Publications

@phdthesis{Gunnarsdóttir2020b,
title = {Nuclear Magnetic Resonance Studies of Lithium Metal Anodes},
author = {Anna B Gunnarsdóttir},
url = {https://www.repository.cam.ac.uk/handle/1810/315711},
year = {2020},
date = {2020-12-14},
abstract = {Lithium metal has received a renewed interest as a promising anode material for next-generation, high-energy batteries owing to its high specific capacity (3860 mAh g-1) and low reduction potential (-3.04 V vs. the standard hydrogen electrode). However, lithium metal batteries suffer from low capacity retention, short cycle life and safety problems associated with microstructural and dendritic growth of lithium. In this work, nuclear magnetic resonance (NMR) spectroscopy is used to understand the effect of the solid electrolyte interphase (SEI) on lithium metal deposition. In situ NMR is used to quantify the lithium microstructures formed during plating and allows the current efficiency and porosity of the structures to be estimated. The effect of the fluoroethylene carbonate (FEC) additive is explored along with a range of plating conditions. NMR measurements show that the isotope exchange between a 6Li-enriched lithium metal and a natural abundance electrolyte depends significantly on the electrolyte and the corresponding SEI. A numerical model is developed to describe the processes during isotope exchange and is discussed in the context of the standard model of electrochemical kinetics. The model is used to extract both an exchange current at the open circuit voltage and the SEI formation current as a function of time. In situ NMR methods are then developed to study ‘anode-free’ lithium metal batteries where the lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The low cycling stability of lithium metal batteries becomes clear when there is no excess of lithium in the cell. The ‘dead lithium’ and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Importantly, the NMR experiments reveal that the dissolution of lithium metal during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. Strategies to mitigate lithium corrosion are explored; the work demonstrating that both polymer coatings and the modification of the copper surface chemistry stabilising lithium metal. Overall, this work demonstrates that the NMR approach offers unique insight into the dynamic processes occurring on lithium metal both during electrochemical measurements and at the open circuit voltage.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Muench2020,
title = {Photodetectors for graphene-based integrated photonics},
author = {Jakob Muench},
url = {https://www.repository.cam.ac.uk/handle/1810/316811},
year = {2020},
date = {2020-09-01},
abstract = {The development of integrated optical circuits has enabled a diverse portfolio of chipscale photonic applications—ranging from data communication over sensing to imaging—that is set to grow further as new device concepts in, for example, quantum information processing and optical neural networks mature. While silicon photonics has emerged as a viable candidate to translate proof-of-principle demonstrations to mass-manufacturing, the fabrication of photonic integrated circuits (PICs) and their subcomponents remains highly heterogeneous, which drives up cost and slows down progress towards faster and more power-efficient performance. In this dissertation, I demonstrate how single-layer graphene (SLG) can bring active functionality to arbitrary passive waveguide platforms, offering a more universal approach to developing integrated photonic components. SLG can be transferred to PICs without the limitations or complexity of traditional deposition or bonding processes. It also stands to outperform conventional semiconductors in terms of speed and spectral operating range due to its ultra-fast carrier dynamics, high carrier mobility, and gapless bandstructure. Using the key component for optical-to-electrical signal conversion—the photodetector—as an example, here I demonstrate graphene-based components on three different PIC platforms. First, I show plasmonic enhanced graphene photodetectors (GPDs) on silicon nitride waveguides, which generate a voltage from optically generated hot-carrier distributions in SLG via the photo-thermoelectric (PTE) effect. Then, I demonstrate PTE-GPDs—fabricated from layered material heterostructures—on a silicon-on-insulator microring resonator. Both detectors operate at Telecom wavelengths (λ = 1.55 μm), are compatible with high-speed (> 10 GHz) operations, and hold the current records in voltage responsivity—R ∼ 12 V/W and R ∼ 90 V/W—for waveguide-integrated GPDs fabricated from chemical vapour deposited and mechanically exfoliated SLG, respectively. I then go on to show how established GPD concepts can be translated to an integrated mid-infrared (λ = 3.8 μm) platform based on sub-wavelength grating waveguides in silicon and study how light-graphene interaction under in-plane incidence can be further optimised to improve GPD performance. Finally, I develop new approaches towards high-quality, scalable SLG on PICs, critically required to advance graphene-based integrated photonics towards industrial production.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Crocker2020,
title = {Design and Application of Nanostructured Flavin Photocatalysts},
author = {Leander Crocker},
url = {https://www.repository.cam.ac.uk/handle/1810/315161},
year = {2020},
