J.K. Walters

An atomic-scale study of the role of titanium in TiO2:SiO2 sol-gel materials

Abstract Neutron diffraction data is used to reveal details of the effect of adding titania to a silica-based sol-gel, including a first direct observation of the TiO correlation. At low Ti contents the data are consistent with the 4-fold oxygen coordination derived from NMR. Even at these concentrations the silica network is shown to have been affected significantly, with bond-length distributions being narrowed, and with a high level of network-OH indicating a qualitative change in the mesoscopic structure associated with interfacial surfaces. Data on a phase separated sample supports the conclusion that the network-OH is associated with the phase separation process.

A spectroscopic study of the structure of amorphous hydrogenated carbon

A range of amorphous hydrogenated carbon (a-C:H) samples have been studied using inelastic neutron spectroscopy (INS) and Fourier transform infrared (FTIR) spectroscopy. Using these complementary techniques, the bonding environments of both carbon and hydrogen can be probed in some detail, with the INS data providing not only qualitative but also quantitative information. By comparing the data from each of the samples we have been able to examine the effects of different deposition conditions, i.e. precursor gas, deposition energy and deposition method, on the atomic-scale structure of a-C:H.

The effect of temperature on the structure of amorphous hydrogenated carbon

The results of a neutron diffraction study on the structure of amorphous hydrogenated carbon a‐C:H are presented up to a maximum temperature of 1000 °C. 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, then graphitic as hydrogen is evolved. Complementary x‐ray diffraction and infrared spectroscopy data are also presented, the infrared data enabling a more detailed discussion of the temperature‐dependent hydrogen environment, and the x‐ray data are used to highlight the change in the carbon network. Comparisons have been made with previous work on similar systems and a brief summary of these results is given.

X-ray diffraction studies of the effects of N incorporation in amorphous CNx, materials

The effects of nitrogen incorporation on the atomic-scale structure of amorphous CNx samples have been studied for 0, 5, 20, and 30 at. % N concentration, by x-ray diffraction. Significant differences in the structure are observed on the incorporation of only 5 at. % N, and the changes in structure continue as further N is added. From the experimental data, we are able to obtain directly the average bond distances and then calculate the average bond angles for each of the samples. The average first neighbor distance shows a gradual decrease from 1.55 A for 0 at. % N, to 1.44 A for 30 at. % N, and a similar trend is observed in the position of the second neighbor peak. This gives a corresponding increase in the average bond angle from 108° to 114°. The results show an increase in the fraction of sp2 bonded carbon atoms with increasing N concentration, and there is evidence for the presence of significant numbers of C≡N and C=N bonds. These results are also consistent with stress, hardness, and optical gap ...

A neutron and X-ray diffraction study of the influence of deposition conditions on the structure of a-C:H

Abstract The atomic scale structure of amorphous hydrogenated carbon (a-C:H) films prepared from an acetylene precursor by plasma enhanced chemical vapour deposition (PECVD) and a fast-atom source (FAS) have been studied by neutron and X-ray diffraction. The effect of beam energy on the structure of the film is investigated, and comparison is made to samples prepared using at fast atom (neutral particle) source, also using acetylene as the precursor. The results show that, in both deposition methods, increasing the beam energy produces a lower total sp2 hybridised carbon content in the film with evidence for a shift from pure olefinic to some aromatic/graphitic bonding in the FAS samples. The high resolution real-space neutron diffraction data allows a direct determination of the single:double bond ratio, and also shows the presence of sp1 hybridised carbon bonding environments.

The effect of hydrogen dilution on the structure of a-C:H

Two a-C:H samples were prepared using a fast-atom deposition system from acetylene and an acetylene/hydrogen gas mixture. Their structure was investigated using neutron and x-ray diffraction and infrared spectroscopy measurements. Compositional analysis shows that a 1:1 mixture results in a change from to a-, i.e. has a very small effect on the composition. The diffraction data also show that the addition of hydrogen to the precursor gas has no significant effect on the average bond distances and angles but shows a small change in the H-C-H and C-C-H correlations between the two samples. However, the infrared data show that there are significant changes in the bonding of hydrogen within the sample - changes which do not affect the average network structure. We observe a decrease in the amount of and groups, and an increase in the fraction of and CH groups, with the formation of a second CH bonding environment in the hydrogen-diluted sample. Therefore, in addition to providing useful structural information on these a-C:H samples, this set of experiments illustrates very well the complementary nature of the data from diffraction and spectroscopic techniques.

The effect of hydrogen dilution on the interatomic bonding of amorphous hydrogenated silicon: carbon

Abstract The effect of hydrogen dilution of the precursor gas mixture on the local bonding environment in glow-discharge deposited a-Si:C:H has been studied by neutron diffraction and inelastic neutron scattering. The neutron diffraction results show a large increase in the silicon-carbon bonding upon hydrogen dilution, at the expense of silicon-silicon bonding. The inelastic neutron scattering provides complementary information on the hydrogen bonding environment. The hydrogen is predominantly bonded in SiH and SiH 2 groups, with a large increase in the SiH 2 group concentration occurring upon hydrogen dilution. The data presented here show that SiH 3 and CH n groups are present as a very small fraction of H bonding sites, if at all.

A new model for tetrahedral amorphous carbon (ta-C)

Abstract We have developed the reverse Monte Carlo (RMC) modeling method to generate two physically acceptable models for tetrahedral amorphous carbon (ta-C). The models have been produced by fitting to experimental neutron diffraction data and by applying various constraints consistent with chemical and physical knowledge of the material. In particular, three different carbon atom types (tetrahedral, planar and linear) have been defined and a new approach to applying coordination number constraints has been developed. The models were generated from an initial random distribution of 3000 atoms and from these we have determined ring number and coordination number distributions that are statistically averaged over the whole structure. We are able to look in detail at the distribution of different types of carbon bonding sites within the box and at the effects of incorporating 5 at.% H into the structure. The new models show that atoms with sp2 bonds tend to cluster and form polymer-like chains, which interlink the regions of sp3 or diamond-like bonding. Also, the relative flexibility of these polymeric chains results in more porous, less dense regions of the network. A comparison with other models for ta-C is also made.

Neutron and x-ray diffraction studies of a-C:N:H

The results of neutron and x-ray diffraction experiments performed on a range of nitrogen-doped amorphous hydrogenated carbon (, where x = 0, 6, 8 and 10 at.%) samples are presented. Changes in the average bond lengths and bond angles are observed as the nitrogen content is varied; however, no qualitative structural differences are seen between the samples. Both CH and NH correlations are seen directly, and N - H bonds are shown to be favoured over C - H at the highest N concentration. Also for the sample with the highest N content, the data provide evidence for the presence of a small quantity of triple bonds, although this is not the dominant bonding environment of N. The data indicate that N incorporation may result in some transformation of C sites to sites, but there is no evidence for N inducing the formation of graphitic clusters, and the structure remains disordered overall.