Characterisation of novel carbonaceous materials synthesised using plasmas

Lau, D 2009, Characterisation of novel carbonaceous materials synthesised using plasmas, Doctor of Philosophy (PhD), Applied Sciences, RMIT University.


Document type: Thesis
Collection: Theses

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Title Characterisation of novel carbonaceous materials synthesised using plasmas
Author(s) Lau, D
Year 2009
Abstract Novel carbon materials such as carbon onions, nanotubes and amorphous carbon (a-C) are technologically important due to their useful properties. Normally synthesised using plasmas, their growth mechanisms are not yet fully understood. For example, the growth mechanism of the high density phase of a-C, tetrahedral amorphous carbon (ta-C), has been a subject of debate ever since its discovery. The growth mechanism of carbon nanostructures such as carbon onions and nanotubes is also not well known. The aim of this thesis is two-fold. Firstly, to provide insight into the growth of carbon films, in particular, the driving force behind the formation of diamond-like bonding in a-C which leads to ta-C. Secondly, to investigate the growth of carbon onions and other sp2 bonded carbon nanostructures such as nanotubes.

To achieve the first aim, carbon thin films were deposited using cathodic arc deposition at a range of ion energies, substrate temperatures and Ar background gas pressures. These films were characterised using electron microscopy techniques to examine their microstructure, density and sp3 content. It was found that the formation of the ta-C is due to a stress-induced transition whereby a critical stress of 6.5±1.5 GPa is needed to change the phase of the film from highly sp2 to highly sp3. Within this region, a preferentially oriented phase with graphitic sheets aligned perpendicular to the substrate surface was found. By investigating the role of elevated temperatures, the ion energy-temperature "landscape" of a-C was mapped. A range of differing types of carbon films were deposited with microstructures determined by a combination of rearrangements that occur during short time scale thermal spikes and long time scale relaxation processes.

The second aim was achieved by examining the microstructures of carbon onions fabricated using pulsed laser ablation and developing a model for growth based on the preparation conditions. Molecular Dynamics (MD) simulations were used to provide support for this growth model. The formation of carbon onions was found to depend critically on the temperature of the plasma used during fabrication. The MD simulations revealed that the formation of carbon onions is largely a self-assembly process in which the onion graphitises from the surface to the core with an optimum growth temperature of 4000 K. Intermediate states included a graphite-like spiroid topology with sp3 bonds interconnecting the shells. The same mechanism was found to apply to carbon nanotubes and other sp2 bonded structures including preferentially aligned graphene sheets in thin films thus providing a general framework for the growth of these types of material.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Applied Sciences
Keyword(s) Carbon composites
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