Au cours des dernières décennies, en raison de sa stabilité́ chimique et thermique exceptionnelle associée à son caractère isolant, le nitrure de bore hexagonal sous forme de nanofeuillets (BNNSs) trouve un grand intérêt dans de nombreuses applications. En effet, il est sérieusement envisagé l’utilisation de ces nanomatériaux comme support de graphene ou pour la fabrication d’hétérostructures horizontales utilisables dans le domaine de la microélectronique pour des applications de nouvelle génération. Il existe deux grandes voies de synthèse de ces nanostructures 2D de h-BN, par dépôt chimique en phase vapeur (CVD), ou par exfoliation d’un monocristal. Dans le but de préparer des BNNS de grande qualité́ chimique et cristalline, notre groupe propose une nouvelle stratégie de synthèse en associant la voie polymère précéramique (PDC) à des techniques de frittage, par Spark Plasma Sintering (SPS) et Hot Isostatic Pressing (HIP). Premièrement, le comportement thermique du précurseur précéramique, le polyborazilène (PBN) a été étudié en conditions dynamiques in-situ. Il a ainsi été mis en évidence, le rôle bénéfique du promoteur de cristallisation (Li3N) sur la qualité cristalline du matériau final. Cependant, une étape de frittage complémentaire reste obligatoire pour parfaire la structuration cristalline du h-BN. Premièrement, un procédé́ de frittage par SPS a été mis en œuvre. Dans cette étude, ont été particulièrement étudiés l’influence de la température de frittage (1200-1950°C) ainsi que la teneur en promoteur de cristallisation (0-10% mass.) sur la qualité cristalline du matériau final. Après optimisation des conditions de synthèse, des pastilles de h-BN composées d’une grande quantité de plaquettes monocristallines de taille d’environ 200 μm2 ont été obtenues. Les caractérisations de ces monocristaux attestent d'une haute qualité́ chimique et cristalline, même si des impuretés, sans doutes dues à l’environnement en graphite dans le SPS, sont détectées par cathodoluminescence. Enfin, des mesures physiques montrent que les BNNSs préparés présentent une constante diélectrique intéressante de 3,9, associée à une résistance diélectrique correcte de 0,53 V/nm. Afin d’augmenter encore la taille des monocristaux préparés, un second procédé́ de frittage, par HIP, a été étudié́. Cette autre combinaison originale conduit alors à des monocristaux de h-BN significativement plus gros (jusqu’à 2000 μm de taille latérale), transparents, incolores et très faciles à exfolier. Ainsi cette nouvelle association de la synthèse de PBN par voie PDCs et du procédé́ de céramisation par HIP nous semble une voie des plus prometteuses pour générer de grands monocristaux de h-BN et des nanofeuillets susceptibles de supporter des hétérostructures à base de graphène. / In the past decades, due to their exceptional chemical and thermal stabilities together with their electrical insulation properties, hexagonal boron nitride nanosheets (BNNSs) have become a promising support substrate for graphene and promoted the incentive of various van der Waals heterostructures. For such applications, BNNSs are generally obtained by Chemical Vapor Deposition (CVD) or exfoliation. In order to achieve high quality and large BNNSs, our group has proposed a novel synthesis strategy based on the Polymer Derived Ceramics (PDCs) route combined with sintering techniques: Spark Plasma Sintering (SPS) or Hot Isostatic Pressing (HIP). Since hexagonal boron nitride (h-BN) crystallization is a key point in the synthesis of high quality BNNSs, efforts have been led to understand the beneficial role of a promotor of crystallization (Li3N), adopting a suitable in situ dynamic approach. It has been established that Li3N does improve the crystallization level of the product, and lower the transformation temperatures from polyborazylene to h-BN. Then, we have further investigate the influence of the SPS sintering temperature (1200-1950°C) and of the crystal promoter content (Li3N, 0-10 wt.%) on BN growth. The tested SPS parameters strongly modify the size of the resulting h-BN flakes. For an optimal Li3N concentration of 5 wt.%, h-BN flakes larger than 200 μm2 (average flake area) have been obtained. A high degree of crystallinity and purity have been achieved, even if the very-sensitive cathodoluminescence technic indicated traces of impurities, probably due to surrounding graphite parts of the SPS. Few-layered BNNSs have been successfully isolated, through exfoliation process. As a final application purpose, further physical measurements have confirmed that SPS derived h-BN exhibits an interesting dielectric constant of 3.9 associated with a dielectric strength of 0.53 V/nm. Due to a very high compact character of SPS-derivative h-BN crystals, the post-exfoliation step is made very difficult, resulting in BNNSs of tens of microns lateral size. Therefore, we have studied another sintering procedure by HIP for the ceramization process. Through this combination, we aim to promote the size of h-BN single crystals, leading to larger size exfoliated BNNSs. Characterizations from bulk crystals to BNNSs have been carried out in three aspects: morphology, lattice structure and chemical composition. This novel attempt has provided us transparent and colorless h-BN single crystals with large lateral size, up to 2000 μm. Besides, BNNSs with high purity have also been confirmed. HIP, as a new ceramization process of PDCs, has to be considered as a promising way to obtain large h-BN single crystals and nanosheets for supporting graphene and 2D heterostructures.

