus, T.; Faupel, F.; Abraham, J.; Bonitz, M., Kinetic Monte Carlo Simulations of Cluster Growth and Diffusion in Metal-Polymer Nanocomposites. In Complex Plasmas, Bonitz, M.; Lopez, J.; Becker, K.; Thomsen, H., Eds. Springer International Publishing: 2014; Vol. 82, pp 321-370. 12. Kovacs, G. J.; Vincett, P. S., Subsurface Particle Monolayer and Film Formation in Softenable Substrates: Techniques and Thermodynamic Criteria. Thin Solid Films 1984, 111, 65-81. 13. Kovacs, G. J.; Vincett, P. S.; Tremblay, C.; Pundsack, A. L., Vacuum

12

Deposition onto Softenable Substrates: Formation of Novel Subsurface Structures. Thin Solid Films 1983, 101, 21-40. 14. Kovacs, G. J.; Vincett, P. S., Formation and Thermodynamic Stability of a Novel Class of Useful Materials: Close-Packed Monolayers of Submicron Monodisperse Spheres Just Below a Polymer Surface. J. Colloid Interface Sci. 1982, 90, 335-351. 15. Amarandei, G.; O’Dwyer, C.; Arshak, A.; Thiele, U.; Steiner, U.; Corcoran, D., Effect of Au Nanoparticle Spatial Distribution on the Stability of Thin Polymer Films. Langmuir 2013, 29, 6706-6714. 16. Kunz, M. S.; Shull, K. R.; Kellock, A. J., Morphologies of Discontinuous Gold Films on Amorphous Polymer Substrates. J. Appl. Phys. 1992, 72, 4458-4460. 17. Ruffino, F.; Torrisi, V.; Marletta, G.; Grimaldi, M. G., Effects of the Embedding Kinetics on the Surface Nano-Morphology of Nano-Grained Au and Ag Films on Ps and Pmma Layers Annealed above the Glass Transition Temperature. Appl. Phys. A 2012, 107, 669-683. 18. Sharp, J. S.; Teichroeb, J. H.; Forrest, J. A., The Properties of Free Polymer Surfaces and Their Influence on the Glass Transition Temperature of Thin Polystyrene Films. Eur. Phys. J. E 2004, 15, 473-487. 19. Weber, R.; Grotkopp, I.; Stettner, J.; Tolan, M.; Press, W., Embedding of Gold Nanoclusters on Polystyrene Surfaces:  Influence of the Surface Modification on the Glass Transition. Macromolecules 2003, 36, 9100-9106. 20. Teichroeb, J. H.; Forrest, J. A., Direct Imaging of Nanoparticle

Embedding to Probe Viscoelasticity of Polymer Surfaces. Phys. Rev. Lett. 2003, 91, 016104. 21. Popok, V. N., Polymer Films with Ion-Synthesized Cobalt and Iron Nanoparticles: Conductance and Magnetism. Rev. Adv. Mater. Sci. 2014, 36, 1-12. 22. Pehrsson, P. E.; Weber, D. C.; Koons, N.; Campana, J. E.; Rose, S. L., Chemical and Physical Interactions in Covalent Polymers Implanted with Transition Metals. Mater. Res. Soc. Symp. Proc. 1984, 27, 429-434. 23. Popok, V. N.; Hanif, M.; Mackova, A.; Miksova, R., Structure and Plasmonic Properties of Thin Pmma Layers with Ion-Synthesized Ag Nanoparticles. J. Polym. Sci. B: Polym. Phys. 2015, 53, 664-672. 24. Maggioni, G.; Vomiero, A.; Carturan, S.; Scian, C.; Mattei, G.; Bazzan, M.; Fernandez, G. D.; Mazzoldi, P.; Quaranta, A.; Della Mea, G., Structure and Optical Properties of Au-Polyimide Nanocomposite Films Prepared by Ion Implantation. Appl. Phys. Lett. 2004, 85, 5712-5714. 25. Popok, V. N.; Khaibullin, R. I.; Toth, A.; Beshliu, V.; Hnatowicz, V.; Mackova, A., Compositional Alteration of Polyimide under High Fluence Implantation by Co+ and Fe+ Ions. Surf. Sci. 2003, 532, 1034-1039. 26. Khaibullin, R. I.; Popok, V. N.; Bazarov, V. V.; Zheglov, E. P.; Rameev, B. Z.; Okay, C.; Tagirov, L. R.; Aktas, B., Ion Synthesis of Iron Granular Films in Polyimide. Nucl. Instrum. Meth. B 2002, 191, 810-814. 27. Ghisleri, C.; Borghi, F.; Ravagnan, L.; Podesta, A.; Melis, C.; Colombo, L.; Milani, P., Patterning of Gold-Polydimethylsiloxane (Au-Pdms)

13

Nanocomposites by Supersonic Cluster Beam Implantation. J. Phys. D: Appl. Phys. 2014, 47, 015301. 28. Ravagnan, L.; Divitini, G.; Rebasti, S.; Marelli, M.; Piseri, P.; Milani, P., Poly(Methyl Methacrylate)-Palladium Clusters Nanocomposite Formation by Supersonic Cluster Beam Deposition: A Method for Microstructured Metallization of Polymer Surfaces. J. Phys. D: Appl. Phys. 2009, 42, 082002. 29. Popok, V. N.; Barke, I.; Campbell, E. E. B.; Meiwes-Broer, K. H., Cluster-Surface Interaction: From Soft Landing to Implantation. Surf. Sci. Rep. 2011, 66, 347-377. 30. Popok, V. N.; Samela, J.; Nordlund, K.; Popov, V. P., Implantation of Kev-Energy Argon Clusters and Radiation Damage in Diamond. Phys Rev B 2012, 85, 033405. 31. Popok, V. N.; Prasalovich, S. V.; Campbell, E. E. B., Nanohillock Formation by Impact of Small Low-Energy Clusters with Surfaces. Nucl. Instrum. Meth. B 2003, 207, 145-153. 32. Carroll, S. J.; Pratontep, S.; Streun, M.; Palmer, R. E.; Hobday, S.; Smith, R., Pinning of Size-Selected Ag Clusters on Graphite Surfaces. J. Chem. Phys. 2000, 113, 7723-7727. 33. Cardia, R.; Melis, C.; Colombo, L., Neutral-Cluster Implantation in Polymers by Computer Experiments. J. Appl. Phys. 2013, 113, 224307. 34. Hanif, M.; Popok, V. N., Magnetron Sputtering Cluster Apparatus for Formation and Deposition of Size-Selected Metal Nanoparticles. In Physics, Chemistry and Applications of Nanostructures, World Scientific 2015; pp 416-419. 35. Kashtanov, P. V.; Smirnov, B. M.; Hippler, R., Efficiency of Cluster

Generation in a Magnetron Discharge. Europhys. Lett. 2010, 91, 63001. 36. Hartmann, H.; Popok, V. N.; Barke, I.; von Oeynhausen, V.; Meiwes-Broer, K.-H., Design and Capabilities of an Experimental Setup Based on Magnetron Sputtering for Formation and Deposition of Size-Selected Metal Clusters on Ultra-Clean Surfaces. Rev. Sci. Instrum. 2012, 83, 073304. 37. Kreibig, U.; Vollmer, M., Optical Properties of Metal Clusters; Springer: Berlin, 1995. 38. Fojan, P.; Hanif, M.; Bartling, S.; Hartmann, H.; Barke, I.; Popok, V. N., Supported Silver Clusters as Nanoplasmonic Transducers for Protein Sensing. Sensors and Actuators B: Chemical 2015, 212, 377-381. 39. Chau, Y. F.; Chen, M. W.; Yeh, H. H.; Wu, F. L.; Li, H. Y.; Tsai, D. P., Highly Enhanced Surface Plasmon Resonance in a Coupled Silver Nanodumbbell. Appl. Phys. A: Mater. Sci. Process. 2011, 104, 801-805. 40. Jain, P. K.; El-Sayed, M. A., Plasmonic Coupling in Noble Metal Nanostructures. Chem. Phys. Lett. 2010, 487, 153-164. 41. Atay, T.; Song, J.-H.; Nurmikko, A. V., Strongly Interacting Plasmon Nanoparticle Pairs:  From Dipole−Dipole Interaction to Conductively Coupled Regime. Nano Lett. 2004, 4, 1627-1631. 42. Mao, P.; Chen, J.; Xu, R. Q.; Xie, G. Z.; Liu, Y. J.; Gao, G. H.; Wu, S., Self-Assembled Silver Nanoparticles: Correlation between Structural and Surface Plasmon Resonance Properties. Appl. Phys. A: Mater. Sci. Process. 2014, 117, 1067-1073. 43. Gong, Y. C.; Zhou, Y.; He, L. B.; Xie, B.; Song, F. Q.; Han, M.; Wang, G.

14

H., Systemically Tuning the Surface Plasmon Resonance of High-Density Silver Nanoparticle Films. Eur. Phys. J. D 2013, 67, 1-6.

