SERS Comparison from Au, Ag, and Au-Ag Alloys: Insights by the First Principles

Surface enhanced Raman scattering (SERS) is presented via a case study of 2,2’-bipyridyl (22BPY) molecules adsorbed on pure Au and Ag as well as on Au-Ag alloy nanodiscs. Experimental SERS spectra from Au and Ag nanodics show similar peaks, but those from Au-Ag alloy reveal new spectral features. The physical enhancement factors due to surface nano-texture were considered by numerical simulations of light intensity distribution for the nano-textured Au, Ag, and Au-Ag alloy surfaces, but found to cause only minor differences. For the chemical insights of enhancement, the density functional theory (DFT) calculations were performed using Au20, Ag20, and Au10Ag10 clusters of a pyramidal structure for SERS modeling. Binding of 22BPY to the clusters was simulated by considering possible arrangements of vertex and planar physical as well as chemical adsorption models. A qualitative match with experimental SERS results for the pure Au and Ag was obtained. DFT models showing spectral shifts of 22BPY SERS on the alloy nanodiscs are presented.