Prediction of Two-Dimensional Janus Transition-Metal Chalcogenides: Robust Ferromagnetic Semiconductor with High Curie Temperature.

Two-dimensional (2D) ferromagnetic semiconductors (FM SCs) provide an ideal platform for the development of quantum information technology in nanoscale devices. However, many developed 2D FM materials present a very low Curie temperature (T), greatly limiting their application in spintronic devices. In this work, we predict two stable 2D transition metal chalcogenides, VSeX (X = S, Te) monolayers, by using first-principles calculations. Our results show that the VSeTe monolayer is a robust bipolar magnetic SC with a moderate bandgap of 0.53 eV, while VSeS is a direct band-gap FM SC with a bandgap of 0.59 eV. Interestingly, the ferromagnetisms of both monolayers are robust due to the V-S/Se/Te-V superexchange interaction, and Ts are about 406 K and 301 K, respectively. Applying biaxial strains, the FM SC to antiferromagnetic (AFM) SC transition is revealed at 5% and 3% of biaxial tensile strain. In addition, their high mechanical, dynamical, and thermal stabilities are further verified by phonon dispersion calculations and ab initio molecular dynamics (AIMD) calculations. Their outstanding attributes render the VSeY (Y = S, Te) monolayers promising candidates as 2D FM SCs for a wide range of applications.