Graphite vs diamond

The two most common allotropes (different atomic structures for the same element) of solid carbon are graphite and diamond. In graphite, the carbon atoms are arranged in sheets, loosely joined by van der Waals forces with a separation between layers of plus or minus 3.4 Å…. Within each sheet, the atoms are arranged in a honeycomb-like lattice, each atom bonded to its three neighbors via strong sp2 covalent bonds.

In diamond, the carbon atoms are linked through sp3 bonds in a cubic lattice. Investigation of the Devil's Canyon meteorite discovered another sp3 allotrope, the lonsdaleite, where carbon atoms are linked in a hexagonal crystal structure. The lonsdaleite is the less common of the two polytypes of diamonds and is not detected free in nature. In meteorite samples, it intermingles within the diamond network showing characteristics consistent with faults in the diamond network.

The graphite to diamond phase transition has been extensively studied for obvious reasons: diamond is a highly desirable material with many applications, from abrasives and coatings to electronics, and carbon is a material abundant in nature.

The transition is driven by the increase in pressure and temperature of any type of graphite sample either by laser driven shocks or directly by the detonation of explosives containing carbon.
Recent experiments have described the dynamics of this process; it occurs on nanosecond timescales at working pressures that depend on the specific type of graphite.
In the last decade, a new set of experiments has shown that the transition from sp2 to sp3 can also be achieved by exciting and heating electrons in Π orbitals. Optical photons (1-4 eV) from a femtosecond laser source are absorbed directly at Π-Π* transitions creating a first population of hot electrons that thermalize to less than 25 fs, reaching electron temperatures that can exceed 5500 K. This electron population is hot enough to populate the interlayer band (energy 4.4 eV above vacuum) from where electrons can undergo thermionic emissions. The phase transition from sp2 graphite to sp3 diamond appears to be caused by instabilities of the graphite lattice induced by the redistribution of electrons from the Π bands, which are linked by bonds, to the interlayer band.

In this work, several graphite samples were irradiated in a vacuum chamber in order to show that nanodiamonds are formed on the surface of the material after irradiation of graphite samples by UV photons with energies greater than 4.4 eV. This finding provides a new and easy-to-implement method for producing nanodiamonds. It also proposes a re-evaluation of the processes for forming nano-diamonds in space.

The work is a collaboration between the Institute of Optics, the U.D. Astronomy and Geodesy (Department of Earth Physics and Astrophysics) of the Complutense University of Madrid, the Joint Center for Ultraviolet Astronomy of the Complutense University of Madrid, the Department of Physics and Astronomy of the McMaster University, the Origins Institute of the McMaster University, the San Pablo-CEU University, the IMDEA Nanoscience-ICTS National Center for Electron Microscopy and the National Center for Biotechnology of Madrid

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