Au_0.5Ag_0.5 Alloy Nanolayer Deposited on Pyramidal Si Arrays as Substrates for Surface-Enhanced Raman Spectroscopy

Silver (Ag) and gold (Au) are the widely used noble metals for surface-enhanced Raman scattering (SERS) applications in the visible range. However, owing to poor chemical stability of silver and interband transition of gold, thereby hindering the detection process, there has always been a need for an alternate material that can provide intense and stable SERS signals by overcoming the above shortcomings. In this work, we report measurements from a SERS substrate based on a Au–Ag alloy as a SERS-active nanolayer on pyramidal Si (P-Si) array surfaces. Fabrication of the substrate is done by following a simple and cost-effective cosputter deposition method on a prepatterned Si surface produced by a chemical etching process. By suitably integrating the merits of SERS activity of Ag, the chemical stability of Au, and the large field enhancement of P-Si, the alloy Au0.5Ag0.5@P-Si exhibits high SERS sensitivity, homogeneity, reproducibility, and chemical stability far beyond those of the individual elements. Using rhodamine 6G (R6G) as a probe molecule, the Au0.5Ag0.5@P-Si substrate shows an ∼28 times enhanced SERS signal as compared to that of pure Au@P-Si and ∼1.5 times that of pure Ag@P-Si. A detection limit down to 10–9 M concentration of R6G is found in our case. Further, an aging study shows the stability of the SERS signal using the proposed Au0.5Ag0.5@P-Si substrates even after a time span of 30 days.