This thesis presents algorithm development in computational chemistry, and applies new computer science concepts to voltammetric simulation. To begin, this thesis discusses why algorithm development is necessary, and inherent problems found in commercial simulation solvers. As a result of this discussion, this thesis describes the need for simulators to keep abreast of recent computational developments. Algorithm development in this thesis is taken through stages. Chapter 3 applies known theory relating to the stripping voltammetry at a macroelectrode to the diffusional model of a microdisk, using finite difference and alternating direction implicit simulation techniques. Chapter 4 introduces the concept of parallel computing, and how computational hardware has developed recently to take advantage of out-of-order calculations, by processing them in parallel to reduce simulation time. The novel area of graphics card simulation for highly parallel algorithms is also explained in detail. Chapter 5 discusses the adaptation of voltammetric finite difference algorithms to a purely parallel format for simulation by explicit solution. Through explicit solution, finite difference algorithms are applied to electrode geometries which necessitate a three-dimensional solution – elliptical electrodes; square, rectangular, and microband electrodes; and dual microdisk electrodes in collector-generator mode. Chapter 6 introduces 'Random Walk' simulations, whereby individual particles in the simulation are modelled and their trajectories over time are calculated. The random walk technique in this thesis is improved for pure three-dimensional diffusion, and adapted to graphics cards, allowing up to a factor 4000 increase in speed over previous computational methods. This method is adapted to various systems of low concentration confined voltammetry (chapter 6.4) and single molecule detection, ultra low concentration cyclic voltammetry (chapter 6.5), and underpotential deposition of thallium on mobile silver nanoparticles (chapter 6.6). Overall, this thesis presents, and applies, a series of algorithm development concepts in computational electrochemistry.

Interest exists in the development of transparent conducting oxide materials, which have diverse applications in areas such as transparent coatings for display technologies, solar cells, and optoelectronics. Since many of the applications require the use of thin film forms, the need is to establish useful experimental approaches to the fabrication of such structures. One relatively new method in this area is oxygen-plasma assisted atomic beam epitaxy (OPABE) in which oxide layers are grown under normal molecular beam epitaxy (MBE) conditions with the addition of an oxygen atom beam to ensure full oxidation of the depositing metallic species. Work in this area has to date mainly focussed on the growth of relatively stable oxides such as ZnO, MgO and In₂O₃ which are the strongly thermodynamically favoured reaction products, across a broad range of reaction conditions. In contrast, the present work is concerned with the growth of Cu2O and a range of delafossite materials, namely CuInO₂, CuCrO₂ and CuGaO₂, which are expected to require much more sensitive control to achieve the desired reaction product. Studies of the OPABE growth of Cu₂O on MgO (100) and MgO (110) substrates have been carried out, using a broad range of physical techniques to characterise the grown Cu₂O deposits. It is demonstrated that CuO is the favoured reaction product at low growth temperatures, although Cu₂O becomes increasingly favoured as the growth temperature increases. Alternatively, it is also shown that a novel bilayer growth method, whereby some pure Cu is deposited prior to oxide growth, can be used to form the desired Cu (I) phase. Varying crystal orientations are seen, depending on the exact growth conditions; core level and valence band X-ray photoelectron spectroscopy (XPS), optical band gap and atomic force microscopy (AFM) measurements are used to characterise the deposits. Further growth investigations of the delafossite compounds CuInO₂, CuCrO₂ and CuGaO₂ using OPABE are also recorded, and for the case of CuInO₂, comparison is also made with the pulsed laser deposition approach. For all three materials systems, oriented crystal growth on basal planes sapphire substrates is seen, with either the (001) plane or the (015) plane orienting parallel to the substrate depending on the growth temperature, provided approximately correct metal fluxes are used as set by the Knudsen-cell temperatures. The typical valence band electronic structure of delafossite materials is observed in all three cases, and XPS peak shifts suggest that the layers can be electrically doped by adding appropriate metal fluxes during growth. AFM measurements show the grown films are relatively rough and it is suggested that the growth mode follows an island growth mechanism in which oriented three dimensional islands formed at the start of growth gradually enlarge and coalesce as the film thickens. Optical absorption measurements are consistent with the generally accepted optical band gaps of the materials concerned.

