This thesis reports the optical and structural properties of GaAs(1-x)Bi(x) alloys grown on GaAs by Molecular Beam Epitaxy (MBE). The photoluminescence (PL) of a GaAs(0.97)Bi(0.03) alloy was measured over a wide range of temperatures and excitation powers. The temperature dependence of the PL peak energy indicated significant exciton localization at low temperatures and the band gap varies more weakly with temperature than in GaAs. An analysis of dominant carrier recombination mechanism(s) was also carried out indicating that radiative recombination is dominant at low temperature. The PL results indicate that dilute fractions of bismuth (Bi) with x < 0.025 improve the material quality of these low temperature growth alloys by reducing the density of gallium (Ga) and/or arsenic related defects. The crystal quality starts to degrade at higher Bi concentration probably due to a significant amount of Bi-related defects, i.e BiGa. However, the room temperature PL intensity continues to increase with Bi content for x up to 0.06 due to the greater band-gap offset between GaAs and GaAs(1-x)Bi(x). To improve the quality of GaAs(1-x)Bi(x) alloys, annealing and growth studies were carried out. At room temperature, the annealed GaAs(1-x)Bi(x) showed a modest improvement (~ 3 times) in PL while the PL peak wavelength remained relatively unchanged. Also, the optimum annealing temperature is Bi composition dependent; for samples with x < 0.048, the optimum annealing temperature is 700 oC but it reduces to 600 oC for higher compositions. Two growth parameters were investigated which are growth rate and As4/Bi beam equivalent pressure (BEP) ratio. It was found that growth rate significantly affects Bi incorporation and the accumulation of surface Bi. Decreasing the As4/Bi BEP ratio has been shown to increase Bi concentration but is limited by the formation of Bi double PL peaks.

This thesis discusses development of new semiconductor quantum dot (QD) devices and materials. Optical spectroscopy of single QDs is employed in order to investigate electronic structure and magnetic properties of these materials. First we realise self-assembled InP/GaInP QDs embedded in Schottky diode structures, with the aim to realise charge control in these nanostructures, which recently provided an important test-bed for spin phenomena on the nano-scale. By varying the bias applied to the diode, we achieve accurate control of charge states in individual QDs, and also characterise the electron-hole alignment and the lateral extent of the exciton wavefunction. Second part of the thesis explores optimum regimes for optically induced dynamic nuclear polarization (DNP) in neutral InGaAs/GaAs QDs. Very efficient DNP under ultra low optical excitation is demonstrated, and its mechanism is explained as the electron-nuclear flip-flop occurring in the second order process of the dark exciton recombination. The final part of the thesis reports on magneto-optical studies of novel individual InPAs/GaInP quantum dots studied in this work for the first time. Here the long-term aim is to realise strong carrier confinement potentially suitable for QD operation at elevated temperatures, e.g. as a single photon emitter. Here we lay foundations for future structural studies of these dots using optically detected nuclear magnetic resonance, and explore regimes for ecient DNP in InPAs dots emitting in a wide range of wavelength 690-920 nm.

Thin films of polymer mixtures made by spin-coating can phase separate in two ways - by forming lateral domains, or by separating into two distinct layers. The latter situation - self-stratification or vertical phase separation - could be advantageous in a number of practical applications, such as polymer photovoltaics. In our experiments, we have used time-resolved small-angle light scattering and light reflectivity during spin coating to study the structure development in PS/PMMA and PFB/F8BT blends, solution cast in toluene. A sample cell was designed, made and mounted on the apparatus to manipulate the evaporation rate. Having solved the Meyerhofer equation for thinning rate and by fitting the model to the experimental data, we are able to extract the evaporation rate of toluene during spin coating. We demonstrate that, by controlling the evaporation rate during the spin-coating process, we can obtain either selfstratification or lateral phase separation in the same system. We relate this to a previously hypothesised mechanism for phase separation during spin coating in thin films, according to which a transient wetting layer breaks up due to a Marangoni-type instability driven by a concentration gradient of solvent within the drying film. Our results show that a high evaporation rate leads to a laterally phase separated structure, while reducing the evaporation rate suppresses the interfacial instability and leads to a self-stratified final film. Using the set up we developed to control the morphology through evaporation rate, we made preliminary photovoltaic devices. It is possible to control the efficiency of the polymer photovoltaics by means of process parameters such as evaporation rate.

