Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali

Corso CFMA. LS-SIMat 1

UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA

Chimica Fisica dei Materiali AvanzatiChimica Fisica dei Materiali Avanzati

Part 6.a – Size effects and applications of metal and Part 6.a – Size effects and applications of metal and semiconductor nanoparticlessemiconductor nanoparticles

Laurea specialistica in Scienza e Ingegneria dei MaterialiCurriculum Scienza dei Materiali



Technological Interest in Metal Technological Interest in Metal NanocrystalsNanocrystals

Novel optical effects Nanoelectronics Biolabelling Single electron devices (capacitors, memory

storage) Polarizers Shape control SERS - molecular detection Catalysis Plasmonics and Optical Chips



HRTEM Ag NanoparticlesHRTEM Ag Nanoparticles

“Monodisperse” 6nm diameter, crystalline silver nanoparticles in water produced by radiolytic reduction. Solution is bright yellow!

Inset: Shows metal atoms with lattice spacingof 2.36Å - same as bulk.



PlasmonsPlasmons



Metal nanostructures & surface Metal nanostructures & surface plasmonsplasmons

Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal’s surface, the properties of surface plasmons—in particular their interaction with light—can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.

WL Barnes, A Dereux and TW Ebbesen, Nature 424, 824 (2003)



Plasma EquationsPlasma Equations

ep

p

ep

p

ee

ti

e

m

Ne

m

Ne

m

eExeExm

e

eEdt

xdm

)(,1)()(

response, dielectric on tocontributi ionic theAdding

.,1)(

.

~ x E, response, harmonic For time

used. becan loss model to termDamping .

0

22

2

2

0

22

2

2

22

2

2

ωω



Dielectric ConstantDielectric Constant

Plasma Dielectric Function

- 2

- 1

0

1

0 0.5 1 1.5 2

Normalized Frequency

No

rma

lize

d E

psi

lon

Reflection

Propagation



Volume EM Waves in MetalsVolume EM Waves in Metals

A “bandgap” exists for propagation due to the negative dielectric constant

2222

2

2

2

2

22

2

22

)(1)(

~

.0

pp

zi

ccc

eE

c

EE

lly.exponentia decays hence imaginary,purely is ,At Ep



Negative Dielectric ConstantE & H out of phase, imaginary

Dispersion of EM in PlasmaDispersion of EM in Plasma

1

2

2

p

c



Surface PlasmonSurface Plasmon Coherent excitation of plasma near the surface of a metal –

coupled to surface EM wave

These are essentially light waves that are trapped on the surface because of their interaction with the free electrons of the conductor (i.e., guided waves)

SPs help us to concentrate and channel light using subwavelength structures.

z

x

propagation

MetalMetal



Dispersion CurvesDispersion Curves Propagation curve in 1 (dielectric medium) does not

cross the surface plasmon dispersion curve

21

21

c

1c

c



SPR excitation techniquesSPR excitation techniques



SPP at planar metal surfacesSPP at planar metal surfaces

Plasmon-polariton excitation produces absorbance peak at specific frequency

Shift in the absorbance spectrum indicates presence of analyte molecules (change in dielectric refractive index)



SPR ApplicationsSPR Applications

Chemical

1. Sensing

2. Reaction kinetics

3. Concentration measurement

4. Mass Spectrometry

5. Equilibrium properties

… and many more

Biological

1. Real time sensing

2. Detection of binding reactions

3. Proteomics

4. Plasma membrane studies

5. Drug delivery techniques

… and many more



SPR-based commercial productsSPR-based commercial productsSPR biosensor Spreeta from Nomadics-Texas Instruments

Applications Beverages Medical diagnostics Food safety Security Water quality Research



Localized Plasmon Resonance @ NPsLocalized Plasmon Resonance @ NPs

Coherent excitation of conduction electrons driven by the E radiation field.

Theoretically modeled by Mie Theory



Optical PropertiesOptical Properties

1 i2 1 i2

Mie Theory(1908)

1 p

2

2 i 1

p2

2 i

Empirically

r 0 a

r r 0

a

r

abs 9V0m3 2

c

2

1 2m 2 22

abs 9V0m3 2

c

2

1 2m 2 22

Drude free electron model

J.H. Hodak, et al. ; J. Phys Chem. B, 104(43), 9954, 2000.

Surface Plasmon Resonance is invariant with respect to the size on the nanoparticle.

The FWHM scales with the radius of the particles. Assumes spherical particle Particle diameter << /10



Different Plasmon ModesDifferent Plasmon Modes

Bulk Plasmons:

Surface Plasmons on Flat Surface:

Surface Plasmons on a Small Sphere:

Surface Plasmons on a Small Ellipsoid:

Intrinsically sensitive to surface perturbations.

“Small” means < 30nm.

L depends on aspect ratio.

0

m

m 2

mL



Dielectric confinement and local field Dielectric confinement and local field amplificationamplification

The value of the local field near the metal nanocrystal is

A huge amplification of the local field occurs near the SP resonance at the pole

EEm

l 2

3

02Re m



Surface Enhanced Raman ScatteringSurface Enhanced Raman Scattering

Surface Enhanced Raman

Scattering (SERS)

RadsSLLSSERS AAINI 22

Normal Raman Scattering

RfreeLSNRS INI



Overview of Novel Effects in Metal Overview of Novel Effects in Metal NanocrystalsNanocrystals

Surface plasmon changes due to electron density.

