Bias-Dependent Chemical Enhancement and Nonclassical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips

Rebecca L.M. Gieseking, Joonhee Lee, Nicholas Tallarida, Vartkess Ara Apkarian, George C Schatz

Research output: Contribution to journalArticlepeer-review

10 Citations (Scopus)

Abstract

Tip-enhanced Raman spectromicroscopy (TERS) with CO-terminated plasmonic tips can probe angstrom-scale features of molecules on surfaces. The development of this technique requires understanding of how chemical environments affect the CO vibrational frequency and TERS intensity. At the scanning tunneling microscope junction of a CO-terminated Ag tip, we show that rather than the classical vibrational Stark effect, the large bias dependence of the CO frequency shift is due to ground-state charge transfer from the Ag tip into the CO π∗ orbital softening the C-O bond at more positive biases. The associated increase in Raman intensity is attributed to a bias-dependent chemical enhancement effect, where a positive bias tunes a charge-transfer excited state close to resonance with the Ag plasmon. This change in Raman intensity is contrary to what would be expected based on changes in the tilt angle of the CO molecule with bias, demonstrating that the Raman intensity is dominated by electronic rather than geometric effects.

Original languageEnglish
Pages (from-to)3074-3080
Number of pages7
JournalJournal of Physical Chemistry Letters
Volume9
Issue number11
DOIs
Publication statusPublished - Jun 7 2018

ASJC Scopus subject areas

  • Materials Science(all)

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