date = {2020-08-01},
abstract = {Flavin compounds are found in Nature within a range of flavin-containing enzymes (flavoenzymes) that are responsible for metabolic, antioxidant and photoreception processes across animal, plant and bacteria kingdoms. Their broad redox and photochemistry has seen the field of flavin-based catalysis emerge as a powerful addition to sustainable catalysis thanks to being cheap, non-toxic and highly active. Flavin photocatalysis in particular has shown great promise in a range of synthetic procedures such as benzylic oxidations, sulfoxidations, decarboxylative transformations, isomerisations and [2+2] cycloadditions to name a few. It has been shown that flavin photocatalysts can be tuned to a specific application either through chemical structure modification or by the design of advanced systems through heterogeneous or polymer carrier attachment. The latter has shown encouraging results to widen flavin photocatalyst applicability within organic synthesis but has lacked in displaying characteristics like flavoenzymes which enable highly efficient and selective catalysis of industrially relevant products with precise stereochemical control. This thesis details the design and application of novel flavin photocatalysts through the combination of polydopamine (PDA). It is shown that PDA not only acts as a carrier of the flavin, but actively engages in the catalytic mechanism and improves flavin photostability. In the first instance, copolymer flavin-polydopamine (FLPDA) nanoparticles were synthesised and their photocatalytic activity was characterised through model oxidation and reduction reactions. This study revealed enzyme-like kinetics of the catalysed reactions and improved photostability of the conjugated flavin moieties. Additionally, the biocompatibility of the nanoparticles was assessed through in vitro cell studies to ensure applicability to other fields such biomedicine or water remediation. Subsequently, the flavoenzyme specific oxidation of indole to indigo and indirubin dyes was explored using the FLPDA photocatalyst. The results showed that the nanoparticle system exhibited higher production of the valuable dyes over a homogeneous flavin photocatalyst. This increase in activity was investigated through reactive oxygen species (ROS) scavenging experiments which revealed that FLPDA’s mechanism of action in this reaction partially resembled natural flavoenzymes and therefore enhanced key product formation. Finally, reduction reactions catalysed by flavoenzymes were investigated using FLPDA and a chiral flavin-polydopamine system (RCPDA). The light-driven reduction of azobenzene dyes was first explored using FLPDA which provided evidence that hydride could be transferred from PDA-conjugated flavin moieties to a substrate upon irradiation in the presence of an electron donor reagent. Next, the reduction of C=C bonds within α,β-unsaturated ketones and aldehydes was explored by initially screening buffered electron donor reagents and homogeneous flavin photocatalysts with key substrates. The optimal conditions and compatible substrates were then applied to the nanoparticle RCPDA system which produced the saturated products in comparable yields to the homogeneous photocatalysts with very low catalyst loading (<1 mol% vs. 10 mol%). The inclusion of a chiral linkage between PDA and flavin and its effect on the stereochemical outcome of the reaction was also explored, with preliminary data showing that adopting such a strategy could enable some enantioselectivity over the product. In summary, this thesis provides new methodologies to design advanced flavin photocatalysts with enzyme-like characteristics that will help to further develop the field of sustainable catalysis.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Burton2020,
title = {Integrated Growth and Process Technology for Atomically Thin Single Crystal Films},
author = {Oliver Burton},
url = {https://www.repository.cam.ac.uk/handle/1810/318017},
year = {2020},
date = {2020-06-26},
abstract = {Graphene has been found to have a wide range of potential applications owing to its unique physical properties. Whilst pioneering lab-scale experiments have demonstrated and leveraged these interesting properties, there is still a significant gap in performance between the one-off exfoliated hero device characteristics and the average characteristics of a large graphene films suitable for scalable fabrication/manufacturing methods. Graphene films deposited by chemical vapour deposition hold the most promise to exploit the unique properties on industrial scales, yielding mobilities approaching that of the current record holding exfoliated graphene based devices. In this work three fundamental issues for the production and application of graphene films are addressed. Firstly, the issue of minute catalyst contamination is systematically studied through the use of Raman spectroscopy, image analysis and film etching to characterise the defect density of the graphene film under different parameters. It was found that the current best method of suppressing graphene nucleation density, an oxidative pretreatment to remove carbon contamination, can supply a Cu catalyst with a low level oxygen that was long undetected due to the limitations of common surface measurements. This oxygen is not only shown to increase the defect density of the graphene films, but also to significantly increase