Graphene features a unique combination of exceptional properties and has emerged as one of the most promising nanomaterials for a variety of applications. The ability to structurally modify graphene with nanoscale precision enables the properties to be further extended. By introducing nanopores in the graphene lattice, nanoporous graphene can be used in high-performance electronic devices or as selective membranes for efficient molecular filtering. Although methods for deterministic nanopatterning already exists, key for the implementation of nanoporous graphene is the development of a scalable and customisable method of patterning graphene that does not require any lithographic mask that is introducing defects. In this project, a novel approach using a nanoporous mask and a broad beam of 20 keV Ar ions has been investigated. Masks with 60-600 nm circular pores have been fabricated, and by irradiating suspended graphene membranes grown by chemical vapor deposition (CVD) through the mask, nanoporous graphene has been deterministically generated. The masks are fabricated using electron beam lithography, and the pattern is highly customisable regarding pore size, pore distribution and areal coverage. In addition to perforating the graphene, the ion beam is also observed to significantly reduce the level of contamination on the graphene membrane. The proposed mechanism is the combination of electronic  sputtering of surface contaminants and the random diffusion that follows, with a low nuclear sputtering yield and to-site pinning of contaminants. An extension of this study could include a more comprehensive characterization of the nanoporous graphene obtained as well as further studies on the dependency of beam parameters.

Porous materials have long been a research interest due to their practical importance in traditional chemical industries such as catalysis and separation processes. The successful synthesis of porous materials requires further understanding of the fundamental physics that govern the formation of these materials. In this thesis, we apply molecular modeling methods and develop novel models to study the formation mechanism of ordered porous materials. The improved understanding provides an opportunity to rational control pore size, pore shape, surface reactivity and may lead to new design of tailor-made materials. To attain detailed structural evolution of silicate materials, an atomistic model with explicitly representation of silicon and oxygen atoms is developed. Our model is based on rigid tetrahedra (representing SiO4) occupying the sites of a body centered cubic (bcc) lattice. The model serves as the base model to study the formation of silica materials. We first carried out Monte Carlo simulations to describe the polymerization process of silica without template molecules starting from a solution of silicic acid in water at pH 2. We predicted Qn evolutions during silica polymerization and good agreement was found compared with experimental data, where Qn is the fraction of Si atoms with n bridging oxygens. The model captures the basic kinetics of silica polymerization and provides structural evolution information. Next we generalize the application of this atomic lattice model to materials with tetrahedral (T) and bridging (B) atoms and apply parallel tempering Monte Carlo methods to search for ground states. We show that the atomic lattice model can be applied to silica and related materials with a rich variety of structures including known chalcogenides, zeolite analogs, and layered materials. We find that whereas canonical Monte Carlo simulations of the model consistently produce the amorphous solids studied in our previous work, parallel tempering Monte Carlo gives rise to ordered nanoporous solids. The utility of parallel tempering highlights the existence of barriers between amorphous and crystalline phases of our model. The role of template molecules during synthesis of ordered mesoporous materials was investigated. Implemented surfactant lattice model of Larson, together with atomic tetrahedral model for silica, we successfully modeled the formation of surfactant-templated mesoporous silica (MCM-41), with explicit representation of silicic acid condensation and surfactant self-assembly. Lamellar and hexagonal mesophases form spontaneously at different synthesis conditions, consistent with published experimental observations. Under conditions where silica polymerization is negligible, reversible transformation between hexagonal and lamellar phases were observed by changing synthesis temperatures. Upon long-time simulation that allows condensation of silanol groups, the inorganic phases of mesoporous structures were found with thicker walls that are amorphous and lack of crystallinity. Compared with bulk amorphous silica, the wall-domain of mesoporous silicas are less ordered withlarger fractions of three- and four-membered rings and wider ring-size distributions. It is the first molecular simulation study of explicit representations of both silicic acid condensation and surfactant self-assembly.

