As optoelectronic components become nano-dimensional, controlling the coupling between light and matter at the nanoscale has become a major technological challenge, as well as the subject of theoretical studies. The aim of this work is threefold. First, to assess the suitability of ferroelectric thin films - Barium Strontium Titanate (BST), and Strontium Barium Niobate (SBN), as active media for plasmonic devices. Second, to find suitable thin film electrodes for such devices, by exploring and optimizing the plasmonic behaviour of already known conductive materials, conductive oxides (Strontium Ruthenate - SRO), and transitional metal nitrides (Titanium nitride - TiN). Third, to optimize the deposition process of metallic (Silver – Ag) films, so as to improve their smoothness, and thus their suitability for plasmonic applications and lithography in general. SBN ceramic targets were sintered. SBN and BST films were deposited by PLD and ellipsometry and normal incidence reflectometry were used to examine their optical tunability. Ellipsometry was further used to measure the effects of the residual strain of the BST thin films on their optical properties. BST and SBN films were found to exhibit a birefringence under bias along the direction of growth. A residual strain variation along the films’ direction of growth was inferred from an observed non-linear change in the refractive index of BST films along that same direction. SRO and TiN films were fabricated using PLD and reactive magnetron sputtering, respectively. The effects of the deposition pressure upon structure, charge carrier concentration and mobility, and optical properties were studied using X-ray diffraction (XRD), Hall-effect measurements, and ellipsometry. The optical properties of SRO were explained based on electron concentration and structure. SRO was confirmed as a promising plasmonic material, for applications in the near infrared range and at elevated temperatures. The influence of the deposition temperature upon the optical properties of TiN films was shown. Films grown at high temperature (800oC) had quasi-metallic optical properties, while films grown at room temperature exhibited well defined plasmon bandwidth, between two distinct Epsilon-near-zero (ENZ) frequencies, which has been linked to the uniform oxidation of the samples). Finally, Ag thin films were deposited using magnetron sputtering, in an Ar/He atmosphere. The effect of the sputtering gas ratio on the films structure, morphology and reflectivity was studied using XRD, Atomic force microscopy (AFM) and visual-range normal incidence reflectometry. The addition of Helium to the sputtering atmosphere was found to reduce the roughness of Ag films and improve their reflectivity, due to the Penning effect present in the Ar-He plasma. The work undertaken has, by developing new plasmonic materials (SRO, oxidized TiN), and expanding the knowledge of the behaviour (BST) and fabrication (Ag) here has paved the way for the development of active plasmonic devices.

There has been a significant interest in polymer light-emitting diodes (PLEDs) due to their potential applications in efficient displays and large area light sources at low cost. In comparison to their red and green counterparts, the relatively poor operational stability of blue PLEDs is the biggest challenge faced in the commercialisation of PLED devices. This thesis is concerned with the stability issues of blue light-emitting fluorene-based conjugated polymers and small molecules towards highly efficient and longer lived blue PLEDs by elucidating underlying degradation mechanisms. Poly(9,9-di-octylfluorene) (F8) is known to have different morphological phases including amorphous, liquid crystalline, crystalline and a phase with an additional degree of ordering termed the β-phase. The effects of different side groups in polyfluorenes are studied to understand their influence on structural and optoelectronic properties. The employment of bulky side-groups restricts the formation of the liquid crystalline and beta phases observed in F8. The photoluminescence quantum yield is larger (from ~35 % to 50 %) for polyfluorene chains with bulkier side-groups and their excitons are found to be longer lived (from ~0.35 ns to 0.45 ns), due to reduced non-radiative decay rate resulted from weaker intermolecular interaction. UV excitation is used as a tool to investigate the photo-stability of blue light-emitting polymers. The photo-stability of different phases of F8 and F8 derivatives are studied under ambient and inert conditions. UV exposure in air causes disorder in the F8 polymer chains due to the formation of defect sites such as fluorenone. Interestingly, the stability of different phases vary and the results show crystalline and beta phases to be most stable, with least degradation in their optoelectronic properties due to their structural ordering. The formation of these ordered phases increases the density of the film thereby decreasing the degradation caused by penetration of oxygen molecules. On the other hand, after UV exposure in nitrogen, there are indications that high energy conformational states of polymer are more reactive to photoexcitation and side-chains are still susceptible to degradation as cross-linking occurs in all films. Polyfluorene with different side-groups attached exhibit a greater stability to UV photoexcitation, as evidence from optical spectroscopy, although these molecules initially have a more disordered structure. This leads to the conclusion that photo-stability is dependent on the chemical structure of the side-groups and this is proven by degradation of the polymers in dilute solutions. The optoelectronic properties and photo and electrical stability of a series of poly-(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (F8-PFB random copolymer) consisting of different fractions of F8 and PFB units are examined. A Raman spectroscopy study reveals that the PFB units are the reactive site for UV degradation in these copolymers, hence, decreasing the fraction of PFB units improves the copolymer photo-stability. The formation of di-radicals on the PFB unit is considered to be the cause of luminescence quenching. By combining knowledge obtained from homopolymer and copolymer degradation; a highly efficient ( > 3 cd/A), deeper blue CIE colour coordinate (0.14, 0.12) and longer-lived device is demonstrated using a model 5 mol % PFB containing random copolymer. Finally, small molecule fluorene stability is studied to determine degradation mechanisms that could be applied to polyfluorene systems. The F8 trimer displays ketone defects when degraded in air. The crystallised F8 and fluorene trimers with bulkier side-groups demonstrate stability greater than that of amorphous phase of F8 trimer. Evidence of chain scissioning after photoexcitation is observed and attributed to the cause of reduced photoluminescence found after degradation in blue light-emitting fluorenes.

