School of Physics - Theses
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ItemDeveloping and applying quantum sensors based on optically addressable spin defects( 2023-04)Quantum sensing aims to further our understanding of the natural world and support an upcoming technological revolution by exploiting quantum properties or systems to exceed the performance of classical sensing. Owing to their convenient modes of operation and strong room temperature quantum properties, optically active spin defects hosted within solid state materials have come to prominence as one of the foremost tools of choice in this landscape. Many applications now aim to leverage dense ensembles of such defects to boost measurement sensitivity or scale up, which places greater emphasis on the quality of the host material and sensor production methods since cherry-picking individual defects is no longer an option. The prototypical example of such a defect is the nitrogen-vacancy (NV) centre in diamond, which exhibits remarkable room temperature spin coherence, bestowed upon it by diamond's material properties. In this thesis, we first look at optimising the production of NV ensembles for quantum sensing, aiming to efficiently and cost-effectively produce sensors capable of performing high sensitivity measurements in two key regimes that will be central to the experimental applications explored later. The topics examined are hyperpolarisation of a nuclear spin ensemble on the diamond surface through coupling to an ultra-near-surface NV layer, and investigating the properties of a van der Waals antiferromagnet through widefield NV microscopy. The demands placed on the NV layer for these applications are diverse from one another, with charge stability and quantum coherence properties being vital for the former, and the ability to scalably and reproducibly create layers of known thickness crucial to the latter. In light of these studies, we finally consider whether a different spin system housed within an entirely separate materials system, the boron-vacancy defect in hexagonal boron nitride, may be a suitable alternative to the well-established NV diamond system. We find that the distinct properties of the new host material provide both advantages and disadvantages compared to diamond, and that this system could allow quantum sensing to find even broader scope in the future. By investigating the link between host material properties and the suitability of a quantum sensor for given applications, this thesis provides a unique perspective on the future of the field, which will likely demand more highly specialised and varied sensors.
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ItemFlexible electrodes for neural recording, stimulation and neurochemical sensing( 2020)This thesis focuses on the development of implantable neural interfaces to perform multifunctional neural recording, neural stimulation and biochemical sensing. Neural interfacing devices using penetrating electrodes have emerged as an important tool in both neuroscience research and medical applications. These implantable electrodes enable communication between man-made devices and the nervous system by detecting and/or evoking neuronal activities. Recent years have seen a rapid development of electrodes fabricated using flexible, ultrathin microwires/microfibers. Compared to the arrays fabricated with rigid materials and larger cross sections, these microwires/microfibers have been shown to reduce foreign body response after implantation, with improved signal-to-noise ratio for neural recording and enhanced resolution for neural stimulation. Carbon fibers (CFs) are considered for implant into particular tissue types since they have small size, cause less tissue damage, and are flexible. CF recording electrodes have shown promise as recording electrodes and have the properties necessary to form sensing electrodes. Micron-scale electrodes such as CFs are expected to evoke localized neural responses due to localized electric fields. CFs are traditionally used with fast-scan cyclic voltammetry to study rapid neurotransmitter changes in vivo and in vitro, as they allow real-time detection of catecholamines with high sensitivity and selectivity. However, they possess narrow usable voltage range, which limits their application for neural stimulation. Additionally, surface fouling occurs with certain neurochemicals potentially obstructing further neurotransmitter adsorption onto the electrode surface. Therefore, they need to be coated with other materials to boost their electrochemical properties for neural stimulation. In this thesis, diamond and diamond-like materials, in particular nitrogen doped ultrananocrystalline diamond (NUNCD) hybrid and boron doped carbon nanowall (B-CNW) are considered as coatings for CFs to enhance properties towards neural interface applications. A focus is finding acceptable properties for recording, stimulation and neurochemical sensing. Novel fabrication techniques were developed to deposit the films onto the surface of CFs. Firstly, the surface of CFs was amine-functionalised and covalent bonds were formed with oxygen terminated nanodiamonds. Films were grown on the treated/seeded fibers using plasma-assisted chemical vapor deposition. To fabricate single fiber electrodes, individual fibers were insulated with capillary glass with 100 micrometer of fiber exposed. The physical and chemical properties of NUNCD hybrid and B-CNW were characterized and studied. The results from electrochemical characterization, in conjunction with both in vitro and in vivo assessments, suggest that these electrodes offer a highly functional alternative to conventional electrode materials for both recording and stimulation, yielding safe charge injection capacities up to 25.08 +-12.37 mC/cm2. To test the capability of electrodes for neural stimulation in vitro, explanted wholemount rat retina was used. The electrodes could elicit localized stimulation responses in the explanted retina. These electrodes with micron -scale cross sections have the potential to improve the spatial resolution for stimulation while minimizing axon bundle activation. In vivo and in vitro single-unit recording showed that the electrodes could detect signals with high signal-to-noise ratios up to 8.7. NUNCD hybrid coated CFs were able to electrochemically detect dopamine with high sensitivity and selectivity. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.