Surface plasmon changes due to shells or adsorbates e.g. biomolecules.

Surface plasmon changes with particle size.

Surface plasmon changes with particle shape.

Single Electron Effects: Coulomb Blockade.



Surface Plasmon SpectroscopySurface Plasmon Spectroscopy

0

0.5

1

1.5

2

-0.12

-0.08

-0.04

0

0.04

300 350 400 450 500

Abso

rbance

Wavelength [nm]

(c)

(b)

(a)

(a) Absorption spectrum of 6nm dia. Silver particles (0.1mM)(b) Predicted difference spectrum following injection of 5µM electrons(c) Experimental difference

spectrum using pulse radiolysis(N2O, 2-propanol)

Blue shift occurs due toIncreased electron concentration.



Shape Effects: Metal Nanorods Shape Effects: Metal Nanorods

200 300 400 500 600 7000.00

0.04

0.08

0.12

Wavelength / nm

C ext

(nm

-2 )

3.53.0

2.52.0

1.5

1 2 3 4

400

500

600

700

Aspect Ratio

Peak P

osi

tion

(n

m)

Longitudinal

Transverse

b = 10 nm

a

b

(Left) Nanorods absorb two colours. The colour depends on the length of the rod. For silver, almost all colours of the rainbow are predicted to be possible.

Gans predicted this effect in 1911.

First proper gold rods made in 1994 to test this.



Shape Control - Polarizers, Liquid Shape Control - Polarizers, Liquid Crystals, NanomechanicsCrystals, Nanomechanics

Aims: Use shape control to create new materials with unusual optical properties

Color of Gold Nanorods depends on aspect ratioand orientation!



Primitive Alignment of Ag RodsPrimitive Alignment of Ag Rods

Focus a laser onto the silver rods and they melt back into yellow nanospheres

Stretch a polymer film with silver rods (10nm x 40nm) and hold it under a polarizer



Nanoshells for expanding SPR’sNanoshells for expanding SPR’s

More sensitive than simple nanoparticles. (gain ~r2/r1)

Resonance frequency is a strong function of geometry

More resonant frequencies possible

Optical resonances of gold-silica core nanoshells as a function of their core/shell ratio.



Nanoshells in bio-applicationsNanoshells in bio-applications

Properties:

Optical activity in bio-compatible wavelengths.

Strong tunable absorption in NIR region (700-1300 nm) (maximum light penetration through tissue in NIR)

Easy conjugation with specific proteins

Chemical / Photochemical stability

Biocompatible, non-toxic to tissue (gold)



NIR photothermal tumor therapyNIR photothermal tumor therapy



Single Electron Devices from NCsSingle Electron Devices from NCs

The capacitance of a small particle isC = Q/V = e/4πeoa in vacuum. To add an electron to the particle costs an energy, U = Q2/2C. If U >>kT, then the usual I-V curve will show jumps. If these jumps can be distinguished then the presence of single electrons can be confirmed, and a storage or logic device created. Low temps are usually needed.

An STM can be used to study the flow of electrons through a singleGold nanoparticle.



Single Electron Devices from NCsSingle Electron Devices from NCs



Metal-ceramic nanocomposite materialsMetal-ceramic nanocomposite materials For inductive components in high-frequency electronic devices

Chemical synthesis of Ni-Fe/SiO2, Co/SiO2, Fe-Co/SiO2, Fe/nickel-ferrite, Ni-Zn-ferrite/SiO2, Fe-Ni/ polymer, and Co/polymer composite magnetic materials

Exchange coupling between nanoparticles large magnetic permeability

Cancellation of magnetic anisotropy

Small parasite currents

Inframat Corporation, Farmington, CT



Nanocrystals are of tremendous interest because they

(i) Have size dependent optical, electronic, catalytic and magnetic properties.

(ii) Quantum size effects provide direct insight into the validity of current models of bonding and structure.

(iii) Numerous applications require miniaturisation.

(iv) QSE may provide the fundamental limits to Moore’s law.

(v) Such materials are fun to play with!

Motivation for Studying Motivation for Studying SemiconductorSemiconductor NanoparticlesNanoparticles



Growing CdSe Quantum DotsGrowing CdSe Quantum Dots

1rapidly inject precursor solution into hot (350oC) trioctylphosphine oxide

dimethyl cadmium+

trioctylphosphine selenide+

trioctylphosphine

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

2heat the reaction mixture at 260oC-

300oC until desired particle size is reached. QuickTime™ and a

Photo - JPEG decompressorare needed to see this picture.

heat

increasing

time

0

0.2

0.4

0.6

0.8

1

300 350 400 450 500 550 600 650 700

Absorbance spectra for a batch of CdSequantum dots

Abs

Wavelength (nm)

Similar preps for: InAs, InP, CdS, ZnS, ZnSe, CdTe, PbS,PbSe, ZnO, alloys and core-shells of these materials.



