In this thesis, an investigation into the optical properties of strongly-coupled inorganic and organic semiconductor microcavities is presented. An analysis ofthe polarisation properties ofthe signal emitted from a GaAs microcavity with InGaAs quantum wells for resonant excitation with one continuous wave laser beam is reported. Polarisation properties of the signal are discussed with respect to the power and linear polarisation of the excitation beam. Near-field spatial images of the signal are also presented. Results from theoretical numerical models are shown to agree with experimental results. Polarisation and spatial emission of the signal have also been studied for resonant excitation ofthe cavity with two independent laser beams. This led to the observation of a polariton state at nil wavevector and at the same energy of that ofthe OPO idlers. New scattering processes to explain this polariton state formation are presented. A variety of strongly-coupled organic semiconductor microcavities has been fabricated and characterised. Strong coupling is reported for cavities containing J-aggregates of a cyanine dye absorbing at near-infrared wavelengths. The result is a step towards the fabrication of hybrid structures containing organic and arsenide-based semiconductors. Here, evidence of a 'hybrid' organic-inorganic polariton state is reported for a cavity containing organic (molecular dye ZnTPP) and inorganic (perovskites) materials. The relative population of the polariton branches in strongly-coupled cavities containing J-aggregates of a cyanine dye have been investigated as a function of Rabi-splitting energy. Experimentally measured photoluminescence emission has been compared with a full quantum-mechanical model. Here, indirect experimental evidence of scattering of upper branch polariton states to the exciton reservoir is presented. This is shown to be more efficient when the energy separation between the polariton state and the uncoupled exciton is in resonance with the molecular vibrational modes of the organic material.

The drive here is to use high pressures and temperatures to make novel alloys, with direct band gaps and tailored lattice constants that act as effective interfaces between optical and electronic devices (for instance, link fibre optics to silicon chips). Ge-Sn binary alloys have been predicted to have such properties but unfortunately these elements are virtually immiscible at ambient pressure. Therefore this study is also of fundamental importance in developing new Ge-Sn phase relations, evaluating their formation properties, structures, stability and solution chemistry. The particular focus of this work is to explore mixing and reactivity of Ge and Sn at high pressures and high temperatures. Two pressure vessels were used, a piston-cylinder (up to 3.5 GPa and 1500 K) and a multianvil press (up to 24 GPa and 1500 K). The reaction products were processed (nano-processed when required with Focused Ion Beam methods). Mixing and reactivity of Ge-Sn was found to be dependent on the electronic properties of the starting elements at the experimental conditions. The two elements were found to separate below the Ge semiconductor-metal transition pressure (~9 GPa), but do not above this pressure.

Conjugated molecular materials offer a range of useful physical properties from highly efficient luminescent behaviour to wire like conduction. This thesis describes the synthesis, molecular and electronic structures of a range of unusually conjugated organic and organometallic compounds. A combination of synthetic and absorption spectroscopic studies, as well as calculations (DFT) were used to probe the electronic structure of the 1, 4-bis(phenylethynyl)benzene and 1,2,4,5-tetraethynyl benzene framework. These revealed that the ground state of l,4-bis(phenylethynyl)benzene type molecules can be described as thermally populated distributions of conformers, while the exited state is best described in terms of planar conformations with considerable acetylenic character. A novel synthesis of 1,1,2,2-tetraethynylethenes has been discovered involving cross-coupling reactions of tetrachloroethene with terminal acetylenes. Reactions of these tetraethynylethenes with [Co(_2)(CO)(_6)(L(_2))][L(_2)= (CO)(_2), dppm] give complexes with one or two (trans) cobalt fragment coordinated to the acetylenic moieties. A theoretical study of the electronic structure of [Ru{(C=C)(_n)R}(L)(_2)Cp] (n = 1-6; L = CO, PH(_3); R = CH(_3), H, C(_6)H(_4)NH(_2)-p, C(_6)H(_5), C(_6)H(_4)NO(_2)-P, CN) has been carried out. The redox potentials of these species may be tuned by the length of the polyynyl ligand, the nature of the supporting ligands, as well the electronic properties of the non-metal end-cap. Electrochemical and spectroelectrochemical analysis of [Ru(_4)(CO)(_11)](Ų4-(RC(_2)C≡CR)(_2)](^0/2-) have revealed an unusual reversible conversion of 62/64 CVE cluster frameworks. This switching behaviour has been modelled using [Ru(_4)(C0)(_11)(Ų4-HC(_2)H)(_2)](^0/2)- via DFT methods.

