Synthetic control of electron-phonon coupling in semiconductor quantum dots

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With this award, the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program is funding Professors Anne Myers Kelley and David Kelley at the University of California at Merced to apply laser-based spectroscopic techniques and computational modeling to study mechanisms whereby light energy is converted to heat in quantum dots (QDs) which are small semiconductor structures containing hundreds to thousands of atoms. The quantitative understanding that will be gained regarding these processes will enable the controlled chemical synthesis of QDs that are optimized for several types of technological applications. These include efficient solar energy capture and conversion, low power dissipation artificial lighting, and laser-based biological imaging for medical diagnostics. UC Merced is a Hispanic Serving Institution and more than half of its undergraduates are first-generation college students, and this research will actively involve both graduate and undergraduate students from traditionally underrepresented groups.This project aims to accurately measure and quantitatively understand the factors that determine the extent of electron-phonon coupling (EPC) in nanometer sized semiconductor quantum dots (QDs) and to synthetically control the extent of EPC. Preliminary calculations suggest that by judicious choice of materials and morphology it is possible to greatly increase or decrease the magnitude of EPC in core-shell and core-alloy-shell structures compared to single component QDs. Both well-known and novel structures based on II-VI semiconductors are synthesized and characterized and their EPC measured through quantitative resonance Raman spectroscopy, including analysis of absolute excitation profiles, overtone intensities, and depolarization ratios,. This study tests the hypothesis that EPC for optical phonons in polar crystals is determined largely by the amount of charge separation produced by electron-hole pair formation via the Fröhlich mechanism, and that it can be varied by controlling the valence and conduction band energies such that the electron and hole wavefunctions have different amounts of overlap. The novelty of this approach is to not only measure EPC accurately for specific materials but also engineer structures that permit the control of EPC.