Typical Absorption and Fluorescence Kinetic Typical Absorption and Fluorescence Kinetic ProfilesProfiles

0

0.5

1

1.5

2

400 500 600 700

Ab

so

rban

ce

Wavelength (nm)

5"

180"

0

20

40

60

80

100

400 450 500 550 600 650 700

Em

issio

n I

nte

nsit

y (

norm

.)

Wavelength (nm)

180"5"

CdSe nucleation in octadecene at 275oC; Growth at 250oC; oleic acid/dodecylamine capping agents (L): Absorption (R): Luminescence vs time



Radius vs TimeRadius vs Time

1.2

1.4

1.6

1.8

2

2.2

2.4

30

32

34

36

38

40

42

44

0 50 100 150 200 250 300

Mean

Cry

sta

llit

e R

ad

ius (

nm

)

FW

HM

(nm

)

Time (s)

Particle growth is slow, distribution narrows

50mM monomer inOctadecene at 275oC



Tweaking…Tweaking… If you make the nucleation go fast enough…

If you can stop the nanocrystals growing…

If you can make them all the same size…

If you can make them as single crystals with no defects…

If you can stop them sticking together…



CdSe Nanocrystals ranging from 1nm to 6nm in diameter - to distinguish so many colours, the size distributions must be very narrow. The growth kinetics must be carefully controlled.

Quantum Size Effect producesQuantum Size Effect produces“Artificial Atoms”“Artificial Atoms”



Size Control is Critical!Size Control is Critical!

123456789

Em

issi

on In

tens

ity (

a.u.

)

Wavelength (nm)

CdS

e Diam

eter (nm)

600

0

300

400 450 500 550 600 650 700



Wannier excitons in bulk semiconducorsWannier excitons in bulk semiconducors Bound electron-hole pairs

that split off the conduction band due to mutual attractive (Coulomb) interaction

Binding energy and degree of localization depend on electrostatic screening

with

and

M

K

n

eEE Gexciton 22

22

222

4

he mm 111 he mmM

2

22

e

nra nB

Bohr radius



Exciton transitions in cuprous oxideExciton transitions in cuprous oxide



Weak Confinement Weak Confinement a a > > aaBB

We will not discuss this region. For NCs small, but larger than the exciton radius, the first evidence of QSE is a slight blue shift of the exciton due to its feeling trapped.

The exciton energy increases until it reaches the band edge..at this point, “normal” excitons no longer exist in the particle..

The shift is not as dramatic as in the strong regime…

Example: CdSe Nanocrystals

HRTEM CdSe Nanocrystal(Courtesy M Bawendi, MIT)



Strong Confinement Regime (Strong Confinement Regime (a < aa < aBB))

Absorption

EgEc

1se

1pe

1de

Ev

1sh

1ph

1dh

Bulk

Energy Smaller than exciton radius in bulk crystal. Treat e,h as separate entities. Spherical box.

Coulomb attraction is less than confinement energy. Varies as 1/a. Brus showed that lowest optical transition obeys:

since

2

22

2

22

2

2 amEE

amEE

h

mlg

hml

e

mlg

eml

aamamEE

ehgss

02

22

2

22

11 4

786.1

22

10



Summary of QSESummary of QSE

E g (R ) E g ( )

2 2

2m * R 2

1 .8e 2

R

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 10 100

En

erg

y (e

V)

NanoCrystal Radius (nm)

CuCl

ZnSe

CdS

CdSe

GaAs

heat

R > 5 nm (bulk)

R < 2 nm R < 3 nm

Filled energy levels

Unoccupied energy levels



Quantum Size Effects producesQuantum Size Effects produces“Artificial Atoms”“Artificial Atoms”

1.5 nm

7 nmNanocrystal Diameter



Applications - NCsApplications - NCs

Tunable lasers

Tunable LEDs

Biolabelling

Smart Glasses



BiolabellingBiolabelling

Advantages of QDs as biolabels

More photostable under laser irradiation. Dyes bleach very quickly.

Narrower spectra so more colours in any spectral region.

Simple excitation spectra, so readily multiplexed with simple optics. Dyes require multiple lasers in microscopes or cytometers.

Common chemistry for all labels.

Note that gold NCs have long been used as biolabels! Today, interest in all types of NCs as labels: magnetic,metallic, fluorescent, mixtures thereof.



Challenges and ConclusionsChallenges and Conclusions

Challenges for Nanocrystal Engineers Shape Control, Core-Shell Synthesis and Passivation. Monodispersity and Scale-Up. Luminescence and Quantum Yield from UV to NIR. QSE theory: matching with computational models.

Challenges for NanoTechnologists Ordering of NCs on different lengthscales. Integration with Top-Down NT processes. Creating Functional nanoscale materials and devices.



Single Nanocrystal SpectroscopySingle Nanocrystal Spectroscopy

Temperature (K) FWHM

300 K

20 K

10 K(low exct. Int185W/cm2)

(15 – 18) nm (50 – 60) meV

(0.06 – 0.3) nm(0.2 – 1) meV

0.04 nm120 eV

Confocal microscope

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