This thesis presents the development of organic field-effect transistors with printed dielectric and semiconductors. The device architecture was bottom-gate bottom-contact. The electrodes were fabricated via standard photolithography. The first milestones were to gravure-coat the dielectric and structure it with photolithography. Dielectric formulations were screened for their ability to photopattern with radical photoinitiators. Variable processing conditions were also investigated such as annealing time and temperature. The preferred formulation and processing conditions gave ca. 130 nm thick dielectric films and 1-2 nm root-mean-square surface roughness. The dielectric films were tested in parallel capacitors and field-effect transistors. For comparison, the dielectric was also spin-cast and processed analogously to the gravure-coated films. Under the same conditions gravure-coating gave films with equal uniformity and insulating properties as spin-casting. Transistors were prepared with thermally-evaporated pentancene and TIPS-pentacene (6,13-bis(triisopropylsilylethynyl) pentacene) via spin- or zone-casting. The measured mobilities were amongst the highest reported in the literature for this material set and device architecture. The highest transistor mobility with TIPS-pentacene was achieved by blending the semiconductor with poly(a-methylstyrene) (PaMS). Device characteristics such as mobility, threshold voltage and sub-threshold swing voltage were calculated and their evolution with blending ratio followed. The semiconductor was either spin-cast onto pre-deposited PaMS layers with different thickness or TIPS-pentacene was blended with PaMS already in the ink. The work concluded with structured gravure-printing of the dielectric and semiconductor. Dielectric inks with different viscosity were printed and print quality investigated. The film thickness ranged between 60 and 500 nm. Exemplary films were used in the fabrication of transistors and complementary inverters. TIPS-pentacene was directly blended with polymer binders and printed.

This thesis describes the development of a process flow to allow the fabrication of organic field effect transistors with self-aligned source and drain electrodes, and sub-micron channel lengths on flexible plastic substrates. This was achieved using a combination of bilayer nanoimprint lithography, gravure printed or photolithographically patterned dielectric, and bilayer self-aligned lithography. Effective channel lengths over an order of magnitude could be defined from 375-3800 nm. Electrode overlaps between gate-drain and gate-source of 100-200 nm are demonstrated, yielding very low channel-length normalised overlap capacitances of 0.1-0.6 fF per μm. The viability and behaviour of these architectures was investigated using zone-cast, gravure and inkjet printed semiconductors. State-of-the-art transition frequencies in the range 1-6 MHz were achieved at operating biases < 30 V. The molecular packing of a zone-cast small molecule p-type semiconductor (TIPS-pentacene) was observed to be influenced by the underlying architecture. Simple techniques were developed to quantify gravure printed film quality. Periodic modulation of printed films was observed to be a function of gravure cliché cell geometry and ink formulation. Both gravure and inkjet printed p- and n-type semiconductors (DPPT-TT and P(NDI2OD-T2)) were studied. Thin gravure printed dielectrics were observed to systematically increase effective mobilities, at the expense of an increase in leakage current. Complementary self-aligned inverters are presented, with a peak gain of 28 achieved for devices operating below 20 V, along with NAND and NOR logic gates operating up to 20 kHz. The viability of gravure printing an indacenodithiophene-benzothiadiazole (C16IDT-BT) on plastic is demonstrated yielding effective mobilities in the range 0.04-0.4 cm squared per V per s at V(DS) < 20 V, exceeding previous reports for the behaviour of this material in a bottom-gate, bottom-contact configuration. Keywords: plastic electronics; organic electronics; flexible; organic field effect transistor (OFET); complementary circuit; inverter; logic gate; printing; gravure; inkjet; zone-casting; photolithography; nano-imprint lithography (NIL); self-aligned; cliché; shim; image analysis; transition frequency; megahertz.

