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

ARTICLE IN PRESS+ MODELG. Gouadec, Ph. Colomban / Progress in Crystal Growth and Characterization of Materialsxx (2007) 1e563.1. Phase identification and phase transitions in nanoparticles11In many nanomaterials, the Raman spectrum remains sufficiently similar to that of the correspondingsingle crystal to facilitate direct identification of the phases [28,32,129e145]. Oncethe Raman spectra are known, phase transitions can be characterized (transition temperature,transition pressure, transition order) through mode variation, much the same way as in bulk materials[92,137,146e158]. Besides, the observation of any theoretically forbidden mode isa very sensitive probe of lattice distortions [131,159].Barborini et al. [157] showed with RS that the structure of gas phase-deposited TiO 2 clustersturned from rutile to anatase whenever they reached 5 nm in diameter. A difference in surfaceenergy usually plays a determinant role in such a phenomenon, as proposed a long time ago byGarvie [160] for zirconia. Similarly, Fray and Payne [158] showed how the temperature of theorthorhombic-tetragonal phase transition of BaTiO 3 ceramics depends on the grain size.3.2. Analysis of amorphous nanodomainsMicro-Raman Spectroscopy is sometimes more powerful than X-ray analysis for detectingand monitoring crystallisation/amorphisation processes in covalent materials [92,100,146,161e164]. Of course, both crystallographers and Raman spectroscopists characterize disorderthrough peak broadening. Yet, while a loss of long distance translational periodicity (of highatomic number atoms) is always associated with broadening for diffraction patterns, only latticeand librational (R 0 ,T 0 ) modes are sensitive to the same ‘‘long distance’’ disorder in RS [165].The width of the other Raman modes is mainly sensitive to the ‘‘local’’ crystal field, more specificallyto the short range order in the first (0.1e0.5 nm) and second (0.5e5 nm) atomic shells.If the ‘‘molecular’’ description of vibrations applies (see Section 2.6), then Raman bendingmodes are even specifically sensitive to local geometric disorientation and Raman stretchingmodes to the neighbouring disorder (particularly atoms from other sublattices or electric defectsresulting from substitutions/vacancies).In fact, diffraction discriminates ‘‘periodical’’ domains from ‘‘disordered’’ ones but does noteasily differentiate clear-cut separations in the real space and progressive orientational disorders(para-crystal), especially in strongly covalent structures such as organic and inorganicpolymers [166, p. 615e620]. In materials, mainly polymers, which present interlocking submicronic‘‘crystalline’’ and ‘‘amorphous’’ conformational domains [167e174], the distinction issometimes possible using mRS. The simplest way to picture the problem is to fit lattice modeswith two components, one representing the amorphous state and the other the crystalline state.The area ratio of the two underlying areas yields a good estimate of the crystallinity [175,176].Fig. 5 illustrates this point with a spectrum of the polyamide 6.6 fibre. The polarization analysisclearly shows the orientational effect of straining (fibre extrusion) on the nanocrystals whereasthe specific analysis of the low frequency components, which show the collective chain movements,facilitates the separate analysis of the amorphous (wide Gaussian band) and crystalline(narrow Lorentzian) phases. These results were used to demonstrate that the mechanical fatigueof the fibres results from the progressive transformation of the amorphous phase [169,170,172].Nanophase separation was also investigated in glasses where the connectivity of constitutivepolyhedra sets the wavenumbers [177]. Some bands could be attributed to definite clusters incomparison with the experimental spectra of reference crystalline phases [177] or first principleDensity Functional Theory (DFT) calculations [94,178].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