Graphene sheets, one atom thick, two dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes - high electronic, thermal conductivities and exceptional mechanical strength. Chemical reduction of graphene oxide (GO) has been considered as a viable route for large scale production of graphene sheets. The reduced graphene oxide (r -GO) sheets although their conductivities are comparatively lower than that of graphene are nevertheless versatile material for applications in thin films and composites. An important consideration in the design of solution processing techniques for the preparation of processable graphene sheets is the dispersibility of GO and r -GO in different solvents, especially aqueous dispersibility. While GO is dispersible in water over a wide range of pH values, r -GO shows poor dispersibility and over a limited range of pH values. Graphene oxide, the oxidized form of graphene, are single atomic layers with lateral dimension that can extend to hundreds of nanometers. The sheets contain a sizable fraction of carbons that are sp3 hybridized and covalently bonded to oxygen in the form of epoxy, carbonyls as well as ionizable hydroxyl and carboxylic functional groups located on the rim of the sheets. The remaining carbons form isolated sp2 graphene like networks. On reduction the oxygen functionalities are removed and the sp2 network partially restored. This thesis focuses on the aqueous dispersibility of GO and r -GO, and describes a strategy to enhance the dispersibility of r -GO by cyclodextrin functionalization. Chapter 1 of the thesis provides a brief review of the synthetic procedures and structure of GO and r -GO while Chapter 2 describes the experimental methods and characterization techniques used in the thesis. The chemistry underlying the aqueous dispersibility of GO and r -GO at different values of pH have been investigated by zeta potential measurements, pH titrations and infrared spectroscopy (Chapter 3). These measurements show that r -GO sheets have ionizable groups with a single pKa value (8.0) while GO sheets have groups that are more acidic (pKa = 4.3), in addition to groups with pKa values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r -GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pKa value by stabilizing the carboxylate anion, resulting in superior water dispersibility. Till recently GO was primarily considered only as an easily available precursor for chemical routes to r -GO but it has now been recognized as an interesting material in its own right. Two such attributes that have attracted wide spread attention are the in- trinsic and tunable fluorescence of GO as well as formation of liquid crystalline phases. Aqueous dispersions of GO exhibit strong pH dependent fluorescence in the visible region that originates, in part, from the oxygenated functionalities present. In Chapter 4, the spectral migration on nanosecond timescales of the pH dependent features in the fluores- cence spectra of GO is described. The changes in the steady state fluorescence spectra with pH have been correlated with the sequence of dissociation events that occur in GO dispersions at different values of pH described in Chapter 3, from time resolved emission spectra (TRES) constructed from the wavelength dependent fluorescence decay curves, it is shown that the migration is associated with excited state proton transfer. Both ‘intramolecular’ and ‘intermolecular’ transfers involving the quasimolecular oxygenated aromatic fragments are observed. Aqueous dispersions of GO constitute a distinctive class of 2D-anisotropic colloids with competing interactions - long range electrostatic repulsion, originating from ionized carboxylic groups located on the rim of the sheets and weak dispersive attractive interactions originating from the un-oxidized sp2 graphitic domains. In Chapter 5, it is shown that, colloidal dispersions of GO are intrinsically frustrated, exhibiting a range of arrested or metastable states, encompassing fluid, glass and gels that coexist with liquid crystalline order. These states can be accessed by varying the relative magnitudes of the repulsive and attractive forces by changing the ionic strength of the medium, by addition of salt and/or the concentration of the dispersion. At low salt concentrations, where long range electrostatic repulsions dominates, the formation of a repulsive Wigner glass is observed while at high salt concentrations, when attractive forces dominate, the formation of gels that exhibits a nematic to columnar liquid crystalline transition. These studies highlights how the chemical structure of GO - hydrophilic ionizable groups and hydrophobic graphitic domains coexisting on a single sheet - gives rise to a rich and complex phase diagram. The poor dispersibility of r -GO in aqueous media limits its use in practical applica- tions. To enhance the dispersibility, r -GO sheets have been functionalized by covalently linking -cyclodextrin ( -CD) cavities to the sheets via an amide linkage (Chapter 6). The functionalized -CD: rGO sheets, in contrast to r -GO, are dispersible over a wide range of pH values (2 - 13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. It is shown that planar aromatic