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Appendix D

Structure and Plasmonic Properties of Thin PMMA Layers with

Ion-Synthesized Ag Nanoparticles Vladimir N. Popok, Muhammad Hanif, Anna Mackova, Romana Mikšovå

Structure and Plasmonic Properties of Thin PMMA Layers with

Ion-Synthesized Ag Nanoparticles

Vladimir N. Popok,1 Muhammad Hanif,1 Anna Mackova,2 Romana Mik�sova2

1Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, 9210 Aalborg, Denmark2Nuclear Physics Institute of the Academy of Sciences of the Czech Republic, 250 68 Rez and J.E. Purkinje University, Ceske

mladeze 8, 400 96 Usti nad Labem, Czech Republic

Correspondence to: V. Popok (E-mail: vp@nano.aau.dk)

Received 1 December 2014; revised 13 January 2015; accepted 15 January 2015; published online 3 February 2015

DOI: 10.1002/polb.23682

ABSTRACT: Silver nanoparticles are synthesized in polymethyl-

methacrylate by 30 keV Ag1 ion implantation with high fluen-

ces. The implantation is accompanied by structural and

compositional evolution of the polymer as well as sputtering.

The latter causes towering of the shallow nucleated Ag nano-

particles above the surface. The synthesized nanoparticles can

be split into two groups: (i) located at the surface and (ii) fully

embedded in the shallow layer. These two groups provide cor-

responding spectral bands related to localized surface plasmon

resonance. The bands demonstrate considerable intensity mak-

ing the synthesized composites promising for plasmonic appli-

cations. VC 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B:

Polym. Phys. 2015, 53, 664–672

KEYWORDS: atomic force microscopy (AFM); carbonization of

polymers; ion implantation; localized surface plasmon resonance;

nanocomposites; nanoparticles; optics; sputtering of polymers

INTRODUCTION In recent years, thin metal/polymer nanocom-posite films attract considerable attention due to a numberof practical applications.1,2 In particular, one can control con-ductance by varying metal species and filling factor (metalconcentration) in a polymer.3–6 Applications combining elec-trical and mechanical properties of metal/polymer compo-sites are of significant interest. Good examples are straingauges and elastomer electrodes.7,8 The latter provides newperspectives for the use in medicine and smart prosthetics.Localized surface plasmon resonance (LSPR) and non-linearoptical properties of polymers with gold, silver, and coppernanoparticles (NPs) attract a lot of research attention.9–12

These materials are considered to be promising for nano-scale plasmonics.9,10,13,14

Formation of metal/polymers composites can be performedby a variety of methods including particle formation byevaporation or different sputtering techniques, chemicaland photo reduction, deposition from cluster beams, andsolutions, and so forth.15 One of the widely usedapproaches is NPs formation by ion implantation. The firstexperiments, which were performed in the middle 1980s,proved the concept of ion synthesis.16 During next 3 deca-des the research on ion-implanted polymers revealed anumber of important phenomena which can be advanta-geous or disadvantageous depending on the purpose of

application. For the NPs nucleation the metal concentrationmust overcome the solubility limit in the polymer. Thus,high-fluence implantation is required. High fluences leadnot only to the accumulation of metal and particle forma-tion but also to damage of the substrate material due to theion stopping.17,18 For low-energy (keV-range) implantationthe nuclear stopping of ions dominates, thus, originatingstructural evolution of organic matrix, in particular, ruptureof chemical bond and scission of polymer chains causingdegassing of volatile compounds. As most polymers arecarbon-based, ion implantation leads to carbonization ofthe matrix accompanied by the formation of conjugatedbonds.18–22 This evolution changes electronic propertiesand charge transport mechanisms. The high metal concen-tration and formation of metal NPs further contribute tothe increase of electrical conductance and can causeinsulator-to-metal transition in the implanted poly-mers.3,4,23,24 Thus, by controlling the implantation parame-ters one can tune the conductance to required values butnot only conductance. High-fluence implantation of transi-tion metals like Fe, Co, Ni, and so forth allows producingmetal/polymer composites with superparamagnetic and fer-romagnetic properties, which can be used for developmentof magnetic data storage media and magneto-sensors.25–28

One more direction of practical importance is related tonanoplasmonics. In the case of coinage metals, NPs

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formation under implantation leads to LSPR.11,29,30

However, the accompanying carbonization and conjugatedbond formation dramatically change dielectric properties ofthe medium surrounding metal NPs that often leads to par-tial or complete quench of LSPR.30–32 Nevertheless, it wasrecently shown by example of ion synthesis of copper/poly-methylmethacrylate (PMMA) that both tuning of theimplantation parameters and appropriate choice of polymermaterial give a possibility to synthesize nanocompositeswith considerable plasmonic properties.33 It is necessary tostress that such approach requires deep understanding ofthe polymer structure evolution under the implantation.

In this article, we present new results on the synthesis of sil-ver NPs in PMMA using low-energy ion implantation. It isexperimentally proved that the ion-synthesized shallowlocated particles tower above the polymer surface due to thesputtering accompanying the high-fluence implantation. Toour best knowledge, a detailed study of this process wasnever presented for polymers and in many cases the surfacebumps related to the nucleated NPs were misinterpreted bythe irradiation-induced materials swelling. The synthesizedparticles provide considerably strong LSPR bands in opticalspectra making the composites promising for optical sens-ing34 and other plasmonic applications.

EXPERIMENTAL

One-mm-thick commercial PMMA substrates were implantedby 30 keV Ag1 ions with fluences F5 6.0 3 1014 – 2.0 3

1017 cm22 at ion current density of 2.0 lA cm22. Theimplantation was performed in a residual vacuum of 1025

Torr. To avoid thermal degradation and polycrystallization ofthe polymer, the sample holder was water-cooled and tem-perature of the polymer targets under the implantation didnot exceed 370 K, that was below the glass transition tem-perature (378 K) of PMMA.

Depth distribution of the implanted silver and change ofcomposition of the implanted PMMA were characterizedby Rutherford back-scattering (RBS) spectrometry andelastic recoil detection analysis (ERDA). The RBS spectrawere measured using a beam of 2.0 MeV He1 ionsrecorded at laboratory scattering angle of 170�. The ERDAspectra were obtained using 2.5 MeV He1 ions. Therecoiled hydrogen atoms were registered at scatteringangle of 30�. Typical intensity of the alpha-particles beamwas 20 nA, thus, allowing to minimize the sample radia-tion damage. To further reduce effects of sample degrada-tion, the spectra were measured for different beam spotsand the final spectrum was obtained by summing the indi-vidual ones.

Atomic force microscopy (AFM) study was performed usingNtegra-Aura nanolaboratory (from NT-MDT). The measure-ments were performed in tapping mode using standard sili-con cantilevers with curvature radius of tip smaller than10 nm. Optical transmittance spectra were obtained by dou-

ble beam Perkin Elmer High-Performance Lambda 1050Spectrometer in a standard configuration.

RESULTS AND DISCUSSION

Structural Evolution of PMMA and Particle FormationFor the Ag-implanted PMMA, the hydrogen depth profilesobtained using ERDA are presented in Figure 1. One can seethat the hydrogen concentration reduces from values close to50 at.%, which is typical for bulk pristine PMMA, to valuesbetween approximately 15–25 at.% at the surface confirmingstrong dehydrogenation of the shallow layer. It is also wellknown for PMMA that the projectiles cause scission of the mac-romolecular chains leading to free radical reactions and forma-tion of volatile compounds other than hydrogen, in particular,carbon monoxide gas.21 Degassing of volatile compounds viaion tracks or microcracks leads to the transformation of theimplanted layer into the carbonized structure with chainfragments of residual PMMA. Degassing, carbonization, andaccompanying formation of conjugated bonds were provedmany times for different ion species18,20–22,35–38 and thereforeare not discussed in detail in this article. Possible degradationscheme showing the polymer evolution towards a polycon-densed system is shown in Figure 2.

Depth profiles of implanted silver obtained by RBS and thosecalculated using stopping and ranges of ions in matter(SRIM) code39 and transport of ions including dynamic com-position changes (TRIDYN) code40 are shown in Figure 3.SRIM predicts the depth profile close to Gaussian shape.However, this type of distribution is far from the experimen-tal profiles because the software disregards the change ofthe target composition (the above described carbonization)and filling the implanted layer with impurity atoms whichbecome essential with the fluence increase. Therefore, use of

FIGURE 1 Depth distributions of hydrogen in PMMA implanted

with different fluences (indicated in the panel) obtained from

ERDA spectra. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

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SRIM is not a good choice for high-fluence implantation ofpolymers. TRIDYN allows to take the composition changesinto account and can provide reasonable estimation for thedepth of concentration maximum as can be seen by compari-son of the simulated and experimental curves for F5 1.0 3

1017 cm22 in Figure 3. However, the experimental dependen-ces are broader and have long inward tails. Similar shapes ofthe depth profiles were earlier found for the high-fluenceimplantation of Co, Fe, and Ni in polyimide, polyetherether-ketone, and polyethylene terephthalate. The tails can beexplained by thermal-stimulated diffusion of metals.28,41,42

The sample surface is heated up by the energetic ions thuscausing a temperature gradient between the shallowimplanted layer and bulk. Besides the thermal-stimulated dif-fusion, one more phenomenon affecting the silver depth pro-files should be considered and it is discussed below.

