# School of Physics - Theses

## Search Results

Now showing 1 - 10 of 203
• Item
Estimation of the cosmogenic activation and measurement of the quenching factor of NaI(Tl) crystal with spectrum-fitting for the SABRE experiment
Mahmood, Ibtihal ( 2022)
Though evidence suggests that 84% of all matter consists of dark matter, its lack of substantial interaction with ordinary matter continues to obscure its exact physical qualities. With the tantalizing prospect of uncovering a rich amount of information about a seemingly fundamental aspect of our Universe, the physics community has attempted to mine this ore of knowledge for the past century. Among these attempts, the use of direct detection experiments to probe the weak interactions between dark and baryonic matter has since mostly yielded null results. An exception to these is the annual modulation signal detected by the DAMA/LIBRA experiment, whose dark matter interpretation remains inconclusive. The Sodium-iodide with Active Background Rejection Experiment (SABRE) will conduct low energy (keV) measurements of dark matter direct detection events using radiopure NaI(Tl) crystals as a model independent test of DAMA's annual modulation signal. In order to so successfully, it is vital that the radioactive background of SABRE's crystals are low enough so that they are more sensitive to WIMP-like events than DAMA/LIBRA and that the crystals' response is properly understood by measuring their scintillating properties beforehand. In this thesis, the radioactive background of SABRE's crystal, due to cosmogenic activation while stored on the surface and during transport to its laboratory site, is estimated. This estimation takes into account the amount of cosmic ray flux and geomagnetic shielding for two possible freight travel scenarios, either by air or sea. The subsequent decay of each considered isotope at their underground site is also considered in order to determine how significantly they would contribute to the background over the lifetime of the experiment. In light of these calculations, recommendations for the storage time and method of travel of SABRE's crystals can be motivated with knowledge on whether the cosmogenic background produced will be sufficiently low for SABRE's purposes. Additionally, the quenching factors of SABRE's NaI(Tl) crystal must also be known to low uncertainty in order to determine the energies of the nuclear scattering interactions. A novel spectrum-fitting methodology was developed and tested to extract the quenching factor from sodium nuclear recoil measurements in NaI(Tl). The method employs Monte Carlo simulated recoil energy spectra to fit measured data in order to account for experiment-specific systematics. This was employed to measure the sodium quenching factors of a commercial NaI(Tl) crystal for recoil energies between 36 and 401 keV. The SABRE experiment will use this method for the measurement of their own crystal's quenching factors.
• Item
Spin-down signatures of young neutron stars
Strang, Lucy Catherine ( 2022)
The spin down of neutron stars has been invoked to explain a wide variety of electromagnetic and gravitational-wave signals. This thesis explores two different signals associated with the spin down of neutron stars, one electromagnetic signal and one gravitational-wave signal. Binary neutron star coalescences, confirmed as the progenitor of at least some short Gamma-ray bursts (sGRBs) in 2017, are predicted to form either a black hole or a highly magnetized neutron star. Up to 20% of sGRBs observed by the Neil Gehrels Swift telescope display prolonged X-ray emission, sometimes called a canonical'' afterglow, consisting of three phases: an initial power-law luminosity decay; a 'plateau', lasting between 10 s and 105 s, during which the X-ray luminosity is approximately constant; and a final power-law decay. Previous authors have noted that the evolution of the canonical light curve is broadly consistent with the expected spin-down luminosity of a neutron star. Key ideas from analytic, one-zone models of plerions (also called pulsar wind nebulae) can be used to model the evolution of a synchtrotron nebula fuelled by the the spin-down luminosity of a neutron star formed in an sGRB. An analytic expression for time-dependent, spatially-averaged electron energy distribution in the nebula is found and used to calculate the light curve and the point-in-time spectra. The light curves predicted by the plerionic model are consistent with the shape and luminosity of the X-ray light curves and reproduce the observed correlation between plateau duration and luminosity (i.e. brighter plateaux end sooner). Furthermore, Bayesian parameter estimation comparing the point-in-time spectra to time-averaged spectra of six Swift sGRBs with canonical X-ray afterglows and of known redshift allows estimation of the parameters of the neutron-star central engine, including its poloidal field strength Bp and its rotation