STRUCTURAL DISORDER IN ION-IRRADIATED CARBON MATERIALS

The effects of ion irradiation on carbon based materials are reviewed laying emphasis on the well known ability of carbon to have different kinds of bonding configuration with the surrounding atoms. It was foun d that two kinds of bonding configuration of the carbon atoms are allo wed in solid amorphous carbon phases. These rearrange the four valence electrons of carbon into sp(2) (trigonal bond) and sp(3) (tetrahedral bond) hybridizations. Driving the trigonal carbon fraction (x), the p hysical and chemical nature of solid carbon materials can change in a dramatic way ranging from metallic (x approximate to 100%) to insulati ng (x approximate to 0%) through semiconductor properties. The amount of the tetrahedral (or trigonal) carbon atoms can be controlled by ion beam irradiation, using suitable conditions and/or introducing foreig n species such as hydrogen or silicon by the implantation technique. I n hydrogenated amorphous carbon (a-C:H) and hydrogenated amorphous sil icon-carbon alloys (a-Si1-xCx:H), the ion beam effects are able to pro duce stable and reproducible compounds, achieved by tuning the hydroge n (silicon) concentration with well defined equilibrium curves between the trigonal carbon fraction and hydrogen (silicon) content. Raman sp ectroscopy and temperature dependent conductivity experiments performe d on these alloys suggest clustering effects in samples with high carb on content (x approximate to 0.5) due to the strong binding energy of the C-C double bond with respect to C-Si and Si-Si. Several models and theoretical studies such as the ''random covalent network'' (RCN) and molecular dynamics calculations have been used to fit the experimenta l results. It is shown that, while RCN models are highly inaccurate be cause of the clustering effects, molecular dynamics calculation data a re very close to the experimental measured physical properties and con firm the ability of the trigonal carbon to cluster in graphite-like ag gregate.