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Transient absorption and Raman spectroscopy of Cadmium Selenide/Zinc Selenide core/shell alloy particles

Abstract

The central premise of this thesis surrounds the effects of smoothing out the interface between core and shell of a nanoparticle system on the multi-exciton auger recombination kinetics as measured by transient absorption spectroscopy. Current research suggests smoothing out the interface between core and shell by alloying at high temperature reduces the auger rate [1], the rate at which multiexcitons combine down to a single exciton, and therefore increases the viability of the system for solar energy devices incorporating multiple exciton generation (MEG) technology. Auger recombination is so detrimental to these technologies because it occurs on a timescale of picoseconds, whereas radiative recombination, the primary mechanism for capturing energy in a nanoparticle and reemitting it, occurs on the nanosecond timescale. If multiexcitons were to be created in a nanocrystal sample with no means of controlling auger recombination, the auger mechanism would reduce these excitons down to one, and subsequent cooling of the hot carrier by phonon release would result in the loss of all of the extra energy that was stored in these excitons. In order to study this phenomenon multiple experimental techniques were performed on the nanoparticle samples, most notably UV-Vis, static fluorescence, Raman and transient absorption spectroscopies. As a result of this research, it is clear that alloying a core/shell particle has little effect on transient absorption kinetics, and that current theory on the subject needs to be reformulated. The experimental data contained herein is compared with calculations using a FORTRAN program developed by Professor David. F. Kelley, and confirms the process of alloying has little to no effect on auger rates of nanoparticles for this system. The calculations were also able to reproduce the UV-Vis and Raman spectra almost quantitatively if given the correct parameters, increasing confidence in the interpretation of the experimental results. This thesis does not aim to discuss what exactly controls the auger rate in nanoparticle systems, but a few conjectures based on sound observation and rooted in solid theory are presented.

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