A methodology was developed for isolation and transfer of 1-4 monolayer graphene from both Cu and Ni foil and Ni/SiO2/Si layered substrate types for use as electron transparent windows in environmental electron microscopy and spectroscopy. The graphene membranes were transferred onto disc "frames" made of stainless steel containing 3-10 μm diameter apertures. Such frames "windowed" with the graphene membrane are assembled into the custom designed environmental cell (e-cell) which contain a specimen immersed in the desired liquid or gaseous environment, and are compatible for imaging with a conventional scanning electron microscope (SEM) (in this case, a Hitachi 4500 SEM). Gold nanoparticles (50 nm) colloidal in water served as model specimens and were observed inside the e-cell using both secondary electron and backscattered electron detectors. An imaging induced radiolysis of water was observed at higher electron doses, which manifested itself in the formation of bubbles growing and coalescing under the enclosed graphene surface. Key SEM imaging parameters responsible for driving the radiolysis phenomena were addressed.

Carbon-based materials have attracted great attention over past few years in hydrogen storage applications. In particular, nanofibrous carbon working as support for sodium alanate exhibits great improvement in the kinetics of H2 releasing/uptaking. Herein, we used graphene with various dopants to simulate the carbon materials and performed a periodic density functional theory study on the impact of the modifications on the hydrogen interaction in the supported sodium alanate. Our results showed that the impact of various defects and dopants can be categorized in groups: (i) Pristine graphene and pentagon-heptagon (5-7) pair defective graphene, as well as nitrogen and sulfur doped graphene do not promote H2 formation. (ii) Carbon vacancies, as well as boron and chlorine doped systems, cause instantaneous H2 formation. (iii) Oxygen, phosphor and fluorine doped graphene led to the formation of a meta-stable di-hydrogen state with a H-H distance of ~ 0.96 Å. In addition, we confirmed the importance of van der Waals interaction in our system.

XAVIER, Leandro Jader Pitombeira. Confinamento eletrônico em bicamadas de grafeno. 2011. 92 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2011. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-10-16T20:16:23Z No. of bitstreams: 1 2011_dis_ljpxavier.pdf: 22751750 bytes, checksum: 0c79760922f8cf3c9ccf576a092c48d7 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-10-20T20:54:33Z (GMT) No. of bitstreams: 1 2011_dis_ljpxavier.pdf: 22751750 bytes, checksum: 0c79760922f8cf3c9ccf576a092c48d7 (MD5) / Made available in DSpace on 2015-10-20T20:54:33Z (GMT). No. of bitstreams: 1 2011_dis_ljpxavier.pdf: 22751750 bytes, checksum: 0c79760922f8cf3c9ccf576a092c48d7 (MD5) Previous issue date: 2011

SILVA, Agmael Mendonça. Estudo teórico das propriedades estruturais, eletrônicas e vibracionais de pontos quânticos de silício e grafeno e cálculos no formalismo DFT aplicados a cristais de ácido úrico. 2010. 148 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-10-15T18:26:42Z No. of bitstreams: 1 2010_dis_amsilva.pdf: 13243236 bytes, checksum: 7a9affef7c51dfd265b97f3957b677d3 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-10-21T20:33:13Z (GMT) No. of bitstreams: 1 2010_dis_amsilva.pdf: 13243236 bytes, checksum: 7a9affef7c51dfd265b97f3957b677d3 (MD5) / Made available in DSpace on 2015-10-21T20:33:13Z (GMT). No. of bitstreams: 1 2010_dis_amsilva.pdf: 13243236 bytes, checksum: 7a9affef7c51dfd265b97f3957b677d3 (MD5) Previous issue date: 2010 / There is a great interest in understanding the electronic properties of nano-structured materials aiming the development of new nano devices, especially how to modify the electronic properties of nano structures already known in a controlled manner. This work shows our studies, which were made in a pure atomistic way by computational simulation, on the electronic, optical and vibrational properties of (a) spherical quantum dots, silicon solid and hollow ones, (b) graphene nanoflakes and (c) crystals of uric acid, anhydrous, mono and dihydrate ones, using methods of Molecular Dynamics, Semiempirical, DFTB+ and DFT. We used the software called AMPAC and the modules of Materials Studio (Accelrys), the Forcite, CASTEP, Gulp and Dmol3 that are states of art in atomistic simulations. From the classical point of view we used Brenner force fields, which allow the formation and breaking of covalent bonds; and from the quantum dots of view, we used the method of density functional and DFTB+. In the study of silicon quantum dots, it was obtained a decrease of the energy gap due to the increase of the radius for massive dots, and