Natural p-n Junctions at the MoS Flake Edges.

Two-dimensional (2D) semiconductors are holding promises as channel materials for field-effect transistors. Compared to traditional three-dimensional (3D) semiconductors whose electronic and optical properties are hindered by dangling bonds and trap states at the surfaces, 2D materials with saturated chemical bonds on the surface maintain the excellent properties even when device thickness scales down to monolayer. However, dangling bonds are unavoidable at their edges, which are often overlooked and should have important effects on the devices. Here, we show that the edges of as-exfoliated and etched MoS are naturally p-type doped and can form p-n junctions with the bulk of the flake. The width of these edge regions is around 20 nm. While their existence could present challenges for the shrinkage of devices, they can be exploited to form rectifying or optoelectronic devices based on a single flake of MoS without the need of an elaborate extrinsic doping process.