Quantum size effects in layered VX2 (X = S, Se) materials: Manifestation of metal to semimetal or semiconductor transition

Most of the two dimensional (2D) transition metal dichalcogenides (TMDC) are nonmagnetic in pristine form. However, 2D pristine VX2 (X = S, Se, Te) materials are found to be ferromagnetic. Using spin polarized density functional theory (DFT) calculations, we have studied the electronic, magnetic, and surface properties of this class of materials in both trigonal prismatic H- and octahedral T-phase. Our calculations reveal that they exhibit materially different properties in those two polymorphs. Most importantly, detailed investigation of electronic structure explored the quantum size effect in H-phase of these materials thereby leading to metal to semimetal (H-VS2) or semiconductor (H-VSe2) transition when downsizing from bilayer to corresponding monolayer.