UV-photon and electrically driven resistance switching in ZnO nanotube   arrays
  Vertically aligned ZnO nanotube arrays fabricated on an ITO substrate are found to exhibit strong persistent photoconductivity (PPC) effect and electrically driven conductance switching behavior, though the latter shows a gradual decay from high conductance state to a low conductance state. Unlike the electrical switching, the PPC cannot be reset or reversed by an electrical pulse. Excitation wavelength dependent conductance measurement indicates the presence of the defect localized states (DLS) ~ 240meV above the valence band edge, in support of the hypothesis that the doubly ionization of these DLS are responsible for the PPC effect. 
Peng Liu, Guangwei She, Zhaoliang Liao, Yao Wang, Zhenzhong Wang,   Wensheng Shi, Xiaohong Zhang, Shuit-Tong Lee and Dongmin...