O Grafeno tem sido estudado há 60 anos, mas só foi desde sua primeira obtenção mediante esfoliação de grafite em 2004 por Novoselov, que obteve grande interesse por parte de pesquisadores, pois tem uma série de notáveis propriedades físicas e químicas que dificilmente são encontradas num mesmo material, o que o torna uma ferramenta de primeira ordem em muitas aplicações de diversos campos. Além disso, sua produção se limita a pequenas folhas, com defeitos e empilhadas formando multicamadas, o qual não permite seu uso em nível industrial. Isso demanda não só que o grafeno seja produzido em grande escala, mas também conservando suas propriedades. O presente trabalho reporta o estudo e estabelecimento de condições para o crescimento de folhas de grafeno, utilizando técnicas de deposição química na fase de vapor a pressão ambiente (APCVD) catalítica, e deposição química na fase vapor assistida por plasma (PECVD), também catalítica, com filmes finos de Níquel e Cobre como metais catalisadores, visto que são as técnicas e metais que tem reportado melhores resultados. Desta forma, esta pesquisa foi encaminhada a um ajuste das variáveis que intervém nas duas técnicas, tais como os gases, seus fluxos e relação entre eles, a temperatura, o tempo de deposição e as espessuras do catalisador. No caso do PECVD, a potência de RF para a geração do plasma e a pressão. Os filmes foram caracterizados por microscopia Raman, que permite ter uma avaliação aproximada do número de camadas e os defeitos presentes no material, e por microscopia eletrônica de varredura (MEV), que permite observar a morfologia das amostras e a possível presença de grafeno, e assim ter certeza da qualidade do grafeno enquanto a continuidade e tamanho das folhas. Além disso, mediante Espectroscopia de raios X por dispersão de energia (EDS), instrumento associado ao MEV, é possível identificar os elementos presentes na amostra em pontos específicos e sua porcentagem. Estes análises revelaram que o grafeno obtido foi de grande área (1 cm2) com alta cristalinidade e poucos defeitos pontuais. / Graphene has been studied for 60 years, but was only since its first achievement by graphite exfoliation in 2004 by Novoselov that got great interest by researches, because it has remarkable physical and chemical properties which are hardly found in a single material, which makes it a first-order tool for many applications in several fields. Besides that, its production is limited to small sheets with defects and stacked in multilayers, which does not allow its use at industrial level, that requires not only a large scale production of graphene but also conservation of its properties. This work reports the study and find suitable conditions for the growth of graphene sheets, using catalytic atmospheric pressure chemical vapor deposition (APCVD) and plasma enhanced chemical vapor deposition (PECVD) techniques and thin nickel and copper films as catalysts. This choice is based on the fact that both, these techniques and the metals had lead to better reported results. Thus, this research is focused on the adjustment of the parameters that intervene in the two techniques, such as precursor gases, their flows and the relationship among them, temperature, deposition time and the catalyst thickness. In the case of the PECVD, the RF power to generate the plasma and the deposition pressure. The films were characterized by Raman spectroscopy, which allows an approximate evaluation of the number of layers and the defects in the material, and by Scanning Electron Microscopy (SEM), which allows to observe the morphology of the deposited layers, and thus to ensure the quality of the graphene as far as the continuity and size of the sheets are concerned. In addition, energy dispersive X-ray spectroscopy (EDS) associated to the SEM instrument was utilized to identify the elements present in particular locations of the sample as well as their percentage. These group of analyses revealed that the obtained graphene achieved areas about 1 cm2 with high crystallinity and low punctual defects.

