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Terahertz Spectroscopy of CdSe Quantum Dots. Fadzai Fungura Cornell College Mentor: Prof Viktor Chikan Kansas State University. What is a quantum dot?. Nanocrystals 2-10 nm diameter semiconductors. What is a quantum dot?. Exciton Bohr Radius Discrete electron energy levels - PowerPoint PPT Presentation
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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
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