Predicting Single-Layer Technetium Dichalcogenides (TcX₂, X = S, Se) with Promising Applications in Photovoltaics and Photocatalysis

One of the least known compounds among transition metal dichalcogenides (TMDCs) is the layered triclinic technetium dichalcogenides (TcX₂, X = S, Se). In this work, we systematically study the structural, mechanical, electronic, and optical properties of TcS₂ and TcSe₂ monolayers based on density functional theory (DFT). We find that TcS₂ and TcSe₂ can be easily exfoliated in a monolayer form because their formation and cleavage energy are analogous to those of other experimentally realized TMDCs monolayer. By using a hybrid DFT functional, the TcS₂ and TcSe₂ monolayers are calculated to be indirect semiconductors with band gaps of 1.91 and 1.69 eV, respectively. However, bilayer TcS₂ exhibits direct-bandgap character, and both TcS₂ and TcSe₂ monolayers can be tuned from semiconductor to metal under effective tensile/compressive strains. Calculations of visible light absorption indicate that 2D TcS₂ and TcSe₂ generally possess better capability of harvesting sunlight compared to single-layer MoS₂ and ReSe₂, implying their potential as excellent light-absorbers. Most interestingly, we have discovered that the TcSe₂ monolayer is an excellent photocatalyst for splitting water into hydrogen due to the perfect fit of band edge positions with respect to the water reduction and oxidation potentials. Our predictions expand the two-dimensional (2D) family of TMDCs, and the remarkable electronic/optical properties of monolayer TcS₂ and TcSe₂ will place them among the most promising 2D TMDCs for renewable energy application in the future.