Optical and surface enhanced Raman scattering responses of densely packed Ag-Au alloy nanoparticle films of varied composition and thickness

Sub-monolayer deposition control of pulsed laser deposition technique has been utilized to grow Ag-Au alloy nanoparticle films of different compositions and mass thicknesses using sequential ablation of pure Ag and Au targets. The films produced at high substrate temperature in single step growth have shown strong localized surface plasmon resonance (LSPR) response, which is systematically red-shifted with Au percentage and film mass thickness. The morphology of the films studied from atomic force microscopy revealed densely packed nanoparticles at all compositions with typical particle number density of ∼1010 - 1011/cm2. The optical response of the films was measured using spectroscopic ellipsometry at different angles of incidence and their effective dielectric response was determined from multiple oscillator modeling. The effective dielectric response was found to well predict the optical transmission spectra and also showed systematic red-shift of interband absorption edge with Au percentage in the films conforming alloy formation, in consistent with the results of X-ray photoelectron spectroscopy. Surface enhanced Raman scattering (SERS) measurements carried out at 488 and 633 nm excitation laser wavelengths revealed that Ag and Ag rich films exhibited relatively higher enhancement of SERS intensity as compared to other composition films. These nanoparticle films were found to be efficient SERS substrates for easy detection of trace dye molecules in nano-molar concentration range. The SERS enhancement was found to depend on film mass thickness and alloy composition dependent LSPR response having close overlap with the laser excitation wavelength.