A eletrônica molecular vem sendo investigada intensivamente por mais de vinte anos. Nesse sentido, as pesquisas científicas estão sendo focadas na busca de estruturas que possam ser utilizadas na construção de dispositivos em escalas nanométricas, que possam substituir a tecnologia tradicional do silício. O objetivo principal deste trabalho foi explorar as propriedades físicas de sistemas a base de grafano, um dos mais promissores materiais para serem usados como nanodispositivos. Para isso, foi realizada uma investigação teórica, baseada em cálculos de primeiros princípios, das propriedades estruturais e eletrônicas do grafeno numa forma pura ou com defeitos intrínsecos e extrinsecos. O primeiro grupo de estruturas investigadas foi o grafeno e grafano como nanofolhas constituídas por elementos do grupo IV da tabela periódica (C, SiC, Si, Ge e Sn). Também foram analisadas as mudanças nas propriedades eletrônicas do grafano do grupo IV com a substituição dos átomos de hidrogênio por flúor. A segunda parte do trabalho explorou as propriedades de defeitos estruturais em grafeno, tais como a monovacância, divacância, trivacância e Stone-Wales, e também o grafeno com dopantes (boro e nitrogênio) em diversas configurações. Todos os cálculos foram feitos utilizando métodos ab initio com base na teoria do funcional densidade. Foram estudadas algumas possíveis aplicações para os grupos de estruturas de grafeno investigados, através da análise de algumas de suas propriedades, tais como as densidades de estados próximas ao nível de Fermi e as estruturas de bandas eletrônicas para cada sistema. / The molecular electronics has been investigated for more than twenty years. In this sense, the scientific research has been focused on the search for structures that could be used in nanoelectronic devices that could replace the traditional silicon technology. The major goal of this work is to explore the physical properties of systems based on graphene, one of the most promising materials to be used in nanoelectronics. For that, an ab initio investigation was carried on the structural and electronic properties of graphene in its pristine form and with intrinsic and extrinsic defects. The first investigation explored the properties of group IV nanosheets (of C, SiC, Si, Ge e Sn), and the modifications on their properties as result of hydrogenation or fluorination. The second part of this work explored the physical properties of structural intrinsic defects in graphene, such as monovacancy, divacancy, trivacancy, and Stone-Wales ones. The work also explored the properties of boron and nitrogen dopants. All the calculations were performed using the ab initio methodology, based on the density functional theory.

Three systems of electrospun composite membranes with piezoelectric polymer polyvinylidene fluoride (PVDF) as matrix incorporating: 1) Carbon based fillers: carbon nanotube (CNT) and graphene oxide (GO); 2) Ceramic based fillers-barium titanate (BT), zinc oxide (ZnO) and nanoclays (halloysite and bentonite); 3) Cellulosic fillers: microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC) at different loadings were prepared by electrospinning process. Influence of filler type and loading on total PVDF crystallinity (Xc), relative fraction of β phase (piezoelectric phase) in total crystalline PVDF (Fβ), volume fraction of β phase in the samples (vβ) and piezoelectric coefficient d33 were characterised and analysed. Correlation between vβ and piezoelectric performance (d33) will be focused by this work. A common situation was observed for the composites-d33 increased while vβ is reduced by the fillers, so it can be concluded that d33 of the composites is not totally up to their vβ, there are other factors that need to be taken into account. For example, for carbon based filler like CNT, it increased electric conductivity of sample during and after electrospinning process, making it easier for charges produced by β crystals from inside of sample to be transferred to surfaces of the sample, and possibly promoting orientation of β crystals in d33 direction, therefore enhanced d33 of the composites though β phase formation was significantly hindered by inclusion of CNT; For piezoelectric ceramic fillers like BT and ZnO, a possible combined piezoelectricity from filler and β phase PVDF might enhanced d33 though less β phase was formed; And for non-piezoelectric and non-conductive fillers, enhancement in orientation of β crystals might play a major role in promotion of d33. Keywords: electrospinning; polyvinylidene fluoride (PVDF); nanocomposites; piezoelectric coefficient d33; energy harvesting.