O poli(etileno-co-álcool vinílico) (EVOH) pertence à família de materiais poliméricos semicristalinos; é dotado de excelentes propriedades de barreira a gases e muito utilizado na fabricação de embalagens para alimentos e outros produtos sensíveis a certos níveis de oxigênio ou dióxido de carbono. Entretanto, o EVOH é muito higroscópico e em condições de alta umidade relativa perde a propriedade de altíssima barreira a oxigênio bem como as propriedades mecânicas. De acordo com literatura, a inclusão de cargas lamelares, como argila e óxido de grafeno, contribui para a melhora significativa das propriedades de barreira a gás e mecânicas do EVOH. Este trabalho estudou os efeitos da incorporação de nanofolhas de óxido de grafeno (GO) nas propriedades dos filmes de EVOH. O GO foi obtido pelo método de Hummer\'s modificado e posteriormente submetido à redução induzida por radiação ionizante. Foram incorporados 0,1- 0,3 % em peso de GO e GO reduzido (RGO) na matriz de EVOH via processo de extrusão, utilizando primeiramente uma extrusora dupla-rosca e após, uma mini extrusora balão de laboratório para a obtenção de filmes flexíveis de EVOH/GO e EVOH/RGO. Os filmes obtidos foram submetidos à radiação ionizante, em acelerador de elétrons de 1,5 MeV, com o objetivo de estudar a contribuição do tratamento por radiação ionizante, nas propriedades finais. As irradiações foram realizadas à temperatura ambiente, em ar, e faixa de dose de radiação 100-250 kGy. As amostras de GO e RGO foram caracterizadas por meio dos ensaios de difração de raios X (DRX), microscopia eletrônica de varredura com fonte de emissão de campo (MEV-FEG), microscopia eletrônica de transmissão (MET), espectroscopia vibracional de absorção no infravermelho com transformada de Fourier (FTIR), espectrometria Raman e termogravimetria (TG). As amostras dos filmes irradiadas e não irradiadas foram caracterizadas por meio de ensaios mecânicos de tração, DRX, MEV-FEG, FTIR, TG, calorimetria exploratória diferencial (DSC), taxa de permeabilidade ao oxigênio (TPO2) e espectroscopia de aniquilação de pósitrons (PALS). Os resultados dos ensaios das amostras de GO e RGO indicaram que o GO obtido pelo método de Hummer\'s modificado foi reduzido por irradiação de raios gama, como a redução dos grupos funcionais contendo oxigênio, grupos epóxi e carboxílicos. As imagens de MEV-FEG das amostras de RGO mostraram folhas separadas sem regiões dobradas e domínios agregados, já o GO apresentou uma superfície com rugosidade e empilhamento de folhas. Os filmes flexíveis de EVOH contendo GO (EVOH/GO) e RGO (EVOH/RGO) apresentaram boa dispersão do GO na matriz de EVOH. Os filmes preparados com RGO (EVOH/RGO) e submetidos à radiação ionizante apresentaram uma dispersão mais homogênea do RGO na matriz e maior adesão interfacial matriz/RGO, e, consequentemente, propriedades mecânicas superiores àquelas obtidas para os filmes de EVOH puro ou de EVOH/GO. / Poly(ethylene-co-vinyl alcohol) (EVOH) belongs to the family of semicrystalline polymeric materials; is endowed with excellent gas barrier properties, it is much used in the research area for food packaging and other products sensitive to certain levels of oxygen or carbon dioxide. However, EVOH is very hygroscopic and the high flow conditions relative to the very high barrier property are oxygen as well as the mechanical properties. According to the literature, an inclusion of lamellar loads, such as clay and graphene oxide, contribute to the strengthening of the gas properties and the mechanical discharges of EVOH. This work was studied the addition graphene oxide (GO) nanosheets into EVOH properties. The GO was obtained by the modified Hummer\'s method and subsequently submitted to the reduction induced by ionizing radiation, 0.1-0.3 % by weight of GO and reduced GO (RGO) were incorporated into the EVOH matrix via the extrusion process, using firstly a double-screw extruder and then a mini-laboratory extruder for obtaining films EVOH / GO and EVOH / RGO. The obtained films were submitted to ionizing radiation, in an electron accelerator of 1,5 MeV, in order to study the contribution of the treatment by ionizing radiation, in the final properties. The irradiations were performed at room temperature in air, and radiation dose range 100-250 kGy. The GO and RGO samples were characterized by X-ray