# Gold Nanofilm-Coated Porous Silicon as Surface-Enhanced Raman Scattering Substrate
## 1 Introduction  
Surface-enhanced Raman scattering (SERS) is a sophisticated analytical technique to detect minute amounts of target analyte, even down to a single molecule, by providing molecule-specific vibrational spectra. The SERS phenomenon is attributed to two effects: Electromagnetic and chemical enhancement mechanisms. Electromagnetic enhancement is due to the enhanced electromagnetic fields localized on or in the immediate vicinity of a nanostructured metallic surface generated by the excitation of localized surface plasmons. On the other hand, chemical enhancement results from the charge transfer between a metal substrate and the adsorbed molecule onto its surface [1; 2; 3]. Moreover, SERS enhancement relieson the spatially localized regions of intense local electromagnetic field \"hot spots\", which are generated at the junctions or gaps of two or more SERS-active substrates, hence providing a huge SERS signal enhancement [4; 5]. Raman signal enhanced by the surface plasmon resonance was originally observed from a metal colloidal substrate, such as silver (Ag) nanoparticles. Due to the unstable motion of solution-based colloidal metal nanoparticles, a planar substrate, such as Au-coated glass substrate, became a favorable and commercially available SERS substrate. However, the advantages of using metallic nanostructure still attract attention in SERS detecting with high sensitivity or selectivity. PSi with high surface-to-volume ratio due to the open nanopores and Si nanostructure was first reported for use as SERS template for the deposition of metallic film by [PERSON] et al. in 2003 [6]. The metallic nanostructure layer on the PSi substrate was fabricated by immersing PSi into aqueous AgNO\({}_{3}\) solution to produce the nanocomposite SERS substrate, which amplified the Raman signal of R6G by ten orders of magnitude. Later on, [PERSON] et al. the pioneer of fabricating and characterizing PSi, introduced a roughened Ag film-coated PSi (Ag-PSi) planar SERS-active substrate by immersion plating of Ag on PSi, where the roughened Ag film produced randomly spaced dendritic structure. The Ag-PSi was further pretreated with mineral acids (HCl or H\({}_{2}\)SO\({}_{4}\)) to minimize the background signals in the SERS spectrum [7]. Different Ag immersion conditions generate different Ag nanostructure on PSi substrate and, consequently, contribute to different SERS amplification. For example, [PERSON] et al. immersed PSi samples into AgNO\({}_{3}\) solution and a successive thermal annealing to create the Ag nanocrystallites, which consequently removed the background Raman noise and further enhanced Raman signal [8]. A simple dipping of PSi into aqueous AgNO\({}_{3}\) solution is still the common and preferred way to fabricate Ag-PSi SERS-active substrates [9; 10; 11; 12; 13]. Despite the simple and easy solution-based process of fabricating metallic nanostructure on PSi, the intensity fluctuation of SERS signal and irregular hot spot generation, due to the inhomogeneous metallic nanostructure [14], have hindered the progress of using metallic film-coated nanostructure as a SERS substrate. In other words, it is difficult to get the quantitative value of Raman signal using these solution-processed SERS substrates. In addition, the generation of metallic nanostructures using solution processing in some instances requires additional reducing agent and extra chemical treatment. The reducing agent may bring more negative electric charges on a SERS substrate, where the negative charge could repulse the targeting molecules and further weaken the Raman signal. These problems could be prevented by utilizing physical vapor deposition of metallic nanofilm on nanostructured template [15; 16]. [PERSON] et al. fabricated PSi-coated Au thin film SERS substrates by electrochemical anodization of the rough back side of the silicon wafer, followed by Au sputtering via PVD technique at room temperature.