Tailoring the Properties of Heterostructures of Monolayers: Epitaxial Growth and Doping

West Virginia University Research Corporation, Morgantown WV

Investigators

Abstract

Nontechnical Description: Heterostructures of semiconductor materials, which provide enhanced electrical and optical characteristics well beyond that of each individual constituent material, have been the key enabling element in modern telecommunication, energy-efficient displays, and energy harvesting technologies. This project explores a new approach to synthesize heterostructures made of sheets of two-dimensional materials, in which atoms within a sheet form strong bonds but interactions between the layers are very weak. Atomic-resolution imaging and calculations are carried out to understand how this characteristic anisotropic bonding facilitates the growth and assembling of the single atomic layers similar to LEGO blocks, thus allowing the design and synthesis of new materials with tailored properties and functionalities beyond the limits of materials that currently exist in nature. This project provides graduate and undergraduate students interdisciplinary training in areas of materials synthesis and characterization at the atomic scale, as well as electronic structure calculations. The open source distribution of electronic structure codes continues to provide the scientific community the benefits of our team's code development efforts. An ongoing Research Experience for Teachers outreach program brings cutting-edge research on two-dimensional materials to high-school students to inspire their interest in science and engineering. Technical Description: This project aims to gain an atomic scale understanding of 1) the inherent grain boundary formation during the van der Waals (vdW) epitaxial growth of two-dimensional transition-metal dichalcogenides, and 2) the doping and bandgap engineering of their heterostructures. Leveraging the capability of integrating molecular beam epitaxy, scanning tunneling microscopy (STM) and atomic force microscopy (AFM), the intrinsic materials properties such as band offsets across lateral junctions and work functions are determined by in-situ tunneling spectroscopy and force-bias spectroscopy, respectively. Layer thickness, doping, and band gaps are further accessed by ex-situ Raman spectroscopy and photoluminescence. The information obtained by these spatially averaged spectroscopic measurements is correlated with atomic scale structural information such as the alignment of vertical junctions, interface intermixing, local strain, and disorder obtained by in-situ atomic resolution STM/AFM imaging, as well as ex-situ transmission electron microscopy. Density functional theory calculations are tightly coupled to the experimental systems of interest, addressing issues related to structural properties (e.g., energetics of the modified vdW growth and the impact of defects and impurities) and electronic properties (interface states, band offsets, charging effects at grain boundaries, and work functions). This integrated approach enables the controlled growth, bandgap engineering, and characterization of heterostructures of monolayers at the atomic scale, all necessary steps towards tailoring their electronic and optical properties.