Atomistic Control over Functional Defects in van der Waals Nanostructures

Part 1: Non-Technical Summary Imperfections (defects) in materials have traditionally been seen as undesirable, especially in electronic materials such as semiconductors. Hence, efforts in science and engineering have focused on eliminating defects in materials for technological applications. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, Profs. Eli and Peter Sutter and their groups at the University of Nebraska pursue a fundamentally different approach, namely to (i) identify possible mechanisms by which specific defects can be generated during materials synthesis; (ii) determine ways of controlling the placement and type of the imperfection in its host crystal; and (iii) uncover the properties emerging from such controlled defects. To realize this vision, the project focuses on a particular class of materials, layered crystals consisting of atomically thin sheets held together by weak van der Waals forces, as well as a "vapor-liquid-solid" synthesis process that promises manifold opportunities for manipulating single defects. If successful, the research could fundamentally change our view of defects in electronic materials, with potentially transformative impact in basic science as well as key sectors of technology. In addition to pursuing these technical objectives, the project provides far-reaching opportunities for training and career development to the participating students. And it aims to help enhance education, diversity, and the participation of underrepresented groups – in particular girls from rural and Native American tribal communities – in Science, Technology, Engineering, and Math (STEM) by organizing annual summer workshops for student/teacher teams from rural Nebraska communities and by developing microprocessor-based teaching aids for use in middle- and high school science classrooms. Part 2: Technical Summary Defects have traditionally been perceived as detrimental (and thus undesirable) in electronic and functional materials. Recent research showed that point defects may provide important new functionality, e.g., for quantum information processing. Extended defects could similarly harbor emerging properties of interest for technology, but identifying and ultimately harnessing functionality from extended defects faces major challenges, including the development of synthesis protocols with innate control over defect placement, orientation, and configuration; and the positioning of tailored single defects in a small host volume so that the defect dominates the overall properties. This project supported by the Solid State and Materials Chemistry program in the NSF’s Division of Materials Research addresses these challenges by taking advantage of opportunities presented by the reduced symmetry of van der Waals crystals and by developing vapor-liquid-solid nanostructure growth processes to obtain a fundamental understanding of extended defect formation and tuning combined with measurements of emerging properties such as electronic structure, optoelectronics, charge transport, and ferroelectricity. Specifically, the research will examine the materials chemistry underlying the formation of defects such as dislocations and stacking faults in nanostructures of layered monochalcogenide semiconductors, their alloys, as well as axial and radial heterostructures. And it aims to identify approaches for transcribing tailored defects into a wide range of other materials, including other layered crystals and conventional 3D-crystalline semiconductors. This will set the stage for systematically exploring the emerging properties of tailored individual line and planar defects in a wide range of nanostructured materials. The successful realization of the project goals could pave the way for a paradigm shift toward the pursuit of crystalline materials where new functionality emerges from single defects with controlled properties. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.