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Novel fluorene-1 ,3,4-oxadiazole and spirobifluorene-1 ,3,4-oxadiazole compounds 202 and 211, respectively, have been synthesised. Compound 202 was blended with MEHPPV in various ratios and incorporated into organic light-emitting diodes (OLEDs).The devices were found to emit light purely from MEH-PPV up to very high doping levels. When the device architecture ITO/PEDOT:PSS/MEH-PPV :202/Ca:Al was used it was found that increasing the amount of202 increases the lifetime of the device. Novel bipolar fluorene- I ,3 ,4-oxadiazole-triphenylamine molecules 237 and 240 were synthesised using silicon protecting groups. When incorporated into devices with architecture ITO/PEDOT:PSS/237 or 240/Ca:Al it was found that the materials facilitated the transport of electrons and holes as well as acting as blue-green emitters with efficiencies of up to 0.26%/0.6 cd A-1. Compound 237 also performed well when blended with MEH-PPV giving rise to efficiencies two orders of magnitude greater than for pure MEH -PPV devices.

Group III-nitride semiconductors are the dominant inorganic solid state light emitting materials, spanning the UV to infra-red spectral range. InGaN/GaN based LEDs and lasers are commercially available and intense research is being pursued to improve their efficiency. One practical approach is the development of functionalised and/or improved materials patterned on a nanometre length scale. This thesis presents the optical, morphological and compositional characterisation of III-nitride based nanostructured light emitters. The III-nitride nanostructures studied are GaN coalesced above arrays of either nanopyramids or nanocolumns, semipolar and nonpolar InGaN QWs on the facets of GaN nanopyramids, and thin epilayers of AlInN and AlInGaN. Spatially resolved optical characterisation of nano-ELOG GaN layers revealed a shift in the exciton-related band edge emission across the coalesced layer. This is related to Si doping and to strain effects. Study of the semipolar {1011} InGaN QWs grown on the facets of GaN nanopyramids identified a blue shift in QW emission energy as the sampled region is moved up the pyramid facets. This shift is found to follow the release of the tensile strain towards the top of nanopyramid. Luminescence properties of nearly lattice matched AlInN epilayers investigated using CL, PL and PLE spectroscopic techniques revealed that the emission and bandgap energy of the AlInN layers are at higher energy than that of GaN. Results obtained from polarisation resolved PL measurements of AlInN epilayers point to two possible implications: the observed higher energy AlInN emission is either related to defects or this emission is due to carrier recombination occurring in InN clusters similar to those of InGaN epilayers. Optical properties of thin AlInGaN epilayers investigated using PL and PLE spectroscopy revealed a redshift in bandgap energy with increase in InN fraction. The observed spatial intensity fluctuations are discussed in terms of the InN compositional fluctuations and inhomogeneous strain effects.

This thesis focuses on the use of high permittivity tunable dielectrics and more specifically sol-gel ferroelectric thin films for low cost, high performance tunable devices such as varactors and filters at RF and microwave frequencies. The top- ics covered include measurement techniques for the characterization of tunable dielectrics at low and microwave frequencies, fabrication processes, electrical and acoustic modeling of thin film ferroelectric varactors, performance optimization using conductive electrodes, realization of tunable microwave circuits and inte- gration of tunable dielectrics with conventional bulk acoustic wave resonators (FBAR). A lead strontium titanate (PST) sol-gel ferroelectric varactor is designed, elec- trically and acoustically modeled and fabricated, displaying dielectric tunability of "'-'75%. A two port automatic extraction technique using MATLAB allowing the de-embedding of parasitic connecting transmission lines, as well as parasitic pads has been developed and presented, yielding accurate dielectric permittivity values in good agreement with literature. The potential factors that may compro- mise the electrical performance of the ferroelectric tunable varactor are analyzed and a novel Au/Ti02 bottom electrode stack process is proposed and shown to improve the RF performance of the tunable varactor lowering the overall metaliza- tion resistance and improving performance, compared to the commonly used Pt electrodes. To establish the possibility of tunable microwave systems integrating sol-gel ferroelectric tunable varactors the following novel microwave devices are designed, modeled and fabricated: A ferroelectric varactor-based RF resonant switch, integrating a thin film sol- gel PST ferroelectric varactor with a high Q micro-machined inductor is fabri- cated. An insertion loss of ",1.5 dB and isolation of ",18 dB have been achieved for a single 7 GHz resonant switch with a device area of 0.6 mm x 1 mm. The intrinsic performance limitations of this type of device due to the ferroelectric thin film are discussed and the implementation of cascaded switches and state-of-the- art ferroelectric materials for further improvement of performance of this device, have been considered and simulated. Tunable band-stop resonators and notch filters using sol-gel PST ferroelectric varactors in a coplanar waveguide (CPW) defected ground structure are fabricated and measured. The PST varactors tune single resonators and 3-pole band-stop filters, operating at the center frequency of 4 and 8 GHz, having a maximum rejection of more than 13.8 dB at the stop band, while the insertion loss at the pass band is less than 3 dB. Full-wave analysis is performed to identify the critical points, where PST varactors are implemented to adjust the resonance frequency of the devices. An optimized fabrication process allows for fabrication of a 3-stage filter with a maximum rejection of 28 dB, albeit with a reduced tuning range, possibly due to DC bias path leakage. Finally, a fabrication approach where a ferroelectric varactor is integrated with a conventional zinc oxide (ZnO) acoustic wave resonator is presented. The approach avoids the piezoelectric thin film degradation due to the ferroelectric annealing by first fabricating the ferroelectric varactor and superimposing the conventional FBAR on top of it. The tuning of the series resonant frequency of a conventional ZnO FBAR with a ferroelectric varactor is demonstrated. Field induced deformation limits the maximum shift of the resonance to 0.45% at 1.5 GHz, for 41% tunability of the ferroelectric varactor, suggesting a big scope for possible improvements in performance by improving the design and fabrication. VIII.

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Light-emitting dendrimers with iridium(III) complex cores have given rise to some of the simplest and most efficient organic light-emitting diodes. However, whilst monochrome devices can be prepared there is currently no method for the patterning of the dendrimer films to give rise to pixelated colour displays. The main aim of this project was to develop methodology for the patterning of dendrimer films. In particular, dendrimers are designed that have an oxetane surface group that can be crosslinked to form patterns by a photo-generated acid.