Raman peak shift

An interesting and powerful new development in Raman spectroscopy of catalysts is the use of a UV laser to excite the sample. This has two major advantages. First, the scattering cross section, which varies with the fourth power of the frequency, is substantially increased. Second, the Raman peaks shift out of the visible region of the spectrum where fluorescence occurs. The reader is referred to Li and Stair for applications of UV Raman spectroscopy on catalysts [40]. 

Sandler J, Shaffer MSP, Windle AH, Halsall MP, Montes-Moran MA, Cooper CA, Young RJ (2003) Variations in the Raman peak shift as a function of hydrostatic pressure for various carbon nanostructures a simple geometric effect. Phys Rev B 67(3) 035417-035418... 

For a 47-pm thick free-standing diamond film, the Raman peak shifted from the interface to the growth surface, as shown in Figure 11.47. 

Figure 9.39 Raman peak shift relationship with the composition change in Sij xGex thin film. The plot also indicates the film thickness effect on the relationship. (Reproduced from M.J. Pelletier, Analytical Applications of Raman Spectroscopy, Blackwell Science, Oxford. 1999 Blackwell Publishing.)...

When a strain is applied to a material, the interatomic distances change, and thus the vibrational frequencies of some of the normal modes change, causing a Raman peak shift [27,28], Amongst specific enhanced peaks in SWNTs, the D band is more sensitive to applied strain. Figure 9 shows a typical Raman peak shift in tension for two extreme strains (i.e. 0 % and 3%). 

The frequency peak number was identified by a Gaussian/Lorentzian function. It can be clearly seen that a 5 cm shift occurred after applying 3% strain. The reproducibility of the data was examined by several tests carried out with the same experimental conditions. Figure 10 shows the results of Raman peak shift in tension for a PAni fiber containing 0.76%w/w SWNTs as a function of applied strain for three samples. The shift in the Raman peak position with strain in tension is negative by 90-130 wave numbers/applied strain. 

The CNTs generally exhibit well-defined Raman peaks. It is possible to use Raman peak shift to characterize the load transfer mechanism of the CNT/poly-mer composites. When a strain is applied to a material, the interatomic distances change, leading to variations in the firequeucy of vibrational modes. Accordingly, Raman spectroscopy can provide useful information related to the load transfer between the polymer matrix and CNTs. The Raman spectrum of SWNTs generally shows characteristic peaks located at 1350 cm (D band), 1550-1605 cm (G band) and 2700 cm (D band) [35-37]. The D-band derives from the disorder-induced mode and its second-order harmonic is D (G ) band. The G-band is associated with the graphite-like in-plane mode. Dresselhaus et al have provided an in-depth review of the Raman spectra of... 

A third example can be taken from analytical chemistry. Absorption and resonance Raman spectra of phenol blue were measured in liquid and supercritical solvents to determine the solvent dependence of absorption bandwidth and spectral shifts. Good correlation between absorption peak shift and resonance Raman bands and between Raman bands and bandwidth of C-N stretching mode were observed while anomalous solvent effect on the absorption bandwidth occnrred in liquid solvents. Large band-widths of absorption and resonance Raman spectra were seen in supercritical solvents as compared to liquid solvents. This was dne to the small refractive indices of the supercritical solvents. The large refractive index of the liqnid solvents only make the absorption peak shifts withont broadening the absorption spectra (Yamaguchi et al., 1997). 

The sensitivity of the phonon frequencies to temperature shows quite clearly the importance of their anharmonicity.42 The width of the Raman peaks, very small at low temperature ( 1cm-1), evolves in parallel with the frequency shift with temperature, which is still a consequence of the phonon-phonon interactions due to the anharmonicity. The fundamental reason for this strong anharmonicity, as well as the importance of the equilibrium-position shifts between 4 and 300 K,45 resides in the weakness of the van der Waals cohesive forces in the molecular crystal. 

The three-pulse-echo peak shift is another two-dimensional echo technique, so far applied only to electronic transitions (122,123). It integrates over r3 and keeps rx and r2 as the time variables. The data are reduced by tracing the maximum in as a function of r2, resulting in a onedimensional decay curve. Although the implementation of this type of echo spectroscopy is quite different, the essential information content is much the same as in the Raman echo approach. 

Fig. 9.30. Raman spectra of an a-Si H/a-Si3N H multilayer showing the dependence of the peak shift on multilayer spacing (Santos et at. 1986).

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