Band structure tuning and charge separation of MNX monolayers and MNX/GaS van der Waals heterostructures

Abstract

Efficient charge separation and band structure tuning are very important for application to photoelectric device. Here, based on first-principles calculations, we propose two-dimensional (2D) MNX (M = Zr, Hf; X = Cl, Br, I) monolayers which can be exfoliated from the corresponding layered bulk phase due to the low cleavage energy. The phonon band structure and the mechanical analysis indicate that 2D MNX monolayers can form free-standing membranes. The calculated results suggest that 2D MNX monolayers are indirect band gap semiconductors with band gaps in the range of 1.55–3.37 eV. Among them, MNI (M = Zr, Hf) monolayers with effective charge separation and moderate band gaps would have potential application to photocatalytic water splitting. In order to realize the effective charge separation for the other MNX monolayers, MNX/GaS (X = Cl, Br) heterostructures are investigated. Our calculations reveal that MNX/GaS heterostructures are typical type-II band alignment, facilitating the separation of photogenerated carriers where electrons and holes are localized in MNX and GaS monolayers, respectively. Furthermore, the band gaps of the MNX/GaS heterostructures are obviously narrowed compared to those of the isolated constituent monolayers. In addition, the band gaps and band edge positions of MNX/GaS heterostructures can further be tuned by biaxial strain to match better with the redox potentials of water. These findings in this study not only enrich the family of 2D materials, but also demonstrate that MNI (M = Zr, Hf) monolayers and MNX/GaS heterostructures are promising candidates for photocatalytic materials.