Indirect-to-Direct Band Gap Crossover in Few-Layer Transition Metal Dichalcogenides: A Theoretical Prediction

Layered transition metal dichalcogenides (TMDs) have been found to exhibit the indirect-to-direct band gap transition when exfoliated from bulk to a single monolayer. Through first-principles calculations, we predict that such a transition can happen at bilayer for 2H-WSe2 and at tetralayer for 2H-WTe2. We find that the transition can be ascribed to the competition between spin-orbit coupling and interlayer coupling interactions, the former leading to appreciable splittings at the K point and the latter to splittings at the Gamma point of the valence band. It is shown that stronger spin-orbit coupling tends to favor transition at a larger number of layers. These results provide insights into the valley degeneracy of the band edges and the valley-dependent optical transitions in few-layer TMDs for quantum control in valley-electronics.