Two-Dimensional Tellurium Nanosheets Exhibiting an Anomalous Switchable Photoresponse with Thickness Dependence.

Two-dimensional (2D) tellurium (Te) was recently predicted to be promising for diverse electronic and optoelectronic applications. However, the synthesis of high-quality 2D Te structures remains challenging, which greatly hinders the exploration of its full properties. Herein, an anomalous photoresponse from negative to positive as a function of thickness in Te nanosheets is reported. Ultrathin Te layers with large size and clean interface were obtained through a topotactic transformation, in which the 2D Te structure was derived from a layered MTe2 (M=Ti, Mo, W) matrix by excessive lithiation. Prominently, the photoresponse in Te nanosheets exhibits negative behavior when the thickness is less than 5 nm, which turns positive as the thickness increases. This unusual photoresponse will shed light on the full exploration of 2D non-layered materials with exotic properties.