the frequency and size of unwanted adlayers in the film. As a result of the study, a simple additional reductive annealing stage was introduced to decrease the oxygen from the Cu foil after it has acted to remove carbon contamination. This new procedure allows for the beneficial effects of the oxygen pretreatment whilst negating any deleterious effects on the final film quality. The second core problem is the control of the topographical and crystallographic texture of the catalyst. It is generally agreed in the literature that Cu(111) is the ideal growth catalyst to produce the highest quality graphene, yet there is significant difficulty in producing flat single crystal Cu at wafer scale whilst still remaining cost effective. This issue is addressed by designing and optimising an integrated wafer scale deposition process for single crystal Cu on sapphire substrates. Previous work in the area has so far relied on lengthy (10~h) high temperature oxygen annealing of the sapphire or high temperature acid baths. Here a novel in-situ deposition demonstrates that these stages can be skipped entirely once the deposition is optimized, giving a wafer-scale low roughness single crystal Cu(111) film ideal for graphene growth. The process is also demonstrated to be transferable to other targeted orientations of Cu when epi-ready MgO substrates are used, allowing for the optimization of the catalyst texture as an explorable parameter space. Finally, the question of which crystallographic orientation of catalyst is best for graphene growth and transfer is answered. The ideal graphene transfer method is proposed to be mechanical exfoliation, limiting the contamination of the catalyst that wet-transfer techniques would otherwise cause. Though Cu(111) is thought to be the best orientation for growth, it is notoriously difficult to mechanically exfoliate, so whilst the graphene may be the best quality on catalyst, it would not perform well after it is removed due to transfer-related cracks and other damage. Using extensive image stitching and alignment to correlate the crystallographic orientation of Cu with the oxidation characteristics, Raman characterisation and propensity for transfer, crystallographic (inverse pole figure) maps are created to show which facet of Cu will result in the highest quality post-transfer graphene. The systematic study demonstrates the effects of Cu oxidation on graphene before and after transfer, with Raman spectroscopy allowing for probing the action of the interfacial oxidation as it decouples the graphene from the catalyst. The study sheds light on the complexity of the Cu-graphene-transfer system, but also shows trends and patterns that point towards the ideal catalyst crystallographic orientation that will yield the best quality graphene after a peeling transfer process.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Ermann2020,
title = {Nanopore-Based Readout of Encoded DNA Nanostructures},
author = {Niklas Ermann},
url = {https://www.repository.cam.ac.uk/handle/1810/312048},
year = {2020},
date = {2020-05-13},
abstract = {Nanopore sensing is a powerful technique for the detection and study of single molecules. The underlying resistive pulse principle relies on molecules modulating an ionic current as they pass through a nanoscale constriction between two reservoirs. The technique has recently been extended to the detection of modifications attached along a single molecule composed of a double strand of DNA. As the strand threads through the nanopore, attachments produce secondary peaks in the already reduced current. The ability to encode information in modification sequences and self-assemble designed strands through DNA nanotechnology opens up promising possibilities ranging from highly parallel single-molecule sensing to DNA data storage. This work investigates experimental methods and analysis techniques to improve the readout of information contained in the position and type of modifications attached along the strand. Experimentally, it is found that fluid flows opposing the movement of DNA strongly promote single-file, unfolded entry into conical glass nanopores. The effect can be induced both with flows due to externally applied pressure and electro-osmotic flows (EOF) along the pore walls. The ability to prevent folded entry into the pore is crucial for decoding of attached modifications as it facilitates the identification and classification of secondary current peaks. In the investigation of analysis methods, Bayesian inference is assessed as a tool to extract information from the current signal. While useful to detect a strand’s folding state, it is found that deep learning-based approaches exhibit superior accuracy where enough training data is available. Two convolutional neural network (CNN) architectures are presented, the first one targeted at the classification of previously known modification sequences and the second one for the identification of modifications at arbitrary positions. The last chapter shows that the deep learning approach is capable of classifying translocation events in real time. A probability calculation reveals how many events are required for a measurement to conclusively show the presence of a certain molecule. Re-measuring molecules by driving them in and out of the nanopore is investigated as a method to improve modification decoding. The thesis concludes with a critical discussion of the presented techniques and their contribution to the development of sensing and data storage applications built on nanopore-based readout of structural modifications on DNA strands.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Nakanishi2020b,
