Nomenclature of sp2 carbon nanoforms

Carbon’s versatile bonding has resulted in the discovery of a bewildering variety of nanoforms which urgently need a systematic and standard nomenclature [1]. Besides fullerenes, nanotubes and graphene, research teams around the globe now produce a plethora of carbon-based nanoforms such as ‘bamboo’ tubes, ‘herringbone’ and ‘bell’ structures. Each discovery duly gains a new, sometimes whimsical, name, often with its discoverer unaware that the same nanoform has already been reported several times but with different names (for example the nanoform in Fig. 1h is in different publications referred to as ‘bamboo’ [2], ‘herringbone-bamboo’ [3], ‘stacked-cups’ [4] and ‘stacked-cones’ [5]). In addition, a single name is often used to refer to completely different carbon nanoforms (for example, the ‘bamboo’ structure in [2] is notably different from ‘bamboo’ in [6]). The result is a confusing overabundance of names which makes literature searches and an objective comparison of results extremely difficult, if not impossible

Fabrication and a Study on the Wetting Properties of Nanostructured Surfaces

Fluid behavior at the microscale exhibits large surface to volume ratios increasing the significance of interfacial phenomena. We have studied two microfluidic phenomena that utilize interplay between microstructure and chemical composition. The first one causes liquid droplets to roll off from surfaces with a very high contact angle. This phenomenon is called superhydrophobic behavior, can be controlled by several tuning parameters. The second one changes the wettability of liquids on a dielectric coated surface with electric potential. The experimental studies were done by first fabricating an ordered array of glass nanocones. Fiber drawing and differential glass etching processes were used to produce cone like structures with lattice constant of 40 μm down to 1.6 μm. The superhydrophobic behavior was first studied and modeled in a series of nanocone wafers of increasing aspect ratio from .3 to 15. The characterization was done by the measurement of the contact and rolling angles. The Wenzel to Cassie transition of wetting states was observed. The contact angles were calculated by using the ‘axisymmetric drop shape analysis’ approach. Next, the study of the electrowetting behavior of two broad categories of structured surfaces was done. One was a low aspect ratio surface exhibiting Wenzel wetting and the other was a high aspect ratio surface exhibiting Cassie wetting. The device for experimental study was prepared by coating additional layers, which included conductive gold and dielectric Parylene-C coatings. Studies were done using silicone oil and air as the ambient medium. Images of drops were taken at different voltages and the contact angles were calculated geometrically. Electrowetting on nanocone arrays was modeled using an energybased approach and the obtained theoretical curves were compared to the experimental ones. Oil helped in achieving a large contact angle change. A qualitative analysis of the electrowetting properties of the surfaces was done based on the voltage-contact angle curves

