Kinetics of Thin Film Growth on van der Waals Surfaces

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Non-technical description: Coatings with sub-micrometer thicknesses are commonly used in a wide variety of applications including but not limited to: automotive, biomedical, catalysis, defense, energy, electronic, and optoelectronic industries. Properties and life-time performance of materials used in these coatings are often dictated by the crystallinity of the materials. Traditionally, improving and/or controlling the crystallinity in coatings require processing at high-temperatures, use of energetic beams, and/or extended annealing for long durations. Atomically-thin sheets of materials such as graphite and hexagonal boron nitride (hBN, also known as white graphite), referred to as van der Waals (vdW) layers, can aid the synthesis of materials with better crystallinity. The goal of this project is to understand the mechanisms leading to enhancing crystallinity on vdW layers. One doctoral student is trained through this project and the research results are taught to undergraduate students in engineering and disseminated through conference presentations and technical publications. By organizing focused workshops at technical conferences, the principal investigator intends to promote awareness of (and find viable solutions to) the challenges faced by graduate students with families.Technical description:The aim of this project is to develop an atomic-scale understanding of the factors controlling the growth of thin solid films on vdW surfaces. Preliminary results obtained by the team suggest that thin films sputter-deposited on hBN layers exhibit better crystallinity than homoepitaxially grown thin films. To understand this intriguing result, it is hypothesized that adatom mobility on vdW surfaces is higher than that on conventional 3D solids and that there exist well-defined crystallographic orientation relations between the substrate, vdW layer, and the deposit. The project uses in situ variable-temperature scanning tunneling microscopy experiments to validate this hypothesis and to test the generality of these observations. Through time-resolved measurements of the thin film growth, the role of the vdW-layer on surface diffusivities and morphological evolution kinetics are quantitatively determined. This project is providing new insights into a potentially technologically important venue for synthesizing high-quality crystalline thin films for a variety of applications. This approach may be the most effective way to improve crystallinity of highly refractory compounds and thermally unstable materials for which increasing temperature has little or deleterious effect on the microstructure and composition of the material.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.