Synthesis of two-dimensional structures based on exfoliation of tungsten disulphide and thin film fabrication using different deposition methods

Abstract

Layered crystalline solids possess chemical bonds that are generally covalent and
in-plane but have weak van derWaals bonds perpendicular to the plane. They can
be readily exfoliated into quasi two-dimensional nanosheets with a thickness of
less than one nanometer, providing a simple and scalable route for the production
of two-dimensional solids. Among two-dimensional materials, transition metal
dichalcogenides (TMDs), with structural formula MX2 (where M is a transition
metal and X a chalcogen) have attracted much interest for their unique properties
such as high carrier mobility and change of the energy band structure due
to physical changes and optoelectronic properties arising from their quasi two dimensional structure. In this work, we aim to find alternative approaches for
fabrication using less environmentally harmful solvents. We use liquid phase processing of bulk layered crystals to produce suspensions of a single layer and few layer of tungsten disulfide, WS2, with water and Triton X-100 as a non-ionic surfactant. After that, we investigated the exfoliated WS2 in low boiling point solvents including isopropanol (IPA), ethanol (EtOH), methanol (MeOH), and acetonitrile (ACN) to successfully find a few layers of WS2. The sample preparation procedure can be broken in two main parts: 1) high shear mixing and 2) centrifugation. The centrifuge process aims to separate the thicker material that has not been fully exfoliated from the WS2 dispersions, stabilized in water. We found that the recycling of previously exfoliated powder gives excellent suspension stability without reaggregation for long periods of more than 8 months, compared to the original powder that was exfoliated for the first time. When we recycle the powder, we are effectively removing small surface contaminants on the flakes from the original powder, and they are not present in the recycled powder. Removing these contaminants produces more stable solutions. We find the dispersed concentration increases with increasing speed of the mixer while the optimum stable dispersions are obtained at shear speeds of 8000 rpm. Dispersions were characterized by absorption spectroscopy, Raman spectroscopy, Scanning Electron Microscopy (SEM), and Transmission electron microscopy (TEM). The concentration of the material, fabricated with optimum shear rates of 5 × 104 s−1, has been determined using optical absorption, giving a WS2 concentration of 0.025 mg/ml. In addition, multiple deposition processes including drop casting, vacuum oven, spin coating, and dip coatingwith non-polar solventwere utilised to investigate the optimal dispersion for nanosheets on the substrate, and determine the best procedure for the fabrication of stable films of WS2 nanosheets.