Tyndall National Institute - Doctoral Theses

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    Laser fabrication of porous, 3D graphene-like carbon from polyimide and sustainable bioplastics
    (University College Cork, 2022-06) Larrigy, Cathal; Quinn, Aidan J.; Iacopino, Daniela; Science Foundation Ireland
    Laser-induced graphitization (LIG) of materials provides a unique and advantageous method for fabrication of 3D, porous, conductive, carbon structures with high surface area. By using laser-irradiation of target substrates, conductive patterns such as electrochemical or chemiresistive sensing elements can be fabricated additively, thus reducing both the energy footprint and cost. However, typically LIG is fabricated on non-sustainable feedstock substrates, most commonly polyimide. Studies toward using alternative and more sustainable substrates have been researched, and in this thesis one such alternative is explored, chitosan-based laser-induced graphene. This thesis aims to show the possibilities of laser-induced graphene fabrication, in examining first the fabrication of LIG on polyimide using a low-cost hobbyist 405 nm laser engraving machine, as an alternative low energy intensive fabrication method. Following this the possibility of using a more sustainable substrate as a target precursor for laser-induced graphitization in the form of a chitosan-based LIG. Finally, a potential application using chitosan-based LIG was demonstrated in the form of chemi-resistive 2-terminal impedance devices for sensing humidity and volatile organic compounds.
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    Micro-transfer printing of micro-structured, ultra-thin light-emitting devices
    (University College Cork, 2023-03) Shaban, Zeinab; Corbett, Brian; Parbrook, Peter James; Science Foundation Ireland
    3D integration of optoelectronic devices is a crucial future technology, which can be applied in the areas of photonic integrated circuits, flexible displays, communication and more. Among the various technologies, micro-transfer printing has emerged as a precise and cost-effective way to assemble devices for 3D integration. To enable this technology, devices must be released from their native substrates, which open up a lot of possibilities. It can achieve integration with flexible or heat-conductive backplanes, as well as heterogeneous integration of multiple materials on a common platform, resulting in miniaturised chips. Also one can benefit from reclaiming and reusing the original substrates to reduce the production cost significantly. On the other hand, GaN devices exhibit unique optical properties in optoelectronics compared to other semiconductors, and GaN-based LEDs have established themselves in a variety of applications, due to their low power consumption, long lifetime, short response time, and high brightness. This thesis has focused on releasing high performance GaN LEDs and addressing their associated issue for micro-transfer printing. The first part of this work is focused on releasing and transfer-printing of GaN LEDs grown on Si substrate. There are several factors that limit the performance and manufacturing of GaN LEDs on Si. One issue is related to the deformation of the released coupons due to their high inbuilt strain, which could result in transfer-printing failures as well as challenges during the post-print integration process. To address this issue, COMSOL software was used to study the stress effect on the devices. Experimentally, the intrinsic deformation of the released LEDs was compensated by using compressed SiNx layers that resulted in flat devices after release. Another issue is related to the low light extraction for GaN LEDs on Si. To solve this problem, the underside of the released LEDs is roughened during the coupon preparation process prior to transfer printing. Furthermore, using the unique properties of transfer printing, the roughened LEDs are printed inside a fabricated reflective trench with 10 μm depth to direct the light to the surface normal. Results showed that roughening along with the reflective trench increased the collected power by a factor of ∼ 7 compared with LEDs on the original substrate. A second part of this study examines the release of GaN-based structures from substrates (i.e. sapphire or bulk GaN) by photoelectrochemical (PEC) etching when pure chemical etching is not possible. A sacrificial layer which can obtain smooth etch surfaces and uniform etching with high selectivity is needed. Also, from the perspective of transfer printing, thick rather than thin sacrificial layers are preferred to facilitate the releasing and picking process. In this work, 300 nm-thick releasing layers comprising of InGaN/AlInN stacks are proposed for PEC etching. The presence of two-dimensional hole gas at the interface of InGaN/AlInN due to the strong polarization field are indicated by modelling and capacitance-voltage measurement. This resulted in a smoother surface with a three times higher etch rate compared to the conventional InGaN/GaN superlattice structures used for PEC etching. Moreover, various electrolytes and post-PEC treatment were studied to improve the surface smoothness. Further work should be done to determine the impact of the adhesion layer in transfer printing on heat generation and device performance. Using the optimized sacrificial layer to release other structures like lasers should also be investigated.
