Distinguishing chemical and electromagnetic enhancement in surface‐enhanced Raman spectra: The case of para‐nitrothiophenol

Abstract

Surface‐enhanced Raman spectra are simulated using a combined classical electrodynamics/real‐time time‐dependent density functional theory approach and compared to experiments. Emphasis is put on discerning between chemical and electromagnetic enhancement. Therefore, three different calculation scenarios are investigated using para‐nitrothiophenol as a test molecule. In the first one, corresponding to electromagnetic enhancement, we simulate the molecule alone with ab initio computations incorporating the electromagnetic field emitted by a nanoparticle. Chemical enhancement is modeled in the second scenario, where we include not only the molecule into the quantum chemistry calculations but also metal atoms of the nanoparticle. Here, any modification of the electromagnetic field due to the nanoparticle is not considered. In the third scenario, the former two setups are combined and demanding simulations of the hybrid system containing the molecule and the metal atoms exposed to a strongly modified electromagnetic field due to the plasmonic properties of the metallic nanoparticles are considered. Results are compared to our experimentally measured spectra. Based on our analysis, we show here on rigorous grounds that the electromagnetic effect leads to increased absolute Raman scattering cross sections but no change of the relative intensities. In contrast, the chemical effect leads to changes in relative peak height and also to newly emerging bands in the spectrum. These findings will have major implications in any study that concerns the interaction of molecules with metallic nanostructures. Copyright © 2013 John Wiley & Sons, Ltd.

Tip-enhanced Raman scattering—Targeting structure-specific surface characterization for biomedical samples

“…Such a high value was due to the resonance effect. Chemical enhancement of para-nitrothiophenol (p-NTP) and Ag sphere was modeled with ab-initio computations with scenarios (a) that incorporated the electromagnetic field emitted by nanoparticle, (b) both metal and molecule atoms were considered, (c) both of the above scenarios are taken together [22]. The results found to be in accordance with SERS experimental work.…”

SERS and DFT investigation of the adsorption behavior of 4-mercaptobenzoic acid on silver colloids

“…While the electromagnetic component describes the characteristics of the substrate, the chemical contribution is related to molecular binding. Thomas et al [19] have also performed theoretical and experimental studies to distinguish EM and CE effects of p-nitrothiophenol adsorbed on the Ag surface. In their approach, classical electrodynamics was coupled with quantum chemistry to include nanoparticle plasmon fields in the calculation of molecular optical properties.…”

Spatial resolution of tip-enhanced Raman spectroscopy – DFT assessment of the chemical effect

“…The best known techniques based on plasmonic enhancement are surface and tip-enhanced Raman scattering (SERS and TERS, respectively) [16][17][18] as well as tip enhanced infrared absorption (TEIRA). 19,20 Signal enhancements observed in these techniques can be rationalized by two phenomena: the electromagnetic effect 8,[21][22][23][24][25][26][27][28] and the chemical effect. 8,9,[29][30][31][32] The former corresponds to the creation of a locally confined strong electric field by an incident light source, i.e.…”

“…8,9,33,34 These three contributions can lead to changes in the shape, spectral position and intensity of Raman bands due to changes in the local environment of the substrate under investigation. 22 Interestingly, several recent TERS experiments have strongly indicated spatial resolutions of 1 nm or smaller (i.e., single molecule resolution). [35][36][37][38][39][40][41] Such high resolutions are unexpected considering the dimensions of the commonly used plasmonic nanoparticles.…”