# Gold Nanofilm-Coated Porous Silicon as Surface-Enhanced Raman Scattering Substrate
## 3 Results and Discussion
This greater intensity of PSi-Au thin film is due to the porous structure of the composite itself, surface roughness, size of the Au nanoparticle structure in thin film, and the agglomerated condition of the Au nanostructure on PSi (Figure 5b), which facilitated the formation of higher number of electromagnetic hot spots at the inter-particle junctions [5; 15]. Moreover, the enhancement factor (EF) value of the fabricated PSi-Au and Si-Au in comparison to the source Si wafer was determined by applying the equation \(EF=\mathrm{I_{SERS}}\,\mathrm{C_{SERS}})/(\mathrm{I_{Raman}}\mathrm{C_{Raman}})\)[27]. Here, \(\mathrm{I_{SERS}}\) is the SERS intensity of the R6G on the PSi-Au or Si-Au substrate, and \(\mathrm{C_{SERS}}\) is the corresponding concentration of the R6G analyte (10\({}^{-6}\) M). On the other hand, \(\mathrm{I_{Raman}}\) stands for the Raman intensity of the R6G analyte on Si wafer, and \(\mathrm{C_{Raman}}\) is the corresponding R6G dye concentration of (10\({}^{-2}\) M). Hence, according to the experimental data (Figure 5c,d), the EF of the PSi-Au and Si-Au substrates was calculated from the average SERS intensities of the major fingerprint peaks of R6G at 612, 1184, 1362, and 1510 cm\({}^{-1}\). The calculated average EF values are \(2.7\times 10^{5}\) for PSi-Au substrate and \(9.7\times 10^{4}\) for PSi-Au film. Figure 4: (**a**) Raman spectra of R6G (10\({}^{-2}\) M) obtained from pure Si wafer, and (**b**) Raman spectra of R6G (10\({}^{-6}\) M) on bare PSi substrate. SERS spectra of R6G (10\({}^{-6}\) M) adsorbed on (**c**) Si-Au thin film and (**d**) PSi–Au thin film. for Si-Au substrate, and the increment of the EF value for the PSi-Au substrate in comparison to Si-Au is around 176% Figure 5c,d. SERS spectra of R6G dye adsorbed on Si wafer coated with 5, 7, and 10 nm thin Au films are presented in Figure 6a. Among the three layers, SERS spectra of 5 and 7 nm Au thin layer-coated Si wafer produced R6G fingerprint peaks which are more intense and well distinguished in comparison to 10 nm thin film. Among the three composites, Si-Au thin film of 7 nm proved the best substrate for SERS enhancement of R6G Raman tag (Figure 6c). On the other hand, SERS spectra of PSi-coated Au thin films with the same thickness as of Si-Au also showed the same trend of SERS intensity (Figure 6b). PSi-Au thin film of 7 nm thickness showed the highest SERS spectra, while 10 nm the least intense spectra, but still well distinguished. However, there is a clear distinguishing feature among the R6G SERS spectra between the two platforms, PSi-Au and Si-Au thin films. The greater SERS intensity of PSi-Au thin films is due to the increased surface roughness and in-homogeneity of Au nanocrystallite distribution on PSi surface, as well as the bigger particle size of nanocrystallites [15]. Furthermore, the in-homogeneity of Au nanocrystals causes the generation of hot spots due to the coupling of the electromagnetic field of the adjacent particles and the consequent enhancement of localized surface plasmon resonance. However, declining of the SERS intensity of R6G signatory peaks exhibited the same trend for both composites, a gradual increase from 5 to 7 nm with the best output, followed by sharp decline of the peak intensity for the 10 nm thickness. The lower SERS intensity at 5 nm Au film thickness could be explained by the presence of smaller Au size due to short e-beam deposition time, hence localization of  
Figure 5: SEM images of (**a**) Si–Au thin films, and (**b**) PSi–Au thin films. (**c**) SERS spectra of both Si–Au and PSi–Au thin film composites, and (**d**) comparison of the SERS peak intensity of the two SERS substrates. R6G to these smaller Au nanocrystals had significant SERS effect [16]. On the other hand, 7 nm Au film obtained after a few more seconds of progression of e-beam deposition generated Au nanocrystal of comparatively bigger size.