Onion-like concentric spherical shells

Intense in situ electron irradiation of particles of carbon soot for tenths of minutes in the electron microscope produces spherical particles consisting of onion-like concentric spherical shells of graphene-like layers [208]. The microscope allows the concentric shell structure to be imaged during its formation. 

Fig. 4. Onion-like graphitic particles formed by three concentric layers (C o, C240, Cs4o) polyhedral (marked P) and spherical (marked S) structures. For clarity, only a half pan of each shell is shown.

Carbon onions are a member of the family of nanometer-scale graphite-like aU-carbon allotropes, the emergence of which was catalyzed by the Nobel Prize-winning discovery of the first member, the fullerene, by Kroto et al. in 1985. Initially, carbon onions were observed by lijima in 1980, and were brought to popular attention by the experiments of Ugarte in 1992. Structurally, they consist of concentric spherically closed carbon shells and receive their name from the close resemblance between their nanoscale structure and the more familiar concentric layered structure of an onion. Closely related to carbon onions is a class of material known as onion-like carbons (OLCs), which include polyhedral nanostructures such as ideal nested fullerenes. This material, rather than ideal spherical carbon onions, can be currently produced in macroscopic quantities, and, hence, be used for future applications. 

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. 

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