Structural and electronic phase transitions in ferromagnetic monolayer VS$_2$ induced by charge doping

Among the known transition metal dichalcogenides (TMDs), monolayer VS$_2$ has attracted particular interest because of its intrinsic ferromagnetism and promising applications as a high-performance functional nonomaterial. Here, using first-principles calculations, we study the electronic and structural phase transitions in the monolayer VS$_2$ induced by charge doping. At the ground state, monolayer VS$_2$ is stabilized in the 2H phase, being a bipolar magnetic semiconductor (BMS) whose valence and conduction states near the Fermi level carry opposite spin polarization. With the increase of hole doping concentration, VS$_2$ will first experience an electronic phase transition from a BMS to a half metal, followed by a 2H-to-1T structural phase transition which concomitantly results in another electronic phase transition from the half metal to a normal metal. However, the electron doping can only induce the BMS-to-half metal electronic transition but will not trigger the structural phase transition. The completely different effects of electron and hole doping on the structural phase transition can be well explained by the energy band diagrams of VS$_2$. These results clearly establish the potential for VS$_2$ utilization in innovative phase-change electronic and spintronic devices.