Heat-treatment effect on the nanosized graphite π-electron system during diamond to graphite conversion

Graphite nanoparticles were prepared by the heat treatment of diamond nanoparticles in the range 900-1600 degreesC. X-ray diffraction, transmission electron microscopy (TEM) and Raman scattering studies indicate that the onset temperature of the diamond-graphite transition is around 1200 degreesC and the complete conversion of diamond to graphite occurs at 1600 degreesC. Based on the structural characteristics the samples are categorized into sp(3)-dominated (as-prepared and 900 degreesC), sp(2):sp(3) mixed-phase (1200 and 1400 degreesC), and sp(2)-dominated systems (1600 degreesC). The larger c-axis repeat distances and the high-resolution TEM images for the sp2:sp3 mixed-phase systems denote the presence of the remnant buckling feature of the diamond (111) planes in the graphene sheets. Magnetic susceptibility and ESR studies suggest the development of itinerant-pi -electron system from the 1200 degreesC and higher-temperature heat-treated samples. The completely graphitized sample reveals the important role of edge-inherited nonbonding pi -electron states in the electronic structure. The Raman G-peak position and the orbital diamagnetism show considerable deviation from the bulk-graphite values, which is explained on the basis of charge transfer from the gaphite pi band to the localized edge states and the resulting shifting of the Fermi level. The enhanced spin-lattice relaxation rates in the case of more graphitized samples heat-treated at 1400 and 1600 degreesC are expected to arise from the involvement of the localized edge-state electrons. In the less-graphitized 1200 degreesC heat-treated sample, however, the corrugated nature of the graphene planes is likely to hinder such fast-relaxation processes.