diffraction (XRD), scanning electron microscopy with field emission source (SEM-FEG), transmission electron microscopy (TEM), vibration absorption spectroscopy in the Fourier transform infrared (FTIR), Raman spectrometry and thermogravimetric (TG). The irradiated and non-irradiated films were characterized by mechanical tests, XRD, SEM-FEG, FTIR, TG, differential scanning calorimetry (DSC), oxygen permeability rate (TPO2) and positron annihilation spectroscopy (PALS). Tests results of GO and RGO samples indicated that the GO obtained by the modified Hummer\'s method was reduced by gamma irradiation, such as the reduction of the oxygen-containing functional groups, epoxy groups and carboxylic groups. The SEM-FEG images of RGO samples showed separate leaves without folded regions and aggregate domains, whereas the GO showed a surface with roughness and stacking of leaves. EVOH flexible films containing GO (EVOH / GO) and RGO (EVOH / RGO) showed good dispersion of GO in the EVOH matrix. The films prepared with RGO (EVOH / RGO) and subjected to ionizing radiation presented a more homogeneous dispersion of RGO in the matrix and higher interfacial matrix / RGO adhesion, and, consequently, superior mechanical properties to those obtained for pure EVOH or EVOH/GO films.

Detection of biomolecules is important for the diagnosis and treatment of diseases. Low concentration detection, specific biomolecule detection, and point-of-care use are appealing characteristics for biosensors because of the possibility of early detection and quick results of specific biomolecules. Furthermore, inexpensive biosensors are appealing so that they are accessible to the general population. The biosensors in this study have the potential to satisfy these characteristics. In this study graphene field-effect transistors (G-FET) were fabricated. Graphene was grown using chemical vapor deposition (CVD) and transferred to a silicon/silicon oxide substrate. The CVD method is the most scalable and cost-effective method of producing graphene for devices. Standard photolithography was used to pattern and then deposit metal electrodes. Two separate experiments were conducted; one using electrostatic attraction to bind protein to the active area of the G-FET to detect the protein poly-L-lysine (PLL) and one using an aptamer modified G-FET to selectively detect the protein thrombin. Analyte was delivered using a homebuilt, pressure driven, microfluidic, mass flow system. Both experiments showed a detection of the protein. The PLL experiment showed a clear change in the effective gate voltage of the G-FET. The thrombin experiment showed a change in the effective gate voltage that varied with differing concentrations of thrombin present. Furthermore, in the thrombin experiment by changing from a thrombin solution back to buffer the effective gate voltage was brought back to its original value. A competing protein was introduced and gave a signal comparable to the signal of a 10 times smaller concentration of thrombin. All of this shows that CVD grown graphene in a FET biosensor can be used for protein detection. Furthermore, the specific detection of thrombin suggests that aptamer modified G-FETs with CVD grown graphene can be used as a protein specific biosensor. / Graduation date: 2013

In this thesis, we present a systematic investigation of the static and dynamic response properties of low-dimensional systems, using a variety of theoretical techniques ranging from time dependent density functional theory to the recursive Green's function method. As typical low-dimensional systems, metal nanostructures can strongly interact with an electric field to support surface plasmons, making their optical properties extremely attractive in both fundamental and applied aspects. We have investigated the energy broadening of surface plasmons in metal structures of reduced dimensionality, where Landau damping is the dominant dissipation channel and presents an intrinsic limitation to plasmonics technology. We show that for every prototype class of systems considered, including nanoshells, coaxial nanotubes, and ultrathin films, Landau damping can be drastically tuned due to energy quantization of the individual electron levels and e-h pairs. Both the generic trend and oscillatory nature of the tunability are in stark contrast with the