## 2 Results and Discussion  
Figure 1 shows a schematic image of the fabrication process of the ZnO and ZnO/ZnS core/shell NW devices. For the fabrication, first, we deposited a ZnO seed layer on a flexible poly(ethylene terephthalate) (PET) substrate with indium tin oxide (ITO) as the bottom electrode using radio-frequency magnetron sputtering (Figure 1a). Subsequently, the ZnO NWs were grown using a hydrothermal method by dipping the substrate into a solution of zinc nitrate hexahydrate (Zn(NO\({}_{3}\))\({}_{2}\)-6H\({}_{2}\)O) and hexamethylenetetramine ((CH\({}_{2}\))\({}_{6}\)N\({}_{4}\)) with a concentration of 0.25 mol/L at 95 \({}^{\circ}\)C for 2 h (Figure 1b). The tilted cross-sectional and top-view field-emission scanning electron microscope (FESEM) images and the low-magnification bright-field and high-resolution transmission electron microscope (TEM) images of the vertically grown ZnO NWs with a diameter of about 50-100 nm are shown in Figure 1a\({}^{\prime}\) and Figure S1, respectively. Thereafter, the ZnS shell was then deposited by immersing the ZnO NWs grown on the substrate in a 0.1 M thioacetamide solution at 90 \({}^{\circ}\)C for 50 min (Figure 1c). To apply strain to the NWs more effectively and to prevent the fracture of the NWs, the NWs were coated with an AZ1512 photoresist film using a spin-coating method. The top layer of the photoresist film was opened using reactive ion etch (RIE) processing, and Au was deposited as the top electrode via e-beam evaporation (Figure 1d). Figure S2 in Supporting Information shows the exposed tips of the polymer-embedded ZnO NW arrays depending on the RIE processing time. Lastly, PET film was positioned on the top of the NW arrays to ensure optimal compression under applied strain. These facile and simple synthesis methods at low processing temperatures enable the use of flexible substrates for the fabrication of devices. The details of the complete fabrication process can be found in the Experimental Section. Figure 2a,b show the tilted FESEM images of the as-grown ZnO and ZnO/ZnS core/shell NWs on the ITO-coated PET substrate, and their corresponding illustrations are schematically shown in the inset of Figure 2a,b, respectively. Most of the ZnO NWs with rational density exhibited a regular hexagonal shape with a smooth surface, whereas the ZnS shell deposited on the tips of the ZnO/ZnS core/shell NWs exhibited a slightly rough surface after the hydrothermal deposition of ZnS. To investigate the structure and composition of the ZnO/ZnS core/shell NWs, we performed the high-angle annular dark field scanning transmission electron microscope energy-dispersive X-ray spectroscopy (HAADF STEM-EDS) analysis. Figure 2c shows the compositional maps collected from the white-boxed area of the HAADF STEM image for Zn, S, and O elements. The core/shell elemental distribution results revealed that a relatively high concentration of S was present at the edge of the NW compared to the core region, confirming the successful synthesis of ZnO/ZnS core/shell NWs. Figure 1: Schematic illustration of the fabrication process of the ZnO and ZnO/ZnS core/shell nanowire (NW)-based devices. (**a**) Preparation of substrate by deposition of ZnO seed layer by RF sputtering on ITO-coated PET substrate. (**a**\({}^{\prime}\)) Top view of scanning electron microscopy (SEM) image of ZnO NW array on substrate. (**b**) ZnO NW array grown using hydrothermal methods, and (**c**) ZnO/ZnS core/shell NW array achieved by synthesis of ZnS shell. (**d**) The ZnO and (**e**) ZnO/ZnS core/shell NW array devices fabricated by Au electrode deposition after RIE processing for opening of spin-coated photoresist layer. The performance of the ZnO and ZnO/ZnS core/shell NWs under UV illumination is shown in Figure 3. Figure 3a,b show the current-voltage (\(I\)-\(V\)) curves of the vertically grown ZnO and ZnO/ZnS core/shell NWs in the dark and under UV LED illumination (365 nm) with a light intensity of 0.78 mW/cm\({}^{2}\).