Semiconductor Belts, Sheets, and Wires Having Idealized Optical and Transport Properties

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William E. Buhro is supported by the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry to perform a thorough experimental study of the synthesis and optical properties of 1D and 2D colloidal semiconductor nanocrystals optimized for long-range charge and energy (exciton) transport. An exciting aspect of 1D and 2D nanocrystals having extended length dimensions, such as quantum wires, quantum belts, nanoribbons, and nanosheets, is their potential for transporting energy and charge over lengths approaching the millimeter scale. As transport is intimately involved in nanoelectronics, nanophotonics, and solar-energy conversion, large potential advantages exist for the use of 1D and 2D nanocrystals in such applications. The semiconductor 1D and 2D nanocrystal fields are now poised for rapid development. As a result of recent advances in surface passivation, some resulting from the prior NSF funding to the PI, quantum wires and nanowires are now available with excellent passivation and optical properties, and further rapid advances appear imminent. The 2D colloidal nanocrystal field is in its infancy, surface passivation and optical properties are initially good, and the field is developing at a very fast pace. A catalog is being constructed of 1D and 2D nanocrystals capable of efficient long-range transport as well as corresponding synthetic and structural strategies. This work strives to solve a long-standing problem with the currently available 1D colloidal nanowires, which are poorly passivated and have correspondingly poor transport properties. The specific goals of the proposed work are to (1) advance core-shell strategies for optimizing the optical properties and photoluminescence efficiencies of semiconductor quantum wires, by oxidative substitution, and by the new colloidal atomic-layer-deposition (c-ALD) method, (2) isolate and further characterize magic-size II-VI nanoclusters to provide experimental structural and spectroscopic data, (3) investigate magic-size nanoclusters as low-temperature nanocrystal precursors, (4) develop methods for gaining control over quantum-belt lengths, widths, and thicknesses, and (5) explore the synthesis and spectroscopic properties of quantum platelets and sheets of II-VI, IV-VI, and I-III-VI semiconductors.There is considerable interest in incorporating semiconductor nanocrystals into next-generation devices for solar-energy conversion. Solar cells constructed from semiconductor nanostructures are expected to be fabricated more economically than the traditional silicon-based devices, and to have other application advantages. A solar cell functions by capturing light energy and converting it to energetic positive and negative electric charges, which are then separated and transported to opposite electrodes in the cell. This provides electrical energy for charging a battery or operating an electrical appliance. The critical steps are thus the efficient separation of the positive and negative charges and the efficient transport of those charges to the electrodes. 1D and 2D semiconductor nanocrystals are targeted for use in new solar-cell designs because they can in principle transport energy and charge over long distances. However, efficient transport requires that charges not be trapped at defect sites in the nanocrystals. This project is aiming to identify and eliminate those trap-site defects, enabling the application of 1D and 2D semiconductor nanocrystals in solar cells, nanoelectronics, and in small-scale devices for light detection and generation. The broader impacts include technological advances to assist in addressing the nation's energy challenge. The PI also has an excellent record of training women and members of underrepresented groups, thereby increasing the diversity of the nation's technological work force. The PI is co-leading an effort at Washington University to increase the retention of undergraduate women in science, technology, engineering, and math (STEM) fields.