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Terahertz Spectroscopy of CdSe Quantum Dots

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

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Terahertz Spectroscopy of CdSe Quantum Dots

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  1. Terahertz Spectroscopy of CdSe Quantum Dots Fadzai Fungura Cornell College Mentor: Prof Viktor Chikan Kansas State University

  2. What is a quantum dot? • Nanocrystals • 2-10 nm diameter • semiconductors

  3. What is a quantum dot? • Exciton Bohr Radius • Discrete electron energy levels • Quantum confinement

  4. Motivation • Semiconducting nanocrystals • are significant due to; • strong size dependent optical properties (quantum confinement) • applications solar cells

  5. Terahertz gap 1 THz = 300 µm = 33 cm-1 = 4.1 meV

  6. Time domain terahertz Spectrometer The pulse width = ΔtFWHM/√2 = 17.6±0.5 fs (A Gaussian pulse is assumed)

  7. Terahertz Signal • 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 Fourier Transform

  8. Terahertz signal

  9. 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

  10. Free carrier Absorption in Quantum Dots

  11. Purification and sample preparation of quantum dots

  12. Experimental procedure & Data analysis time domain: frequency domain: Power transmittance Relative phase √T(ω), Φ(ω) Complex refractive index (nr(ω) + i.nim(ω)) No Kramer-Kronig analysis!!!

  13. Changes upon charging large quantum dot: Intrinsic Imaginary Dielectric constant • The frequency dependent complex dielectric constants determined • by experimentally obtained • Frequency dependent absorbance and refractive index. The complex dielectric constant = (nr(î) + ini(î))2 • 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

  14. Surface phonon Shift of resonance of tin doped Agreement with charged QDs Results

  15. Results

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