Facile fabrication of asymmetric surfaces via mechanochemistry

We demonstrate a mechanochemical approach to fabrication of a bidirectional surface on a doped silicon wafer. In the initial step of our fabrication procedure, a high-density polyethylene (HDPE) stylus is pressed and drawn on the Si surface. This rubbing action provides a mechanochemical treatment of the Si surface, which unexpectedly alters the etching direction in the subsequent metal-assisted chemical etching step. Hence, Si nanowires of two different orientations can be patterned conveniently by a rub-write step. The mechanism of bidirectional Si nanowire formation is investigated. This anisotropy is exploited to propel water droplets on a fluidic track.