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Example of electric field localization in colloids and sharp point samples. The field intensity depends on the inter-particle distance and particle shape. |
When a molecule is adsorbed onto, or lies close to a roughened metal surface, a large enhancement in Raman signal can be observed. Enhancements of 5-6 orders of magnitude over regular Raman scattering can be observed dependent on the molecule being analysed. The enhancement comes from the increase of the local optical field intensity in the proximity of sharp points of textured metals such as Au, Ag, Cu, or in the nano-scale gaps between colloidal particles. As well as signal enhancement, one of the other main advantages of SERS over normal Raman is that fluorescence, which interferes with the recording of Raman scattering, is quenched if the molecule is adsorbed on the surface of the metal.
The effect was observed for the first time in 1974 with pyridine molecules absorbed on to an electrochemically roughened silver surface. The main theories behind the SERS process are attributed to two linked mechanisms, namely electromagnetic and chemical enhancement.
It is largely agreed that majority of the enhancement results from the electromagnetic contribution. When a metallic surface is roughened, plasmons are localised on the surface. The plasmon properties – such the wavelength and width of its resonance – depend on the nature of the metal surface and on its geometry. When incident light is directed on to the roughened surface it leads to the excitation of the surface plasmons and consequently the electromagnetic field of the light at the surface becomes greatly increased. Due to this, the Raman scattering of the molecule adsorbed on the surface is amplified. The second contribution is due to a chemical effect and is due to the mixing of the orbitals of the adsorbed molecule and the metal atoms. This second effect is thought to be much weaker than the electromagnetic effect, however it is thought to exist as the magnitude of the enhancement in most cases can not be explained by the electromagnetic contribution alone.
SERS process steps: (1) laser light incident on the metal substrate (2) plasmons excitation (3) light scattered by the molecule (4) Raman scattered light transferred back to plasmons and scattered in air (5)
