Plasmon response and structure of nanocrystalline diamond powder

High-resolution transmission electron microscopy, electron diffraction and parallel electron-energy-loss spectroscopy are used to analyse nanocrystalline diamond powder. Grains of diameter in the range 2-10 nm were found aggregated together; the grain boundaries were essentially a grossly disordered (amorphous) intergranular phase. Analysis of the plasmon loss function indicated a mass density of 3.30 g cm(-1), compared with 3.51 g cm(-3) for a chemically vapour-deposited diamond. The core loss spectra showed virtually pure sp(3) bonding overall although some exposed surfaces were coated with two or three graphitic layers. Two peaks were observed in the low energy loss function; one at 34 eV was characteristic of the volume plasmon typically observed in crystalline diamond, and a second peak at approximately 23 eV for larger grains shifted to lower energies as the particle size decreased (to 19.5 eV for 2.8 nm diameter) and at the same time increased in intensity, becoming stronger than the volume plasmon for a 2.8 nm crystal. These results are interpreted using theoretical results for surface plasmons in small spherical particles.