Influence of chalcogen composition on the structural transition and on the electronic and optical properties of the monolayer titanium trichalcogenide ordered alloys

Based on first-principles spin-polarized density functional theory, we investigate the effects of chalcogen composition on the structural, electronic, and optical properties of monolayer (where X and X′ = S, Se, Te) ordered alloys with values of x of 0, 0.167, 0.333, 0.500, 0.667, 0.833, and 1. We determine the optimized geometry for all possible substitutional adsorptions of chalcogen atoms for each x composition, and identify the energetically most stable allotropes as a function of composition. Our extensive analysis reveals that the structural and electronic properties depend on the chemical composition of the monolayers, and the band gap of TiX₃ nanosheets can be tuned by adjusting the ratio of chalcogen compositions. While substitutional doping of tellurium atoms into TiS₃ or TiSe₃ monolayers results in a semiconductor-metal transition, the alloys remain a semiconductor under the transition from TiS₃ to TiSe₃ with band gaps which are very suitable for optical devices and infrared detectors. We also find that each TiS_3(1-x)Se_3x structure has an anisotropic dielectric function. Because of the anisotropy of the dielectric function, they can be useful materials for application in the transition metal trichalcogenide-based nanoelectronics industry in the future.