## 3 Results and Discussions  
Figure 2a-e shows top view scanning electron microscope (SEM) images of the self-assembled double-metallic NPs (NPs 1, NPs 2, NPs 3, NPs 4, and NPs 5) on the u-GaN layer, respectively. The NP \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Ag Thickness** & **Au Thickness** \\ \hline NPs 1 & 4 nm & - \\ NPs 2 & 3 nm & 1 nm \\ NPs 3 & 2 nm & 2 nm \\ NPs 4 & 1 nm & 3 nm \\ NPs 5 & - & 4 nm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thicknesses of Au/Ag multilayers to form the double-metallic NPs. Figure 1: Schematic illustrations of the sample preparation process. (**a**) Double-metallic nanoparticles (NPs) on an undoped GaN (u-GaN) layer for measuring the resonant frequency of localized surface plasmon (LSP) modes. (**b**) Double-metallic NPs on a green light-emitting diode (LED) for measuring the enhancement factor (EF) of the photoluminescence (PL) intensity. sizes were all similar for all samples except for NPs 1. More specifically, the average diameter of the NPs was estimated to be \(\sim\)33 nm for NPs 1, but \(\sim\)40 nm for the others. From the analysis given in References [34; 35], the resonant frequency of the LSPs is strongly dependent on the size and density of the NPs. Thus, in this study, we took into account only the Ag/Au ratio of the NPs, regardless of the size or density effects of the NPs. Figure 2(a) shows the absorption spectra for NPs 1, NPs 2, NPs 3, NPs 4, and NPs 5 on the u-GaN layer, as shown in Figure 0(a). The absorption spectra are strongly dependent on the Ag/Au ratio in double-metallic NPs. Because the resonant wavelength of the LSP mode corresponds to the wavelength at peak absorption, the resonant wavelength of the LSP mode for each NP can be obtained, and are plotted in Figure 2(b) as a function of the ratio of Ag film thickness to total film thickness, i.e., Ag film thickness/4 nm. The dashed line is the fifth-order polynomial fitting curve. The data fit very well. The inset picture in Figure 2(b) shows different colored samples due to the different absorption characteristics of each NP. The resonant wavelength of the LSP mode for NPs 1, which was composed of only Ag, corresponded to \(\sim\)516 nm, whereas that of NPs 5, which was composed of only Au, corresponded to \(\sim\)630 nm. These values were similar to those obtained in previous research reports [35; 36]. Additionally, the resonant wavelength of the LSP mode can be tuned by adjusting the ratio of Ag to Au in the NPs within the wavelength range of 516 nm to 630 nm. Thus, we could match the resonant wavelength of the LSP to green- and yellow-emitting LEDs by tuning them. In addition, when the Ag NPs and Au NPs were separately formed, dual peaks in the absorption spectrum were observed, i.e., one corresponding to Ag NPs and the other corresponding to Au NPs, as presented in Reference [36]. However, we observed only a single peak in the absorption spectrum, as shown in Figure 2(a). This implies that Au and Ag were well mixed and that the formed NPs were double-metallic. To investigate the effect of LSPs on PL efficiency, we measured the PL spectrum of the LED without NPs (reference), the LED with NPs 1, the LED with NPs 2, the LED with NPs 3, the LED with NPs 4, and the LED with NPs 5. For external optical pumping, a 405-nm laser diode was used, and the peak PL wavelength of the reference was approximately 528 nm. Figure 3(a) shows the PL spectra for the samples, and Figure 3(b) shows the EFs, i.e., the integrated PL intensities of the SP-enhanced LEDs over the integrated PL intensities of the conventional LED for the samples. Note that the PL intensities in  
Figure 2: Top view SEM images of (**a**) NPs 1 on the u-GaN layer, (**b**) NPs 2 on the u-GaN layer, (**c**) NPs 3 on the u-GaN layer, (**d**) NPs 4 on the u-GaN layer, and (**e**) NPs 5 on the u-GaN layer. Figure 4a depended on the measurement location, and the difference was about 5% to 7%. As Figure 4b shows, the highest EF was observed in the LED with NPs 2.