Near-field surface-enhanced Raman spectroscopy of dye molecules adsorbed on silver island films

We report the combined use of scanning near-field optical microscopy and surface-enhanced Raman scattering (SERS) to obtain spectral, spatial and chemical information of molecular adsorbates with subwavelength lateral resolution. Near-field SERS spectra of cresyl fast violet and rhodamine 6G on silver substrates are obtained with short exposure times (less than 100 s) and a signal-to-noise ratio of >20. Spectra from as few as ≈300 molecules or less than 10−2 monolayers adsorbed on about ten silver nanoparticles can be recorded using 200 nm tip apertures. For the near-field spectra, a local Raman enhancement factor of 1013 or greater can be derived from a comparison with fluorescence measurements.