CAREER: Mapping and Manipulating Lattice Relaxation in Moire Superlattices of Group VI Transition Metal Dichalcogenides

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Non-technical description:Structural distortions have a significant impact on the properties of atomically thin materials. Understanding how two-dimensional (2D) lattices deform on an atomic scale and how these deformation processes can be manipulated is essential for tailoring their behavior in next-generation device technologies and developing material systems with new functionalities. Moiré superlattices, comprised of vertically stacked 2D sheets with a small rotational offset or lattice mismatch, are a class of 2D structures in which natural lattice relaxation processes and resulting strain are closely linked to changes in observed optical, electronic, and photonic properties. This project aims to elucidate the structural mechanisms driving moiré superlattice relaxation and to investigate how relaxed moiré architectures and their emergent physics can be precisely modified by external stimuli, such as an electric field or mechanical force. These research efforts are integrated with education and outreach initiatives that seek to broaden participation in STEM education and scientific research, including the expansion of funded research opportunities for undergraduate transfer students at the University of California at Berkeley and the development of scientific discussion sessions for incarcerated students at Mount Tamalpais College at San Quentin State Prison.Technical description:The unique, tunable electronic band structures and resultant properties of two-dimensional moiré superlattices are highly sensitive to intrinsic structural relaxation processes and corresponding accumulation of intralayer strain. Precise structural characterization of these materials and thorough understanding of their relaxation mechanisms are therefore critical to harnessing their potential in novel (opto)electronic device platforms. Efforts to probe the structure of moiré materials have previously been complicated by the fact that the layers of interest are often buried within complex multi-component heterostructures, as required for device fabrication. As such, existing descriptions of lattice relaxation are largely qualitative and mechanistic pictures are based purely on simulations. To address this challenge, the research aims in this CAREER project utilize interferometric four-dimensional scanning transmission electron microscopy (4D-STEM), a diffraction-based imaging methodology developed by the PI’s research group specifically for measuring mechanical deformations and strain in moiré structures, including those in typical device architectures. The primary goals of this work are (1) to quantitatively map out mechanical deformations that govern relaxation in moiré bilayers composed of semiconducting group VI transition metal dichalcogenides (TMDs) and (2) to perform operando measurements on the perturbation of relaxed TMD moirés and their intrinsic strain fields in the presence of an external electric field or uniaxial mechanical strain. A combination of photoluminescence spectroscopy, electronic transport measurements, and theoretical calculations supplement the imaging experiments to correlate the observed structures with emergent optical and electronic properties. This work deepens the understanding of fundamental structure–property relationships in TMD moiré superlattices and provides a framework for leveraging structural distortions and strain as tuning knobs for modifying the (opto)electronic behavior of these systems.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.