A prototype nano-photodiode has been demonstrated based on heterojunctions between ZnO nanowires and p-Si substrate. The electrical and photoelectrical performances of the ZnO/Si structure have been characterized by a conducting atomic force microscopy at nanometer spatial resolution. The photoelectrical measurements demonstrate that the photodiode has high sensitivity and selectivity to UV light. (c) 2006 Elsevier B.V. All rights reserved.