Abstract
Surface Enhanced Raman Spectroscopy presents a rapid, non-destructive method to identify chemical and biological samples with up to single molecule sensitivity. Since its discovery in 1974, the technique has become an intense field of interdisciplinary research, typically generating >2000 publications per year since 2011. The technique relies on the localised surface plasmon resonance phenomenon, where incident light can couple with plasmons at the interface that result in the generation of an intense electric field. This field can propagate from the surface from the metal-dielectric interface, so molecules within proximity will experience more intense Raman scattering. Localised surface plasmon resonance wavelength is determined by a number of factors, such as size, geometry and material. Due to the requirements of the surface optical response, Ag and Au are typical metals used for surface enhanced Raman applications. These metals then need to have nano features that improve the localised surface plasmon resonance, several variants of these substrates exist; surfaces can range from nanoparticles in a suspension, electrochemically roughened electrodes to metal nanostructures on a substrate. The latter will be the focus of this review, particularly reviewing substrates made by oblique angle deposition. Oblique angle deposition is the technique of growing thin films so that the material flux is not normal to the surface. Films grown in this fashion will possess nanostructures, due to the atomic self-shadowing effect, that are dependent mainly on the deposition angle. Recent developments, applications and highlights of surface enhanced Raman scattering substrates made by oblique angle deposition will be reviewed.
Highlights
Raman scattering is the inelastic scattering of light, first discovered in 1928 [1], from molecular vibrations
The methods finite difference time domain (FDTD) and discrete dipole approximation (DDA) will be considered, while finite element methods have been used in spectroscopy or scattering (SERS) studies, there is little in literature regarding oblique angle deposition (OAD) derived
While changes in geometries were observed as a temperature dependence during oblique angle depositions, there is little in literature that studies this temperature dependence with other nanostructures [15,53], perhaps this will be a new area that will be expanded in the near future
Summary
Raman scattering is the inelastic scattering of light, first discovered in 1928 [1], from molecular vibrations. It was found that in Raman scattering, the energy shift of the scattered photon is either higher in energy than the incident photon (anti-Stokes shift) or lower energy (Stokes shift). The shift will be unique to the type of molecular vibrations, spectral ‘fingerprints’ can be used to identify samples. Raman spectroscopy can be used to identify the differences between natural and synthetic diamonds [2]. One major weakness of this technique is that Raman scattering is a naturally weak effect. Advanced Raman techniques such as surface enhanced Raman spectroscopy or scattering (SERS) are capable of getting around this issue
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