# Gold Nanofilm-Coated Porous Silicon as Surface-Enhanced Raman Scattering Substrate
## 3 Results and Discussion
It is to be noted that the 10 nm Au-PSi, though having rough surfaces, showed no specific nanocrystalline shape, nor the compact arrangement. Due to the surface morphology of PSi, which depends on the crystallographic orientation of the crystal silicon wafers, as well as the nanopores and the surface roughness features of PSi which are defined by the etching conditions, a large number of nucleation sites are generated for the nucleation and growth of Au nanostructures than the smooth Si surface, hence influencing the morphology of the deposited Au thin films [15; 26]. To verify the ability of PSi-Au as SERS substrate, a SERS study was conducted using bare PSi as a control substrate, while Si wafer with and without Au thin film as the reference platforms. Figure 3 shows the schematic of each substrate for Raman measurement and the corresponding Raman spectra without normalization. Obviously, the PSi-Au exhibited the best performance as a SERS substrate. Figure 4 shows the normalized Raman spectra from different experimental conditions for better comparison of the Raman signal observed from different SERS substrates. In the study, bare Si wafer treated with a high concentration of R6G (10\({}^{-2}\) M) and the corresponding Raman spectra were characterized with major fingerprint peaks of R6G at 612, 1184, 1366, and 1512 cm\({}^{-1}\) (Figure 4a). However, the bare PSi treated with a lower concentration of R6G dye (10\({}^{-6}\) M) did not produce any distinct and characterized peaks (Figure 4b). On the other hand, coating of both Si and PSi templates with Au thin films, and subsequent treatment with the lowest concentration of R6G dye (10\({}^{-6}\) M), produced intense and more R6G fingerprint peaks (Figure 4c,d). R6G assigned peaks for the Si-Au substrate were revealed at 614, 771, 1187, 1311, 1364, 1510, and 1648 cm\({}^{-1}\), while for PSi-Au substrate, the peaks were observed at 614, 773, 1186, 1311, 1362, 1509, and 1648 cm\({}^{-1}\). Figure 4c,d exhibited a little peak shifting only in few instances. However, the vibrational spectra of R6G for both normal Raman and SERS study comply with previously reported studies [15; 27]. The intensity of the well distinguished peaks generated on both SERS platforms demonstrated a remarkable enhancement,  
Figure 2: SEM images of Si–Au thin films of (**a**) 5 nm, (**b**) 7 nm, and (**c**) 10 nm; and PSi–Au thin films of (**d**) 5 nm, (**e**) 7 nm, and (**f**) 10 nm. which is due to the electromagnetic enhancement and the chemical enhancement, due to the charge transfer mechanism between the adsorbed R6G and Au crystallites in the corresponding Au thin film substrate. The huge electromagnetic enhancement is due to the hot spots generated at the junctions between the platform components or among the Au crystallites assembled in the thin film [5; 28]. Figure 3: Schematic of the (**a**) Si, (**b**) Si-Au, (**c**) PSi, and (**d**) PSi-Au substrates and the corresponding Raman spectra plotted without normalization for the different concentration of R6G dye. Moreover, SERS study of PSi-Au versus Si-Au was conducted to understand the impact of the porous structure of PSi on SERS effect. From the SEM topography, structural differences between the two platforms were also observed. Si-Au thin film was featured by homogenous distribution of the Au particles with smooth surfaces (Figure 5a). On the contrary, PSi-Au thin film was characterized by the tubular particle structures in aggregated form with rough surfaces (Figure 5b). However, both Si-Au and PSi-Au thin films produced identical, well distinguished, and intense SERS spectra with R6G fingerprints peaks (Figure 5c). R6G adsorbed on PSi-Au thin film produced more intense peaks than Si-Au thin film (Figure 5d).