period P0 at birth, and injection parameters within the shock, including the energy range of the relativistic electrons and their power-law index. All six sGRBs favour a neutron star with Bp ~ 1011 T and P0 ~ s, consistent with the prediction the neutron star should be highly magnetized and rapidly spinning. We also apply the point-in-time spectra to four time-averaged spectra taken at four separate epochs in the X-ray afterglow of GRB130603B and infer the evolution of the magnetic field in the synchrotron bubble B. We find the evolution of B is slower than the expected evolution of the far-field limit of the stellar magnetic field. Rotating, non-axisymmetric neutron stars spin down via the emission of continuous gravitational waves which may be detectable by current terrestrial interferometers such as the advanced Laser Interferometric Gravitational-wave Observatory (LIGO) and advanced Virgo. Young core-collapse supernova remnants are likely hosts of young neutron stars and are common targets for wide-band directed searches for continuous gravitational waves targeting non-pulsating neutron stars. In this work, we present the results for two searches for continuous waves from neutron stars in young supernova remnants using a hidden Markov model (HMM). The HMM tracking scheme models the frequency evolution as a random walk with secular spin down and remains sensitive in the presence of stochastic spin wandering similar to that observed in pulsar timing observations. A search targeting twelve neutron stars in young supernova remnants in the second observing run (O2) of advanced LIGO using an HMM tracking scheme identifies 1012 potential candidates, 18 of which survive a series of standard vetoes. Further assessment of the 18 survivors based on their dependence on sky position and Doppler modulation confirms they are all consistent with terrestrial noise. A second search, conducted with the the LIGO-Virgo-KAGRA (LVK) collaboration, targets fifteen neutron stars in young supernova remnants in the first half of the third observing run (O3a) of advanced LIGO and advanced Virgo using three search pipelines, including an HMM tracking scheme, and reports no candidates consistent with an astrophysical origin after a rigorous veto and follow-up process. The HMM tracking scheme sets the first 95% confidence limits on gravitational-wave strain, h095%, for these targets with a random-walk signal model, reaching a sensitivity of h095% = 2.64 x 10-25 at 172 Hz for G353.6-0.7. The constraints on h095% are converted to upper limits on neutron-star ellipticity below 10^-5 above 150 Hz and constrain the maximum amplitude of internal r-mode oscillations below 10^-3 above 150 Hz.
• Item
A Taste of Flavour and Neutrino Physics with Scalar Leptoquarks
Bigaran, Innes Elizabeth ( 2022)
Flavour physics is the branch of particle physics that examines the structure of the flavour sector of the Standard Model. This sector, describing fermion masses and their mixing, involves a large number of free parameters which are determined via experimental input. Thus, understanding the nature of the flavour sector provides a key motivation for many theories beyond the Standard Model. The accidental lepton-flavour symmetry of the Standard Model need not be preserved in extended models. Although neutrinos are massless particles in the Standard Model, and thus their flavour is conserved, strong experimental evidence of neutrino flavour oscillations requires that neutrinos are actually massive. Since those masses, though nonzero, are constrained to be tiny, it is well motivated that they are generated in some exotic way. This observation highlights the need for new physics to explain lepton-flavour violation in the neutrino sector. In this thesis, we explore not only neutrinos and their flavour violation, but also how this violation could manifest in the charged-lepton sector. Extensions to the Standard Model discussed in this thesis centre around hypothetical particles called leptoquarks, which directly couple quarks and leptons. Their interactions naturally lead to violation of lepton flavour symmetries, and imbue a sense of linkage between these two classes of Standard Model fermions. Moreover, the simple nature of scalar leptoquark extensions motivate us to consider where these could fall within the larger framework of unified models of nature. In particular, these could explain the structure of the (presently semi-empirical) flavour sector. Chapter 1 provides background on the Standard Model of particle physics, and outlines the relevant conventions adopted in this thesis. Chapter 2 reviews the present landscape of the flavour and neutrino sectors, including an overview of experimental results to guide the ensuing work. Chapter 3 centres on understanding divergences in the Standard Model, and how one can extend this theory of nature within a framework of effective field theory. Chapters 4, 5 and 6 present a series of original studies