Graphene, a single layer of graphite, owing to its excellent mechanical, electrical, and thermal properties, has evolved as an exceptional nanomaterial in the past decade. It holds great promise in developing various novel applications from biomedical to structural composites. However, several challenges remain in realising the great potential of this material; one being the bulk scale production of graphene. This thesis has been concerned with production of pristine few-layer graphene (FLG) using liquid phase exfoliation (LPE) of graphite in various solvent media and exploring the applications of graphene-based composite coatings as optical Raman-strain sensors. LPE of natural graphite using bath sonication was used to produce highly stable pristine FLG in 1-methyl-2-pyrrolidinone (NMP) and N,N-dimethylformamide (DMF). Atomic force microscope (AFM) was used to analyse the exfoliation efficiency and lateral dimensions, while Raman spectroscopy provided an insight about the quality of the graphene flakes. Moreover, the potential for dynamic light scattering (DLS) as an efficient in situ characterisation technique for estimating the lateral dimensions of graphene flakes in dispersions was demonstrated. LPE was also employed to explore various routes to produce pristine graphene in aqueous media which can be used for toxicity studies. Aqueous dispersions were prepared by a solvent exchange method of graphene originally in organic solvents (NMP and DMF) using dialysis, achieving 0.1 v/v% organic solvent levels. Pristine aqueous graphene dispersions were also prepared by directly exfoliating graphite in biocompatible surfactant (TDOC- Sodium taurodeoxycholate) and biomolecules (Phosphatidylcholine and human serum albumin) solutions. Cell culture studies by collaborators revealed that solvent-exchanged and TDOC-exfoliated pristine FLG displayed minimal toxicity and albumin-exfoliated FLG hardly any cytotoxicity, whereas phosphatidylcholine-exfoliated FLG was cytotoxic. Raman spectroscopy is a well-established technique used to study the local deformation of carbon-based composites by following the shift rates of the Raman 2D band with strain. Raman active strain coatings were produced from epoxy composites made with the FLG produced by LPE in organic solvents and by electrochemical exfoliation method. The deformation experiments on these coatings revealed little or no strain sensitivity, due to several factors such as length of flakes, processing history, graphene loading, defects in graphene and alignment of flakes within the composites. As an alternative, composite coatings made from chemical vapour deposition (CVD) graphene were investigated. Excellent strain sensitivity was observed upon various cyclic deformational sequences and Raman mapping over 100 × 100 µm area. In comparison to the commercially available wide area strain sensors, CVD graphene composite coatings with a calculated absolute accuracy of ~ ± 0.01 % strain and absolute resolution of ~ 27 microstrains show promise for wide area Raman-based strains sensors.

abstract: Liquid-phase exfoliation (LPE) is a straightforward and scalable method of producing two-dimensional nanomaterials. The LPE process has typical been applied to layered van der Waals (vdW) solids, such as graphite and transition metal dichalcogenides, which have layers held together by weak van der Waals interactions. However, recent research has shown that solids with stronger bonds and non-layered structures can be converted to solution-stabilized nanosheets via LPE, some of which have shown to have interesting optical, magnetic, and photocatalytic properties. In this work, two classes of non-vdW solids – hexagonal metal diborides and boron carbide – are investigated for their morphological features, their chemical and crystallographic compositions, and their solvent preference for exfoliation. Spectroscopic and microscopic techniques are used to verify the composition and crystal structure of metal diboride nanosheets. Their application as mechanical fillers is demonstrated by incorporation into polymer nanocomposite films of polyvinyl alcohol and by successful integration into liquid photocurable 3D printing resins. Application of Hansen solubility theory to two metal diboride compositions enables extrapolation of their affinities for certain solvents and is also used to find solvent blends suitable for the nanosheets. Boron carbide nanosheets are examined for their size and thickness and their exfoliation planes are computationally analyzed and experimentally investigated using high-resolution transmission electron microscopy. The resulting analyses indicate that the exfoliation of boron carbide leads to multiple observed exfoliation planes upon LPE processing. Overall, these studies provide insight into the production and applications of LPE-produced nanosheets derived from non-vdW solids and suggest their potential application as mechanical fillers in polymer nanocomposites. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

