Prediction of two-dimensional narrow-gap Janus TiOXY (X, Y = Cl, Br, I; X ≠ Y) monolayers for electronic and optoelectronic applications.

Two-dimensional (2D) transition metal oxyhalides have garnered significant interest due to their unique physical properties and promising potential applications. By using density functional theory and the non-equilibrium Green's function method, the anisotropic mechanical, electronic, optical, and transport properties of the Janus TiOXY (X, Y = Cl, Br, I; X ≠ Y) monolayers are systematically investigated. The proposed Janus TiOClBr, TiOBrI, and TiOClI monolayers exhibit favorable thermodynamic, dynamic, and mechanical stabilities, respectively. The Young's modulus of the Janus TiOClBr, TiOBrI, and TiOClI monolayers is 43.19-75.40 N m-1, 37.14-69.06 N m-1, and 40.11-66.26 N m-1, while their Poisson's ratios are 0.10-0.37, 0.12-0.38, and 0.11-0.34, respectively. The band gap of the semiconducting TiOClBr, TiOBrI, and TiOClI monolayer is 1.36 eV, 0.34 eV, and 0.46 eV, respectively. The Janus TiOClBr monolayer is found to be insensitive to the in-plane biaxial strain, while an indirect semiconductor-to-semimetal transition may be observed in the Janus TiOBrI and TiOClI monolayers under 4% and 4.9% compressive strain, respectively. The anisotropic optical properties as a function of photon energy of the Janus TiOClBr, TiOBrI, and TiOClI monolayers have been demonstrated. Remarkably, the constructed nanoelectronic devices based on the Janus TiOXY monolayers possess highly-anisotropic transport properties, such as the current-voltage curves, transmission spectra and local projected density of states (PDOS). The obtained results demonstrate that the 2D Janus TiOXY monolayers can be used as potential candidate platforms for novel nanoelectronic and optoelectronic applications.