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

The Journal of Physical Chemistry Letters
Rebecca L M GiesekingGeorge C Schatz

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.

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Citations

May 19, 2020·Applied Spectroscopy·Jeremy F SchultzNan Jiang
Jul 24, 2020·Journal of Physics. Condensed Matter : an Institute of Physics Journal·Jeremy F SchultzNan Jiang
Nov 24, 2020·Journal of Computational Chemistry·Rebecca L M Gieseking
Jan 26, 2021·Journal of the American Chemical Society·Kai BraunAlfred J Meixner
Dec 16, 2020·The Journal of Chemical Physics·Ran Chen, Lasse Jensen
May 4, 2021·Nano Letters·Shuyi LiuTakashi Kumagai
Sep 4, 2019·ACS Nano·Judith LangerLuis M Liz-Marzán
Nov 24, 2018·Journal of the American Chemical Society·Melissa L ClarkClifford P Kubiak
Mar 12, 2020·The Journal of Physical Chemistry Letters·Chih-Feng WangPatrick Z El-Khoury

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