Cette thèse démontre le potentiel dont dispose l’exfoliation en phase liquide du graphite dans le but d’obtenir des feuillets de graphène dispersés dans un solvant organique. Ainsi le mécanisme d’exfoliation a été étudié en profondeur, en particulier, l’influence de plusieurs paramètres (température, puissance et solvants). Le choix de ses paramètres se montre crucial dans le contrôle du procédé, et pour l’obtention des feuillets de graphène ayant une taille ciblée. Il est donc possible de fabriquer des nano-feuillets de quelques dizaines de nanomètre qui en plus possèdent des propriétés de photoluminescence.Dans le but de comprendre le mécanisme d’exfoliation en phase liquide assistée par des molécules, une nouvelle approche a été mise au point : l’approche supramoléculaire. Cette approche se base sur l’utilisation de surfactants d’un nouveau type. En effet, les molécules sélectionnées possèdent une longue chaine alkyle. Cette chaine s’adsorbe sur la surface du graphène et permet de stabiliser les feuillets lors de l’exfoliation. L’influence de la taille de la chaine alkyle de ces molécules lors de l’exfoliation a été vérifiée. De plus, ces molécules ont été équipées de différentes fonctions supramoléculaires afin qu’elles puissent former des dimères sur la surface du graphène. L’ajout de ces molécules augmente non seulement le rendement d’exfoliation mais aussi le nombre de mono-feuillets présents dans ces dispersions. Ces dispersions présentent des propriétés conductrices lorsqu’elles sont déposées sur des substrats. Une nouvelles méthode de déposition a été mise au point afin d’améliorer et d’augmenter la conductivité mais aussi le pourcentage de transparence. / This thesis demonstrates the potential of exfoliation of the graphite in the liquid phase in order to obtain graphene sheets dispersed in an organic solvent. Thus the exfoliation mechanism has been studied, in particular, the influence of several parameters (temperature, power and solvents). The choice of parameters is actually crucial for the control of the process, and to obtain graphene sheets having a targeted size. It is therefore possible to manufacture nanosheets of several tens of nanometers, which in addition exhibit photoluminescence properties.In order to understand the exfoliation mechanism in liquid phase assisted by molecules, a new approach has been developed: the supramolecular approach. This approach is based on using a new type of surfactant. Indeed, the selected molecules carry a long alkyl chain. This chain is adsorbed on the surface of graphene and can stabilize the sheets during exfoliation. The influence of the size of the alkyl chain of these molecules during exfoliation was verified. Furthermore, these molecules have been equipped with various supramolecular functions, which can form dimers on the surface of graphene. The addition of these molecules not only increases exfoliation performance but also the number of mono-layers present in these dispersions. These dispersions have conductive properties when deposited on substrates. A new deposition method was developed to enhance and increase conductivity but also the percentage of transparency.

Made available in DSpace on 2016-03-15T19:36:49Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-04-17 / The different methods of obtaining two-dimensional materials are being researched intensively, due to their promising physical and chemical properties. Among the methods of obtaining graphene, the liquid phase exfoliation (LPE) of graphite is proving to be a relatively simple and efficient process for the production of flakes of high quality and large scale. It is primarily based on the separation of the layers of graphite in liquids, such as common organic solvents and aqueous surfactant solutions. In this work the exfoliation of graphite was performed in liquid phase by sonication in aqueous suspension in the presence of an industrial reagent. A comparison with suspensions not using the polymeric surfactant indicates that its presence is necessary, because it prevents the re-agglomeration of the layers after sonication, through the multipolar and electrostatic repulsion mechanism. This result coincides with the reports of most recent works on liquid-phase exfoliation of graphite. / Os diferentes métodos de obtenção de materiais bidimensionais estão sendo pesquisados intensamente, devido a suas promissoras propriedades físicas e químicas. Entre os métodos de obtenção de grafeno, a esfoliação em fase líquida (LPE) de grafite está demonstrando ser um processo relativamente simples e eficaz de produção de flocos de alta qualidade e em larga escala. Ela se baseia principalmente na separação das camadas de grafite em líquidos, tais como solventes orgânicos comuns e soluções surfactantes aquosas. Neste trabalho foi realizada a esfoliação de grafite em fase líquida através da sonificação em suspensão aquosa em presença um reagente industrial. A comparação com resultados usando suspensões sem o agente surfactante polimérico indica que a presença deste é necessária, pois evita a reaglomeração das camadas após a sonificação, através do mecanismo de multipolo e repulsão eletrostática. Este resultado coincide com os reportes dos trabalhos mais recentes realizados sobre esfoliação em fase líquida de grafite.

Depuis la découverte des propriétés physiques et électroniques du graphène, un très grand nombre de méthodes visant à produire et modifier chimiquement le graphène ont été développées afin d'étendre et améliorer ses capacités en vue de futures applications. Les travaux réalisés au cours de cette thèse ont portés sur une méthode exfoliation du graphite en phase liquide assistée d'une réaction de cycloaddition réversible. Cette approche repose sur la réaction de Diels-Alder entre le graphite et un diène masqué très réactif, et se révèle être très efficace dans des solvants organiques volatils qui ne permettent pas l'exfoliation directe du graphite. L'introduction de groupements fonctionnels sur le diène a permis de moduler les propriétés de surface de films de graphène, ainsi que de post-fonctionnaliser les feuillets de graphène afin d'apporter une plus-value au matériau. / Since the discovery of the exciting properties of graphene, many techniques to produce and chemically modify graphene have been developed in order to expand and improve its properties in view of future applications. The study presented in this thesis focus on a process for the chemically-assisted exfoliation of graphite based on a reversible cycloaddition reaction. It relies on the Diels-Alder reaction between graphite and highly reactive masked diene, and it is effective even in solvents that are otherwise ineffective for exfoliation of graphite. Furthermore, it is possible to introduce functional groups on the diene, thereby enabling the tuning of the surface properties of graphene, as well as the post-functionalization of graphene sheets.

