The Structure of Amorphous Hydrogenated Carbon by Neutron Diffraction
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THE STRUCTURE OF AMORPHOUS HYDROGENATED CARBON BY NEUTRON DIFFRACTION D.W. HUXLEY, P.J.R. HONEYBONE, R.J. NEWPORT, W.S. HOWELLS* and
J. FRANKS** Physics Laboratory, University of Kent, Canterbury, CT2 7NR, Kent, UK. *Neutron Division, Rutherford Appleton Laboratory, Didcot, OXll OQX, UK. **Diavac ACM Ltd., 2 Brookfield Avenue, Ealing, London, W5 1LA, UK.
ABSTRACT Neutron diffraction data from a large, off-substrate sample of amorphous hydrogenated carbon (a-C:H) is presented and discussed. The material is prepared using a fast-atom deposition system using acetylene as the precursor gas. The experiments were performed on the ISIS pulsed neutron source (Rutherford Appleton Laboratory, UK) which is capable of yielding data over an exceptionally wide dynamic range; this ensures a real-space resolution sufficient to resolve directly, for the first time, contributions from the principle C-C bond types. Precise details on the C-H correlations are also revealed by the data, including the presence of molecular hydrogen trapped within distorted spheroidal cages. Quantitative complementary data on the vibrational states of the bonded hydrogen, derived from inelastic neutron scattering (INS) using a simple force-field model, is also presented. In particular, the INS data is used to provide a reliable estimate of the CH:CH2 ratio. INTRODUCTION Amorphous materials have a history of successful technological exploitation extending back more than two decades and in that period our knowledge of their properties has grown steadily. However, the materials at the core of this continuing fundamental and technological interest are relatively complex and a large number of important questions concerning their properties remain unanswered. Moreover, new and novel materials continue to be generated which open up the range of questions still further. We seek to address ourselves to the central problem of understanding the relationship between the observed (bulk) properties of these materials and their structure at the su'b-microscopic, or atomic level. A material that offers a particularly intriguing, and certainly one of the broadest ranges of technological potential is amorphous hydrogenated carbon, a-C:H (also referred to as "diamond-like carbon") which may be prepared harder, denser and more resistant to chemical attack than any other solid hydrocarbon. These properties, along with optical properties such as the optical gap and refractive index may be varied by changing the deposition parameters, which has led to a large number of potential applications [1]. The structure giving rise to these useful properties is not yet fully 3 2 understood, with current models involving clusters of sp carbon linked by sp carbon. The reviews of Robertson [2] and Angus et al. [3] give a fuller account of Mat. Res. Soc. Symp. Proc. Vol. 270. 01992 Materials Research Society
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these and other models. The role played by hydrogen in determining the properties of a-C:H is crucial to a full understanding of the material. Zou et al. [4] have shown that hi
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