Development of new probes based on carbon nanocones for near-field microscopies

La microscopie à champ proche permet l'étude topographique et des propriétés physiques (électrique, mécanique, etc.) de la surface d'un matériau à l'échelle nanométrique. Pour ce faire, l'échantillon étudié est balayé en surface par une sonde (ou pointe) dont les caractéristiques géométriques (comme le rayon de courbure de l'apex et le facteur de forme) et les propriétés physiques (mécanique, électrique etc.) doivent être adaptées pour garantir une résolution suffisante et une représentation fidèle de la surface. Cependant, les sondes actuelles présentent des limitations importantes vis-à-vis de la résolution apportée, dans les artefacts possibles d'imagerie, et dans leur adaptabilité concernant leur utilisation dans différents modes (qu'ils soient conducteurs ou non). Ces limitations sont causées principalement par le type de matériau utilisé (par exemple le silicium ou le nitrure de silicium, en standard, ou des nanotubes de carbone), ainsi que par les procédés de fabrication employés pour structurer la géométrie des sondes. Dans ce travail, nous étudions le potentiel de nanocônes de carbone (morphologie carbonée graphénique en forme de cônes à haut facteur de forme et d'apex nanométrique) pour différents modes de microscopie à champ proche. Ces nanocones présentent d'excellentes propriétés mécaniques (forte liaison C-C) et électriques. Ces derniers ont déjà été testés avec succès et brevetés en tant qu'émetteurs d'électrons pour les canons à émission de champ froid équipant les microscopes électroniques par transmission les plus performants. Ces diverses caractéristiques des nanocônes (facteur de forme, apex nanométrique, conductivité, stabilité mécanique, forte cohésion atomique) et d'autres (hydrophobicité, inertie chimique, morphologie multi-échelle micro-nano...) font qu'ils pourraient également constituer une solution prometteuse pour concevoir des sondes potentiellement supérieures aux sondes existantes, qu'elles soient standard ou plus spécifiques comme celles en nanotubes de carbone, pour divers types de microscopie à champ proche, notamment en termes de résolution spatiale et durabilité. Dans une première partie, cette thèse est dédiée à la synthèse de nanocônes de carbone individuels suivant une méthode originale de synthèse nommée ToF-CVD (Time of Flight - Chemical Vapor Deposition). Le travail révèle des mécanismes de formation complexes mettant en jeu d'une part les mécanismes de nucléation en phase hétérogène spécifiques de la CVD du carbone pyrolytique, et d'autre part des mécanismes de mouillabilité impliquant de phénomènes connus du domaine comme l'instabilité de Plateau-Rayleigh. Le montage des nanocônes sur des supports dédiés en tant que sondes pour microscopies à champ proche est ensuite réalisé, suivi par des études de caractérisation (SEM, TEM, spectroscopie RAMAN) pour évaluer leurs caractéristiques initiales du point de vue géométrique et structural et leur évolution vis-à-vis des conditions opératoires requises à la fois lors du montage et pour les différents modes de microscopie à champ proche étudiés. Dans une seconde partie, le potentiel des nanocônes de carbone en tant que sondes pour des modes de microscopie à champ proche non conducteurs comme le mode topographie (microscopie à force atomique - AFM) et le mode "Peak Force Quantitative Nano Mechanical" (PF-QNM), et pour des modes conducteurs comme pour la microscopie à effet tunnel (STM), la microscopie à force atomique conducteur (c-AFM), la microscopie à force Kelvin (KFM) est évalué. Cette évaluation est faite sur la base de (i) leurs performances ; (ii) leur durabilité ; (iii) leur versatilité. La finalité ultime est de comparer la performance des sondes-nanocônes de carbone par rapport à des sondes commerciales. Les nanocônes de carbone se révèlent être de véritables sondes multimodes avec peu d'équivalents actuels. Des améliorations sont cependant nécessaires et possibles, ce pour quoi des directions sont proposées.Near-field microscopy allows studying the topography and the physical properties (electrical, mechanical, etc.) of a material surface at nanoscale. For such a purpose, the sample surface is scanned by a probe (or tip) which geometric characteristics (such as the apex radius and the aspect ratio) and the physical properties (mechanical, electrical, etc.) must be suitable to ensure a sufficient resolution and a reliable representation of the surface. However, the current probes have significant limitations regarding the resolution, the possible imaging artifacts, as well as their ability to be used in different modes (conductive and non-conductive). These limitations are caused mainly by the type of material used (for example silicon or silicon nitride, for standard probes, or carbon nanotubes), as well as by the manufacturing processes used to structure the geometry of the probes. In this work, we study the potential of carbon nanocones (graphenic carbonaceous morphology with conical shape with high aspect ratio and nanosized apex) for different modes of near-field microscopy. These nanocones exhibit excellent mechanical (strong C-C bond) and electrical properties. They have already been successfully tested and patented as electron emitters for the cold-field-emission guns which equip the most performing transmission electron microscopes. These various characteristics of the nanocones (aspect ratio, nanosized apex, conductivity, mechanical stability, strong atomic cohesion) and others (hydrophobicity, chemical inertia, multiscale micro-nano morphology...), make that they could also constitute a promising solution for designing probes potentially superior to existing probes, either standard or more specific such as those in carbon nanotubes, for various types of near-field microscopy, in particular in terms of spatial resolution and durability. In the first part, this thesis is dedicated to the synthesis of individual carbon nanocones using an original synthesis method called ToF-CVD (Time of Flight Chemical Vapor Deposition). The work reveals complex formation mechanisms involving the heterogeneous phase nucleation mechanisms specific of the CVD deposition of pyrolytic carbon on the one hand, and well-known wetting mechanisms such the Plateau-Rayleigh instability on the other hand. The mounting of the nanocones on dedicated supports as probes for near-field microscopies is then carried out, followed by characterization studies (SEM, TEM, RAMAN spectroscopy) to assess their starting characteristics from the geometry and structure point of view, and their evolution under the operating conditions required for both the probe fabrication and for the different near-field microscopy modes studied. In a second part, the potentiality of carbon nanocones as probes for non-conductive modes such as topographic mode (atomic force microscopy - AFM) and "Peak Force Quantitative Nano Mechanical" (PF-QNM) mode, as well as for conductive modes such as scanning tunneling microscopy (STM), conductive atomic force microscopy (c-AFM), and Kelvin force microscopy (KFM) is evaluated. This evaluation is made on the basis of (i) performances; (ii) durability; (iii) versatility. The final goal is to compare the performance of the carbon nanocone probes with other commercial probes. Carbon nanocones reveal to truly be multimode probes with few existing counterparts nowadays. Improvements are needed and possible, for which directions are proposed