This thesis reports upon synchrotron based luminescence studies of wide band gap semi-conductors and organic thin films. The optical and structural electronic properties of cubic and hexagonal boron nitride have been studied using X-ray Excited Optical Luminescence (XEOL) and Optically Detected X-ray Absorption Spectroscopy (OD-XAS). UV/visible emission was identified in both h-BN and c-BN with additional exciton related deep UV emission for the former. UV excited luminescence measurements were used to determine the band gap energies of h-BN and c-BN, these were found to be 5.96eV ± 0.04eV and 6.36eV ± 0.03eV respectively. Spatially resolved XEOL and OD-XAS was used to investigate c-BN microcrystals revealing lateral differences in luminescence and local structure. Synchrotron/laser pump probe spectroscopy was applied to investigate defect states in h-BN and c-BN. Subsequent correlation to the XEOL emission was made proving these defects states to be responsible for the lowest energy emission bands in both materials. Angular resolved NEXAFS, photo-luminescence (PL) and OD-XAS was used to characterise spin coated thin organic films of poly(phenylamine). The material was shown to exhibit preferred orientation within the film, but spatially resolved imaging OD-XAS revealed lateral variation in the molecular orientation. Electrospray deposition was developed for the growth of thin organic semiconducting films in ultra high vacuum. PL and OD-XAS studies were carried out on pure and mixed films of tetra sulfonated copper phthalocyanine (tsCuPc) and poly(ethyleneoxide) (PEO). Only the mixed complexes displayed infrared emission resulting from disstacking of the tsCuPc by the PEO within the film.

The general theme of this thesis is the investigation of light harvesting materials as solar absorbers. The chapters reported within this thesis can be divided into two domains. The first half concerns issues for the electroformation of uniform copper indium diselenide (CISe) thin films and the latter half investigates surface electrochemical processes exhibited by mesoporous, nanocomposite thin films. Several strategies were investigated concerning the formation of reproducible, large area (ca. 10 cm2), uniform CISe thin films. The examination of a novel substrate for the electrodeposition of CISe is discussed in chapter 2. Here molybdenum coated glass slides undergo a pre-selenisation to form MoSe2/Mo/glass. The MoSe2 is inert and resistant to chemical attack as well as degenerate in nature. Photoactive CISe films were formed on MoSe2/Mo electrodes to demonstrate their potential as substrates for devices. The use of a rocking disc electrode for the electroformation of large area (ca. 12.1cm2) copper and CISe films is shown in chapters 3 and 4 respectively. The rocking disc electrode induces mass transport by rocking (vibrating) the cell. The induced convection was investigated by monitoring limiting currents of RuII/III(NH3)63+ and potentiostatic deposition of uniform copper films free from pitting and pores was performed. Under rocking disc conditions it is possible to attain a 1:1 ratio in CuIn films using a tartrate based plating bath. Films are selenised to form CISe and as a by-product CuxSe. The photoactivity of the films is non-uniform and patchy due to the presence of CuxSe and/or pin holes which occur from either under or over etching the film. Various post selenisation treatments were performed to improve the photoactivity of the films such as the dilution of etchant, re-annealing the CISe film and the use of an oxidising agent in conjugation with the etchant. The second part of this thesis investigates the surface electrochemistry of nanocomposite films Fe2O3 and di-ruthenium-bis(benzimidazolyl)pyridine – TiO2 which are shown in chapters 5 and 6, respectively. Mesoporous Fe2O3 thin films exhibit a higher oxidation state which is likely to be a key intermediate for the oxidation of water. By using the spectro-electrochemical methods potentialmodulated spectroscopy and light induced spectroscopy it is possible to attain in situ transmission spectra of this state. The spectra correspond to a surface trapped hole which has been reported by others. The frequency dependence of this surface state was investigated and correlated with theory. In the final experimental chapter films composed of an inorganic di- ruthenium-bis(benzimidazolyl)pyridine complex (Ru2) and TiO2 nanoparticles were produced. The films exhibited an immobilised Ru signal. Using focus ion beam methods the creation of a nano-trench in a film of ITO was undertaken. The immobilisation of this material within the trench allowed bi-potentiostatic experiments to be conducted which evaluated the charge diffusion processes. An apparent diffusion coefficient of Dapp = 2.5 x 10-15 ms-2 was attained using generator and collector experiments. The potential use of these junction electrodes for other nanocomposite materials is highlighted.

No description available.

The first part of this thesis describes transport measurements of long quantum wires, which are affected by disorder. The resulting additional features in the conductance are characterised, and the results are discussed in the context of the Luttinger liquid model. Realistic strategies for controlling disorder in long wires are suggested, which should eliminate many of the problems associated with experimental studies of Luttinger liquids. Disorder effects are further investigated using a new lithography technique called Erasable Electrostatic Lithography (EEL). A scanning probe tip at a fixed voltage is used to locally charge surface states above a long disordered quantum wire. This allows the potential of the disordered wire to be manipulated, with the creation of microconstrictions and quasi-bound states inside the wire. The importance of electron-electron interactions in short 1D systems was demonstrated in 1996 by the discovery of the 0.7 structure. This is an additional quasi-plateau in the conductance at a value of around 0.7(2<i>e</i>²/<i>h</i>) and is a universal phenomenon in quantum wires. The main result of this thesis is the discovery of non-quantised conductance structures at the crossings of spin-split 1D subbands which have similar characteristics to the 0.7 structure. We call these new structures 0.7 analogues. It is shown that the 0.7 analogue is accompanied by a spontaneous splitting and abrupt restructuring of energy levels in the region of the crossing, which is thought to be an exchange effect. We believe that this gives valuable new insight into the origin of the 0.7 structure.