that highlight the potential impact of scalar leptoquarks on the structure of the flavour and neutrino sectors. Chapter 4 explores the viability of scalar leptoquarks to generate large corrections to charged-lepton dipole moments. We identify the mixed-chiral scalar leptoquarks (S1 and R2) capable of generating chirally-enhanced and sign-dependent contributions to lepton magnetic moments (as favoured by present measurements). We find that TeV scale particles are capable of addressing present anomalies in the magnetic dipole moments of the electron and the muon. Moreover, signals of these models in the muon electric dipole moment are found to be within reach of future experimental programs. Chapter 5 presents a next-to-minimal scalar leptoquark model capable of reconciling recent experimental B-anomalies, and of radiatively generating neutrino masses. Building upon a single leptoquark model for addressing these B-anomalies, we combine two existing neutrino-mass models (containing the leptoquarks S1 and S3, and a vector-like quark) and find that this hybrid model is able to ameliorate the anomalies in b to s transitions, charged-current b decays, and the muon magnetic moment. Furthermore, it is capable of generating radiative neutrino masses consistent with experimental values. Chapter 6 involves the study of a discrete flavour-group model built around a scalar S1 leptoquark extension. Beginning with a GF = D17 x Z17 flavour group, we outline how this model is capable of generating the textures of charged-fermion masses and mixings, as well as the leptoquark couplings required to address anomalies in charged-current b decays and the muon magnetic moment.
• Item
All-electrical Quantum Sensing with Silicon Carbide Devices
Lew, Christopher Tao-Kuan ( 2022)
Silicon carbide (SiC) is a complimentary metal-oxide-semiconductor (CMOS) compatible wide bandgap semiconductor most notably used in high power electronics. Furthermore, SiC is a promising quantum materials platform able to host a wide variety of spin defects exhibiting long spin decoherence times. To leverage the full potential of SiC electronics for future quantum technologies, this thesis conducts a series of detailed investigations on electrically active spin defects in SiC with a particular emphasis on utilising these spin defects for quantum sensing applications measured using the electrically detected magnetic resonance (EDMR) technique. EDMR is a highly sensitive spectroscopic technique able to readout a small ensemble of electron spins in a fully fabricated electronic device by utilising the spin- dependent recombination (SDR) mechanism. The magnetic field sensitivity of a processing-induced spin defect in a SiC diode device was first explored. Instead of utilising the spin resonance response, the electromagnetic irradiation-free hyperfine-induced spin-mixing response situated at zero magnetic field is used for magnetometry. It is shown that the magnetic field sensitivity can be enhanced by at least an order of magnitude by employing a balanced detection scheme for common- mode rejection and above bandgap optical illumination for photogeneration of electron-hole pairs. These two methods are not limited to the processing- induced spin defect studied here and may be applied to other spin defects utilising the EDMR technique. Taking advantage of the wide selection of existing commercial SiC devices available, we then investigated the temperature dependence of the hyperfine- induced spin-mixing response in a commercial SiC power transistor device. As SiC electronics are typically operated under extreme environmental conditions, it is important to understand how the spin-mixing response changes with temperature and how changes in the device response can affect the spin-mixing response. Although a complex temperature dependence was observed for the spin-mixing response convoluted with changes in the device characteristics, a linear change in the signal linewidth with temperature was observed, which may have further implications for quantum sensing of temperature fluctuations. Lastly, we have developed a multi-stage lock-in modulation pulsed EDMR (pEDMR) measurement scheme to study the spin dynamics of processing- induced spin defects in SiC. This alleviates some of the stringent experimental constraints associated with the pEDMR technique and allowed us to charac- terise the spin dephasing and decoherence times, which form the basis of more complex sensing protocols of DC and AC magnetic fields, respectively. The relatively long room temperature spin decoherence time on the order of microseconds demonstrates that SiC is indeed a suitable quantum materials platform for quantum sensing applications. The series of studies presented in this thesis shed new insight on several different aspects of quantum sensing utilising electrically detected spin defects and provides the first steps toward the realisation of an all-electrical SiC quantum sensor.