title = {From Micro- to Nano-porous Cellular Materials with Layered 2D Microstructure},
author = {Kenichi Nakanishi},
url = {https://www.repository.cam.ac.uk/handle/1810/302857},
year = {2020},
date = {2020-04-25},
abstract = {A large body of work has been committed to studying the unique properties of 2D materials such as graphene, with advancements in both the material quality and scale of mechanical exfoliation and chemical vapour deposition (CVD) methods. These emergent 2D materials have recently been engineered as the cell walls in three-dimensional structures, but their superb material properties are yet to be fully realized in this new form. This thesis investigates the CVD processing of a range of catalytic templates to open new routes towards the controlled fabrication of graphitic foams and lattices. As part of a full feedback loop, mechanical characterization of these unique cellular materials was undertaken in order to examine their deformation and failure mechanisms, including capturing their behaviour in a new hierarchical model framework. These novel structures have the potential to combine the properties of structured porous materials, i.e. low density, high geometric surface area, permeability and mechanical stability, with the intrinsic properties of 2D materials such as enhanced electrical and thermal conductivity, high mechanical strength and stiffness as well as resistance to damage from extreme temperatures and chemical attack. Such high quality 2D-material based cellular structures have manifold potential applications in electrochemistry, catalysis and filtration. Herein, freestanding graphitic foams are fabricated across a range of relative densities, and their uniaxial compressive responses are measured to investigate the operative deformation and failure mechanisms that govern the mechanical response of such foams. For this purpose, a hierarchical micromechanical model is developed, which traces the deformation of the hollow cell struts to the axial stretching of the cell walls. The waviness of the multilayered graphitic wall increases the axial compliance of each cell wall, and it is established that axial straining within the cell wall occurs by interlayer shearing. Crucially, this mechanism demonstrates that the continuum properties of such foams are dictated by the weak out-of-plane shear properties of the layered cell wall material, leading to a large knockdown in the macroscopic mechanical properties of the foam. Ordered graphene gyroid lattices possessing nanoscale unit cell sizes are then fabricated and characterized through a combination of nanoindentation and a multi-scale finite element analysis (FEA) study. These structured nanolattices were found to be highly conductive and possessed a high degree of elastic recovery and strength owing to the structural efficiency afforded by the stretching-dominated cellular architecture. However, the nanoscale interlayer shearing deformation mechanism was again found to be active in the cell walls of these structures, attenuating the continuum response of the lattice. The hierarchical micromechanical model developed herein rationalizes why CVD-grown multilayer graphitic foams and lattices possess diminished continuum elastic moduli and yield strengths in comparison to the exemplary in-plane mechanical properties of 2D materials, presenting a first step towards the understanding of porous materials whose cell walls are comprised of emergent 2D materials. In addition, the direct shrinkage of commercial polymer foams and 3D printed templates is used herein to offer a very simple and low-cost method for reaching identically-shaped structures with sub-200 μm unit cell sizes. The conformal addition of different thicknesses of alumina is shown to control the level of isotropic shrinkage, reducing the shrinkage ratio from 125x to 4x after addition of 25 nm of alumina, while inducing a surface stress mismatch that drastically increases the surface roughness of the material. Furthermore, efficient graphitization was demonstrated through the use of an electrolessly deposited Nickel film, resulting in the formation of a conductive multilayer graphenic coating at temperatures below 1100°C. These processes present the flexible production of multifunctional cellular materials with sub-mm unit cells, tuneable size, roughness and conductivity. A final study investigates the preparation of a nascent 2D material, WS2, through the use of a deconstructed metal organic chemical vapour deposition (MOCVD) process which allowed insight into the role of each process step. The catalytic effect of an Au substrate is unambiguously demonstrated, which allowed for a reduction in the precursor partial pressures required to nucleate and grow WS2 by over an order of magnitude in comparison to competing methods. This enabled the efficient low-pressure growth of WS2 films with low levels of carbon contamination. Furthermore, the reaction process developed herein exhibited a self-limiting monolayer growth behaviour with exposure cycles lasting just 10 minutes, a significant improvement over prior MOCVD processes requiring growth times in excess of 1 hour. These insights foster our understanding of the key underlying mechanisms of WS2 growth for future integrated manufacturing of transition metal dichalcogenides (TMDCs) and other 2D materials.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Datta2020b,
