Structural engineering brings new electronic properties to Janus ZrSSe and HfSSe monolayers

Interfacing effects within emergent two-dimensional (2D) materials are of fundamental interest and are at the center of applications in nanoelectronics. Thus, out-of-plane and in-plane heterostructures as well as electronic heterostructures with phase boundaries and large-angle (60 degrees) grain boundaries (GBs) of Janus ZrSSe and HfSSe are studied in this work using first-principles calculations. The out-of-plane heterostructures of T-ZrSSe and T-HfSSe illustrate quite weak interfacing interactions, thus the electronic properties are, unusually, more like the superposition of individual monolayers. The in-plane heterostructures of T-ZrSSe and T-HfSSe, interestingly, exhibit an indirect-direct band gap transition and type-II band alignment, which correspond to boosted optical properties and spatially separated excitons. For the in-plane electronic heterostructures that are constituted by T-ZrSSe and H-ZrSSe, semiconductor-metal crossover occurs due to the polar discontinuity across the T-H phase boundary, and they behave as one-dimensional metallic wires embedded in otherwise semiconducting Janus ZrSSe, creating a one-dimensional electron/hole gas. This also indicates a strategy for stabilizing the unstable and/or metastable monolayer via the phase boundary. As a result of the zero formal bulk polarization of the T-phase ZrSSe, the metallicity of 60 degrees GBs originates mainly from the edge atoms rather than from the polar discontinuity.