Self-Trapped Excitons and Room-Temperature Ferroelectricity in Quasi-One-Dimensional Semiconductor BiSeBr for Polarization-Sensitive Optoelectronics

Bismuth-based chalcohalides are emerging as a promising family of materials for optoelectronics due to their intrinsically anisotropic optical and electronic properties. However, the lack of high-quality crystals has hindered access to their intrinsic physical properties. Here, we present the synthesis and comprehensive optoelectronic characterization of quasi-one-dimensional (1D) van der Waals (vdW) BiSeBr crystals. This work establishes a previously unreported synergy, the coexistence of ferroelectricity and self-trapped excitons (STEs) in a single quasi-1D semiconductor, and its exploitation for multifunctional polarization-sensitive photodetection. The low-dimensional chain structure of BiSeBr breaks inversion symmetry, giving rise to out-of-plane ferroelectric behavior and a second-harmonic generation signal. This symmetry breaking, together with strong electron–phonon coupling, promotes the formation of self-trapped excitons. The wavelength-dependent linear dichroism reversal phenomenon reflects a k -space rotation of the dominant transition dipole moment orientation, a general feature of low-symmetry semiconductors that is further enhanced here by the ferroelectric structural distortion. The BiSeBr nanowire photodetector translates these physical mechanisms into device functionality. It exhibits broadband photoresponse from ultraviolet to near-infrared (350 to 1060 nm), with a responsivity of 2.4 × 10 4 A W –1 , a specific detectivity of 3.3 × 10 13 Jones, and a fast response time of 389 μs. The detector resolves a clear polarization sensitivity reversal between the visible and near-infrared regions. These results establish BiSeBr as a platform for broadband polarization-sensitive photodetection in which multifunctionality is not achieved by stacking disparate materials, but emerges from the intrinsic synergy between ferroelectricity and strong electron–phonon coupling within a single quasi-one-dimensional vdW crystal.