Ultrafast Dynamics in Solids and Semiconductor Nanoparticles

*技术摘要*本研究项目旨在通过直接实时检测巨磁电阻(CMR)材料和半导体量子点的结构动力学来了解它们的结构-性质/功能相关性。利用PI实验室开发的飞秒电子衍射(FED)新技术，将在原子时空分辨率下探测结构动力学。与FED一起，还将进行相关自由度的飞秒时间分辨光学光谱测量，以彻底表征这些材料的动力学行为。要解决的问题和问题包括：(1)CMR材料中光诱导相变的动力学：涉及哪些步骤和相关的时间尺度？Jahn-Teller失真在这些光致相变中起什么作用？(2)半导体量子点中电-声子耦合动力学：光沉积能量从载流子转移到晶格的速度有多快？这个时间尺度如何取决于量子点的大小、类型和结构，以及表面条件？声子瓶颈的作用是什么？这些研究将提供这些动力学的实时和原子水平的视图，并有可能完全揭示相关的物理机制和结构-功能相关性，否则这些相关性是不可能提取的。该项目将研究和教育相结合，以培养学生在凝聚态物理和超快科学的前沿领域掌握尖端技术和现代方法。物质的性质在很大程度上取决于它的微观结构，即组成原子在原子水平上的排列方式。一个众所周知的例子是碳及其同素异形体，如石墨、钻石和巴克球C60。同样，涉及组成原子和/或分子重排的结构变化和转变决定了自然界中许多重要的过程，从看似简单的过程，如熔化到复杂的生物反应。在原子时空尺度上探索结构动力学引起了人们极大的兴趣。在我们之前由NSF资助的研究中，我们开发了一种独特的飞秒电子衍射工具，并展示了它在原子运动的时间和长度尺度上探测结构动力学的能力。在这里，我们将使用这种新技术来研究几个具有基本物理意义和有趣的技术含义的凝聚态物理问题。它们包括巨磁电阻材料中光致结构转变的动力学和半导体纳米晶体中光生载流子的弛豫。前一项研究将揭示所涉及的基本步骤，从而对过渡路径有新的认识。后者的研究将有助于理解纳米晶体中的能量再分配，并有助于提高纳米晶体太阳能电池的能量转换效率。参与的本科生和研究生将获得当代凝聚态物理和超快科学前沿的培训和知识，并将为在学术界、工业界和国家实验室的职业生涯做准备。

****Technical Abstract****This research project seeks to understand the structure-property/function correlation in colossal magnetoresistive (CMR) materials and semiconductor quantum dots by directly examining their structural dynamics in real time. The structural dynamics will be probed at the atomic spatiotemporal resolution using a novel technique of femtosecond electron diffraction (FED) developed in PI's laboratory. In conjunction with FED, femtosecond time-resolved optical spectroscopy measurements in the relevant degrees of freedom will also be conducted to thoroughly characterize the dynamical behavior in these materials. Issues and questions to be addressed include: (1) Dynamics of photoinduced phase transition in CMR materials: What are the steps involved and what are the associated timescales? What is the role of Jahn-Teller distortion in these photoinduced phase transition? (2) Dynamics of electron-phonon coupling in semiconductor quantum dots: How fast is the photo-deposited energy transferred from charge carrier to lattice? How does this time scale depend on the size, the type and structure of quantum dots, and the surface conditions? What is the role of phonon bottleneck? These research will provide a real-time and atomic-level view of these dynamics and hold the potential to fully reveal the related physical mechanisms and the structure-function correlations that would otherwise be impossible to extract. This project integrates research and education to train students in cutting-edge techniques and modern methods in the forefront areas of condensed matter physics and ultrafast science. ****Non-Technical Abstract****The property of matter is largely determined by its microscopic structure, that is, how the constituent atoms are arranged at the atomic level. A well-known example is the carbon and its allotropes, such as graphite, diamond and buckyball C60. Likewise, structural changes and transformations involving the rearrangement of constituent atoms and/or molecules dictate many important processes in nature, ranging from seemingly simple processes such as melting to complex biological reactions. Probing structural dynamics at the atomic spatiotemporal scales has attracted considerable interest. In our previous research funded by NSF, we developed a unique tool of femtosecond electron diffraction and demonstrated its capability of probing structural dynamics at the time and length scales of atomic motions. Here, we will use this new technique to study several condensed matter physics problems of both fundamental physical importance and interesting technological implications. They include the dynamics of photo-induced structural transformation in colossal magnetoresistive materials and the relaxation of photo-generated carrier in semiconductor nano-crystals. The former study will reveal the elementary steps involved, thus gaining new insight to the transition pathways. The latter research will contribute to the understanding of the energy redistribution in nano-crystals and help improving the energy conversion efficiency in nano-crystal based solar cells. Participating undergraduate and graduate students will acquire training and knowledge in the forefronts of contemporary condensed matter physics and ultrafast science, and will be prepared for careers in academia, industry, and national laboratories.