What is a quantum dot?

• Nanocrystals• 2-10 nm diameter• semiconductors

What is a quantum dot?

• Exciton Bohr Radius• Discrete electron

energy levels• Quantum

confinement

Motivation• Semiconducting nanocrystals

are significant due to;

strong size dependent optical properties (quantum confinement)

• applications solar cells

Terahertz gap

1 THz = 300 µm = 33 cm-1 = 4.1 meV

Time domain terahertz Spectrometer

The pulse width = ΔtFWHM/√2 = 17.6±0.5 fs (A Gaussian pulse is assumed)

Terahertz SignalTerahertz Signal

Fourier Transform

To obtain the response of the sample to the THz radiation 2 measurements are made

•THz electric field transmitted through the empty cell •THz electric field transmitted through the sample cell

Terahertz signal

Doping

• Intentionally adding impurities to change electrical and optical properties

• Add free electrons to conduction band or free holes in valence band

• Tin and Indium dopants

Free carrier Absorption in Free carrier Absorption in Quantum DotsQuantum Dots

Purification and sample preparation of quantum dots

Experimental procedure & Data analysisExperimental procedure & Data analysis

))(exp()()(

~)(

~

)(

)(

00

iTE

E

tE

tE

√T(ω), Φ(ω) Complex refractive index (nr(ω) + i.nim(ω))

No Kramer-Kronig analysis!!!

Power transmittance Relative phase

time domain: frequency domain:

Changes upon charging large quantum Changes upon charging large quantum dot: Intrinsic Imaginary Dielectric dot: Intrinsic Imaginary Dielectric constantconstant

2 3 4 5 6 7-0.5

0.0

0.5

1.0

1.5

2.0

im(

)

Frequency (THz)

3.2 nm uncharged 3.2 nm charged

2 3 4 5 6 7

0

2

4

6

8

im(

)Frequency (THz)

6.3 nm uncharged 6.3 nm charged

•For the charged samples Frohlich Band diminishes: A broader and weaker band appears•The reason of this is the presence of coupled plasmon-phonon modes

Nano Lett., Vol. 7, No. 8, 2007

The complex dielectric constant = (nr(î) + ini(î))2

The frequency dependent complex dielectric constants determined by experimentally obtained• Frequency dependent absorbance and refractive index.

Results

1 2 3 4 5 6 7 8

0.0

0.5

1.0

1.5

2.0

indium doped undoped tin doped

Ab

sorb

ance

THz

• Surface phonon

• Shift of resonance of tin doped

• Agreement with charged QDs

Results

2 3 4 5 6 7 81.26

1.28

1.30

1.32

1.34

Ref

ind

ex

THz

Un Sn In

Semiconductor Quantum DotsSemiconductor Quantum Dots

Justin Galloway2-26-07

Department of Materials Science & Engineering

I. Introduction

II. Effective Mass Model

III. Reaction Techniques

IV. Applications

V. Conclusion

OutlinOutlinee

HowHow Quantum Dots

Semiconductor nanoparticles that exhibit

quantum confinement (typically less than 10 nm

in diameter)

Nanoparticle: a microscopic particle of an

inorganic material (e.g. CdSe) or organic material

(e.g. polymer, virus) with a diameter less than

100 nm

More generally, a particle with diameter less than

1000 nm

1. Gaponenko. Optical properties of semiconductor nanocrystals 2. www.dictionary.com

PropertiPropertieses

Properties of Quantum Dots Compared to Organic Fluorphores?High quantum yield; often 20 times brighter

Narrower and more symmetric emission spectra

100-1000 times more stable to photobleaching

High resistance to photo-/chemical degradation

Tunable wave length range 400-4000 nm

http://www.sussex.ac.uk/Users/kaf18/QDSpectra.jpg

CdSe

CdTe

J. Am. Chem. Soc. 2001, 123, 183-184

ExcitatioExcitationn

Excitation in a Semiconductor

The excitation of an electron from the valance

band to the conduction band creates an electron

hole pair

h e (CB)h(VB)

E=h

optical detector

semiconductor

E

EVB

CBE h=Eg

Creation of an electron hole pair where h is the photon energy

exciton: bound electron and hole pairusually associated with an electron trapped in a localized state in the band gap

