Chemical Vapour Deposition of Graphene & Carbon Nanotubes
The underlying dispersive growth kinetics, and characterisation of thermal chemical vapour deposited (CVD) graphene grown using Cu foils, PVD Cu thin films on Si and Si/SiO2, and non-catalytic thermal-CVD on GaN, and SiO2 is investigated. Similarly, the heterogeneous catalysis of carbon nanotubes, synthesised by thermal and plasma enhanced chemical vapour deposition (left), is also studied using supergrowth Al2Ox/Fe bilayer (above) and ITO/Ni dot array catalysts. Conditions for large-area, wafer-scale synthesis of monolayer graphene, single-wall CNTs and multi-wall CNTs is investigated with wafer-scale transfer and characterisation considered. This research addresses the key issues and challenges for technology up-scaling and pragmatic integration. Working as an Aixtron Research Fellow, and in close collaboration with their Cambridge research facility, we have developed catalyst structures and growth recipes for large area chemical vapour deposition of mono- and multi-layer graphene, MWCNTs and SWCNTs from R&D scale samples, as small as 5x5 mm, up to industry-scale 300 mm wafers (right) using micro-CVD reactors and load-lock commercial systems.
- "Evolutionary Kinetics of Graphene Formation on Copper", Nano Lett., 13, 3, 967-974, (2013).
Dry Alignment of Carbon Nanotubes
Photonic crystals and metamaterials elicit a number of novel optical properties not found in nature. Understanding and developing protocols to achieve accurate alignment of CNTs,en masse, offers one possible route to fabricate such enigmatic materials. Here, I am principally concerned with dry-transfer techniques to preserve the as-synthesised CNTs electronic structure. Here I research dielectrophoresis - based on anisotropic torque induction-, gas alignment, as well as solid-state extrusion (right), an effect attributed to high inter-tube van der Waals binding. with the optical properties of the resulting aligned nanoscale, pseudo-regular graphitic material investigated.
- "Dry-Transfer of Aligned Multiwalled Carbon Nanotubes for Flexible Transparent Thin Films", J. Nanomat. 10.1155/2012/272960, (2012).
- "Probing The Electronic Structure Of Multi-Walled Carbon Nanotubes by Transient Optical Transmittivity", Carbon, 57, 50-58, (2013).
- "Plasma Enhanced Chemical Vapour Deposition of Horizontally Aligned Carbon Nanotubes", Materials, 6, 6, 2262-2273, (2013).
Carbon Nanotube- & Graphene-based Field Emission Electron & X-Ray Sources
Field emission electron sources permeate health care, border control, and graphical user interface / display technologies. Field Emission is the liberation of electrons from an electron-dense solid into a vacuum, under the application of an intense electric field. It is a wholly quantum mechanical phenomenon and can be succinctly described in adequate detail using fairly simplistic descriptors based on the free-electron model. A potential difference is applied between an electron emitting surface (cathode) and a counter electrode (anode). Electrons confined to the Fermi Sea Fowler-Nordheim tunnel through the electric-field-narrowed triangular potential barrier thereby producing collimated beams of electrons. Graphitic nanocarbons, and graphene in particular, are low-Z materials and have one of the lowest secondary electron and backscattered electron yields reported to date, which in the case of graphene is a consequence of its linear dispersion. Subsequently, graphitic nanocarbons are near ideal electron emitters and electron transparent media. The energy barrier associated with atomic migration and surface diffusion is much larger than in metallic emitters, making CNTs highly stable. Similarly, the sp2 bonding in CNTs are significantly stronger than the metallic bonds of ubiquitous refractory metal electron emitter tips, allowing CNTs to carry current densities of up to 109 A/cm2 and the ability to withstand extremely intense electric fields (several V/nm) necessary for high current applications, specifically X-ray generators, with the CNTs remaining exceptionally stable at temperatures of up to 2000 K. CNTs are physically inert to sputtering and chemically inert to surface work-function adjusting absorbates. The combination of high temperature and high electric fields in metals induce field-sharpening by surface diffusion and electro migration. My research focuses on the development of novel nano-micro scale ballasting structures to prevent avalanche run-away and consequent destruction of the CNT emitters. I also investigate potential new nanocarbon triode structures based on graphene and CNTs, as well developing nanostructured field emission displays. CNTs are particularly resilient to bulk mobilisation making them temporally stable and particularly well suited for highly parallel electron lithography systems, travelling wave tubes, microwave amplifiers, displays, and X-ray sources (right).
- "In-Situ Deposition of Sparse Vertically Aligned Carbon Nanofibres on Catalytically Activated Stainless Steel Mesh for Field Emission Applications", Dia. & Rel. Mater., 23, 66-71, (2012).
- "Hot Electron Field Emission via Individually Transistor-Ballasted Carbon Nanotube Arrays", ACS Nano, 6, 4, 3236-3242, (2012).
- "A Graphene-Based Large Area Surface-Conduction Electron Emission Display", Carbon, 56, 255-263, (2013).
- "Field Emission Characteristics of Contact Printed Graphene Fins", Small, In Press, (2013).
- "Highly Electron Transparent Graphene for Field Emission Triode Gates", Adv. Funct. Mater., In Press, (2013).