Terahertz Spectroscopy of CdSe Quantum DotsCdSe量子的太赫兹光谱.pptVIP

Fadzai Fungura Cornell 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 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 Quantum Dots Purification and sample preparation of quantum dots Experimental procedure Data analysis √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 dot: Intrinsic Imaginary Dielectric constant 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(?) + ini(?))2 The frequency dependent complex dielectric constants determined by experimentally obtained Frequency dependent absorbance and refractive index. Results Surface phonon Shift of resonance of tin doped Agreement with charged QDs Results * * * * * * * * * * * * * * * * * * * *