Structural and optical properties of WS2 prepared using sulfurization of different thick sputtered tungsten films

Our activities were focused on the preparation of WS2 films on sapphire substrates by sulfurization of different thick sputtered W films. The influence of very thin W films in the range from 4 nm to 12 nm on the structural, morphological and optical properties of WS2 was investigated. XRD measurements revealed a polycrystalline nature with hexagonal symmetry and randomly connected nanocrystals with grain size about 6 nm for all WS2 films. Using Raman spectroscopy with a 532 nm laser excitation, the presence of characteristic E-2g(1) and A(1g) vibration modes was recorded and the multilayered nature of the prepared WS2 films was confirmed. FESEM observations revealed randomly oriented lamellar and flake-shaped microstructures with the basal plane of the WS2 crystallites. Thinner WS2 films (20 and 24 nm) showed highly dense horizontally aligned flakes. On the other hand, thicker WS2 films (33 and 42 nm) indicated a granular surface and the WS2 crystallites grew perpendicularly to the substrate surface. All examined WS2 films were transparent from 30 to 78% in the spectral range of 500 to 900 nm and showed a direct bandgap of 2.3 eV.