expectations of the semiclassical surface scattering picture. For a more realistic environment of low-dimensional systems, the effect of a dielectric substrate is considered to mimic the experimental setup. We have studied the dispersion of various plasmon excitations in metal thin films with growth substrates. Our results qualitatively reproduce the experimentally observed plasmon spectra of the Mg/Si systems. The underlying physics for the formation of various absorption peaks can be understood with a simple hybridization concept. Based on this concept, the coexistence of surface and bulk plasmons in experimental observation turns out to be a clear evidence for the existence of multiple-multipole surface plasmons due to the quantum confinement in thin films. To step into more confined worlds, we choose the real two-dimensional material graphene as our representive system, which is a semi-metal with zero band-gap. As the first step, the static electric response of graphene is investigated by exploring its transport properties. We have studied the pseudospin valve effect in bilayer graphene nanoribbons. The pseudospin degree of freedom is associated with the electron density in two layers and can be controlled by external gate electrodes. We find that the conductance of nanoribbons shows different behaviors compared with infinite systems due to the appearance of edge states and quantum confinement. Remarkably, a large on-off ratio can be achieved in nanoribbons with zigzag edges, even when the Fermi energy lies in the bulk energy gap. The influence of possible edge vacancies and interface conditions is also discussed. Finally, we discuss the possibility of using plasmon excitations to detach the graphene from its growth substrate, where the dynamic electric response of the graphene-metal system is expected to play a central role.

In the last decades several new nanostructures of carbon have been discovered, including carbon nanotubes (CNTs), and the recently discovered 2-dimensional graphene. These new materials exhibit extraordinary and unique properties—making them extremely interesting both for fundamental science and for future applications. It is, however, of crucial importance to develop new and improved fabrication and processing methods for these carbon nanomaterials. In this thesis the concept of applying solution chemistry and solution-based techniques to fabricate and to deposit graphene and other carbon nanomaterials is explored. An area-selective deposition method was developed for CNT and carbon-coated iron nanoparticles. By utilizing organic functionalization the properties of the nanomaterials were tuned, with the purpose to make them soluble in a liquid solvent and also enable them to selectively adsorb to non-polar surfaces. The first step of the functionalization process was an acid treatment, to introduce defects in the materials. This method was also used to create defects in so-called carbon nanosheets (CNS). The effect of the defect formation on the electric properties of the graphene-like CNS was studied; it was found that the resistance of the CNS could be reduced to 1/50 by acid treating of the sample. Also, the effect of the created defects on gas adsorption to the surface of the CNS has been investigated. This was done using atomic layer deposition (ALD) of TiO2 on the CNS, and a clear change in nucleation be-havior was seen due to the acid-treatment. Furthermore, a solution-based new method for fabrication of graphene was developed; this method combines intercalation of bromine into graphite with ultrasonic treatment to exfoliate flakes into a solvent. From the solvent the flakes can be deposited onto an arbitrary substrate. Several important parameters in the method were investigated in order to optimize the process. One important parameter proved to be the choice of solvent in all steps of the procedure; it was shown to influence sonication yield, flake size, and deposition results. Toluene was identified as a suitable solvent. A mild heat-treatment of the starting material was also identified as a way to increase the exfoliation yield. Using this method, fabrication of few-layer graphene sheets was achieved and areas down to 3 layers in thickness were identified—this is in the very forefront of current solution-based graphene fabrication techniques.

