Growth, electronics, processing and toxicology of carbon nanotubes

The growth of carbon nanotubes by chemical vapour deposition has been investigated using sodium chloride for facile post-CVD removal of the catalyst support. Saturated methanol vapour and acetylene gas have been used to grow tubes from bimetallic Co/Mo salts and cobalt-based nanoparticles at temperatures below 900°C. Multi-walled nanotubes and carbon fibres have been produced. Absence of single-walled tubes in the carbon product is shown to be characteristic of catalysts prepared using sodium chloride and other inorganic-salt supports, and the factors contributing to this outcome are explored. It is shown that sodium chloride actively inhibits carbon deposition and the formation of single-walled tubes in specific catalytic systems. L-α-phosphatidylcholine, a model for pulmonary surfactant, has been shown to disperse carbon nanotubes to a degree commensurate with commonly employed nanotube surfactants (e.g. Triton X-100 and SDS). The results obtained using this model surfactant have been used in understanding the potential interaction between nanotubes and the lung environment, and the relevance to issues surrounding the toxicology of carbon nanotubes is discussed. In collaboration with co-workers at the Scottish Microelectronics Centre, single-walled carbon nanotubes have been exposed to CF4 and SF6 plasmas to control their degree of functionalisation before substitution with 1,2-diaminoethane. The degree of amino functionalisation has been shown to depend on the degree of initial fluorination rather than oxygen or carbon defects and thereby presents a replicable route to n-type doping. The different types of nanotube-fluorine bonding produced by the plasma processes (e.g. covalent, semi-ionic) have been investigated as well as the effect of different plasmas on the doping process. Electrical characterisation has shown p-type semiconducting behaviour, for CF4 functionalised tubes and n-type semiconducting behaviour for amino functionalised tubes. The degree of n-type behaviour increases with the amount of nitrogen attached.