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Metallic nanocrystals such as cobalt or gold are encapsulated by spherical graphitic shells under high-temperature electron irradiation. The irradiation promotes a heavy contraction of the carbon onions. The contraction forces the metal atoms to migrate outwards through the shells, even without further irradiation. This leads to a gradual but complete displacement of the encapsulated crystals. In-situ observation in an electron microscope allows shrinkage of the encapsulated crystals and migration of the atoms through the shells to be monitored. The experiment shows that spherically curved graphene layers are permeable to metal atoms.

Electron irradiation-induced basal plane disordering in single- and multi-shell carbon nanotubes and onions is found to be inhibited at irradiation temperatures above 300°C. Irradiation-induced defects anneal out due to the thermally activated migration of interstitials at elevated temperatures and leave behind perfectly coherent shells of high tensile stability. Continuous loss of atoms as a result of sputtering induces a surface tension which can be identified as the origin of the formation and self-compression of spherical concentric-shell carbon onions.

The synthesis of BCN nanotubes and boron doped (1 to 5 at.% B) carbon nanotubes is achieved by an electric arc-discharge between an anode made of homogeneous BC 4 N and a cathode made of graphite. The BC 4 N anode is prepared by pyrolysis of polymer precursors. The effect of boron doping is seen to increase the tube lenghts up to 100 μm and to improve graphitization. Boron and nitrogen are enriched at the surfaces of tubes with overall BCN composition. The observed structure of BCN tubes consists of concentric cylinders of BC 2 N and pure carbon.