molecules adsorbed on the r -GO sheet as well as nonplanar molecules included in the tethered -CD cavities have their fluorescence effectively quenched by the -CD: rGO sheets. The -CD: rGO sheets combine the hydrophobicity associated with r -GO along with the hydrophobicity of the cyclodextrin cavities in a single water dispersible material. Resonance Raman spectroscopy is a powerful analytical tool for detecting and identi- fying analytes, but the associated strong fluorescence background severely limits the use of the technique. In Chapter 7, it is shown that the cyclodextrin functionalized -CD: rGO sheets, described in Chapter 6, provides a versatile platform for resonance Raman detection. Planar aromatic and dye molecules that adsorb on the r -GO graphitic domains and non-planar molecules included within the tethered -CD cavities have their fluorescence effectively quenched. Using the water dispersible -CD: rGO sheets, it is possible to record the resonance Raman spectra of adsorbed and included organic chromophores directly in aqueous media without having to extract or deposit on a substrate. The Raman signal intensities show a linear dependence with the concentration of analyte present in water. This is significant, as it allows for the identification and estimation of organic analytes present in water by resonance Raman spectroscopy.

Graphene is a single layer of carbon atoms arranged in honeycomb lattice. Over a long period of time it was treated as a hypothetical material to understand the properties of other allotropes of carbon, such as graphite, carbon nanotube etc. Half decade back, a single layer of graphene was finally isolated and since then the field has observed a flurry of activities. Low energy excitations in graphene are massless Dirac Fermions and quantum electrodynamic effects can be observed at room temperature in graphene, which makes it very popular among the condensed matter community. In addition graphene also shows many interesting mesoscopic effects, which is the focus of the present work. We study the electronic, magnetic and structural properties of the graphene nanostructures. The entire thesis based on the results and findings obtained from the present investigation is organized as follows. Chapter 1: provides a general introduction to the properties of graphene and graphene based nanostructures. Chapter2:describes the theoretical tools used in this thesis to investigate the properties of graphene nanoribbons. The first two chapters are meant to give the reader an overview about the field of graphene and a few of the computational techniques commonly used to investigate the properties of graphene. The following chapters are the new findings reported in this thesis. Chapter3:shows how zigzag graphene nanoribbons respond in a non-linear fashion when edges are subjected to some external potential such as magnetic field. Such response originates from the edge states present in zigzag ribbons and thus not observed in armchair nanoribbons. In the limit of ribbon width W→∞, an edge magnetic field produces a moment of ~ 1/3 per edge atom even for an infinitesimally small field, which is clearly a signature of non-linear response. Response of a finite width nanoribbon is size dependent and also depends on ln(V), the applied field. This is akin to Weber-Fechner law of audio visual perceptions. It is interesting to note that nature does provide a “quantum realization” of this in the form of biological sensing organs like the ear and eye. The magnetic response is found to scale inversely with the ribbon width. Chapter4:deals with the magnetic properties of the zigzag graphene nanoribbon. This is also a special property of the geometry of the zigzag edges and not observed in armchair nanoribbons. Our investigation reveals that the electron-electron repulsion (Hubbard U) energy creates a delta function like edge magnetic field in zigzag graphene nanoribbons. Starting from this, magnetic properties of zigzag graphene nanoribbons can be qualitatively and quantitatively explained from the non-linear response of zigzag nanoribbons. Zigzag graphene nanoribbons can exist in two possible ‘magnetic states’: antiferro (AF) where the two opposite edges have antiparallel magnetic moment and ferro (FM) where moment is parallel in the two opposite edges. First we describe the properties of undoped zigzag nanoribbons. They have AF ground state. Continuum theory can explain the size dependent bandgap and magnetic moment of the ground state. We present the first explicit derivation of the gap. Then we show that hole doping can change the ground state to FM, which is metallic. Thus the system has the property of magnetoresistance, which can be exploited by doping zigzag graphene nanoribbons externally with some gate voltage or internally by some electron acceptor element, such as boron. The critical doping for transition depends inversely with the ribbon width. We have found that the ‘phase transition’ on hole doping is a common phenomena for zigzag terminated nanostructures, such as hexagonal nanodots. Chapter5:discusses the effects of random edge shapes and random potential (Anderson disorder) on the magnetic properties of zigzag graphene nanostructures. Defects