The high-fluence implantation leads not only to the embed-ment of metal into the polymer but also causes sputtering ofsurface. It was shown elsewhere that the implantation oflight ions such as H1, B1, and N1 with energies of 100–300

keV in PMMA causes decrease of the sample thickness for300–600 nm for F5 (1.0–5.0) 3 1016 cm22.43 Sputteringthicknesses of PMMA in secondary ion mass spectroscopyexperiments were found to be of the same order of magni-tude. For example, it was shown in ref. [44] that Cs1 ionswith fluence of about 1.0 3 1016 cm22 can sputter over 1-mm-thick layer of pristine PMMA. Carbonization of PMMAinduced by the nitrogen ion implantation with F5 2.0 3

1016 cm22 reduced the sputtering rate almost twice.However, it was still very significant value.

In this work, thickness of the sputtered layer is studiedusing AFM by measuring the step at the boundary betweenthe implanted surface area and the one covered by a metalclamp (i.e. completely shielded from implantation). It isfound that 30 keV Ag1 ion implantation causes sputtering ofrelatively thick layer. 3D image of the transition between theun-implanted and implanted areas can be seen in Figure 4.The left-bottom part of the image shows the implanted areawith typical granular structure, which will be discussed later.The top-right part corresponds to virgin polymer. Differencein height between these two areas is measured to be about470–500 nm for F5 1.0 3 1017 cm22. Same height of thestep is obtained using Dektak XT stylus profiler andContourGT optical microscope providing vertical resolutionof images. Taking into account that the sputtering is gradualprocess and it occurs during the entire implantation period,it becomes clear that silver atoms shallow-imbedded in thebeginning of the implantation can be sputtered together withthe polymer components at the later stage. Thus, we have asituation with gradual removing of surface material which isan equivalent of moving the surface position. Hence, theexperimentally found near-surface location of maxima of sil-ver concentration and the profiles broadening (see Fig. 3)are related to gradual sputtering of the target during theimplantation.

The sputtering also affects the resulting metal filling factor.As one can see in Figure 3, the doubling of F from 7.5 3

1016 to 1.5 3 1017 cm22 does not lead to any significant

FIGURE 3 Experimentally obtained using RBS and simulated

using SRIM and TRIDYN codes depth distributions of silver in

PMMA implanted with different fluences indicated in the panel.

[Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]

FIGURE 4 AFM height image of transition area between un-

implanted top-right and implanted (with fluence of 1.0 3 1017

cm22) bottom-left parts of PMMA.

FIGURE 2 Schematic representation of composition and struc-

tural evolution of PMMA under ion implantation.

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increase of silver concentration in the implanted layer. RBSdata allow recalculation of the measured Ag concentrationsto so-called equivalent fluences (the fluences that would cor-respond to the measured concentrations) which are pre-sented in Table 1. The equivalent fluences are found tosaturate at the level of (1.3–1.4) 3 1016 cm22. In otherwords, the increase of fluence above this value does not leadto a rise of metal concentration in the implanted layerbecause the accompanying sputtering removes the shallowembedded impurity.

AFM images of the pristine and implanted PMMA surfacesare shown in Figures 5–7. Typical roughness of un-implanted polymer samples is found to be around 1–2 nmfor 2 3 2 mm2 areas. Sometimes it is possible to seetrenches or some other surface imperfections, for examplerelated to large linear bundles of macromolecular chains, asshown for example in Figure 5. They locally increase theroughness. The root mean square value (RMS) for this imageis found to be 3.5 nm. Implantation with fluences of up toabout 5.0 3 1015 cm22 does not cause any considerablechange of surface topography. Therefore, AFM images corre-sponding to these fluencies are not presented. At fluence of7.5 3 1015 cm22 one can observe appearance of individualhemispherical bumps [see Fig. 6(a)] which provide strongcontrast in the phase image [see Fig. 6(b)] indicating signifi-cant difference in materials properties of the bumps com-pared to the rest of surface. Increase of fluence to 1.0 3

1016 cm22 leads to growth of the bumps and they cover theentire surface [Fig. 7(a)]. Implantations with very high fluen-ces of 7.5 3 101621.5 3 1017 cm22 revile very similar sur-face topography [see examples in Fig. 7(b,c)]. Phase images(not shown) provide contrast of the bumps similar to thecase depictured in Figure 6(b). Heights of bumps can befound in corresponding histograms from which it is seenthat the mean value is around 3 nm for fluence of 7.5 3

1015 cm22. However, it is increases and stabilizes at the levelof 14–15 nm for the higher fluences.

It is well known that metal implantation with high fluencesinto dielectrics leads to nucleation of NPs. In the case of lowimplantation energies, the particles are formed in the shal-low layer and due to the accompanying sputtering they starttowering above the surface. This effect was demonstrated fora number of different combinations of metal species anddielectric materials.35,45–48 However, it has not been studiedin detail for polymers. Towering should be especially pro-nounced for organic substrates because sputtering rates for

light chemical elements composing polymers are two tothree times higher compared to the incorporated metalatoms.39 Additionally to sputtering, compaction of theradiation-damaged layer due to the degassing and carboniza-tion effects can be one more mechanism promoting thetowering.49

Therefore, it can be concluded that the hemispherical bumpsobserved by AFM are the nucleated silver NPs which are jut-ted out. For F5 7.5 3 1015 cm22 the bumps are small, meanheight hmean 5 3.1 nm. The presence of a long tail in theheight distribution [see Fig. 6(c)] indicates that hmean corre-sponds to the case when the very top parts of NPs areobserved while there are a number of particles towering formore than 3 nm. At higher fluences, the sputtering uncoversa larger part (surface fraction) of every nanoparticle.Therefore, there is an increase in the height of bumps asshown in the histograms of Figure 7. One can also see thatthe height distributions follow the Gaussian fits well.

As stated above in the part discussing RBS data, the fluenceincrease above 1.4 3 1016 cm22 does not lead to rise of themetal filling factor due to the sputtering of the shallowimplanted silver. Thus, it can be assumed that conditions forparticle growth are very similar for different fluence above1.4 3 1016 cm22. This suggestion is in good agreement withthe images and histograms presented in Figure 7 showingvery similar in height NPs. Unfortunately, using AFM it isimpossible to find the fraction of each particle that towersabove the surface, whether it is 1/3, 1/2, or higher. However,some estimates will be given below when discussing opticalproperties of the nanocomposites.

Optical Properties of CompositesOptical transmittance spectra of the pristine PMMA andselected implanted samples are shown in Figure 8.Implantation with low fluences (see as example the spec-trum corresponding to 2 3 1015 cm22) decreases transmit-tance especially in the UV-blue spectral range and shifts the

TABLE 1 Implantation Fluences and Equivalent Ones

Recalculated from the Measured Silver Concentration in PMMA

Implantation

Fluence (cm22)

Calculated

Fluence (cm22)

7.50 3 1016 1.29 3 1016

1.00 3 1017 1.39 3 1016

1.25 3 1017 1.32 3 1016

1.50 3 1017 1.34 3 1016

FIGURE 5 AFM height image of pristine PMMA surface. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

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absorption band edge to higher wavelengths. Same tenden-cies were observed for the implantation of different speciesinto PMMA, for instance Si,50 N, and Ar,36 and into otherpolymers: polyethylene, polyamide, polystyrene, polypropyl-ene, CR-39, and so forth.51–54 These spectral changes are ori-ginated by the polymer structural evolution which isdescribed above. Major contribution is given by the carbon-ization. With fluence increase, conjugated systems extend inthe growing carbonized regions thus causing decrease of theoptical band gap.18,36,55 The carbonization and formation ofconjugated bonds enhance absorption in the visible spectralinterval. Visually, the color of the samples changes to lightbrown at low fluences and then gradually evolves towardsdark brown at high fluences.

With fluence increase, a weak and broad absorption bandat k � 460 nm appears as can be seen for the sample

implanted with F5 7.5 3 1015 cm22 which corresponds tothe case of observation of slightly towered NPs [Fig. 6(a)].Earlier studies allow to assign this band to LSPR on silverNPs embedded into the radiation-modified PMMA.30 For flu-ences between 1.0 3 1016 and 1.0 3 1017 cm22 the bandintensity increases and one can observe one more band atlower k. Wavelengths of the transmittance minima (bandextremuma) are presented in Table 2. To better visualizeLSPR bands and eliminate contribution of carbonization,some of the spectra presented in Figure 8 are convertedinto absorption and spectrum 1, corresponding to signifi-cant carbonization, is subtracted from the spectra 2–5. Newgraphs with well pronounced absorption bands are shownin Figure 9.

