( A) The low magnification image shows bumps or islands of hundreds of nm marked with a red arrow that are formed after UV irradiation of the sample (the change in the contrast is due the thickness gradient of the sample produced during FIB preparation). It also calls for a re-evaluation of the processes to form nanodiamonds in space.Īnnular bright field (ABF) images of the irradiated HOPG sample the magnification of the images increases from ( A) to ( D) panels. This finding provides a new and easy-to-implement method to produce nanodiamonds. In this work, we show that nanodiamonds are formed after the irradiation of graphitic samples (HOPG and polycrystalline graphite) by UV photons with energies above 4.4 eV. Also, the graphite layered structure may render unstable by inducing excited holes 15. The relevance of graphite lattice distortion for the stabilization of defects in the graphite layered structure is well known 14. The sp 2 graphite to sp 3 diamond phase transition seems to be caused by instabilities of the graphitic lattice induced by the redistribution of electrons from the π bands with bonding character to the interlayer band. This electronic population is sufficiently hot to populate the interlayer band (energy 4.4 eV above vacuum) from where electrons may undergo thermoionic emission. 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 K 13. In the last decade, a new set of experiments have proven that the sp 2 to sp 3 transition can also be achieved by exciting and heating electrons in π orbitals 10, 11, 12, 13. 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 graphite 8. The transition is driven by increasing the pressure and the temperature of any kind of graphitic sample 5, 6 either by laser driven shocks 7, or directly, by the detonation of carbon-containing explosives 8, 9. Diamond is a highly desirable material with many applications, from abrasives and coatings to electronics, and carbon is an abundant material in nature. The phase transition from graphite to diamond has been extensively studied. In meteoritic samples, it comes intermingled within the diamond network showing features consistent with faults in the diamond network 4. Lonsdaleite is the less common of the two diamond polytypes and it is not detected free in nature. The investigation of the Cañón del Diablo meteorite uncovered another sp 3 allotrope, the lonsdaleite, where carbon atoms are bonded in a hexagonal crystalline structure 1, 2, 3. In diamond, carbon atoms are bonded through sp 3 bonds in a cubic network. Within each sheet the atoms are disposed in a honeycomb lattice, each atom linked to the three neighbours through strong covalent sp 2 bonds. In graphite, carbon atoms are arranged in sheets, weakly bound together by van der Waals forces with an interlayer separation of ~ 3.4 Å. The two most common allotropes of solid carbon are graphite and diamond. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands. Electron energy loss spectroscopy of the islands show that the sp 2 to sp 3 hybridation transition is a surface effect. 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. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. They induce it by increasing either pressure or temperature on graphite. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. 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. The phase transition from graphite to diamond is an appealing object of study because of many fundamental and also, practical reasons.
0 Comments
Leave a Reply. |