Diamond-graphite hybrids and the nature of amorphous carbon and diamond-like carbon
- PDF / 439,878 Bytes
- 5 Pages / 576 x 792 pts Page_size
- 10 Downloads / 202 Views
The nature of amorphous carbon has been explored by molecular mechanics by examining the structures of species such as Cg4Hx and CisoH*, wherein the percentage of sp3 carbons is progressively increased in a graphitic network. The nature of diamond-like carbon has been similarly investigated by examining the structures of C84HX and Q02H1 where the percentage of sp2 carbons is varied in an sp3 network. The dependence of the average coordination number as well as the sp31 sp2 atom ratio on the atom fraction of hydrogen has been investigated in light of the random covalent network model.
I. INTRODUCTION Interest in carbon structures has increased enormously in recent years, especially because of the discovery of the fullerenes and related forms of carbon containing carbon atoms with hybridization somewhere between sp2 and sp3 and carbon-carbon bond distances between those of single and double bonds.1 An equally fascinating aspect of carbon pertains to the host of possible hybrid structures between graphite and diamond, involving mixtures of sp2 and sp3 hybridized carbon atoms. Thus, amorphous carbon (a-C: H) involves a graphitic network containing a fairly high percentage of sp3 carbons, the latter being hydrogenated.2'3 Molecular dynamic studies4 suggest amorphous carbon to contain around 15% sp3 carbon. Tetrahedral amorphous carbon (ta-C) or hydrogenated tetrahedral amorphous carbon (ta-C: H), broadly referred to as diamond-like carbon (DLC), containing 10-25% sp2 carbon is another important form of carbon.5'6 The conditions for preparing amorphous carbon differ from those of DLC. Furthermore, DLC is very much harder than amorphous carbon which is more like graphite. We have been interested in exploring the possible structures of amorphous carbon, diamond-like carbon, and such diamond-graphite hybrids, in view of the obvious academic importance besides technological relevance. There are indeed some proposals in the literature regarding the structures of such hybrids,2'5"7 some of which may exhibit novel electronic properties.7 Thus, Balaban et al? have considered hybrid structures that consist of cubic or hexagonal diamond blocks interconnected by blocks or layers of graphitic strips with zig-zag (acenic) or arm-chair (fibonacenic) edges.
a)
Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
Downloaded: 13 Mar 2015
These authors have not, however, examined the relative stabilities of the alternate structures. In addition to visualizing diamond-graphite hybrid structures, the nature of which we know so little, it is of value to understand their structures in terms of the dependence of the average coordination number as well as the sp3/sp2 atom ratio on the hydrogen atom fraction, x H , the hydrogen atoms being present on the surface and the edges of diamond-type structures. Some of the amorphous carbons are known to contain 20-60% hydrogen, the percentage varying with the sp31 sp2 ratio.8'9 We have employed molecular mechanics to obtain t
Data Loading...