The aims of this work is to design low noise electronics for optical sensing and X‐ray spectroscopy using Sheffield‐grown Avalanche photodiodes(APD). A transimpedance amplifier(TIA) for a 2.0 μm LIDAR system is designed and tested as part of a project funded by ESA. Numerical analysis is provided for the TIA in addition to SPICE and experimental analysis. Characterisation of the TIA shows that a noise equivalent power of less than 100 fW/√Hz can be achieved with an optimised InAs APD. Preliminary results of a TIA‐InAs module at 2.0 μm is presented. A low noise charge sensitive preamplifier(CSP) with a novel local feedback is designed and characterised. The CSP shows a better noise performance than commercially available CSP such as the CoolFet 250. The CSP is also characterised for APD dark current of up 4 μA and the CSP is found to behave well for such relatively high dark current. Discrepancies between the SPICE model and measured characteristic of the CSP’s input JFET is presented and discussed. The first ever Aluminium Indium Phosphide (AlInP) APD X‐ray spectroscopy measurement is presented in this work. AlInP is the widest band material that can be grown latticematched on a GaAs substrate. Due to its wide bandgap, AlInP can offer reverse dark current of less than 2 pA at gain of 100 for a 200um device, making it desirable for room temperature operation. An energy resolution of 647 eV is obtained for AlInP APD coupled to the CSP and exposed to 55Fe X‐rays. Using the CSP presented in this work, previously reported GaAs/AlGaAs APD is characterised and compared with results obtained using a commercial CSP. A 21% improvement in X‐ray energy resolution is reported, despite degradation in the APD.

This thesis presents a detailed description of the fabrication process, spectral characteristics and applications of in-fibre gratings of normal structures, such as fibre Bragg gratings and long period gratings, and tilted structures of small, large and 45 angle tilted fibre gratings. All these in-fibre gratings were fabricated by UV-laser inscription in standard telecom single mode fibres. The key part of this research work is the fabrication and systematic spectral and sensitivity characterisation of the fibre Bragg gratings (FBGs) and long period gratings (LPGs). Their temperature sensitivities were compared for different wavelength ranges from near-IR to mid-IR. The LPGs, which have multiple transmission loss peaks were characterised for temperature and refractive index sensitivities for each transmission loss peak, obtaining the correlation between the cladding mode order and sensitivity. The results of this investigation have enabled to select the best LPGs for two bio sensing applications: (i) investigation of Foetal Bovine Degradation due to change in temperature and (ii) sensing different haemoglobin concentrations. The other major contribution of this Ph.D. research is the systematic approach used in fabricating and characterising tilted fibre gratings (TFGs) with small, large and 45°angle tilted structures. All these types of TFG have been investigated in terms of inscription methods, spectral characteristics, polarisation properties and thermal responses. The three fabrication techniques used to inscribe TFG structures, two-beam holographic, phase-mask and amplitude-mask, have been fully discussed. The TFGs were subjected to various temperature sensing experiments to evaluate their responses and how the temperature change could affect their performance in real environment. In addition, for the small and large angle TFGs, their refractive index (RI) sensing characteristics have been investigated to show their unique RI sensing capability to the surrounding medium. And due to the unique polarisation property of large angle TFG, it was employed in an all fibre twist sensor. Finally, a chemical sensing application was evaluated using a pair of large angle TFGs forming a high sensitivity interferometer. Based on their unique optical properties, a power tapper working at 800nm wavelength region has been demonstrated using 45°-TFGs. The in-fibre tapper system was characterised for its dispersion, side-tapped beam width and side-tapped power variation along the grating length. This system was then used as a temperature sensor, showing side-tapping functionality. Finally, as another major contribution, the 800nm 45°-TFG combined with CCD array were developed into an optical fibre signal interrogation system and evaluated for FBG temperature sensing, which clearly demonstrated the design concept of an in-fibre spectrometer of low cost, compact structure and high function. In collaboration with Bern University of Applied Sciences, Bangor University and Jiangnan University, the 800nm 45°-TFG was first used to develop an OCT system for bio sensing applications.