and disorders in the form of edge shape randomness and random potentials arising from substrate are very common in graphene. Our study reveals that edge state magnetism is very robust to shape randomness of the terminating edges of nanostructures; as long as there are three to four repeat units of a zigzag edge, the edge state magnetism is preserved. We also discover some “high energy” edges (ones where the edge atoms have only one nearest neighbor) can have very large moments compared to even the zigzag edges. Edge magnetism is also found to be robust to relatively small Anderson disorders, because a slowly varying small potential does not scatter the edge states. Chapter6:reveals how edge functionalization by O atom and OHgroup changes the properties of the zigzag graphene nanoribbons. Functionalization by various different molecules is a very popular method of tuning the properties of graphene. We have shown that it is possible to tune the properties of zigzag graphene nanoribbons by edge functionalization. Further, we have found that structures with clustered functionalization leads to “spatially” varying electronic structure, which can lead to interesting possibilities for electronic devices. Chapter7:describes structural stability, electronic and magnetic properties of graphene nanoribbons in presence of topological defects such as Stone-Wales defects. Our study reveals that the sign of stress induced by a SW defect in a graphene nanoribbon depends on the orientation of the SW defect with respect to the ribbon edge and the relaxation of the structure to relieve this stress determines its stability. Local warping or wrinkles arise in graphene nanoribbon when the stress is compressive, while the structure remains planar otherwise. The specific consequences to armchair and zigzag graphene nanoribbon can be understood from the anisotropy of the stress induced by a SW defect embedded in bulk graphene. We also have found localized electronic states near the SW defect sites in a nanoribbon. However, warping results in delocalization of electrons in the defect states. We have observed that, in zigzag graphene nanoribbons magnetic ordering weakens due to the presence of SW defects at the edges and the ground state is driven towards that of a nonmagnetic metal.

This research fabricated and tested various graphene-related 1D, 2D and 3D materials. We described how using specifically designed graphene foam (GF) as templates can transform its unique structures and excellent properties to new materials. Graphene, GF, Polypyrrole (PPY), Polyaniline (PANI), PPY-GF, PANI-GF, SiC foam, SiC nanowires and SiC nanoflakes will be described in this thesis. The chemical vapour deposition method was used to produce graphene and GFs. PPY-GF, PPY, PANI and PANI-GF were prepared by both chemical and electrochemical (Chronopotentiometry) methods. SiC foams were produced by a low-cost carbothermal reduction of SiO with GF, and then the SiC nanoflakes were separated from SiC nanowires and purified via a multistep sonication process. The synthesised materials were characterised by a variety of techniques such as SEM, EDX, XRD, TEM, Raman, AFM and TGA. The electrochemical properties of the materials were measured in a three electrode cell using cyclic voltammetry (CV), galvanostatic charge-discharge and A.C impedance spectroscopy techniques. The mechanical properties of the GF and SiC foams were investigated by conducting compression tests under in-situ SEM imaging. The as-produced graphene in this research was few layer graphene with layer number varies from 2 to 15. The GFs was found to be extremely light weight with an average density value of 5 mg cm-3. Using GF as electrode materials for supercapacitors, we obtained 100% capacity retention after 10,000 of charge-discharge cycles. The PPY-GF composite electrode exhibited an outstanding specific capacitance of 660 Fg-1, which is superior to the performance of most of the existing PPY-CNT, PPY-graphite and PPY-Graphene electrodes reported to date. In contrast to the PPY which shows a big structure degradation and a 30% capacity loss after only hundreds of CV cycles, the PPY-GF composite showed nearly 100% capacity retention after 6,000 cycles of charge-discharge. Our post-test characterisations showed no structural loss for the GF and PPY-GF. The excellent pseudocapacitive performance of the electrodes was found to be related to three key parameters: the open porosity feature of the GF which provides short pathways for ion diffusion and charge transportation, the dual charge storage mode in the composite, and the excellent mechanical properties of the GF. Due to its high flexibility and void spaces, the GF played successfully the role as a holder and stabilizer for the electroactive materials in protecting them from any structural degradation during the repeated ion intercalation-de-intercalation processes. In the SiC project, we have successfully created extremely light-weighted SiC foams with a density range of 9-20 mg cm-3, with various shapes, by using the GF as templates. These foams are the lightest reported SiC structures, and they consist of hollow trusses made from 2D SiC and long 1D SiC nanowires growing from