In 1978 it was discovered, largely through the work of Fleischmann, Van Duyne, Creighton, and their coworkers that molecules adsorbed on specially prepared silver surfaces produce a Raman spectrum that is at times a millionfold more intense than expected. This effect was dubbed surface-enhanced Raman scattering (SERS). Since then the effect has been demonstrated with many molecules and with a number of metals, including Cu, Ag, Au, Li, Na, K, In, Pt, and Rh. In addition, related phenomena such as surface-enhanced second-harmonic generation, four-wave mixing, absorption, and fluorescence have been observed. Although not all fine points of the enhancement mechanism have been clarified, the majority view is that the largest contributor to the intensity amplification results from the electric field enhancement that occurs in the vicinity of small, interacting metal particles that are illuminated with light resonant or near resonant with the localized surface-plasmon frequency of the metal structure. Small in this context is gauged in relation to the wavelength of light. The special preparations required to produce the effect, which include among other techniques electrochemical oxidation-reduction cycling, deposition of metal on very cold substrates, and the generation of metal-island films and colloids, is now understood to be necessary as a means of producing surfaces with appropriate electromagnetic resonances that may couple to electromagnetic fields either by generating rough films (as in the case of the former two examples) or by placing small metal particles in close proximity to one another (as in the case of the latter two). For molecules chemisorbed on SERS-active surface there exists a "chemical enhancement" in addition to the electromagnetic effect. Although difficult to measure accurately, the magnitude of this effect rarely exceeds a factor of 10 and is best thought to arise from the modification of the Raman polarizability tensor of the adsorbate resulting from the formation of a complex between the adsorbate and the metal. Rather than an enhancement mechanism, the chemical effect is more logically to be regarded as a change in the nature and identity of the adsorbate.

Surface-enhanced spectroscopy

Surface raman spectroelectrochemistry: Part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode

The modification of the absorption, emission, and Raman scattering intensities of molecules near metal surfaces is discussed with emphasis on the last. These modifications are usually referred to as surface selection rules. We show that in Raman scattering, the ‘‘rule,’’ which states that only those modes will be active which belong to the irreducible representation to which α′zz also belongs, is only approximately obeyed. The circumstances under which modes which are represented by nondiagonal derived polarizibility tensors are ‘‘surface‐active’’ are considered, as well as the geometrical strategies which should be followed in studying the Raman spectra of molecules adsorbed on flat, metal surfaces.

Surface selection rules

SMALL clusters of silicon, containing between 2 and 100 atoms, have been studied extensively both because of their intrinsic interest from the point of view of chemical structure and bonding and because of the potential technological applications of cluster-assembled materials1–3. Ab initio quantum-chemical calculations predict that very small clusters should have structures markedly different from that of the crystalline phase4–12. Experiments on two- (ref. 13), three- (ref. 14) and four-atom15 clusters have allowed some comparison with theoretical predictions, but structural information on larger clusters has been only indirect16. Here we report on structural studies of size-selected Si4 , Si6 and Si7 clusters prepared and isolated by low-energy deposition into a solid nitrogen matrix. Surface plasmon-polariton enhanced Raman spectroscopy17–20 yields well resolved vibrational spectra for each of these clusters in which the vibrational frequencies agree well with those predicted for optimized structures calculated by ab initio methods. We confirm that Si4 is a planar rhombus, and find that Si6 is a distorted octahedron and Si7 a pentagonal bipyramid.

Raman spectra of size-selected silicon clusters and comparison with calculated structures

This paper reports the enhancement of long-term oxidation of copper at room temperature by a graphene coating. Previous studies showed that graphene is an effective anticorrosion barrier against short-term thermal and electrochemical oxidation of metals. Here, we show that a graphene coating can, on the contrary, accelerate long-term oxidation of an underlying copper substrate in ambient atmosphere at room temperature. After 6 months of exposure in air, both Raman spectroscopy and energy-dispersive X-ray spectroscopy indicated that graphene-coated copper foil had a higher degree of oxidation than uncoated foil, although X-ray photoelectron spectroscopy showed that the surface concentration of Cu(2+) was higher for the uncoated sample. In addition, we observed that the oxidation of graphene-coated copper foil was not homogeneous and occurred within micrometer-sized domains. The corrosion enhancement effect of graphene was attributed to its ability to promote electrochemical corrosion of copper.

http://pubs.acs.org/doi/pdf/10.1021/nn402150t

Robebt G. Greenler

Papers

Method for obtaining the Raman spectrum of a thin film on a metal surface