Highly textured films of layered metal disulfide 2H-WS2 - Preparation and optoelectronic properties

Highly textured films of 2H-WS2 can be obtained by sulfurization of up to 4 mu m thick WO3 layers in the presence of hydrogen using amorphous (quartz glass, glassy carbon) or crystalline (sapphire, muscovite, highly oriented pyrolytic graphite) substrates. Best conditions have been found employing (00.1) oriented sapphire substrates with a 5 nm thick nickel layer interposed between substrate and oxide film (0.5 to 4 mu m thickness) and a reaction temperature ranging from 973 to 1173 K. Depending on time the crystallites, oriented with their hexagonal basal planes parallel to the substrate, exhibited a lateral extension of up to 20 mu m and a thickness of less than or equal to 300 nm. Conductivity measurements of the films showed a p-type conductivity in the range from 0.1 to 3 Omega(-1) cm(-1) and a lateral mobility as high as 105 cm(2) V-1 s(-1) at room temperature. The conductivity type has been confirmed by ultraviolet photoelectron and x-ray photoelectron spectroscopy which were compared with n-type single crystals. A freestanding film pealed off from a Pt coated quartz substrate and mounted on a brass holder was investigated photoelectrochemically. Using a 0.2 M Fe2+/Fe3+ redox electrolyte in 0.5 M H2SO4 an open-circuit voltage of approximate to 100 mV and a short-circuit current of 5 mA/cm(2) has been detected for the first time.