META 2021, META'12

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Activated Vibrational Modes and Fermi Resonance in Tip-Enhanced Raman Spectroscopy
Hongxing Xu

Last modified: 2011-12-05


Tip-Enhanced Raman spectroscopy (TERS) is a high-sensitivity and high spatial-resolution optical analytical technique with nanoscale resolution beyond the diffraction limit of light, which was discovered more than one decade ago by Stöckle et al. [1]. In TERS a sharp metal tip is used to create a “hot site” to excite the localized surface plasmons and consequently enhance the electromagnetic field and Raman signals in the vicinity of the tip apex [1-6]. Since the tip can be moved three dimensionally to control the position of “hot site” and the corresponding enhancement factor by changing the gap distance between the tip and the substrate, TERS thus overcomes one of the most severe restrictions in the application of surface enhanced Raman scattering (SERS), which usually requires roughness of metal surfaces or aggregations of metal nanoparticles to create “hot sites”, but hardly to be controlled. TERS may solve a wide variety of problems in high vacuum (HV) single crystal surface science, electrochemistry, heterogeneous catalysis, microelectronics, and tribology. In order to conduct spectroscopy sensitive to ambient conditions on a variety of specimens, the use of an HV environment is mandatory [5]. 

In this paper, by exploring the advantages of TERS, we investigate this long-standing issue, the appearance of normally unseen Raman modes of p-aminothiophenol (PATP) molecules, in SERS. Using PATP molecules adsorbed on a gold substrate as prototypical examples, we observed the Raman spectra in high vacuum tip-enhanced Raman spectroscopy (HV-TERS), which are very different from normal Raman spectra of this compound. Raman-active symmetric modes and IR-active asymmetric modes of DMAB are simultaneously observed in the HV-TERS Raman spectra. Much narrower and much more activated (from IR-active) Raman modes in our TERS study than previously reported SERS spectra of DMAB gives more complete vibrational spectroscopic information and confirm strongly the putative production of DMAB. The activation of IR active modes to be Raman active is primarily attributed to enhanced electromagnetic field gradient effects within the gap region of the tip-surface geometry. The enhanced dynamical responses further result in the splitting of some vibrational modes caused by Fermi resonances in the vibrational spectra involving the coupling of a fundamental mode and the overtone or a combinational mode. These findings help to establish the power of TERS and substantially broaden the versatility of TERS as a promising technique for single-molecule spectroscopy.