Determination of bonding in amorphous carbon films: A quantitative comparison of core-electron energy-loss spectroscopy and 13C nuclear magnetic resonance spectroscopy

Abstract

The direct determination of carbon–carbon bonding in amorphous carbon thin films has been performed by core‐electron energy‐loss spectroscopy and cross‐polarization magic‐angle spinning ¹³C nuclear magnetic resonance (NMR) for a series of varying band‐gap amorphous carbon films (1.1–3.0 eV). The amorphous carbon films were prepared using rf plasma discharge with either capacitive or inductive coupling. The core‐electron energy‐loss spectroscopy is shown to be quantitative by measuring the relative number of unoccupied π* and σ* states available to the C 1s core electron. Determination of the percentage of sp² and sp³ bonded carbon atoms follows from the measured ratio of π* and σ* states. If unity cross section for the C 1s to π* and σ* core losses is assumed, the agreement with high‐resolution ¹³C NMR determination of the sp² and sp³ content is very encouraging. The low band‐gap material (1.1 eV) has the largest sp² content (55–60%) which decreases with increasing band gap (3.0 eV, 12–19%). The orientation dependence of the intensities of the π* and σ* loss features is not as strong as found in highly oriented pyrolytic graphite. The observation of the orientation dependence suggests that there is a tendency for the π* orbitals to be parallel to the substrate.