## 2 Results and Discussion
In particular, the change in the output current with a change in the compressive strain plotted (Figure 4c,d) indicates that the increase in the current levels of the ZnO/ZnS core/shell NWs (red dots) was higher than that in the current levels of the ZnO NWs (magenta dots) when increasing the compressive strain at biases of \(-5\) V and \(5\) V. To compare the output currents of the ZnO and ZnO/ZnS core/shell NWs under external compressive strain, we calculated the relative increase in the output current, defined as \(\frac{\Delta I}{b}=\frac{I_{\mathrm{strain}}-I_{0}}{I_{0}}\), where \(I_{\mathrm{strain}}\) and \(I_{0}\) are the current with and without a certain external strain, respectively [32]. The \(\Delta I/I_{0}\) value of the ZnO/ZnS core/shell NWs under a compressive strain of \(-0.15\%\) at \(5\) V was \(91.1\%\), whereas that of the ZnO NWs was \(42.7\%\) under the same condition. These results can be attributed to the distribution of electrons and holes arising from the type-II band structure of the ZnO/ZnS core/shell NWs, as well as the piezopotential changes across the junction interfaces. Generally, the piezopotential is generated in ZnO NWs when external strain is applied by bending the substrate. In the case of the ZnO/ZnS core/shell NWs, changes in the piezopotential caused by the application of strain can modulate the charge transport across the junction interface not only between ZnO NWs and electrodes but also at the heterojunction interface between the ZnO core and the ZnS shell. That is, electrons in ZnO/ZnS core/shell NWs can be transported more easily owing to the synergy effect of charge separation caused by the type-II band alignment, as well as the piezopotential changes at the heterojunction interface of core/shell NWs by external strain. Figure 4: Typical \(I\)–\(V\) curves of **(a)** ZnO and **(b)** ZnO/ZnS core/shell NWs under dark condition and \(365\) nm UV illumination (\(0.78\) mW/cm\({}^{2}\)) as the compressive strain changes from \(0\) to \(-0.15\%\). Dark current and photocurrent of the ZnO and ZnO/ZnS core/shell NWs as a function of the compressive strain at applied biases of **(c)**\(-5\) V and **(d)**\(5\) V.  
To further investigate the piezo-phototronic effect in the ZnO and ZnO/ZnS core/shell NW devices, we measured the strain-dependent photocurrent of both devices under UV illumination (365 nm and 0.78 mW/cm\({}^{2}\)). Compared with the \(I\)-\(V\) curves under the dark condition, the \(I\)-\(V\) curves under UV irradiation revealed that the photocurrents of both devices increased with an increase in the external compressive strain in a range from 0 to \(-\)0.15% (Figure 4a,b). At applied voltages of \(-\)5 V and 5 V, the photocurrents of the ZnO/ZnS core/shell NWs (\(-\)21.3 uA and 68.4 uA) under a compressive strain of \(-\)0.15% were 2.47 and 4.53 times higher than those of the ZnO NWs (\(-\)8.63 uA and 15.1 uA) (Figure 4c,d). To compare the performance of both ZnO NWs and ZnO/ZnS core/shell NWs as a photodetector, we calculated the responsivity and external quantum efficiency (EQE) of the devices under varying compressive strains using previously reported equations [21]. The responsivity and EQE of the ZnO/ZnS core/shell NWs under \(-\)0.15% of compressive strain at 5 V were 1.95 A/W and 6.65%, respectively, whereas those of the ZnO NWs under the same conditions were 0.4 A/W and 1.44%, respectively. The higher photoresponsivity and EQE values of the ZnO/ZnS core/shell NWs compared to those of the ZnO NWs under compressive strain were attributed to the separation of the photogenerated charge carriers due to the type-II band alignment, as well as strain-induced piezopolarization charges at the junction interfaces. These piezoelectric charges can effectively modulate the barrier height at the junction between ZnO NWs and the metal electrode, and they can induce the enhanced transport of photogenerated electrons and holes and a reduced recombination probability for the charge carriers.