Electronic structure studies of undoped and nitrogen-doped tetrahedral amorphous carbon using high-resolution electron energy-loss spectroscopy

The electronic structure of undoped and nitrogen-doped tetrahedral amorphous carbon (ta-C) has been studied using high-resolution electron energy-loss spectroscopy in transmission to determine the local bonding of the carbon and nitrogen atoms. We found that the carbon and nitrogen atoms are mainly sp3-bonded for nitrogen contents up to 10 at. %. However, the data analysis is also consistent with some N atoms occupying trivalent σ or pyrrole configurations. The real (ε1) and imaginary parts (ε2) of the dielectric function were derived over a wide energy range carrying out a Kramers–Kronig analysis of the loss spectra. The ε2 spectra show two features due to the excitation of the π and σ valence states. The π feature is very small in ta-C deposited at 100 eV, consistent with its large sp3 content (∼85%). We have determined the ratios of sp2/sp3-hybridized atoms in the samples by both the C 1s edge and the optical sum rules.