Material Genome Explorations and New Phases of Two-Dimensional MoS₂, WS₂, and ReS₂ Monolayers

Two-dimensional transition metal dichalcogenides have attracted intense interests in recent years. Existing studies have fully explored the properties of their ground-state structures, but their global energy landscapes are still not well understood. The global energy landscape is important for understanding the experimental synthesis and thermal dynamic properties as well as for discovering new phases. This work uses material genome techniques (huge global search and data mining) to explore the global energy landscapes and new phases of two-dimensional MoS₂, WS₂, and ReS₂ monolayers at ab initio level. Our results show that their energy landscapes have two or three major funnels, each of which consists of a few local minima with hexagonal, octahedral, quadrilateral, nanoribbon structures as well as their distorted and hybrid structures. The global-minimum structures are confined in deep funnels while the higher-energy minima stay in flat funnels and can transform to other minima at high temperatures. A few new phases with high geometry symmetry are found, e.g., quadrilateral phase, nanoribbon phase, distorted hexagonal phase, 4-2-coordination phase, and Pmm2 space group phase. These new phases exhibit novel phonon and electron properties (e.g., direct Γ point band gap and distorted Dirac cone), and could be the new candidates for devices applications. We further demonstrate that these phases could have better stability with charge doping and thus could be fabricated in experiments.