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Raman Effect Brings Nano up to Size : Raman/Rayleigh Imaging of Nanosized Materials

June 08, 2003

Materials made with (or from) nanophases received considerable attention in the last few years but their characterisation is not easy. Unique properties (conductivity, diffusion, reactivity, sintering, mechanical strength….) have been reported for nanostructured materials, all of which result from the interfacial characteristics. Raman scattering is a unique tool for the characterization of such materials, providing information on the chemical bond arrangement and short-range order in amorphous, nanocrystalline or nanosized phases. Usual X-Ray diffraction techniques fail to analyse nanosized/nanocrystalline materials: small size effect and short-range disorder are hardly distinguished and long counting times are required. On the contrary, Raman spectroscopy has demonstrated its capability to analyse low crystallinity and amorphous materials. Most of the properties controlled by the particle size (electrical conductivity, mechanical strength, etc) are correlated to Raman parameters, which can therefore be used to predict these properties.

Raman spectroscopy is an optical technique and can therefore be performed through different optical devices: camera lenses, microscopes, etc. The size of the laser beam determines the surface analysed in one shot. Typically ~1000 mm2 are illuminated in a macro configuration. The laser spot is reduced down to ~1 mm2 for measurements with long focus, high magnification microscope objectives.

A sample “image” based on one Raman (or Rayleigh) parameter can be obtained by mapping the sample using a XY moving plater, with a lateral resolution close to 0.1mm. Raman spectroscopy is a unique tool to analyse nanophases dispersed in a matrix (e.g. pigments of ceramic glaze, precipitates in a fibre coatings, etc), surface-formed nanophases (corrosion mechanisms, semiconductors) and solid-state devices.

IMAGING THE NANOPHASES
Raman map of a SiC SA3® (Ube) fibre section has been made ; the grain size has been calculated from the D’/G bands intensity ratio. Smaller grains are located in the core. This results from the heterogeneous spinning, curing and firing of the polysilane precursor. Because of the strong resonant character of C-C bond Raman spectrum, the penetration depth is small and the technique offers a unique tool to map surfaces. In this case Rayleigh images, recorded using the elastic light scattering (i.e. the “zero” wavenumber Raman spectrum) are very informative and are very useful to select appropriate regions for the recording of Raman images. Note the counting time required for a Rayleigh image is an order of magnitude shorter than for a Raman image.

CNRS Institut des Sciences Chimiques Seine-Amont

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