A New Look at Classic Materials Systems: Advanced Synchrotron X-ray Characterization of Colloidal Nanocrystals

非技术摘要：尽管半导体纳米晶体在光伏、固态照明和纳米电子学方面显示出巨大的应用前景，但组装的器件的成功将主要取决于纳米晶体组装的导电性。用于表征这些系统的常规实验探测器通常不会探索对原子物种敏感或在现场进行的技术。因此，当务之急是确定新的实验探针，这些探针可以提供关于NC的关键信息，而这些信息无法通过其他方法实现。该项目在美国国家科学基金会固体和材料化学项目的支持下，使用最先进的X射线表征方法来阐明NC文献中一系列尚未回答的问题，包括NCS中的阳离子交换机制以及配体化学对NCS中载流子动力学的影响。该项目的重点是操作台/原位软和硬X射线吸收光谱以及超快X射线吸收光谱。这些技术在很大程度上还没有被用来探索半导体NCS的化学和物理，预计它们将为回答关于这些材料中的成键和电荷转移动力学的重要问题提供重要的见解。反过来，这项研究提案的结果将对制造下一代纳米电子材料的能力产生强烈的影响。这项研究利用了表面科学与技术实验室(LASST)内庞大的基础设施，该实验室是一个跨学科中心，汇集了来自物理、化学、电气与计算机工程和化学与生物工程的研究人员。该研究项目提供物理、化学和材料科学领域的高级研究生培训，特别是超高真空表面科学和材料表征，包括高级光源和高级光子源的同步加速器研究。此外，位于LASST的研究实验室通过参观方式展出，将向许多贫困学生介绍尖端乐器，其中包括大量的美洲原住民。技术摘要：该项目由美国国家科学基金会的固体和材料化学项目支持，对半导体纳米晶体(NCS)中电荷传输和阳离子交换等过程背后的基本机制进行了前所未有的研究。研究活动集中在操作烷/原位软和硬X射线吸收光谱以及超快X射线吸收光谱。这些技术在很大程度上还没有被用来探索半导体NCS的化学和物理，预计它们将为回答关于这些材料中的成键和电荷转移动力学的重要问题提供重要的见解。反过来，这项研究提案的结果将对制造下一代纳米电子材料的能力产生强烈的影响。这项工作的变革性特征是使用尖端的X射线技术来探测NC最常见的环境：溶液阶段。考虑到NCS中控制载流子密度和迁移率之间的相互作用将在制造高导电性的NC器件中发挥重要作用，该项目的研究目标集中在两个相关的研究重点领域：(A)经历阳离子交换过程的NC材料的原位X射线光谱和(B)表面配体对NCS中界面电荷载流子动力学的影响，如超快X射线光谱所探测的那样。感兴趣的体系材料包括作为原型纳米材料的CdSe。然而，所研究的一般机制将导致对新材料的研究，例如其他二元半导体，如PbSe或金属纳米结构，如银或铅。该研究项目提供物理、化学和材料科学领域的高级研究生培训，特别是超高真空表面科学和材料表征，包括高级光源和高级光子源的同步加速器研究。

Nontechnical Abstract:Although semiconductor nanocrystals (NCs) show great promise for applications in photovoltaics, solid state lighting, and nanoscale electronics, the success of the assembled devices will be determined primarily on how well an assembly of NCs can conduct electricity. The conventional experimental probes used to characterize these systems typically do not explore techniques that are either sensitive to the atomic species or that are performed in situ. It is imperative, therefore, to identify new experimental probes that can provide crucial information on NCs that cannot be achieved via other methods. This project, supported by the Solid State and Materials Chemistry program at NSF, employs state-of-the-art x-ray characterization methods to elucidate a wide range of unanswered problems in the NC literature, including the mechanism of cation exchange in NCs and effects of ligand chemistry on charge carrier dynamics in NCs. The project focuses on both operando/in situ soft and hard x-ray absorption spectroscopy and well as ultra-fast x-ray absorption spectroscopy. These techniques, for the most part, have not been exploited to probe the chemistry and physics of semiconductor NCs and it is anticipated that they will provide crucial insight to answer important questions regarding the bonding and charge transfer dynamics in these materials. In turn, the results from this research proposal will have a strong impact on the ability to fabricate next generation nano-electronic materials. This research leverages the vast infrastructure within the Laboratory for Surface Science and Technology (LASST), an interdisciplinary center that brings together researchers from Physics, Chemistry, Electrical & Computer Engineering, and Chemical & Biological Engineering. This research project provides advanced graduate training in the fields of physics, chemistry, and materials science, specifically in ultra-high vacuum surface science and materials characterization including synchrotron research at the Advanced Light Source and Advanced Photon Source. In addition, the research laboratory, which is housed in LASST, is on display via tours and will introduce many underprivileged students, including a substantial Native American population, to cutting edge instrumental tools. Technical Abstract:This project, supported by the Solid State and Materials Chemistry program at NSF, provides an unprecedented look into the fundamental mechanisms behind processes such as charge transport and cation exchange in semiconductor nanocrystals (NCs). The research activities focus on both operando/in situ soft and hard x-ray absorption spectroscopy and well as ultra-fast x-ray absorption spectroscopy. These techniques, for the most part, have not been exploited to probe the chemistry and physics of semiconductor NCs and it is anticipated that they will provide crucial insight to answer important questions regarding the bonding and charge transfer dynamics in these materials. In turn, the results from this research proposal will have a strong impact on the ability to fabricate next generation nano-electronic materials. The transformative feature of this work is the use of cutting-edge x-ray techniques to probe NCs in their most common environment: solution phase. Armed with the knowledge that the interplay between controlling carrier densities and mobilities in NCs will play a strong role in making highly conductive NC devices, the project research objectives focus on two related research thrust areas: (a) in situ x-ray spectroscopy of NC materials undergoing cation exchange processes and (b) effects of surface ligands on the interfacial charge carrier dynamics in NCs as probed by ultrafast x-ray spectroscopy. Materials of systems of interest include CdSe as the archetypal nanomaterial. The general mechanisms that are investigated, however, will lead to the study of new materials, such as other binary semiconductors like PbSe or metallic nanostructures such as Ag or Pb. This research project provides advanced graduate training in the fields of physics, chemistry, and materials science, specifically in ultra-high vacuum surface science and materials characterization including synchrotron research at the Advanced Light Source and Advanced Photon Source.