Band Gap (energy barrier)

E

EVB

CBE

band-to-band recombination

recombination atinterband trap states (e.g. dopants, impurities)

E

EVB

CBEE=h

radiative recombination

non-radiative recombination

recombination processes

radiative recombination photonnon-radiative recombination phonon (lattice vibrations)

e (CB)h(VB) h

ReleasReleasee

Recombination of Electron Hole Pairs

Recombination can happen two ways:

radiative and non-radiative

Band gap of spherical particles

The average particle size in suspension can be obtained from the absorption onset using the effective mass model where the band gap E* (in eV) can be approximated by:

Egbulk - bulk band gap (eV), h - Plank’s constant (h=6.626x10-34 J·s)

r - particle radius e - charge on the electron (1.602x10-19 C)me - electron effective mass - relative permittivitymh - hole effective mass 0 - permittivity of free space (8.854 x10-14 F cm-1) m0 - free electron mass (9.110x10-31 kg) Brus, L. E. J. Phys. Chem. 1986, 90, 2555

E* Egbulk

22

2er2

1me m0

1

mhm0

1.8e40r

0.124e3

2 40 21

me m0

1

mhm0

1

ModelModel Effective Mass ModelDeveloped in 1985 By Louis Brus

Relates the band gap to particle size of a spherical

quantum dot

Brus, L. E. J. Phys. Chem. 1986, 90, 2555

ModelModel Term 2The second term on the rhs is consistent with the

particle in a box quantum confinement model

Adds the quantum localization energy of effective mass

me

High Electron confinement due to small size alters the

effective mass of an electron compared to a bulk materialConsider a particle of mass m confined in a potential well of length L. n = 1, 2, …

En n222

2mL2 n2h2

8mL2

For a 3D box: n2 = nx2 + ny

2 + nz2

0 Lx

Pot

entia

l Ene

rgy

E* Egbulk h2

8r21

mem0 1

mhm0

1.8e2

40r 0.124e4

h2 20 21

mem0 1

mhm0

1

Brus, L. E. J. Phys. Chem. 1986, 90, 2555

ModelModel Term 3 The Coulombic attraction between electrons and holes

lowers the energy

Accounts for the interaction of a positive hole me+ and a

negative electron me-

E* Egbulk h2

8r21

mem0 1

mhm0

1.8e2

40r 0.124e4

h2 20 21

mem0 1

mhm0

1

Electrostatic force (N) between two charges (Coulomb’s Law):

Consider an electron (q=e-) and a hole (q=e+)The decrease in energy on bringing a positive charge to distance r from a negative charge is:

E e2

40r2dr e2

40r

r

F q1q2

40r2 Work, w = F·dr

ModelModel Term InfluencesThe last term is negligibly small

Term one, as expected, dominates as the radius is

decreased

0

1E

nerg

y (e

V)

0 5 10

d (nm)

term 3

term 2

term 1

Conclusion: Control over the particle’s fluorescence is possible by adjusting the radius of the particle

Mod

ulus

ModelModel Quantum Confinement of ZnO &

TiO2

ZnO has small effective masses quantum effects can

be observed for relatively large particle sizes

Confinement effects are observed for particle sizes <~8

nm

TiO2 has large effective masses quantum effects are

nearly unobservable

3

4

Eg

(eV

)

250

300

350

400

ons

et (

nm

)

0 5 10

d (nm)

ZnO

3

4

Eg

(eV

)

250

300

350

400

on

set (

nm

)

0 5 10

d (nm)

TiO2

TheThe

MakinMakingg

Formation of

Nanoparticles

Varying methods for the synthesis of

nanoparticles

Synthesis technique is a function of

the material, desired size, quantity

and quality of dispersionSynthesis Techniques• Vapor phase (molecular beams, flame synthesis etc…• Solution phase synthesis

•Aqueous Solution•Nonaqueous Solution

Semiconductor NanoparticlesII-VI: CdS, CdSe, PbS, ZnSIII-V: InP, InAsMO: TiO2, ZnO, Fe2O3, PbO, Y2O3

Semiconductor Nanoparticles Synthesis:Typically occurs by the rapid reduction of organmetallic precusors in hot organics with surfactants

some examples of in vitro imaging with QDs (http://www.evidenttech.com/)

TheThe

MakinMakingg

Nucleation and Growth

C. B. Murray, C. R. Kagan, and M. G. Bawendi, Annu. Rev. Mater. Sci. 30, 545, 2000.

Figure 1. (A) Cartoon depicting the stages of nucleation and growth for the preparation of monodisperse NCs in the framework of the La Mer model. As NCs grow with time, a size series of NCs may be isolated by periodically removing aliquots from the reaction vessel. (B) Representation of the simple synthetic apparatus employed in the preparation of monodisperse NC samples.