No description available.

No description available.

As organic semiconductor materials advance in both performance and stability, opportunities to integrate them into commercial applications increase considerably. The main benefits of organic semiconductor-related technology are the realisations of large area, flexible, cheap electronics. Already, organic light emitting diodes (OLEDs) are being integrated into small screens for mobiles phones, MP3 players, digital cameras, and high-resolution micro-displays. Such portable devices favour the high light output of OLEDs for easier readability in sunlight as well as their low power drain. However, at this time, the issue of low field effect mobilities still haunt these materials, limiting their application into low speed circuits. More recently, the introduction of the next generation of small molecule-based organic materials has increased the possibility of attaining high field effect mobilities. The majority of the focus is on pentacene as this has been shown to demonstrate effective mobilities of up to 1.5 cm2y-1s-l. Unfortunately, this type of pentacene was evaporated which does not seem feasible for large scale manufacturing. More recently, modified pentacene materials have surfaced allowing them to be dissolved in common solvents, one such example being triisopropylsilyl-pentacene (TIPSpentacene). Thin film transistors (TFTs) made with this material have been reported to have mobilities as high as 1.2 cm2y-ls-1 and onloffratios of 108. Materials such as TIPS-pentacene are now preferred as they can be integrated into low-cost manufacturing techniques such as inkjet printing and roll-to-roll processing. One of the major prospective applications for these organic materials is the integration into radio frequency identification tags (RFID); these operate at 13.56 MHz. This is a great challenge as the rectification stage will require devices with high mobilities to enable carriers to follow the signal, thus gaining the maximum amount of energy from an inductively coupled magnetic field. It is not clear as to whether Schottky diodes or gated-transistors will be required here. The advantage of gated-transistors is the simple incorporation into the fabrication process. Schottky diodes with these materials require thicker films which are incompatible with spin coated thin film transistors. This thesis focuses on three of the main components for a potential organic RFID tag: the tag antenna, the schottky diode and the organic thin film transistor. These are all vital components in the successful operation of an RFIO tag. The antenna is imperative for the power supply as it absorbs energy due to inductive coupling with the RFID reader antenna. The Schottky diode is important for the frontend/rectification stage, converting AC power to a DC supply voltage for an organic chip. The thin film transistor is hugely important as it is the backbone for logic and memory. The fundamental background into inductive coupling based RFID systems is explored and discussed. Major components such as the reader, tag and control system are introduced, while their role and importance are also looked into. Operational principles such as near field inductive coupling for systems functioning at 13.56 MHz are featured, involving the issues of data transfer and power supply. From here, the concept of mutual inductance is explored in detail, as well as highlighting the fact that most RFID systems of this nature comprise lower coupling, as low as I %. The core theory behind predicting the chip voltage on the tag is also explained, to illustrate just how many design parameters are involved and how they affect the performance of an RFID system. The challenges presented to organicbased RFID tags are also summarised and discussed. The numerous charge transport models proposed so far to represent conduction in organic semiconductors are assessed. These models include variable range Miller Abraham hopping and Poole-Frenkel mechanisms. Currently, an outright universal understanding of carrier transport is yet to be widely agreed. An analytical model is developed to demonstrate the carrier density dependence of mobility that is generally observed in organic semiconductors. An empirical relationship between mobility and carrier density, known as the Universal Mobility Law (UML) is recognised. The polycrystalline-based theory which consists of deriving expressions for quasi-drift and quasi-diffusion regions of operation is explained. TIPS-pentacene is utilised here for the first time to test the model. The fabrication procedure for creating bottom-gate bottom-contact organic thin film transistors is covered, with aluminium as the gate material, a high-K alumina gate dielectric and gold for the source and drain contacts. The transistors were fabricated and characterised in a clean non-vacuum environment. The effect of solvent choice is also investigated. comparing tetralin and toluene solvents. The field effect mobility of the charge carriers calculated were approximately 0.02 cm2Ns with threshold voltages ranging from -1V to +1V depending on the chosen solvent. The on/off current ratio estimated from the transfer characteristics were found to be six order of magnitude. Schottky diodes made with TIPS-pentacene show onloff current ratios three to four order magnitude higher than the P3HT devices, suggesting they are much more suitable for rectification circuits. The introduction of mixing the TIPSpentacene material with binders such as poly( a-methyl styrene ) (PAMS) and polystyrene (PS) produced some intriguing results. Both types of binders produced much smoother drop cast films than TIPS films without any binder applied, indicating that the binders were definitely improving the film morphology. Different surface treatments were also employed to help further increase the performance of the devices. It appeared that applying oxygen plasma to the bottom contact definitely helped adhesion. However, chemical treatments such as pentafluorobenzenethiol (PFBT) and I-hexadecanethiol (HDT) either did not affect performance or severely inhibited it.