Oxide Coated Nano-Porous Ag Thin Film for Chemical Sensing By Gas Flow Sputtering

Abstract

A completely non-destructive analytical method is one of the great advantages of Raman spectroscopy. However, low efficiency of Raman signals should be improved to be widely used for detecting less than ppm of chemicals. Surface enhanced Raman scattering (SERS), which is a valuable technique for detecting the target substance very sensitively, is a method of amplifying Raman signals by the plasmon coupling effect at the gap between the nanoparticles. For this reason, materials and the nano-structure such as particle size, shape, gap and density were major factors for determining the performance of the SERS substrate.In this study, gas flow sputtering (GFS) was used for fabricating SERS substrates. The nano-porous structure was observed using scanning electron microscope (SEM) and crystal structures were also analyzed by X-ray diffraction (XRD). The density of the nano-porous structure was calculated using the thickness of the film and the mass difference before and after deposition.Silver (Ag) was widely studied as SERS substrate, due to excellent SERS effect and lower price than that of other noble metals. However, Ag has a significant degradation in SERS effect with time which appears to be a problem caused by oxidation of the Ag surface. Owing to this problem, fabricating a highly reproducible SERS substrate is a challenging issue and many researchers are conducting study.To solve this problem, Ag dense film and oxide film were sequentially deposited. Then, Ag nano-porous thin film was deposited on the oxide film to form a dielectric nano-gap between Ag dense film and Ag nano-porous thin film. Also, to protect from oxidation, Ag nano-porous thin film was coated with a thin oxide film using in-situ co-sputtering. The structure of nano-porous thin film was controlled by varying the sputtering power, time, process pressure.As a result, rhodamine 6G of less than 0.5 uM was successfully detected and it showed much improved Raman signal compared with single Ag nano-porous thin film. In addition, the aging problem, which was the most important issue, also reduced noticeably.