Light-matter interactions on

nanocrystals

Outline

Motivation

Light-matter interactions in nanoparticles

Metal nanoparticles

Semiconductor nanoparticles

Simple models

Measuring optical properties

Semiconductor nanocrystals in litterature

Motivation Nanoparticles (NP) are attractive because

they bridge the gap between bulk and molecules

They exhibit interesting properties: catalytic, electrical, optical, magnetic, ferroelectric, etc.

Much research has been done on the synthesis and understanding of their properties

Not much research has been done on understanding their concrete structure and its correlation with their properties, i.e. optical

Effects of the Structure

What are the effects of phase segregation, dopant distribution, geometry, interfaces in heterostructures,

etc. on the optical properties of NPs?

B. Patrick, H. C. Ham, Y. Shao-Horn, L. F. Allard, G. S. Hwang, and P. J.

Ferreira, Chemistry of Materials 25, 530 (2013).

Structure of a CdSe/ZnS core/shell NP

J. McBride, J. Treadway, L. C. Feldman, S. J. Pennycook, and S. J.

Rosenthal, Nano Letters 6, 1496 (2006)

Matter-light interaction Impossible to apprehend as a single concept Multiplicity of processes explainable by classic and/or

quantum mechanics

Preliminary distinction of phenomena:

Metal NanoparticlesScattering & Absorption

Electrons in metals described as an electron gas or jellium model

E.g. Au NPSurface plasmon

resonance enhances

absorption or scattering

for characteristic s

- --

- ---

+++ ++

- --- - --

++ + ++

Stephan Link; Mostafa A. El-Sayed; J. Phys. Chem. B 1999, 103, 4212-4217.

UVvis absorption spectra of 9, 22, 48, and 99 nm gold nanoparticles in water

Semiconductor Nanoparticles or Quantum Dots (QD)

Size-quantization effect

Caruso, Frank. "Chapter 2. Semiconductor Nanoparticles." Colloids and Colloid Assemblies: Synthesis,

Modification, Organization and Utilization of Colloid Particles. Weinheim: Wiley-VCH, 2004. N. pag.

Properties of Semiconductor NP

The Band Gap of NP increases as we reduce its size, r

If r < rB ; rB is the exciton Bohr radius

=2

21

+

1

=2

21

Excitons

Quantum of excitation

Electron and hole bound by Coulomb force

Exciton

Confinement

energy for the

excited electron

Confinement

energy for the

hole

Eg

Excitons

Frenkel exciton

Small High Coulomb interaction Small rB

Common in alkali halides and organic molecules

Wannier exciton

Large Small Coulomb interaction Large rB

Physical problem: Schrdinger eq. hydrogen atom + lattice potential + electrons and holes correlations

Semiconductors

Simple Quantum Confinement

Models

Effective Mass Approximation or Particle-in-Box Model

Linear Combination of Atomic Orbitals (LCAO)

Particle in a Box

Parabolic dispersion relation for the electron

=22

2

=22

2 21.8 2

=

Confinement

energy

Coulomb

interaction

Particle in a Box

At small r, the model breaks

Broken line: particle-in-box model; solid line: tight-binding

model; squares: experimental dataY. Wang and N. Herron, Journal of Physical Chemistry 95, 525 (1991).

LCAO

Quantum dots are

considered as large

molecules

Good for small

particles

For large particles it

becomes too complex

LCAO for a Si nanoparticle

M. L. Steigerwald and L. E. Brus, Accounts of Chemical Research 23, 183

(1990).

Optical Properties of QD

Absorption and Photoluminescence (PL) Absorption of hex Emission hPL; ex< PL

Intrinsic and extrinsic effects on photoluminescence

Intrinsic: Band-edge or near band-edge transitions

Extrinsic: Impurities, dopants, defects, etc.

Radiative vs. non-radiative relaxation processes

E.g. PL Spectra of ZnO Nanostructures

Nanostructures: (1) tetrapods, (2) needles, (3) nanorods, (4) shells, (5) highly

faceted rods, and (6) ribbons/combs

Djurii, Aleksandra B., and Yu Hang Leung. "Optical properties of ZnO nanostructures." Small 2.89 (2006): 944-961.

