Electronic phase transitions in transition metal dichalcogenides

Transition metal dichalcogenides are two-dimensional materials that have attracted attention in recent years for their applications in electronics, spintronics, and optoelectronics. These materials are characterized by their potential to exhibit electronic phase transitions as a result of intrinsic and extrinsic effects. The aim of this work is to study the electronic phase transitions in transition metal dichalcogenides using density functional theory. The first hypothesis of the research is that the band gap energy of the semiconductor VS_2 can be tuned by uniaxial and biaxial strains and that mechanical deformation can lead to a transition from the semiconductor phase to the metallic phase. The results of the study show that the band gap energy of VS_2 increases under both uniaxial and biaxial tensile strain, and the transition from semiconductor to metallic phase is achieved by tensile strain. The second hypothesis is that VSSe, which has not yet been produced experimentally, is a semiconductor in a monolayer structure and the band gap energy closes with the increase in the number of layers. The investigations revealed that VSSe is a stable and ferromagnetic semiconductor in a monolayer structure, the band gap energy decreases with increasing number of layers and it transitions from semiconductor to half-metallic phase. In the last part of the thesis, it is investigated how the band structure of semiconducting monolayer VSSe is affected by external electric field and internal magnetic ordering. The results indicate that the external electric field is able to modulate the band gap energy of VSSe and trigger the semiconductor-metallic phase transition when sufficient field is applied. Moreover, it is observed that the band gap energy decreases with the transition from ferromagnetic to antiferromagnetic order and there are significant changes in the band structure. An analysis of the spin-orbit coupling reveals that VSSe exhibits valley polarization. In conclusion, the results of this work provide important information for phase engineering, valleytronics and spintronics applications of VS_2 and VSSe.