School of Physics - Theses

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End date: 2021 × Start date: 2021 × Type: PhD thesis × Subject: Color centre × 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.