Appearance of the LSPR band for F5 7.5 3 1015 cm22 indi-cates that concentration of implanted silver becomes high

FIGURE 6 AFM (a) height and (b) phase images of PMMA implanted with fluence of 7.5 3 1015 cm22. Panel (c) shows histogram

of height distribution for bumps presented in panel (a). Solid line represents Gaussian fit. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

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enough for the nucleation of NPs. Enhancement of the bandintensity for higher fluences is related to further NPs growthand increase of particle density in the implanted layer. Asfound by AFM (Fig. 7), there is towering of very shallowlocated NPs for high fluences. Therefore, one can condition-ally separate all NPs into two groups: located on the surface

and those fully embedded into the substrate. Particles ofthese two groups are exposed to different dielectric environ-ments. The embedded NPs are fully surrounded by the car-bonized PMMA while the surface ones are only partlyincorporated in the material; significant (may be even major)fraction of particle surface is exposed to air.

This difference in surroundings can be taken into account inthe simulations of NPs optical extinction by introducing aneffective medium with varying dielectric constant emed count-ing different contributions of dielectric constants of amor-phous carbon (a-carbon) eC, PMMA epol, and air eair. Dielectricconstants of PMMA and air are not wavelength-dependentfor the interval of interest and equal to 2.6 and 1.0, respec-tively.56 Dielectric constant of a-carbon is known to varywith wavelength in the UV–visible interval.57 For calculationsof optical extinction efficiency of a silver particle the follow-ing equation is used58,59

FIGURE 7 AFM height images and histograms of height distribution for PMMA implanted with fluences (a) and (d) 1.0 3 1016

cm22, (b) and (e) 7.5 3 1016 cm22, (c) and (f) 1.0 3 1017 cm22. Solid lines in histograms represent Gaussian fit. [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 8 Optical transmittance spectra of PMMA implanted

with different fluences indicated in the panel. The spectrum of

un-implanted (pristine) PMMA is shown for reference. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

TABLE 2 Optical Absorption Bands for PMMA Implanted with

Different Fluences

Fluence (cm22) k1 (nm) k2 (nm)

7.5 3 1015 – 460 6 5

1.0 3 1016 361 6 5 453 6 5

7.5 3 1016 373 6 3 457 6 3

1.0 3 1017 369 6 3 449 6 3

The extremuma are found by calculation of first derivative of the experi-

mentally measured transmittance.

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Qext524pRe3=2med

keim

e2im1ðer12emedÞ2; (1)

where R is the NP radius, er and eim are the real and imagi-nary parts of the silver particle dielectric constant,respectively.60

The spectra calculated for different media surrounding NPsare shown in Figure 10. The calculation of optical extinctionefficiency for NP with diameter d5 20 nm in air gives aband with maximum at k 5 354 nm. If we assume that thepolymer is fully converted into a-carbon, thus, emed 5 eC andthe maximum of NP extinction should be shifted to 516 nm(see Fig. 10). This wavelength is far from both experimen-tally found bands (see Table 2). However, gradual decreaseof a-carbon contribution and increase of PMMA part lead tobackward shift of the extinction peak to lower k and for thecase of emed 5 0.4eC 1 0.6epol the peak is at 454 nm (see Fig.10) which agrees well with k2 in the experiments (see Table2). Thus, it can be suggested that the observed absorptionband at around 450–460 nm is related to NPs fully (oralmost fully) imbedded into the carbonized PMMA.Additional argument supporting this conclusion is that thisis the only band present in the spectrum corresponding toF5 7.5 3 1015 cm22. For this sample, we see only tinybumps on the surface [Fig. 6(a)]. Hence, the absolute major-ity of NPs is embedded in the substrate and there are onlyfew of them slightly towered above the surface. It is worthmentioning that the ratio between a-carbon and PMMA 0.4/0.6 is just the best fit which not necessarily exactly corre-

sponds to the real ratio. However, it indicates considerablecarbonization of the polymer.

The best fit of the extinction efficiency to the experimental bandwith k1 is obtained if we consider emed5 0.80eair1

0.06eC1 0.14epol (see Fig. 10, corresponding maximum at k 5

370 nm). This case would correspond to a particle with a majorfraction located above the surface, and only a small part is embed-ded into the carbonized polymer. Thus, it can be suggested thatthe band at low k in the experimental spectra is related to LSPRof the surface-located NPs.

It is worth noting that we also performed calculations forsmaller (down to 10 nm) and large (up to 30 nm) in diameterparticles and different effective media. Despite the choice of aparticle size of 20 nm is somehow arbitrary it gives one of thebest fits to the experimental spectra. If one assumes that thisdiameter is close to the mean size of real nucleated NPs agood agreement of the simulations with the AFM measure-ments can also be seen. The found heights of about 14–15 nm(see histograms in Fig. 7) would correspond to the particlewith diameter of about 20 nm towered above the surface levelfor about 3/4 of its diameter, thus, mostly exposed to air. Ofcourse, the relatively high deviations in real particle sizes,which are caused by statistical processes under the implanta-tion and nucleation, should be taken into account and, there-fore, we cannot make a conclusion on the exact agreementbetween the simulated and real sizes. Nevertheless, the generalarguments allowing to separate the particles into two groupsexposed to different environments must be valid.

CONCLUSIONS

Silver nanoparticles are synthesized in shallow PMMA layersusing high-fluence ion implantation with energy of 30 keV.

FIGURE 9 Normalized optical absorption spectra of PMMA

implanted with different fluences indicated in the panel. The

spectra are obtained by conversion of transmittance into

absorption and subtraction of spectrum corresponding to flu-

ence of 2.0 3 1015 cm22 from other spectra to minimize contri-

bution of carbonization into optical absorption and enhance

LSPR bands. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

FIGURE 10 Calculated Qext of Ag NP with d 5 20 nm in differ-

ent environments: air, PMMA, a-carbon, mixture of 0.80air-

1 0.06a-C 1 0.14PMMA and mixture of 0.4a-C 1 0.6PMMA.

[Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]

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The maximum silver concentration is measured by RBS to beat the depth of about 10–20 nm depending on the fluence.These depths agree well with the simulations using TRIDYN.It is experimentally found that the implantation causes sig-nificant dehydrogenation of the near-surface layer leading tocarbonization. High-fluence implantation also causes sputter-ing of the surface polymer layer which can reach 500 nm inthickness for fluence of 1.0 3 1017 cm22. The implanted sil-ver also becomes partly sputtered. Thus, the competitionbetween the metal embedding and sputtering at high-fluenceimplantation leads to the limit of the possible metal fillingfactor in the near-surface layer. In other words, an increaseof implantation fluence above approximately 1.4 3 1016

cm22 is found to be pointless for any further rise of silverconcentration in the shallow layer of PMMA.

The sputtering and possibly carbonization-related compac-tion originate towering of the shallow nucleated NPs abovethe surface which is proved by AFM. Transmittance opticalspectra of the synthesized silver/PMMA nanocompositesrevel presence of two absorption bands assigned to localizedsurface plasmon resonance of NPs. The calculations of opti-cal extinction efficiency of Ag NPs suggest that the bands arerelated to two groups of particles: (i) located at the surfaceof the radiation-modified PMMA and (ii) fully embedded intothe carbonized polymer. Thus, low-energy ion implantationmethod can be used not only for the nucleation of NPs fullyembedded into material but also for the formation ofsurface-located particles. The choice of polymer with rela-tively low carbon content (which is PMMA in our case) helpsto reduce the implantation-induced carbonization, thus pro-moting relatively intense LSRP absorption bands and demon-strating good potential capability of the synthesizedcomposites to be used for plasmonic application. For exam-ple, the surface located particles make these polymer nano-composites attractive for utilization as transducers foroptical sensing of biomolecules.

ACKNOWLEDGMENTS

RBS and ERDA measurements have been performed at theCenter of Accelerators and Nuclear Analytical Methods(CANAM) infrastructure by A.M. and R.M. who acknowledge thesupport by project P108/12/G108. We are grateful to A.L.Stepanov from Kazan Physical-Technical Institute in Russia forproviding the implanted PMMA.

REFERENCES AND NOTES

1 F. Faupel, V. Zaporojtchenko, T. Strunskus, M. Elbahri, Adv.

Eng. Mater. 2010, 12, 1177–1190.

2 K. I. Winey, R. A. Vaia, Mater. Res. Soc. Bull. 2007, 32, 314–

319.

3 M. C. Salvadori, M. Cattani, F. S. Teixeira, I. G. Brown, Appl.

Phys. Lett. 2008, 93, 073102.

4 G. Du, A. Burns, V. N. Prigodin, C. S. Wang, J. Joo, A. J.

Epstein, Phys. Rev. B 2000, 61, 10142–10148.

5 Y. Wang, L. B. Bridwell, R. E. Giedd, J. Appl. Phys. 1993, 73,

474–476.

6 F. S. Teixeira, M. C. Salvadori, M. Cattani, I. G. Brown, J.

Appl. Phys. 2009, 105, 064313.

7 G. Di Girolamo, M. Massaro, E. Piscopiello, L. Tapfer, Nucl.

Instrum. Methods Phys. Res. B 2010, 268, 2878–2882.

8 G. Corbelli, C. Ghisleri, M. Marelli, P. Milani, L. Ravagnan,

Adv. Mater. 2011, 23, 4504–4508.

