School of Physics - Theses
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ItemNo Preview AvailableAn all-optical voltage imaging platform using charge-sensitive fluorescent defects in diamond( 2021)The understanding of electrogenic cells and their networks in vitro plays a major role in the development of therapies targeting maladies of the cardiovascular and nervous systems. Continued advancement of this understanding relies on the use of tools which can measure electrical potentials with ever-increasing spatio-temporal resolution and scale. However, present voltage imaging technologies cannot simultaneously achieve sub-cellular resolution while recording over whole-network spatio-temporal scales, which leads to incomplete descriptions of network function and dysfunction. In this thesis, we report on the early stage development of a fundamentally new tool for quantitative voltage imaging which can overcome the physical limitations of the present state of the art. The central goal of the work is to investigate the extent to which charge-state transitions of fluorescent point defects in wide-bandgap semiconducting materials can be exploited to image bio-electrical activity in vitro. Here, we use the nitrogen-vacancy centre in diamond as an experimental platform for exploring this topic. We describe the design, fabrication, and testing of a novel voltage imaging chip technology which relies on optically detecting the charge-state conversion of near-surface nitrogen-vacancy ensembles in response to electrical potentials in solution. These ensembles are localised within nanoscale diamond p-n junctions and embedded within the tips of diamond nanopillars, which we refer to as ‘optrodes’ in analogy with standard electrodes. Through several generations of diamond optrode arrays, voltage sensitivities are increased by more than three orders of magnitude. The work culminates in a device possessing a voltage sensitivity only one order of magnitude less than that of commercial high-density multi-electrode array systems, and we calculate that the sensitivities of future devices can be improved beyond that of state-of-the-art technologies. Concurrent to the development of physical devices, a nonlinear Poisson solver was constructed to study the effects of dopant concentrations and depth distributions, as well as the effects of diamond surface defects, on voltage sensing performance. Informed by this modelling, a new method for precisely tuning the surface chemistry of diamond for optimal charge-state sensing was developed, enabling the fabrication of a device containing high-density nitrogen-vacancy ensembles stabilised entirely in the neutral and positive charge-states. To our knowledge, this is the first ever realisation of high-density nitrogen-vacancy ensembles exclusively comprised of positive and neutrally-charged defects. The thesis concludes with a discussion of known and possible pathways for increasing the performance of future devices. In addition, we highlight some possible directions for fundamental research which may help to further advance this emerging technology. This work establishes the feasibility of using charge-sensitive fluorescent defects in diamond for all-optical imaging of electrical potentials with both high spatial resolution and over large spatial and temporal scales.
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ItemA Study of Silicon on Diamond( 2021)In modern-day technology, silicon and silicon-based materials play a key role in the production of computing parts, specifically, the transistors within the chips. The exponential densification of transistors has caused the excess heat generated during operation to significantly hamper chip performance. This has led to the rise of hyperbolic language which refers to this problem as the `silicon apocalypse', and the material as `dark silicon', on account of each chip mostly being turned off to limit heating. The focus of this work lies in creating a silicon-based heterostructure with diamond, which has an unparalleled thermal conductivity, to effectively extract and dissipate heat directly from the chip. To realise such a structure, this work explored the usage of single crystal diamond as a substrate for silicon growth. As diamond had been previously grown on silicon carbide with limited coherence, shown in literature by other authors, the work begins by investigating the interface between 3C-SiC and a diamond substrate both experimentally and theoretically. It was found that 3C-SiC could form a coherent interface with diamond across several nanometres and that, through atomistic simulation, it was possible to realise greater regions of coherence. In the fabrication process, which involved heating thermally deposited silicon within an ultra high vacuum chamber, it was discovered that thin film dewetting became a major hurdle for long range coherence. This dewetting phenomenon and the behaviour of silicon on diamond during annealing then became the second major focus of this thesis to better understand how, or if, a coherent interface could be achieved between these two materials. To diminish the possibility of dewetting, which occurs with very thin films, PECVD silicon was chosen as an accessible method to time-efficiently deposit thicker layers. In spite of the benefits of PECVD silicon, crystallisation through annealing did not yield single crystal silicon. Instead, minimal interaction was found below 600C between the diamond substrate and silicon, which exhibited behaviour in line with PECVD silicon on other substrates as reported in the literature. However, at elevated temperatures around 1000C, a novel form of diamond etching was observed. The results gleaned from this work indicated that 3C-SiC/diamond heterostructures with a coherent interface could be produced, and thus, future applications are possible. The formation of a silicon and diamond heterostructure, however, was hampered by thin film dewetting and so further testing is needed to determine whether a coherent interface could be achieved using processing techniques applied in the case of 3C-SiC, but with thicker layers. By utilising a more accessible deposition method, namely PECVD, thicker layers became readily available on diamond substrate. Under thermal treatment, the PECVD silicon was found to crystallise into a polycrystalline layer, and at temperatures exceeding 1000C, even begin etching the diamond substrate. These series of studies bring to light several fundamental details of a silicon and diamond interface that are a prerequisite to the utilisation of diamond substrates in the future, especially with silicon-based materials.
