![]() Intriguingly, one of these structural phases is semiconducting, whereas the others are metallic, unlike in the case of group V TMDs, where electrically activated metal-to-metal structural phase transitions have been demonstrated in multilayer TaS 2 (ref. This polymorphism sets group VI TMD monolayers apart from other 2D materials such as graphene and hexagonal BN 17, 18, 19. In an energy context, these materials hold promise as hydrogen evolution catalysts when certain features are exposed to the reacting environment 11, 12, 13, 14, 15, 16.Ī special but often overlooked feature of group VI TMD monolayers is that they have more than one possible 2D crystal structure. The mostly semiconducting 2 subset of TMDs where the transition metal M is Mo or W (both in group VI) has received the greatest amount of attention in the pursuit of applications, including ultrathin flexible electronics 2, 3, 4, 5, 6, 7, 8 and valleytronics 9, 10. Among the layered crystals amenable to isolation of atomically thin monolayers is a large family of transition metal dichalcogenides (TMDs) having the chemical formula MX 2, where M is some transition metal and X stands for S, Se or Te. The discovery of a mechanical exfoliation method 1 for two-dimensional (2D) crystals has led to the discovery of fundamentally new physics, and it was a watershed in the search for the materials that will take the centre stage in tomorrow’s technology. The potential for mechanical phase transitions is predicted for all six studied compounds. We identify a range from 0.3 to 3% for the tensile strains required to transform MoTe 2 under uniaxial conditions at room temperature. Based on state-of-the-art density functional and hybrid Hartree–Fock/density functional calculations including vibrational energy corrections, we discover that MoTe 2 is an excellent candidate phase change material. Here we discover that mechanical deformations provide a route to switching thermodynamic stability between a semiconducting and a metallic crystal structure in these monolayer materials. If controllable, such a transition could bring an exciting new application space to monolayer materials beyond graphene. Some of these monolayers exhibit tantalizing hints of a poorly understood structural metal-to-insulator transition with the possibility of long metastable lifetimes. Mo- and W-dichalcogenide compounds have a two-dimensional monolayer form that differs from graphene in an important respect: it can potentially have more than one crystal structure. ![]()
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