Horizontal dashed lines represent the critical concentration for nucleation and the saturation concentration

TheThe

MakinMakingg

Capping Quantum Dots

Due to the extremely high surface area of a

nanoparticle there is a high quantity of “dangling

bonds”

Adding a capping agent consisting of a higher band

gap energy semiconductor (or smaller) can eliminate

dangling bonds and drastically increase Quantum YieldWith the addition of CdS/ZnS the Quantum Yield can be increased

from ~5% to 55%

Shinae, J. Nanotechnology. 2006, 17, 3892

Synthesis typically consisted of lower concentrated of precursors injected at lower temperatures at slow speeds

TheThe

MakinMakingg

Quantum Dot Images

Quantum dot images prepared in the Searson Lab

using CdO and TOPSe with a rapid injection

455000x560000x

770000x

Quantum Dot Ligands Provide new Insight into erbB/HER receptor – Mediated Signal TransductionUsed biotinylated EGF bound to commercial quantum dots

Studied in vitro microscopy the binding of EGF to erbB1 and erbB1 interacts with erbB2 and erbB3

Conclude that QD-ligands are a vital reagent for in vivo studies of signaling pathways – Discovered a novel retrograde transport mechanism

Nat. Biotechnol. 2004, 22; 198-203

A431 cell expressing erbB3-mCitrine

Dynamics of endosomal fusion

ApplicatioApplicationn

QD’s

ApplicatioApplicationn

Cartoon of

assay

Fluoresence data for all 4 toxin assays at high concentrations

Anal. Chem. 2004, 76; 684-688

QD’s

Multiplexed Toxin Analysis Using Four Colors of Quantum Dot FluororeagentsDemonstrated multiplexed assays for toxins in the same well

Four analyte detection was shown at 1000 and 30 ng/mL for each toxin

At high concentrations all four toxins can be deciphered and at low concentrations 3 of the 4

Gao et al., “In vivo cancer targeting and imaging with semiconductor quantum dots,” Nat. Biotechnol. 22, 969 (2004).

Quantum Dot ImagingQDs with antibodies to human prostate-specific membrane antigen indicate murine tumors developed from human prostate cells15 nm CdSe/ZnS TOPO/Polymer/PEG/target

ApplicatioApplicationn

QD’s

BiologicaBiological l

ParticlesParticles

Magnetic Nanoparticles

Nano-sized magnetic particles can be

superparamagnetic

Widely Studied – Suggested as early as the 1970’s

Offers control/manipulation in magnetic field

J. Phys. D: Appl. Phys. 36, 2003; 167-181.

Science 291, 2001; 2115-2117.

An Attractive Biological Tool

Co has higher magnetization compared to magnetite and maghemite

Magnetic Nanoparticles: Inner Ear

Targeted Molecule Delivery and Middle

Ear ImplantSNP controlled by magnets while transporting a payload

Studies included in vitro and in vivo on rats, guinea pigs and human cadavers

Demonstrated magnetic gradients can enhance drug delivery

Perilymphatic fluid samplesfrom animals exposed to magnetic forces

Perilymphatic fluid from the cochleaof magnet-exposed temporal bone

Audiol Neurotol 2006; 11: 123-133

ApplicatioApplicationn

Magnetic

Particles

Magnetic

Particles

WhatWhat

isis

MQD ?MQD ?

