Synthesis and Characterization of Two-Dimensional Materials

Abstract

Since the isolation and discovery of the electronic properties of graphene in 2004, two-dimensional (2D) materials have emerged as a new class of materials that can enable next-generation applications. Hexagonal boron nitride (h-BN), has the potential to make transformative advances in ultra-high permeance, size selective membranes for applications such as hydrocarbon separations, proton exchange membranes, and even hydrogen isotope separations, however the generation of nanometer scale pores in the material remains a challenge due to h-BN’s ceramic material properties. In this dissertation, a thorough literature analysis is performed on all existing methods to synthesize h-BN, ranging from flux-based synthesis of high-quality bulk h-BN, analogous to graphite, to the synthesis of monolayer and few-layered films. Monolayer h-BN synthesis via chemical vapor deposition (CVD) on copper is explored as route to make atomically-thin membranes with ceramic properties, useful for separations in harsh thermal and chemical environments. Size-selective membrane performance is found to vary with the CVD growth temperature, offering a bottom-up method to integrate nanopores in the material. The mechanism of h-BN synthesis on iron is then investigated, where the co-solubility of boron and nitrogen into the catalyst foil significantly alters reaction kinetics. Insights into the unique reaction mechanism are presented. Using machine-learning, an ideal parameter space for growing monolayer h-BN is identified. The separation of hydrogen isotopes is then demonstrated on CVD-grown h-BN, and we find that h-BN grown on iron allows for comparable separation performance as pristine exfoliated h-BN, whereas h-BN synthesized on Cu exhibits significantly less selectivity due to intrinsic nanopores in the material. Membrane-based defect characterization is also performed on monolayer MoS2, enabling centimeter-scale analysis of sub-nm to nanometer vacancy defects in the material. The bottom-up generation of carbon defects in the h-BN lattice is demonstrated during the CVD process, ultimately useful as another method of pore creation, or intentional carbon doping towards generating single-photon emitters. Finally, a novel and scalable synthesis process is demonstrated to grow flux-quality h-BN on a planer substrate with deterministic thickness control, circumventing the challenges associated with flake exfoliation methods.