Advances in bottom up and top down fabrication of nano carbon architectures and their applications
2017-02-26T22:50:42Z (GMT) by
In this thesis three different carbon allotropes are examined through bottom up and top down approaches: carbon nanotubes (CNT), carbon nanofibers (CNF) and reduced graphene oxide (rGO). The CNTs and CNFs are synthesized by Chemical Vapour Deposition (CVD); CNTs were typically grown using a substrate of thin film of iron deposited on silicon, while CNFs were grown from a novel Ni-doped metal organic framework (MOF). Patterned and electrically conducting rGO structures are fabricated by focused ion beam induced deoxygenation on Graphene oxide (GO) films and micro-super-capacitor devices are constructed and evaluated. Arrays of aligned CNTs were integrated into fluidic micro-channels through microfabrication (photolithography and focused ion beam milling) approach. Ion-current measurements through these devices indicate currents of 0.93 nA per tube which is four orders of magnitude higher than the theoretical current through a standard pore. The CNFs are grown on a scaffold of MOF-5 in which a percentage of the Zn metal centres have been isomorphously substituted with Ni. The quality and diameter distribution of the CNFs are studied with varying percentages of Ni substitution up to 50% substitution. The CNFs are found to template the MOF microstructure, while introducing the electrically conductive CNF - an ideal situation for capacitive energy storage. The hierarchically porous CNF demonstrate a capacitance of 24 F/g and increases with increasing concentration of Ni in the parent MOF; however as Ni is still present, activation of the nickel centres in an alkaline environment leads to a pseudo capacitance of 247 F/g making it an effective electrode material. A novel top down approach for patterned reduction of graphene oxide (GO) is reported, which utilises the kinetic energy of incoming ions, specifically gallium ions, to preferentially sputter away the oxygen species due to differing volatility and bond strengths to carbon, leading to reduction. The ions are fired at the sample in the form of a focused ion beam allowing for the direct writing of conductive reduced graphene oxide region with nanometre spatial resolution. The effect of ion bombardment is also studied in detail uncovering distinct regimes of amorphisation and reduction with varying ion fluence. This is used to determine the optimal parameters for the reduction of the graphene oxide and makes use of them to directly write planar micro super capacitors into a graphene oxide film. These micro capacitors were evaluated and display a high specific capacitance (112 mF/cm2) as well as high specific energy densities (3.9 × 10-6 Wh/cm2) and ultra-high power densities (179 W/cm2), as yet unseen in this field.