Layer-by-layer thinning of MoS₂ by plasma

The electronic structures of two-dimensional materials are strongly dependent on their thicknesses; for example, there is an indirect to direct band gap transition from multilayer to single-layer MoS₂. A simple, efficient, and nondestructive way to control the thickness of MoS₂ is highly desirable for the study of thickness-dependent properties as well as for applications. Here, we present layer-by-layer thinning of MoS₂ nanosheets down to monolayer by using Ar⁺ plasma. Atomic force microscopy, high-resolution transmission electron microscopy, optical contrast, Raman, and photoluminescence spectra suggest that the top layer MoS₂ is totally removed by plasma while the bottom layer remains almost unaffected. The evolution of Raman and photoluminescence spectra of MoS₂ with thickness change is also investigated. Finally, we demonstrate that this method can be used to prepare two-dimensional heterostructures with periodical single-layer and bilayer MoS₂. The plasma thinning of MoS₂ is very reliable (with almost 100% success rate), can be easily scaled up, and is compatible with standard semiconductor process to generate heterostructures/patterns at nanometer scale, which may bring out interesting properties and new physics.