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    Development of electromagnetic vibration energy harvesters as powering solution for IoT based applications
    (University College Cork, 2022-09-20) Paul, Kankana; Roy, Saibal; Amann, Andreas; Kennedy, Peter; Science Foundation Ireland; Horizon 2020
    The drive towards building pervasive intelligence encompassing urban as well as rural environments has paved the way for the Internet of Things (IoT), which has reshaped our regular lifestyle alleviating the dependence on wired communication systems since its inception. The inexorable advancement in low to ultra-low power electronics have steered the rapid growth of the IoT platform expanding into several application fields. With the ongoing implementation of 5G (Fifth generation) and the emergence of 6G (Sixth generation) wireless technology on the horizon, the explosive growth of IoT connected devices reinforces the requirement of a robust and reliable power solution for the deployed wireless communication platforms. Utilizing distributed clean energy sources, especially the ubiquitous mechanical energy available in environment through dedicated transducers in the form of vibration energy harvesters (VEHs) to power the IoT-based wireless sensor platforms is a sought after alternatives to batteries in the forthcoming IoT applications. The potential of the resonant/linear VEHs have been limited owing to the narrow operable frequency bandwidth as well as due to the lack of intelligent device designs that aids to yield large electrical power from the provided mechanical energy. In this thesis, a concertina shaped linear VEH spring architecture has been exploited to instigate large amplitudes of oscillation, which aids to yield a high power density (455.6μW/cm3g2) at resonance from a relatively small device footprint. From the application perspective, this concertina-VEH has been utilized to power the electronics interface and enhance the performance of a NFC (Near Field Communication) based wireless sensor platform which offers the benefits of low power consumption and on call data acquisition through this short range NFC based communication protocol. Such a robust autonomous wireless sensing platform offers the potential to be used in a large number of IoT based applications. Despite of the large deliverable power obtained from the resonant VEH, the energy extraction drops dramatically as the excitation frequency deviates from the resonance condition, which is inevitable owing to the random nature of vibrations. A novel broadband VEH with tapered spring geometry has been developed as a part of this thesis to address this issue. Nonlinear restoring forces arising from the stretched springs enables the VEH to generate large power over a considerably wide bandwidth (45Hz of hysteresis width that is the difference of the jump down and jump up frequency with 1g excitation amplitude) of operable frequencies. Suitable power management strategies have been proposed to enhance the energy extraction capabilities. The nonlinear VEH has been successfully used to harness mechanical energy from the broadband vibrations of a car; the extracted energy is fed to a wireless sensor platform that reports on ambient temperature and humidity. This self-powered sensing system opens up the scope for exploiting this technology for monitoring food and medicinal quality during transportation while the VEH extracts mechanical energy from the transporting vehicle and perpetually powers the wireless sensor node. Multiple nonlinearities arising from the stretching of the VEH spring as well as from the interaction of repulsive magnets have been introduced into the energy harvester, which gives rise to coexisting multiple energy branches. Not all of these energy states are achieved through the typical excitation frequency routine, some of these energy states are rather hidden. Experimentally a route to achieve these hidden energy branches have been explored in this work. Suitable frequency routines have been designed to achieve and sustain these higher energy states. A useful graphical representation has been introduced in the form of ‘eye diagrams’ that essentially estimates the transaction of energy from mechanical to electrical domain, and provides deep insight of the dynamical features of each energy branches, based on time resolved measurements of acceleration and voltage. A mathematical model has been developed to investigate the intricate complexities of the nonlinear system, which supports the experimental findings. One of the major impediments in miniaturizing high-efficiency macroscale VEHs into MEMS (Micro-Electro-Mechanical-System) scale is the lack of matured technology for the CMOS (Complementary-Metal-Oxide-Semiconductor) compatible integration of magnets and the adverse effect of scaling on the permanent hard magnets. A part of the presented work investigates the effect of patterning continuous thin films of magnets into micromagnet array. With detailed analytical framework and exhaustive finite element analysis, the shape, size and distribution of these micromagnets have been optimized to maximize the stray magnetic field emanating from each edge of these magnets. Novel MEMS device topologies comprising of linear/nonlinear MEMS springs, micromagnet arrays and copper microcoil have been proposed which systematically maximizes the electromagnetic interaction between the micromagnets and the integrated coil that in turn translates into large deliverable power. In addition to the developed device prototypes and demonstrations, this thesis further provides a firm roadmap that highlights the potential routes for enhancing the energy harvesting capabilities through highly integrated MEMS scale VEHs as well as for improving system level integration to establish these VEHs as a reliable and sustainable alternative of batteries in IoT applications.