The processes by which anisotropic colloidal and nanoscale particles may come together to form ordered monodisperse structures are not well understood. The canonical example of such a system is provided by the assembly of virus capsids, in which tens to thousands of particles of one or a few types assemble efficiently into ordered, highly symmetrical shells. Other examples include a wide variety of protein oligomers, and there is interest in producing analogous systems of synthetic particles. In this thesis I study the self-assembly of simple model particles, consisting of spheres decorated with attractive patches. I consider in detail the assembly of clusters of particles corresponding to the Platonic solids. For the majority of these structures assembly is found to be efficient over a wide range of parameter space. The optimal conditions represent a compromise between thermodynamic stability and kinetic accessibility. We consider two versions of the model, with and without constraints on the torsion angle of bound particles. In both cases the structures with triangular faces are found to assemble most easily. In the absence of torsional constraints dodecahedra will not assemble under any set of parameters as a result of the preferential formation of disordered aggregates. With torsional constraints included all of the Platonic solids assemble successfully and the behaviour of the model is considerably changed. In particular disordered aggregates become far less favourable. I explore possible methods of assembling larger structures, either via “hierarchical” assembly where small clusters are first assembled and then used as building blocks in another stage of assembly, or by a templating method in which an inner cluster acts as a template for a larger outer cluster. These approaches are studied using the “Virtual Move Monte Carlo” cluster move algorithm, the behaviour of which we examine in some detail.

This thesis is concerned with furthering our understanding of the basis of visible region photocatalytic activity exhibited by doped TiO2-based materials. A range of experimental techniques including high resolution X-ray photoemission spectroscopy and diffuse reflectance spectroscopy are used to investigate electronic structure and an attempt is made to link these results to the observed photocatalytic activity. Both anionic (N) and cationic (Rh and Sn) dopants are investigated. [See pdf file for full abstract].

The behaviour of colloidal particles in two-dimensional (2D) systems is addressed in real space and time using magnetic fields, optical tweezers and optical video microscopy. First, the fluctuations of a grain boundary in a 2D colloidal crystal are analysed. A real space analogue of the capillary fluctuation method is derived and successfully employed to extract the key parameters that characterise the grain boundary. Good agreement is also found with a fluctuation-dissipation based method recently suggested in simulation. Following on from analysis of the interface fluctuations, the properties of the individual grain boundary particles are analysed to investigate the long standing hypothesis that suggests that grain boundary particle dynamics are similar to those in supercooled liquids. The grain boundary particle dynamics display cage breaking at long times, highly heterogeneous particle dynamics and the formation of cooperatively moving regions along the interface, all typical behaviour of a supercooled liquid. Next, the frustration induced by confining colloidal particles inside a pentagonal environment is investigated. The state of the system is adjusted via two separate control parameters: the inter-particle interaction potential and the number density. A gradual crystalline to confined liquid-like transition is observed as the repulsive inter-particle interaction potential is decreased. In contrast, re-entrant orientational ordering and dynamical effects result as the number density of the confined colloidal particles is increased. Finally, the dynamics of colloidal particles distributed amongst a random array of fixed obstacle particles is probed as a function of both the mobile particle and fixed obstacle particle number densities. Increasing the mobile and the obstacle particle number density drives the system towards a glass transition. The dynamics of the free particles are shown to behave in a similar way to the normal glass transition at low obstacle density and more analogous to a localisation glass transition at high obstacle density.

Amperometric gas sensors are widely used for environmental and industrial monitoring. They are sensitive and cheap but suffer from some significant limitations. The aim of the work undertaken in this thesis is the development of ‘intelligent’ gas sensors to overcome some of these limitations. Overall the thesis shows the value of ionic liquids as potential solvents for gas sensors, overcoming issues of solvent volatility and providing a wide potential range for electrochemical measurements. Methods have been developed for sensitive amperometry, the tuning of potentials and especially proof-of-concept (patents Publication numbers: WO2013140140 A3 and WO2014020347 A1) in respect of the intelligent self-monitoring of temperature and humidity by RTIL based sensors. Designs for practical electrodes are also proposed. The specific content is as follows. Chapter 1 outlines the fundamental principles of electrochemistry which are of importance for the reading of this thesis. Chapter 2 reviews the history and modern amperometric gas sensors. Limitations of present electrochemical approaches are critically established. Micro-electrodes and Room Temperature Ionic Liquids (RTILs) are also introduced in this chapter. Chapter 4 is focused on the study of analysing chronoamperometry using the Shoup and Szabo equation to simultaneously determine the values of concentration and diffusion coefficient of dissolved analytes in both non-aqueous and RTIL media. A method to optimise the chronoamperometric conditions is demonstrated. This provides an essential experimental basis for IL based gas sensor. Chapter 5 demonstrates how the oxidation potential of ferrocene can be tuned by changing the anionic component of room temperature ionic liquids. This ability to tune redox potentials has genetic value in gas sensing. Chapters 6 and 7 describe two novel patented approaches to monitor the local environment for amperometric gas detection. In Chapter 6, an in-situ voltammetric ‘thermometer’ is incorporated into an amperometric oxygen sensing system. The local temperature is measured by the formal potential difference of two redox couples. A simultaneous temperature and humidity sensor is reported in Chapter 7. This sensor shows advantageous features where the temperature sensor is humidity independent and vice versa. The Shoup and Szabo analysis (Chapter 4) requires ‘simple’ electron transfer and as such the reduction of oxygen in wet RTILs can be complicated by dissolved water. Chapter 8 proposes a method to stop oxygen reduction at the one electron transfer stage under humid conditions by using phosphonium based RTILs to ‘trap’ the intermediate superoxide ions. Chapters 9 and 10 report the fabrication of low cost disposable electrodes of various geometries and of different materials. The suitability of these electrode for use as working electrodes for electrochemical experiments in aqueous, non-aqueous and RTIL media is demonstrated. Their capability to be used as working probes for amperometric gas sensing systems is discussed.