the trusses, edges and defect sites. The 1D SiC nanowires, being confirmed as 3C-structure, appeared in a variety of shapes and sizes and are highly flexible; the 2D SiC is hexagonal, and upon breakup the resulting 2D nanoflakes have an average size of 2 µm and a thickness value of 2-3 nm which is 5-9 layers of SiC. They, to the best of our knowledge, are probably the thinnest and largest reported SiC flakes. Ultimately, in this research we have successfully produced two extremely lightweight and simultaneously strong foams: the GF and SiC foam. We have explored the GFs by efficiently addressing a key issue in the cycle life of energy storage devices, by creating an ideal architecture of such 3D GF-based electrodes. We have developed a completely novel 3D SiC structure made from continuously linked 2D layered SiC reinforced with 1D SiC nanowires. In-situ compression studies have revealed that both the GF and SiC foams can recover significantly, up to 85% in the case of GF, after compression strain exceeding 70%. The SiC foam did not experience any dramatic failure under the compression loads, as do in conventional ceramics. Compared with most existing lightweight foams of similar density, the present 3D SiC exhibited superior compression strengths and an significantly enhanced strength-to-weight ratio.

This thesis describes the transport properties observed in thermally treated graphite oxide (GO), which holds promise as an economical route to obtaining graphene. Graphene is a material consisting of a single atomic plane of carbon atoms and was first isolated as recently as 2004. Several isolation techniques have been investigated, including mechanical exfoliation, chemical vapor deposition, and the reduction (by various methods) of chemically synthesized graphite oxide. Two fundamental questions are pursued in this work. The first is concerned with the maximum electrical conductivity that can be achieved in atomically thin reduced graphite oxide samples (rGO). As produced, GO is insulating and of little use electronically. By heating and exposure to reducing atmospheres, however, the conductivity can be increased. Through the lithographic definition and fabrication of four-point contact structures atop microscopic samples of GO, the resistance of the sample can be monitored in situ as the reduction process takes place. It was discovered that the resistance of few-layer GO could be decreased by an order of magnitude when heated to 200 °C and subsequently cooled back to room temperature in forming gas. Final resistivities were on the order of 0.5 Ω-cm. An ambipolar field effect was observed in the thermally treated samples, with resistance decreasing by up to 16 % under a substrate bias of ±20 V. Mobilites were inferred to be on the order of 0.1 cm²/V-s. It was also found that the presence of forming gas during reduction decreased the resistance of the GO samples by roughly one half. The second question that this work begins to answer is concerned with the distance that electrons can travel in such thermally-reduced GO before spin-randomizing scattering. The answer can be elucidated with the aid of magnetoresistance measurements using ferromagnetic contacts to inject a spin-polarized current through the sample. The observation of the magnetoresistive effect with the contacts separated by a certain distance can be taken as evidence of a spin coherence length in the material of at least that distance. Though this experiment has not yet been carried out, progress has been made toward its possibility; specifically in the fabrication and characterization of independently switchable magnetic contacts. By exploiting magnetic shape anisotropy, contact pairs have been fabricated and demonstrated to differ in magnetic coercivity by up to 8 Oe. / Graduation date: 2011

The effects of octadecylamine-functionalized reduced graphene oxide (FRGO) on the frictional and wear properties of diglycidylether of bisphenol-A (DGEBA) epoxy are studied using a pin-on-disk tribometer. It was observed that the addition of FRGO significantly improves the tribological, mechanical, and thermal properties of epoxy matrix. Graphene oxide (GO) was functionalized with octadecylamine (ODA), and then reduction of oxygen-containing functional groups was carried out using hydrazine monohydrate. The Raman and x-ray photoelectron spectroscopy studies confirm significant reduction in oxygen-containing functional groups and formation of ODA functionalized reduced GO. The nanocomposites are prepared by adding 0.1, 0.2, 0.5 and 1.0 wt % of FRGO to the epoxy. The addition of FRGO increases by more than an order of magnitude the sliding distance during which the dynamic friction is ≤ 0.1. After this distance, the friction sharply increases to the range of 0.4 - 0.5. We explain the increase in sliding distance during which the friction is low by formation of a transfer film from the nanocomposite to the counterface. The wear rates in the low and high friction regimes are approximately 1.5 x 10-4 mm3/N·m and 5.5 x 10-4 mm3/N·m, respectively. The nanocomposites exhibit a 74 % increase in Young’s modulus with 0.5 wt. % of FRGO, and an increase in glass transition and thermal degradation temperatures.

