Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 48-50). Also available in electronic version.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.

This thesis is concerned with the research of the electron field emission properties of carbon based materials. Low emission threshold fields have been observed from both amorphous carbon thin films and carbon nanotubes. The emission mechanism can be subdivided into two groups depending on the type of electric field enhancement. These are the amorphous carbon flat films with non-geometric field enhancement and carbon nanotubes with high surface geometric field enhancement. Amorphous carbon thin films are deposited using an rf-plasma enhanced chemical vapour deposition technique. Changing the deposition conditions such as the addition of Argon or Nitrogen modifies the electronic properties. This induces variations in the sp2 concentration and its distribution within the films. The electron field emission properties from amorphous carbon thin films show a close relationship to its sp2 configuration. A model based on non-geometric field enhancement is proposed to explain the variation in the field emission characteristics. Nano-structured amorphous carbon films custom "designed" using ion beam assisted deposition with sp2 cluster sizes of around 60 nm have also been investigated. The field emission threshold field was shown to be controlled by the film's intrinsic stress and the local carbon density. With increasing stress, there is a concomitant increase in the local density, which is postulated to decrease the distance between the carbon graphitic "planes". This results in enhancement of the electron emission at lower fields. Stress within the films also induces changes to the band structure of the nano-structured carbon which are beneficial to the field emission process. Field emission from carbon nanotubes that are embedded in a polymer matrix has been investigated. The emission threshold fields are observed to be dependent on the nanotube density. The effect of electric field screening is used to explain the reduction of field enhancement observed in these films with increasing nanotube density. The field emission properties are compared with those films which have vertically aligned and in e-beam fabricated nanotube arrays. Results indicate that field emission properties from non-aligned nanotube films are comparable in performance to the best designed arrays in the literature. Although this study shows carbon based materials to have superior field emission properties, integrating the cathodes to fabricate commercial devices could prove to be very challenging.

The discovery of carbon nanotubes has stimulated intensive research on the synthesis, modification and physical properties of these novel carbon materials due to their unique and one dimensional pore structure, rolled graphitic layers as well as their potential application in sensors, separations, electronic devices, gas storage and quantum dots. Research has suggested that the role of nitrogen in the synthesis of carbon nanotubes could be either to enhance the formation of graphitic layers on the catalyst or to increase the separation of the graphitic layers from the catalyst. The studies further suggested that if nitrogen is incorporated into the structure then the presence of nitrogen in the pentagonal defects would induce the bending and distortion of the carbon nanotubes, whilst nitrogen in substitutional sites would not produce major distortion of the nanotubes. Well-aligned carbon nanotubes were grown from the pyrolysis of ferrocene, toluene and small fractions of various amines, aromatic amines, diamines and amides by nebulized spray pyrolysis at temperature of 8500C. Transmission electron microscopy and scanning electron microscopy reveal that the carbon nanotubes possess bamboo-like-structures and that the nanotubes are well-aligned. The nanotubes ranged from 36 nm to 121 nm in diameter and 65 μm to 625 μm in length depending on the specific growth conditions such as concentration of various nitrogen containing hydrocarbons, growth time and flow rates of the carrier gasses. Raman spectra show the characteristics bands at 1336 cm-1 (D-band) and 1583 cm-1 (G-band). The G-band modes are larger than the D-band modes, suggesting that the carbon nanotubes synthesized In the presence of nitrogen additives are more graphitic than those synthesized without nitrogen addition. TGA analysis under static air environment, showed that the carbon nanotubes are stable within a temperature range of 40 – 5600C and in the at temperature range of 590 – 8000C, the mass loss of carbon nanotubes is constant. The residual metal content in the carbon nanotubes was found to be approximately 20%, when nanotubes were synthesized by the pyrolysis of toluene and ferrocene and about 7% after addition of nitrogen containing hydrocarbons to the catalyst. Selected carbon nanotubes were purified using a microwave purification method. The mass loss between unpurified carbon nanotubes and purified nanotubes were found to be insignificant. The diameters of the carbon nanotubes were found to have been reduced drastically, which suggest that defects were removed during the purification process. The data generated in this study revealed that nitrogen containing hydrocarbons acted as co-promoters of CNTs during the synthesis of carbon nanotubes. / Mr. L.M. Cele Prof. N. Coville

