Abstract
The phase transition from graphite to diamond is an appealing object of study because of many fundamental and also, practical reasons. The out-of-plane distortions required for the transition are a good tool to understand the collective behaviour of layered materials (graphene, graphite) and the van der Waals forces. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. They induce it by increasing either pressure or temperature on graphite. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. We show high resolution electron microscopy images of pyrolytic carbon evidencing the dislocation of the superficial graphitic layers after irradiation and the formation of crystallite islands within them. Electron energy loss spectroscopy of the islands show that the sp2 to sp3 hybridation transition is a surface effect. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands.
Highlights
The phase transition from graphite to diamond is an appealing object of study because of many fundamental and practical reasons
Carbon atoms are arranged in sheets, weakly bound together by van der Waals forces with an interlayer separation of ~ 3.4 Å
Recent experiments have resolved the dynamics of this process; it occurs in time scales of nanoseconds at working pressures that the depend on the specific type of graphite; 19 GPa in highly ordered pyrolytic graphite (HOPG) and 228 GPa in polycrystalline g raphite[8]
Summary
The phase transition from graphite to diamond is an appealing object of study because of many fundamental and practical reasons. Two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. They induce it by increasing either pressure or temperature on graphite. Optical photons (1–4 eV) from a femtosecond laser source are directly absorbed into π–π* transitions creating a first population of hot electrons that thermalize in less than 25 fs; they achieve a Boltzmann’s statistical distribution reaching electron temperatures that may exceed 5500 K13 This electronic population is sufficiently hot to populate the interlayer band (energy 4.4 eV above vacuum) from where electrons may undergo thermoionic emission. The graphite layered structure may render unstable by inducing excited h oles[15]
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