The motivation behind this work was the exploration of the possibility of diamond deposition on steel substrates for low friction and low wear applications. Materials such as tungsten carbide are commercially available as diamond coated tools, where the diamond coating greatly extends the tool lifetime and performance. The diamond deposition on steel differs in terms of limitations to the diamond deposition on tungsten carbide. The main limitations of steel are its sensitivity to elevated temperatures which are commonly used for diamond deposition and a large difference in the thermal expansion coefficients of steel and diamond. Overcoming those challenging limitations would result in an introduction of competitive products for many applications. This project was a pioneering work in diamond deposition on steel substrates at Aston University in co-operation with Teer Coatings Ltd (Miba goup). The main focus was on the use of an interlayer as a facilitator of enhanced diamond growth and its adhesion towards the steel substrate. Particular attention was given to amorphous carbon coating being a buffer layer for subsequent diamond growth, followed by the investigation of diamond film growth on tungsten coated steel substrates. Interlayers were deposited using the magnetron sputtering technique at Teer Coatings. Diamond thin films were deposited at Aston University using microwave plasma chemical vapour deposition (CVD) with methane and hydrogen as a deposition gas mixture. Investigation of diamond growth from amorphous carbon films coated on steel substrates was found despite the initial promising results to provide low diamond nucleation coverage resulting samples with a sparse population of diamond crystals. The focus of the study changed into an investigation of diamond growth on steel substrates coated with metallic interlayers. As an enhancement for diamond nucleation a pre-treatment of seeding the substrates with nanocrystalline diamond particles, transferred onto the substrates by immersion into a diamond suspension, was developed and used further in this work. Tungsten coating was chosen as the main interlayer material for its diffusion barrier properties, carbide formation, specific thermal expansion coefficient and no inclination to hydrogen embrittlement. The direct tungsten deposition onto a substrate was found problematic and was initially solved by the development of a structured CrW interlayer (1 μm thick) on which an optimization of diamond CVD deposition conditions was performed. The need for a reliable temperature measurement resulted in creation of a setup with thermocouple mounted at the bottom of a substrate holder and a suitable calibration of the setup to be able to calculate the temperature of the substrate surface. CrW was found to have poor adhesion properties and a new MoW interlayer (1 μm thick) possessing excellent adhesion characteristics was developed. The diamond films deposited using previously optimised diamond deposition conditions was found to be at 785 °C. The ≈250 nm thick diamond films showed a good adhesion strength while the MoW interlayer was proved to be an effective diffusion barrier. The previously optimised diamond deposition conditions were found to deteriorate the steel substrate’s properties and further low temperature diamond deposition conditions were optimised for diamond growth at 650 °C. The resulting ≈250 nm thick films showed poor diamond adhesion characteristics due to weaker bonding between diamond and the substrate. The steel substrate did not undergo any softening during the diamond deposition. The effect of different diamond deposition temperatures, as well as the different thickness of the MoW interlayer on stress within diamond film, was studied. Lowest amount of compressive stress of 1.6 GPa was found for a sample coated with the thickest MoW (8.3 μm) and diamond deposition conditions at 650 °C. The sample showed superior adhesion upon Rockwell C indentation, while poor adhesion was observed by means of scratch testing using WC ball as an indenter.

Integrating sphere based multipass cells, unlike typical multipass cells, have an optically rough reflective surface, which produces multiple diffuse reflections of varying lengths. This has significant advantages, including negating scattering effects in turbid samples, removing periodicity of waves (often the cause of etalon fringes), and simple cell alignment. However, the achievable pathlength is heavily dependent on the sphere wall reflectivity. This presents a challenge for ongoing in-situ measurements as potential sphere wall contamination will cause a reduction in mean reflectivity and thus a deviation from the calibrated pathlength. With this in mind, two techniques for pathlength calibration of an integrating sphere were investigated. In both techniques contamination was simulated by creating low reflectivity tabs e.g. ≈5x7mm, that could be introduced into the sphere (and removed) in a repeatable manner. For the first technique, a four beam configuration, adapted from a turbidity method used in the water industry, was created using a 5cm diameter sphere with an effective pathlength of 1m. Detection of methane gas was carried out at 1650nm. A mathematical model was derived that corrected for pathlength change due to sphere wall contamination in situ, thus enabling gas measurements to continue to be made. For example, for a concentration of 1500ppm of methane where 1.2% of the sphere wall was contaminated with a low reflectivity material, the absorption measurement error was reduced from 41% to 2% when the model was used. However some scenarios introduced errors into the correction, including contamination of the cell windows which introduced errors of, for example, up to 70% if the particulate contamination size was on the order of millimetres. The second technique used high frequency intensity modulation with phase detection to achieve pathlength calibration. Two types of modulation were tested i.e. sinusoidal modulation and pulsed modulation. The technique was implemented using an integrated circuit board which allowed for generation of modulation signals up to 150MHz with synchronous signal processing. Pathlength calibration was achieved by comparison of iii the phase shift for a known length with the measured phase shift for the integrating sphere with unknown pathlength over a range of frequencies. The results for both modulation schemes showed that, over the range of frequencies detected, 3-48MHz, the resultant phase shift varied as an arctangent function for an integrating sphere. This differed from traditional single passes where frequency and phase have a linear relationship.

