Stripping Method for WS2 Preparation

Bulk tungsten disulfide (WS₂) can be stripped by physical and chemical methods, which are classified as mechanical and stripping method, and lithium-ion intercalation method. In recent years, in order to obtain large-area, high-quality monolayer tungsten disulfide films, researchers have tried to grow monolayer tungsten disulfide films on ingot substrates and then exfoliate them by atomic or molecular intercalation methods.

The mechanical stripping method is a simple physical method used to produce WS₂ films. First, the transparent tape is applied to the tungsten disulfide bulk laminate to be peeled. The tape is then used to repeatedly peel off the bulk material to make the laminate sheet thinner. Finally, the tungsten disulfide film is transferred from the tape to the target substrate (e.g. SiO₂/Si), and the tape is slowly peeled off after a rest period to ensure that the tungsten disulfide film adheres well to the target substrate.

The prepared monolayer TMD materials have few defects and high crystallinity, and thus have good optoelectronic properties such as high luminescence efficiency and high carrier mobility. However, the large-area films are difficult to peel off, which limits their practical application and their repetition is low.

The liquid phase exfoliation method is based on the use of ultrasound to generate cavitation bubbles or shear forces to separate layered materials from each other. Then, the materials are dispersed in a solvent to obtain a two-dimensional monolayer and layer fewer materials. The choice of solvent is the most critical factor in the liquid phase exfoliation method.

The surface tension of tungsten disulfide is about 40 mJ.m^-2, so a suitable solvent must be selected to successfully exfoliate the WS₂ material. The most commonly used solvents are propanol, ethanol, and water. In order to shorten the sonication time and improve the stability of the suspension, Adilbekova et al. used ammonia as the solvent for liquid phase exfoliation. The liquid phase exfoliation method for tungsten disulfide films can be used to prepare tungsten disulfide in large quantities, but the thickness of tungsten disulfide films produced by this method is not uniform and the purity of the films is low.

The lithium-ion intercalation method uses a lithium-containing solvent (e.g., n-butyl lithium) to intercalate into tungsten disulfide-layered nanomaterials, forming many intermediate compounds. These intermediate compounds increase the interlayer distance of WS₂ and weaken the van der Waals forces between the layers, leading to peeling. Then, the materials were washed and dried to obtain monolayer or multilayer tungsten disulfide nanosheets. Ghorai et al. intercalated tungsten disulfide at room temperature in the presence of an inert solvent, hexane or toluene, using lithium halide as a lithium source.

(Photo source: Rosanna Mastria et al/Nature)

The intercalation of lithium ions in a two-dimensional layered material leads to interlayer separation, which is an electrochemical stripping method. The greatest advantage of this method is the high yield of tungsten disulfide films and the large film area. However, defect formation, high cost, long process time, and high sensitivity to environmental changes are some of the disadvantages of this process.

Cited Article: Ding J, Feng A, Li X, et al. Properties, preparation, and application of tungsten disulfide: A review[J]. Journal of Physics D: Applied Physics, 2021, 54(17): 173002.

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