Nanoscale properties of thin twin walls and surface layers in piezoelectric WO3−x

Reduced WO3 single crystals have been investigated by conductive and piezoresponse force microscopy. The reduced crystals show a piezoelectric active surface layer with a noncentrosymmetric tetragonal structure which is different from the bulk of the crystal. The domain walls carry a high current while the bulk remains insulating. Twin-related interfaces are atomically thin, the upper bound being less than 10 nm. The reduced surface layer remained in the piezoelectric state even after several weeks, while the current in the twin boundaries was reduced by re-oxidization. This layer shows a significant piezoelectric activity with a piezoelectric coefficient of about 7.9 pm/V.