SERS enhancement, sensitivity and homogeneity studies on bi-metallic Ag-Cu films through tuning of broad band SPR towards red region

Abstract

The objective of this work is to obtain a strong and a broad visible surface plasmon resonance (SPR) and also to tune the wavelength towards red region in thermally evaporated bimetal Ag-Cu film which will act as relatively sensitive surface enhanced Raman spectroscopy (SERS) substrate for the application in bio molecule detection. For the optimization, the morphological property including surface roughness, number-particle density, size distribution and inter-particle distances were altered through varying Cu content in Ag-Cu and temperature by vacuum thermal annealing. The effective dielectric constant of Ag-Cu films is studied from theoretical simulation using extended Maxwell garnet method demonstrating a strong plasmonic character as compared to pure Ag and Cu films. The elemental composition and the surface oxidation states of Ag-Cu film are probed through X-ray photoelectron spectroscopy (XPS). SERS study of RhG molecules exhibit higher enhancement factor for Ag0.75Cu0.25 film (annealed at 100 °C) as compared to pure Ag and Cu by the factor of 1.6 and 50 respectively and congruently the limit of detection was found to be 10−9 M. The higher SERS homogeneity with the least standard deviation of 18% on RhG molecules was studied through Raman mapping and bar plot. A broad and visible SPR band optimized towards red region on Ag0.75Cu0.25film may improve the sensitivity of bio-molecules detection.