Domain Architectures and Grain Boundaries in Chemical Vapor Deposited Highly Anisotropic ReS2 Monolayer Films.

Recent studies have shown that vapor phase synthesis of structurally isotropic two-dimensional (2D) MoS2 and WS2 produces well-defined domains with clean grain boundaries (GBs). This is anticipated to be vastly different for 2D anisotropic materials like ReS2 mainly due to large anisotropy in interfacial energy imposed by its distorted 1T crystal structure and formation of signature Re-chains along [010] b-axis direction. Here, we provide first insight on domain architecture on chemical vapor deposited (CVD) ReS2 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 ReS2, establish domain architecture of high symmetry ReS2 flakes, and determine Re-chain orientation within subdomains. Results also provide a first atomic resolution look at ReS2 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 b-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 ReS2 for the first time for tangible applications in photonics and optoelectronics where direction-dependent dichroic and linearly polarized material properties are required.