Au/Ag SERS active substrate for broader wavelength excitation

Self-organized Ag nanoparticles arrays capped with Au layer are produced by depositing Ag nanoparticles on ion beam produced ripple patterned Si substrate followed by Au deposition. Au/Ag combination of nanoparticles shows intermediate plasmonic properties of Ag and Au nanoparticles by overcoming the associated drawbacks. Such combination redshifts the Localized Surface Plasmon Resonance (LSPR) compared to pure Ag and makes them suitable to excite the surface plasmon by both 532 nm and 785 nm laser for a wider range SERS application. The optimized Au deposition decreases the interparticle gap and elongates the nanoparticles along the ripple direction and makes a larger redshift in the LSPR. FDTD simulation also revealed that the electric field enhancement is higher along the ripple direction for both exciting lasers and shows that the LSPR of the combined Au/Ag system is shifted towards 785 nm. The limit of detection of Crystal Violet using the optimized Au/Ag substrate is found to be 10−10 M for 532 nm laser and 10−8 M for 785 nm laser, respectively. The substrate shows good spatial uniformity with a relative standard deviation of less than 10% with SERS enhancement factors in the order of 104-105 for both lasers. These studies suggest that the as-prepared substrate is a promising candidate for detecting molecules sensitive to different wavelengths of laser excitation.