Cantilever tip near-field surface-enhanced Raman imaging of tris(bipyridine)ruthenium(II) on silver nanoparticles-coated substrates

The near-field surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) images of tris(bipyridine)ruthenium(II) adsorbed on a silver nanoparticles-coated substrate were obtained with a scanning near-field optical microscope (SNOM, or near-field scanning optical microscopy, NSOM) using a cantilever tip. In comparison with the most widely used fiber tip for SNOM, the cantilever tip has higher optical throughput and better thermal stability, making it more suitable for detecting the extremely low Raman signal in the near-field spectroscopic investigations. Our preliminary results show that the near-field SERS with the higher spatial resolution can provide richer fingerprint information than the far-field SERS. A comparison of the two types of images shows that there are more SERS than SEF hot spots, and the two types of hot spots do not overlap. More surprisingly, the near-field SERS spectra differ from the far-field SERS spectra obtained on the same sample in the band frequency and relative intensities of some major Raman bands, and some IR-active bands were observed with the near-field mode. These results are explained mainly by the electric field gradient effect and heterogeneous polarization character that operate only in the near-field SERS.