Pathway for the Transformation from Highly Oriented Pyrolytic Graphite into Amorphous Diamond

We report the discovery of a novel pathway for the transformation from highly oriented pyrolytic graphite foils into amorphous diamond platelets. This pathway consists of three stages of neutron irradiation, shock compression, and rapid quenching. We obtained transparent platelets which show photoluminescence but no diamond Raman peak, similar to the case of amorphous diamond synthesized from ${\mathrm{C}}_{60}$ fullerene. Wigner defects formed by irradiation are considered to make a high density of diamond nucleation sites under shock compression, of which growth is suppressed by rapid quenching.

Figure 1

Electron and optical micrographs of neutron-irradiated HOPG recovered after the shock compression and rapid quenching. (a) Dark and bright areas correspond to the original graphite area and the gold foil, respectively. Division of the dark areas by bright lines indicates an occurrence of a fracture of the specimen into tiles. Some of the tiles were stripped off, exposing the gold foil surface. (b) An oblique view of a tile. The cross section of the tile is seen to be homogeneous, implying a disappearance of the initial layered structure of graphite along the $c$ axis. (c) A tile riding on fractured tiles. The tile is optically transparent, and then splitting lines below the tile can be seen. (d) Interference patterns observed for transparent tiles.

Figure 2

Raman spectra for the original and neutron-irradiated HOPG specimens and a change of the Raman spectrum for the irradiated one after the shock compression. (a) Spectrum of the original HOPG showing the first-order band ($G$ band) near $1580{\mathrm{cm}}^{-1}$ and the second-order Raman band near $2700{\mathrm{cm}}^{-1}$. (b) Spectrum of the neutron-irradiated HOPG showing a broad band in a range from 800 to $1700{\mathrm{cm}}^{-1}$. The original $G$ band and the second-order Raman band mostly disappeared. (c) Representative spectrum of the neutron-irradiated HOPG after the shock compression showing broad bands near 1600 and $2200{\mathrm{cm}}^{-1}$ and a remarkable increase of the background intensity.

Figure 3

Electron micrograph of HOPG specimen recovered after the shock compression. The shock experiment was done in almost the same pressurized condition done for the neutron-irradiated HOPG. The specimen is broken into pieces with jagged edges but still preserves the initial layered structure.

Figure 4

Irradiation-produced defects proposed by recent experimental and theoretical studies. Interlayer defects of single interstitial, interstitial clusters such as di-interstitials, and cross-linking defects are presented in the figure. Dislocation dipoles with an eightfold ring or a pair of sevenfold and fivefold rings at the ends should induce the disordering of graphite by the buckling and rotation of the basal plane.