This thesis explores the application of two-dimensional (2D) materials such as graphene and single layer hexagonal boron nitride (h-BN) which are produced by liquid phase exfoliation for use in printed electronics and energy composite applications. In Chapter 2 I give a broad overview of the electrical, mechanical and optical properties of 2D materials among other nanomaterials that were used in the thesis such as carbon nanotubes and conductive polymers. Additionally I review the techniques and theory behind the exfoliation and dispersion of functional layered materials. In Chapter 3 I present the coating and printing techniques which were used in this thesis along with the experimental techniques and methods which I use to characterise my inks, films and devices. Chapter 4 is the first experimental chapter of the thesis and demonstrates the printing of 2D material heterostructures to create fully printed dieletrically gated field effect transistors with 2D materials on textile and polymer substrates. In this chapter I also demonstrate reprogrammable volatile memory, p and n type inverters, complementary inverters, and logic gates which pave the way to fully printed integrated circuits, operational at room temperature and pressure with 2D materials processed in liquid. In Chapter 5, I review spray coating (a highly industrial scalable printing technique), in terms of the optimisation of its parameters to achieve thin films of nanomaterials on three-dimensional (3D) surfaces. I then demonstrate that it is possible to create large area (∼750 cm2) transparent conducting films around curved surfaces with spray coating enabling a semi-transparent (around 360°) spherical touch sensor for interactive devices. Chapter 6 explores printed photonics for applications in terahertz (THz) frequencies. Here I demonstrate the feasibility of liquid phase exfoliated graphene to create THz saturable absorbers (SAs) which could enable many applications in THz frequencies such as tomography or time-resolved spectroscopy that require mode-locked (i.e. enabling a train of short pulses to be derived from continuous-wave operation) THz pulses. I also demonstrate that these SAs can be inkjet printed on demand providing unprecedented compactness in a quantum cascade laser system. Finally in Chapter 7, I look at the application of graphene in microbial fuel cells (MFC). I demonstrate that enhanced MFC output arises from the interplay of the improved surface area, enhanced conductivity, and catalytic surface groups of a graphene based electrode. As a final step graphene based anodes and cathodes which were entirely platinum free were combined to create an environmentally sustainable energy source.

Grafen v kombinaci s komplexy kovů by mohl poskytnout nové přísutpy v hybridních materiálech založených na grafenové bázi a v oblasti molekulárního magnetismu. Obě témata jsou velmi diskutovaná jako taková, nicméně, výzkumu vedoucího k možnosti jejich míchání není mnoho. Vzorky byly připraveny sonikací grafitu v kapalné fázi, což vedlo k exfoliaci grafenu. Následně byly nadeponovány pomocí modiikované Lagmuirovy– Schaeferovy depoziční metody různé komplexy kovů na substrát pokrytý grafenem. Klíčovým krokem bylo určení vlastností takto připraveného materiálu. Proto byly následně vlastnosti takového hybridního materiálu charakterizovány pomocí vysokofrekvenční elektronové paramagnetické rezonance (angl. HFEPR), rastrovacího elektronového mikroskopu, (angl. SEM), Ramanovou spektroskopií a čtyř bodovou metodou měření odporu. V této práci jsme potvrdili naši presumpci, že je možné vytvořit hybridní materiál smícháním exfoliovaného grafenu s molekulárními magnety pro získání nových magnetických a elektronických vlastností, které by mohly být využity v další generaci detektorů a elektroniky.

