Synthetic control of electron-phonon coupling in semiconductor quantum dots

凭借这一奖项，大分子、超分子和纳米化学(MSN)计划资助加州大学默塞德分校的Anne Myers Kelley和David Kelley教授应用基于激光的光谱技术和计算模型来研究量子点(QD)中光能转化为热的机制。量子点是包含数百到数千个原子的小型半导体结构。对这些过程的定量了解将使量子点的受控化学合成成为可能，这些量子点针对几种类型的技术应用进行了优化。其中包括高效的太阳能捕获和转换、低功耗人工照明，以及用于医疗诊断的基于激光的生物成像。加州大学默塞德分校是一家西班牙裔服务机构，超过一半的本科生是第一代大学生，这项研究将积极纳入传统上代表不足的群体的研究生和本科生。本项目旨在准确测量和定量了解决定纳米级半导体量子点(QD)中电子-声子耦合(EPC)程度的因素，并综合控制EPC程度。初步计算表明，通过合理选择材料和形貌，与单一组分量子点相比，核壳和核-合金-壳结构中的EPC的大小是可能的。合成和表征了基于II-VI半导体的已知和新型结构，并通过定量共振拉曼光谱测量了它们的EPC，包括绝对激发谱、泛音强度和退偏率的分析。这项研究验证了这样一种假设，即极性晶体中光学声子的EPC主要取决于通过Fröhlich机制形成电子-空穴对所产生的电荷分离量，并且可以通过控制价带和导带能量来改变EPC，从而使电子和空穴波函数具有不同的重叠量。这种方法的新颖之处在于不仅可以准确地测量特定材料的EPC，而且还可以设计允许控制EPC的工程结构。

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.