Two dimensional (2D) materials are poised to revolutionize the future of optics and electronics. The past decade saw intense research centered around graphene. More recently, the tide has shifted to a bigger class of two-dimensional materials including graphene but more expansive in their capabilities. The so called ‘2D material zoo’ includes metals, semi-metals, semiconductors, superconductors and insulators. The possibility of mixing and matching 2D materials to fabricate heterostructures with desirable properties is very exciting. To make devices with superior electronic, optical and thermal properties, we need to understand how the electrons, phonons and other quasi particles interact with each other and exchange energy in the femtosecond and nanosecond timescales. To measure the timescales of energy distribution and dissipation, I used ultrafast pump-probe spectroscopy to perform time-domain measurements of optical absorption. This approach allows us to understand the impact of manybody interactions on the bandstructure and carrier dynamics of 2D materials. After a brief introduction to femtosecond laser spectroscopy, I will explore the transient absorption dynamics of three classes of 2D materials: intrinsic graphene, graphene-hBN heterostructures and Transition Metal Dichalcogenides (TMDs). We will see that using pumpprobe measurements around the high energy M-point of intrinsicgraphene, we can extract the value of the acoustic deformation potential which is vital in characterizing the electron-acoustic phonon interactions. In the next part of the thesis, I will delineate the role of the substrate in the cooling dynamics in graphene devices. We will see that excited carriers in graphene on hBN substrates cool much faster that on SiO2 substrates due to faster decay of the optical phonons in graphenehBN heterostructures. These results show that graphene-hBN heterostructures can solve the hot phonon bottleneck that plagues graphene devices at high power densities. In the last part, I will demonstrate the role of phonon induced bandgap renormalization in the carrier dynamics of TMD materials and measure the timescale of phonon decay through the generation of low-energy phonons and transfer to the substrate. This study will help us understand carrier recombination in TMD devices under high-bias conditions which show great potential in opto-electronic applications such as photovoltaics, LEDs etc.

The prototypical exciton model of two interacting Dirac particles in graphene was analysed by Sabio et al, Phys. Rev. B 81, 045428 (2010), and it was found that in one of the electron-hole scattering channels the total kinetic energy vanishes, resulting in a singular behaviour. We show that this singularity can be removed by extending the quasiparticle dispersion, thus breaking the symmetry between upper and lower Dirac cones. The dynamics of an electron-electron pair are then mapped onto that of a single particle with negative mass and anisotropic dispersion. We show that the interplay between dispersion and repulsive interaction can result in the formation of bound, Cooper-pair-like, metastable states in double-layered hybrid structures. We analyse these states by calculating their binding energies, decay rates into the free- electron continuum and semiclassical trajectories. We also analyse the problem of bi-electron pairing with the inclusion of the two dominant many-body effects at zero temperature: screening of the Coulomb interaction by the Dirac sea, and reduction of the available phase space due to Pauli blocking of transitions into the states below the Fermi level. We show that these effects result in strong renormalization of the binding energy, but do not destroy the metastable states. Thus the binding energies are strongly dependent on the chemical potential owing to the combined effects of screening and Pauli blocking. Hence, the quasibound resonances can be tuned by electrostatic doping.

Nowadays, low carbon economy becomes a significant topic over the world. Due to the decreasing amount of fossil energy source and the worsening environmental pollution, traditional energy sources should be transferred to renewable energy sources. A transition to renewable energy will require radical changes to systems and technologies for energy storage. Lithium ion (Li-ion) batteries are now considered as the most important electrochemical energy source for portable devices, electrical vehicles and expected to be used in grid electrical energy storage. Beside on Li-ion batteries, sodium ion (Na-ion) batteries are another promising energy source, which have the advantages in cost, safety and environmental factors, and they could be used for stationary energy storage systems and large vehicles. Tin-based nanocomposites are promising to replace the traditional graphite for Li-ion batteries to achieve a higher battery performance. In 2005, Sony Corporation launched the first Sn-based anode Li-ion batteries (Nexelion) to obtain a 50% increase in volumetric capacity over the conventional battery, which marked Li-ion batteries to enter into a new cutting edge. However, Sn-based materials faced with challenges. The battery performance was limited by a low cycling life and low rate performance, and methods should be devised to overcome these shortcomings. In this thesis, SnO2-based nanocomposites, including the graphene-SnO2, the carbon-coated graphene-SnO2 and the carbon-coated nanostructured SnO2 have been prepared and investigated as anodes for Li-ion and Na-ion batteries. The microstructure, electrochemical performances and even the degradation mechanisms have been investigated as the effects for different composite materials. Chapter 4 reports an amorphous carbon coated graphene-SnO2 composite which exhibited an enhanced cycling stability. In previous researches, the performance enhancements of that type of materials were commonly attributed to the carbon coating enhancing the electronic conductivity. However, it is found that the carbon coating deeply relates to the microstructure stability of the active materials, the performance enhancement can be attributed to the enhancement of structural stability. Chapter 5 reports same composites with various graphene to amorphous carbon mass ratios. In this chapter, we try to find out the optimized composition and understanding the different roles of graphene and amorphous carbon in that type of composites. It is found that an optimised graphene to carbon mass ratio can effectively enhance the structural stability and the electrode conductivity. Chapter 6 reports a carbon-coated flower-like nanostructured SnO2 for Na-ion battery application, which has been demonstrated to have a high reversible capacity and high rate performance. The carbon coating is found to help in the formation of a high quality solid electrolyte interface (SEI) layer on the surface of the active materials. These researches focus on modifying SnO2 and SnO2-based materials by carbon coating technologies, which aim to develop novel electrode materials to obtain a better battery performance for Li-ion and Na-ion batteries.

