Raman Spectroscopy of nanomaterials - institut de chimie et des ...

ARTICLE IN PRESS+ MODEL8 G. Gouadec, Ph. Colomban / Progress in Crystal Growth and Characterization of Materialsxx (2007) 1e562.5. Resonant Raman Spectroscopy (RRS)If the energy of the laser excitation (usually in the UV-nIR range) is high enough to approachthose of the various electronic states of the material (in other words if the material is coloured),Raman spectra may be mingled with photoluminescence spectra arising from excited electroniclevels. The consequence is the strong enhancement of some vibrational modes (near-resonance/resonance Raman scattering) but one should notice that the probed chemical bonds are ina markedly disturbed state [22,26,27,76e78]. The light penetration is reduced to a few tensof nanometers only (Dz coloured in Fig. 2b), which makes mRS a good method for surface analysis[23,79]. 7 If films or fibre-reinforced composites made of absorbent materials are polished withsurface to growth direction and surface to fibre axis angles slightly different from, respectively,90 and 0 , then in-line scans of the samples provide high spatial resolution across the filmthickness [83e85] or the fibreematrix contact region (interphase) [22].The penetration depth d of the light is directly related to the wavelength-dependent coefficientof linear absorption a l (cm 1 ):d ¼ l excitation4pnk ¼ 1a l ðlÞð8ÞIn Eq. (8), n and k are, respectively, the refraction and extinction indexes. The penetrationdepth d is indeterminate in the (frequent) lack of absorption coefficients but switching excitationwavelengths close to the electronic absorption threshold can help separate surface frombulk Raman contributions [37]. This is illustrated in Fig. 4, where polyaniline fibres spectrarecorded with red, green and blue laser lines, are compared [86]. The bottom spectrum correspondsto the surface of these fibres (maximum electronic absorption) where a ‘‘type I’’monoclinic form of polyaniline is dominant [87]. By contrast, the bulk of the fibre (as seenwith blue excitation on the top spectrum) contains more of the type II orthorhombic form. Followingthe same principle, Shen and Pollak [49] and Yakimova et al. [48] used multiple laserlines to measure spectra at different depths below the surface of semiconductor films.Note that when the absorption is very high, the temperature may rise at the point of laser impact,even for a few mW/mm 2 irradiation. This effect is reduced if the sample is either dispersed in a nonabsorbingmatrix, put in a rotating cell or observed in a low temperature cryostat but often results ina wavenumber shift or even a chemical degradation of the sample (oxidation, decomposition, etc.).It is then mandatory to calibrate the thermal effects [88e90]. Besides, whenever the absorbingphase is not dispersed in a transparent matrix, a significant part of the scattered light intensitymay be reabsorbed.2.6. Analysis of ‘‘isolated’’ units: the molecular schemeEq. (4) corresponds to a description where the vibrations in the solids are pictured as collectivewaves but another description is possible for solids with different bond strengths. In this ‘‘molecular’’description, clusters of strong covalent bonds are isolated from one another by weaker ionicbonds and, thus, become the relevant vibrational unit (localized vibrations). All atoms from thisunit must exclusively belong to it (including, for example, oxygen atoms from polymerised7 Resonance Raman spectra recorded with different wavelengths of excitation are also a way of characterizing excitonsand polarons in semiconductors [80,81] and conducting polymers [82].Please cite this article in press as: G. Gouadec, Ph. Colomban, Prog. Cryst. Growth Charact. Mater. (2007),doi:10.1016/j.pcrysgrow.2007.01.001