• Item
Belle II Silicon Vertex Detector and a measurement of B → D**lν decays at Belle
Webb, James Maitland ( 2021)
The Belle II Silicon Vertex Detector (SVD) is a silicon strip detector designed to possess a high irradiation tolerance and short shaping time, making the detector suitable for operation at the high luminosity SuperKEKB collider. In this thesis, the construction of the inner most layer of the detector ''Layer-3'' and subsequent electrical characterisation of the devices are described. Each of the 11 Layer-3 ladders produced were of a high electrical quality, with a strip failure rate of less than 0.2%, demonstrating each of the ladders to be a suitable candidate for installation into the Belle II detector. In the early stages of the detector commissioning phase, numerous high occupancy regions were discovered on the origami sensors. This problem was identified to be caused by crosstalk between control lines on the pitch adapters and the electrodes of the sensor beneath. An algorithm was developed to identify events in which these clusters were present, such that further studies into the affect of the crosstalk clusters could be performed. In particular, the impact on the track finding performance was studied in the search of an offline software approach to mitigating the crosstalk clusters. It was found that the signal-to-noise Ratio (SNR) of the crosstalk clusters were distinct from clusters deposited by signal tracks and an SNR cut based approach demonstrated an improvement to the tracking computation time of order 10%, and a slight improvement to the track parameter resolution. The hit occupancy in the SVD is expected to continually increase as the instantaneous luminosity of SuperKEKB increases over the course of the experiment. As a means of reducing the exponentially growing number of 2D hit candidates which are supplied to the track finding software, detector information was utilised to filter background events. Through exploiting cluster charge, cluster time, and cluster size correlations between each side of the strip detector, a quality index was assigned to each of the reconstructed 2D hits. The quality index of the 2D hits was included in the track candidate multivariate classifier (MVC), having the second highest impact of all the included variables. Through inclusion as a feature of the MVC, the purity of the global track quality ranking was improved. Additionally, a measurement of the semi-inclusive $B\rightarrow D^{**}\ell\nu$ rates, (where $\ell$ denotes either an electron or a muon) were obtained from the entire 711 $fb^{-1}$ Belle data-set. $B\rightarrow D^{**}\ell\nu$ decays are of particular interest due to the uncertainty in the branching fractions calculated by previous measurements. A more precise measurement is of importance for the difference between the inclusive charmed semileptonic decay rate and the sum of the exclusive charmed semileptonic decays (the gap problem'') and for improving the precision of future measurements of $\mathcal{R}(D)$ and $\mathcal{R}(D^{*})$, where new physics may be observed. The tag-side $B$ meson is fully reconstructed in a hadronic decay mode with the latest \ac{BDT} tagging algorithm. The measured branching fractions are $\mathcal{B}(B^{+}\rightarrow D^{-}\pi^{+}\ell^{+}\nu) = (0.396 \pm 0.014 \pm 0.020)\%$, $\mathcal{B}(B^{+}\rightarrow D^{*-}\pi^{+}\ell^{+}\nu) = (0.509 \pm 0.019 \pm 0.030)\%$, $\mathcal{B}(B^{0}\rightarrow \bar{D}^{0}\pi^{-}\ell^{+}\nu) = (0.364 \pm 0.020 \pm 0.020)\%$, $\mathcal{B}(B^{0}\rightarrow \bar{D}^{*0}\pi^{-}\ell^{+}\nu) = (0.589 \pm 0.030 \pm 0.040)$. Each of which are in agreement with current world averages, apart from $\mathcal{B}(B^{+}\rightarrow D^{*-}\pi^{+}\ell^{+}\nu)$, which falls below the world average by 1.8$\sigma$. Each of these measurements offer a higher precision than previous results.
