Domain Architectures and Grain Boundaries in Chemical Vapor Deposited Highly Anisotropic ReS<sub>2</sub> Monolayer Films

Recent studies have shown that vapor phase synthesis of structurally isotropic two-dimensional (2D) MoS<sub>2</sub> and WS<sub>2</sub> produces well-defined domains with clean grain boundaries (GBs). This is anticipated to be vastly different for 2D anisotropic materials like ReS<sub>2</sub> mainly due to large anisotropy in interfacial energy imposed by its distorted 1T crystal structure and formation of signature Re-chains along [010] <i>b</i>-axis direction. Here, we provide first insight on domain architecture on chemical vapor deposited (CVD) ReS<sub>2</sub> domains using high-resolution scanning transmission electron microscopy, angle-resolved nano-Raman spectroscopy, reflectivity, and atomic force microscopy measurements. Results provide ways to achieve crystalline anisotropy in CVD ReS<sub>2</sub>, establish domain architecture of high symmetry ReS<sub>2</sub> flakes, and determine Re-chain orientation within subdomains. Results also provide a first atomic resolution look at ReS<sub>2</sub> GBs, and surprisingly we find that cluster and vacancy defects, formed by collusion of Re-chains at the GBs, dramatically impact the crystal structure by changing the Re-chain direction and rotating Re-chains 180° along their <i>b-</i>axis. Overall results not only shed first light on domain architecture and structure of anisotropic 2D systems but also allow one to attain much desired crystalline anisotropy in CVD grown ReS<sub>2</sub> for the first time for tangible applications in photonics and optoelectronics where direction-dependent dichroic and linearly polarized material properties are required.