9 C. Buhal, S. P. Withrow, C. W. White, D. B. Poker, Annu. Rev.

Mater. Sci. 1994, 24, 125–158.

10 E. Stratakis, E. Kymakis, Mater. Today 2013, 16, 133–146.

11 A. L. Stepanov, In Metal-Polymer Nanocomposites; L.

Nicolais, G. Garotenuto, Eds.; Wiley: Hoboken, 2005; pp 241–

263.

12 A. Heilmann, U. Kreibig, Eur. Phys. J. Appl. Phys. 2000, 10,

193–202.

13 S. A. Maier, P. G. Kik, L. A. Sweatlock, H. A. Atwater, Mater.

Res. Soc. Symp. Proc. 2003, 777, T7.1.1–T7.1.12.

14 K. A. Willets, R. P. Van Duyne, Annu. Rev. Phys. Chem.

2007, 58, 267–297.

15 G. Compagnini, Appl. Surf. Sci. 2004, 226, 216–225.

16 P. Pehrsson, D. C. Weber, N. Koons, J. E. Campana, S. L.

Rose, Mater. Res. Soc. Symp. Proc. 1984, 27, 429–434.

17 T. Venkatesan, L. Calcagno, B. S. Elman, G. Foti, In Ion

Beam Modification of Insulators; P. Mazzoldi, G. W. Arnold,

Eds.; Elsevier: Amsterdam, 1987; pp 301–379.

18 V. N. Popok, In Surface Science Research; Ch.P. Norris, Ed.;

Nova Sci. Publ.: New York, 2005; pp 147–193.

19 G. Marletta, F. Iacona, In Materials and Processes for

Surface and Interface Engineering; Y. Pauleau, Ed.; Kluwer:

Dordrecht, 1995; pp 597–640.

20 V. N. Popok, I. I. Azarko, R. I. Khaibullin, A. L. Stepanov, V.

Hnatowicz, A. Mackova, S. V. Prasalovich, Appl. Phys. A 2004,

78, 1067–1072.

21 A. Kondyurin, M. Bilek, Ion Beam Treatment of Polymers;

Elsevier: Amsterdam, 2008.

22 V. N. Popok, I. I. Azarko, V. B. Odzhaev, A. Toth, R. I.

Khaibullin, Nucl. Instrum. Methods Phys. Res. B 2001, 178,

305–310.

23 P. A. Lee, T. V. Ramakrishan, Rev. Mod. Phys. 1985, 57, 287–

337.

24 V. N. Popok, Rev. Adv. Mater. Sci. 2014, 36, 1–12.

25 V. Y. Petukhov, N. R. Khabibullina, M. I. Ibragimova, A. A.

Bukharaev, D. A. Biziaev, E. P. Zheglov, G. G. Gumarov, R.

Muller, Appl. Magn. Reson. 2007, 32, 345–361.

26 R. I. Khaibullin, V. N. Popok, V. V. Bazarov, E. P. Zheglov, B.

Z. Rameev, C. Okay, L. R. Tagirov, B. Aktas, Nucl. Instrum.

Methods Phys. Res. B 2002, 191, 810–814.

27 R. I. Khaibullin, B. Z. Rameev, C. Okay, A. L. Stepanov, V. A.

Zhikharev, I. B. Khaibullin, L. R. Tagirov, B. Aktas, In

Nanostructured Magnetic Materials and Their Applications; B.

Aktas, L. Tagirov, F. Mikailov, Eds.; Kluwer: Dordrecht, 2004; pp

33–64.

28 A. Mackova, P. Malinsky, R. Miksova, H. Pupikova, R. I.

Khaibullin, V. F. Valeev, V. Svorcik, P. Slepicka, Nucl. Instrum.

Methods Phys. Res. B 2013, 307, 598–602.

29 H. Boldyryeva, N. Umeda, O. A. Plaksin, Y. Takeda, N.

Kishimoto, Surf. Coat. Technol. 2005, 196, 373–377.

30 A. L. Stepanov, V. N. Popok, I. B. Khaibullin, U. Kreibig,

Nucl. Instrum. Methods Phys. Res. B 2002, 191, 473–477.

31 G. Maggioni, A. Vomiero, S. Carturan, C. Scian, G. Mattei,

M. Bazzan, C. de Julian Fernandez, P. Mazzoldi, A. Quaranta, G.

Della Mea, Appl. Phys. Lett. 2004, 85, 5712–5714.

JOURNAL OFPOLYMER SCIENCE WWW.POLYMERPHYSICS.ORG FULL PAPER

WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2015, 53, 664–672 671

32 A. L. Stepanov, R. Kiyan, A. Ovsianikov, V. I. Nuzhdin, V. F.

Valeev, Y. N. Osin, B. N. Chichkov, Appl. Phys. A 2012, 108,

375–378.

33 V. N. Popok, V. I. Nuzhdin, V. F. Valeev, A. L. Stepanov, J.

Mater. Res. 2015, 30, 86–92.

34 C. de Juli�an Fern�andez, M. G. Manera, J. Spadavecchia, G.

Maggionia, A. Quarantac, G. Matteia, M. Bazzan, E. Cattaruzza,

M. Bonafinia, E. Negroa, A. Vomieroa, S. Carturana, C. Sciana,

G. Della Meac, R. Rella, L. Vasanelli, P. Mazzoldi, Sensors

Actuators B 2005, 111–112, 225–229.

35 V. N. Popok, Rev. Adv. Mater. Sci. 2012, 30, 1–26.

36 R. Gupta, V. Kumar, P. K. Goyal, S. Kumar, Appl. Surf. Sci.

2012, 263, 334–338.

37 A. Kondyurin, M. Bilek, Nucl. Instrum. Methods Phys. Res. B

2011, 269, 1361–1369.

38 V. Hnatowicz, J. Vacik, V. Svorcik, V. Rybka, V. Popok, O.

Jankovskij, D. Fink, R. Klett, Nucl. Instrum. Methods Phys. Res.

B 1996, 114, 81–87.

39 J. F. Ziegler, J. P. Biersack, M. D. Littmark, The Stopping

and Ranges of Ions in Matter; Lulu Press: Morrisville, 2008.

40 W. Moller, W. Eckstein, J. P. Biersack, Comput. Phys.

Commun. 1988, 51, 355–368.

41 V. N. Popok, R. I. Khaibullin, V. V. Bazarov, V. F. Valeev, V.

Hnatowicz, A. Mackova, V. B. Odzhaev, Nucl. Instrum. Methods

Phys. Res. B 2002, 191, 695–699.

42 A. Mackova, J. Bocan, R. I. Khaibullin, V. F. Valeev, P.

Slepicka, P. Sajdl, V. Svorcik, Nucl. Instrum. Methods Phys.

Res. B 2009, 267, 1549–1552.

43 F. F. Komarov, A. V. Leontyev, V. V. Grigoryev, M. A.

Kamishan, Nucl. Instrum. Methods Phys. Res. B 2002, 191,

728–732.

44 R. Kallweit, M. Baur, P. Eichinger, H. Strack, Nucl. Instrum.

Methods Phys. Res. B 1991, 59/60, 1288–1291.

45 A. L. Stepanov, V. N. Popok, Surf. Coat. Technol. 2004, 185,

30–37.

46 A. L. Stepanov, V. N. Popok, D. E. Hole, I. B. Khaibullin,

Appl. Phys. A 2002, 74, 441–446.

47 C. Okay, B. Rameev, R. I. Khaibullin, M. Okutan, F. Yildiz, V.

N. Popok, B. Aktas, Phys. Stat. Sol. (a) 2006, 203, 1525–1532.

48 V. N. Popok, A. V. Gromov, V. I. Nuzhdin, A. L. Stepanov, J.

Non Cryst. Solids 2010, 356, 1258–1261.

49 D. M. R€uck, J. Schulz, N. Deusch, Nucl. Instrum. Methods

Phys. Res. B 1997, 131, 149–158.

50 G. B. Hadjichristov, V. Ivanov, E. Faulques, Appl. Surf. Sci.

2008, 254, 4820–4827.

51 I. Kozlov, V. Odzhaev, I. Karpovich, V. Popok, D. Sviridov, J.

Appl. Spectrosc. 1998, 65, 390–394.

52 A. Kondyurin, R. Khaybullin, N. Gavrilov, V. Popok, Vacuum

2002, 68, 341–347.

53 L. Calcagno, G. Foti, Nucl. Instrum. Methods Phys. Res. B

1991, 59/60, 1153–1158.

54 N. Shekhawat, S. Aggarwal, A. Sharma, S. K. Sharma, S. K.

Deshpande, K. G. M. Nair, J. Appl. Phys. 2011, 109, 083513.

55 D. V. Sviridov, Russ. Chem. Rev. 2002, 71, 315–327.

56 Handbook of Chemistry and Physics; W. M. Haynes, Ed.;

CRC Press: Boca Raton, 2013.

57 M. Gioti, S. Logothetidis, Diam. Relat. Mater. 2003, 12, 957–

962.

