Nitrogen driven structural transformation in carbon nitride materials

The variation of local bonding as a function of nitrogen concentration in plasma-assisted pulsed-laser deposited carbon nitride films has been systematically studied. Time-of-flight (TOF) mass spectroscopy and electron energy loss spectroscopy (EELS) were combined to identify ablation conditions that produce highly sp3-hybridized diamond-like-carbon (DLC) for typical carbon nitride growth pressures. EELS studies of carbon nitride films grown using these optimal conditions demonstrate that there is a structural transformation from ∼70 to 0% sp3-bonded carbon as the nitrogen concentration increases from 12 to 17%. Density measurements show that this transformation is accompanied by a density decrease from 3.3 to 2.1 g/cm3. Hartree–Fock and density functional calculations on nitrogen substituted diamond clusters show that there is a strong preference to form sp2-bonded carbon when the local nitrogen concentration is larger than 12 atomic percent. These experimental results and calculations suggest that amorphous carbon nitride structures with highly sp3-hybridized carbon are unstable.