Two dimensional (2D) transition metal dichalcogenides (TMDs), compounds with chemical composition MQ, where M is a transition metal, and Q = S, Se, Te, are promising materials for applications in photovoltaic devices, mainly due to their tunable band gaps, strong light-matter interaction, and the potential to design van der Waals (vdW) heterostructures through the stacking of different layers. (1) Therefore, a comprehensive understanding of the electronic properties of these materials and their heterostructures, such as band gaps and band alignments (2), encompassing their wide range of compositions, is an important step for the development of novel heterojunctions based on 2D TMDs for solar energy harvesting. TMDs from Ti-, V-, and Cr-groups are commonly obtained in layered crystal structures and therefore the properties of 2D materials based on these compounds have been widely explored. (3) However, diverse structural phases are known for TMDs based on the remaining transition metals, and the investigation of the physical and chemical properties of 2D TMDs from Fe-, Co-, Ni-, and Cu-groups has been restricted. Therefore, in order to contribute to the understanding of their electronic properties, we performed an investigation of 36 MQ compositions (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au; Q = S, Se, Te), by means of density functional theory calculations. We employed semi-local (GGA-PBE) and hybrid (HSE06) exchange-correlation functionals, using the vdW corrections of the D3 formulation, and our simulations were performed with the Vienna Ab initio Simulation Package (VASP). For each composition, lowest energy crystal structures were selected among a set of layered phases and a set of and non-layered phases, both resulting from 11 crystal structures previously experimentally reported for TMDs. The electronic band gaps of the selected crystal structures in their equilibrium geometries were screened, and 17 semiconductors were identified among the monolayers, all from the Fe- and Ni-groups. The conduction and valence band offsets of these monolayers were obtained, and we found that the trends can be explained based on the changes on the M-d and Q-p energy levels and on the variations of bond lengths. We applied the Anderson rule to classify the heterojunctions formed with the semiconductor monolayers, which mainly fall in the type-II classification, and type-I junctions are more common when Ni-group monolayers are used. To further screen the potential of these 2D TMDs for photovoltaics applications, the power conversion efficiencies of solar cells based on the identified type-II heterostructures were estimated, and we obtained values that are comparable to and higher than those that can be estimated for the widely studied junctions of Mo- and W-based TMDs. R. Besse acknowledges financial support from Grant 2017/09077-7, São Paulo Research Foundation (FAPESP).
