The role of lattice defects in the formation of new carbon structures under electron irradiation

The paper summarizes electron irradiation and imaging studies of carbon nanoparticles. The electron microscope is used here as a nanolaboratory for carrying out in situ experiments on an atomic scale. It is demonstrated that irradiation of graphitic precursors within a wide range of temperatures can lead to the formation of new and unexpected structures. Irradiation-induced atom displacements in graphene layers generate vacancies and defects such as non-six-membered rings. Vacancies permit the coalescence of adjacent nanoparticles or the penetration of foreign atoms through the layers. Defect combinations in the shape of five- or seven-membered rings are the origin of curvature of the layers and enable the structures to form closed onion-like particles that show self-compression and, eventually, diamond nucleation under irradiation. The irradiation-induced phase transformation of graphite to diamond at ambient pressure is another phenomenon that has been discovered and is studied in detail by in situ electron microscopy. From such experiments, a non-equilibrium phase diagram of carbon can be deduced by varying irradiation intensity and specimen temperature and observing the stability of the respective phases. It is shown that irradiation can lead to a decrease in entropy of the irradiated systems and hence to self-organized ordering phenomena.