Preparation and applications of mechanically exfoliated single-layer and multilayer MoS₂ and WSe₂ nanosheets

Although great progress has been achieved in the study of graphene, the small current ON/OFF ratio in graphene-based field-effect transistors (FETs) limits its application in the fields of conventional transistors or logic circuits for low-power electronic switching. Recently, layered transition metal dichalcogenide (TMD) materials, especially MoS₂, have attracted increasing attention. In contrast to its bulk material with an indirect band gap, a single-layer (1L) MoS₂ nanosheet is a semiconductor with a direct band gap of ∼1.8 eV, which makes it a promising candidate for optoelectronic applications due to the enhancement of photoluminescence and high current ON/OFF ratio.Compared with TMD nanosheets prepared by chemical vapor deposition and liquid exfoliation, mechanically exfoliated ones possess pristine, clean, and high-quality structures, which are suitable for the fundamental study and potential applications based on their intrinsic thickness-dependent properties. In this Account, we summarize our recent research on the preparation, characterization, and applications of 1L and multilayer MoS₂ and WSe₂ nanosheets produced by mechanical exfoliation. During the preparation of nanosheets, we proposed a simple optical identification method to distinguish 1L and multilayer MoS₂ and WSe₂ nanosheets on a Si substrate coated with 90 and 300 nm SiO₂. In addition, we used Raman spectroscopy to characterize mechanically exfoliated 1L and multilayer WSe₂ nanosheets. For the first time, a new Raman peak at 308 cm^-1 was observed in the spectra of WSe₂ nanosheets except for the 1L WSe₂ nanosheet. Importantly, we found that the 1L WSe₂ nanosheet is very sensitive to the laser power during characterization. The high power laser-induced local oxidation of WSe₂ nanosheets and single crystals was monitored by Raman spectroscopy and atomic force microscopy (AFM). Hexagonal and monoclinic structured WO₃ thin films were obtained from the local oxidization of single- to triple-layer (1L-3L) and quadruple- to quintuple-layer (4L-5L) WSe₂ nanosheets, respectively. Then, we present Raman characterization of shear and breathing modes of 1L and multilayer MoS ₂ and WSe₂ nanosheets in the low frequency range (<50 cm^-1), which can be used to accurately identify the layer number of nanosheets. Magnetic force microscopy was used to characterize 1L and multilayer MoS₂ nanosheets, and thickness-dependent magnetic response was found. In the last part, we briefly introduce the applications of 1L and multilayer MoS₂ nanosheets in the fields of gas sensors and phototransistors.