Excellence in Research: Dynamics of Photogenerated Charge Carriers in Mixed Dimensional van der Waals Heterostructures

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Nontechnical Description: Atomically thin two-dimensional semiconductor nanostructures are an intriguing alternative to bulk semiconductors that underpin much of current optical and electronic technology. These nanostructures, which include graphene and transition metal dichalcogenides, exhibit weaker bonding in the out-of-plane direction and can thus be integrated with a broad range of organic and inorganic semiconductors without the materials compatibility requirements limiting conventional semiconductor devices. Mixed dimensional systems combining two dimensional nanostructures with zero- and one-dimensional nanostructures offer new degrees of freedom to engineer devices by tailoring the properties of each component. Significant questions remain, however, regarding the movement of charge carriers within and between the different types of nanostructures. This research is using multiple experimental tools to elucidate the properties of excited charge carriers in mixed dimensional systems. This project provides opportunities for undergraduate and graduate student researchers drawn from the predominantly underrepresented minority student population of North Carolina Central University and expands institutional research capabilities. Materials and research methods studied as part of this project are infused into undergraduate and graduate physics courses, introducing nanoscience and materials science to a broader group of students. Technical Description: The scientific goal of this project is to improve the fundamental understanding of carrier dynamics and charge transport in mixed-dimensional van der Waals heterostructures integrating two-dimensional transition metal dichalcogenides (MoS2, WS2) with one-dimensional inorganic metal chalcogenide (CdSxSe1-x) semiconductor nanowires and zero-dimensional organic semiconductors (pentacene, zinc phthalocyanine). The local morphologies and band energies of the zero- and one-dimensional components is expected to significantly impact the dynamics of photogenerated charge carriers in these heterostructures. The experimental approach of this project focuses on fabrication of heterostructures with controlled morphologies and band offsets, and characterization of these heterostructures using correlated ultrafast optical microscopy, Kelvin probe microscopy, and time resolved millimeter wave photoconductivity measurements to track the evolution of photoexcited charge carriers across a wide range of time and length scales. These measurements address multiple steps that occur during relaxation of charge carriers, including exciton dissociation, charge transfer across the interface, lifetimes of charge transfer states and carrier diffusion through comparison of data from heterostructures and individual components, and shed light on the influence of nano- or micro-scale morphology through spatially resolved mapping.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.