Quantum Mechanical Effects in High-Resolution Tip-Enhanced Raman Imaging

Tip-enhanced Raman spectroscopy (TERS) with atomically sharp tips can achieve subnanometer spatial resolution due to confinement of the plasmonic electric field. Although enhancement of the local electromagnetic field is generally the dominant enhancement mechanism, chemical interactions may substantially modify the TERS intensity on the subnanometer scale. Modeling these chemical interactions requires a quantum mechanical treatment of both the molecule and the metal tip. The semiempirical INDO/CIS model reproduces the TD-DFT optical spectra of Ag nanoclusters and allows straightforward decomposition of the TERS enhancement into electromagnetic and chemical contributions. In a prototypical Ag_n nanowire–CO₂ system, we show that chemical interactions on a subnanometer scale substantially modify the TERS intensity, suppressing the signal of the Raman-active stretching mode and enhancing the signal of the Raman-inactive mode. The enhancement profile in the TERS images results from changes in three distinct factors: (1) the effective electric field on the nanowire, (2) the extent of ground-state charge transfer, and (3) the extent of mixing between σ-type and π-type molecular orbitals due to symmetry-breaking. These results suggest that a subtle interplay between several quantum mechanical effects are critical to understand the origins of TERS images on the subnanometer scale.