  Quasi-one-dimensional (quasi-1D) materials are a newly arising topic in
low-dimensional researches. As a result of reduced dimensionality and enhanced
anisotropy, the quasi-1D structure gives rise to novel properties and promising
applications such as photodetectors. However, it remains an open question
whether performance crossover will occur when the channel material is
downsized. Here we report on the fabrication and testing of photodetectors
based on exfoliated quasi-1D BiSeI thin wires. Compared with the device on bulk
crystal, a significantly enhanced photoresponse is observed, which is
manifested by a series of performance parameters, including ultrahigh
responsivity (7 x 10$^4$ A W$^{-1}$), specific detectivity (2.5 x 10$^{14}$
Jones) and external quantum efficiency (1.8 x 10$^7$%) when $V_{\textrm {ds}}$
= 3 V, $\lambda$ = 515 nm and $P$ = 0.01 mW cm$^{-2}$. The conventional
photoconductive effect is unlikely to account for such a superior
photoresponse, which is ultimately understood in terms of the increased
specific surface area and the photogating effect caused by trapping states.
This work provides a perspective for the modulation of optoelectronic
properties and performance in quasi-1D materials.
