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

Abstract

Experimental evidence suggests an extremely high, possibly even sub-molecular, spatial resolution of tipenhanced Raman spectroscopy (TERS). While the underlying mechanism is currently still under discussion, two main contributions are considered: The involved plasmonic particles are able to highly confine light to small spatial regions in the near-field, i.e. the electromagnetic effect, and the chemical effect due to altered molecular properties of the sample in close proximity to the plasmonic tip. Significant theoretical effort is put into the modelling of the electromagnetic contribution by various groups. In contrast, we previously introduced a computational protocol that allows for investigation of the local chemical effect -including nonresonant, resonant and charge transfer contributions -in a plasmonic hybrid system by mapping the sample molecule with a metallic tip model at the (time-dependent) density functional level of theory. In the present contribution, we evaluate the impact of static charges localized on the tip's frontmost atom, possibly induced by the tip geometry in the vicinity of the apex, on the TERS signal and the lateral resolution. To this aim, an immobilized molecule, i.e. tin(II) phthalocyanine (SnPc), is mapped by the plasmonic tip modeled by a single positively vs. negatively charged silver atom. The performed quantum chemical simulations reveal a pronounced enhancement of the Raman intensity under non-resonant and resonant conditions with respect to the uncharged reference system, while the contribution of charge transfer phenomena as well as of locally excited states of SnPc is highly dependent on the tip's charge.

Chemical Enhancement vs Molecule–Substrate Geometry in Plasmon-Enhanced Spectroscopy

“…Furthermore, recent theoretical insights were provided on the local chemical sample-nanoparticle interactions in plasmon-enhanced spectroscopy addressing the entire CM. − The interactions include nonresonant, resonant, and charge-transfer contributions, entirely based on quantum chemical simulations. Computational results indicate the enhancement of specific vibrational modes by several orders of magnitude, merely originating from the CM. − Especially in the scope of TERS, considerable amplification of specific modes might be connected to charges localized at the apex atom of the tip’s frontmost plasmonic nanoparticle, as shown very recently by our group.…”

“…We follow our recently reported fully quantum mechanical approach to assess the entire CM in plasmon-enhanced Raman spectroscopy comprising nonresonant, resonant, and charge-transfer contributions. − Thereby, we consider exclusively A-term contributions, while B-term contributions are currently not implemented in our custom code, see section Computational Details. These first principle methods provide a theoretical complement to our experimental results.…”

“…In addition to the simulations addressing the photophysical properties of the isolated CoPc, quantum chemical calculations were performed on a CoPc immobilized on a gold(111) surface to mimic the chemical effect in the scope of SERS. Therefore, our lately established computational approach to assess nonresonant and resonant contributions of the so-called “chemical effect” in tip-enhanced Raman spectroscopy ,, was adapted to describe plasmonic hybrid systems in the frame of SERS. In the first step, one CoPc molecule was optimized on top of a Au(111) slab, comprising 147 gold atoms within three layers (3 × 7 × 7), using the CP2K 7.1 program .…”

Single-Molecule Surface-Enhanced Raman Spectroscopy

“…It is worth mentioning that the mechanism of some experimental phenomena of SM-SERS has not yet been fully explained. Several reports have proposed that some previously ignored physical mechanisms, such as Rayleigh scattering, discrete interaction, and quantum interaction need to be considered in SM-SERS [23][24][25][26].…”

“…Indeed, the mechanism of SERS is not completely clear, especially for single-molecule events. It has been proposed that some previously ignored physical effects such as Rayleigh scattering, discrete interaction, and quantum interaction deserve to be considered in the mechanism [23][24][25][26]. Thus, it is necessary to extend the SERS theory to take physical effects into account.…”