Semiconductor to metal transition in bilayer transition metals dichalcogenides MX2 (M = Mo, W; X = S, Se, Te)

We report by means of ab initio density functional theory based calculations that the semiconducting energy gap of bilayer transition metal dichalcogenides (TMDs) can be reduced by applying mechanical strains, tuning interlayer distance and applying an external electric field. Our results suggest that in-plane strains cause semiconductor to metal (S-M) transitions in bilayer sheets. These transitions, however, strongly depend on the types of applied strain. The energy gap of semiconducting TMDs gets reduced continuously by reducing the bilayer separation, eventually rendering them metallic at a critical value of interlayer distance. Electrically gated semiconducting bilayer TMDs are also found to show a reduction in the band gap when increasing the magnitude of the electric field to result in band gap closure at a critical value of the field. S-M transitions are also found to occur irrespective of the types of stacking between the two layers of bilayer TMDs. The possibility of tuning the energy gap in a controlled way over a wide range of energy makes TMDs potential candidates for tunable nanoelectronics.