(color online) (a) I–V characteristics with and without localized UV irradiation for ohmic-typed sensor. (b) Current as a function of time recorded at a fixed voltage of 2 V under localized periodic UV irradiation at different power densities for ohmic-typed sensor. (c) I–V characteristics with and without localized UV irradiation for Schottky-type sensor. (d) Current as a function of time recorded at a fixed voltage of 2 V under periodic UV light irradiation at different power densities for Schottky-typed sensor. (e) Schematic diagram of the mechanism for sensor response to localized UV irradiation at the third leg of a tetrapod. (f) Schematic energy band diagram of the surface potential of ZnO for sensor with a Schottky contact.