The C͂¹B₂ ←　X͂¹A₁ electronic transition of SeO₂ has been investigated under high resolution, at a rotational temperature of around 10 K, using the technique of Laser Excitation Spectroscopy. The vibrationally-resolved survey spectrum contained around 100 new bands in addition to the bands which had been reported in a previous study of the same region (G.W. King and P.R. McLean, J. Mol. Spec. 51, 1974). In the light of this new spectrum a number of bands have been reassigned, most significantly the O⁰₀ band, and a number of progressions have been extended. This led to a revised determination of the vibrational constants of the excited state, and a more acceptable estimate of v'₃ than was suggested in the previous work. These reassignments and extensions of existing assignments accounted for only a small fraction of the newly observed bands; those remaining are thought to be due to a different electronic transition which lies in the same region as the C͂¹B₂ ←　X͂¹A₁ transition. The 1³₀, 1²₀ and 1¹₀ bands of the C͂¹B₂ ←　X͂¹A₁ transition were also recorded at rotational resolution and analysed using the method of ground state combination differences. The 1³₀ band was found to be perturbed, which was one of the major factors which prompted the survey study described above. From the analysis of these bands the rotational constants of the excited state were determined and hence the geometry of the SeO₂ molecule in the given vibrational levels of the ¹B₂ excited state was calculated. This in turn enabled the rotational constants and the geometry of the (00) vibrational level of the excited state to be estimated. This work confirms that the symmetry of the excited state is ¹B₂ and the transition studied is C͂¹B₂ ←　X͂¹A₁. An additional band around 31957 cm⁻¹ was also recorded at rotational resolution, which was initially though to be the O⁰₀ band, on the basis of King and McLean's assignments. However in the light of the reassignments the nature of this band is not known, and attempts to assign it as vibrationally cold band of the C͂¹B₂ ←　X͂¹A₁ transition were unsuccessful, implying that it is probably either a hot band of the C͂¹B₂ ←　X͂¹A₁ transition or a band belonging to different electronic transition.

Zeeman deceleration is an experimental technique which allows for the manipulation of open-shell atoms and molecules in a supersonic beam thus producing mK-cold, velocity-tunable beams of particles in selected quantum states. The method relies on the Zeeman interaction between paramagnetic particles and time-varying, inhomogeneous magnetic fields generated by pulsing high currents through an array of solenoid coils. This thesis describes the construction and implementation of a supersonic beam setup including a 12-stage Zeeman decelerator. The Zeeman decelerator follows an original design that makes it possible to replace individual deceleration coils. Using ground-state hydrogen atoms as a test system, it is shown that the transverse acceptance in a Zeeman decelerator can be significantly increased by generating a rather low, temporally varying quadrupole field in one of the solenoid coils. An electron-impact source was constructed and optimised enabling, for the first time, the Zeeman deceleration of metastable helium atoms in the 23S1 state, with an up to 40 % decrease in the kinetic energy of the beam. It is shown that the pulse duration for electron-impact excitation needs to be matched to the acceptance of the decelerator in order to attain a good contrast between the decelerated and undecelerated parts of the beam. Experimental results are rigorously analysed and interpreted using three-dimensional numerical particle trajectory simulations. A phase-space model provides, for the first time, a means to estimate the six-dimensional phase-space acceptance in a Zeeman decelerator and to find optimum parameter sets for improved Zeeman deceleration schemes. The approach also reveals a hitherto unconsidered velocity dependence of the phase stability which is ascribed mainly to the rise and fall times of the current pulses that generate the magnetic fields inside the deceleration coils. In the future, it is planned to combine the Zeeman decelerator with a source of cold atomic and molecular ions to study chemical collisions at low temperatures. A hybrid magnetic guide consisting of permanent magnet assemblies (Halbach arrays) in hexapole configuration and a set of current-carrying wires is proposed and simulated as an interface between these setups. The design promises very efficient velocity selection, a high degree of quantum-state selection and a nearly complete removal of residual carrier gas. Prospects for using magnetic hexapole focusing in front of the Zeeman decelerator are discussed. The work represents a major step towards the study and control of chemical reactivity of paramagnetic species in the low-temperature regime and it will help in the testing of fundamental chemical reaction theories.