58 Ch. Voisin, N. Del Fatti, D. Christofilos, F. Vallee, J. Phys.

Chem. B 2001, 105, 2264–2280.

59 K. L. Kelly, E. Coronado, L. L. Zhao, G. C. Schatz, J. Phys.

Chem. B 2003, 107, 668–677.

60 P. B. Johson, R. W. Christy, Phys. Rev. B 1972, 6, 4370–4379.

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POLYMER SCIENCE

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141

Appendix E

Supported Silver Clusters as Nanoplasmonic Transducers for

Protein Sensing P. Fojan, M. Hanif, S. Bartling, H. Hartmann, I. Barke, V.N. Popok

Supfor

P. Foa Depab Instit

a r t

ArticleReceivReceivAcceptAvaila

KeywoClusteMetalLocalisProteinTransd

1. In

Dereseacal chpresesensiventiadvansorsa plaprocegreator to

AmimpotrolsA bioto bea signof vaface

∗ CoE-

http://0925-4

Sensors and Actuators B 212 (2015) 377–381

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

ported silver clusters as nanoplasmonic transducersprotein sensing

jana, M. Hanifa, S. Bartlingb, H. Hartmannb, I. Barkeb, V.N. Popoka,∗

rtment of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, 9210 Aalborg, Denmarkute of Physics, University of Rostock, Universitätplatz 3, 18051 Rostock, Germany

i c l e i n f o

history:ed 10 October 2014ed in revised form 30 January 2015

a b s t r a c t

Transducers for optical sensing of proteins are prepared using cluster beam deposition on quartz sub-strates. Surface plasmon resonance phenomenon of the supported silver clusters is used for the detection.It is shown that surface immobilisation procedure providing adhesion of the silver clusters to quartz and

y coupand thcluste

tion. Ars has

applyiinteres

d sincee toothe tanaturers gen sizes, beinbiosenely higr of aslectiven of tare chthroulutiontage o

ed 31 January 2015ble online 16 February 2015

rds:r beam depositionnanoparticlesed surface plasmon resonancesucers for optical sensing

functionalisation of cluster surfaces for antibodprotein detection. Focus was put on these tasksconditions for coupling of the antibodies to thethe plasmon absorption band used for the detecthat immobilisation of antibodies on silver clustebate and detect an antigen of interest. Hence, byimmobilisation scheme the sensing of protein ofspectroscopy method.

troduction

velopment of nanosensors is a rapidly growing field ofrch. The increasing interest arises from the unique physi-aracteristics and properties on the nanoscale that are not

nt in bulk materials. Therefore, nanodevices are able to delivertivity, which is orders of magnitude higher compared to con-onal sensor technologies, and supply additional performancetages like short response time and portability [1]. Nanosen-

also allow for building integrated systems, thus providingtform for intelligent devices having significant data storing,ssing and analyzing power. Intelligent nanosensors have apotential to become very attractive as autonomous systemsbe spread out in a large number to form networks.

ong nanosensors, biorecognition systems are of significantrtance for environmental, bioprocess and food quality con-

demonstrated anpowerful label-frdetection is thatdetected in theirused as transducthey are similar iand proteins, thustate-of-the-artprovide a relativare still a numbea reliable and setransducer, desigsation chemistry

NPs preparedsalt containing soA major disadvan

as well as for medical and pharmaceutical applications [2,3].sensor is an analytical device that interfaces a biological objectrecognised with a physical or chemical transducer to generateal which is then registered and analysed. There are a number

rious approaches in realisation of detection [4]. Localised sur-plasmon resonance (LSPR) biosensors were among the first

rresponding author. Tel.: +45 99409229; fax: +45 99409235.mail address: vp@nano.aau.dk (V.N. Popok).

and short shelf life timof their sensing properhigh tendencies to aggcontrol surface coveragcluster beams have bee[6,7]. One of the main aters are first formed inof flexibility and precissize. Thereafter, the clustrate with control of s

dx.doi.org/10.1016/j.snb.2015.01.131005/© 2015 Elsevier B.V. All rights reserved.

ling are the key issues for cluster stability ande processes have been optimised. In particular,rs are developed providing an enhancement oftomic force microscopy study allows to suggestbeen achieved, thus giving a possibility to incu-

ng the developed preparation stages and proteint can be assured using a relatively simple optical

© 2015 Elsevier B.V. All rights reserved.

e then they have gradually become a veryl. One of the great advantages of label-freerget molecules are not altered, i.e. they areal forms. Nanoparticles (NPs) are typicallynerating optical signals. At the same timeto some organic molecules such as enzymesg ideal transducers used in detection. Manysors utilising LSPR were demonstrated toh degree of sensitivity [5]. However, therepects to be considered in order to producesensor. Among them formation of a stable

he detection scheme and surface immobili-allenging tasks.gh sol–gel processes starting with differents are the most widely used as transducers.f this approach is the relatively low stabilitye of the particles, leading to a rapid decayties. There is also a poor size selection and

lomeration of NPs. Additionally, it is hard toe by NPs. Alternatively, NPs deposited from

n demonstrated to be an attractive approachdvantages of this technique is that the clus-a gas phase that provides both a high level

ion in the control of cluster composition andsters can be deposited on the required sub-

urface coverage. Deposition which is carried

378 7–381

out iter bof thor nspec

Inof trters

devereasisatiappl

2. E

Sbase[10,1prodstrathighone

Depoalmosetudevibetwtionsize

menselecdesc

Qdepoof thstratthe fQuarquenorgagrouin a

pres(APTfor 3of APshowcharclustgrousion

S1 mM

ctionali

ubseqcted

undd to sn gro

se gro 1-eth-hydrentlyroteinncubations tr the

used wof samy–antickenence oorrespe but

d by

this

ping

ance. wer

P. Fojan et al. / Sensors and Actuators B 212 (2015) 37

n vacuum allows avoiding contamination. Moreover, the clus-eam technique gives a possibility to control the kinetic energye particles thus providing conditions for pinning of clusters

anostructuring of surfaces [8,9], in other words widening thetrum of possible applications.

the current paper, we present first results on the formationansducers for protein sensing using deposition of silver clus-on modified quartz surfaces. The research is focused on thelopment of the surface immobilisation procedure to provide

onable adhesion of the silver clusters to quartz, functional-on of cluster surfaces for protein coupling and testing theicability of the sensing scheme utilising LSPR.

xperimental

ilver clusters were produced using the experimental setupd on magnetron sputtering which is described in detail in1]. A silver target of 99.99% purity was used for the clusteruction. Cluster deposition was carried out on quartz sub-es with dimensions 10 mm × 10 mm at room temperature in

vacuum at a background pressure of ca. 1 × 10−8 mbar. Thus,produces pure supported silver NPs on the quartz surface.sited at low kinetic (so-called thermal) energies NPs preservest spherical shape with a slight tendency to oblate [10]. The

p allows for size selection of clusters with a relative standardation of ∼9–13% for particles of various diameters in the rangeeen 5 and 23 nm [11]. However, to test the principle of detec-

and develop methods for surface immobilisation, the preciseof clusters is considered to be not essential for the first experi-ts. Therefore, silver clusters were deposited without exact sizetion in this work. Mean sizes and the size distribution will beribed below.uartz substrates were modified (functionalised) prior to thesition. A series of earlier experiments led us to the elaboratione methodology to improve the cluster adhesion to the sub-es in relation to stability against dipping in solutions used inollowing steps of transducer formation and protein deposition.tz substrates have been cleaned with ethanol and subse-tly treated for 30 min in an ozone cleaner to remove residual

nic materials and to increase the surface density of hydroxylps. Directly after the ozone treatment the samples were placeddesiccator and evacuated (subjected to low vacuum) in theence of a mixture of toluene/3-aminopropyltrimethoxysilaneMS) at a ratio of 3:1. The gas phase deposition was carried out0 min to cover the surface with approximately one monolayer

Fig. 2. Fun

for 30 min and sresidual not reahave been driedselectively bounprovides reactioTo activate thefreshly preparedhydrochloride/N20 min. Subsequstrates and a psubstrates. The iAfter the incubaproteins used fothey have been

Three series

classical antibodantibody and chof inversed sequmin, then the cinversed schemnot be recogniseproteins used inity of the developractical import

The samples

TMS. A schematic picture of the quartz surface modification isn in Fig. 1. After this surface functionalisation with positively

ged amine groups, the quartz substrates have been used forer deposition as described above. The presence of the amineps was found to be significantly improving the silver NPs adhe-

to the substrate.ubstrates with as-deposited clusters were incubated with a

11-mercaptoundecanoic acid (11-MUA) solution in ethanol

mentioned steps in patomic force microsctroscopy. For AFM stuutilised. The measuremcommercial Si cantile10 nm and a spring

transmission spectra wmance Lambda 1050

Fig. 1. Gas phase deposition of APTMS on quartz (surface functional

sation of silver cluster surfaces using 11-MUA.

uently washed with pure ethanol to remove11-MUA. The 11-MUA modified substrateser a stream of nitrogen. 11-MUA becomesilver NP by the sulphur-containing end andups for coupling of proteins (see Fig. 2).ups the samples were incubated with a

yl-3-[3-dimethylaminopropyl]carbodiimideoxysuccinimide (EDC/NHS) mix ratio 1:1 for

the mix has been removed from the sub- solution has been added on top of thetion period for protein solutions was 30 min.he samples have been thoroughly rinsed. All

experiments are commercially available andithout further purification.ples were prepared: the first one follows

igen scheme (with anti-chicken egg albumin egg albumin as antigen), the second one isf protein deposition (first chicken egg albu-onding antibody) and the third one is also

with lysozyme as the antigen, which shouldthe anti-chicken egg albumin antibody. Thework are chosen only to test the applicabil-detection approach and they are not of high

e characterised after each of the above-

reparation of the transducer system usingopy (AFM) and optical transmission spec-dies, an Ntegra-Aura (NT-MDT) system wasents were performed in tapping mode using

vers with curvature radius of tip better thanconstant of approximately 26 N/m. Optical

ere obtained by a Perkin Elmer High Perfor-spectrometer in the interval of wavelengths

isation).