<p>  </p> <p>Graphene is a two-dimensional (2-D) sheet of <em>sp2</em> hybridized carbon atoms with extraordinary thermal, electrical, and mechanical properties. Among numerous sophisticated and costly synthesis techniques including chemical vapor deposition (CVD), SiC and microwave plasma; liquid-phase exfoliation (LPE) has been one of the most widely used techniques for low-cost and large scale graphene synthesis since it was first reported in 2008. LPE involves the use of liquid media to exfoliate graphite precursors directly into mono- or few-layered graphene. Stable dispersions of few-layered graphene are desirable for thin-film deposition on a large scale but are limited by the use of polar organic solvents with high boiling points and unfavorable toxicity profiles. This limitation can be overcome by milling and exfoliation of graphene nanoplatelets (GrNPs) in ethyl acetate (EtOAc) and acetone, volatile solvents with low toxicity profiles and modest environmental impact. Solvent-assisted grinding of pristine GrNPs on a horizontal ball mill followed by sonication produces concentrated suspensions up to 356 µg/mL that remain stable at room temperature for a minimum of 6 weeks without the addition of surfactants. Exfoliated graphene layers have an average thickness of 4.5 nm which corresponds to 10–12 layers of graphene on Si/SiO2 substrates. EtOAc and acetone-based dispersions of exfoliated graphene can be deposited uniformly using conventional airbrush equipment as low-boiling point solvents evaporates instantaneously after deposition. This deposition method also provides freedom regarding to target substrate and overcomes any substrate related limitations observed in other techniques. Practical demonstrations of spray-coated graphene films include (i) conductive surfaces with sheet resistance as low as 1 kΩ/sq, and (ii) solid contacts for disposable and low-cost nitrate-selective electrodes, with high reproducibility in the voltage readouts across multiple sensors.</p>

Cette thèse est consacrée à la production de matériaux 2D en phase liquide, en utilisant des approches pouvant permettre la production en masse de graphène et de matériaux apparentés. Notre objectif est de surmonter certains problèmes critiques pour le traitement et l'utilisation pratique des encres à base de matériaux 2D et de fournir une compréhension approfondie de la relation structure-propriétés dans ces matériaux, constituant des étapes obligatoires pour leurs applications futures. Cette thèse porte principalement sur l'UILPE et l'exfoliation électrochimique du graphène et du disulfure de molybdène (MoS2), qui ont été choisis comme matériaux prototypes à 2 dimensions. Les approches synthétiques sont combinées à une caractérisation physico-chimique des matériaux produits, à l'aide de techniques telles que l'AFM, la microscopie électronique, la spectroscopie XPS et Raman, ainsi qu'à une caractérisation électrique. Des applications dans le domaine de la détection et de l'électronique ont été explorées et ont permis de démontrer que des approches d'exfoliation en phase liquide pouvaient être utilisées pour obtenir un contrôle précis des propriétés des matériaux 2D ouvrant la voie à leur intégration en tant que matériaux actifs dans de nouveaux dispositifs multifonctionnels. / This thesis is devoted to the production in liquid-phase of two-dimensional materials, by using approaches that may enable mass production of graphene and related materials. We aim to overcome some issues that are critical for the processing and practical use of 2D materials-inks and to provide a deep understanding of the structure-properties relationship in such materials being mandatory steps toward their future applications. This thesis mainly focuses on ultrasound-induced liquid-phase exfoliation and electrochemical exfoliation of graphene and molybdenum disulfide, which have been chosen as prototypical 2D materials. The synthetic approaches have been combined with a multiscale physico-chemical and electrical characterization of the produced materials, by employing techniques such as AFM, XPS and Raman spectroscopy. Applications in the field of sensing and electronics have been explored and allowed to demonstrate that liquid-phase exfoliation approaches can be conveniently employed to achieve a fine control on the properties of 2D materials paving the way to their integration as active materials in novel multifunctional devices.

This master thesis work is performed at ABB Corporate Research Center inVästerås. The aim of this study is to investigate Ag:graphene composites as slidingelectrical contacts, suitable for use in e.g. tap-changers. Three different graphenematerials, all produced by a low-cost exfoliation process, are evaluated in this study. The results are compred to an ongoing work on Ag:GO (graphene oxide) composites. This material has shown very good tribological properties, however it hasbeen difficult to handle during sintering processing. The goal of this study is to geteven better tribological, electrical and mechanical properties than Ag:GO, and also todevelop a new powder-metallurgical method to produce the Ag:graphene composites.The study also investigates the influence of graphene flake size and concentration aswell as microstructure of the Ag:graphene composites. This report focuses on aninvestigation of the graphene raw material quality from the suppliers, and friction,wear and resistance analysis of the composites. This is done by using Ramanspectroscopy, SEM with EDS, LOM, tribometer tests and resistivity analysis. Raman and SEM analyses show that none of the supplied LEG materials are ofhigh-quality G (single or bilayer), but rather multi-layer graphene or even graphite.Small amounts of graphene added to Ag gave extremly low friction (μ&lt;0.2 vs. pureAg μ~1.3, 5 N load and 5 cm/s speed). The composite manufacturing process hadcritical steps, which have to be optimized, to obtain low values of friction. Severedegassing of the composites was observed for some sampes, but the samples stillmaintained good friction values. SEM and EDS analyses of 2dfab’s wear track show abuild-up thin carbon-containing tribofilm on the Ag surface. Indicating that G ispresent, and works as a lubricant, creating good tribological properties. The resultsfrom this project may for sure be of importance for future ABB products in specificindustrial applications.