Submitted in fulfillment of the requirements of the Degree in Chemistry, Durban University of Technology, 2017. / High prevalence and mortality cases of prostate cancer (PCa) have increased around the world, particularly in developing countries. Several forthcoming factors have been revealed nowadays, one of them is due to the incapability of the diagnostic methods to produce reliable results, which impacts negatively on cancer-treatment. However, a sensitive diagnosis of PCa cells remains a challenge in the field of biosensors. Emerging whole-cell detection as biosensing targets has opened up avenues for successful cancer diagnostics, due to high selectivity among other cells. A switchable and flexible surface-based graphene material is one of the techniques that revolutionized smart biodevice platforms in biosensor technology. In this present study, a covalently linked poly-(N-isopropylacrylamide) (PNIPAM) to graphene oxide surface has been employed as “on/off”-switchable aptamer-based sensor for the detection of PC3 whole-cancer cell. The constructed surface has benefitted from PNIPAM, as the thermal-stimulus agent, which allows the coil-to-globule transitions by triggering temperature changes. When the system is above its lower critical solution temperature (LCST) of 32oC, PNIPAM will exist as hydrophobic -globular state providing an “on” binding region for the whole-cell, reaching the interactions on the biosurface. The “off” binding systems is only possibly when the PNIPAM turns into extended-state by lowering its temperature below LCST. The first principle studies have successfully characterized the electronic behavior with particular emphasis of PNIPAM monomer functions along with the description of the structural energetics of complex through density functional theory (DFT). Docking studies have further been performed to predict a plausible binding aptamer toward the protein-representative PCa cell. To better understand the prospect of an aptamer-based tunable biosensor, molecular dynamics (MD) highlighted the behavior of PNIPAM-grafted GO in exhibiting a globular and extended conformations at above and below LCST, permitting the biomolecules to interact with each other as well as to avoid interactions, respectively. Experimental studies have been included to validate the theoretical predictions by fabricating real-biosensor systems using electrochemical impedance technique, resulting a low-detection limit down to 14 cells/mL. Engagement between theoretical and experimental studies delivered an enhanced tunable-biosensor performance for the detection of whole cell prostate cancer. / M

The work described in this thesis concerns development, synthesis and characterisation of new molecular compounds and materials based on the carbon allotropes fullerene (C60) and graphene. A stepwise strategy to a symmetric ferrocene-linked dumbbell of fulleropyrrolidines was developed. The versatility of this approach was demonstrated in the synthesis of a non-symmetric fulleropyrrolidine-ferrocene-tryptophan triad. A new tethered bis-aldehyde, capable of regiospecific bis-pyrrolidination of a C60-fullerene in predominantly trans fashion, was designed, synthesised and reacted with glycine and C60 to yield the desired N-unfunctionalised bis(pyrrolidine)fullerene. A catenane dimer composed of two bis(pyrrolidine)fullerenes was obtained as a minor co-product. From the synthesis of the N-methyl analogue, the catenane dimer could be separated from the monomeric main product and fully characterised by NMR spectroscopy. Working towards organometallic fullerene-based molecular wires, the N-unfunctionalised bis(pyrrolidine)fullerene was coupled to an activated carboxyferrocene-fullerene fragment by amide links to yield a ferrocene-linked fullerene trimer, as indicated by mass spectrometry from reactions carried out at small scale A small library of conjugated diarylacetylene linkers, to be coupled to C60 via metal-mediated hydroarylation, was developed. Selected linker precursors were prepared and characterised, and the hydroarylation has been adapted using simple arylboronic acids. Few-layer graphene was prepared and dip-deposited from suspension onto a piezoelectric polymer substrate. Spontaneous side-selective deposition was observed and, from the perspective of non-covalent interaction, rationalised as being driven by the inbuilt polarization of the polymer. Aiming for selectively edge-oxidized graphene, a number of graphitic materials were treated with a combination of ozone and hydrogen peroxide under sonication. This mild, metal-free procedure led to edge-oxidation and exfoliation with very simple isolation of clean materials indicated by microscopy, spectroscopy, and thermogravimetric analysis.

