School of Physics - Theses

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Type: PhD thesis × Start date: 2021 × End date: 2021 × Subject: Diamond × Subject: Nitrogen vacancy ×

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    An all-optical voltage imaging platform using charge-sensitive fluorescent defects in diamond
    McCloskey, Daniel ( 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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    Development of widefield nitrogen-vacancy centre microscopy for application to condensed matter systems
    Lillie, Scott Eric ( 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.