Tuning the topographical parameters of Si pyramids for a better surface enhanced Raman response

Abstract

Development of facile routes for the fabrication of surface enhanced Raman substrates (SERS) along with optimal conditions for a high enhancement factor are significant from an application perspective of SERS. Despite steady efforts to establish high SERS signals, cost effectiveness without compromising the enhanced and robust Raman signal remains a major challenge. To address this aspect, herein, we try to tune the topographical aspects of Si pyramidal textures in pursuit of efficient SERS substrates. These pyramidal surfaces are deployed as a pre-template for adopting a SERS substrate using a cost-effective wet chemical etching method. By controlling the etching time, various topographical parameters namely base size, height, pyramidal number density and uniformity of pyramidal textures are modulated. To make all the surfaces SERS active, a Au (50%)–Ag (50%) alloy nanolayer is post-deposited over them. Furthermore, SERS behavior of all the surfaces is investigated by using Rh6G dye as an analyte molecule. In addition to the high density of hot spots in terms of pyramidal number density, base size and uniformity shows a strong correlation in deciding the substantial SERS response. Furthermore, we find a high enhancement factor (∼1.42 × 10⁸) for the substrate consisting of dense, small and uniformly sized pyramids. Finite Difference Time Domain (FDTD) simulations done on similar structures corroborate our results. Additionally, universal applicability of the proposed substrate is also verified by detecting methylene blue and methyl parathion analyte molecules. These substrates are much cheaper (∼5 USD for 1 × 1 cm²) in comparison with commercially available Klarite SERS substrates (∼100 USD for 2 × 2 mm²). We believe this work provides a critical insight into the design of potential SERS substrates using a significantly cost-effective wet chemical etching process.