We perform a comprehensive first-principles study of the electronic properties of phosphorene nanoribbons, phosphorene nanotubes, multilayer phosphorene, and heterobilayers of phosphorene and two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayer. The tensile strain and electric-field effects on electronic properties of low-dimensional phosphorene nanostructures are also investigated. Our calculations show that zigzag phosphorene nanoribbons (z-PNRs) are metals, regardless of the ribbon width while armchair phosphorene nanoribbons (a-PNRs) are semiconductors with indirect bandgaps and the bandgaps are insensitive to variation of the ribbon width. We find that tensile compression (or expansion) strains can reduce (or increase) the bandgap of the a-PNRs while an in-plane electric field can significantly reduce the bandgap of aPNRs, leading to the semiconductor-to-metal transition beyond certain electric field. For single-walled phosphorene nanotubes (SW-PNTs), both armchair and zigzag nanotubes are semiconductors with direct bandgaps. With either tensile strains or transverse electric field, similar behavior of bandgap modulation can arise as that for a-PNRs. It is known that multilayer phosphorene sheets are semiconductors with their bandgaps decreasing with increasing the number of multilayers. In the presence of a vertical electric field, the 2 bandgaps of multilayer phosphorene sheets decrease with increasing the electric field, and the bandgap modulation is more significant with more layers. Lastly, heterobilayers of phosporene with a TMDC (MoS 2 or WS 2 ) monolayer are still semiconductors while their bandgaps can be reduced by applying a vertical electric field as well.

We have performed a comprehensive first-principles study of the electronic and magnetic properties of two-dimensional (2D) transition-metal dichalcogenide (TMD) heterobilayers MX2/MoS2 (M = Mo, Cr, W, Fe, V; X = S, Se). For M = Mo, Cr, W; X = S, Se, all heterobilayers show semiconducting characteristics with an indirect bandgap with the exception of the WSe2/MoS2 heterobilayer which retains the direct-bandgap character of the constituent monolayer. For M = Fe, V; X = S, Se, the MX2/MoS2 heterobilayers exhibit metallic characters. Particular attention of this study has been focused on engineering the bandgap of the TMD heterobilayer materials via application of either a tensile strain or an external electric field. We find that with increasing either the biaxial or uniaxial tensile strain, the MX2/MoS2 (M = Mo, Cr, W; X = S, Se) heterobilayers can undergo a semiconductor-to-metal transition. For the WSe2/MoS2 heterobilayer, a direct-to-indirect bandgap transition may occur beyond a critical biaxial or uniaxial strain. For M (=Fe, V) and X (=S, Se), the magnetic moments of both metal and chalcogen atoms are enhanced when the MX2/MoS2 heterobilayers are under a biaxial tensile strain. Moreover, the bandgap of MX2/MoS2 (M = Mo, Cr, W; X = S, Se) heterobilayers can be reduced by the vertical electric field. For two heterobilayers MSe2/MoS2 (M = Mo, Cr), PBE calculations suggest that the indirect-to-direct bandgap transition may occur under an external electric field. The transition is attributed to the enhanced spontaneous polarization. The tunable bandgaps in general and possible indirect-direct bandgap transitions due to tensile strain or external electric field make the TMD heterobilayer materials a viable candidate for optoelectronic applications.

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 MX 2 (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 (TiX 2 , ZrX 2 , HfX 2 ), the bandgap increases with the tensile strain, but for ZrX 2 and HfX 2 (X=S, Se), the bandgap starts to decrease at strain 6% to 8%. For the group-VB TMDCs (TaX 2 ), the tensile strain can either induce the ferromagnetism or enhance the existing ferromagnetism. For the group-VIB TMDCs (CrX 2 ) the direct-to-indirect bandgap transition is seen upon application of the tensile strain, except CrTe 2 whose bandgap decreases with the tensile strain even though the direct character of its bandgap is retained. Lastly, for the group-IIIB TMDCs (ScX 2 ) 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.