Tip-Enhanced Raman Spectroscopy of Graphene Nanoribbons on Au(111) Akitoshi Shiotari, †,‡ Takashi Kumagai,* ,† and Martin Wolf † † Department of Physical Chemistry, Fritz-Haber Institute of the Max-Planck Society, Faradayweg 4-6, 14195 Berlin, Germany ‡ Department of Chemistry, Graduate School of Science, Kyoto University, 606-8502 Kyoto, Japan * S Supporting Information ABSTRACT: 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 × 10 5 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. ■ INTRODUCTION Tip-enhanced Raman spectroscopy (TERS), a technique combining scanning probe microscopy (SPM) with Raman spectroscopy, is a powerful variant of surface-enhanced Raman spectroscopy (SERS). 1-6 In TERS a sharp metallic tip is used to generate a localized electromagnetic field (plasmon polar- iton) in the gap between the tip and surface under laser illumination, which can lead to a giant enhancement of Raman scattering of adsorbates by a factor of 10 6 -10 9 . 7 The enhancement is often classified into chemical and electro- magnetic effects; the former is essentially determined by electronic properties of the adsorbates, while the latter is equally effective for all species. Therefore, TERS promises a high sensitivity and selectivity for chemical identification at the nanoscale. A remarkable spatial resolution of a few tens of nanometers has already demonstrated by mapping Raman signals from dye molecules adsorbed on flat metal surfaces during the past decade. 7 Especially, TERS measurements under ultrahigh-vacuum (UHV) conditions offer the additional advantages of extremely high spatial resolution and direct comparison between local TER spectra and SPM images on atomically well-defined surfaces. 8-10 Recent UHV-TERS measurements at cryogenic temperatures demonstrate the direct observation of a single molecule along with vibration spectroscopy with an exceptional high stability. 11 Such a single molecule Raman spectroscopy will provide not only the structural details (chemical identification) but also new insights into the local dynamics of adsorbates. Although the capability to measure vibrational spectra at room temperature is also one of the distinct advantages of TERS, thermal fluctuations (instability) of the system make the characterization much more difficult. It is known that the spectral features in TERS frequently show a temporal fluctuation in the intensity, 12-18 which is termed as “blinking”. This blinking is also commonly observed in SERS and causes “on-off” features of vibrational structures in the Raman spectra on the time scale of seconds. 19-24 Previous studies have pointed out several origins of blinking; thermal diffusion of adsorbates 12-15,20-22 and/or of contaminations into hot spots, 18,22 changing adsorbate orientation, 15-17 charge transfer between the tip and adsorbates, 14 photoinduced fragmentation of molecules, 23 and intramolecular vibronic coupling. 24 It should be noted, on the other hand, that the blinking was not observed in some cases. 8,11,25 The blinking is a subtle but serious problem because it causes some ambiguities in the interpretation of TER spectra and related to the practical performance. Therefore, it is important for better understanding of TERS to reveal the detailed mechanism. Here we report a TERS study of graphene nanoribbons (GNRs) prepared on a Au(111) surface at room temperature under UHV conditions. The bottom-up fabrication of GNRs using on-surface polymerization provides precisely defined molecular structures on nanometer length scales. 26 The electronic and vibrational properties of the well-defined GNRs can be controlled via their width 27,28 and edge structure, 29 i.e., zigzag or armchair. We observed the character- istic vibration modes of GNRs in both the far- and near-field Raman spectra, whereby a strong enhancement of the intensity up to 4 × 10 5 is observed in the near-field. On the other hand, blinking of the Raman signal is observed in the near-field Received: March 25, 2014 Revised: May 13, 2014 Published: May 13, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 11806 dx.doi.org/10.1021/jp502965r | J. Phys. Chem. C 2014, 118, 11806-11812