Highly Anisotropic GeSe Nanosheets for Phototransistors with Ultrahigh Photoresponsivity.

2D GeSe possesses black phosphorous-analog-layered structure and shows excellent environmental stability, as well as highly anisotropic in-plane properties. Additionally, its high absorption efficiency in the visible range and high charge carrier mobility render it promising for applications in optoelectronics. However, most reported GeSe-based photodetectors show frustrating performance especially in photoresponsivity. Herein, a 2D GeSe-based phototransistor with an ultrahigh photoresponsivity is demonstrated. Its optimized photoresponsivity can be up to ≈1.6 × 10 A W. This high responsivity can be attributed to the highly efficient light absorption and the enhanced photoconductive gain due to the existence of trap states. The exfoliated GeSe nanosheet is confirmed to be along the [001] (armchair direction) and [010] (zigzag direction) using transmission electron microscopy and anisotropic Raman characterizations. The angle-dependent electric and photoresponsive performance is systematically explored. Notably, the GeSe-based phototransistor shows strong polarization-dependent photoresponse with a peak/valley ratio of 1.3. Furthermore, the charge carrier mobility along the armchair direction is measured to be 1.85 times larger than that along the zigzag direction.