Composite with A Novel Structure forComposite with A Novel Structure for ActiveActive Sensing in Sensing in LivingLiving cellscells

SilicaZnS

CdSe

Co

① Cobalt coreCobalt core : active manipulation

diameter : ~10 nm

superparamagnetic NPs

→ manipulated or positioned by an external field without aggregation in the absence of an external field

② CdSe shellCdSe shell : imaging with

fluorescence thickness : 3-5 nm

visible fluorescence (~450 – 700 nm)

ability to tune the band gap

→ by controlling the thickness, able to tune the emission wavelength, i.e., emission color

④ Silica shellSilica shell : bio-

compatibility &

functionalization with

specific targeting group thickness : ~10 nm

bio-compatible,

& non-toxic to live cell functions

stable in aqueous environment

ability to functionalize its surface

with specific targeting group

MMagnetiagneticc

QQuantuuantumm

DDotot

③ ZnS shellZnS shell : electrical passivation

thickness : 1-2 nm

having wider band gap (3.83 eV) than CdSe (1.91 eV)

enhancement of QY

→ CdSe (5-10%) CdSe/ZnS (~50%)

Rap-Rap-UpUp

Conclusions

The effective mass model give an excellent

approximation of the size dependence of

electronic properties of a quantum dot

Recent synthesis advances have shown

many quantum dot reactions to be robust,

cheap, and safe then previously thought

Quantum dots offer wide range electronic

properties that make them an attractive tool

for biological and medical work

MQD’s improve afford in vivo manipulation

expanded the applicability of quantum dots

From an Atom to a SolidPhotoemission spectra of negative

copper clusters versus number of atoms in the cluster. The highest energy peak corres-ponds to the lowest unoccupied energy level of neutral Cu.

Typically, there are two regimes:

1) For < 102 atoms per cluster, the energy levels change rapidly when adding a single atom (e. g. due to spin pairing).

2) For > 102 atoms per cluster, the energy levels change continuously (e. g. due to the electric charging energy (next slide).

Energy below the Vacuum level (eV)

3d 4s

Energy Levels of Cu Clusters vs. Cluster Radius R

ΔE = (E- ER) 1/R (charged sphere)

Solid Atom

The Band Gap of Silicon

Nanoclusters

Bulk Silicon

3 nm : Gap begins to change

GaAs

The Band Gap of Silicon

Nanoclusters

3 nm : Gap begins to change

Increase of the Band Gap in Small

Nanoclusters by Quantum Confinement

Conduction Band

Valence Band

k2 k1Gap

Size Dependent Band Gap in CdSe Nanocrystals

The Band

Gap of CdSe

NanocrystalsSize:

Photon Energy vs. Wavelength:

h (eV) = 1240 / (nm)

Beating the size distribution of quantum dots

Quantum dots formed by thin spots in GaAs layers

Termination of nanocrystals

Critical for their electronic behavior

From Giulia Galli’s group

H-terminated Si nanocrystal:

Electrons stay inside,

passivation, long lifetime

Oxyen atom at the surface:

Electrons drawn to the oxygen

Fluorine at the surface:

Complex behavior

Single Electron Transistor

A single electron

e- tunnels in two

steps from source

to drain through

the dot.

The dot replaces

the channel of a

normal transistor

(below).

dote- e-

electrons

Nanoparticle attracted electrostatically to the gap between source and drain electrodes.The gate is underneath.

Designs for Single

ElectronTransistors

Small (10 nm) for operation around room temperature

Large (≈ m) for operation at liquid He temperature

Quantum Dots as Artificial Atoms in Two Dimensions

Filling electron shells in 2D

* The elements of this Periodic Table are named after team members from NTT and Delft.

*

Magnetic Clusters

“Ferric Wheel”

Magnetic Nanoclusters in Biology

The Holy Grail of Catalysis: Reactions at a Specific Nanoparticle

Want this image chemically resolved.

Have chemical resolution in micro-spectroscopy via X-ray

absorption but insufficient spatial resolution.

Fischer-Tropsch process converts coal to fuel using an iron catalyst.

Di and Schlögl

De Smit et al., Nature (2008)

4 Mn + 1 Ca

The Oxygen Evolving Complex

Instead of rare metals with 5d or 4d electrons, such as Pt, Rh, Ru,

one finds plentiful 3d transition metals in bio - catalysts: Mn, Fe .

Nature does it by necessity. Can we do that in artificial photosynthesis ?

Biocatalysts = Enzymes

The active Fe6Mo center of nitrogenase,

Nature’s efficient way of fixing nitrogen.

Most biocatalysts consist of a protein with a small metal cluster at the active

site.