When approaching a metal surface, the electronic structure of Rydberg atoms or molecules is perturbed by the surface potential and at close enough distances resonant ionisation of the Rydberg electron into the conduction band of the surface can occur. It is possible to interfere in this process and steer the ionisation distance by making use of the polarisability of the Rydberg orbital in the presence of electric fields. The resulting ions from the surface can extracted via electric fields and subsequently detected via well established ion detection schemes. The question of how this charge-transfer process is affected by different properties of the surface (both electronic and structural) represents the main aspect of the work presented in this thesis. At first, the charge transfer of atomic hydrogen Rydberg atoms with a flat gold metal surface is investigated. While such a surface might appear homogeneous, stray fields are present in its vicinity due to local variations in the surface work function. The surface ionisation process as a function of applied electric field is therefore measured experimentally and the results are compared with classical Monte-Carlo simulations (which include stray field effects). This way the possibility to utilize Rydberg states as a probe of the magnitude of such stray fields is demonstrated. To investigate the effect the surface structure can have on the ionisation process, the interaction of Rydberg atoms with surfaces covered by nanoparticles is investigated. Surface ionisation is measured at a 5 nm nanoparticle monolayer surface and it is shown that population transfer between surface- and vacuum-oriented Rydberg states occurs. In addition, results are presented, which suggest a dependence of the ionisation process on the relative size of Rydberg orbital and nanoparticle. Furthermore, charge transfer between a Rydberg state and discrete electronic states at the surface vacuum interface are investigated by performing experiments with a Cu(100) band-gap semiconductor surface. By analysing surface ionisation as a function of collisional velocity ionisation rates can be determined and are subsequently compared with theoretical predictions. The potential of identifying resonant ionisation is thereby demonstrated. Last, a new method to produce 2s atomic hydrogen via mixing of the 2s and 2p state in an electric field is proposed and first experimental results are presented, thus demonstrating viability of the idea. The experiments presented in this thesis represent the most in depth analysis of the charge-transfer process between atomic hydrogen Rydberg states and a range of different surfaces to date. As such, they demonstrate the potential of utilizing the unique properties of Rydberg states and their applicability as surface probes. In addition, these results pave the way for further experiments involving thin films or the phenomenon of quantum reflectivity.

We study the low-temperature physics arising in models of a strongly correlated, tunnel-coupled double quantum dot (DQD), particularly the two-impurity Anderson model (2AIM) and the two-impurity Kondo model (2IKM), employing a combination of physical arguments and the Numerical Renormalisation Group. These models exhibit a rich range of Kondo physics. In the regime with essentially one electron on each dot, there is a competition between the Kondo effect and the interdot exchange interaction. This competition gives rise to a quantum phase transition (QPT) between local singlet and Kondo singlet phases in the 2IKM, which becomes a continuous crossover in the 2AIM as a result of the interlead charge transfer present. The 2IKM is known to exhibit two-channel Kondo (2CK) physics at the QPT, and we investigate whether this is also the case for the 2AIM at the crossover. We find that while in principle 2CK physics can be observed in the 2AIM, extremely low temperatures are required, such that it is unlikely that 2CK physics will be observed in an experimental DQD system in the near future. We have studied the effect of a magnetic field on the 2AIM and the 2IKM, finding that both the zero-field QPT in the 2IKM and the zero-field crossover in the 2AIM, persist to finite field. This presents the possibility of observing 2CK physics in an experimental DQD at finite field, but we find that the temperatures required to do so are extremely low. We show that longer even-numbered chains of spins also exhibit QPTs at finite field, and argue that a 2N-spin chain should undergo N QPTs as field is increased (starting deep in the local singlet phase at zero field). We have also carried out a joint theoretical-experimental study of a carbon nanotube based DQD, in collaboration with Dr. Mark Buitelaar et al. The agreement between experimental and theoretical results is good, and the experiments are able to access the crossover present in the 2AIM at finite field. Furthermore, the experiments show the wide range of physics exhibited by DQD systems, and illustrate the utility of such systems in probing correlated electron physics.