## 2 Results and Discussion
The Schottky junctions can be formed at the interface regions between metals and semiconductors, such as ZnO/ITO, ZnO/Au, and ZnS/Au, because the work functions of ITO(\(\Phi_{ITO}\)) and Au(\(\Phi_{Au}\)) are larger than those of ZnO(\(\Phi_{ZnO}\)) and ZnS(\(\Phi_{ZnS}\)) [36; 37]. In particular, the energy-band diagram of the ZnO/ZnS core/shell NWs with a type-II band structure exhibited potential energy gradients with a staggered alignment at the heterojunction interface between ZnO and ZnS (Figure 6b). These potential energy gradients can induce the separate confinement of the electrons and holes in the conduction band of ZnO core and the valence band of ZnS, respectively. Thus, this ideal distribution of charge carriers, where the electrons were confined in the ZnO NWs and the holes were confined in the ZnS shell, contributed to the enhanced charge transport. In addition, upon exposure to UV light, the recombination probability of the ZnO/ZnS core/shell NWs reduced because of the spatial separation of the photogenerated electrons and holes due to the type-II band alignment. In contrast, the recombination probability of the photogenerated carriers of the ZnO NWs increased because of their large number of surface states, which resulted in a low photocurrent. When the substrate was bent into a convex configuration, the compressive strain was introduced along the \(c\)-axis direction of the ZnO NWs, and the corresponding piezopotential distribution was also induced. Figure 6c,d show that the positive piezoelectric charges were generated at the bottom of the tip of the ZnO NWs, and they induced the downward bending of the conduction and valence bands of the ZnO near the ITO. This indicates that the positive piezopotential modified the local SBH at the ZnO/ITO interfaces of both the ZnO and ZnO/ZnS core/shell NWs, thus facilitating the electrical transport. These features indicate that the piezoelectric charges and potential generated along the ZnO NWs play a crucial role in determining the movements of the electrons, which affects the properties of the device. In particular, in the case of the ZnO/ZnS core/shell NWs, a  
Figure 5: (**a**) The calculated Schottky barrier height (SBH) change as a function of the applied external compressive strain at 5 V. (**b**) Schematic illustration of vertically grown ZnO/ZnS core/shell NW devices, showing the direction of the applied strains. negative piezopotential at the heterojunction interfaces between ZnO and ZnS can eliminate the electron trap state barrier (\(\Delta\Phi_{1}\)), which is unfavorable for charge transport (Figure 6d). This indicates that the electrical transport properties of ZnO/ZnS core/shell NWs can be further improved by applying an external strain. Figure 6e,f show the change in the energy-band diagrams after UV light irradiation under applied strain. Similarly, strain-induced positive piezoelectric charges under UV irradiation reduced the SBH at the interfacial region between the ZnO and ITO. Consequently, more photogenerated electrons can be transferred from ZnO to ITO electrodes, resulting in an increase in the photocurrent and responsivity. The charge carrier transport of the ZnO/ZnS core/shell NWs was significantly enhanced in the type-II alignment due to the combined effect of the piezopotential and photo-excitation, thereby resulting in a higher current and responsivity change when the NWs were simultaneously subjected to strain and illumination. However, it is important to note that the photogenerated electrons that reside in the conduction band of ZnO are attracted to a positive piezopotential, resulting in a partially weakened piezopotential [34]. Thus, the dependence on strain  
Figure 6: Energy-band diagrams of the ZnO NW and ZnO/ZnS core/shell NW devices (**a,b**) without strain and (**c,d**) with compressive strain (**e,f**) under simultaneous application of compressive strain and UV illumination, respectively.