Graphite, vibrational bands

Raman spectroscopy A nondestructive method for the study of the vibrational band structure of materials, which has been extensively used for the characterization of diamond, graphite, and diamond-like carbon. Raman spectroscopy is so far the most popular technique for identifying sp bonding in diamond and sp bonding in graphite and diamond-like carbon. 

The half-widths of the second harmonic bands 2vd and 2vg are 56 and 64 cm, respectively (Fig. 7.9a, b). This is practically the same frequency shift as observed for the maxima of these bands relative to similar bands observed in the single crystalline graphite sample (Fig. 7.8a, b). The half-widths observed for the first-order vibration bands v and Vg are 56 and 21 cm correspondingly. The too narrow width of the observed tone bands does not allow for a more definite conclusion as concerning the appearance of cooperative effects in the vibration modes of the SWCNT. Nevertheless, a significant role of nonlinear interactions of their excitations shows the importance of their wave properties for the Raman spectra interpretation. 

It is convenient to make a comparison of the different vibration bands of SWCNT and graphite after deconvolution of the corresponding spectra into elementary... 

Carbon nanotubes and graphite are excellent model systems to address fundamental issues related to physical materials science. This is due to relative simplicity of these materials containing just one type of atoms and small number of vibration bands as well as possibility of variation of the contributions from sp and sp hybrid states of carbon atoms within the same system. Raman spectroscopy is very important and powerful tool for the study and characterization of graphitic materials and carbon nanotubes especially. In this article, we give a short review of the achievements of the Raman spectroscopy in the study of the physical properties of carbon nanotubes during the last decade. 

The structure of vibration bands of the first and the second order in SWCNT Raman spectra has also been studied for ordered and disordered forms of graphite. This was accomplished by decomposition of the complex spectral bands into constituting components. We found proximity of spectral positions in most of spectral components of the nanotubes and graphite and considerable variation of their intensities. This also demonstrates variation of the electronic polarizabilities and can explain anomalous shifts of the harmonic bands 2vq and 2vd for nanotubes in comparison to corresponding bands of a single crystalline graphite. Narrow width of the low frequency mode Vrbm 160 cm leads to reproduction of the G-band structure in the sum harmonic band Vg+Vrbm" 1750 cm while the complex stmcture of the broad Vp band is remarkably reproduced in the Vq+Vg sum tone. The narrow width of SWCNT s 2vd and 2vg harmonics in the Raman spectra may be related to group synchronism effects [72]. 

The major vibrational bands in graphite are separated into four main groups which can be approximately described as local in-plane stretches at 1400-1600 cm", local in-plane bends at 600-900 cm", sheet deformations at 400-500 cm" and the rigid sheet modes below 100 cm". ... 

Modification (i.e. decoration) of graphite surfaces also results in considerable enhancement of vibrational bands of the carbon substrate in addition, modes of electrolyte solution constituents may be observed. In the case of a glassy carbon surface that is electrochemically activated by repeated electrode potential, cycling deposition of silver micro- and nanocrystals results in SER spectra, as shown in Fig. 5.78. 

Another valuable advantage of Raman spectroscopy, which is unique, is its capability of being used to characterise carbon species, in particular graphitic and amorphous carbon this can be of value to many degradation and pyrolysis studies. Perfectly ordered graphite is characterised by a Raman-active vibrational mode that occurs at 1,575 cm-1 this band is usually referred to as the C7 band. With increasing disorder in the carbon, a new band, the D band, appears at... 

Eberhardt et al. (22a) have studied the photoemission of oxygen physi-sorbed on graphite at 10 K. The photoemission spectra exhibit vibrational structure in the 2n band. From calculations based on Franck-Condon factors, the authors conclude that on the graphite surface the equilibrium distance of the oxygen nuclei is decreased by 0.065 A relative to the gas phase. This would also be consistent with a partial electron withdrawal from oxygen antibonding orbitals into available orbitals in the graphite. 

Raman spectra were recorded using a Spex Triplemate spectrograph equipped with a diode array detector. The 514 nm line of an Ar+ laser was used for excitation. The Raman spectra displayed a band at approximately 1600 cm-1 due to C-C vibrations of graphite-like structures and a band at 1365 cm 1 due to imperfections of the graphite lattice and to amorphous carbon. The width of the 1600 cm"1 band in the Raman spectra has been reported to be inversely proportional to the degree of graphitization [5,6]. 

For Raman scattering measurement, a freshly cleaved sample is directly illuminated with the Ar-ion laser, and the resulting spectrum, accumulated during 10 min, is shown in Fig. 23. The band at 1580 cm 1 corresponds to the in-plane C-C breathing mode of the whole graphite lattice, namely the E2g mode. The band at 2730 cm-1 is an overtone of a lower-energy vibration, and... 

We have analyzed the influence of the annealing temperature, structural disorder, and the frequency of a continuous excitation laser radiation Vl on the first- and the second-order Raman spectra of several nanostructured carbon materials including single-wall carbon nanotubes (SWCNT), SWCNT-polymer composites, and nanostructured single-crystalline graphites. Consideration of the high-order nonlinear effects in Raman spectra and anharmonicity of characteristic Raman bands (such as G, G, and D modes) provides important information on the vibration modes and collective (phonon-like) excitations in such ID or 2D confined systems... 

One can see that G-band is characterized with the highest intensity in the spectra (Fig. 7.6a). It should be noted as well that the SWCNT 2vd band is noticeably higher in intensity in comparison with the main D band, which can be seen from the lower curve 1. Broadband noise is typically not strong in Raman spectra of SWCNT but became more noticeable for the bulk systems (graphite and polymer-SWCNT composite, curves 2 and 3 in the Fig. 7.6a). SWCNT possess also a low-frequency vibration mode at Vrbm 160 cm corresponding to radial oscillations of carbon atoms in plane of the cross section, which exhibits a strong variation with nanotube diameter [7]. The SWCNT vibration spectra show a sum harmonic signal Vg+Vrbm with a spectral shape, which will be discussed below. 

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