Two-dimensional Janus NbOClI: A promising anisotropic unit for multifunctional optoelectronic devices

Recently, the synthesis of a ${\mathrm{NbOCl}}_{2}$ monolayer has been reported, revealing notable anisotropic properties and linear dichroism [Guo et al., Nature (London) 613, 53 (2023)]. Inspired by this progress, we have engineered a Janus NbOClI monolayer by breaking the out-of-plane spatial inversion symmetry in ${\mathrm{NbOCl}}_{2}$. Through rigorous first-principles calculations, we present a detailed and comprehensive investigation of its mechanical and optical characteristics. We find that it exhibits a substantial out-of-plane piezoelectric response (${d}_{31}=0.87$ pm/V), driven by atomic electronegativity differences and Peierls distortion. The intrinsic electric field within NbOClI enables robust photocatalytic water-splitting capabilities across a broad pH spectrum, independent of external strain. Capitalizing on its pronounced linear dichroism, we construct a polarization-sensitive photodetector that leverages the photogalvanic effect to produce an anisotropic, self-powered photocurrent. Notably, this photocurrent is significantly amplified by a strain gradient, achieving self-rectification via the synergistic coupling of flexoelectric and piezoelectric fields. Building upon these achievements, a proof-of-concept three-channel optical communication scheme is shown to amplify the output efficiency of binary signals by an additional $50%$. Our results present a fascinating functional coupling architecture that simultaneously enables excellent force-electric conversion, efficient photocatalytic water splitting, polarization-sensitive self-powered photodetection, and multichannel optical communication in Janus NbOClI-based optoelectronic devices. This brings us closer to realizing the next generation of chip-scale integrated optoelectronics.