No description available.

No description available.

No description available.

This thesis describes the design, construction and application of a novel vacuum system for the preparation of thin films of organic charge-transfer compounds. The method of thermal evaporation was used for four materials: tetrathiafulvalene (TTF) and three of its derivatives, dimethyltetrathiafiilvalene (DiMe-TTF), trimethyltetrathiafiilvalene (TriMe-TTF) and bis(ethylenedithio)tetrathiafiilvalene (BEDT-TTF). The resulting thin layers were characterised using optical and electron microscopy, infrared/visible spectroscopy and dc conductivity measurements down to 77K.Thin films of tetrathiafulvalene, after doping with iodine, exhibited a maximum value of dc, in-plane room temperature conductivity σ of 8.0+2.4 S cm(^-1). Semiconducting behaviour was exhibited over the range 77-300 K with AE = 0.09+0.02 eV. The effect of the deposition rate on fihn morphology is reported. TTF iodide layers were also prepared by co- evaporating the two components. These films exhibited a maximum conductivity of 2.9+0.4 S cm(^-1) at room temperature. Again, semiconducting behaviour was noted over the range 77- 300 K with AE = 0.2+0.02 eV. A comparison of the optical, structural and electrical properties of the two types of films is made. DiMe-TTF and TriMe-TTF thin films were also successfully prepared. Doping with iodine resulted in in-plane, dc room temperature conductivities of 10(^-6) and 10(^-7) S cm(^-1), respectively. These values, together with data from optical spectroscopy, suggested that both salts were in the full charge-transfer state. (BEDT-TTF) iodide thin films were deposited by evaporating the organic compound and subsequent doping. Doped films possessed a dc, in-plane room temperature conductivity of 10(^-3) S cm(^-1).Annealing these layers at 60ºC resulted in an increase in conductivity with a final value of 1.6 S cm(^-1). Semiconducting behaviour over the range 77-300 K was exhibited by the annealed films (ΔE = 0.028 eV).Finally, thin film transistors, incorporating TTF and BEDT-TTF doped layers, were fabricated and their electrical characteristics measured.

This thesis presents an investigation into the factors which control the gain and amplification properties in conjugated materials. Conjugated polymers and perylene dyes are highly fluorescent, are easy to process into thin films, and exhibit strong amplification over a broad gain bandwidth making them ideal for use in lasers and amplifiers. The stimulated emission created when thin films of the red emitting polymer poly(2-methoxy-5-(2'-ethylhexyloxy)- p-phenylenvinylene) (MEH-PPV) were photoexcited with high energy laser pulses was investigated. This was characterised by a dramatic narrowing of the emission spectrum which has been assigned to amplified spontaneous emission (ASE). The emission was found to have a gaussian profile and the gain coefficient was found to be 4 cm-1.The temperature dependence of the absorption, photoluminescence and ASE of films of MEH-PPV was measured. The effect of film morphology on the photophysical properties was investigated by using films cast from two spinning solvents, chlorobenzene (CB) and tetrahydrofuran (THE). Film morphology was found to greatly affect the temperature dependence. A particularly important property is the spectral position of the ASE and the factors which affect it. By controlling the film thickness close to the cut-off thickness for waveguiding in the polymer film it was shown that the peak position of the ASE could be tuned by 31 nm. Modelling of the waveguide modes in the polymer films was used to explain this effect. The cut-off wavelength for each film was measured and good agreement with the theory was found. In order to investigate ways in which energy transfer could be used to control the emission, two perylene dyes were used as a donor-acceptor pair in a host matrix of poly methymethacralate (PMMA). The position of the ASE was found to depend on the acceptor concentration. Measurements of the photoluminescence quantum yield and time-resolved luminescence measurements showed that the energy transfer coefficient was 5x10(^11)mol(^-1)d(^3)

No description available.