In this thesis a variety of topics concerning 2D materials that have been separated from bulk layered crystals are discussed. Throughout the thesis, single and few layers of graphene, fluorinated graphene, MoS2 and WS2 are used. Two new methods of freely suspending 2D materials are presented as well as a method of removing the background from optical images. This aids contrast measurements for the determination of the number of layers. Fluorinated graphene is found to be sensitive to beta radiation; the resistance of fluorinated graphene transistors is shown to decrease upon exposure to the radiation. This happens due to the carbon-fluorine bond breaking. The sp3 hybridised structure of the fluorinated graphene is reduced back into the sp2 hybridised structure of pristine graphene. The superconducting properties of molybdenum-rhenium are characterised. It is shown to have a transition temperature of 7.5 K. It is also discovered that the material has a resistance to hydrofluoric acid; the acid etches nearly all other superconducting materials. This makes MoRe a possible candidate to explore superconductivity in conjunction with high mobility suspended graphene. To see if the material is compatible with graphene, a supported Josephson junction is fabricated. A proximity induced super current is sustained through the junction up to biases of ∼ 200 nA. The temperature dependence of the conductivity is measured for both suspended MoS2 and WS2 on a hexagonal boron nitride substrate. The dominant hopping mechanism that contributes to the conductivity at low temperatures is found to be Mott variable range hopping, with the characteristic T−1/3 dependence. The hopping transport is due to impurities that are intrinsic to the crystals, this is confirmed by comparing the results with those of supported devices on SiO2.

Neste estudo foi investigado o processo de redução térmica do óxido de grafeno em médio vácuo como uma rota viável de baixo custo econômico para obtenção do óxido de grafeno reduzido para aplicação em supercapacitores. O objetivo principal foi estudar a influência da temperatura de processamento no grau de redução do óxido de grafeno utilizando um sistema de vácuo com bomba mecânica de duplo estágio. O processamento constituiu na exposição do óxido de grafeno em várias temperaturas (200, 400, 600, 800 e 1000 °C) com pressão reduzida (10-2mbar) condição de médio vácuo. Foram utilizadas técnicas convencionais para caracterização dos materiais precursores e processados, tais como: microscopia eletrônica de varredura (MEV), difração de raios-X (DRX) e espectroscopia no infravermelho com transformada de Fourier (FTIR). Com os resultados deste estudo foi demostrado que é possível obter o óxido de grafeno reduzido utilizando um sistema de vácuo com bomba mecânica de duplo estágio e temperaturas de processamento superiores a 200°C. / In this study the process of medium vacuum thermal reduction of the graphene oxide as a low cost route for obtaining reduced graphene oxide has been investigated. The main objective was to study the influence of the processing temperature on the degree of reduction of the graphene oxide using a vacuum system with two stage backing pump. The processing was carried out by exposing the graphene oxide at various temperatures (200, 400, 600, 800 e 1000 °C) with reduced pressure (10-2 mbar). Conventional techniques have been employed to the characterization of the starting and processed materials, such as: scanning electron microscopy (SEM), X-ray diffraction and Fourier transformed infrared spectroscopy (FTIR). With the results of this study it has been demonstrated that it is possible to obtain the reduced graphene oxide using a vacuum system with a two stage backing pump and processing temperatures superior to 200°C.