contrary behavior to the hollow dots, when we fixed one point and varied only the radius of the hole. In relation to the graphene nanoflakes, it was obtained the stability of structures by the Dynamics Molecular, verifying that they keep their flattened form up to 1000 K; over 3400 K structures begin to have their links broken. The HOMO-LUMO energy gaps are sensitive to edges. Analysis of spin states revealed that only the triangular nanoflakes with zigzag edge have excess of electrons with alpha spin, however symmetry dependent. The modes of vibration for structures with nC ~ 50 were obtained and it was observed that rectangular nanoflake displays absorption bands in common with zigzag nanoflakes in two ranges of the infrared spectrum. Finally for the uric acid crystals, we observed that the lattice parameters for the dihydrate crystal are less consistent with experimental values. The gap of the crystal of uric acid, anhydrous and mono ones, is direct (~ 3.18 eV and 3.16 eV, respectively) and of the dihydrate is indirect (~ 2.89 eV). The 2p orbitals are the largest contributors to the density of states. Water has great influence in the conduction band of the dihydrate crystal. There is an anisotropic behavior in relation to the study of the optical properties of these crystals along four directions of incidence of the electric field, where the anisotropy is more accentuated to the dihydrate. The studies fit in the theme of the role of Instituto de NanoBioEstruturas & Simulação NanoBioMolecular [NANO(BIO)SIMES], one of the National Institutes of Science and Technology funded by CNPq from the beginning of 2009, which aims to develop research activities and high-quality human resource training in nanobiostructures and nanobiomolecular simulation. / Com a finalidade do desenvolvimento de novos nanodispositivos, há um grande interesse em conhecer as propriedades eletrônicas de materias nanoestruturados. Sobretudo, como modificar as propriedades eletrônicas de nanoestruturas já bem conhecidas de forma controlada. Com este objetivo, muitas metodologias e experimentos tem sido desenvolvidos. Neste trabalho, estudamos de forma inteiramente atomística através de simulação computacional as propriedades eletrônicas, ópticas e vibracionais de (a) pontos quânticos esferéricos maciços e ocos de silício, (b) nanoflocos de grafeno e (c) cristais de ácido úrico anidro, mono e dihidratado utilizando métodos de Dinâmia Molecular, Semiempírico, DFTB+ e DFT, para tanto utilizamos o programa AMPAC e os módulos do Materials Studio (Accelrys), o Forcite, CASTEP, Gulp e o Dmol3 que são estados de arte em simulações atomísticas. Do ponto de vista clássico utilizamos campos de força Brenner, que permite a formação e rompimento de ligações covalentes; do ponto de vista quântico, utilizamos o método do funcional da densidade e DFTB+ . No estudo dos pontos quânticos silício obteve-se uma diminuição do gap de energia em função do aumento do raio para os pontos maciços, e comportamento contrário para os pontos ocos, quando fixamos um ponto e variamos tão somente o raio do buraco. Para os nanoflocos de grafeno obteve-se por meio de Dinâmica Molecular a estabilidade das estruturas, averiguando que até 1000K elas conservam sua forma plana; acima de 3400K as estruturas começam a ter suas ligações rompidas. Os gaps de energia HOMO-LUMO são sensíveis às bordas. A análise dos estados de spins revelou que somente os nanoflocos triangulares com borda zigzag possuem excesso de elétrons com spin alfa, dependente no entanto da simetria. Os modos de vibração para estruturas com nC ~ 50 foram obtidas e observou-se que nanofloco retangular exibe bandas de abosorção em comum com nanoflocos zigzag em dois intervalos do espectro infravermelho. Finalmente para os cristais de ácido úrico, observou-se que os parâmetros de rede para o cristal dihidratado são menos coerentes com valores experiemntais. O gap do cristal de ácido úrico anidro e mono é direto (~ 3.18 eV e 3.16 eV, respectivamente) e do dihidratado é indireto (~ 2.89 eV). Os orbitais 2p são os maiores contribuintes à densidade de estados. A água tem bastante influência na banda de condução do cristal dihidratado. Há um comportamento anisotrópico quando do estudo das propriedades ópticas destes cristais ao longo de quatro direções de incidência do campo elétrico, sendo a anisotropia mais acentuada para o dihidratado. As pesquisas realizadas enquadram-se na temática de atuação do Instituto de NanoBioEstruturas & Simulaçãoao NanoBioMolecular [NANO(BIO)SIMES], um dos Institutos Nacionais de Ciência e Tecnologia financiados pelo CNPq a partir do início de 2009, que visa desenvolver atividades de pesquisa e formação de recursos humanos de alto nível em nanobioestruturas e simulação nanobiomolecular.