Semiconductor Nanocrystals in

Litterature

II-VI semiconductor are the most extensively investigated. CdS, Eg=2.45 eV, Eg=2.9 eV @ r=2.6 nm [1] CdSe, Eg=1.74 eV, Eg2.25 eV @ r=2 nm [3] CdTe, Eg=1.49 eV, Eg2.0 eV @ r=2 nm [3] ZnSe, Eg=2.7 eV, Eg=3.18 eV @ r=2.7 nm [1] ZnS, Eg=3.7 eV, Eg=4.1 eV @ r=2.7 nm [2] ZnTe, Eg=2.25 eV ZnO, Eg=3.35 eV, Eg 3.5 eV @ r=5.5 nm [4] PbSe, Eg= 0.26

[1] Beri, Rupinder K., et al. "Band-gap engineering of ZnSe quantum dots via a non-TOP green synthesis by use of organometallic selenium compound."

Current Applied Physics 10.2 (2010): 553-556.

[2] Bera, Debasis, et al. "Quantum dots and their multimodal applications: a review." Materials 3.4 (2010): 2260-2345.

[3] Baskoutas, Sotirios, and Andreas F. Terzis. "Size-dependent band gap of colloidal quantum dots." Journal of applied physics 99.1 (2006): 013708.

[4] Lin, Kuo-Feng, et al. "Band gap variation of size-controlled ZnO quantum dots synthesized by solgel method." Chemical Physics Letters 409.4 (2005): 208-211.

Semiconductor Nanocrystals in

Litterature

Group III-V Quantum Dots have been studied extensively, but not as colloidal nanocrystals

QD created by Stranski-Krastanov epitaxial growth

Optoelectronic applications

Qdot

Wetting layer

Semiconductor Nanocrystals in LitteratureIII-V Group

Difficult to synthetize NP due to covalent nature of bonds

Large exciton Bohr radii

Some Synthesized nanocrystals: [1]

InP

GaP

InAs

GaAs

[1] Caruso, Frank. "Chapter 2. Semiconductor Nanoparticles." Colloids and Colloid Assemblies: Synthesis, Modification, Organization and

Utilization of Colloid Particles. Weinheim: Wiley-VCH, 2004. N. pag.

Metal Oxides

Less research on their optical properties with some exceptions (ZnO, TiO2)

Material Phases Eg (eV) PL (nm) NP sizes (nm) RemarksMgO 7 360- 475 3-10SiO2 SiO2 ~10 300-645 6 to 400

Aluminum oxide

-Al2O3 9.4 266-740 125, 32 Importance of impurity atoms on PL spectra

Vanadium oxide

VO2 0.7Semiconductor-metal phase transition @68. Research done on size effect on transition.

V2O5 2.9 460 -593 7 Most (table phase. Possible to synthesize nanostructuresChromium

oxideCr2O3 3 385 3-50

Used in industry as pigment and catalyst, possible to synthesize NPs

Manganese Oxide

MnO 4.2324, 359,

3863-50

Size and shape dependent magnetic behavior, possibility of synthesizing in different shapes, Mott insulator

Mn3O4 315-834 3, NW Study of magnetic properties-Mn2O3 1.9-3.3 281 - 649 35-40, NW

Iron oxide

Fe3O4 3-50

-Fe2O3 2.7-3.2 570-886 12-48Significant literature of PL on NP. Size of NP very important on PL properties. Magnetic properties, magneto-optical effects. Structural changes dependent on size

-Fe2O3 10,60 Many physico-chemical studies done on it.

Cobalt Oxide Co3O4 1-1.9 600 3-150, NWSynthesis of nanoparticles possible. Nanoparticles are considered harmful to humans and environment.

Nickel Oxide NiO 3.5- 4.3 320-350 3-50, NWMott insulator, nanoparticles considered very toxic. NP have been studied due to applications in catalysis.

Copper oxideCu2O 2.1-2.6 370-1000 2-45 Most studied semiconductor, in history. Non-toxic and abundantCuO 1.2-2.8 398-527 5-30

Tin oxide SnO2 3.6 180-1596 3CeO2 CeO2 3.31-5 369 - 467 30