L'utilisation de l'énergie des micro-sources de production d'électricité est un concept prometteur qui consiste à récolter des sources d'énergie faible et diffuse présent dans notre environnement pour l’alimentation de systèmes autonomes. Le nombre en croissance de nouveaux appareils miniaturisés et communicants dans les domaines civils et militaires devrait accentuer le phénomène de dépendance énergétique et ouvre de nouveaux marché.Parmi les éventuelles sources d’énergies renouvelables, l’énergie solaire est la source la plus prometteuse car elle est potentiellement la plus puissante et la mieux répartie. Le développement de ces systèmes de récupération des micro-sources d’énergie passe par de faibles coûts avec substrat souple (papier,polymère) et des matériaux facilement exploitables. Après la récupération de l’énergie, il est nécessaire pour les systèmes autonomes de stocker l'électricité.Dans cet objectif, les supercondensateurs sont les candidats idéaux. En effet, Le principal avantage des supercondensateurs par rapport aux batteries est leur haute densité de puissance (la collecte rapide de l’énergie) ainsi qu'une longue durée de vie. La thèse concerne donc la fabrication d’un supercondensateur et in fine le couplage avec une cellule solaire. Les travaux concernent spécifiquement l’étude de l'oxyde de graphène (GO) synthétisé par la méthode Hummers et Marcano, de sa réduction en oxyde de graphène réduit (RGO) par les voies chimique et électrochimique et de réalisation du supercondensateur. Dans ce projet, les propriétés de l'oxyde de graphène réduit (RGO) seront optimisées lors de l'étape de réduction et le matériau sera mis en forme dans une structure sandwich (RGO/ électrolyte /RGO) ou interdigité Mots clés: Graphène,supercondensateur, oxyde de graphène, micro-source d'énergie / The use of micro-power generation energy is a promising concept that consists in harvesting low and diffuse energy sources present in our environment for the supply of autonomous systems. The growing number of new miniaturized and communicating devices in civil and military fields should accentuate the phenomenon of energy dependence and open up new markets.Among possible sources of renewable energy, solar energy is the most promising source because it is potentially the most powerful and best distributed. The development of these micro-energy recovery systems involves low costs with flexible substrate (paper, polymer) and easily exploitable materials. After energy recovery, it is necessary for the autonomous systems to store electricity.For this purpose, supercapacitors are ideal candidates. Indeed, the main advantage of supercapacitors over batteries is their high power density (fast energy collection) as well as a long cycle life. The thesis concerns the manufacture of a supercapacitor and ultimately coupling with a solar cell. The work specifically concerns the study of graphene oxide (GO) synthesized by the Hummers and Marcano methods, its reduction in reduced graphene oxide (RGO) by chemical and electrochemical routes and the realization of supercapacitor. In this project, the properties of reduced graphene oxide (RGO) will be optimized during the reduction step and the material will be shaped into a sandwich structure (RGO / electrolyte / RGO) or interdigitated.Keywords: Graphene,supercapacitor, graphene oxide,energy micro-source

Graphene Nanoplatelets (GNP) was incorporated into polyamide 6 (PA6) matrix by melt compounding method and the enhancements in the properties of the nanocomposites were studied. Response Surface Methodology (RSM) was employed to assist in the study of processing conditions in melt compounding. RSM analysis revealed that the GNP concentrations to be the most significant term to affect the tensile modulus and crystallinity followed by the screw speed whereas the residence time was found to be non-significant. GNP with 5 Î1⁄4m (G5) and 25 Î1⁄4m (G25) were used in the GNP aspect ratio study. The average flake size of G5 and G25 to was measured to be 5.07 Î1⁄4m and 22.0 Î1⁄4m, respectively with the G5 distributed narrowly whereas the G25 exhibit broad distribution. TGA analysis shown that HT25 is more thermally stable compared to G25 due to some remnants lost during thermal treatment and this was confirmed