  We have performed a systematic first-principles study of the effect of
tensile strains on the electronic properties of early transition-metal
dichalcogenide (TMDC) monolayers MX2 (M = Sc, Ti, Zr, Hf, Ta, Cr; X = S, Se,
and Te). Our density-functional theory (DFT) calculations suggest that the
tensile strain can significantly affect the electronic properties of many early
TMDCs in general and the electronic bandgap in particular. For group IVB TMDCs
(TiX2, ZrX2, HfX2), the bandgap increases with the tensile strain, but for ZrX2
and HfX2 (X=S, Se), the bandgap starts to decrease at strain 6% to 8%. For the
group-VB TMDCs (TaX2), the tensile strain can either induce the ferromagnetism
or enhance the existing ferromagnetism. For the group-VIB TMDCs (CrX2) the
direct-to-indirect bandgap transition is seen upon application of the tensile
strain, except CrTe2 whose bandgap decreases with the tensile strain even
though the direct character of its bandgap is retained. Lastly, for the
group-IIIB TMDCs (ScX2) in the T metallic phase, we find that the tensile
strain has little effect on their electronic and magnetic properties. Our study
suggests that strain engineering is an effective approach to modify electronic
and magnetic properties of most early TMDC monolayers, thereby opening an
alternative way for future optoelectronic and spintronic applications.
