Controlled Plasma Thinning of Bulk MoS₂ Flakes for Photodetector Fabrication

The electronic properties of layered materials are directly determined based on their thicknesses. Remarkable progress has been carried out on synthesis of wafer-scale atomically molybdenum disulfide (MoS₂) layers as a two-dimensional material in the past few years in order to transform them into commercial products. Although chemical/mechanical exfoliation techniques are used to obtain a high-quality monolayer of MoS₂, the lack of suitable control in the thickness and the lateral size of the flakes restrict their benefits. As a result, a straightforward, effective, and reliable approach is widely demanded to achieve a large-area MoS₂ flake with control in its thickness for optoelectronic applications. In this study, thick MoS₂ flakes are obtained by a short-time bath sonication in dimethylformamide solvent, which are thinned with the aid of a sequential plasma etching process using H₂, O₂, and SF₆ plasma. A comprehensive study has been carried out on MoS₂ flakes based on scanning electron microscopy, atomic force microscopy, Raman, transmission electron microscopy, and X-ray photoelectron microscopy measurements, which ultimately leads to a two-cycle plasma thinning method. In this approach, H₂ is used in the passivation step in the first subcycle, and O₂/SF₆ plasma acts as an etching step for removing the MoS₂ layers in the second subcycle. Finally, we show that this technique can be enthusiastically used to fabricate MoS₂-based photodetectors with a considerable photoresponsivity of 1.39 A/W and a response time of 0.45 s under laser excitation of 532 nm.