Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides and electronic properties in their heterostructures.

Transition metal dichalcogenides have generated an immense amount of interest and recent work for applications in optoelectronics, sensing, etc... Despite the number of studies, the diversity of their chemical bonding characteristics and charge transfer is not well understood. Using density functional (perturbation) theory we determine and compare their static (Bader) and dynamic (Born) charges. The dynamic charge of the transition metal dichalcogenides with trigonal symmetry are anomalously large, while in their hexagonally symmetric counterparts, we find a counterintuitive sign, i.e., the transition metal takes a negative charge. This phenomenon, observed in a few compounds in the past, but never analyzed, is understood by investigating the perturbative response of the system, and by investigating the hybridization of the molecular orbitals near the Fermi level. The charges and electronic properties of heterostructures composed of symmetry-equivalent Dichalcogenides are also investigated, showing competing effects due to strain and interlayer polarization.