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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Chemistry of single-wall carbon nanotubes : studies in purification, non-covalent modifîcation by tertiary phosphines and covalent functionalisation via nucleophilic reduction

Sun, Anil January 2007 (has links)
Carbon nanotubes (CNTs) have attracted considerable research interest owing to their exciting properties and potential for a wide range of applications. However, major challenges must be overcome before these applications can be realised. As-prepared CNT material contains a significant proportion of impurities, such as amorphous carbon, fullerenes and metal catalyst particles. The raw CNT material must be purified before the CNTs can be studied and utilised. Also, CNTs tend to aggregate into bundles or "ropes”, and have poor solubility in common solvents, making their handling and processing extremely difficult. Also, many applications require individually separated CNTs. To improve the solubility of CNTs, and amenability to processing on a large scale, chemical modification of CNT surfaces is necessary. To this end, non-covalent as well as covalent strategies have been developed. However, chemical modification may perturb the electronic structure of CNTs, thereby compromising their interesting properties. The challenge, therefore, is to develop chemical modification routes that improve CNT solubility while not seriously affecting their properties. In this work, we firstly study the problem of purification of as-produced CNT material. We have resolved a major controversy concerning the use of oxidising acids for purifying CNTs, which has profound implications for the spectroscopy and subsequent chemical modification of the CNTs. Secondly, we have developed a route for the non- covalent modification of CNTs by tertiary phosphines. This method has the advantages of significantly improving the solubility of CNTs in organic solvents while being extremely simple, not seriously perturbing the CNT electronic structure, as well as not rendering large areas of the CNT inaccessible^ Thirdly, we describe a method for the covalent derivatisation of CNTs based on reduction, followed by electrophilic substitution. This route is considerably more facile and versatile than other covalent functionalisation methods reported to date, and does not cause significant disruption of the CNT electronic structure. Finally, we demonstrate the covalent attachment of formyl (-CHO) groups to CNT walls, which could potentially open the gateway for a plethora of of coupling and modification reactions.
2

Effective mass modelling of excitons in semiconductor nanocrystals

Tyrrell, Edward J. January 2013 (has links)
This thesis investigates the properties of exciton states in semiconductor nanocrystals using effective mass models. We focus on type-I core-shell nanocrystals in which the staggered band alignments of the core and shell material mean the lowest energy states for electrons and holes lie in different spatial regions, giving rise to spatially indirect excitons. The technological potential of type-I! nanocrystals provides motivation for understanding single exciton states which determine many important optical properties. In the first research chapter we study CdTe/CdSe and CdSe/CdTe nanocrystals using a single-band and (2,6)-band effective mass model. The (2,6)-band model is based on a multiband k·p theory previously developed for spherical quantum dot heterostructures. We calculate exciton energies as a function of the core radius a and shell width as of the heterostructure, and assign six exciton transitions in the experimental absorption spectra of CdTe/CdSe nanocrystals. The second research chapter is concerned with strained ZnTe/ZnSe nanocrystals. The (2,6)-band model is modified to incorporate strain using a continuum elasticity model. Exciton energies from absorption spectra are compared with the predictions of the strained and unstrained nanocrystal models, showing that they only describe the lowest exciton energy of one of the three size series. Improved agreement is found for the change in exciton energy due to a particular shell width, with the strained nanocrystal model giving much better fits to the as-dependence. The as-dependence of nanocrystals with alloyed heterointerfaces is better described by the unstrained nanocrystal model, indicating alloying relaxes the strain in this system. In the final chapter we model spatial correlations between the electron and hole in CdTe/CdSe and CdSe/CdTe nanocrystals using a configuration interaction approach developed in the framework of the (2,6)-band k·p theory. We find that the single-particle basis can, be restricted without changing t~e resulting exciton energies significantly; using this decoupled configuration ihter~ction approach we calculate exciton energy shifts due to correlation as a function of aand as. Dielectric confinement increases correlation for many heterostructure designs by shifting carrier wavefunctions away from the surface, and the interparticle Coulomb interaction leads to large changes in radial probability density for the uniform dielectric constant case. Dielectric confinement affects the correlated hole more than the electron so that excitons in the CdSe/CdTe heterostructure are more affected by the dielectric environment than those in the CdTe/CdSe heterostructure. The overall behaviour of the correlated charge density is due to the net effect of the type-I! spatial confinement, interparticle Coulomb attraction, dielectric confinement and single-particle electronic structure.
3

