Plasma Processing Studies with Application to Carbon Nanotube Fluorination

Author: Anders Jack Barlow

Barlow, Anders Jack, 2012 Plasma Processing Studies with Application to Carbon Nanotube Fluorination, Flinders University, School of Chemical and Physical Sciences

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The fluorination and growth of carbon nanotubes using plasma is a possible means towards controlling their properties. To develop a deeper understanding of these processes, a series of fundamental experiments were performed that studied sulphur hexafluoride (SF6) plasma, its exposure to carbon nanotubes and finally the growth of nanotubes within a plasma environment. Plasma was characterised using a single cylindrical Langmuir probe, while carbon nanotube modification and growth treatments were studied using X-ray photoelectron spectroscopy, Raman spectroscopy and electron microscopy. Methods that allowed the fluorination and growth of nanotubes with desired properties are discussed. Using an established Langmuir probe apparatus, plasma ignited within sulphur hexafluoride was characterised. Initially, a curious instability was observed within the main plasma density that oscillated with a frequency of tens of Hertz. This instability was characterised with respect to experimental parameters such as applied power, gas pressure and probe position within the plasma chamber. From this research the instability is hypothesised to manifest as a wave that moves radially throughout the chamber from the plasma source with a speed of ~16 ms-1. The instability interfered with Langmuir probe experiments by inducing fluctuations in the measured probe currents. The experiment was refined to account for this interference and allow the characterisation of the plasma with respect to experimental parameters. The electron density within SF6 plasma was observed to increase with absorbed rf power. Only marginal variations were observed with gas pressure. Typical densities were found to be ~108 cm-3. Ion densities display a stronger dependence on the gas pressure in comparison with rf power and overall were significantly greater than that of the electron density, ~109 cm-3, despite the assumption that plasma maintains quasineutrality. This is attributed to the strong electronegativity of the SF6 molecule. The space potential of SF6 plasma remained largely constant irrespective of applied rf power and gas pressure, while the floating potential is found to decrease with increasing power and pressure. For most SF6 plasma the electron temperature is found to remain constant across the powers studied, although for high pressures the presence of a second population of high temperature electrons is observed. Single-walled carbon nanotubes were exposed to sulphur hexafluoride plasma resulting in the attachment of fluorine moieties to the structures. Initial analysis found that smaller diameter nanotubes result in greater levels of functionality. Nanotubes with metallic band structure are also observed to show a greater susceptibility to the plasma fluorination mechanism. By varying the absorbed rf power the extent of these functionalities could be tuned, with a general increase in fluorination as power was increased. The oxygen content on the nanotube surfaces acted to interfere with the fluorination mechanism whereby increased oxygen content at the surfaces prior to a treatment resulted in a greater amount of fluorine attachment after exposure. This observation was then used to develop another level of control over the fluorination mechanism. Sulphur hexafluoride plasma containing low amounts of either oxygen or water vapour produce much greater levels of functionality. The cause of this was elucidated as an enhanced breakdown of the parent sulphur hexafluoride molecule resulting in a greater concentration of reactive fluorine ions in the plasma and thus greater reactivity overall. Pure sulphur hexafluoride plasma resulted in carbon-fluorine bonding at the nanotube surface that was a mixture of covalent and semi-ionic types, thereby decreasing the overall quality of the functionalised sites. Through the addition of oxygen, either into the plasma or at the nanotube surface via pretreatments, the relative amounts of covalent and semi-ionic bonding could be tuned. Pretreatments using oxygen plasma result in almost purely covalent carbon-fluorine bonds. The plasma chamber was modified to enable the growth of carbon nanotubes from mixed methane/argon plasma. The resultant nanotube growth displayed predominantly single-walled nature with very high homogeneity. The yield was very low however, with the nanotubes hypothesised to be very short in length. The variation of experimental parameters, such as time and temperature, afforded the growth of nanotubes with tailored diameters. Through Raman spectroscopy a transition from single- to multi-walled growth was observed to occur with increasing growth time and decreasing growth temperature. For temperature this is explained through a decrease in the energy of growth species at the catalytic sites and thus insufficient energy for the formation of single-walled nanotubes. The transition observed for growth time is explained through the enhanced etching of the smaller diameter nanotubes by plasma species, an effect that the larger multi-walled nanotubes could withstand for longer periods of time.

Keywords: carbon,nanotube,plasma,sulphur hexafluoride,XPS,Langmuir probe,synchrotron
Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2012
School: School of Chemical and Physical Sciences
Supervisor: Assoc. Prof. Jamie S. Quinton