Carbon nanotubes/fibres for high field applications

• Main Author: Bell, M. S.
• Published: University of Cambridge 2006
• Subjects: 530.41
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596534

The physical processes occurring during nanotube deposition were investigated, and a nanotube growth model explained. A mass spectrometer was used to investigate the plasma chemistry. The role of NH₃ in suppressing C₂H₂decomposition and encouraging nanotube formation was demonstrated, and an optimum condition for clean nanotube production determined to be at a gas ratio of approximately 20% C₂H₂: 80% NH₃. The ability to selectively deposit nanotubes and nanofibres at pre-defined locations on a flat silicon substrate was demonstrated, and shaping of the silicon substrate using anisotropic wet chemical etching identified as a way to improve the field amplification factor of a nanotube device. Using a self-aligning process, individual nanofibres were successfully deposited on the tips of etched silicon pyramids. The importance of removing any molecular adsorbates from carbon nanofibres before commencing field emission measurements was shown. Following conditioning to remove adsorbates, an array of nanofibres on a flat substrate was measured to have a field amplification factor of approximately 400, and an array of nanofibres on a shaped substrate was measured to have a field amplification factor of approximately 1400. High voltage experiments resulted in irreversible field evaporation of atoms from the tips of the carbon nanofibres, causing a reduction in the field amplification factor. The vertical growth rate of carbon nanofibres during nanofibre deposition was shown to be proportional to the electric field in the plasma close to the tip of the growing nanofibre. An observed tailing-off of the vertical growth rate of isolated carbon nanofibres was attributed to the changing morphology of the catalyst particle resulting in the precipitation of amorphous carbon, which covers the particle and prevents further growth.