No description available.

Organic semiconductors have attracted worldwide interest for the past two decades. The properties of these molecules can be easily manipulated and exploited, and furthermore benefit from chemical versatility, mechanical flexibility and low cost. This has led to a remarkable success in the field of plastic electronics and molecules have found numerous device applications such as photovoltaic cells (PV), organic light emitting diode (OLED), organic field effect transistor (OFET) and sensors. Organic semiconductors have recently become of considerable interest for spintronic applications, due to the long spin relaxation times and magneto-resistive effects observed in these systems. In order to fully exploit the advantages of these molecules for spintronic applications, it is essential to explore molecular routes towards all organic spin valves and search for molecule-based magnets as alternatives to conventional spin injector/detector such as La0.67Sr0.33 MnO3 (LMSO) and Co. The scope of this thesis is to investigate the way to control the functional properties and in particular the magnetic interactions and coercivities in molecular thin films, with an emphasis on the charge-transfer salt, [MnTPP][TCNQ], and a ferromagnetic system, FePc (including mixed H2Pc:FePc), respectively, fabricated by organic molecular beam deposition (OMBD). Although the magnetic couplings are currently limited to cryogenic temperature, it is shown that it is possible to engineer exotic physical properties in these mixed films, where the magnetism seen as an intrinsic property to the functional molecules shows a strong dependence on the local chemical structure and spatial displacement for the magnetic ions, which can be manipulated by addition of electron acceptor and non-magnetic substituent. Compared to conventional magnetic semiconductors, this approach is a molecular route towards tuneable magnetic properties, allowing one to directly control the magnetic interactions by varying the film composition via co-deposition, a desirable property that is obtained in the film form and readily exploited in all organic spintronic applications.

Metal oxides are a group of materials that have shown great promise in improving the efficiency of devices based on organic materials through inclusion in the contact structures of such devices. In this work, the deposition technique of spray pyrolysis is developed for use for the deposition of doped zinc oxide films and molybdenum oxide films. Spray pyrolysis is of great interest as a technique for use in the organic electronics field due to its cost, scalability and compatibility with other solution processing techniques. This makes the technique particularly interesting for use in devices intended for large scale applications such as lighting The study of doped zinc oxide focuses on its application as an alternative transparent conducting layer to the standard indium tin oxide layer. The zinc oxide layer was doped with aluminium to increase its conductivity, and the effect of lithium doping was investigated with the intent of improving the conductivity of the layers further. Annealing of the layers in a nitrogen environment was found to produce layers of a similar conductivity to that of indium tin oxide and the lithium doping was found to result in higher conductivities in annealed layers. The study of molybdenum oxide focuses on its application as a hole injection layer included in the anode contact of organic light emitting diodes. The deposition temperature was found to have a large effect on the resulting device efficiencies. This effect was determined to be due to variation of the work function and ion ratios present in the molybdenum oxide layer with deposition temperature. This work resulted in the fabrication of devices with efficiencies double that of the standard solution processed hole injection layer.

