Fullerenes carbon onion shell sizes

The question remains on the size of the innermost shell. An estimate can be made by measuring the diameter in projections of entirely or partially filled carbon onions. The values obtained this way frequently correspond to the diameter of Ca). It is not possible, however, to exclude fullerenes of similar size hke C50, with certainty aU the more as the low stability observed for these species in their isolated... 

The carbon onions produced in Ugarte s experiments have almost perfect spherical shape, and the size of the innermost shell is approximately equal to the size of the most abundant fuUerene, Cgo-Fullerene molecules, which contain 60n carbon atoms (where n is a nonzero positive integer), display an icosahedral shape. The ideal structure of a fullerene molecule containing 240 carbon atoms is shown in Figure 10.2. The icosahedral shape of these fullerenes originates from the local curvature introduced by the 12 pentagons present in their structure. This icosahedral shape becomes clearly visible as the size of fullerene molecules increases. 

In contrast to the spherical carbon onions observed in the first experiments by Ugarte, OLC particles were subsequently produced with polyhedral facets, more closely matching the polyhedral structures predicted from the consideration of nested fullerene structures described above. These polyhedral onion-like particles were synthesized by vacuum heat treatment of carbon sooF and diamond nanoparticles." Figure 10.5 presents HRTEM images of the polyhedral OLC particles produced in the experiments of Kuznetsov et al. The range of synthesis methods available has led to the production of different types of OLC. In addition to their shape, such carbon onions can be characterized by other parameters, such as the number of concentric shells, the spacing between adjacent shells, the size of the innermost shell, and the presence of different types of defects. 

Actually, X-ray diffraction in general is a useful supplement to high-resolution electron microscopy because it measures the mean characteristics of the whole sample, whereas the HRTEM can only cover ejctremely small sections of it. The XRD results confirm the sp -character of carbon atoms in onions, low particle size as well as a certain deviation from perfect crystaUinily. This imperfection results from the nonordered packing of individual shells, the different sizes of individual onions, and the defects present in the fullerene shells. 

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