Spatial and temporal variation of surface-enhanced Raman scattering at Ag nanowires in aqueous solution

The spatial and temporal variation of local field enhanced Raman scattering (SERS) at Ag nanowires (NWs) in aqueous solution is presented for an improved understanding of the NW structure-SERS enhancement capability relationship. Crossed Ag NWs and Ag NW bundles are found to have SERS enhancement factors much higher than single Ag NWs because of the higher density of interstitials formed by strong surface plasmon coupling when the wires are close to each other. The role of the interstitials of Ag NWs is enhanced by using unpurified Ag NWs containing Ag nanoparticles or decorating the Ag NWs surface with gold nanoparticles using galvanic replacement reaction and electroless deposition methods. This leads to an improved SERS enhancement capability as compared to purified single Ag NWs. Raman imaging reveals a different temporal response of the SERS signal in aqueous solution in comparison to the photoluminescence background of Ag NWs in the absence of Raman-active molecules. Such a different temporal response can be potentially used to separate the SERS signal from the fluorescence background. The Discrete Dipole Approximation (DDA) method is used for the first time to calculate the local field intensity of two crossed and parallel Ag NWs. Heterogeneities in the SERS spatial distribution of the interstitials and their incident-light polarization dependence are illustrated by comparing the SEM image of a selected unpurified Ag NW bundle with its Raman image.