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Zhang, Jianhong (2010): Control of Electronic Coupling and Optical Properties in Quantum Dot Solids. Dissertation, LMU München: Fakultät für Physik



In this thesis, we investigate the electronic coupling in quantum dot (QD) solids, optical anisotropies of nanowires (NWs) with diameters comparable to the wavelength of light, and the propagation of light in nanoribbon waveguides. In particular, we demonstrate a new mechanism to control the electronic coupling in QD solids thermomechanically, and how size controls the optical anisotropies in NWs. We firstly demonstrate that the electronic coupling in QD solids can be controlled by a new thermomechanical mechanism. This mechanism is realized by controlling the expansion and shrinkage of the interstitial material in the QD solids, which in turn controls the distance and distance-dependent electronic coupling between semiconductor nanocrystals (SNCs). Photoluminescence (PL) and TEM investigation demonstrate the tuning of the band gap emission in individual polycrystalline NWs and densely packed SNCs via this mechanism. At low temperature, temperature-induced blueshift in densely packed SNC film and redshift in polycrystalline NWs were realized. This is qualitatively different from bulk CdTe and isolated CdTe SNCs. The electronic coupling between the nearest SNCs for sub-nm distances agrees well with semiempirical calculations. Size dependence of optical anisotropies in NWs is demonstrated in this work. We found optical anisotropies in NWs with diameters comparable to the wavelength of light in the NW, i.e., beyond the electrostatic limit, are much lower than those of NWs in electrostatic limit. Finite-difference time domain calculations, with realistic parameters for the CdTe NWs, for excitation and PL anisotropy were carried out. It was found that the optical anisotropies of NWs display a strong size dependence when the NW is beyond the electrostatic limit. Changing the diameter allows tuning the polarization anisotropy from its maximum, predicted by the electrostatic limit, to zero. The optical anisotropies of a NW are determined by the diameter-wavelength ratio, the material dispersion, as well as the local refractive index of the surrounding. In addition, the optical anisotropies can be transferred into macroscopically aligned NW arrays, and the anisotropies of the NW arrays are determined by the optical anisotropies of isolated NWs, the disorder of the NWs in the film, the local environment and multiple scattering in the thick film. Furthermore, we show that self-assembled nanoribbons can serve as single-mode waveguides for the propagation of PL light. Calculations show that the minimum width needed for single-mode operation is approximately 150 nm, which agrees well with SEM measurements. The loss in the nanoribbon waveguides was quantitatively determined. Re-absorption was demonstrated in the nanoribbon waveguides to be a major contribution in the loss mechanism. Losses in the nanoribbon waveguide are on the same order of magnitude as for plasmon waveguides.