The development of scalable and cost-efficient solar cell technologies constitutes a priority in the field of photovoltaics research. Semiconductor sensitised solar cells (SSSC) and hybrid organic-inorganic perovskite solar cells (HOPSC) are two emerging technologies that have been actively pursued in the search for the most efficient, cost-competitive, stable and nontoxic photovoltaic devices. The HOPSCs have been polarising the attention of the photovoltaics community in the past three years due to their ever-increasing efficiencies, currently exceeding 20% while SSSCs have increased in popularity in the last decade, showing a steady increase of their efficiency and emergence of new materials implemented as light sensitisers. Improvement of these novel technologies relies on understanding of the physical properties of the materials components and on efficient strategies towards the discovery of novel compounds. In this thesis, we address these requirements by modelling the electronic structures of novel light sensitisers from first principles. We focus on the analysis of two groups of materials, the hybrid organic-inorganic lead-iodide perovskites and the metal chalcogenides of the stibnite family. We study the electronic structure of CH3NH3PbI3 within density functional theory and obtain the quasiparticle band gap for this material within the GW method in good agreement with experiment. Further, we conduct a systematic study of the interplay between the electronic properties of hybrid organic-inorganic perovskites and the structure of the inorganic perovskite network. As a result, we obtain a simple strategy for tuning the band gap of perovskite light absorbers by changing the size of the central cation, and propose AsH4PbI3 and SbH4PbI3 as potential novel light sensitisers for HOPSC. The second part of the thesis is dedicated to the study of the electronic structure of four isostructural metal chalcogenides of the stibnite family, stibnite, antimonselite, bismuthinite and guanajuatite. A study of the quasiparticle band gaps of these materials is conducted, similar to the approach used for CH3NH3PbI3. By using the Shockley-Queisser analysis, we obtain that all four materials are promising for application as light sensitisers.

For the last 40 years a natural demand for faster, more complex, and therefore, more functional electronic systems, has been the fundamental driving force behind the miniaturisation of the complementary metal oxide semiconductor transistor. The formation of highly conducting, ultra-shallow, p-type junctions is a key component for the source/drain contact and extension regions of the p-channel metal oxide semiconductor transistor. However, the requirements are becoming increasingly more difficult to achieve as technology advances. In fact, new ways of achieving device improvements are being considered. One method currently being implemented within industry is a switch from bulk silicon substrates to silicon on insulator (SOI). Therefore, it is important for any new techniques to be SOI compatible. The most commonly used p-type dopant, boron, suffers from process related phenomena which hinders the creation of such shallow junctions. During annealing interstitial defects remaining from the implantation process impede the junction formation through a defect-dopant interaction, which reduces the electrical activation and enhances the junction depth - the exact opposite to what is required! This thesis studies a technique which generates an excess of vacancy defects (a vacancy is essentially a missing silicon atom). The vacancies counteract the effect via an interstitial-vacancy recombination mechanism, thus reducing their detrimental effect on the subsequent boron implant. A detailed investigation into the optimisation of such a technique has been achieved through Monte Carlo simulations and experimental studies on diffusion, electrical activation and lattice damage in bulk silicon and SOI. It has been shown that it is possible to optimise the boron and vacancy generating implants to achieve a near "diffusionless" process, producing a junction depth of around 17nm, with an extremely high level of electrical activation (~5x1020cm-3) at low annealing temperatures. Furthermore, the junction is extremely thermally stable (600-900&deg;C) giving rise to a large process window for ease of integration. Overall, this optimised technique rivals competing processes with a much lower equipment cost and "footprint" making it potentially a highly viable alternative to the current preferred methodology within industry.