ZnO NB was bent resulting in an asymmetric strain across the thickness of the NB. As demonstrated in our recent work, a bent ZnO NB could produce a piezoelectric potential across the NB due to the strain-induced piezoelectric effect. With the stretch and compression effects of a deformed ZnO NB, a positively charged and a negatively charged surfaces were produced at the outer and inner bending surfaces of a ZnO NB. Conse- quently, a piezoelectric field was built across the ZnO 共NB Fig. 3共b兲兲. These ionic charges are immobile on the ZnO NB surface without releasing the strain. This trapping effect low- ers the available carrier densities in ZnO NBs. On the other hand, the negative charged surface of ZnO NBs acted as a back gate which repulses free electrons away from the sur- face under the electrostatic effect. Therefore, a depletion re- gion was formed in space close to the negative charged sur- face. This reduces the size of the conducting channel in the ZnO NB. This also accounts for the decrease in conductivity of the bent ZnO NB. These two mechanisms are schemati- cally presented in Figs. 3共c兲 and 3共d兲, which together may be responsible to the observed decrease in conductivity. The depletion region formed by the negative charged surface and the consequent gating effect to the conduction channel in ZnO NB is analogous to the case of applying a gate voltage across a ZnO NB in a typical FET, e.g., PE-FET. In our experiment, bending of the ZnO NB comes from the swell- ing of polymers, which is a result of chemical process. When the sample was dehydrated in dry air, the strain was released and the ZnO NB returns to its original shape