| Abstract
| - Localized surface plasmon resonance (LSPR) is a key optical property of metallic nanoparticles.The peak position of the LSPR for noble-metal nanoparticles is highly dependent upon the refractive indexof the surrounding media and has therefore been used for chemical and biological sensing. In this work,we explore the influence of resonant adsorbates on the LSPR of bare Ag nanoparticles (λmax,bare). Specifically,we study the effect of rhodamine 6G (R6G) adsorption on the nanoparticle plasmon resonance becauseof its importance in single-molecule surface-enhanced Raman spectroscopy (SMSERS). Understandingthe coupling between the R6G molecular resonances and the nanoparticle plasmon resonances will providefurther insights into the role of LSPR and molecular resonance in SMSERS. By tuning λmax,bare through thevisible wavelength region, the wavelength-dependent LSPR response of the Ag nanoparticles to R6G bindingwas monitored. Furthermore, the electronic transitions of R6G on Ag surface were studied by measuringthe surface absorption spectrum of R6G on an Ag film. Surprisingly, three LSPR shift maxima are found,whereas the R6G absorption spectrum shows only two absorption features. Deconvolution of the R6Gsurface absorption spectra at different R6G concentrations indicates that R6G forms dimers on the metalsurface. An electromagnetic model based on quasi-static (Gans) theory reveals that the LSPR shift featuresare associated with the absorption of R6G monomer and dimers. Electronic structure calculations of R6Gunder various conditions were performed to study the origin of the LSPR shift features. These calculationssupport the view that the R6G dimer formation is the most plausible cause for the complicated LSPRresponse. These findings show the extreme sensitivity of LSPR in elucidating the detailed electronic structureof a resonant adsorbate.
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