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

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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.