## 2 Results and Discussion
It is important to note that ln\(I\) is linearly correlated with \(V\) in the intermediate bias range for both positive and negative currents under dark and illumination conditions, as shown in the inset of Figure 3a,b, which corresponds to the typical \(I\)-\(V\) characteristic of a back-to-back Schottky barrier structure [31]. After the devices were irradiated with a UV light of 365 nm from the bottom of the device, as illustrated in Figure 3c, the photocurrents of both devices increased. For example, at an applied voltage of 5 V, the dark current and photocurrent of the ZnO NWs were 58.4 nA and 0.45 \(\upmu\)A, respectively, whereas those of the ZnO/ZnS core/shell NW arrays were 154 nA and 5.4 \(\upmu\)A, respectively. This indicates that the photocurrent of the ZnO/ZnS core/shell NWs was 35 times larger than the dark current, whereas the photocurrent of the ZnO NWs was 8 times larger than the dark current, implying that the photoconductivity of the ZnO NWs can be significantly enhanced by forming a shell layer on the core NWs. Figure 2: Scanning electron microscopy (SEM) images of (**a**) ZnO and (**b**) ZnO/ZnS core/shell NWs. (**c**) High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) image of ZnO/ZnS core/shell NW with energy-dispersive X-ray spectroscopy (EDS) composition maps for Zn, S, and O. The larger enhancement in the output current of the ZnO/ZnS core/shell NWs compared to that of the ZnO NWs can be ascribed to the type-II band alignment of the ZnO/ZnS core/shell NWs, as shown in Figure 3d. It is well known that a number of surface states exist on the surface of ZnO NWs, facilitating the recombination of the photogenerated electrons with the photogenerated holes [12]. By coating ZnO NWs with a ZnS shell to form a core/shell structure, the surface state density of the NWs can be modified, and the charge separation of the photogenerated electrons and holes can occur owing to the formation of the type-II band alignment. The type-II band induced the spatial separation and confinement of electrons and holes into the ZnO core and ZnS shell, respectively, and it suppressed the recombination of photogenerated electron-hole pairs under UV illumination [17]. Hence, the charge carriers contributing to the photocurrent in the ZnO/ZnS core/shell NWs were significantly higher than those in the ZnO NWs. Furthermore, the type-II band alignment enabled the generation of electron-hole pairs not only via the direct band-to-band absorption in ZnO but also via the indirect type-II transition from the valence band maximum of ZnS to the conduction band minimum of ZnO under UV irradiation, which is a wavelength below the bandgap of ZnS, thus resulting in the enhanced photocurrent. To compare the piezotronic and piezo-phototronic effects of the ZnO and ZnO/ZnS core/shell NW devices, the current-voltage (\(I\)-\(V\)) curves were systematically measured by applying external compressive strains under a UV illumination with a wavelength of 365 nm with a power intensity of 0.78 mW/cm\({}^{2}\). With an increase in the applied external compressive strain from 0 to \(-\)0.15%, the output current of both the ZnO and ZnO/ZnS core/shell NWs increased cumulatively (Figure 4a,b). Figure 3: Current–voltage (\(I\)–\(V\)) characteristics of (**a**) ZnO NWs and (**b**) ZnO/ZnS core/shell NW arrays in dark condition and under 365 nm UV illumination (0.78 mW/cm\({}^{2}\)). Inset: ln\(I\)–\(V\) curves in the dark and under UV illumination. (**c**) Schematic of cross-sectional view of ZnO/ZnS core/shell NW devices with illumination source of 365 nm UV light and (**d**) its corresponding type-II energy-band diagram.