# Gold Nanofilm-Coated Porous Silicon as Surface-Enhanced Raman Scattering Substrate
## 3 Results and Discussion  
Freshly prepared PSi is metastable and reactive due to the presence of hydrogen-terminated Si-H\({}_{\mathrm{x}}\) (x = 1, 2, 3) on the external layer of the PSi, therefore vulnerable to react with the impurities present in the electrolytes used in etching process or from the air of the material storage environment B. At ambient condition, the unstable Si-H\({}_{\mathrm{x}}\) undergoes spontaneous oxidation; hence, it is important to coat the PSi surface to improve its stability. In this study, Au nanofilm of different thicknesses were deposited on the PSi substrate by e-beam evaporation, in which the surface of the pores was coated by immediate electroless deposition of Au ions via simultaneous Au reduction and Si oxidation reactions. The surface roughness feature of the PSi provides a large number of nucleation sites due to the lower free energy for nucleation than the Si flat surface B. (1999). Therefore, large numbers of Au nuclei are generated on the PSi surface, which consequently facilitate the formation of Au nanocrystallites on roughened PSi surface at a higher rate in comparison to the conventional flat Si surface. In addition,the edge of PSi induces a stronger electric field, resulting in a higher density of Au crystallites due to the edge specific surface defect that heightens the reactivity and stimulates the Au reduction [23]. The fabricated Si-Au and PSi-Au SERS substrates were first investigated by XRD technique. XRD spectra of Si-Au sample produced four diffraction peaks at 20 = 38.45\({}^{\circ}\), 44.51\({}^{\circ}\), 64.85\({}^{\circ}\), and 77.75\({}^{\circ}\), while PSi-Au sample also represented the four diffraction peaks at the same position with very little blue-shifting at 20 = 38.21\({}^{\circ}\), 44.39\({}^{\circ}\), 64.61\({}^{\circ}\), and 77.69\({}^{\circ}\) (Figure 1). All four peaks are attributed to standard Bragg reflections (crystal planes) of Au (111), Au (200), Au (220), and Au (311) of Au face centers cubic (fcc) lattice, respectively [24]. The intense peak at 20 = 38.21\({}^{\circ}\) indicates the preferential growth in Au (111) direction due to preferred orientation of the Au particle. Moreover, the mean size of the Au nanocrystallites of both composites was determined from XRD data using [PERSON]'s equation [24]:  
\[\mathrm{D}=\ \frac{0.94\ \lambda}{\mathrm{FWHM\ Cos\ \theta}} \tag{1}\]  
where D is the average size of the nanocrystallites, 0.94 is the Scherer constant, \(\lambda\) = wavelength of the X-ray sources, FWHM = the width at half maximum of the XRD peaks registered at \(\theta\) degree, and \(\theta\) = peak position (radians). It was calculated that the average size of the Au nanocrystallites in Si-Au and PSi-Au samples are equal to 5.8 nm and 10.4 nm, respectively, which dictates that the average size of the Au nanocrystallites in PSi-Au is larger than in Si-Au. In contrary, the crystallinity percentage of Si-Au (82.1%) is greater than the PSi-Au structure (74.8%). It is also to be noted that the width of the diffraction peaks of Si-Au is greater and less intense in compare to the intense and narrow diffraction peaks of PSi-Au (Figure 1). This result is due to the size of the crystallites, as large crystallite causes sharp reflections, while small crystal leads to broader reflections [25]. Figure 1: XRD spectra of Si–Au and PSi–Au thin film structures. Scanning electron microscope (SEM) topography of the fabricated Si-Au and PSi-Au with different thicknesses is shown in Figures 2a-c and 2d-f, respectively. SEM top view of Si after Au deposition was featured by the quality smooth surface and homogenous distribution of Au for all of the three Au-Si films of 5, 7, and 10 nm thickness, which in fact relies on the crystallographic orientation and smooth surface roughness of the starting Si substrate [16]. On the other hand, the surface view of the PSi substrate coated with Au nanofilm of different densities shows irregular/tubular morphology; 7 nm Au-PSi shows the morphology of almost globular-shaped particles with agglomerated state.