The use of interference microscopy has enabled the direct observation of transient concentration profiles generated by intracrystalline transport diffusion in nanoporous materials. The thus accessible intracrystalline concentration profiles contain a wealth of information which cannot be deduced by any macroscopic method. In this paper, we illustrate five different ways for determining the concentration-dependent diffusivity in one-dimensional systems and two for the surface permeability. These methods are discussed by application to concentration profiles evolving during the uptake of methanol by the zeolite ferrierite and of methanol by the metal organic framework (MOF) manganese(II) formate. We show that the diffusivity can be calculated most precisely by means of Fick’s 1st law. As the circumstances permit, Boltzmann’s integration method also yields very precise results. Furthermore, we present a simple procedure that enables the estimation of the influence of the surface barrier on the overall

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Within this general context, this talk will consider the use of novel nanoporous materials as the basis for adsorption based separations [3] that will range from concentrated mixtures to direct capture of carbon dioxide from air. An overview of different classes of materials will show how these can be tailored to such a wide range of conditions. The sheer scale of the task leads to having to optimize systems and speed up processes, which in turn brings in diffusion limitations.

Hydrogen storage remains a major technological barrier to the widespread adoption of hydrogen as an energy source. Organic polymers offer one potential route to useful hydrogen storage materials. Recently, Frechet and his coworkers described a series of hypercrosslinked polymers with high surface area and studied their surface properties and hydrogen storage capacities. McKeown and his coworkers studied a class of materials termed Polymers of Intrinsic Microporosity (PIMs) which are also based on a "hypercrosslinked" concept. We enchained N-substituted maleimide and functionalized stilbene alternating copolymers into a "hypercrosslinked system" to achieve high rigidity, high surface areas, high aromatic content and good thermal stability. Hypercrosslinked copolymers of N-(3-methylphenyl)maleimide (3MPMI), 4-methyl stilbene (4MSTBB), vinylbenzyl chloride (VBC) and divinyl benzene (DVB) were synthesized. Scanning electron micrographs (SEM) show the copolymers are porous and some examples have shown surface areas over 1200 m²/g. We have also found the incorporation of 3MPMI and 4MSTBB improves the thermal stability and raises the glass transition temperature of the copolymer. However, the incorporation of 3MPMI and 4MSTBB decreases the hypercrosslinking density and therefore causes a decrease in the copolymer surface area. The systematic study of styrene (STR) – vinylbenzyl chloride (VBC) – divinyl benzene (DVB) networks indicates that a low density of chloromethyl groups leads to a decrease in surface area. Therefore, we are continuing to investigate other monomers, such as N-substituted maleimide and functionalized stilbene containing chloromethyl groups, in order to enhance thermal stability while maintaining surface area. In order to increase the enthalpy of hydrogen adsorption and thus raise the temperature of hydrogen storage, the monomer N,N-dimethyl-N',N'-diethyl-4,4'-diaminostilbene (4,4'DASTB-3MPMI) which contains electron donating groups was incorporated into hypercrosslinked polymer particles. Hypercrosslinked polymer (4,4'DASTB-3MPMI)1.0(VBC)98.5(DVB).50 exhibits a surface area of 3257 m²/g. / Master of Science