Composite fibers comprised of 5:95 wt ratio of ultra-short single walled carbon nanotubes (US-SWCNT):polyacrylonitrile (PAN) were spun using a dry-jet wet-spinning method followed by oxidative stabilization at 285 °C. The as-spun and stabilized composite fibers exhibited a 50 and 40 % increase, respectively, in modulus when compared to neat PAN. The vacuum pressure impregnation (VPI) method was employed to reinforce SWCNT fibers. SWCNT fibers were impregnated with polyamic acid (PAA) solution at 100 psi followed by thermal imidization to obtain fibers reinforced with polyimide (PI). The tensile strength was increased form 68 to 215 MPa for SWCNT fibers after VPI and imidization. Surfactant-wrapped chemically converted graphene (CCG) sheets obtained from the hydrazine reduction of GO were functionalized by treatment with aryl diazonium salts. The functionalized nanosheets disperse readily in polar aprotic solvents. A one-pot method has also been developed for reducing GO and simultaneously functionalizing it with alkyl and aryl groups. The alkyl functionalized reduced GO shows higher solubility in organic solvents when compared to GO. Graphene-filled PI composite films were prepared by solution blending of GO and PAA, casting the mixture and imidizing the films by heating up to 400 °C resulting in composite films that exhibit up to a ∼75 % increase in modulus and low moisture uptake. At 2 wt % loading GO, the composite films exhibit a conductivity of 1.25 × 10 -5 S/cm. The layer-by-layer (LbL) assembly technique was also employed in the fabrication of thin film composites of CCG and PI. The assembly was driven by the acid-base interaction between the aniline moieties on functionalized CCG and the carboxyl groups of the PAA. A simple fluid-phase processing method to obtain single to few layers of graphene without the aid of sonication has been developed. Graphene is spontaneously exfoliated from graphite and dissolved at isotropic concentrations as high as ∼1000 ppm in chlorosulfonic acid. The dissolution mechanism in superacids is protonation and electrostatic repulsion. The utility of this simple exfoliation process is further extended to diazonium functionalization of graphene allowing access to edge-functionalized graphenes with a minimal disruption of the graphitic network on the basal plane.

Since the discovery of the buckyball in 1985, researchers have imagined its potential in fields ranging from materials science to medicinal chemistry. The unique size, shape and hydrophobicity of C 60 fullerene endow it with the ability to interact with biological superstructures such as enzymes and membranes making it attractive as a potential pharmacophore. In this regard, we have developed a new, simple route to water soluble fullerene amino acids, both alkyl and aryl, through the dipolar addition of azido starting materials. The synthesis of our phenylalanine derivative, including the chromatographic purification, requires only one day for its completion. We have subsequently used our C 60 phenylalanine derivative in the synthesis of a series of C 60 peptides for the purposes of enzyme inhibition, specifically human immunodeficiency virus Type 1 protease, a critical viral enzyme responsible for the maturation of the virus and a popular target of medicinal chemists. We have demonstrated the ability of our C 60 amino acids and peptides to inhibit HIV-1 PR in a cell-free fluorescence based assay at low nanomolar concentrations. Graphite, or specifically graphene, has recently come to the forefront of nanomaterials research due to it similar scale, properties, and reaction pathways as other more costly carbon nanostructures such as carbon nanotubes. We have demonstrated the high yield functionalization of graphitic starting materials through the thermal decomposition of azido amino acids to their corresponding nitrene. The result is an inexpensive, highly functionalized, carbon based scaffold.