by EDX and CHNS analysis. XRD profiles of the PA6-G-NC illustrate typical peaks of PA6 crystals phase as well as pure graphite characteristic peak. PA6-G25-NC observed to exhibit slightly higher peak intensity compared to PA6-G5-NC suggesting more formation of PA6 crystals. Similar improvement was observed on PA6-HT25-NC compared to PA6-G25-NC indicating more formation of PA6 crystals due improved dispersion of HT25. DSC on PA6-G25-NC showed higher cooling temperature and crystallinity compared to PA6-G5-NC due to larger surface area of the G25. Similarly, PA6-HT25 showed better improvement in crystallinity over PA6-G25-NC due to increase nucleation sites by the HT25. The thermal conductivity of PA6-G25-NC is slightly higher than the thermal conductivity of PA6-G5-NC but not significant considering the G25 is 5 times larger than G5. Instead, no significant difference was observed between PA6-HT25-NC and PA6-G25-NC. Addition of GNP increased the thermal stability of the PA6-G-NC systems under both nitrogen and air atmospheres regardless of the GNP aspect ratio. The viscoelastic properties showed insignificant difference between PA6-G5-NC and PA6-G25-NC. The inefficient improvement by G25 might be due to agglomeration formed during processing. The storage modulus and tan Î ́ of PA6-HT25-NC decreased but the Tg significantly improved compared to PA6-G25-NC. This was assumed to be because of improved dispersion of HT25 but reduced interfacial interaction after the heat treatment. The shear storage modulus, G’ and complex viscosity, |η*| were observed to increase with increasing GNP content with more pronounced improvement seen on PA6-G25-NC compared to PA6-G5-NC. However, no network percolation threshold was observed until 20 wt.% of GNP. The poor interfacial interaction of HT25 resulted in lower G’ and |η*| compared to G25. Tensile test results showed typical improvement with PA6-G25-NC having higher tensile modulus compared to PA6-G5-NC. Further enhancement was obtained with PA6-HT25-NC suggesting improved dispersion and volume of constrained chains mobility despite the poor surface interaction. Comparison with Halphin-Tsai modulus revealed that the effective modulus to be 150 GPa for G5 and 200 GPa for G25. The water uptake measurement results showed that GNP reduced the water uptake percentage and diffusion coefficient especially with G25. The test conducted on saturated PA6-G-NC results in improved thermal conductivity due to the high thermal conductivity of water but the viscoelastic and tensile properties severely reduced due to plasticisation effect.

Since the isolation of graphene in 2004, a large amount of research has been directed at 2D materials and their applications due to their unique characteristics. This thesis delivers pioneering developments on the applications of graphene and graphene oxide (GO) on electromagnetic ranges such as radio frequency, microwave frequency and THz bands, and specifically 2D materials based antennas, absorbers, sensors and etc. This thesis focuses on exploring electromagnetic applications of monolayer graphene, printed graphene and graphene oxide. In study of monolayer graphene applications, the theoretical and simulation studies are carried out to design tunable terahertz (THz) absorbers, tunable microwave wideband absorbers, and reconfigurable antennas, etc. These studies on the applications of monolayer graphene have proved prospective potentials of graphene in THz sensing, RCS reduction, and reconfigurable antennas. This thesis also presents pioneering advances on electromagnetic applications of printed graphene. Among these works, low-cost highly conductive and mechanically flexible printed graphene is developed for radio frequency (RF) applications. For the first time, effective RF radiation of printed graphene is experimentally demonstrated. Based on these results, applications of printed graphene including RFID (radio frequency identification) tags, anti-tampering RFID, EMI shielding, flexible microwave components such as transmission lines, resonators and antennas, conformable wideband radar absorbers, graphene oxide based wireless sensors, etc. are developed and experimentally demonstrated. This work significantly expands applications of graphene in electromagnetic areas.