title = {Synthesis of nanostructured materials in deep eutectic solvents},
author = {Sukanya Datta},
url = {https://www.repository.cam.ac.uk/handle/1810/307748},
year = {2020},
date = {2020-02-12},
abstract = {Deep eutectic solvents (DES), a new generation of non-toxic, eco-friendly and biodegradable solvents discovered in 2001 consist of a variety of cations and anions. These new sub-category of ionic liquids present interesting properties due to the hydrogen bonding between their components and are being currently explored for a number of applications such as nanomaterial synthesis, electrodeposition, metal processing etc. Particularly interesting is their use as solvents for the synthesis of nanomaterials. The DES are especially attractive due to the lack of toxic emissions because of their negligible vapour pressure in comparison to the volatile organic compounds. However there is a lack in the understanding on the role played by DES to produce nanomaterials. Herein, the role of reline, an eutectic mixture of choline chloride and urea, one of the most popular DES, is elucidated during the synthesis of different families such as gold (noble metal), vanadium pentoxide (transition metal oxide), ceria and zirconia (ceramic oxides). The work focuses on gaining a holistic understanding on the interaction of reline with different precursors. It is found that reline can act as an ‘all in one’ platform such as template reagent, reducing agent, solvent platform and morphology directing agent in the synthesis of nanomaterials. It is important to highlight that there is no addition of any external additives in contrast to previously published work. It is demonstrated here that reline can actively form gold nanoparticles by reducing a salt precursor and simultaneously stabilize the nuclei, delaying their growth, leading to highly monodispersed small gold particles. In the case of metal oxides, the templating role of reline is elucidated to produce different morphologies of vanadium pentoxide upon altering the water ratio in reline. Reline has been also shown to direct the growth of 1D ceria-zirconia nanorods. The effect of reline on the crystal phase of the zirconia nanomaterial is also investigated herein.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}

@phdthesis{Connolly2020,
title = {Synthesis and Applications of Monolithic Metal-Organic Frameworks},
author = {Bethany Connolly},
url = {https://www.repository.cam.ac.uk/handle/1810/300989},
year = {2020},
date = {2020-02-04},
abstract = {Metal-organic frameworks (MOFs) are a diverse family of coordination polymers which result from the crystalline self-assembly of metal ions/oxide clusters with multi-dentate organic linkers. Despite outstanding academic results, these porous materials have not entered the industrial sphere; their powder morphology renders them unsuitable for most potential applications. A new class of monolithic MOF was recently reported - monolithMOF combine the single-crystal density and porosity of the theoretical MOF with the functionality of a pelletised material. Correspondingly, outstanding results were achieved for two MOFs, with monolithHKUST-1 (Cu3(benzene-1,3,5-tricarboxylate)2) setting a benchmark in natural gas storage while monolithZIF-8 (Zn(2-methylimidazolate)2) demonstrated a capacity to host catalytic nanoparticles, NP@monolithMOF, for practical and environmental water purification. With the aim of expanding on these preliminary results, the synthesis of new monolithic MOFs was explored. Zr-MOFs, renowned for their industrially favourable chemical, thermal and mechanical stability, were targeted. A synthetic procedure was developed for the preparation of the archetypal Zr-MOF UiO-66 (Zr6O4(OH)4(1,4-benzenedicarboxylate)6) as a robust and porous monolith, monolithUiO-66. A novel capacity to control bulk density and pore-size distribution in the macroscopic monolith, via the synthetic inclusion of non-crystalline mesoporosity, was also developed. The generality of this synthetic procedure towards Zr-MOFs in general was further explored including functionalised UiO-66 analogues as well as non- isostructural Zr-MOFs. The capacity of monolithMOFs to host nanoparticles was determined through analysis of the effects of nanoparticle surface functionality on doping level of monolithZIF-8. Subsequently, the immobilisation of mono-, bi- and tri-metallic nano-objects in monolithZIF-8 was pursued for a range of possible industrial applications. The more general capability of monolithic MOFs to host nanoparticles was determined via the synthesis of Pd@monolithHKUST-1. Finally, the feasibility of the new materials, monolithUiO-66 and Pd@monolithHKUST-1, towards possible industrial gas storage applications (CH4, CO2 and H2) was recorded. Benchmark volumetric CH4 and H2 storage capacities were collected in each monolith, and the dependence of these capacities on e.g. pore-size distribution and doping level were also elucidated. Overall, the synthesis of new monolithMOF and NP@monolithMOF composites was comprehensively explored, developing novel and practical materials for prospective industrial applications.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}