Controlled synthesis and transfer of large-area WS₂ sheets: From single layer to few layers

The isolation of few-layered transition metal dichalcogenides has mainly been performed by mechanical and chemical exfoliation with very low yields. In this account, a controlled thermal reduction-sulfurization method is used to synthesize large-area (∼1 cm²) WS₂ sheets with thicknesses ranging from monolayers to a few layers. During synthesis, WO _x thin films are first deposited on Si/SiO₂ substrates, which are then sulfurized (under vacuum) at high temperatures (750-950 C). An efficient route to transfer the synthesized WS₂ films onto different substrates such as quartz and transmission electron microscopy (TEM) grids has been satisfactorily developed using concentrated HF. Samples with different thicknesses have been analyzed by Raman spectroscopy and TEM, and their photoluminescence properties have been evaluated. We demonstrated the presence of single-, bi-, and few-layered WS₂ on as-grown samples. It is well known that the electronic structure of these materials is very sensitive to the number of layers, ranging from indirect band gap semiconductor in the bulk phase to direct band gap semiconductor in monolayers. This method has also proved successful in the synthesis of heterogeneous systems of MoS₂ and WS₂ layers, thus shedding light on the controlled production of heterolayered devices from transition metal chalcogenides.