# Enhancing SERS Intensity by Coupling PSPR and LSPR in a Crater Structure with Ag Nanowires
## 3 Results and Discussion
### Coupling of the Ag Crater Structure and AgNWs  
To generate strong field enhancement by coupling with the LSPR and PSPR, 5, 10, and 15 cm\({}^{3}\) of AgNWs dispersion was spray-coated on the inside of the crater and the flat, respectively. The morphology of AgNPs was measured using FE-SEM, as shown in Figure 3a-f.  
The AgNPs were coated almost uniformly on the entire surface of both the flat and the crater. Additionally, as the amount of the AgNPs dispersion was increased, the density of AgNPs and the number of AgNW junctions also increased. However, under  
Figure 3: Top-view images from the field-emission scanning electron microscope (FE-SEM) of the flat surface (**a–c**) and inside of the crater (**d–f**) coated with different amount of AgNPs dispersion, where (**a,d**) used 5 cm\({}^{3}\), (**b,e**) used 10 cm\({}^{3}\), and (**c,f**) used 15 cm\({}^{3}\). Schematic representation of the area on (**g**) the flat surface and (**h**) in the crater.  
Figure 2: (**a**) Schematic of the fabrication process of the GaAs crater structure with the Ag film. (**b**) Optic microscope image of the crater and depth profile. (**c**) Raman spectra on the crater and flat structure with an exposure time of 20 s and pyridine solution of 0.1 M. FDTD simulation at (**d**) the crater and (**e**) the flat structure.  
the same conditions, the density of AgNPsNWs inside the crater was lower than that of the flat, which is explained by the difference in area as shown in Figure 3g,h. Here, areas A and B represent the flat area of the top surface of the crater and the inclined area inside the crater, respectively. Area B is approximately 1.72 times larger than Area A. Therefore, if the same amount of AgNPsNWs dispersion is coated, the density of AgNPs inside the crater is lower.  
Figure 4a shows the Raman spectrum of 0.1 M pyridine molecules on the flat surface when coated with 5, 10, and 15 cm\({}^{3}\) of AgNPsNWs dispersion.  
Figure 4: Raman spectra measured in the 0.1 M pyridine solution on (**a**) the flat surface and (**b**) in the crater structure coated with different amount of AgNPsNWs dispersion. (**c**) Relative Raman intensities of the pyridine peak at 1003 cm\({}^{-1}\) according to the amount of AgNPsNWs dispersion.  
Similar to the previous result, the Raman intensity appeared at the peak positions of 1003 cm\({}^{-1}\) and 1034 cm\({}^{-1}\) for the aqueous pyridine solution. The Raman intensity was amplified as the amount of the coated AgNWs dispersion was increased due to the strong plasmon coupling between the Ag film and the AgNWs, and among the AgNWs. Figure 4b shows the Raman spectrum of the pyridine molecules adsorbed inside the crater under the same conditions as those in Figure 4a. The SERS intensity inside the crater structure was amplified as the amount of AgNWs dispersion increased. Therefore, the high density of AgNWs on both the flat surface and in the crater forms numerous hot spots, contributing to the strong SERS enhancement. Figure 4c shows the Raman intensity on the flat and inside the crater structures at the pyridine peak position of 1003 cm\({}^{-1}\), according to the amount of AgNWs. For the crater coated with 15 cm\({}^{3}\) of AgNWs dispersion, the SERS intensity was enhanced by approximately 17.4 times more than it was on the flat surface without the presence of the AgNPs. Despite the low density of AgNWs inside the crater, the higher Raman signal than that of the flat surface can be explained by the SERS effect in the crater structure, as demonstrated above. This is due to the strong coupling between the LSPR generated by the densely coated AgNWs and the PSPR generated by the crater, which is a metal microstructure. In addition, if the density of AgNPs in the crater increases, a larger SERS effect can be expected.