Preparation of nanostructures for electron field emission applications

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

Preparation of nanostructures for electron field emission applications

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

Fusion of carbon nanotubes for fabrication of field emission cathodes

Consolidated carbonaceous samples prepared by spark plasma sintering of multi-walled carbon nanotubes are analyzed, and the effect of the heating regime on their morphology, density, thermal stability, electron field emission and adhesive behavior studied. The trend in the field emission properties of these samples is explained by the changes in the mobility of the nanotube tips. The effect of such changes in the number of free nanotube tips is also deduced from micro-adhesion data, obtained from pull-off tests using atomic force microscopy

Extending the Utility of Conducting Polymers through Chemisorption of Nucleophiles

The investigation of poly­(3,4-ethylenedioxythiophene) (PEDOT) exposed to several example amines has shown that they bind to the conducting polymer through a nucleophilic attack on the positively charged carbon atoms. The PEDOT films were polymerized using the vacuum vapor phase polymerization (VPP) technique, and their electrical and optical properties subsequently modified by adsorbing aniline, ammonia or urea. Analysis of the surface chemistry shows that the reversibility of the binding depends on the nature of the amine, although a portion is chemisorbed to the PEDOT. This mechanism allows the polymer surface to be decorated with biomolecules or nanoparticles, as demonstrated by attachment of poly­(allylamine) coated silica nanoparticles to the PEDOT. This understanding provides the opportunity to control PEDOT properties, and opens the pathway to extend the utility of these electroactive, optoactive, and bioactive materials

Ultrathin polymer films for transparent electrode applications prepared by controlled nucleation

The vacuum vapor phase polymerization (VPP) technique is capable of producing conducting polymer films with conductivities up to 3400 S cm−1. However, the method is not able to produce robust nano-thin films as required for transparent conducting electrode (TCE) applications. We show that with the addition of aprotic solvents or chelating agents to the oxidant mixture, it is possible to control the polymerization rate, and nucleation, in the VPP process. This provides the opportunity of altering the grain size and depositing conducting polymer films with a thickness of 16 to 200 nm with resulting optical transmission within the range 50−98% that are robust enough to endure the post polymerization processing steps. The figure of merit (FoM), which is used to quantify a film’s suitability for TCE applications, results in values from 12 to 25. This result indicates that the nanofilms outperform most of the previously reported graphene films and approaches the accepted industry standard for TCE applications.

Using oxygen plasma treatment to improve the performance of electrodes for capacitive water deionization

An oxygen plasma treatment was employed to modify the surface of carbon electrodes used in capacitive deionization (CDI). X-ray photoelectron spectroscopy analysis of samples showed that oxygen plasma is mainly attaching oxygenated groups on the PTFE binder used in these electrodes. By functionalizing the binder it can increase the hydrophilicity of the electrode surface and increase the available specific surface area. 2.5 min of plasma treatment resulted in the largest improvement of CDI performance of electrodes. Thermodynamic study of CDI performance showed that the modified electrodes followed Langmuir and Freundlich isotherms resulting from the increased interaction between the enhanced electrodes and water. The kinetic study showed that the CDI process followed a pseudo-first order adsorption kinetics. The calculated adsorption rate constants suggested that plasma modification can accelerate ion adsorption of electrodes.

Large Area Nanostructured Arrays: Optical Properties of Metallic Nanotubes

In this study, large area metallic nanotube arrays on flexible plastic substrates are produced by templating the growth of a cosputtered alloy using anodized aluminum oxide membranes. These nanotube arrays are prepared over large areas (ca. squared centimeters) by reducing the residual stress within the thin multilayered structure. The nanotubes are approximately 20 nm in inner diameter, having walls of <10 nm in thickness, and are arranged in a close packed configuration. Optically the nanotube arrays exhibit light trapping behavior (not plasmonic), where the reflectivity is less than 15% across the visible spectra compared to >40% for a flat sample using the same alloy. When the nanotubes are exposed to high relative humidity, they spontaneously fill, with a concomitant change in their visual appearance. The filling of the nanotubes is confirmed using contact angle measurements, with the nanotubes displaying a strong hydrophilic character compared to the weak behavior of the flat sample. The ability to easily fabricate large area nanotube arrays which display exotic behavior paves the way for their uptake in real world applications such as sensors and solar energy devices

Large Area Nanostructured Arrays: Optical Properties of Metallic Nanotubes

In this study, large area metallic nanotube arrays on flexible plastic substrates are produced by templating the growth of a cosputtered alloy using anodized aluminum oxide membranes. These nanotube arrays are prepared over large areas (ca. squared centimeters) by reducing the residual stress within the thin multilayered structure. The nanotubes are approximately 20 nm in inner diameter, having walls of <10 nm in thickness, and are arranged in a close packed configuration. Optically the nanotube arrays exhibit light trapping behavior (not plasmonic), where the reflectivity is less than 15% across the visible spectra compared to >40% for a flat sample using the same alloy. When the nanotubes are exposed to high relative humidity, they spontaneously fill, with a concomitant change in their visual appearance. The filling of the nanotubes is confirmed using contact angle measurements, with the nanotubes displaying a strong hydrophilic character compared to the weak behavior of the flat sample. The ability to easily fabricate large area nanotube arrays which display exotic behavior paves the way for their uptake in real world applications such as sensors and solar energy devices

Polymeric material with metal-like conductivity for next generation organic electronic devices

The reduced pressure synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) with sheet-like morphology has been achieved with the introduction of an amphiphilic triblock copolymer into the oxidant thin film. Addition of the copolymer not only results in an oxidant thin film which remains liquid-like under reduced pressure but also induces structured growth during film formation. PEDOT films were polymerized using the vacuum vapor phase polymerization (VPP) technique, in which we show that maintaining a liquid-like state for the oxidant is essential. The resulting conductivity is equivalent to commercially available indium tin oxide (ITO) with concomitant optical transmission values. PEDOT films can be produced with a variety of thicknesses across a range of substrate materials from plastics to metals to ceramics, with sheet resistances down to 45 Omega/square (ca. 3400 S.cm(-1)), and transparency in the visible spectrum of >80% at 65 nm thickness. This compares favorably to ITO and its currently touted replacements

Vapor phase synthesis of conducting polymer nanocomposites incorporating 2D nanoparticles

The one step fabrication of nanocomposite films of conducting polymers with 2D nanoparticles is investigated in this study. Specifically, the inclusion of nanomaterials (single layer graphene, single layer molybdenum disulfide) within PEDOT is achieved using the vapor phase polymerization (VPP) technique. This facile process allows for the formation of thin films of the order of less than 200 nm, which display a wide range of enhanced properties (mechanical, optical, and electrochemical). Herein, in a typical example with added graphene (\u3c0.003% w/w), the in-plane modulus of the film is increased to 145 GPa (ca. 65% increase above PEDOT−Tos) without any decrease in light transmission or lowering of conductivity. Furthermore, the nanocomposite outperforms both the PEDOT−Tos film and a Pt substrate in the reduction of oxygen when acting as an air-electrode