Tunable Wetting Property in Growth Mode-Controlled WS Thin Films.

We report on a thickness-dependent wetting property of WS/AlO and WS/SiO/Si structures. We prepared WS films with gradient thickness by annealing thickness-controlled WO films at 800 °C in sulfur atmosphere. Raman spectroscopy measurements showed step-like variation in the thickness of WS over substrates several centimeters in dimension. On fresh surfaces, we observed a significant change in the water contact angle depending on film thickness and substrate. Transmission electron microscopy analysis showed that differences in the surface roughness of WS films can account for the contrasting wetting properties between WS/AlO and WS/SiO/Si. The thickness dependence of water contact angle persisted for longer than 2 weeks, which demonstrates the stability of these wetting properties when exposed to air contamination.