Since the prediction of Liu and Cohen [Science 245, 841 (1989)] of the potential extraordinary mechanical properties of crystalline β-C3N4, many authors have attempted its synthesis. However, in most cases, the obtained materials are amorphous phases with a complex bonding structure. Their characterization is complicated due to the absence of a reference compound, the lack of long-range order, and the poor knowledge about their bonding structure. In this article, we present H1, C13, and N15 solid-state nuclear magnetic resonance (NMR) measurements for the determination of the bonding types in amorphous CNx films. NMR measurements do not require long-range order and are able to clearly identify the signals from the sp2- and sp3-bonded phases. The analysis of the data obtained by other characterization techniques, such as infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, and x-ray absorption near-edge spectroscopy on the same sample, based on the information acquired by NMR, enables the description of a structure model for the studied amorphous-CNx phase prepared by dc-magnetron sputtering and to revise the interpretation found in the literature.
Various hydrogenated amorphous carbon films have been analyzed by C13 and H1 nuclear magnetic resonance (NMR) spectroscopies. The films have been deposited from acetylene or cyclohexane by dc plasma enhanced chemical vapor deposition, at various dc biases and gas pressures. The total hydrogen content has been measured by forward recoil elastic scattering (FRES). 13C NMR investigations have been performed in various configurations: high power decoupled to determine the sp2:sp3 carbon ratio, cross polarized at magic angle contact spinning with different contact times to provide information on carbon atoms directly bound to hydrogen, and with dipolar dephasing to study the quaternary carbon atoms. By performing the 13C and 1H NMR measurements on the same samples, it was possible to resolve for the first time the seven different forms of unprotonated and protonated CHx carbon, for both sp2(x=0,1,2) and sp3(x=0,1,2,3) carbon hybridizations, as well as the ratio between bound and unbound hydrogen. The results are discussed and compared with previously published FRES and Fourier transform infrared (FTIR) data. It is shown that NMR and FTIR combined with FRES do not agree systematically on the ratio of bound/unbound hydrogen.
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