## 3 Results and Discussion
### SERS and Surface Morphology of AuNP-Decorated ZnO NRs  
Studies have shown the enhanced Raman signal using Au- or Ag-decorated ZnO NRs, in which the 3D shape of Au or Ag offers a \"hot spot\" and enhances the Raman signal in the resonance and nonresonance mode [10; 12; 14; 16]. The CT occurs in the interface between metal and ZnO, with positively charged metal and negatively charged ZnO [20; 21]. The Raman signal of the analyte absorbed on the ZnO nanostructure with an increased surface-to-volume ratio was further enhanced because of additional absorption from the LSPR mode (resonance) and CT (nonresonance). Therefore, to investigate the ability of the SERS effect using metallic NPs, we decorated ZnO NRs with AuNPs through PLIP. Figure S2 depicts the Raman signal of R6G using Au-decorated ZnO NRs at concentrations ranging from 10\({}^{-6}\) to 10\({}^{-9}\) M. Comparing to the undecorated ZnO NR SERS substrate (Figure 3b, annealed T = 150 \({}^{\circ}\)C), AuNPs did not have any effect on the enhancement of Raman signal. Figure 5 illustrates the SEM images of Au-decorated ZnO NRs, which indicated the nonhomogeneous distribution of AuNPs. Moreover, AuNPs aggregated on the top and between ZnO NRs. Studies have proved that the surface morphology of decorated metallic NPs plays a critical role in enhancing the Raman signal [10; 12; 14; 16]. Only the 3D and isolated metallic NPs enhanced SERS effectively and further increased the Raman signal of the analyte. The surface morphology of ZnO NRs directly affects the size and the shape of grown AuNPs. Therefore, we grew AuNPs on unannealed ZnO NRs using the same PLIP method. Figure 6a depicts the Raman signal of R6G using Au-decorated ZnO NRs as the SERS substrate, in which ZnO NRs were fabricated using unannealed seed layers. Notably, the Raman signal of R6G was enhanced by reducing the full-width of half maximum. Figure 6b illustrates the SEM images of Au-decorated ZnO NRs, which reveal that AuNPs were independently grown on the  
Figure 4: SEM images of ZnO NRs fabricated using seed layers annealed at (**a**) 100 \({}^{\circ}\)C, (**b**) 150 \({}^{\circ}\)C, (**c**) 200 \({}^{\circ}\)C and (**d**) 250 \({}^{\circ}\)C.  
top and sidewall of ZnO NRs. The surface morphology of Au-decorated ZnO NRs fabricated using annealed (Figure 5) and unannealed seed layers was compared (Figure 6b), which revealed that the annealed seed layers enhanced the growth of the ZnO NRs with (002) c-axis orientation and improved the Raman signal because of higher surface-to-volume ratio and density of ZnO NRs. However, the c-axis orientation and high density of ZnO NRs were favorable to the growth of closely packed and aggregated AuNPs and inhibited the growth of independent AuNPs, which hinders the formation of \"hot spots\" for SERS [20]. By contrast, ZnO NRs grown using unannealed seed layers resulted in ZnO NRs with a large diameter and horizontal (100) and (101) orientations, which are ineffective SERS substrates because of the low density and surface-to-volume ratio. However, AuNPs could be grown homogeneously and independently on ZnO NRs, such that AuNPs offers effective \"hot spots\" for SERS enhancement. Previous studies have used complex and time-consuming methods to achieve 3D metallic NPs on the ZnO nanostructure for enhancing SERS signals [10; 12; 14; 16]. Our method is a simple method of fabricating Au-decorated ZnO NR SERS substrates in a short time.  
Figure 5: SEM images of Au-decorated ZnO NRs of different magnification.