Abstract
This work focuses on the conditions under which multiple carbon nano-fibres (CNFs) can be grown radially on a single metal catalyst, known as carbon nano-octopi (CNO) structures, and expands on the evaluation for this type of growth in order to produce large surface areas at the low temperatures that are compatible with large area electronic processing. It was found that the CNF growth was promoted by the use of an adhesive tape initially utilised to fasten a carbon cloth coated with a nickel catalyst onto a silicon support. Compositional characterisation of the catalyst using electron energy loss spectroscopy (EELS), revealed copper in the bulk of the nickel catalyst and in some surface locations near the catalyst edges. An investigation of the growth mechanism was necessary to increase the CNO yield and produce homogeneous large-area growth, and was facilitated by the control and reduction of the number of experimental parameters in the growth runs. It was observed that CNO growth can occur under a range of temperatures and copper to nickel ratios, however, the growth resulted in a single CNF per catalyst in the absence of the polymer, the copper metal and/or in the case of a large copper to nickel ratio on the catalyst surface. Growth would not occur at all in the absence of acetylene. It was concluded that both the copper and the polymer of the adhesive played a role in the growth mechanism of the CNOs: the polymer enabled catalyst faceting, allowing for multiple carbon leg formation, and the active nickel regions on the surface of the cupro-nickel catalyst served to separate the growing carbon legs. Maximising the specific yield and surface area of carbon nanostructures can help to improve the efficiency of a range of devices. CNO forests with higher yields and higher specific surface areas were achieved by optimising growth parameters such as temperature, catalyst annealing time and catalyst thickness. It was found that the radial growth of thinner, longer, more numerous legged CNOs is promoted by the choice of the size of the catalysts on which they are grown. It was also found that the diameter and length of the nanofibres depend on the catalyst diameter which can be tuned by controlling the annealing time of the catalyst. The surface area that can be obtained from the octopus geometry was optimised at a critical catalyst size of 125 nm through growth of a large number of octopus legs. Carbon films consisting of CNOs resulted in relatively low electrical resistivity, highlighting the possibility of their use as high surface area electrical contacts.