Semiempirical quantum-chemical methods can provide insight the surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) magnitude and mechanism that is not readily available experimentally. A semiempirical INDO/SCI approach allows us to decompose the total SERS enhancement factor at each wavelength into electromagnetic and chemical terms, with proper treatment of resonant charge-transfer contributions to the enhancement. The method also enables us to determine the dependence of the enhancement on the electrochemical potential. Using this method, we show that both the electromagnetic and chemical enhancements are strongly wavelength-dependent, contrary to the assumptions made in earlier DFT-based models.<br/><br/>Our INDO/SCI-based model has also contributed to understanding various experimental results. In TERS imaging with a CO-terminated plasmonic Ag tip, the TERS intensity is bias-dependent. Our model reveals that the bias dependence is due to chemical enhancement, where a positive bias tunes a charge-transfer excited state close to resonance with the Ag plasmon. This change 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. Similarly, strong SERS enhancement on organic substrates can be attributed to strong chemical enhancement when a charge-transfer excited state is near resonance with the incoming light.

Semiempirical modeling of SERS and TERS enhancement mechanisms

2019 Fall National Meeting

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