1. The Correlation between Diameter and Two Photon Absorption Cross Section of II-VI Semiconductor Quantum Dots . The Confined Acoustic Phonon Study of II-VI Semiconductor Nanoparticles
Date Issued
2008
Date
2008
Author(s)
Yang, Meng-Ju
Abstract
Part 1: The size-dependent two photon absorption (TPA) cross section of II-VI semiconductor quantum dots (QDs) has been investigated. As the size of the QDs increased, the TPA cross section was found to be empirically related via a power-law proportionality of 3.5 and 5.6 to the diameters of CdSe and CdTe QDs. These results can be rationalized by a theoretical model of two-photon excitation in a system incorporating excitons and defects.art 2: With spatial confinement, low dimensional systems such as QDs and nanowires (NWs) exhibit not only electronic but also acoustic energy quantization. In 1882, Lamb had already studied the acoustic normal modes confined in a homogeneous free sphere. Two types of modes, torsional and spheroidal (SPH), were derived from the stress-free boundary condition on the spherical surface. Taking advantage of the charge separation structure in the CdSe/CdTe core-shell type-II QDs, we experimentally observed and verified the existence the resonance-enhanced dipolar interaction between terahertz (THz) photons and their corresponding eigenmode (l=1). We also observed that the piezoelectricity of CdSe QDs can induced THz photon absorption related to inactive breathing mode (l=0). From our THz absorption spectrum, we found that the corresponding absorption cross section of CdSe/CdTe QDs shows a D4 (D: diameter) dependence, and the frequency of absorbed photons is inversely proportional to D of the QDs and agrees with that of dipolar active mode. The absorption peak of CdSe QDs which related to breathing mode also shows that the frequency of peak is inversely to the D of the CdSe QDs. We can covert the THz photons into a phonon of the same frequency through charge separation of CdSe/CdTe and the piezoelectricity of CdSe.
Subjects
quantum dots
nanorods
core/shell structure
two photon absoptioption
confined acoustic phonon
Type
thesis
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