Membrane-based gas separation processes are an area of interest owing to their high industrial demand for a wide range of applications, such as natural gas purification from CO2 or H2, and N2 or O2 separation from air. This thesis is focused on developing and investigating polymeric-based membranes. Firstly, novel mixed matrix membranes (MMMs) were prepared, incorporating few-layer graphene in the polymer of intrinsic microporosity PIM-1. Secondly, novel polyphenylene-based polymers of intrinsic microporosity (PP-PIMs) were synthesised. An optimum preparation method of graphene/PIM-1 MMMs (GPMMMs) was established from numbers of experiments. In this study, graphene exfoliation was a step towards GPMMM preparation. Starting from graphene exfoliation in chloroform, as a good solvent for PIM-1, enhancement in graphene dispersibility was obtained with addition of PIM-1. This result helped in GPMMM preparation with high graphene content (up to 4 wt.%). Characterizations techniques such as Raman spectroscopy and scanning electron microscopy (SEM) of GPMMMs, confirmed the few layer graphene content, with morphology changes in the polymeric matrix compared to pure PIM-1.Gas permeability results of GPMMMs showed an enhancement in permeability with low loading graphene (0.1 wt.%) using a relatively low permeability PIM-1 batch, due to high water content. However, less influence of graphene incorporation on permeability was observed with a highly permeable PIM-1 batch. Reduction in permeability over time, termed an ageing effect, is known for a polymer of high-free volume like PIM-1. However, the enhancement of GPMMMs permeability after eight months storage was shown to be retained. Novel PP-PIMs were prepared from novel precursors using a series known organic reactions. PP-PIMs were divided into two groups of polymers based on their polymerization reactions. A group of polymers were prepared from condensation polymerization between bis-catecol monomers and tetrafluoroterephthalonitrile (TFTPN). Another group of polymers were prepared from Diels Alder polymerization between monomers of terminal bisphenylacetylene groups and bis tetraphenylcyclopentadienones (TPCPDs). All of which yielded polymers with apparent BET surface area in the range 290-443 m2 g-1.

Doctor of Philosophy / Physics / Christopher M. Sorensen / We demonstrated that gram quantities of pristine graphene nanosheets (GNs) can be produced via detonation of a hydrocarbon. This one-step and catalyst-free method is eco-friendly and economical for the production of GNs. The hydrocarbons detonated were C₂H₂, C₂H₄, C₃H₈ and CH₄ in the presence of O₂. The carbon products obtained from the detonation were analyzed by XRD, TEM, XPS and Raman spectroscopy. Depending upon the ratio of O₂ to C₂H₂, the GNs of size up to ~ 250 nm, SSA up to ~ 200 m²/g and yield up to 70% with 2-3 layers' stack have been obtained so far. N₂O was determined as a good alternative to O₂ as an oxidizer to produce GNs by detonating C₂H₂ with it. A two-color pyrometer was designed and calibrated to measure the temperature of the detonation of hydrocarbons. The measured detonation temperatures were in between 2700 K and 4300 K. Along with the high detonation temperature, the composition of precursor hydrocarbon was observed to be crucial as well to determine its suitability to detonate with oxidizer to produce GNs. The hydrocarbons C₂H₂ and C₂H₄ were determined as the suitable precursors to produce GNs whereas detonation of C₃H₈ yields mere amorphous carbon soot and CH₄ gives no solid carbon while detonated with O₂. It has been proposed that the hydrocarbons with C/H≥0.5 are suitable for GNs production by detonation method. Highly oxidized graphene nanosheets (OGNs) were produced by solution-based oxidation of GNs prepared via a controlled detonation of acetylene at O₂/C₂H₂=0.8. The produced OGNs were about 250 nm in size and hydrophilic in nature. The C/O ratio was dramatically reduced from 49:1 in the pristine GNs to about 1:1 in OGNs, as determined by X-ray photoelectron spectroscopy. This C/O in OGNs is the least ever found in all oxidized graphitic materials that have been reported. Thus, the OGNs produced from the detonated GNs with such high degree of oxidation herein yields a novel and promising material for future applications.