No description available.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg, 2016 / Technologies for the separation of CO2 from flue gas require a feat of engineering for efficient achievement. Various CO2 capture technologies, including absorption, adsorption, cryogenics and membranes, have been investigated globally. The absorption technology uses mainly alkanolamine aqueous solutions, the most common being monoethanolamine (MEA); however, further investigation is required to circumvent its weakness due to degradation through oxidation, material corrosion and high energy costs required for regeneration. Attractive advantages in adsorption technology, including the ability to separate the more diluted component in the mixture with a low energy penalty, have been a motivation for many researchers to contribute to the advancement of adsorption technology in CO2 capture. The challenge in CO2 adsorption technology is to design a hydrophobic and biodegradable adsorbent with large CO2 uptake, high selectivity for CO2, adequate adsorption kinetics, water tolerance, and to require low levels of energy for regeneration processes. The existing adsorbent such as activated carbon, silica gel, zeolites, metal organic frameworks and others, have been ineffective where moisture occurs in flue gas. This work provides an advanced adsorption technology through a novel adsorbent, MWNT-PAA, designed from the noncovalent functionalization of multi-walled carbon nanotubes (MWNTs) by polyaspartamide (PAA) as product of amine grafted to polysuccinimide (PSI). Three types of PAA were prepared using ethylenediamine (EDA), 1, 3 propanediamine (PDA) and monoethanolamine (MEA) drafted to PSI to give PSI-EDA, PSI-PDA and PSI-MEA respectively. The CO2 adsorption capacity was 13.5 mg-CO2/g for PSI-PDA and 9.0 mg-CO2/g for PSI-MEA, which decreased significantly from PSI where the CO2 adsorption capacity was 25 mg-CO2/g. PSIEDA was selected as PAA, because the CO2 adsorption capacity was 52 mg-CO2/g which doubled from PSI. The polymer polyethylenimine (PEI), the most commonly polymer used in CO2 capture, was found to be non-biodegradable, while the polymer PAA showed the presence of CONH as a biodegradable bond functionality, occurring in the MWNT-PAA, as confirmed through Fourier Transform Infrared (FTIR) analysis. The adsorbent MWNT-PAA was demonstrated to have a water tolerance in the temperature range 25-55 ℃, where CO2 adsorption capacity increased with the increase of water in the adsorbent. The highest CO2 adsorption capacity recorded was 71 mg-CO2/g for the moist MWNT-PAA using 100% CO2 and 65 mg-CO2/g for the mixture of 14% CO2 with air. Under the same conditions, the dry MWNT-PAA adsorbed 70 and 46 mg-CO2/g respectively (100%, 14% CO2). The 2 regenerability efficiency of the MWNT-PAA absorbent was demonstrated at 100 ᵒC; after 10 cycles of adsorption-desorption 99% of adsorbed gas was recovered in the desorption process. The heat flow for the thermal swing adsorption system resulted in the net release of heat over the complete cycle; a cycle includes adsorption (heat release) and desorption (heat absorbance). Thus, this MWNT-PAA adsorbent demonstrates an advantage in terms of overall energy efficiency, and could be a competitive adsorbent in CO2 capture technology.

Application of carbon nanomaterials like fullerene, carbon nanotubes, and graphene in solar cells using solution processable methods presents a great potential to reduce the cost of producing electricity from solar energy. However, carbon nanotubes and graphene materials are predominantly metallic and this limits their function in organic photovoltaic devices (OPVs) where semiconducting behavior is required. Doping of carbon nanomaterials is a well-known method for making them semiconducting. Doping of carbon nanomaterials with nitrogen and boron can tune their properties to suit the requirements for use in photovoltaic applications as n-type and p-type semiconducting materials, respectively. Indeed, the use of nitrogen doped and boron doped carbon nanotubes in organic solar cells together with fullerene acceptors can improve the current density of the OPV devices. Nitrogen doping of carbon nanotubes can be achieved by using nitrogen-containing precursor materials during chemical vapor deposition. However the doping of carbon nanotubes with nitrogen does not automatically make them n-type materials; they remain metallic unless a large amount of quaternary type nitrogen is incorporated in the carbon nanotubes. In this work we have developed a method to control the type of nitrogen that is incorporated in CNTs by using an appropriate synthesis temperature and use of oxygen-containing carbon precursors during the chemical deposition of carbon nanotubes. Quaternary N was incorporated in a CVD process when high temperatures and a high concentration of O in the precursor materials were used. We also showed that the type and amount of N can be changed from pyrrolic and pyridinic-N-oxide to pyridinic N and quaternary N by annealing N doped carbon nanotubes at temperatures above 400°C. At temperatures above 800°C most of the nitrogen is converted to quaternary nitrogen. N-CNT thin films were used in OPVs so as to modify the ITO electrode and transform it into a 3D electrode. The resulting effect was an improved short circuit current density in the devices containing an N-CNT thin film that was placed on top of the ITO electrode. A reduction in efficiency losses in OPVs at increasing light intensity was observed in the NCNT ITO modified electrode OPVs. This is a remarkable finding when considering that one of the main problems hindering commercialization of OPVs is the loss of efficiency at high light intensities. We related these effects to the efficient charge collection by the modified ITO electrode. Incorporation of N-CNTs in the bulk heterojunction layer of the OPV device resulted in poor performance when compared to an OPV device made without N-CNTs. This effect is caused by shorting of the OPVs. We used a method of incorporating N-CNTs whilst minimizing shorting and this showed potential for better performance. A study on the attempted doping of graphene with B to make it a p-type material showed that in the presence of a nitrogen carrier gas, BN instead of B was incorporated in graphene. This remarkable finding enabled us to grow a p-type graphene with a possible a band gap opening. This was corroborated by XPS and Raman spectroscopy studies of the material. This BN doped graphene material showed potential as a possible replacement of PEDOT:PSS as a hole transport material in OPVs. The BN doped graphene material can match the performance of PEDOT:PSS when the level of BN doping in graphene is increased.