• Item
Real-time Detection and Identification of Pathogens and Pollutants through Optical Imaging
Qazi, Farah ( 2021)
• Item
Dense matter and magnetic fields in neutron stars
Anzuini, Filippo ( 2021)
The life cycle of main sequence stars with masses in the range 8 - 25 solar masses ends in a supernova explosion, whose remnant is a dense compact object called neutron star. The huge gravitational field of neutron stars, counterbalanced by the pressure of strongly degenerate matter, combined with intense magnetic fields offer the opportunity to probe matter in extreme conditions, far beyond the reach of contemporary laboratory experiments. The wealth of observations of neutron stars gradually reveals more facets of their nature, although much is yet to be discovered. The aim of this work is to study the interplay of two ingredients of neutron star physics: dense matter and magnetic fields. The properties of ultra-dense matter can be inferred from the thermal radiation emitted by isolated neutron stars and magnetars. Detailed models of the atmosphere, surface, crust and core are required to determine how information about dense matter in the core is filtered out by the outer layers. Often, several key quantities that determine the thermal luminosity are unknown, such as the mass and radius, or the chemical composition and magnetic field configuration. The ionization state and emission model adopted for the stellar atmosphere and the chemical composition of the outer envelope (i.e. a layer with a typical thickness of ~ 100 m) affect the thermal radiation produced at the surface. Strong magnetic fields modify the heat transport in the atmosphere and in the crust and can decay, producing high Joule heating rates, complicating the interpretation of thermal luminosity observations in terms of the internal chemical composition and the neutrino emission processes active in the deeper regions of the core. Chapters 2 and 3 study the thermal radiation produced by neutron stars with cores hosting unconventional particles such as hyperons by performing state-of-the-art magneto-thermal evolution simulations. The influence of the magnetic field on the thermal evolution is examined for several plausible initial magnetic field configurations, and the thermal luminosity is compared with the data of thermally emitting, isolated neutron stars and magnetars. It is found that (i) internal heating is required by stars with and without hyperon cores, regardless of the composition of the outer envelope, if direct Urca processes activate in stars with masses ~ 1 solar mass and neutrons are superfluid in a large fraction of the core; and (ii) the thermal power produced by the dissipation of crustal electric currents sustaining the magnetic field can hide the effect of fast cooling processes related to the appearance of hyperons in the core, making hard to infer the chemical composition of neutron stars from thermal luminosity data. Although the intense magnetic field of neutron stars plays a central role in their phenomenology, the internal field is poorly known, and often one relies on simplistic magnetic topologies. Additionally, neutron stars may have internal velocity fields generated by differential rotation that affect the magnetic field configuration. One important question is whether in such conditions the magnetic field can force the fluid to rotate uniformly with the solid crust, or whether the fluid can be in a state of differential rotation on long, viscous time-scales. Due to computational limitations, performing detailed magneto-thermal simulations with the addition of evolving internal velocity fields of several fluid components is a nontrivial task. However, one can gain some insight into the magnetic configuration in the presence of internal flows by modeling the star as a spherical shell containing a single, idealized and electrically conducting fluid. In Chapter 4 we study the implications of internal fluid flows on the magnetic field configuration by applying a constant rotational shear between the inner and outer boundaries. It is found that differential rotation tangles the magnetic field lines and produces small-scale toroidal flux tubes containing bundles of closed toroidal field lines in proximity of the magnetic equator. In these toroidal flux tubes, the fluid velocity is set by viscosity rather than by the magnetic field, allowing differential rotation in neutron star interiors to persist on long, viscous time-scales. Hyperons are not the only unconventional particles that may appear in neutron star cores. Typical densities in massive stars may be sufficiently high for quarks to deconfine, and crystalline phases arising from inhomogeneous condensation of quarks may form. In Chapter 5 we develop an analytic approximation for the free energy of deconfined quark matter and study its ground state in the presence of strong magnetic fields and at high temperatures, which may be characteristic of neutron star binary mergers. It is shown that the magnetic field and temperature compete in enlarging and reducing respectively the region of the phase diagram where inhomogeneous phases of quark matter are favored, which correspond to the regions where the neutrino emissivity of quark matter increases due to the activation of direct Urca processes.
