In this work, we report an ab initio investigation based on density functional theory of the structural, energetic and electronic properties of 2D layered chalcogenides compounds based in the combination of the transition-metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and chalcogenides (S, Se, Te) in three polymorphic phases: trigonal prismatic (2H), octahedral (1T) and distorted octahedral (1T d ). We determined the most stable phases for each compound, verifying the existence of the 1T d phase for a small number of the compounds and we have also identified the magnetic compounds. In addition, with the determination of the exfoliation energies, we indicated the potential candidates to form one layer material and we have also found a relation between the exfoliation energy and the effective Bader charge in the metal, suggesting that when the materials present small exfoliation energy, it is due to the Coulomb repulsion between the chalcogen planes. Finally, we analyzed the electronic properties, identifying the semiconductor, semimetal and metal materials and predicting the band gap of the semiconductors. In our results, the dependence of the band gap on the d-orbital is explicit. In conclusion, we have investigated the properties of stable and metastable phases for a large set of TMD materials, and our findings may be auxiliary in the synthesis of metastable phases and in the development of new TMDs applications. arXiv:1903.08112v1 [cond-mat.mtrl-sci]

Transition metal dichalcogenides (TMDs) offer a platform for obtaining two-dimensional materials with excellent properties for diverse applications. However, the exploration of the properties of two-dimensional TMDs based on transition metals from Fe, Co, Ni, and Cu groups is scarce. Therefore, to contribute to the understanding of these materials, we performed a density functional theory investigation of 36 MQ 2 compounds (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Ag, Au; Q = S, Se, Te), employing for each of them layered and nonlayered crystal structural phases previously reported for TMDs. We found that layered crystal structures are energetically favored for Ni group compounds and that the intralayer octahedral coordination has lower energy than the trigonal prismatic phase for all compositions. The layered phases Fe and Ni group compounds have weak interlayer binding dominated by van der Waals interactions, whereas the remaining materials have high exfoliation energies. We identified 17 semiconductor monolayers among the lowest energy layered phases, with band gaps that vary from 0.45 to 2.62 eV, and their valence and conduction band offsets are mainly determined by the positions of M d-states and Q p-states, which contribute both to the valence and conduction edge states. Semiconductor heterojunctions that can be formed with the stacking of monolayers were mostly classified into type-II band alignments, whereas type-I heterojunctions are more likely formed with Ni group TMDs. Estimates for the power conversion efficiency of solar cells based on the type-II heterojunctions resulted in 10 systems with efficiency > 15%, suggesting potential application in photovoltaic devices. This study unveils the understanding of the properties of TMDs of the groups 8–11, paving the way for the design of their van der Waals heterostructures.

Vertical stacking of two-dimensional materials with weak van der Waals (vdW) interactions has laid the ground for breakthroughs in physics as well as in technological applications. Although vdW interactions dominate interlayer binding, interlayer electronic coupling may not be negligible and can lead to properties beyond the superposition of constituent monolayers. Here, studying heterobilayers of transition-metal dichalcogenides (MQ 2; M = Mo, Ni, Pt; Q = S, Se) by means of density functional theory calculations, we show two mechanisms that influence the band gaps of vdW heterostructures beyond the Anderson rule: (1) interfacial hybridization (mainly involving out-of-plane states, such as chalcogen p z -states), which leads to an upshift in the valence band maxima and accordingly a decrease in the band gap. (2) Formation of an interfacial electric dipole, resulting in an effective gap increase in type-II junctions. While the former is material specific, depending on the proximity of p z -states to each other and the valence band maxima, the latter can be generally described using a model based on the charge density decay outside the monolayers and the pristine band edge positions with respect to the vacuum level, irrespective of junction type.