Terahertz Spectroscopy of CdSe Quantum Dots

Fadzai FunguraCornell College

Mentor: Prof Viktor Chikan Kansas State University

Terahertz Spectroscopy of CdSe Quantum Dots

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

iT

EE

tEtE

√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 70

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

Abs

orba

nce

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

inde

x

THz

Un Sn In