Origin of Hydrophobicity in FIB-Nanostructured Si Surfaces

Surface morphology has been demonstrated to influence the tribological properties at different scales, but the phenomena which occur at the nano-scale have not been completely understood. The present study reports on the effect of Focused Ion Beam nano-patterning on coefficient of friction (CoF) and adhesion of Si(001) surface covered by native oxide. Regular arrays of nano-grooves reduce both CoF and adhesion, related to a hydrophobic character of the patterned surface, but this effect disappears as soon as the separation among the nano-structures approaches the microscopic scale. The dependence of this hydrophobic effect on the pitch is not linked to the corresponding contact area. It has been found that each nano-structure is surrounded by a low-friction region which extends some hundreds nm from it. For pitch of 125 and 250nm these low-friction regions completely overlap, generating a consistent decrease in CoF and adhesion, while for pitch of 500 and 1000nm its effect is negligible. The low-friction regions were not observed in humidity free ambient, indicating that they are the origin of patterns hydrophobicity.