In Situ Wettability Characterization of Chemically Heterogeneous Surfaces Probed by Ionic Liquid Contact Angle in Vacuum: Pentacene on Single-Crystal SrTiO₃(001)

Intrinsic surface wettability of pentacene thin films with different island coverages grown on single-crystal SrTiO₃(001) was investigated by an in situ contact angle measurement in a high vacuum using ionic liquid as a probe fluid. The contact angle at the quasi-equilibrium state, which was measured at a certain time after its fast decay depending on the liquid viscosity, increased with the deposition amount of pentacene and saturated when the surface was fully covered with pentacene islands. The observed island coverage dependence of the in situ contact angle was well-fitted by a three-component Cassie equation composed of surface areas of not only the bare substrate and island pentacene but also adsorbed pentacene. The importance of the in situ contact angle measurement was confirmed by the result of an air exposure experiment, where the contact angle took almost irrelevant values to the integral coverage of pentacene, suggesting the significant effects of airborne contaminants on the surface wettability.