P. Fojan et al. / Sensors and Actuators B 212 (2015) 377–381 379

Fig. 3.

alised

� = 30of 0.3standdiam

3. Re

Sito haof thon APadhetion oThis atanceseen

sensiA

tionadeposampheigh

Fig. 4antiboantige

tical abantibodd by ch

12 nmter coamplMUA aecreasuppora ofrate the presence of a LSPR absorption band atin Figs. 5–7 (dash-doted curves). These absorptionned from transmission measurements for partic-e spectrum corresponding to the functionalised

Optical transmittance spectra of virgin quartz and one with surface function-by APTMS (silane groups).

0–750 nm with data interval of 1 nm and acquisition time2 s for every measurement. The spectrometer was used in aard configuration providing beam spot size of about 2 mm ineter at the sample location.

sults and discussion

lver clusters deposited on a bare quartz substrate were foundve very low adhesion to the surface leading to removal of mostem when immersing into solutions. The clusters depositedTMS-functionalised substrates show significantly improved

sion. The most probable mechanism is through the forma-

Fig. 5. Normalised opclusters followed by

min antibody followeof the spectra.

be between 8 andvery similar cluscan be seen by exver NPs with 11-see only a small dresistance of the

Optical spectclearly demonst� ≈ 390–400 nm

spectra are obtaiular samples. Th

f polarisation interaction between the amine groups and NPs.dditional APTMS layer on the surface decreases the transmit-

of the substrates uniformly and only by about 0.5% as can bein Fig. 3, i.e. the presence of APTMS does not affect the opticaltivity.typical AFM image for the clusters as-deposited on the func-lised quartz is shown in Fig. 4a. Since the clusters weresited without size-selection, mean sizes slightly vary fromle to sample. These sizes (diameters) were estimated from thet of the particles assuming near-spherical shape and found to

. AFM images of substrates (a) with as-deposited clusters, (b) afterdy–antigen incubation (classical scheme) on clusters and (c) aftern–antibody incubation (inversed scheme) on clusters.

quartz (covered by APeliminate contributionmonic features. One caon the wavelength scThis difference is relatebetween the series of sa

Fig. 6. Normalised optical abclusters followed by antibodmin antibody followed by chfor six times compared to th

sorption spectra of quartz substrates with as-depositedy–antigen incubation scheme (anti-chicken egg albu-icken egg albumin). Straight base line is shown for one

. In the experiments, we use substrates withverages which are below one monolayer ase in Fig. 4a. After the functionalisation of sil-nd deposition of antibodies or antigens we

se in the NPs’ coverage, demonstrating goodrted clusters when treated wet chemically.

the samples with as-deposited clusters

TMS) is subtracted from every spectrum to of the substrate and emphasise the plas-n see that the position of LSPR maximum

ale varies slightly from sample to sample.d to a small variation in mean particle sizesmples. For quantitative comparison of LSPR

sorption spectra of quartz substrates with as-depositedy–antigen incubation scheme (anti-chicken egg albu-icken egg albumin). Albumin concentration is reducede case presented in Fig. 5.

380 P. Fojan et al. / Sensors and Actuators B 212 (2015) 377–381

Fig. 7. Normalised optical absorption spectra of quartz substrates with as-depositedclusters followed by inversed antigen–antibody incubation scheme (chicken eggalbumin followed by anti-chicken egg albumin antibody).

Table 1Wavelength of LSPR maximum �m and band intensity for spectra in Fig. 5. Relativeerror for intensity is calculated using standard deviations for optical measurementsand found to be ±0.05.

Sample �m (nm) Intensity (rel. un.)

As-deposited Ag clusters 390 2.32After antibody deposition 426 2.53After albumin deposition 430 2.77

Table 2Wavelength of LSPR maximum �m and band intensity for spectra in Fig. 6. Relativeerror for intensity is calculated using standard deviations for optical measurementsand found to be ±0.05.

Sample �m (nm) Intensity (rel. un.)

As-deposited Ag clusters 393 2.61After antibody deposition 435 3.04After albumin deposition 439 3.16

Table 3Waveerrorand fo

Sam

As-AfteAfte

intenas a

“redtra.

maxresp500

tionit is

AsignFigs.the fiIn thof th

shift to longer wavelebe clearly seen in Tabltively. The subsequenfurther small red shift

change of the band intmin concentration. Thcase presented in Fig.

One can clearly see a laalbumin concentration

11-MUA moleculesgroup. The other endused to form a covalening chemisorption of

antibody to the NP chaenhancement of the Senhancement may beNP and 11-MUA, howare required. Chicken

chicken egg albumin aantibody changes the

fore, the spectral shiftsee an increase in the

tration dependent, thuthe transducers. It is wspectral changes the dsubstantial effect on thmethods, the definingin this work, results in

An AFM image of a

classical scheme can bslightly decreased comters (see Fig. 4a) but threpresenting NPs are inof the antibodies (ca. 4can suggest that one

NP, then providing a cdency of a single moleclier found for a few difNPs supported on graies located on the indlarger bumps and mor

For the inversed scin Fig. 7), the small siz

of thlustersive a

physin Fig.

s is relrface.

ies aree sam

posite. 7 anLSPR

ns andnmen

furtheelengtthe LSlbumidies are not directly coupled to the nanoparticles

length of LSPR maximum �m and band intensity for spectra in Fig. 7. Relative for intensity is calculated using standard deviations for optical measurement

und to be ±0.05.

ple �m (nm) Intensity (rel. un.)

deposited Ag clusters 398 2.15r albumin deposition 421 1.60r antibody deposition 423 1.71

sities, a base line is introduced for every spectrum. It is definedstraight line tangential to spectral minima in the “blue” and” regions. An example is shown in Fig. 5 for one of the spec-Intensity of plasmon band is measured from this line to theimum and the data are presented in Tables 1–3 for Figs. 5–7,ectively. One can also observe a very weak band at aroundnm which is tentatively attributed to cluster-cluster interac-. However, it is not essential for the current study and, therefore,not discussed further.fter the deposition of proteins the optical spectra changeificantly. The difference between the cases presented in

selective coatingcoupled to the cscenario of masthe clusters andAs one can see itopography. Thiers the entire sualbumin antibodNPs changing ththat with as-depresented in Figintensity of the

the band broadedielectric envirobody moleculesinterval of wavbut intensity of

assumed that asince the antibo

5 and 6 and that in Fig. 7 is in sequence of protein deposition. Inrst two cases, the classical antibody–antigen scheme is used.e third one, the inversed scheme is applied. The attachmente antibodies to Ag NPs causes broadening of the LSPR band,

one cannot see any spthe presence of albumtion of albumin is posproteins.

ngth and increase of the absorption that canes 1 and 2 as well as in Figs. 5 and 6, respec-t deposited of chicken egg albumin leads toof the plasmon band (see Tables 1 and 2). Theensity is found to be dependent on the albu-e albumin/antibody ratio is 2:1 (high) for the5 and 1:3 (low) for the case shown in Fig. 6.rger increase in the band intensity for higher, thus, being indicative of albumin detection.

are attached to silver NPs via the sulfhydryl of the molecule with its carboxyl group ist amide bond to the antibody, thus, provid-

the protein. Strong chemical bonding of thenges the dipole characteristics leading to anPR absorption as seen in Figs. 5 and 6. The

caused by the charge transfer between theever additional studies of this phenomenonegg albumin is smaller compared to the anti-ntibody and its subsequent attachment to theNP–antibody interaction only a little, there-

of the band is small. However, we are able toband intensity which is found to be concen-s, demonstrating the detection of albumin byorth noting that due to the relatively small

efinition of the absorption intensity may havee quantitative evaluation. Among alternative

intensity from the base line, which is chosen most consistent numbers.

