DOE OSTI2017
The SERS spectra of small organic molecules can be very sensitive to the physisorption or chemisorption of the molecules onto metallic surfaces. However it is challenging to model the results, as typical SERS substrates have a broad distribution of possible surface sites, and the populations of these sites and the SERS enhancement factors associated with them are very difficult to determine, especially when electromagnetic hot spots dominate the observed spectra. However it is important to account for these effects if one is to make a quantitative interpretation of measured SERS spectra. Indeed, the influence of different sites is important to both the electromagnetic and chemical mechanisms in SERS, where the former mechanism is concerned with local field enhancement effects arising from plasmon excitation, while the latter arises from the effect of chemisorption induced charged transfer on the Raman intensities, as can be modeled using static limit Raman intensity calculations[1]. Extensive studies have been performed on simple organics like pyridine bound to gold and silver clusters, and it was found that both the chemical and electrodynamic enhancement factors associated with binding pyridine in various orientations result in noticeable changes in the SERS spectra compared to the gas phase spectrum of pyridine.[2] One of the most important benchmark molecules for SERS and TERS studies is Rhodamine 6G (R6G). This is a strongly fluorescent dye molecule in solution, but when adsorbed onto SERS substrates, its fluorescence is quenched, and the resulting SERS intensities are among the largest observed for any molecule. As a result R6G has played a crucial role in the development of single molecule SERS, and in the development of SERS substrates. [3, 4] Previously, the SERS spectrum of R6G has been modeled as if it were a gas phase molecule. Jensen and Schatz[5] incorporated resonant effects as well as vibronic effects and found that resonance excitation to the S1 excited state of R6G resulted in good agreement with spectra obtained from SERS experiments. In another study, intermolecular charge transfer was studied with R6G bound to a Ag2 cluster.[6] It is unclear if a cluster model as small as two silver atoms can accurately capture the surface effects of a nanoparticle with thousands of atoms. Recent single molecule, ultra high vacuum, tip enhanced Raman spectroscopy (SM-UHV-TERS) measurements for R6G[3] were moderately reproduced by gas phase resonance Raman calculations. Of particular importance in this work was the comparison of TERS spectra at room temperature with those at a low temperature of 10K. It was found that lowering the temperature led to both sharpening of the vibrational spectra, and shifts in the spectra (up to 20 cm-1 with some lines shifting red, some blue and some not at all), suggesting that the molecule transitions from a largely gas phase molecule at room temperature to being strongly chemisorbed at 10K. A potential energy distribution analysis performed by Chulhai and Jensen[3] showed that the modes that were shifted from the gas phase had vibrational activity associated with the ethylamine component of R6G, which is the likely moiety associated with Raman calculations. What remains unresolved concerning the SM-UHV-TERS studies of R6G is direct evidence that the chemical binding of R6G to a silver nanoparticle surfaces causes frequency shifts and/or intensity changes in the Raman scattering intensities of specific modes, especially those involving vibrations of the ethylamine moiety. The theoretical work described in this chapter directly addresses this issue by calculating Raman spectra for R6G interacting with a 20-atom silver cluster, and with a flat Ag(111) surface. Tetrahedral Ag20 is chosen for part of these studies as it provides a model nanoparticle structure that has a well-defined fragment of a (111) surface, as well as having edges and vertices that can be used to model coordinately unsaturated sites on a particle. In addition, Ag20 has been found to provide a reasonable reference structure for determining the chemical contribution to SERS enhancements.[1] Two different geometries of the Ag20/R6G system are considered in the present study, corresponding to two different binding configurations of R6G to a pristine Ag(111) surface. In addition, we study adsorption of R6G on a flat Ag(111) surface of using plane wave DFT calculations to determine optimized geometries and normal mode properties.
77 NANOSCIENCE AND NANOTECHNOLOGY↗