• Item
Entanglement in superconducting quantum devices and improving quantum circuit compilation
Mooney, Gary John ( 2021)
Quantum computing has the potential to solve many computational problems more efficiently than classical computing. It is expected to expand our ability to simulate and solve difficult and classically intractable problems in many fields of research and industry. However we are currently in the noisy intermediate-scale quantum (NISQ) era of quantum computing where devices are relatively small and suffer significant levels of noise. In this thesis we report on experiments on IBM Quantum devices, which are among the most advanced quantum devices in the world, to assess the growing capabilities of quantum computing. We focus on the ability to prepare and detect sizeable multi-qubit entangled states, which is a critical milestone for quantum physical platforms. In our first set of experiments, graph states of 20, 53 and 65 qubits are prepared on IBM Quantum devices and are shown to be fully bipartite entangled. Before these experiments, the largest cluster of qubits shown to be bipartite entangled on a gate-based quantum computer was 16 qubits. In another experiment, a Greenberger-Horne-Zeilinger (GHZ) state is prepared over all 27 qubits of the ibmq_montreal device and is demonstrated to exhibit the stronger genuine multipartite entanglement (GME), the largest state shown to exhibit GME to date. These results highlight the positive advancement of quantum computing technology towards the physical realisation of sizeable and complex quantum algorithms. Physical quantum devices usually have limited connectivity. So they typically have coupling maps that specify which pairs of qubits support direct application of two-qubit gates. Mapping quantum circuits to physical devices requires qubit states on the device to move to target qubits via a sequence of SWAP gates to satisfy two-qubit gate coupling map requirements. One approach to finding optimised SWAP schedules involves solving a problem called multi-qubit pathfinding (MQP). We introduce an algorithm that finds optimal solutions to this problem. It primarily minimises the total SWAP gate circuit depth and secondarily minimises the accumulated gate errors. The algorithm is benchmarked on a variety of quantum hardware layouts. The run time appears comparable to current state-of-the-art algorithms while additionally optimising with respect to accumulated gate error and the algorithm is flexible with respect to variations on the problem. Large scale complex quantum algorithms will require robust error-correcting protocols performed over encoded logical qubits. Only a small set of gates can be applied transversally to encoded quantum states, and only a subset of unitary gates can be fault-tolerantly applied via resource-expensive distillation procedures. Together, these gates can be multiplied in sequence to generate any unitary gate to arbitrary precision, where sequences are found using a gate synthesis procedure. We assign resource costs to various base gates and perform cost-optimal single-qubit gate synthesis with the standard Clifford+T base gate set along with additional gates from the Clifford hierarchy. We show that by including higher orders of the Clifford hierarchy, cost savings of over 50% could potentially be achieved. This suggests that adapting current synthesis algorithms to support higher order Clifford hierarchy base gates and individually assigned resource costs could provide substantial benefits.
• Item
Oxygen Terminated Nanodiamond Photoelectrodes for Neuromodulation
Falahatdoost, Samira ( 2021)
Neuromodulation is used for the treatment of a number of neural impairments that hinder cell activation and proper information transfer. The treatment of psychiatric disorders, Alzheimer’s and Parkinson’s disease, motor function disorders, blindness due to retinal degeneration, dystonia, and epilepsy are some of the examples of clinical applications of neuromodulation. Beyond the widely used approach of using wired electrodes for neuromodulation, various methods have been explored. One of them is to use light to wirelessly control neural networks. When used within safe intensity limits, light can stimulate or modulate the function of neurons. Among the wide variety of optical neuromodulation and stimulation methods is the use of photosensitive thin films or particles as transducers to convert the incident light into electrical signals. It has been claimed that this method can be less invasive than electrically driven stimulation and has the added benefit of scalability in both stimulation location and resolution. Moreover, compared with the optogenetic neuromodulation technique, photoactive surfaces have the capability to eliminate the need for genetic modification to introduce photosensitive proteins to the target neural tissues and possible adverse immune system responses. In this technique, photo-excited charge carriers in an electrode are used to stimulate neural tissue. Various materials have been used for this purpose such as conductive polymers, photoconductive silicon, and semiconducting quantum dots, but many of these materials are not ideal because they lack sufficient biostability and are in some cases toxic. Diamond-based materials are excellent candidates owing to their