The nature of the transitions that occur between the quantized plateaux observed in the quantum Hall effect (QHE) have been classified as second order quantum phase transitions. These transitions occur between the localized and the extended states found within a Landau level band of energies. The theory of the critical phenomena associated with these quantum Hall transitions (QHTs) predicts a universal behaviour irrespective of any microscopic detail of the two-dimensional system (2DES) within which they are observed such as carrier concentration or mobility. This proposed universality of QHTs can be verified by measuring the value of certain critical exponents governing the transitions. If valid, these critical exponents should be measured as a universal constant in all instances. This thesis investigates the universality of QHTs using a finite-size scaling theory and attempts to address disagreements that exist in the literature on the critical exponents associated with QHTs. The scaling theory of QHTs presented here involves experimental studies based on varying either the temperature of the 2DES or the frequency of the applied electric field. It was found that the critical exponents of QHTs are not universal across all systems investigated. It is shown that changing the nature of disorder within the system influences the value of the critical exponent measured. In general, it was found that the experimental observation of quantum criticality, as expected from the critical phenomena theory of QHTs, depends on the competition between three key length scales characterizing the 2DES; the size of system, the phase coherent length and the typical size of the electron clusters forming within the system. A study on the limit of the observation of the QHE is also undertaken in the millimetre wave regime. It was found that localization within the 2DES, and as a result the QHE, is destroyed at frequencies below the millimetre wave regime for a GaAs based 2DES.

The introduction of a new technique for metrology of thin liquid films to give both the profile of the exterior surface and information on the thickness of the film is the main focus of this research. The proposed approach is based on the use of fringe projection system with a narrow band laser illumination and a high concentration of fluorescent dye dissolved in the fluid in order to generate fluorescence emission from minimum thickness of the film (i.e. the top few microns). The method relies on calculation of an interference phase term and the modulation depth of the fringes created by means of a twin fibre configuration. The characterisation of candidate fluorescent dyes in terms of absorption, related to the depth of penetration of the incident light into the dye and their fluorescence emission efficiency is presented and their application in full field imaging experiments is evaluated. A strong focus of the technique proposed is its flexibility and versatility allowing its extension to phase stepping techniques applied to determine the (fringe) phase map from static and dynamic fluids. Some experiments are carried out using the best dye solution in terms of fluorescence emission and light depth penetration. On the basis of the phase-height relationship achieved during the calibration process, the proposed measurement system is applied for the shape measurement of some static fluids. The profile of the exterior surface of these fluids is investigated by means of phasestepping technique and the resolution of the measurements is estimated. Furthermore a flow rig set-up based on inclined system (gravity assisted) is presented in order to test the shape measurement system in presence of real liquid flows. Different liquid flow thicknesses are processed and analysed. Example data will be included from some fluid films of known geometry in order to validate the method.

In this thesis, theoretical electronic structure methods have been used to study systems of interest to materials science and engineering. The many-body problem of quantum mechanics has been reviewed, and it has been explained that the density functional theory (DFT) of Hohenberg, Kohn and Sham is a very practical approach to solving it. The core details of DFT have been explicitly laid out. Having then demonstrated an awareness of the many different powers and varied capabilities of DFT in predicting material properties, systems laying at the current frontiers in nanoelectronics (chapters 3 and 5) and theoretical surface science (chapter 4) have then been focused on. In chapter 3, the behaviour of metal adatoms on graphene substrates has been predicted using DFT. From adatom binding energy and migration energy calculations, it has been theoretically suggested that single Cr, Au and Al adatoms diffuse randomly on graphene at room temperature until they collide with edge sites or defects, where they form stable bonds. This prediction has been used to explain experimental electron microscopy data which shows that metal adatoms evaporated onto graphene by chemical vapour deposition (CVD) have only ever been observed at edge sites and defects, and never on the pristine regions. In chapter 4, a new methodology has been developed for predicting the energies of step defects on crystalline solid surfaces, and it has been applied to steps on the (110) surface of TiO2 rutile. The limitations of currently published methods of calculating step energies have been explained in detail, and it has been demonstrated that this new method is much more reliable. The method has been used to predict the shape of a terrace island on the (110) surface of TiO2 rutile, and the prediction has been found to compare well with published experimental electron microscopy data. In chapter 5, current progress on an ongoing project has been summarised which investigates whether there is an energetic advantage to multiple substitutional nitrogen dopants in graphene occupying the same sublattice. The results are inconclusive so far, although it has been shown so far that magnetic effects are unlikely to be playing a role. In chapter 6, the accomplishments of this thesis have been summarised and future directions suggested.

No description available.