Metal supported carbon nanostructures for hydrogen storage

Carbon nanocones are the fifth equilibrium structure of carbon, first synthesized in 1997. They have been selected for investigating hydrogen storage capacity, because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures. The aim of this thesis was to study the effect of impregnation conditions on metal catalyst dispersion and to investigate whether the metal loaded cones had improved hydrogen storage characteristics. Pre-treatment of carbon nanocones with hydrogen peroxide was carried out, followed by metal decoration in aqueous solution by an incipient wetness technique. Two methods of reducing the metal catalyst have been applied: in hydrogen at room temperature (RT) and in an aqueous solution of NaBH4. X ray diffraction (XRD) technique confirmed the complete metal reduction and transmission electron microscope (TEM) analysis showed that the reduction technique affected the catalyst dispersion. Very fine dispersions of ca. 1 nm diameter metal clusters at catalyst loadings of 5 wt% were achieved and high dispersions were retained for loadings as high as 15 wt%. Hydrogen uptakes at RT were measured and an increase with metal loading was observed. In comparison the same route of pre-treatment and metal impregnation has been done over graphite nanofibres (GNF) and the hydrogen uptake showed an adsorption superior of the cumulative contribution of the substrate and metal catalyst attributing this to hydrogen spillover. The GNF have been impregnated also with another metal catalyst Ni showing as well the phenomenon of hydrogen spillover. The attempt to impregnate the carbon nanocones with a mixture of Pd-Ni, Pd-Cu and Pd-Ag resulted in an increase of hydrogen uptake for the first two but a decrease for the last of these. The carbon nanocones have been also impregnated with a Mg organometallic precursor dibutyl magnesium (DBM) and then decomposed without the use of hydrogen environment synthesizing successfully MgH2. The stoichiometry and the enthalpy of this decomposition have been studied. Furthermore, the DBM has been mixed with another hydride LiALH4 and the decomposition reaction of the complex hydride has been studied

Metal supported carbon nanostructures for hydrogen storage

Carbon nanocones are the fifth equilibrium structure of carbon, first synthesized in 1997. They have been selected for investigating hydrogen storage capacity, because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures. The aim of this thesis was to study the effect of impregnation conditions on metal catalyst dispersion and to investigate whether the metal loaded cones had improved hydrogen storage characteristics. Pre-treatment of carbon nanocones with hydrogen peroxide was carried out, followed by metal decoration in aqueous solution by an incipient wetness technique. Two methods of reducing the metal catalyst have been applied: in hydrogen at room temperature (RT) and in an aqueous solution of NaBH4. X ray diffraction (XRD) technique confirmed the complete metal reduction and transmission electron microscope (TEM) analysis showed that the reduction technique affected the catalyst dispersion. Very fine dispersions of ca. 1 nm diameter metal clusters at catalyst loadings of 5 wt% were achieved and high dispersions were retained for loadings as high as 15 wt%. Hydrogen uptakes at RT were measured and an increase with metal loading was observed. In comparison the same route of pre-treatment and metal impregnation has been done over graphite nanofibres (GNF) and the hydrogen uptake showed an adsorption superior of the cumulative contribution of the substrate and metal catalyst attributing this to hydrogen spillover. The GNF have been impregnated also with another metal catalyst Ni showing as well the phenomenon of hydrogen spillover. The attempt to impregnate the carbon nanocones with a mixture of Pd-Ni, Pd-Cu and Pd-Ag resulted in an increase of hydrogen uptake for the first two but a decrease for the last of these. The carbon nanocones have been also impregnated with a Mg organometallic precursor dibutyl magnesium (DBM) and then decomposed without the use of hydrogen environment synthesizing successfully MgH2. The stoichiometry and the enthalpy of this decomposition have been studied. Furthermore, the DBM has been mixed with another hydride LiALH4 and the decomposition reaction of the complex hydride has been studied