Failure Modes of Organic Devices

Sessford, Joanna Ruth January 2008 (has links)
Research into Organic Electronics has increased dramatically over the past 20 years. Organic Electronics cannot compete with single crystal silicon for speed and circuit complexity, but it can be regarded as a competitor because of its potential to produce very large area circuits at low process temperatures and low cost. Potential applications for conjugated polymer devices include photovoltaic cells (low cost solar panel), optical amplifiers, memory, displays (in conjunction with e-paper and large area LCD's) and radio frequency (RFID). Many of these devices will use Thin Film Transistors (TFTs). TFT characteristics are therefore discussed with 'an emphasis on the hysteresis observed; the results observed indicate that one ofthe causes ofhysteresis is ion movement. A number of different current-voltage equations have been derived for the drain current channel. It is important to develop these equations to determine the effect of treating the real variation in the concentration of carriers, normal to the interface. In addition, how this large variation in the density of carriers gives rise to an enhancement of carrier mobility at the interface is approached, has been examined. It has been shown that for devices fabricated using PTAA; the most appropriate equation is that which assumes the exponential distribution of states depends on electron concentration. MOS capacitors are important test structures; they are useful because of the light they shed on electrical instability in organic devices, MOS capacitors characteristics have therefore been discussed before the hysteresis they display has been examined. The following mechanisms of hysteresis in MOS capacitors have been examined: carrier hopping, carrier percolation, gate leakage current, ion movement in the semiconductor and instability in the gate dielectric. The main causes of instability appear to be oxygen ions in the semiconductor and in the gate dielectric. The hysteresis observed in Schottky diodes has also been examined; contributing factors appear to include ion movement and diffusion. Values for Meyer Neldels energy for ; . Schottky diodes fabricated using Lisicon (undoped) have been calculated to be 5.0.10-21 ; this value increases when dopant is added; this is expected as doping introduces more states and at high levels of doping leads to Fermi level pinning. As Organic Electronics is growing rapidly into an industry, it is vital to have aroadmap; to allow equipment manufacturers and semiconductor technologists to plan targets for the future, this piece of work provides a roadmap for Organic Electronics. One of the major potential uses of Organic Electronics is believed to be in the area of auto hI/rf tags, for example, the replacement of the barcode, the roadmap therefore concentrates on this area; it has been predicted that by the year 2030 full barcode replacement will be available at an economic cost.
4

The meatgrinder : an efficient current-multiplying inductive energy storage and transfer circuit

Pimperton, M. G. January 1990 (has links)
The meatgrinder is a high-efficiency inductive energy storage and transfer circuit which may be used to supply high-current pulsed power requirements in applications such as electromagnetic propulsion. It overcomes the inherent 25% efficiency limit when transferring energy between uncoupled inductors and simultaneously provides current multiplication. An unloaded six-step demonstration circuit has been used to multiply current from 7A to 76A at an efficiency of 44%, and a single-step demonstration circuit has been used to multiply the current in an uncoupled load induct or from lOA to 30A, the efficiency of energy transfer being 31%. Both circuits use power MOSFETs for switching. These circuits have been used in conjunction with theoretical analysis and computer simulation to study the design and performance of the meatgrinder. Investigations have been carried out in order to confirm the basic theory, to clarify the details of circuit operation, and to provide the information necessary for future feasibility studies.
5

Low temperature (<150°C) hydrogenated amorphous silicongrown by PECVD with source gas heating

Cross, Richard Barrie Michael January 2005 (has links)
Low temperature « 150°C) hydrogenated amorphous silicon grown by PECVD with source gas heating Richard Barrie Michael Cross Hydrogenated amorphous silicon (a-Si:H) is a semiconductor that is widely used in a variety of applications. A particularly important development has been the incorporation of this material into thin film transistor (TFT) arrays for the active matrix addressing of liquid crystal displays. Plasma Enhanced Chemical Vapour Deposition (PECVD) is one of the most successful techniques currently in use for the deposition of device quality a-Si:H. However, there is an increasing desire to improve process compatibility with low cost, plastic substrates. This entails trying to reduce the deposition temperature from approximately 250 - 300°C to below 150 °C, whilst maintaining material quality. This thesis describes the design of a novel, low temperature PECVD system incorporating the facility to pre-heat the deposition source gases. The physical and electronic properties of a-Si:H deposited at <150°C are investigated, and the performance of TFT structures incorporating optimised material as the active layer is described. It is shown that the physical properties of a-Si:H produced at a substrate temperature of 125°C with the source gas line heated to 400 °C are commensurate with films deposited at 250-300 °C. The hydrogen content of the optimised film was found to be 10.5 %, with a Tauc bandgap of 1.66 eV. Pre-heating of the source gases also leads to an increase in the proportion ofhydrogen bonded in the monohydride configuration. It is suggested that the diffusion of the film-forming gaseous species is enhanced by this technique, resulting in a reduction in the degree of disorder within the film and hydrogen elimination. Consequently, the concentration of hydrogen and the Tauc bandgap also decrease, leading to an increase in photoconductivity of one order of magnitude. TFTs exhibit a switching ratio of 105 , which is approximately an order of magnitude smaller than high temperature a-Si:H TFTs, but a comparable OFF current of approximately 10.12A. However, the field effect mobility of these devices is very poor (10.3 cm2V·1s·I). This is thought to be due to a high interface state density at the boundary between the low temperature, gas-heated a-Si:H layer and the high temperature silicon nitride gate insulator.
6

