Growth of two-dimensional WS2 thin films by pulsed laser deposition technique

Tungsten disulfide (WS2) is a promising material for potential applications in next-generation 2D-electronics. However, for such applications large-area films will be required. Fabrication techniques involving exfoliation give films of high crystal quality but suffer from the problem of extremely small size of the films which makes it very challenging to realize real devices for industrial applications. In this study, we are reporting the growth of large area two-dimensional (2D) films of WS2 using pulsed laser deposition technique. A polycrystalline WS2 target was ablated using a pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns). The ablated material was deposited over a c-axis aligned single crystal sapphire substrate maintained at 500 degrees C in an ambient vacuum of 10(-5) Torr. Thickness of the resulting films was controlled by controlling the number of incident laser pulses. Detailed characterization of the films was carried out using several state-of-the-art techniques including atomic force microscopy, Raman spectroscopy, and ultraviolet-visible spectroscopy. Relationship between the number of monolayers in the WS2 films and the spacing between the A(1g)(Gamma) and E-2g(1) (Gamma) Raman peaks (Delta f) was determined. Effect of post annealing on the optical properties of the films was also investigated.