The structure of amorphous hydrogenated carbon

The structure of several amorphous hydrogenated carbon (a-C:H) samples has been studied in detail using time-of-flight neutron diffraction, inelastic neutron scattering, infrared spectroscopy and reverse Monte Carlo (RMC) computer modelling. Supplementary work has also included combustion analysis. The results are presented as evidence for a new structural model for a-C:H.

The high-resolution real-space neutron diffraction data allows direct determination of the single:double bond ratio, and also shows the presence of sp1 hybridised carbon bonding environments in some samples. There is limited evidence for the presence of molecular hydrogen "trapped" within the amorphous network. The spectroscopic data is then used to provide information on the C-H bonding environments, so that using a combination of experimental techniques a detailed picture of the atomic scale structure has been produced.

For the hand carbon samples, prepared using acetylene and propane, the carbon-atom sites are found to be predominantly sp2 bonded, with a single:double bond ratio for carbon-carbon bonds of about 2.5:1. The effect of beam energy on the structure of the material is also investigated, and comparison made between samples prepared using a fats-atom (neutral particle)source and those prepared by plasma enhanced chemical vapour deposition, from acetylene. The results show that in both deposition methods, increasing the beam energy produces a lower total sp2 hybridised carbon content in the material with evidence for a shift from pure olefinic to some aromatic/graphitic bonding in one sample. This trend to a more aromatic bonding environment is also observed in samples prepared from a cyclohexane precursor.

The spectroscopy results show that for all samples the hydrogen bonding environments are similar, although there is some evidence for changes in the distribution of hydrogen within the network with deposition energy. The spectra for all the samples show similarities to those for the polymeric materials, polyethylene and rubber.

In addition, the results of a study of the structure of a-C:H up to a maximum of 1000c are presented. The data show clearly the effect on atomic correlations of elevated temperatures, with the initial room-temperature amorphous network (a mixture of single bonds and olefinic double bonds) becoming progressively aromatic, the graphite as hydrogen is evolved. Infrared spectroscopy results would seem to indicate that sp3 CH is the primary source of hydrogen for effusion, such that, on annealing, molecular hydrogen is formed wherever there are two neighboring hydrogen atoms. Structural transformations are seen to occur throughout the heating process.

Finally, the RMC method has been used to produce a model for the structure of a-C:H, by fitting to experimental data from neutron and X-ray diffraction experiments. The RMC method was implemented with the introduction of additional constraints on the minimum number of atoms in a ring, and on the maximum coordination number. Once the data has been fitted, it is possible to generate model partial pair distribution functions, bond angle distributions, coordination number distributions, etc.. By fitting all the experimental data sets simultaneously, there is sufficient information to generate a viable "physical" model for the structure of these materials. The effects of increasing number density within the model have also been investigated, and this confirmed that the density is a crucial parameter in the modelling process.