Application of sampling techniques to the phase-controlled thyristor cycloconverter

Hanley, G. A. January 1976 (has links)
Low frequency distortion components of the output voltage of a cycloconverter are largely responsible for the restriction on its practical range: of frequencies, and the object of this thesis is to show that these components can be attenuated by the application of sampling techniques to the control system. After a general description of the operation and control of the cycloconverter, the distortion of the output waveform due to low frequency components is discussed. Under these circumstances, the fundamental repetition frequency of the waveform is less than the wanted output frequency, and two methods of determining it for given input and output frequencies are developed. The characteristics and properties of the low frequency distortion components, and the requirements for attenuating them are analysed. The particular effects on the magnitudes of these components due to operation of the cycloconverter in the inhibited mode, rather than the circulating-current mode, are examined and the requirements for attenuating them are identified. It is shown that the communications engineering processes of pulse width modulation and of natural sampling can be identified in the control of the cycloconverter. Regular sampling is more widely used in communications engineering, and its effect on the low frequency distortion components in the cycloconverter output is compared with natural sampling. A modified control method for the inhibited cycloconverter is then developed to attenuate these components. Digital computer programs were written to test the effect of introducing modifications to the control of the cycloconverter, and the more significant results are given in graphical and tabulated form. An experimental cycloconverter, with an inhibition control circuit designed for this project, was constructed to check the validity of the computer programs. The design details are described, and the experimental results are discussed.
7

Phase locked loop analysts for steerable antenna arrays

Soo, C. B. January 2008 (has links)
The purpose of this work was to produce novel Phase Locked Loops (PLL) architectures for antenna array applications. The operational characteristics of these PLLs architectures are presented and are analysed theoretically. The first type ofPLL ar.c. hitecture presented is called the Phase Conjugating Locked Loops (PCLL). Three different topologies for this type are designed to provide the phase conjugation operation necessary for retrodirective antenna array action. PCLL .Type 1 operates under the circumstance that input radio frequency roRF is different . from the VCO output frequency rovco while Type 2 and 3 operate when both ofthese frequencies are equal to each other. I I The second type of PLL architecture, which is called the Variable Phase Locked Loop (VPLL), is designed to operate in transmit only operation as a spatial phase shifter. The beamforming operation of a VPLL based antenna array is obtained by baseband tuning ofthe VCO voltage. The last type of PLL architecture studied, the Phase Buffering Locked Loop (PBLL), is suitable for self-tracking receive only operation. The embodiment of this PLL is architecturally similar to that of PCLL Type 1 with the exception that a down' converter mixer is included after the VCO and an inverter is inserted before the VCO. For each of the PLL architectures comprehensive derivations of the output phase noise jitter noise equations including the effects of the non-linearity and non-idealistic mixer characteristics is presented. These RMS phase noise jitter equations are utilised in order to evaluate the steady state error of each PLL topology. Following the investigation of noise performance, the stability of PLL architectures is examined. Bode Plot and Polar Plot have been deployed to facilitate the investigation. The final investigation concentrates on the impact of multiple signals on PBLL self-tracking receiver antenna array.
8

Type-II gallium antimonide quantum dots in gallium arsenide single junction solar cells