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    Development and characterisation of macro-disc and micro-band electrodes for electrochemical sensing applications
    (University College Cork, 2022) Madden, Julia; Galvin, Paul; O'Riordan, Alan; Thompson, Michael; Science Foundation Ireland; Electronic Components and Systems for European Leadership
    The aim of this PhD thesis was to investigate potential next generation sensor platforms for electrochemical biosensor developments, specifically towards health monitoring applications. With increasing interest in the integration of miniaturised electrodes with minimally invasive and wearable devices, this thesis sought to explore electrodes fabricated using three different technologies for the construction of electrochemical biosensors: Silicon microfabrication, Laser scribing, and dispense printing. The first experimental section aimed to investigate the use of a single ultramicroband electrode fabricated on silicon for mediator-free glucose monitoring in bio-fluid environments. Six ultramicroband electrodes, a counter electrode and reference electrode were fabricated using standard microfabrication methods i.e. lithography and etching techniques. Glucose oxidase was selected as a model enzyme to attach onto a platinum modified gold microband electrode by electropolymerisation with an o-phenylenediamine/ß-cyclodextrin layer. The resulting microband biosensor demonstrated on-chip glucose detection in buffer based media. When applied to foetal bovine serum the sensor displayed a reduced sensitivity. The second experimental section explores the use of laser-scribed graphitic carbon for flexible sensing applications. A facile fabrication method was assessed involving electrodeposition of platinum followed by two casting steps to functionalise electrodes. This study examined the chronoamperometric response of the enzymatic lactate sensor whilst the flexible polyimide substrates were fixed at a curvature (K) of 0.14 mm-1. No noticeable change in signal response was observed in comparison to calibrations obtained with a flat substrate (K=0 mm-1), suggesting potential opportunities for sensor attachment or integration with oral-care products such as mouth swabs and mouth guards. Both laser scribed graphitic carbon and Ag/AgCl modified-laser scribed graphitic carbon were examined as reference electrodes for chronoamperometric lactate measurements. This device was applied for measuring lactate concentrations in artificial saliva and diluted sterile human serum. Finally, this study investigates the potential for a low cost additive printing tool to enable the fabrication of electrochemical sensor devices. To do this, electrodes were designed and printed onto polyimide substrates. Reproducibility between electrode dimensions was assessed using 3D microscopy. Standard electrochemical characterisation techniques were employed to study the reproducibility between electrode electrochemical response. Functionality was also assessed whilst electrodes were fixed were fixed at a curvature (K) of 0.14 mm-1. Finally, a simple casting approach was applied to the dispense printed working electrode to construct a lactate biosensor for a proof of concept electrochemical sensor demonstration.
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    Atomistic simulation and analysis of novel group IV semiconductor alloys and devices
    (University College Cork, 2023-01-01) Dunne, Michael D.; O'Reilly, Eoin P.; Broderick, Christopher; Schulz, Stefan; Science Foundation Ireland
    A long held goal of the semiconductor community is the development of a direct gap silicon (Si) compatible material to enable the seamless integration of optical and electronic components on a single chip. The dominant elemental group-IV semiconductors silicon and germanium are the mainstays of current microelectronics, but their fundamental indirect gaps pose a roadblock to the development of active photonic components. The alloying of germanium with other group-IV elements, such as tin or carbon, has come into focus in recent years in pursuit of developing a direct gap alloy. Band engineering of germanium is attractive owing to the small difference between the indirect L6c-Γ8v and direct Γ7c-Γ8v band gaps of germanium which is only 140 meV. Alloying opens the possibility of reducing the Γ7c state below that of the L6c state leading to a direct gap alloy. Initial work on Ge1−xCx alloys have predicted the formation of a direct gap upon incorporation of dilute quantities of C (<1%), though there has not yet been an experimental demonstration of direct gap behaviour. Ge1−xSnx alloys have attracted greater research interest owing to the experimental demonstration of direct gap behaviour for a range of Sn compositions. Previous theoretical work suggested the transition from indirect to direct band gap occurs in a composition range of 6-11% Sn, while recent research indicates that a direct band gap emerges continuously with increasing x due to alloy band mixing. This atomistic effect, which is neglected in the widely-employed virtual crystal approximation (VCA), results in the alloy conduction band (CB) edge possessing hybridised character that evolves continuously from indirect (Ge L6c-like) to direct (Ge Γ7c-like) with increasing x. In this thesis we present a theoretical analysis of electronic structure evolution in the highly- mismatched dilute carbide group-IV alloy Ge1−xCx by adopting an atomistic approach encompassing calculations of the electronic structure using the semi-empirical tight-binding method. We demonstrate that C incorporation strongly perturbs the conduction band (CB) structure by driving hybridisation of A1-symmetric linear combinations of Ge states lying close in energy to the CB edge. These calculations describe the emergence of a “quasi-direct” alloy band gap, which retains a significant admixture of indirect Ge L-point CB edge character. The trends identified by our calculations are markedly different to those expected based on a recently proposed interpretation of the CB structure based on the band anti-crossing model. For Ge1−xSnx alloys we are interested in the impact of the previously overlooked alloy effects have on the band to band tunneling in the alloy. We achieve this using non-equilibrium Green’s function (NEGF) band-to-band tunneling (BTBT) calculations based on atomistic tight-binding electronic structure calculations. We then extend this analysis to look at the effect of Sn incorporation on the current characteristics of TFET devices. We demonstrate that CB mixing strongly modifies the complex band structure, driving complex band anti-crossing that – for Sn compositions at which the band gap is assumed indirect in the VCA – strongly increases the BTBT generation rate G. Our results highlight the importance of atomistic effects in determining the electrical properties of Ge1−xSnx alloys