The near field response of molecules coupled with plasmons at atomistic resolution.

The interaction between nanoparticles on mirror (NPoM) nanostructures and molecules is of great significance for the development of plasmon-enhanced spectroscopy (PES) techniques. However, the coupling mechanism between resonantly excited molecules and plasmonics has not been fully understood. In this work, we took viologen molecules within an Au plasmonic nanocavity (AuNC) as an example to illustrate how resonant molecules influence the near-field distributions. We found that the near-fields are highly enhanced and the near-field distributions are altered when the monocationic viologen (V⁺˙) is in resonance. In the AuNC, the near-field enhancement of a molecule is significantly enhanced by the adjacent molecules. However, the average near-field enhancements experienced by each molecule decrease with the increasing coverage of the molecular monolayer. Furthermore, the contributions of molecules to the near-field enhancement initially increase and then decrease as coverage increases. The interactions between the molecules and the nanocavity exhibit negative contributions to near-field enhancement. Overall, this work offers valuable insights into the impact of resonantly excited molecules on near-field enhancements in nanocavities and offers guidance for tuning excitation wavelength. We propose that the resonance state and coverage of molecules are critical to improving the sensitivity and specificity of PES techniques.