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Gurpreet Singh / The inevitable depletion of fossil fuels and related environmental issues has led to exploration of alternative energy sources and storage technologies. Among various energy storage technologies, rechargeable metal-ion batteries (MIB) are at the forefront. One dominant factor affecting the performance of MIB is the choice of electrode material. This thesis reports synthesis of paper like electrodes composed for three representative layered materials (van der Waals sheets) namely reduced graphene oxide (rGO), molybdenum disulfide (MoS₂) and hexagonal boron nitride (BN) and their use as a flexible negative electrode for Li and Na-ion batteries. Additionally, layered or sandwiched structures of vdW sheets with precursor-derived ceramics (PDCs) were explored as high C-rate electrode materials. Electrochemical performance of rGO paper electrodes depended upon its reduction temperature, with maximum Li charge capacity of 325 mAh.g⁻¹ observed for specimen annealed at 900°C. However, a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. More importantly, annealing of GO in NH₃ at 500 °C showed negligible cyclability for Na-ions while there was improvement in electrode's Li-ion cycling performance. This is due to increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH₃. Further enhancement in rGO electrodes was achieved by interfacing exfoliated MoS₂ with rGO in 8:2 wt. ratios. Such papers showed good Na cycling ability with charge capacity of approx. 225.mAh.g⁻¹ and coulombic efficiency reaching 99%. Composite paper electrode of rGO and silicon oxycarbide SiOC (a type of PDC) was tested as high power-high energy anode material. Owing to this unique structure, the SiOC/rGO composite electrode exhibited stable Li-ion charge capacity of 543.mAh.g⁻¹ at 2400 mA.g⁻¹ with nearly 100% average cycling efficiency. Further, mechanical characterization of composite papers revealed difference in fracture mechanism between rGO and 60SiOC composite freestanding paper. This work demonstrates the first high power density silicon based PDC/rGO composite with high cyclic stability. Composite paper electrodes of exfoliated MoS₂ sheets and silicon carbonitride (another type of PDC material) were prepared by chemical interfacing of MoS₂ with polysilazane followed by pyrolysis . Microscopic and spectroscopic techniques confirmed ceramization of polymer to ceramic phase on surfaces on MoS₂. The electrode showed classical three-phase behavior characteristics of a conversion reaction. Excellent C-rate performance and Li capacity of 530 mAh.g⁻¹ which is approximately 3 times higher than bulk MoS₂ was observed. Composite papers of BN sheets with SiCN (SiCN/BN) showed improved electrical conductivity, high-temperature oxidation resistance (at 1000 °C), and high electrochemical activity (~517 mAh g⁻¹ at 100 mA g⁻¹) toward Li-ions generally not observed in SiCN or B-doped SiCN. Chemical characterization of the composite suggests increased free-carbon content in the SiCN phase, which may have exceeded the percolation limit, leading to the improved conductivity and Li-reversible capacity. The novel approach to synthesis of van der Waals sheets and its PDC composites along with battery cyclic performance testing offers a starting point to further explore the cyclic performance of other van der Waals sheets functionalized with various other PDC chemistries.

abstract: The continuous random network (CRN) model of network glasses is widely accepted as a model for materials such as vitreous silica and amorphous silicon. Although it has been more than eighty years since the proposal of the CRN, there has not been conclusive experimental evidence of the structure of glasses and amorphous materials. This has now changed with the advent of two-dimensional amorphous materials. Now, not only the distribution of rings but the actual atomic ring structure can be imaged in real space, allowing for greater charicterization of these types of networks. This dissertation reports the first work done on the modelling of amorphous graphene and vitreous silica bilayers. Models of amorphous graphene have been created using a Monte Carlo bond-switching method and MD method. Vitreous silica bilayers have been constructed using models of amorphous graphene and the ring statistics of silica bilayers has been studied. / Dissertation/Thesis / Doctoral Dissertation Physics 2014

abstract: Proton exchange membrane fuel cells have attracted immense research activities from the inception of the technology due to its high stability and performance capabilities. The major obstacle from commercialization is the cost of the catalyst material in manufacturing the fuel cell. In the present study, the major focus in PEMFCs has been in reduction of the cost of the catalyst material using graphene, thin film coated and Organometallic Molecular catalysts. The present research is focused on improving the durability and active surface area of the catalyst materials with low platinum loading using nanomaterials to reduce the effective cost of the fuel cells. Performance, Electrochemical impedance spectroscopy, oxygen reduction and surface morphology studies were performed on each manufactured material. Alkaline fuel cells with anion exchange membrane get immense attention due to very attractive opportunity of using non-noble metal catalyst materials. In the present study, cathodes with various organometallic cathode materials were prepared and investigated for alkaline membrane fuel cells for oxygen reduction and performance studies. Co and Fe Phthalocyanine catalyst materials were deposited on multi-walled carbon nanotubes (MWCNTs) support materials. Membrane Electrode Assemblies (MEAs) were fabricated using Tokuyama Membrane (#A901) with cathodes containing Co and Fe Phthalocyanine/MWCNTs and Pt/C anodes. Fuel cell performance of the MEAs was examined. / Dissertation/Thesis / Masters Thesis Technology 2014