Single-walled metal oxide nanotubes have emerged as an important class of 'building block' materials for molecular recognition-based applications in catalysis, separations, sensing, and molecular encapsulation due to their vast range of potentially accessible compositions and structures, and their unique properties such as well-defined wall structure and porosity, tunable dimensions, and chemically modifiable interior and exterior surfaces. However, their widespread application will depend on the development of synthesis processes that can yield structurally and compositionally well-controlled nanotubes. Moreover, such processes should be amenable to scale-up and preferably operate via benign chemistries under mild conditions. There is currently very little knowledge on the molecular-level 'design rules' underlying the engineering of such materials. The capability to engineer single-walled tubular materials would lead to a range of structures, with novel properties relevant to diverse applications. In this thesis, main objectives are to discover the first molecular-level mechanistic framework governing the formation and growth of single-walled metal-oxide nanotubes, apply this framework to demonstrate the engineering of nanotubular materials of controlled dimensions, and to progress towards a quantitative multiscale understanding of nanotube formation. The class of aluminosilicate (AlSiOH)/germanate (AlGeOH) nanotubes are of particular interest to us, and serve as the exemplar materials for single-walled metal oxide nanotubes. They can be synthesized in pure form from inexpensive and easily accessible reactants at low temperatures (95 ˚C) from aqueous solutions. The synthesis of nanotubes occurs on a time-scale of hours to days, making them an ideal model system to study the nanotube formation mechanism. In Chapter 2, the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature, in the formation of single-walled aluminosilicate nanotubes is reported. The structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm, are characterized by electrospray ionization (ESI) mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. DFT calculations revealed the intrinsic curvature of nanoscale intermediates with bonding environments similar to the structure of the final nanotube product. It is shown that curved nano-intermediates form in aqueous synthesis solutions immediately after initial hydrolysis of reactants at 25 ˚C, disappear from the solution upon heating to 95 ˚C due to condensation, and finally rearrange to form ordered single-walled aluminosilicate nanotubes. Integration of all results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles. Then, in Chapter 3, new molecular-level concepts for constructing nanoscopic metal oxide objects are demonstrated. The diameters of metal oxide nanotubes are shaped with Ångstrom-level precision by controlling the shape of nanometer-scale precursors. The subtle relationships between precursor shape and structure and final nanotube curvature are measured (at the molecular level). Anionic ligands (both organic and inorganic) are used to exert fine control over precursor shapes, allowing assembly into nanotubes whose diameters relate directly to the curvatures of shaped precursors. Having obtained considerable insight into aluminosilicate nanotube formation, in Chapter 4 the complex aqueous chemistry of nanotube-forming aluminogermanate solutions are examined. The aluminogermanate system is particularly interesting since it forms ultra-short nanotubes of lengths as small as ~20 nm. Insights into the underlying important mechanistic differences between aluminogermanate and aluminosilicate nanotube growth as well as structural differences in the final nanotube dimensions are provided. Furthermore, an experimental example of control over nanotube length is shown, using the understanding of the mechanistic differences, along with further suggestions for possible ways of controlling nanotube lengths. Ultimately, it is desired to produce the single-walled aluminosilicate nanotubes on a larger scale (e.g., kilogram or ton scales) for technological application. However, a quantitative multiscale understanding of nanotube growth via a detailed growth model, is critical to be able to predict and control key properties such as the length distribution and concentration of the nanotubes. Such a model can then be used to design liquid-phase reactors for scale-up of nanotube synthesis. In Chapter 5, a generalized kinetic model is formulated to describe the reactions leading to formation and growth of single-walled metal oxide nanotubes. This model is capable of explaining and predicting the evolution of nanotube populations as a function of kinetic parameters. It also allows considerable insight into meso/microscale nanotube growth processes. For example, it shows that two different mechanisms operate during nanotube growth: (1) growth by precursor addition, and (2) by oriented attachment of nanotubes to each other. In Chapter 6, a study of the structure of the nanotube walls is presented. It has usually been assumed in the literature that the nanotube wall is free of defects. A combination of 1H-29Si and 1H-27Al FSLG-HETCOR, 1H CRAMPS, and 1H-29Si CP/MAS NMR experiments were employed to evaluate the proton environments around Al and Si atoms during nanotube synthesis and in the final structure. The HETCOR experiments allowed to track the evolving Si and Al environments during the formation of the nanotubes from precursor species, and relate them to the Si and Al coordination environments found in the final nanotube structure. The 1H CRAMPS spectra of dehydrated aluminosilicate nanotubes revealed the proton environments in great detail. Integration of all the NMR results allows the structural assignment of all the chemical shifts and the identification of various types of defect structures in the aluminosilicate nanotube wall. In particular, five main types of defect structures are identified arising from specific atomic vacancies in the nanotube structure. It is estimated that ~16% of Si atoms in the nanotube inner wall are involved in a defect structure. The characterization of the detailed structure of the nanotube wall is expected to have significant implications for its chemical properties and applications. Chapter 7 contains concluding remarks, as well as suggestions for future directions in the engineering of single-walled nanotube materials.

