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 \ifmmode^\circ\else\textdegree\fi{}C. X-ray diffraction, transmission electron microscopy (TEM) and Raman scattering studies indicate that the onset temperature of the diamond-graphite transition is around 1200 \ifmmode^\circ\else\textdegree\fi{}C and the complete conversion of diamond to graphite occurs at 1600 \ifmmode^\circ\else\textdegree\fi{}C. Based on the structural characteristics the samples are categorized into ${\mathrm{sp}}^{3}$-dominated (as-prepared and 900 \ifmmode^\circ\else\textdegree\fi{}C), ${\mathrm{sp}}^{2}{:sp}^{3}$ mixed-phase (1200 and 1400 \ifmmode^\circ\else\textdegree\fi{}C), and ${\mathrm{sp}}^{2}$-dominated systems (1600 \ifmmode^\circ\else\textdegree\fi{}C). The larger c-axis repeat distances and the high-resolution TEM images for the ${\mathrm{sp}}^{2}{:sp}^{3}$ 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-\ensuremath{\pi}-electron system from the 1200 \ifmmode^\circ\else\textdegree\fi{}C and higher-temperature heat-treated samples. The completely graphitized sample reveals the important role of edge-inherited nonbonding \ensuremath{\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 graphite \ensuremath{\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 \ifmmode^\circ\else\textdegree\fi{}C are expected to arise from the involvement of the localized edge-state electrons. In the less-graphitized 1200 \ifmmode^\circ\else\textdegree\fi{}C heat-treated sample, however, the corrugated nature of the graphene planes is likely to hinder such fast-relaxation processes.