<p>Graphene is a novel carbon material with great promise for a range of applications due to its electronic and mechanical properties. Its two-dimensional nature translates to a high sensitivity to surface chemical interactions thereby making it an ideal platform for sensors. Graphene's electronic properties are not degraded due to mechanical flexing or strain (Kim, K. S., et al. nature 07719, 2009) offering another advantage for flexible sensors integrated into numerous systems including fabrics, etc. </p><p>We have demonstrated a graphene NO2 sensor on a solid substrate (100nm SiO2/heavily doped silicon). Three different methods were used to synthesize graphene and the sensor fabrication process was optimized accordingly. Water is used as a controllable p-type dopant in graphene to study the relationship between doping and graphene's response to NO2. Experimental results show that interface water between graphene and the supporting SiO2 substrate induces higher p-doping in graphene, leading to a higher sensitivity to NO2, consistent with theoretical predications (Zhang, Y. et al., Nanotechnology 20(2009) 185504). </p><p>We have also demonstrated a flexible and stretchable graphene-based sensor. Few layer graphene, grown on a Ni substrate, is etched and transferred to a highly stretchable polymer substrate (VHB from 3M) with preloaded stress, followed by metal contact formation to construct a flexible, stretchable sensor. With up to 500% deformation caused by compressive stress, graphene still shows stable electrical response to NO2. Our results suggest that higher compressive stress results in smaller sheet resistance and higher sensitivity to NO2. </p><p>A possible molecular detection sensor utilizing Surface Enhanced Raman Spectrum (SERS) based on a graphene/gallium nanoparticles platform is also studied. By correlating the enhancement of the graphene Raman modes with metal coverage, we propose that the Ga transfers electrons to the graphene creating local regions of enhanced electron concentration modifying the Raman scattering in graphene.</p> / Dissertation

Due to a lack of fundamental knowledge about the role of molecular structures in molecular electronic devices, this research is focused on the development of instruments to understand the relation between device design and the electronic properties of electroactive components. The overall goal is to apply this insight to obtain a more efficient and reliable scheme and greater functional control over each component. This work developed a fabrication method for porphyrinoids on graphene-based field effect transistors (FETs), and a chemical sensing platform under an ambient environment by integrating a tip-enhanced Raman spectroscope (TERS), atomic force microscope (AFM), and electronic testing circuit. The study is divided into three aspects. The first is aimed at demonstrating fabrication processes of nanoscale FETs of graphene and porphyrinoid composites based entirely on scanning probe lithography (SPL). A nanoshaving mechanism was used to define patterns on octadecanethiol self-assembled monolayers on gold film evaporated on graphene flakes, followed by metal wet etching and/or oxygen plasma etching to develop patterns on Au films and graphene, respectively. The integrity and optoelectronic properties were examined to validate the processes. The second area of study focused on the development of the chemical sensing platform, enabling chemical changes to be monitored during charge transports under an ambient environment. The localized Raman enhancement was induced by exciting surface plasmon resonance in nanoscale silver enhancing probes made by thermal silver evaporation on sharp AFM tips. As the system was designed along an off-axis illumination/collection scheme, it was demonstrated that it was capable of observing molecular decomposition on opaque and conductive substrates induced by an electric bias. The third line of work proposed a novel TERS system and a probe preparation method. Silver nanowires mounted on AFM tips were used to locally enhance the Raman scattering. The observed Raman enhancement allows quick chemical analysis from a nanoscale region, and thus enables chemical mapping beyond the diffraction limit. Compared with other TERS geometries, the new optical design not only allows analysis on large or opaque samples, but also simplifies the design of the optical components and the alignment processes of the setup.

Many diseases have been associated with oxidative stress (OS) which is caused when the production of reactive oxygen species (ROS), such as superoxide (O2•-) and hydroxyl radical (•OH), overcome the scavenging efficiency of living organisms. It is known that ROS production is worsened during traumas related to ischemic events and subsequent reperfusion in which the treatment with fast and effective antioxidants is critical to prevent cell and tissue damage. PEG-HCCs are carbon nanoparticles that showed O2•- and •OH scavenging properties according to electron paramagnetic resonance (EPR) experiments and peroxyl scavenging properties based on oxygen radical absorbance capacity (ORAC) assays. The O2•- quenching capability was also examined in vivo using a mild traumatic brain injury (mTBI) model complicated with hypotension. As result of the PEG-HCCs treatment, the cerebral blood flow (CBF) was restored while normalizing O2•- and nitric oxide (NO•) levels, primarily in the cerebral vasculature