Transition from an indirect type-I to a direct type-II bandgap in α-tellurene/Ca(OH)2 heterostructures with excellent optical properties

Abstract

Exploring van der Waals heterostructures (vdWHs) based on two-dimensional (2D) materials is of importance for designing new materials or tuning the properties of existing materials. By virtue of first-principles calculations, we propose a type-I vertical 2D α-tellurene/Ca(OH)₂ vdWH with an indirect bandgap of 1.10 eV, in which both the CBM and VBM originate completely from tellurene. In addition, we also show that such a vdWH presents excellent optical absorption and an unexpectedly hyperbolic nature, which is mainly determined by tellurene. Hyperbolic materials play a central role in nanophotonics. In tellurene and Te-based materials, we for the first time report this natural hyperbolic property, which was previously unknown. More importantly, under in-plane biaxial strain or vertical strain, the α-tellurene/Ca(OH)₂ bilayer transforms into a direct bandgap type-II vdWH, in which both the CBM and VBM located at the Γ point are contributed by the tellurene and Ca(OH)₂ monolayers, respectively. Meanwhile, the application of strain can also be used to tune the optical absorption and the frequency range of the hyperbolic property. Our findings extend the applications of tellurene and Te-based vdWHs in nanodevices and provide some guidance for investigating the related vdWHs.