high corrosion resistance, good biocompatibility, and excellent charge injection properties and recent publications demonstrate their electrical response to illumination. A key feature of diamond-based materials is the ability to control their properties by controlling their surface termination. Surface modification is used to alter nanodiamond properties such as chemical affinity, and electrical and optical properties. In particular, the transfer of photoexcited electrons from the conduction band of the nanodiamond to the adlayer at the nanodiamond surface depends on the surface termination of this material. By tuning these surface properties, a diamond can be an effective photocathode for neural stimulation/modulation. Earlier works have demonstrated that oxygen terminated diamond displays a charge-balanced capacitive charge transfer when it is illuminated in saline solution. In capacitive charge transfer, no photoexcited electrons are transferred across the interface and the diamond/electrolyte adlayer can be modelled as a simple electrical capacitor. On the other hand, hydrogen-terminated diamond exhibits Faradic charge transfer, which may be due to the transfer of photoexcited electrons to the adlayer because of the negative electron affinity of the hydrogen-terminated surface. This suggests that the properties of oxygen terminated diamond are more favourable for neural stimulation/modulation. The capacitive charge transfer mechanism is known to minimize both electrode damage and cell degradation which should be avoided in neural stimulation. This thesis starts by examining the properties of oxygen terminated detonation nanodiamond (O-DND) particles with the size of 20 nm to 170 nm. Earlier works have demonstrated that such particles can be incorporated into living cells. When coupled with the high surface area to volume ratio for nanoparticles, this system is potentially attractive for optically induced cell stimulation/modulation. Whilst the presence of a photogenerated charge accumulation layer on oxygen-terminated diamond films has been demonstrated, this has not been experimentally observed in O-DND particles. In this project, furnace annealing is used to oxygen terminate DND powder and to investigate the effect of light on O-DND particles’ surface charge in an aqueous solution. During these experiments, two new techniques are developed. In the 1st method, the electric double layer around the nanoparticles is probed whilst suspended in saline solution and without the need for any electrical connection to the nanoparticles. To do this a 4-electrode electrochemical impedance spectroscopy technique is used to measure the complex impedance of the O-DND nanoparticles suspended in the saline solution as a function of frequency. This technique shows good sensitivity to the surface termination of DND and using this technique, the capacitance and resistance of the particles suspended in the saline solution were extracted. Building on this method, the effect of light on the EDL of the nanoparticles is investigated. Whilst the capacitance and resistance of particles in saline solution is measured, the changes in particle capacitance and resistance due to light illumination are too small to be measured within the standard error range. In the 2nd method, the zeta potential of particles in a solution is measured using the laser doppler electrophoresis technique. The zeta potential directly relates to the surface charge of the nanoparticles, and by measuring it, the light-induced changes in the surface charges may be observed. Here a conventional Zetasizer is modified to allow measurement of the zeta potential while the samples are illuminated with an optical fibre. Using this technique, the changes in surface charge is characterized as a function of different surface terminations, but no changes in the zeta potential under illumination are detected within the sensitivity of the technique. Possible reasons for the lack of observable changes in zeta potential under illumination are discussed. In the 2nd part, this work is focused on the photoresponse of nitrogen-doped ultrananocrystalline diamond, under the assumption that the defect levels created by the nitrogen doping contribute to a photoresponse at longer wavelengths (around 800 nm) which makes it a favourable material for photostimulation. The same surface termination is employed for the nitrogen-doped ultrananocrystalline diamond (N-UNCD) films to evaluate oxygen terminated N-UNCD as a biocompatible photoactive surface for neural stimulation. The oxygen annealing time is optimized to gain the maximum electrochemical capacitance for photoelectrodes, and the electrochemical properties of samples are investigated. Moreover, the electrochemical capacitance of N-UNCD samples oxygen terminated with different techniques is measured and compared with the oxygen annealed sample. The oxygen annealed sample exhibits the greatest electrochemical capacitance and can be optimized to reach a value of about 30 mF cm-2, 6 times higher than other techniques used in this thesis and also previously reported Pt electrodes and comparable to sputtering iridium oxide electrodes. This enhancement is suggested to be due to a combination of factors, including oxygen surface functionalities, graphitic grain boundary etching, and the