We report on near-field Raman spectra of a single nanocrystal of adenine molecules using a silver-coated apertureless probe tip. Raman signal is amplified due to surface plasmon polaritons localized at the metallic tip by a factor of more than 2700 times compared with the far-field Raman signal. Plasmonic near-field Raman spectrum showed eight Raman bands assigned to the normal modes of adenine molecule based on the density functional theory calculations. Vibrational frequencies of some Raman bands are observed to have shifted to the values of the corresponding bands, observed using conventional surface enhanced Raman-scattering spectrum. We found that these frequency shifts are caused by the transient states of the adenine-silver complexes by analyzing vibration mode of the complexes; that is, the near-field Raman spectra of adenine agree with Raman spectra of the complexes which are calculated by reducing the bond distance between an adenine molecule and a silver atom. Repulsive forces calculated from reduction of the bond distance were equal to the atomic force applied to the adenine molecule in our Raman NSOM experiment. All results support that the active Raman shift occurs owing to the deformation of adenine molecules pressurized by the silver atoms of the tip.
A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is difficult due to the strict limitation of spatial resolution due to the wave nature of light. While there have been attempts to overcome the diffraction limit by using nonlinear response of materials, near-field optical microscopy could provide better detecting accuracy. In this paper, we present molecular distribution nano-imaging colored by Raman-scattering spectral shifting, which is probed with a metallic tip. The metallic probe tip has been used to enhance the optical field only in the vicinity of probe tip. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS), while in this case a single metallic tip works for the field enhancement in nanometer scale.