## 2 Results and Discussion
These results verify that the ZnO/ZnS core/shell NWs exhibited excellent optical sensing properties with more than a 4-fold increase in the absolute photocurrent and responsivity under external compressive strain owing to the piezo-phototonic effect and their type-II band structure. Furthermore, we calculated the relative increase in the photocurrent under an external strain condition, defined as \(\frac{\Delta I_{p}}{I_{p,0}}=\frac{I_{p,a}-I_{p,0}}{I_{p,0}}\), where \(I_{p,s}\) and \(I_{p,0}\) are the photocurrent (indicated by subscript \"\(p\)\") with and without a certain external strain under UV light, respectively. The \(\Delta I_{p}/I_{p,0}\) values of the ZnO/ZnS core/shell NWs and ZnO NWs under a compressive strain of \(-\)0.15% at 5 V were 11.7 and 32.3%, respectively. In addition, the \(\Delta I_{p}/I_{p,0}\) values were lower than the \(\Delta I/I_{0}\) values without UV irradiation. These results can be understood as a piezoelectric screening effect, wherein the piezopotential generated in the NWs wanes owing to the photogenerated charge carriers [33]. In particular, in the case of the ZnO/ZnS core/shell NWs, we can infer that the piezoelectric screening effect was further increased under UV illumination owing to the type-II band alignment. Thus, while the absolute photocurrent of the ZnO/ZnS core/shell NWs was 4.5 times higher than that of the ZnO NWs, the relative increase in the ZnO/ZnS core/shell NWs by strain was smaller than that in the ZnO NWs. To further understand the physical modulation mechanism, we derived the piezo-induced Schottky barrier height (\(\Delta\)SBH) changes from the \(I\)-\(V\) curves using the classic Schottky formular \(\Delta\)SBH = \(-\)\(kT\)ln(\(I\)strain/\(I\)free) [34; 35]. Figure 5a reveals that the calculated \(\Delta\)SBH was linearly correlated with the applied strain, indicating that the strain-induced piezopotential can effectively modulate the SBH. When the substrate was bent into a convex configuration by mechanical deformation, as shown in the schematic in Figure 5b, compressive strain was applied in the growth direction of the \(c\)-axis of the ZnO NWs. Consequently, the NWs undergoing stress generated a piezoelectric potential difference along the ZnO NWs under axial strain, and this localized piezopotential modified the SBH at the metal-junction interfaces, thus adjusting the charge transport and separation at the interface through piezotronic and piezo-phototronic effects under UV irradiation [32]. As shown in Figure 5a, the obtained \(\Delta\)SBH of both ZnO NWs and ZnO/ZnS core/shell NWs increases monotonously with an increase in the external compressive strain from 0 to \(-\)0.15%. These results demonstrate that positive piezoelectric polarization charges were generated at the interface of Schottky contact and that they can effectively reduce the Schottky barrier, resulting in an increase in the current flow. In particular, it was observedthat the \(\Delta\)SBH slope of both devices under UV irradiation decreased compared to that under the dark condition, which could be attributed to the reduction in the piezopotential generated in the ZnO NWs by the screening effect of the photogenerated charge carriers [34]. In addition, although the \(\Delta\)SBH slopes of the ZnO NWs and the ZnO/ ZnS core/shell NWs under UV irradiation were similar, we observed a significant increase in the absolute photocurrent of the ZnO/ZnS core/shell NWs with an increase in the applied strain. To elucidate the piezotronic and piezo-phototronic behavior of the ZnO and ZnO/ZnS core/shell NWs, energy-band diagrams with the corresponding realignment of the band position under compressive strain are illustrated in Figure 6. Figure 6a,b show the energy-band diagram of the unstrained ZnO and ZnO/ZnS core/shell NWs with back-to-back Schottky contact at the external bias.