Le graphène épitaxié sur des substrats métalliques est un modèle prometteur pour le développement de nouveaux systèmes hybrides, dans lesquelles les effets d'interface peuvent être exploités pour concevoir de nouvelles propriétés. L'insertion d'espèces entre le graphène et son substrat, une opération connues sous le nom d'-«intercalation», est une approche très puissante à cet égard. Avec l'aide des outils de la physique des surfaces, nous avons étudié trois systèmes graphène/métal, dont deux sont des systèmes hybrides intercalés, et l'autre est un candidat pour un tel système : (i) le graphène/Ir(111) intercalé avec un oxyde ultra-mince, (ii) graphène/Ir(111) intercalé avec des couches sub-atomiques du cobalt et (iii) de graphène sur Re(0001). Nous avons montré que certains défauts, en particulier les ridules (délamination du graphène de son substrat) et d'autres régions courbées du graphène, jouent un rôle crucial, non anticipé, dans le processus d'intercalation. Nous avons également observé que l'intercalation se déroule d'une manière nettement différente sous ultravide et à pression atmosphérique. Dans le premier système, des espèces contenant de l'oxygène entrent à l'extrémité ouverte des ridules et diffusent au long de ces ridules pour former des nano-rubans d'oxyde. Ces rubans modifient le dopage électronique du graphène, ce qui se traduit également par des changements substantiels dans la réponse optique inélastique (Raman) du graphène. Dans le second système, l'efficacité de l'intercalation est apparue dépendante de l'interaction graphène-métal, laquelle varie entre les domaines de graphène orientés différemment sur_(111). Dans ce système, les sites d'entrée pour les espèces intercalées, des régions courbées dans le graphène, ont pu être identifiés grâce à l'observation in_operando (en cours de croissance) du processus. Enfin, la croissance de graphène dans un troisième système (graphène/Re(0001)), a été étudiée afin de permettre le développement de futurs systèmes graphène/Re hybrides supraconducteurs. Dans ce système, nous avons proposé deux voies de croissance, l'une étant basé sur un processus de croissance en surface d'un monocristal massif de Re(0001), l'autre reposant sur la ségrégation en surface, activée thermiquement, du carbone dissout à haute température dans des films minces de Re sur saphir. / Epitaxial graphene grown on metal substrates is a promising platform for developing new hybrid systems, in which interface effects can be exploited to engineer novel properties. The insertion of foreign species between graphene and its substrate, referred to as “intercalation”, was shown very powerful in this respect. With the help of surface science tools, we have studied three graphene/metal systems, two of which are intercalated hybrid systems, and the other is a candidate for such a system: (i) graphene/Ir(111) intercalated with an ultrathin oxide, (ii) graphene/Ir(111) intercalated with cobalt (sub) atomic layers, and (iii) graphene on Re(0001). We found that some defects, especially wrinkles (linear delaminations of graphene from its substrate) and other curve graphene regions, play a crucial, yet unanticipated role in the intercalation process. We also found that the intercalation proceeds in a markedly different fashion under ultra-high vacuum and under atmospheric pressure. In the first system, oxygen-containing species were found to intercalate via the open end of wrinkles, to diffuse along then, and to form oxide nanoribbons along wrinkles accordingly. These ribbons modify the charge density of graphene, which also translates into substantial changes in the inelastic (Raman) optical response of graphene. In the second system, the efficiency of intercalation proved to be dependent on the graphene-metal interaction, which varies between differently oriented graphene domains on Ir(111). In this systems the entry sites for intercalated species could be identified, thanks to in operando observation of the process, as curved regions in graphene. Finally, graphene growth in a third system, graphene on Re(0001), was addressed in order to enable the future development of graphene/Re superconducting hybrids. In this system, we proposed two growth routes, one being a surface-confined process, on bulk single-crystal Re(0001), and the other being a temperature-induced segregation of carbon dissolved at high temperature in thin Re(0001) films on sapphire.