In this thesis, we use rst principles density functional theory (DFT) to study the energetics, structural and electronic properties of hydrogenated and lithiated bilayer graphene material systems. The newly developed four variants of the non-local van der Waals (vdW) exchange-correlation functionals (vdW-DF, vdW-DF2, vdW-DF C09x and vdW-DF2 C09x) are employed to explore all the possible con gurations of hydrogen adsorption at 50% and 100% coverage on a 1 1 unit cell. The results obtained are also compared with the GGA PBE functional. For 50% hydrogen coverage, 16 unique con gurations are identi ed in the unrelaxed state. Formation energy analysis reveals six possible energetically favourable con gurations with three low-energy competing con gurations. It is found that the properties of hydrogenated bilayer graphene greatly depend on the hydrogen con guration. For instance, the formation of a hydrogen dimer within the layers decouples the structure, whereas the dimer formation outside surfaces does not have a signi cant in uence on the van der Waals forces; thus the bilayers remain coupled. In this coupled con guration, the vdW-DF C09x functional predicts the lowest formation energy and shortest interlayer separation, whereas the GGA PBE functional gives the highest formation energy and largest interlayer distance. The reasons behind the variation of these functionals are discussed. Two of the three low-energy competing con gurations exhibit semimetallic behaviour, whereas the remaining con guration is a wide band gap material. The wide band gap structure is found to undergo a hydrogen-induced spontaneous phase transformation from hexagonal to tetrahedral (diamond-like) geometry. We conclude that this wide band gap con guration represents a viable template for synthesizing nanodiamonds from graphene by hydrogenation. At 100% coverage, ten unique hydrogen con gurations are identi ed from a 1 1 unit cell. All exchange-correlation functionals predict nine of the structures to have negative formation energies. From these nine structures, three low-energy competing structures are noted and found to be wide band gap semiconductors, whereas the other con gurations exhibit either a semimetallic or metallic character. Although a 1 1 unit-cell is able to present a clear picture for the interaction between hydrogen and graphene, our results reveal that it limits the occurrence of other interesting physics. The cell size was increased to 2 1, to identify other low-energy con gurations that are not possible in a 1 1 cell. The identi ed con gurations have shown physically interesting hydrogen arrangements such as chair-like, zigzag-like and boat-like con gurations. Furthermore, our results reveal that hydrogenation reduces the elastic properties of the pristine structures. We further perform a systematic investigation of the e ects of lithium (Li) on AA and AB stacking sequences of bilayer graphene. Two Li atoms are considered to examine the e ects of the Li-Li interaction on bilayer graphene, and a total of 12 unique con gurations for AB and 9 for AA stackings are identi ed. The vdW-DF consistently predicts the highest formation energies, whereas vdW-DF2 C09x gives the lowest. Unlike in the case of the pristine structures, it is noted that for lithiated bilayer graphene, GGA PBE gives comparable results to the other functionals. One of the Li intercalated con gurations undergoes a spontaneous translation from the AB to AA stacking, and is found to be the most energetically stable con guration. We therefore conclude that Li favours the AA stacking, and that con guration represents a feasible template for experimentally synthesizing and characterizing a Li-based anode material. We noticed that all identi ed Li con gurations exhibit metallic behaviour. Lastly, we found that the intercalated Li dimer weakly interacts with the graphene layers, whereas the intercalated isolated Li atom exhibits strong interaction. / Thesis (PhD)--University of Pretoria, 2014. / gm2014 / Physics / unrestricted

