Bias-Dependent Highly Anisotropic Photoresponse of a Non-d<sup>0</sup> Ferroelectric NbOCl<sub>2</sub> Monolayer.

Cui, Qilong; Xu, Shaowen; Xu, Siqi; Chenwei, Xianbo; Jia, Fanhao; Li, Ruixue; Xu, Gaofeng; Li, Yuan · Nano Lett · 2025

basic_science · Level V

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Abstract

NbOCl<sub>2</sub> 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 NbOCl<sub>2</sub> monolayer, aiming to understand its non-d<sup>0</sup> 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 NbOCl<sub>2</sub> as a foundational system for the design of next-generation optoelectronic devices that harness large in-plane electric polarization in layered ferroelectric semiconductors.