Layer-controlled, wafer-scale, and conformal synthesis of tungsten disulfide nanosheets using atomic layer deposition

The synthesis of atomically thin transition-metal disulfides (MS ₂) with layer controllability and large-area uniformity is an essential requirement for their application in electronic and optical devices. In this work, we describe a process for the synthesis of WS₂ nanosheets through the sulfurization of an atomic layer deposition (ALD) WO ₃ film with systematic layer controllability and wafer-level uniformity. The X-ray photoemission spectroscopy, Raman, and photoluminescence measurements exhibit that the ALD-based WS₂ nanosheets have good stoichiometry, clear Raman shift, and bandgap dependence as a function of the number of layers. The electron mobility of the monolayer WS₂ measured using a field-effect transistor (FET) with a high-k dielectric gate insulator is shown to be better than that of CVD-grown WS₂, and the subthreshold swing is comparable to that of an exfoliated MoS₂ FET device. Moreover, by utilizing the high conformality of the ALD process, we have developed a process for the fabrication of WS₂ nanotubes.