This thesis contains primarily a study of CdTe/CdS heterojunction solar cells^ chiefly using photoluminescence spectroscopy. These solar cells show a good potential for commercial power generation in the near Aiture and are of interest to several major companies. A vital but little understood step in the manufacturing process is: annealing the cells in the presence of chlorine prior to back contact application. Studies are performed on a selection of thin film CdTe/CdS cells subjected to CdCl(_2) anneals of different duration. A chemical bevel etch was used to study the spectra at different depths into the sample and laser intensity arid sarhple temperature variations to identify the mechanisms behind the observed photoluminescence peaks. Evidence was found for the CdCl(_2) anneal promoting sulphur diffusion and subsequent grain boundary passivation in the CdTe, leading to increased minority carrier lifetimes and device efficiencies Attempts to obtain electroluminescence from the CdTe/CdS solar cells were madei By using current pulses electroluminescence was obtained in the 780-850nm range with discernible spectral features. Photoluminescence (PL) studies were also performed on a single crystal of CdTe grown to an unprecedented size of approx. 5 cm diameter at Durham university by a multi-tube seeded vapour method of crystal growth. Much higher resolution spectra were: obtained for this than for the solar cells. Several peaks were identified arid the mechanisms responsible were theorised. By taking slices of the crystal boule the PL spectra at different points-throughout the bulk of the crystal were determined. A series of high quality ion-implanted CdTe crystals were also studied! by intensity and temperature dependent PL in order to obtain a better understanding of the effects of known concentrations of known impurities on the PL spectra of CdTe. Specific PL features associated with certain dopants were observed.

<p>Thin liquid films are often studied by reducing the Navier-Stokes equations</p><p>using Reynolds lubrication theory, which leverages a small aspect ratio</p><p>to yield simplified governing equations. In this dissertation a plate</p><p>coating application, in which polydimethylsiloxane coats a silicon substrate,</p><p>is studied using this approach. Thermal Marangoni stress</p><p>drives fluid motion against the resistance of gravity, with the parameter</p><p>regime being chosen such that these stresses lead to a stable advancing front.</p><p>Additional localized thermal Marangoni stress is used to control the thin film;</p><p>in particular, coating thickness is modulated through the intensity of such</p><p>localized forcing. As thermal effects are central to film dynamics, the dissertation</p><p>focuses specifically on the effect that incorporating temperature dependence</p><p>into viscosity, surface tension, and density has on film dynamics and control.</p><p>Incorporating temperature dependence into viscosity, in particular,</p><p>leads to qualitative changes in film dynamics.</p><p>A mathematical model is developed in which the temperature dependence</p><p>of viscosity and surface tension is carefully taken into account.</p><p>This model is then</p><p>studied through numerical computation of solutions, qualitative analysis,</p><p>and asymptotic analysis. A thorough comparison is made between the</p><p>behavior of solutions to the temperature-independent and</p><p>temperature-dependent models. It is shown that using</p><p>localized thermal Marangoni stress as a control mechanism is feasible</p><p>in both models. Among constant steady-state solutions</p><p>there is a unique such solution in the temperature-dependent model,</p><p>but not in the temperature-independent model, a feature that</p><p>better reflects the known dynamics of the physical system.</p><p>The interaction of boundary conditions with finite domain size is shown</p><p>to generate both periodic and finite-time blow-up solutions, with</p><p>qualitative differences in solution behavior between models.</p><p>This interaction also accounts for the fact that locally perturbed solutions,</p><p>which arise when localized thermal Marangoni forcing is too weak</p><p>to effectively control thin film thickness, exist only for a discrete</p><p>set of boundary heights.</p><p>Modulating the intensity of localized thermal Marangoni forcing is</p><p>an effective means of modulating the thickness of a thin film</p><p>for a plate coating application; however, such control must be initiated before</p><p>the film reaches the full thickness it would reach in the absence of</p><p>such localized forcing. This conclusion holds for both the temperature-independent</p><p>and temperature-dependent mathematical models; furthermore, incorporating</p><p>temperature dependence into viscosity causes qualitative changes in solution</p><p>behavior that better align with known features of the underlying physical system.</p> / Dissertation

