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

The Migration of Metal Atoms through Carbon Onions. Banhart, F., Redlich, P., Ajayan, P.M.: Chemical Physics Letters, 1998, 292[4-6], 554-60