Morphology and electronic structure in nitrogen-doped ultrananocrystalline diamond

Ultrananocrystalline diamond (UNCD) thin films consist of 2–5 nm grains of pure ${\mathrm{sp}}^3$ -bonded carbon and ∼0.5-nm-wide grain boundaries with a disordered mixture of ${\mathrm{sp}}^2$ - and ${\mathrm{sp}}^3$ -bonded carbon. UNCD exhibits many interesting materials properties that are a direct consequence of its nanoscale morphology. In this work, we report the changes in morphology induced in UNCD by the addition of nitrogen gas to the ${\mathrm{Ar/CH}}_4$ microwave plasma, as studied using high-resolution transmission electron microscopy and nanoprobe-based electron energy-loss spectroscopy. Both the grain size and grain-boundary widths increase with the addition of ${\mathrm{N}}_2,$ but the overall bonding structure in both regions remains mostly unchanged. These results are used to explain the variation of materials properties of nitrogen-incorporated UNCD films.