In the last decade, carbon nanotubes (CNTs) and Graphene nanoplatelets (GNPs) have attracted a lot of attentions in various polymeric and ceramic composite systems, in an effort to improve their mechanical and functional properties. Al2O3 has attracted considerable interests in ceramics community, in particular as a matrix material for composite fabrications. The high stiffness, excellent thermal stability and chemical resistance of Al2O3 make it practically a very important engineering material, and if we can overcome its brittleness issue, its applications will be much wider. Adding CNTs as a reinforcement to the Al2O3 matrix to improve the toughness is one of the most promising methods. Similarly, GNPs have recently also been shown to be very promising for the same purpose. It has been demonstrated that by adding a mixture of the 2D-GNPs and 1D-CNTs into a polymer matrix, the toughest or strongest man-made ropes have been made. However, the homogenous dispersion of CNTs or GNPs is more of a challenge in a ceramic matrix than in polymeric matrices, owing to the tendency of CNT agglomerations and more steps are needed to completely transfer the useful properties of CNTs and GNPs into ceramics. In this thesis, nanocomposites of Al2O3 reinforced with a hybrid of GNTs (a blend of GNPs and CNTs) were first fabricated. The hybrid GNT reinforcements were mixed with the Al2O3 using a wet chemical technique under ultrasonic treatment. The effects of varied GNT contents on the microstructural features and mechanical properties of the nanocomposites were then investigated. It is found that the well-dispersed GNT fillers resulted in high sintered densities (>99%) in the composites, whilst the fracture mode alteration, grain refinement and improved flexural strength of the composites are all associated with the inclusion of CNTs and GNPs. The average fracture toughness of the nanocomposites reached up to 5.7 MPa·m1/2, against 3.5 MPa·m1/2 of the plain Al2O3, and the flexural strength improved from 360 MPa to 424 MPa respectively, at a hybrid addition of 0.5 wt% GNPs and 1 wt% CNTs. The toughening mechanisms attributed with the unique morphologies and structures of the GNT fillers were also discussed based on analyses on the morphology, grain sizes and fracture mode. The effects of hot-pressing (HP) and spark plasma sintering (SPS) methods on the grain size, microstructural features, and mechanical behaviour of GNT-reinforced Al2O3 nanocomposites were then comprehensively studied. Identical overall reinforcement contents at various GNP/CNT ratios were selected to prepare the composites using both HP and SPS. Highly densified samples (>98%) were obtained at 1650°C under 40 MPa in Ar atmosphere, with dwell times of 1 h and 10 min for HP and SPS respectively. Both types of sample showed a mixture of inter- and trans-granular fracture behaviour. A 50% grain size reduction was observed for samples prepared by HP, compared with the SPS samples. Both types of samples achieved a high flexural strength and fracture toughness of > 400 MPa and 5.5 MPa·m1/2, respectively, whilst the properties of the SPS samples peaked at relatively lower GNT contents than those of the HP samples. Based on analyses of the morphology, grain sizes and fracture mode, similar toughening mechanisms for both types of sample were observed, involving the complex characteristics of the combined GNT fillers. The tribological performance of the HPed pure Al2O3 and its composites containing various hybrid GNT contents was further evaluated under different loading conditions using a ball-on-disc method. Benchmarked against the pure Al2O3, the composite reinforced with a 0.5 wt% GNP exhibited a 23% reduction in the friction coefficient along with a promising 70% wear rate reduction, and a hybrid reinforcement consisting of 0.3 wt.% GNPs + 1 wt.% CNTs resulted in even better performance, with a 86% reduction in the wear rate. The extent of damage to the reinforcement phases caused during wear was studied using Raman spectroscopy. The wear mechanisms for the composites were analysed according to the mechanical properties, brittleness index and microstructural characterization. The combination between GNPs and CNTs contributed to the excellent wear resistance properties for the hybrid GNT-reinforced composites. The GNPs played an important role in the formation of a tribofilm on the worn surface by exfoliation; whereas the CNTs contributed to the improvement in fracture toughness and prevented the grains being pulled out during the tribology test. Finally, Graphene Oxide (GO) was used to replace the GNPs in the hybrid, to prepare Al2O3-GONT nanocomposites, by adopting a new sol-gel processing, in addition to powder mixing. It has been found that sol-gel process leads to an impressive grain size reduction of 62%, the fracture toughness and flexural reached 6.2 MPa·m1/2 and 420 MPa (i.e. 70% and 14% improvement), respectively, than those of pure Al2O3, which even marginally outperformed the previously optimised Al2O3-GNP nanocomposites by 8% in fracture toughness. The success of our new sol-gel strategy opens up new opportunities for choosing hybrid reinforcements for the fabrication of advanced ceramic nanocomposites.