Nanowires from Layered van der Waals Crystals: Opportunities for Tuning Structure and Function in 1D-2D Hybrid Nanostructures

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Part 1: Non-Technical SummarySemiconductor nanowires are of technological interest as a class of functional components for electronics, light emission, energy conversion, or quantum computation, which can be mass-produced with exceptionally high crystal quality by simple growth processes. To date, research has almost exclusively focused on nanowires of traditional semiconductor materials, such as silicon or germanium, in which the atoms are tightly bonded to their neighbors in a three-dimensional crystal. This project, which is supported by the Solid State and Materials Chemistry program at NSF, explores opportunities in tuning structure and properties that arise in nanowires of layered semiconductors, in which atoms are tightly bonded in atomically thin sheets that are in turn held together by much weaker forces. Coordinated efforts in studying the synthesis of such layered nanowires and in exploring their distinct electronic and optoelectronic properties aim to uncover relationships between crystal structure, electronic structure and the flow of electrical current, as well as light absorption and emission, which are import for technological applications. In addition to the targeted technical advances, the project provides far-reaching educational and training opportunities for the involved graduate and undergraduate students. It includes dedicated outreach to high-school students from rural areas that gives student-teacher teams the opportunity to participate in hands-on activities related to the research. Under the guidance of the researchers the students prepare new science learning materials, which they bring back to their school to share their experience with their peers and to help build excitement for careers in STEM disciplines.Part 2: Technical SummaryExtensive research on planar 2D and layered chalcogenide semiconductors has been driven by outstanding electronic, optoelectronic, mechanical and chemical properties of these materials. In contrast, little attention has been paid to the possibility of combining the concepts of vapor-liquid-solid (VLS) nanowire growth and van der Waals (vdW) epitaxy to realize layered chalcogenide nanowires. The few existing studies have treated those systems on an equal footing with conventional 3D-crystalline nanowires without addressing the opportunities for materials design that arise in vdW crystals. This project, which is supported by the Solid State and Materials Chemistry program at NSF, explores the unique crystallographic degrees of freedom of vdW nanowires and their control via tailored VLS growth processes. Examples of distinct properties include layering along different directions, facile materials integration due to lifted lattice matching requirements, and a tendency toward spontaneous interlayer twist that adds up to an overall chirality and whose magnitude is controlled by the wire diameter. Coordinated growth and characterization of selected materials with focus on anisotropic metal monochalcogenide semiconductors provide insight into the atomistic growth mechanisms of vdW nanowires and heterostructures. Structure and morphology are correlated to single-nanowire charge transport, and to optoelectronic properties probed by cathodoluminescence spectroscopy at resolution below the exciton radius to address the role of layer stacking and twist, confinement, excitonic effects, as well as interfacial carrier transfers in heterostructures.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.