The objectives of the research undertaken have been to investigate the properties of semiconductor lasers operating at around 1.3 mum. The aim of the investigation is to suggest modifications which give rise to improved operating characteristics especially in the high temperature (approaching 85 &deg;C) range. The investigation can be divided into 2 sections: a theoretical approach and an experimental section. The theoretical study examined the performance of compressively strained InGaAsP/InP multiple quantum-well lasers emitting at 1.3 mum in order to investigate the important factors and trends in the threshold current density and differential gain with strain, well width and well number. Structures with a fixed compressive strain of 1% but variable well width, and also with a fixed well width but variable strain from 0% to 1.4% have been considered. It has been found that there is little benefit to having compressive strains greater than 1 %. For structures with a fixed 1% compressive strain and unstrained barriers, an optimum structure for lowest threshold current density and a high differential gain has been found to consist of six 35 A quantum-wells. In addition, compensated strain (CS) structures with compressive wells and tensile barriers have been examined. It is shown that the conduction band offset can be significantly increased and the valence band offset reduced in such structures, to give band-offset ratios comparable with aluminium based 1.3 mum devices. The gain calculations performed suggest that there is little degradation in the threshold carrier density or differential gain due to these alterations in the band offsets; and hence a better laser performance is expected due to a reduction in thermal leakage currents due to the improved electron confinement. The experimental study concentrates on looking at certain key design parameters to investigate their effect on the laser performance. These design parameters range from the number of quantum-wells to the device length. The experimental study confirms the conclusions drawn in the theoretical investigation that the optimum structure for a 1.3 mum InGaAsP laser for low threshold current, high efficiency and high characteristic temperature operation consists of six 1% compressively strained 50 A quantum-wells in a device of medium length (approx. 450 mum). The inclusion of a high reflection coating on one facet provides further improvement in the device performance, but increases the production cost dramatically. Also investigated in the experimental section is the effect of changing the device material from InGaAsP to InGaA1As. The results discussed do not offer firm evidence of any improvement in the device characteristics in switching from a P-based to an Al-based structure. This is mainly due to the added complication of switching to a RWG structure from a BH structure. Another explanation for the relatively poor performance of InGaAsP 1.3 mum lasers has been examined. That is leakage of the carriers out of the well region. Evidence of a leakage current has been seen primarily in devices with a low number of quantum-wells. A novel measurement technique has been demonstrated, which should prove useful for obtaining a numerical value for the leakage current in semiconductor lasers. The results presented suggest that leakage current is not significant for a 9 well device until operating at temperatures above around 373 K. This is supported by evidence supplied by the spontaneous emission spectra.

Pyroelectric activity is exhibited by materials which possess a spontaneous temperature-dependent electric polarisation. These materials generate a current as their temperature is changed. Many classes of organic materials exhibit pyroelectric activity but only if processed in such a way that a non-centrosymmetric arrangement of dipole results. When deposited as alternate layers by the Langmuir-Blodgett (LB) technique a macroscopically polar assembly is formed. To date, the best performance has been achieved by alternately depositing two materials, one containing acid groups and the other containing amine groups. Calixarenes are one family of materials which are particularly good vehicles for the acid and amine groups. Alternate layer LB films of acid- and amine-substituted calixarenes have high pyroelectric coefficients and form extremely robust films. Fourier transform infrared (FTIR) spectroscopy is a useful tool in examining the properties of thin film samples. Using the FTIR techniques of attenuated total reflection (ATR) and reflection-absorption infrared spectroscopy (RAIRS) it is possible to study the behaviour of the acid and amine groups within the pyroelectric samples. This thesis describes the pyroelectric properties of a series of calix[8]arenes. The dependence of the pyroelectric coefficient on temperature, film thickness and substituent chain length is analysed. The infrared spectra show that the acid and amine groups interact by proton transfer but also that the remaining acid groups form either facing dimers with the amine or sideways dimers between themselves. The spectra do not change with temperature. This demonstrates that the films are thermally stable. Additionally, this invariance shows that the pyroelectric activity in these films does not arise from a change in the proton transfer as has been previously postulated. Theoretical calculations undertaken predict that the source of the dipole change required for the level of pyroelectric activity seen is likely to be a change in distance between the acid and amine groups. Further observations, quantitatively examined by curve fitting techniques, show that the greater the number of proton-transferred pairs, the lower the pyroelectric coefficient. Thus, only the temperature-dependent separation of the acid and amine pairs which have not undergone proton transfer is responsible for the pyroelectric activity in these systems.