Mahajumi, Abu Syed January 2013 (has links)
The novel idea of GaSb quantum dots (QDs)1 quantum rings (QRs) stacked layers single junction solar cells have been investigated for the examination and enhancement of the infrared photo response. Initially the investigation used photoluminescence to probe the optical properties of a type-II material interface between GaSb/GaAs using optimum growth temperature for QDs/QRs with two different growth modes (Stranski-Krastanow (SK) and exchange growth); and two different GaSb deposition thickness (1.5ML and 2.IML). The photoluminescence spectra of the stacked epilayers confirmed that the dominant radiative recombination mechanism was band-to-band in the GaSb QDs/QRs stacked layers. Excellent structural quality is observed in each sample with no threading dislocations (by Transmission Electron Microscopy (TEM)). The composition of the QRs is close to 100 % GaSb with high purity GaAs centres. The ring density per layer is approximately 1010 rings/cm2 with no significant variation in size or density in the separate layers. II Rapid thermal annealing (RTA) has been used to tailor the optical properties of 10-layer stacks of type-II GaSb self-assembled QDs and QRs embedded within GaAs grown by molecular beam epitaxy. An increase in PL emission intensity and a blue shift in peak energy in both types of QD stacks were observed, along with changes in the activation energy for PL quenching. These effects were attributed to Sb-As intermixing and size effects with corresponding changes in the band structure and an increase in the oscillator strength associated with the transformation towards type-I behaviour. It has been concluded that postgrowth rapid thermal annealing can be used to tune the spectral response and control carrier recombination and escape properties of stacked GaSb QD for more effective use in devices such as solar cells and lasers. The final part of the investigation examined the properties of multi-layer QDs/QRs single junction solar cells (SC) to obtain an understanding of the operation and characteristics of the devices. Three kinds of solar cells were fabricated; one is intrinsic layer with ? layers of QDs/QRs, another comprises 10 layers and the final one is control cells (without QDs/QRs).
9

Synthesis of High Quality Low-toxic Semiconductor Nanocrystals

Xu, Shu January 2008 (has links)
New approaches for the synthesis of highly luminescent InP and InP/ZnS nanocrystals were developed by stepwise systematic investigation of the parameters during the reaction stages. The parameters, including solvents, precursors, ligands, capping agents, protic agents, Lewis acids and bases and temperature were discussed in detail in different chapters. The investigation processes helped increase understanding of understand the reaction and surface passivation mechanisms and to develop convenient synthesis approaches. Highly luminescent InP NCs were prepared with an in-situ indium chloride complex in the presence of zinc carboxylates or zinc dithiocarbamates - convenient nucleation initiators and stabilisers. The nanocrystals prepared covered a wide photoluminescence emission range from blue to the near infra-red. This synthesis method also allowed the in-situ growth of highly luminescent InP/ZnS core-shell nanocrystals as well. The principles of selection of reagents are also applicable in the synthesis of other nanocrystals. The as-prepared high-quality InP and InP/ZnS nanocrystals have been exploited in applications in the fields of light emitting diodes (LEOs) and bioimaging.
10

Novel methods of preparation and characterisation of zeolite thin films

Wee, Lik Hong January 2008 (has links)
Over the past years, the synthesis of zeolite films has attracted considerable attention due to their potential applications as membranes, sensors, and insulators with low dielectric constant. Therefore, the preparation of high quality defect free zeolite films with controlled film orientation and thickness is highly desired. Silicalite-l nanocrystals were functionalised via ultrasonic treatment in alcohol (methanol, ethanol, 2-propanol and I-butanol) suspensions. The formation of methoxy groups on the external surface of silicalite-l was confirmed by nitrogen adsorption porosimetry analysis. Pressure-area Langmuir isotherms and BAM analysis confirmed the enhanced hydrophobicity of alcohol modified silicalite-l nanocrystals and film formation at the air-water interface, respectively. Silicalite-l films were successfully fabricated onto silicon substrates for the first time using the Langmuir-Blodgett (LB) method. SEM and AFM characterisations confmned that the monolayer nature of the LB films extended over an area of several mm2 • Secondary growth of the monolayers LB seeds produced smooth and continuous polycrystalline zeolite film with preferred b-orientation evidenced by XRD analysis. In addition, we have also demonstrated the synthesis of continuous and homogeneous silicalite-l films by steaming of spin-coated precursor coatings according to the SEM images. This simplified method benefits from the limited amount of structure directing template used, facile recovery of the supported films and negligible amounts of waste products. We have also shown the first successful use of synchrotron IR spectroscopy in grazing incidence mode for the characterisation of zeolite precursor LB and spin-coated films on gold-coated glass substrates. The spectra confirmed the existence of incipient zeolite nanoparticles with MFI type structure in clear silicalite-l synthesis solution. The supported LB zeolite precursor particles were used as seed layers to prepare highly crystalline and uniformly oriented silicalite-l films by secondary growth. HR-TEM images confirmed that the steamed LB precursor films were ultra-thin, < 10 nm thick but structural characterisation is required. The results confirmed the nm dimensions of the silicalite-l precursor species.

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