Experimental study on polarized surface enhanced resonance Raman scattering of rhodamine 6G adsorbed on porous Al2O3 substrates

Abstract

The polarization properties of surface enhanced resonance Raman scattering (SE(R)RS) of rhodamine 6G molecules, adsorbed to a hexagonally ordered gold nanostructure, are studied with the purpose to discriminate between adsorption sites with different plasmonic properties. The nanostructure is based on a self-organizing hexagonally ordered porous Al(2)O(3) substrate sputter-coated with gold. Each hexagonal subunit has D(6h) symmetry, where the symmetry center may act as an isotropic site, whereas the six narrow gaps between the individual Au hemispheres may act as hot-spots. The variation of the depolarization ratio (DPR), measured in resonance for the eight most prominent vibrational modes of the xanthene moiety, is analyzed by rotating the sample. According to theory, the DPR of the SE(R)RS signal obtained from molecules physisorbed in the isotropic sites deviates from the DPR originating from molecules physisorbed in the hot-spots in two ways: 1. The DPR associated with the isotropic sites depends differently on the rotation angle than the DPR associated with the hot-spots. 2. The DPR of the SE(R)RS signal obtained from molecules physisorbed in the isotropic sites depends on the nature of the Raman modes, whereas it for molecules physisorbed in the hot-spots is independent of the nature of the Raman modes. By applying the latter in the analysis of the polarized SE(R)RS data, we conclude that the dominating SE(R)RS signal comes from molecules adsorbed in the hot-spots. However, since the DPR's obtained for Raman modes of different symmetry are slightly different, the SE(R)RS signal must contain an additional contribution. Our analysis shows that the small mode-dependent SE(R)RS signal most likely comes from molecules adsorbed in the isotropic sites. The general result that can be derived from the present study is that by measuring the polarization properties in SE(R)RS and SERS it is possible to discriminate between adsorption sites with different plasmonic properties present in a highly symmetric nanostructure, even when the magnitude of the different contributions are highly different. The consequence of the insufficient spatial resolution with respect to a detailed mapping of the substrate often encountered in unpolarized SE(R)RS and in two-photon luminescence microscopy may thereby be circumvented.

Surface‐enhanced Raman scattering properties of Lewis acid–base complexes ofp‐hydroxybenzoic andp‐mercaptobenzoic acids with CO2in the presence of silver

“…For comparison, in Figs S3 and S4, we also displayed the spectra for the Ag 3 complexes. As discussed previously, these types of complexes are able to form well‐organized interfacial intermolecular systems. Hence, we have determined the depolarized light scattering properties for specific vibrational modes, directly involved in the intermolecular interactions.…”

“…Because these types of complex are able to form well‐organized interfacial intermolecular systems, Raman light‐scattering properties were also calculated for important Raman‐active bands of the complexes. The differential cross sections of depolarized Raman scattering observed at right angles to the incident beam are determined by the activities, whereas the depolarization ratios for both natural and plane‐polarized light are, respectively, given by where and Δα ' are the derivatives of the average and anisotropic dipole polarizabilities.…”

Analysis of temporal evolution of quantum dot surface chemistry by surface-enhanced Raman scattering

“…Under ambient conditions, Ag thin film was oxidized, ended up with a shiny white appearance. In this case, weak C-C stretching modes were also observed from R6G at 1140, 1450 and at 1680 cm −1 33 40 , which proves the improved enhancement of Ag/Ag 2 O thin films with respect to Ag thin films. As a function of film thickness, we found that the highest enhancement was observed from 10 nm Ag/Ag 2 O films (the enhancement observed from 100 nm films were also similar to that of 50 nm films, therefore it is not shown here for brevity).…”

Plasmonic nanostructures for surface enhanced spectroscopic methods

“…Instead of using polymer or silica spheres as the template, porous Al 2 O 3 produced by different anodizing techniques can be applied. [332][333][334] These nanoporous templates are versatile platforms for sensing, storage and separation applications and are created by a two-step anodization of a thin aluminum film on the surface of silicon substrates (anodic aluminum oxide, AAO). The porous size and depths are controlled by the anodic oxidation.…”