sample with antibody–antigen deposited in ae seen in Fig. 4b. The coverage of silver NPs ispared to the sample with as deposited clus-

e height and lateral dimensions of the bumpscreased. Taking into account the lateral sizes

nm) and supported clusters (8–12 nm), oneor two antibodies become attached to eachoupling of albumin molecules. The same ten-ule coupling to an individual cluster was ear-ferent types of proteins immobilised by goldphite [12,13]. In the case of several antibod-ividual NP the image would represent muche significant changes in the topography.heme of protein incubation (the case shownes of albumin proteins (1–2 nm) cause non-

e substrate. It means that albumin is not onlys, but also fills the gap between them. Thislbumin deposition (with immobilisation onsorption on the quartz) is confirmed by AFM.4c, the clusters are hardly recognised in theated to the fact that chicken egg albumin cov-The sequentially incubated anti-chicken egg

consequently situated randomly around theple topography dramatically compared to

d clusters. According to the optical spectrad corresponding parameters in Table 3, theband after albumin deposition is decreased,

experiences a red shift due to the change int for NPs. Sequential deposition of the anti-r reduces the transmittance over the whole

hs (entire spectrum in Fig. 7 is shifted up)PR band is affected very little. It can be still

n molecules are attached to antibodies but

ecific change in the plasmon band related toin. Thus, it can be concluded that no detec-sible for the inversed deposition scheme of

P. Fojan et al. / Sensors and Actuators B 212 (2015) 377–381

Fig. 8.

clusterlowed

Thdepoappliular pTo fuples iwith

the snisedwhicbodyof waresulindiccase

raphyin Fig

4. Co

Trsilversilverdetecsubstoptimformcoup“optito suual Nto attalbumtion sappliticulaprotea simapprois of

curre

References

[1] J.T. Devreese, Importannanotechnology, MRS B

[2] M.A. Arugula, A. Simonlenges and perspectives

[3] B. Van Dorst, J. Mehta,

R. Blust, J. Robbens,

and environmental bio1178–1194.

[4] L. Su, W. Jia, C. Hou, Y. L1799.

inman,

lasmon.seri, H.le scie

rgetic cli. Eng. R

uckovic cobalt2009) 2Prasaloergy clu5–153..N. Popbilities

n and di. Instruopok, Mof size-. Gurin,es, Worouchakof prote.ung, Imol. 25 (

his Ph97. He

is postdgy he mhe joincame abiologigenera

raduateThen, hersity oorg Un

interebeam t

ceived ha Ph.D. sf cataly

btained a Ph.D.

in mole

his Ph.Dsity), fosin, Ma

Rostocctural af-assem

ceived

(BSU) in Between 1999 and 2011 he was affiliated with the Universityeden and the University of Rostock in Germany having dif-

Normalised optical absorption spectra of quartz substrates with as-depositeds followed by inversed antigen–antibody incubation scheme (lysozyme fol-

by anti-chicken egg albumin antibody).

us, our study demonstrates that providing an appropriatesition and immobilisation protocol is very essential for thecation of silver NPs as transducers for optical sensing of partic-roteins, i.e. for providing conditions for a sensitive detection.

rther proof this concept we prepared one more series of sam-n which lysozyme was used instead of albumin and incubatedthe same antibody as before. In other words, we carried outequential deposition of two proteins which cannot be recog-

by each other. The optical spectra are presented in Fig. 8, fromh one can see that the deposition of lysozyme and then anti-

decreases the sample transmittance over the whole intervalvelengths and causes the red shift of the plasmon band. It alsots in slight and unspecific damping of the band intensity, thus,ating no possibility for detection of the antigen similar to theof inversed deposition scheme shown in Fig. 7. Surface topog-

of the sample (not shown) is very similar to that presented. 4c.

nclusion

ansducers for optical sensing of proteins are prepared using cluster beam deposition on quartz substrates. The supported NPs exhibit a specific LSPR band used in the followingtion scheme. The conditions for functionalisation of both therate prior to the deposition and cluster surface after that areised providing considerable NP adhesion to quartz as well as

ation of chemical bonds coupling the antibodies to NPs. Thisling enhances the intensity of the LSPR band that is used as ancal signature” for sensing. Our AFM study of the samples allowsggest that there is immobilisation of an antibody on individ-P. Bonding of the antibody to NP then provides a possibilityach and detect the antigen of interest, which is chicken eggin in the current study. It is proven that appropriate prepara-

tages and immobilisation schemes are the key issues for thecation of silver NPs as transducers for optical sensing of par-r proteins. Thus, by applying the correct protocol the assuredin detection with high sensitivity can be reached while usingple optical spectroscopy method. The developed detection

26 (2011) 1788–[5] A. Abbas, M.J. L

resonance and p(2011) 169–175

[6] K. Wegner, P. Pitool for nanoscaR439–R459.

[7] V.N. Popok, Enelayers, Mater. Sc

[8] V.N. Popok, S. Vping of energeticPhys. Rev. B 80 (

[9] V.N. Popok, S.V.

of small low-enB 207 (2003) 14

[10] H. Hartmann, VDesign and capaing for formatiosurfaces, Rev. Sc

[11] M. Hanif, V.N. Pand deposition

Gaponenko, V.Sof Nanostructur

[12] J.A. Collins, C. Xirimmobilisation

(2004) 197–208[13] R.E. Palmer, C. Le

Trends Biotechn

Biographies

Peter Fojan receivednology, Austria in 19organisms. During hment of Biotechnolomodelling. In 2004,

of AAU where he beare centred around

cells and surfaces in

agents.

Muhammad Hanif gUniversity in 2008.

the Norwegian UnivPh.D. student at AalbHis current researchticles using cluster

biosensors.

Stephan Bartling re2010. He is currently

the characterisation oconditions.

Hannes Hartmann o2011. He is currentlyexcitation processes

cles and molecules.

Ingo Barke obtained

TU Dortmund UniverUniversity of Wiscona senior researcher intems at surfaces, struand excitations in sel

Vladimir N. Popok resian State University

as researcher at BSU.of Gothenburg in Sw

ach may be transferred to other proteins, of which sensinghigher practical importance compared to those used in thent experiments.

ferent positions. Since 2011Denmark. Main fields of resnanostructures and nanocomproperties.

381

ce of nanosensors: Feynman’s vision and the birth ofull. 32 (2007) 718–724.ian, Novel trends in affinity biosensors: current chal-, Meas. Sci. Technol. 25 (2014) 032001.K. Bekaert, E. Rouah-Martin, W. De Coen, P. Dubruel,Recent advances in recognition elements of foodsensors: a review, Biosens. Bioelectron. 26 (2010)

ei, Microbial biosensors: a review, Biosens. Bioelectron.

Q. Cheng, Sensitivity comparison of surface plasmon-waveguide resonance, Sens. Actuators B: Chem. 156

Vahedi Tafreshi, P. Milani, Cluster beam deposition: ance and technology, J. Phys. D: Appl. Phys. 39 (2006)

uster ion beams: modification of surfaces and shallow 72 (2011) 137–157.

, J. Samela, T. Järvi, K. Nordlund, E.E.B. Campbell, Stop- clusters and formation of radiation damage in graphite,05419.

vich, E.E.B. Campbell, Nanohillock formation by impactsters with-surfaces, Nucl. Instrum. Methods Phys. Res.

ok, I. Barke, V. von Oeynhausen, K.-H. Meiwes-Broer,of an experimental setup based on magnetron sputter-eposition of size-selected metal clusters on ultra-cleanm. 83 (2012) 073304.agnetron sputtering cluster apparatus for formation

selected metal nanoparticles, in: V.E. Borisenko, S.V. C.H. Kam (Eds.), Physics, Chemistry and Applicationsld Scientific, Singapore, 2015, in press.i, R.E. Palmer, J.K. Heath, C.H. Jones, Clusters for biology:ins by size-selected metal clusters, Appl. Surf. Sci. 226

mobilisation of proteins by atomic clusters on surfaces,2007) 48–55.

.D. in Biotechnology at the Graz University of Tech- initially worked on industrial genetics of eukaryoticoc time at Aalborg University (AAU) at the Depart-oved into the area of protein physics and moleculared the Department of Physics and Nanotechnologyn Associate Professor in 2009. His research interestscal and small molecules and their interactions withl, for medical, sensor applications and as antibacterial

d in Physics and Mathematics at Bahauddin Zakariyae obtained M.Sc. in Condensed Matter Physics fromf Science and Technology in 2011. Since 2012 he is aiversity under the supervision of Dr. Vladimir Popok.sts are in formation of size-selected metal nanopar-echnique and their application for development of

is Diploma in Physics at the University of Rostock intudent at the same university. His research interests aretically active metal oxide nanoparticles under reactive

his Diploma in Physics at the University of Rostock instudent at the same university. His research deals withcular aggregates and the coupling between nanoparti-

. in Physics in 2004 at the University of Dortmund (nowllowed by several years as postdoctoral fellow at the

dison and at the University of Rostock. Since 2013 he isk. His research interests include low-dimensional sys-nd electronic properties of clusters and nanoparticles,bled molecular aggregates and hybrid nanostructures.

his Master in Physics and Ph.D. degrees from the Belaru- Minsk in 1990 and 1995, respectively. Then he worked

he is an Associate Professor at Aalborg University inearch interest are related to formation and study ofposite materials with focus on electronic and optical