Clusters graphite

Onion-like graphitic clusters have also been generated by other methods (a) shock-wave treatment of carbon soot [16] (b) carbon deposits generated in a plasma torch[17], (c) laser melting of carbon within a high-pressure cell (50-300 kbar)[l8]. For these three cases, the reported graphitic particles display a spheroidal shape. 

The optimum coefficients in Equation 2 are Eq -37.8363 and E =-0.5685 au. Eq is very close to the value -37.8366 found for the pure graphite clusters. The value of E corresponds to a contribution of 47.1 kcal/mol to the total energy of Cg 2Hg for each bonded H-atom (the hydrogen atom has an energy of -0.4935 au in the oasis set used). Alternatively, the parameter values can be interpreted as 83.3 kcal/mol per C-C bond, 88.7 kcal/mol per C-H bond. 

The optical gap of a-C H films was found to continuously decrease with increasing self-bias [42]. The gap shrinking was found to be strongly correlated to the variation of Raman spectra that is related to the increase of the graphitic clusters present in the a-C H films. Accordingly, the electrical resistivity of C H films was found to strongly decrease with substrate bias [43]. 

This picture was found to be consistent with the comparison of Raman spectra and optical gap of a-C H films deposited by RFPECVD, with increasing self-bias [41], It was found that both, the band intensity ratio /d//g and the peak position (DQ increased upon increasing self-bias potential. At the same time, a decrease on the optical gap was observed. Within the cluster model for the electronic structure of amorphous carbon films, a decrease in the optical gap is expected for the increase of the sp -carbon clusters size. From this, one can admit that in a-C H films, the modifications mentioned earlier in the Raman spectra really correspond to an increase in the graphitic clusters size. 

Results on Raman spectroscopy thus show that nitrogen incorporation, at least for a large enough N content, results in the increase of the graphitic clusters. This is contrary to the formation of an amorphous solid related to the -C3N4 phase, which presumes sp -C hybridization and no clustering effects. 

It is important to stress that nitrogen incorporation in a-C H films always result, at least above a certain level, in a strong decrease in the tetrahedrally bonded carbon atom fraction. Raman spectroscopy also gives support to this observation, because the increase in the size of graphitic clusters only can proceed with also increasing sp fraction. 

Figure 5.12 GALDI mass spectrum of shellac (from methanol solution). Peaks at m/z—570 are related to esters of aliphatic hydroxy acids with sesquiterpenoid carboxylic acids (see text and Table 5.4). Signals marked with crosses are contaminants in the spectrometer that accumulated over time (m/z 413, 469, and 507) peaks marked ( ) are contaminating graphite clusters from the matrix (m/z 264, 276, 288).

Carbon atoms released by the support upon heating contaminate the surface of Pd particles to make them inaccessible to adsorbates [31-33,35]. At temperatures as high as 600-730 °C, carbon ad-atoms aggregate into extended graphitelike crystallites, which build up capsules around metal particles and prevent further sintering of the catalyst. Aggregation of carbon ad-atoms into graphite clusters at the Pt surface is observed at 630-930 °C in [73] and at 1100°C in [30]. 

Watanabe et al. [154] suggested that the peak at 1150 cm" is due to a kind of hydrogenated-]ike carbon with alternate C=C and C—C bonding. In addition to the earlier reports [155], some unusual peaks have been observed that may not be explained only by a graphite cluster model embedded in an sp network. Earlier,... 

A unique carbon composite material is formed when polymers are ion implanted. The rapid deceleration of high-energy ions within the polymer results in a material that is a mixture of carbon structures. This carbon-based material has different phases that are the result of different energy loss processes as the ion decelerates in the material. Toward the mean range of the ion, the material maintains much of its polymer character, but closer to the surface the material is a mixture of amorphous carbon and graphite clusters. 

NakadairaM, Saito R, KimuraT, Dresselhaus G, Dresselhaus MS. Excess Li ions in a small graphite cluster. J Mater Res. 1997 12 1367-75. 

Raman spectroscopy can and is used to help develop and quality control DLC films that are resistant to certain types of the more damaging wear mechanisms by ensuring that the carbon film has the desired chemical and physical structure. Differences in the chemical structure result in measurable differences in the Raman spectrum. Explanations of observations of correlations between the Raman spectrum and structure-wear properties generally refer to the structural models for amorphous carbon thin films proposed by Robertson and O Reilly [23]. They proposed that amorphous carbon films consist of p -bonded, graphitic clusters interconnected by a network of sp bonds. 

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