First-principles study of direct band gap semiconductors XS2 (X = Zr and Hf) with orthorhombic symmetry

Transition metal dichalcogenides have attracted intense interest due to their potential applications as electronic and optical devices. Here, we propose two XS2 (X = Zr and Hf) compounds within the orthorhombic Pnma phase, denoted as Pnma-ZrS2 and Pnma-HfS2 , as promising candidates for thin-film solar cells and nanoelectronics. Both Pnma-ZrS2 and Pnma-HfS2 are found to be semiconductors with desired direct band gaps of 1.07 and 1.24 eV, respectively, which lead to large visible-light absorption. The absorption coefficients of the first absorption peak for the Pnma-XS2 are about two times larger than the corresponding one for the P (3) over bar m1 phase with a 1T-CdI2 -like structure. Such Pnma-XS2 polymorphs are predicted to be mechanically, dynamically, and thermodynamically stable up to 500 K, and they are highly realizable in experiments since a phase transition from the P (3) over bar m1 phases to the Pnma phases could occur under moderate temperatures, based on our calculations.