Surface enhanced Raman scattering in thin films: the importance of Raman analyte-plasmonic particle inverse geometry

Abstract

Most of the successful applications of surface enhanced Raman scattering (SERS) involves placing the Raman analyte molecule over the SERS substrate. This conventional geometry of SERS does not work when the Raman analyte is in the form of a thin film. In this report, we experimentally demonstrate the importance of the rarely explored inverse geometry wherein a plasmonic particle is placed over an analyte thin film for SERS study. Initially, as a case study, the effect of size, concentration, and distribution of gold nanoparticles (AuNPs) on the SERS of Si wafer was performed. The AuNPs, prepared by optimized annealing of direct current sputtered Au, were characterized by scanning electron microscopy, atomic force microscopy, and UV-Visible spectrophotometry. Finally, as an application, SERS in inverse geometry was successfully performed with an electron-beam evaporated Si thin film. For the first time, a working formula has been proposed to determine the experimental enhancement factor (EEF) for the inverse geometry of SERS. The values of EEF were estimated to be 1526 and 3274 respectively for Si wafer and Si thin film for the similar distribution of AuNPs of average size 52 nm. This study provides an insight into the characterization of thin films.