Bias-Dependent Highly Anisotropic Photoresponse of a Non-d 0 Ferroelectric NbOCl 2 Monolayer

NbOCl2 is an emerging 2D ferroelectric with unique potential in designing multifunctional optoelectronics. Here, we carried out a density functional theory study combined with non-equilibrium Green’s function formalism for the NbOCl2 monolayer, aiming to understand its non-d0 in-plane ferroelectricity and the bias-dependent highly anisotropic photoresponse. Dual-mode pseudo-Jahn–Teller distortions, comprising a Peierls-like mode and a ferroelectric mode, induce large band gap opening and predominantly ionic spontaneous polarization, respectively. Under the external bias, the polar direction (Nb–O–Nb) photoresponse is universally enhanced and exhibits a notable offset. In contrast, the nonpolar direction (Nb–Cl–Nb) demonstrates a distinct field-dependent behavior: an initial enhancement at low fields followed by suppression at higher fields, attributable to electrode state misalignment. Our work demonstrates NbOCl2 as a foundational system for the design of next-generation optoelectronic devices that harness large in-plane electric polarization in layered ferroelectric semiconductors.