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Experimental evidence suggests an extremely high, possibly evensub-molecular, spatial resolution of tip-enhanced Raman spectroscopy. While theunderlying mechanism is currently still under discussion, two maincontributions are considered: The involved plasmonic particles are able tohighly confine light to small spatial regions in the near-field, i.e. theelectromagnetic effect, and the chemical effect due to altered molecularproperties of the sample in close proximity to the plasmonic tip. Significanttheoretical effort is put into the modelling of the electromagneticcontribution by various groups. In contrast, we previously introduced acomputational protocol that allows for investigation of the local chemicaleffect -- including non-resonant, resonant and charge transfer contributions --in a plasmonic hybrid system by mapping the sample molecule with a metallic tipmodel at the (time-dependent) density functional level of theory. In thepresent contribution, we evaluate the impact of static charges localized on thetip's frontmost atom, possibly induced by the tip geometry in the vicinity ofthe apex, on the TERS signal and the lateral resolution. To this aim, animmobilized molecule, i.e. tin(II) phthalocyanine (SnPc), is mapped by theplasmonic tip modeled by a single positively vs. negatively charged silveratom. The performed quantum chemical simulations reveal a pronouncedenhancement of the Raman intensity under non-resonant and resonant conditionswith respect to the uncharged reference system, while the contribution ofcharge transfer phenomena as well as of locally excited states of SnPc ishighly dependent on the tip's charge.

Are charged tips driving TERS-resolution? A full quantum chemical approach