Tip-Enhanced Raman Spectroscopy of Graphene Nanoribbons on Au(111)

We report tip-enhanced Raman spectroscopy of graphene nanoribbons (GNRs) fabricated on Au(111) by the on-surface polymerization technique under ultrahigh-vacuum conditions. The 0.74 nm wide armchair GNRs are directly observed by scanning tunneling microscopy at room temperature, and the characteristic vibration modes of GNRs appear in both the far- and near-field (tip-enhanced) Raman spectra. The Raman scattering is enhanced by up to 4 × 105 in the near-field, while a strong intensity fluctuation (blinking) frequently emerges in the time series of the near-field spectra. From the STM observation of a stable adsorption structure of GNRs under the laser illumination and statistical analysis of the intensity fluctuation, we attribute the origin predominantly to thermal fluctuations of the effective radius of the Au tip apex that induces the localized plasmonic field. The intensity distribution is qualitatively reproduced with a simple theoretical model in which the tip apex is approximated by ideal metal sphere.