The chemical effect goes resonant – a full quantum mechanical approach on TERS

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.

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“…For a more detailed discussion of the uncharged SnPc-Ag system obtained using a similar computational protocol, see Ref. 66.…”

Section: Resultsmentioning

confidence: 99%

“…To simulate the tip-enhanced Raman spectroscopy (TERS) experiment, the same protocol as previously published by our group 35,66 was employed. In that, an immobilized molecule, i.e tin(II) phthalocyanine (SnPc), is mapped by a silver tip, in this case modelled by a single silver atom, along a three-dimensional grid.…”

Section: Computational Detailsmentioning

confidence: 99%

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

Fiederling

Kupfer

Gräfe

2021

The Journal of Chemical Physics

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“…TERS is a near-field method to spectroscopically analyze a wide range of chemical and biological samples with a high spatial resolution of a few nanometers, only limited by the tip apex size and shape (generally of diameter~10 nm). The tip acts as an individual plasmonic nanostructure and is scanned over the sample using a scanning probe microscopy technique, as comprehensively explained by Fiederling et al [54]. It provides a localized region of SERS enhancement when brought within a few nanometers from the sample [53].…”

Section: Theoretical Background Of Surface Enhanced Raman Spectroscopy Techniquesmentioning

confidence: 99%

A Comprehensive Review on Raman Spectroscopy Applications

et al. 2021

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Raman spectroscopy is a very powerful tool for material analysis, allowing for exploring the properties of a wide range of different materials. Since its discovery, Raman spectroscopy has been used to investigate several features of materials such carbonaceous and inorganic properties, providing useful information on their phases, functions, and defects. Furthermore, techniques such as surface and tip enhanced Raman spectroscopy have extended the field of application of Raman analysis to biological and analytical fields. Additionally, the robustness and versatility of Raman instrumentations represent a promising solution for performing on-field analysis for a wide range of materials. Recognizing the many hot applications of Raman spectroscopy, we herein overview the main and more recent applications for the investigation of a wide range of materials, such as carbonaceous and biological materials. We also provide a brief but exhaustive theoretical background of Raman spectroscopy, also providing deep insight into the analytical achievements.

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“…Illumination by light excites a coherent oscillation of the surface conduction electrons in these materials that results in an enhanced and confined electric field at their surface. 33,34 Although these early works attributed the enhanced Raman signal to the enhanced electric field located at the probe tip's apex, alternative enhancement mechanisms have also been proposed, including chemical enhancement 35 and a lightning rod effect. 36 Following the trends established in SERS, three of the first groups focused on dye molecules.…”

Section: Brief Historymentioning

confidence: 99%