removal of trans-polyacetylene (TPA) and hydrogen from the sub-surface layer during the oxygen annealing process. N-UNCD exhibits a photoresponse at longer wavelengths, hence it is possible to employ Near-infrared (NIR) light for photoexcitation. NIR light has a higher penetration depth and less phototoxicity than the lower wavelength which makes N-UNCD a favourable material for in vivo photostimulation. The oxygen annealed N-UNCD, which displays a very high surface capacitance, is evaluated in terms of its photoresponse to NIR light. Under optimal conditions, a capacitive photocurrent of 3.7 uA/W is achieved, higher than previously reported photocurrent values of optically driven N-UNCD electrodes. This translates to an approximate 200 times increase in the photocurrent compared with the as-grown sample. Surface sensitive spectroscopy techniques reveal that these orders of magnitude enhancement in photocurrent are due to the formation of a diamond-rich capping layer as the result of preferential etching of graphite at the grain boundaries. It has been suggested that the surface treatments in reactive oxygen resulted in changes in the surface functional groups, which modulate the surface Fermi level. These results hold significance for applications of oxygen terminated N-UNCD photoelectrodes for neuromodulation applications. For neuromodulation, the surface of the electrode must support neuronal growth as well as had perfect biostability. The stability test results show that oxygen annealed N-UNCD photoelectrodes have remarkable stability when stimulated over many cycles in saline. Moreover, the surface of oxygen annealed N-UNCD displays significant biocompatibility and encourages neuron growth without the necessity of promoters, indicating that it is highly biocompatible. When studying the neuronal growth on oxygen annealed N-UNCD surfaces, light stimulation is found to greatly enhance neuronal growth, with better survival rates and improved neurite outgrowth. As the light illumination does not show a significant impact on the control samples, the improved neuronal growth on oxygen annealed N-UNCD films is tentatively concluded to be due to their photoelectric responses. This is the first evidence that implies the potential for using the photoresponse from oxygen terminated N-UNCD to improve neuronal growth. These results suggest that light could be used to direct the growth of specific neural networks. Possibly in the future oxygen terminated N-UNCD photoelectrodes could find applications for neural network regeneration and nervous system repair.
• Item
A spectroscopic and chromatographic study of the photochemical properties of daylight fluorescent paint
Hinde, Elizabeth ( 2009)
Daylight fluorescent pigments fade rapidly, accompanied by a chronology of colour change. Fluorescence is a photo-physical phenomenon which involves emission of light from an excited state. Fluorescent dyes thus have a high susceptibility of being promoted to an excited state; a characteristic in the case of organic fluorophores which infers vulnerability toward photo-bleaching. Multiple organic fluorescent dyes are routinely incorporated into a given daylight fluorescent pigment, to either additively fluoresce or interact through energy transfer. The organic fluorescent dyes employed invariably differ in photo-stability, and upon loss of each species of fluorophore an abrupt colour change is observed. The collective result of this fading behaviour is that in a short period of time a daylight fluorescent paint layer will be of a different hue, devoid of luminosity. As consequence it is almost impossible to colour match a faded daylight fluorescent paint layer without the hues diverging asynchronously, or ascertain the original palette of a daylight fluorescent artwork after a protracted period of time. The predicament is exacerbated by the fact that there is no standard method in cultural material conservation, of documenting daylight fluorescent colour in a painting photographically or colorimetrically. The objective of this thesis is to investigate the photochemical behaviour of daylight fluorescent pigments, to ensure best practice in the preservation of artworks that contain daylight fluorescent paint. Fluorimetrie and chromatographic analysis of the DayGlo daylight fluorescent pigment range at the constituent dye level, prior to and during an accelerated light ageing program formed the basis of the experimental. Given the limited selection of fluorescent dyes suitable for daylight fluorescent pigment manufacture, it is anticipated that the results attained for the DayGlo range will be applicable to all daylight fluorescent media encountered in cultural material. Experimental data revealed the manner in which the fluorescent dyes behind each DayGlo daylight fluorescent pigment were formulated, and provided explanation for the 1colour changes observed upon fading. A prognosis of when and why a daylight fluorescent palette experiences hue shift and the implications this has for display is presented. Methodology for imaging daylight fluorescence, identification of the constituent fluorescent dyes in a daylight fluorescent pigment and colour matching a daylight fluorescent paint layer are presented and applied in-situ, to case studies possessing a daylight fluorescent palette.