Roles of silver nanoclusters in surface-enhanced Raman spectroscopy

Abstract

The cause for the huge enhancement factors of surface-enhanced Raman spectroscopy (SERS) by the addition of small silver nanoclusters is theoretically investigated focusing on the difference between resonance Raman activity and surface plasmon effects. First, the resonance and off-resonance Raman spectra are calculated using the incident light wavenumbers of the low-lying charge transfer excitations for the surface (S) and vertex (V) complexes of pyridine molecule attaching to three small silver nanoclusters: Ag 5 , Ag 10 and Ag 20. As a result, it is found that the incident radiation dramatically increases the resonance Raman activities with the enhancement factors up to 10 12. This indicates that the resonance Raman effects are dominant in the enhancement factors of SERS, at least when to use small silver clusters. It is also found that the resonance Raman spectra significantly depend on the adsorption sites given in S or V complexes, and on the inclusion or exclusion of the long-range correction for density functional theory, irrespective of the size of the silver clusters. The electromagnetic field enhancement effects called "surface plasmon effects" are also examined for the Ag 20 cluster to confirm this conclusion. Consequently, the enhancement in the electric field are roughly evaluated as less than one for the static polarizability of this small cluster. It is, therefore, concluded that the resonance Raman activity effect is dominant in the huge SERS enhancement factors for, at least, small silver nanoclusters.

Adsorption Geometry of Alizarin on Silver Nanoparticles: A Computational and Spectroscopic Study

“…Further insights on the adsorption geometry of AZ on silver surface could be theoretically achieved by performing calculations modeling the substrate with as slab or increasing the number of atoms [73][74][75][76][77][78][79][80][81][82][83] and experimentally by using other techniques such as X-ray photoelectron spectroscopy (XPS) and Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy [38,39,43,84].…”

A Comprehensive Review on Raman Spectroscopy Applications

“…This phenomenon could be explained using two different theories-the electromagnetic and chemical theories. The electromagnetic theory has strong experimental support [27] and will be the only one discussed in this section.…”

Insight into SERS Chemical Enhancement Mechanism of Fungicide Thiram Adsorbed on Silver Nanoparticles

“…An optimal scale factor for harmonic frequencies being 1.0353 was applied [53]. The Ag 20 pyramidal cluster which has been recommended by several computational works in literature is used to reproduce the AgNPs surface model [37][38][39][40][41][42][43][44]52]. Geometries of all possible interaction configurations between the THR and Ag 20 clusters are optimized in the gas phase with tight convergence criteria and an ultrafine integration grid and loose symmetry.…”

“…Several papers have been published on this topic, including studies on large and complicated molecules like Tinuvin P [32], crystal violet [33], paroxetine [34], amoxicillin [35], and L-valine phosphonate dipeptides [36]. Many authors have utilized pyramidal Ag 20 or Au 20 clusters as models to reproduce the NPs surfaces [37][38][39][40][41][42][43][44], although it has recently been reported that these models are not the most stable cluster isomers [45]. Despite this harsh simplification, these works provide valuable insight into the interaction of different molecules onto SERS substrates.…”