Preparation of nanostructures for electron field emission applications

Abstract

Carbon nanostructures are a very promising option as new materials in field emission (FE) devices. In this thesis, a number of current challenges in the application of such materials in FE devices are addressed. A new microwave-based method was developed to improve the FE properties of commercial carbon nanotubes (CNT), and methods to produce macro-scale ensembles of CNTs as either networks or composites, were developed. Research was also carried out to develop a simple and economic method for preparation of carbon nanofibers and graphene sheets. At each step, a combination of analytical techniques including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, electron microscopy, thermogravimetry, as well as FE analyses, were used to determine the resultant material properties. In order to modify the FE properties of CNTs, two new microwave-based, treatments were developed. XPS and Raman results of samples prepared by conventional acid treatments or after thermal treatments were compared to the microwave-treated materials. It was found that the microwave-based methods resulted in less damage to the CNT structure and more uniformity in chemical functional groups on CNTs, compared to other treatments. It was also found that the microwave-plasma method was able to produce a “sheet-like” graphitic material between CNTs. Since the only carbon source in the MW-plasma process was the CNTs, a mechanism based on unzipping of CNTs was proposed and confirmed by XPS and Raman spectroscopy results. The formation of this new nanostructure was found to show considerably enhanced FE properties, compared to those of the CNTs. The unzipped nanotubes have additional sharper and thinner emitting tips (edges) with sp3 hybridisation of the carbon atoms on the edges, which is the likely reason for the significant improvement of FE properties in these post-treated samples. A number of methodologies aimed at fabrication of efficient CNT-based FE arrays were examined. In the first approach, nanocomposites based on CNTs or graphene nanosheets and conductive polymers were prepared. The most stable CNT/ polypyrrole nanocomposites were achieved by direct electropolymerisation on the surface of a membrane acting as spacer in the FE process, with the functional groups on the surface of CNTs acting as the necessary counter-ions. The FE measurement of these samples showed that limiting the emitting surface caused the turn on electrical field to increase, however the stability of emission was also improved. The other approach for the preparation of the macro-scale emitters was based on forming a network of CNTs. Webs based on spinnable CNT forests were drawn over a graphitic substrate. The FE studies of these samples showed a direct relationship between the density and thickness of the webs and FE properties, and an inverse relationship was found between the length of CNTs and their FE performance. By defining a tip number parameter, a direct linear relationship was observed between tip number and emission current. It was found that increasing the number of layers of CNT webs did not result in any improvement in FE performance, whilst double-layered samples with the two layers vertical to each other was found to significantly improve the FE performance. The final approach for producing macro-scale samples involved producing CNT networks by molecularly-fusing CNTs using the high temperature, spark plasma sintering (SPS) process. The networks prepared by this method showed that by increasing the sintering temperature from 1000C to 2000C, the CNT network became more packed and dense with better graphitic structure. However, improved fusion of the nanotubes at high temperatures led to the loss of freedom of CNT tips, and reduced FE properties. In the last aspect of this research, the microwave-based method previously used for modification of CNTs, was employed for synthesis of carbon-based nanomaterials themselves. It was found that synthesis of nanofibers could be catalysed by the presence of metallic catalyst nanoparticles which can be produced by the microwave-plasma process itself, from an incorporated solid metallic coupon. It was found that changing the carbon source from polystyrene to polyethylene (with higher hydrogen to carbon ratio) in the absence of a catalytic trigger can also be used for preparation of graphene nanosheets. FE studies of these samples showed that samples containing nanofibres exhibit better FE properties than graphene-based samples, although thermal treatment of the graphene samples did result in an improvement in field emission properties. It was also shown that depositing of the nanostructures during the production process results in better FE properties, in comparison to samples prepared by coating the substrate by solution casting from a dispersion of nanoparticles, due to the vertical alignment of the directly-coated nanostructures