Current research in the field of nano-electronics is directed towards device miniaturization in order to find ways to increase the speed of electronic devices. The work presented in this dissertation is on the electronic transport properties of single walled carbon nanotube (SWNT) ropes synthesized by laser ablation. The measurements were performed on devices with different geometries; namely SWNT mats, metal incorporated (aligned individual and bundled) SWNTs and lastly on aligned pure SWNTs from low temperatures up to room temperature. The work was performed so as to gain an understanding on how best to utilize SWNTs in the semiconductor industry towards miniaturization. Such an understanding would ultimately highlight if SWNTs can be considered as a viable alternative to the current silicon-based technology, which seems to be approaching its physical limit. For a mat of SWNTs, 3D-Variable range hopping is the principal conduction mechanism from 2 K – 300 K. The magneto-resistance was found to be predominantly negative with a parabolic nature which converts to a linear nature as the temperature is increased. The negative MR is a consequence of quantum interference and the positive upturn is attributed to wave function shrinkage at low temperatures as described by the Efros-Shklovskii model. The hopping ranges of the electrons for a SWNT mat increases as the temperature decreases due to manifestation of quantum effects and reduced scattering. It was also found that metal incorporation does not alter the properties of the SWNT significantly. SWNT ropes aligned by di-electrophoresis across a 1 micron gap between gold micro-electrodes, exhibit Tomonaga-Luttinger liquid (TLL) like behaviour, within the 80 K – 300 K temperature range. The effects of confinement and electron-electron interaction unique to one dimension were identified in electronic transport as a non-universal power law dependence of the differential conductance on temperature and source-drain voltage. Ballistic conductance at room temperature was confirmed from the high frequency transport of the SWNT devices. The complex impedance showed some oscillatory behaviour in the frequency range 6 to 30 GHz, as has been predicted theoretically in the Tomonaga-Luttinger Liquid model. The observation of Luttinger Liquid behaviour demonstrates the outstanding nature of these one-dimensional molecular systems. In these devices the charging Coulomb energy of a single particle played a critical role in the overall device performance. This study can be used to understand the nature of dynamics of plasmons which are the charge carriers in a TLL system and how Coulomb interactions can be used to design highly tuneable systems for fabrication of single molecule devices. The incorporation of metal onto individual SWNT ropes does not alter its electronic properties significantly but the properties of the bundled metal incorporated SWNT ropes are altered. This study has found that under optimized conditions SWNTs might be a viable option for incorporation in nano electronics devices. Individual SWNT ropes promise better devices compared to SWNT mats and further work should be done on individual SWNTs.

Thesis (Ph. D.)--Uppsala universitet, 2006. / Description based on contents viewed Feb. 5, 2007; title from title screen. Includes bibliographical references (p. 53-57).