TY - JOUR
T1 - Bias-Dependent Chemical Enhancement and Nonclassical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips
AU - Gieseking, Rebecca L.M.
AU - Lee, Joonhee
AU - Tallarida, Nicholas
AU - Apkarian, Vartkess Ara
AU - Schatz, George C.
N1 - Funding Information:
This research was supported by the grant of NSF Center for Chemical Innovation dedicated to Chemistry at the Space-Time Limit (CHE-1414466).
PY - 2018/6/7
Y1 - 2018/6/7
N2 - 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.
AB - 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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U2 - 10.1021/acs.jpclett.8b01343
DO - 10.1021/acs.jpclett.8b01343
M3 - Article
C2 - 29782171
AN - SCOPUS:85047480494
VL - 9
SP - 3074
EP - 3080
JO - Journal of Physical Chemistry Letters
JF - Journal of Physical Chemistry Letters
SN - 1948-7185
IS - 11
ER -