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.