On the electrical activity of sp2-bonded grain boundaries in nanocrystalline diamond

By means of tight-binding molecular dynamics simulations we find that the ground-state atomic structure of a typical high-energy grain boundary in diamond is highly disordered with a large fraction of sp(2) bonded atoms. This structure gives rise to localised bands within the band gap. We describe how multiphonon assisted hopping conduction can arise from such electronic states in high-energy grain boundaries, giving in turn a basis for the experimentally observed conductivity and electron field emission in nanocrystalline diamond. Simulated electron-energy-loss spectra indicate correlations between the disordered atomic structure and features of the electronic structure.