First-Principles Exploration of Two-Dimensional Transition Metal Dichalcogenides Based on Fe, Co, Ni, and Cu Groups and Their van der Waals Heterostructures

Abstract

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 MQ2 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.