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ItemDevelopment of widefield nitrogen-vacancy centre microscopy for application to condensed matter systems( 2021)Microscopy based on the nitrogen-vacancy centre in diamond is an emerging technology that may soon find regular application in characterising a broad range of systems. The development of nitrogen-vacancy microscopy has been benchmarked by key proof-of-concept experiments, each demonstrating the great potential of the technique, without necessarily delivering unique insights into the system studied. In this thesis, we aim to further the development of this emerging technology by applying it to study a range of delicate and complex condensed matter test systems. Specifically, we take a widefield approach which utilises a dense ensemble of near-surface nitrogen-vacancy centres to image magnetic and electric fields over a 100 um field of view with diffraction-limited resolution and good sensitivity. In each application presented, we study physics both intrinsic to the test systems and arising from their interaction with our imaging technique. These observations inspire refinements to our approach that enhance the utility of the technique. Chapter 1 reviews the physics underpinning microscopy with nitrogen-vacancy centres in diamond, and surveys key achievements in the literature. Chapter 2 details our approach to widefield nitrogen-vacancy microscopy. Chapter 3 applies widefield magnetic imaging to ultrathin depositions of nanoparticles on the diamond surface. We observe a strong magnetic noise associated with metallic depositions, even when the deposited material is less than 1 nm thick. This study demonstrates the great sensitivity of widefield nitrogen-vacancy microscopy, allowing the detection of minute sample volumes. Chapter 4 studies magnetic and electric fields associated with graphene field-effect transistors fabricated on the diamond surface. We demonstrate current density mapping under different doping conditions, a significant extension of device studies published at the time, and identify strong electric fields associated with the gating that affect both the spin resonances and photoluminescence of the sensing ensemble. These first two studies motivate the use of a deeper and thicker nitrogen-vacancy ensemble that protects against the short-range noise contributed by the metallic nanoparticles, which mimic common contaminants, and protects the sensing layer from device-related electric fields. Chapter 5 extends our widefield imaging technique to low temperatures (4 K) by incorporating a closed-cycle cryostat into the apparatus, which we use to study a low-critical-temperature system, superconducting niobium wires. By imaging Abrikosov vortices and superconducting transport at various laser powers, we quantify local heating in the sample caused by the excitation laser. This observation motivates a refined nitrogen-vacancy imaging substrate that includes a reflective metallic layer at the diamond surface and an insulating oxide layer, onto which the target sample is deposited. This design mitigates the local heating caused by the excitation laser, an essential component of the imaging technique, and leaves the sample electrically isolated, facilitating advanced studies of low-temperature physics. Chapter 6 deploys this refined nitrogen-vacancy substrate to characterise the magnetic properties of two free-standing ultrathin ferromagnetic materials. We directly quantify the layer-dependent magnetisation of a van der Waals ferromagnet, VI3; a capability lacking in established characterisation techniques. Finally, we identify a threshold size (20 nm) below which a non-layered ultrathin material, magnetite, ceases to show ferromagnetism. This thesis furthers the development of widefield imaging using nitrogen-vacancy centres in diamond, enabling a range of future applications.
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ItemDiamond Quantum Sensing in Biological Systems( 2020)Biological discovery is fuelled by technological advances. As a new process is developed, a period of exploration tests the waters to uncover what the novel technology can reveal. This thesis presents the application of quantum sensing with negatively-charged nitrogen vacancy (NV) centres in diamond to real biological systems and questions. NV sensing provides a means to probe systems, including biosystems, in ways unavailable with other techniques. This is most evident in the NV’s magnetic sensitivity, along with a collection of attributes that lend it to biological settings. Many questions about biomagnetism remain unanswered, and the advent NV sensing affords a new avenue to explore these questions. In this thesis, we begin by establishing the techniques and capabilities of NV magnetic imaging in diamond. An introduction to magnetometry precedes a description of diamond, the properties of the NV centre, and quantum measurement protocols. We then dive into an animal model of iron biomineralisation, a sea mollusc with extraordinary teeth. Biomineralisation is an area of current interest that has previously received little attention from the angle of its intrinsic magnetic properties. Next, we see how nitrogen vacancy sensing can be applied to the improvement of magnetic tools used in the biosciences. These tools are seeing an explosion of new activity across multiple disciplines, so ways to evaluate them will prove valuable. Then, we examine the enigmatic mechanism behind animal magnetoreception. A remarkable sense known to be possessed by many disparate animal species, magnetoreception remains a near complete mystery. Finally, we consider the challenges and limitations of diamond-based temperature sensing in biological systems. This interdisciplinary endeavour has brought about exciting results including the first subcellular magnetic profiling of a eukaryotic system, vector magnetic images of developing biominerals, new protocols for the assessment of magnetic materials and avenues for new bioassays, and a window to a recently-discovered organelle with a suggested role in animal magnetoreception. With the current trajectory of quantum sensing with NV centres in diamond, the future for biological discovery looks bright.