Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H- M X 2 semiconductors ( M = Mo, W; X = S, Se, Te)

Using the first-principles calculations, we explore the electronic structures of 2H-$M{X}_{2}$ ($M$ $=$ Mo, W; $X$ $=$ S, Se, Te). When the number of layers reduces to a single layer, the indirect gap of bulk becomes a direct gap with larger gap and the band curvatures are found to lead to the drastic changes of effective masses. On the other hand, when the strain is applied on the single layer, the direct gap becomes an indirect gap and the effective masses vary. Especially, the tensile strain reduces the gap energy and effective masses while the compressive strain enhances them. Furthermore, the much larger tensile stress leads to become metallic.