La conversion thermoélectrique a suscité un regain d'intérêt en raison des possibilités d'augmenter l'efficacité tout en exploitant les effets de taille. Par exemple, les nanofils montrent théoriquement une augmentation des facteurs de puissance ainsi qu'une réduction du transport des phonons en raison d'effets de confinement et/ou de taille. Dans ce contexte, le diamètre des nanofils devient un paramètre crucial à prendre en compte pour obtenir des rendements thermoélectriques élevés. Une approche habituelle consiste à réduire la conductivité thermique phononique dans les nanofils en améliorant la diffusion sur les surfaces tout en réduisant les diamètres.Dans ce travail, la caractérisation thermique d'une forêt dense de nanofils de silicium, germanium, silicium-germanium et alliage Bi2Te3 est réalisée par une méthode 3-omega très sensible. Ces forêts de nanofils pour le silicium, le germanium et les alliages silicium-germanium ont été fabriqués selon une technique "bottom-up" suivant le mécanisme Vapeur-Liquide-Solide en dépôt chimique en phase vapeur. La croissance assistée par matrice et la croissance par catalyseurs en or des nanofils à diamètres contrôlés ont été réalisés à l'aide d'alumine nanoporeuse comme matrice. Les nanofils sont fabriqués selon la géométrie interne des nanopores, dans ce cas le profil de surface des nanofils peut être modifié en fonction de la géométrie des nanopores. Profitant de ce fait, la croissance à haute densité de nanofils modulés en diamètre a également été démontrée, où l'amplitude et la période de modulation peuvent être facilement contrôlées pendant la fabrication des matrices. Même en modulant les diamètres pendant la croissance, les nanofils ont été structurellement caractérisés comme étant monocristallins par microscopie électronique à transmission et analyse par diffraction des rayons X.La caractérisation thermique de ces nanofils a révélé une forte diminution de la conductivité thermique en fonction du diamètre, dont la réduction était principalement liée à une forte diffusion par les surfaces. La contribution du libre parcours moyen à la conductivité thermique observée dans ces matériaux "bulk" varie beaucoup, Bi2Te3 ayant une distribution en libre parcours moyen (0,1 nm à 15 nm) très faible par rapport aux autres matériaux. Même alors, des conductivités thermiques réduites (~40%) ont été observées dans ces alliages attribuées à la diffusion par les surfaces et par les impuretés. D'autre part, le silicium et le germanium ont une conductivité thermique plus élevée avec une plus grande distribution de libre parcours moyen. Dans ces nanofils, une réduction significative (facteur 10 à 15 ) a été observée avec une forte dépendance avec la taille des nanofils.Alors que les effets de taille réduisent la conductivité thermique par une meilleure diffusion sur les surfaces, le dopage de ces nanofils peut ajouter un mécanisme de diffusion par différence de masse à des échelles de longueur atomique. La dépendance en température de la conductivité thermique a été déterminée pour les nanofils dopés de silicium afin d'observer une réduction de la conductivité thermique à une valeur de 4,6 W.m-1K-1 dans des nanofils de silicium fortement dopés avec un diamètre de 38 nm. En tenant compte de la conductivité électrique et du coefficient Seebeck calculé, on a observé un ZT de 0,5. Avec l'augmentation significative de l'efficacité du silicium en tant que matériau thermoélectrique, une application pratique réelle sur les appareils n'est pas loin de la réalité. / Thermoelectric conversion has gained renewed interest based on the possibilities of increasing the efficiencies while exploiting the size effects. For instance, nanowires theoretically show increased power factors along with reduced phonon transport owing to confinement and/or size effects. In this context, the diameter of the nanowires becomes a crucial parameter to address in order to obtain high thermoelectric efficiencies. A usual approach is directed towards reducing the phononic thermal conductivity in nanowires by achieving enhanced boundary scattering while reducing diameters.In this work, thermal characterisation of a dense forest of silicon, germanium, silicon-germanium and Bi2Te3 alloy nanowires is done through a sensitive 3ω method. These forest of nanowires for silicon, germanium and silicon-germanium alloy were grown through bottom-up technique following the Vapour-Liquid-Solid mechanism in Chemical vapour deposition. The template-assisted and gold catalyst growth of nanowires with controlled diameters was achieved with the aid of tuneable nanoporous alumina as templates. The nanowires are grown following the internal geometry of the nanopores, in such a case the surface profile of the nanowires can be modified according to the fabricated geometry of nanopores. Benefiting from this fact, high-density growth of diameter-modulated nanowires was also demonstrated, where the amplitude and the period of modulation can be easily tuned during the fabrication of the templates. Even while modulating the diameters during growth, the nanowires were structurally characterised to be monocrystalline through transmission electron microscopy and X-ray diffraction analysis.The thermal characterisation of these nanowires revealed a strong diameter dependent decrease in the thermal conductivity, where the reduction was predominantly linked to strong boundary scattering. The mean free path contribution to the thermal conductivity observed in the bulk of fabricated nanowire materials vary a lot, where Bi2Te3 has strikingly low mean free path distribution (0.1 nm to 15 nm) as compared to the other materials. Even then, reduced thermal conductivities (~40%) were observed in these alloys attributed to boundary and impurity scattering. On the other hand, silicon and germanium have higher thermal conductivity with a larger mean free path distribution. In these nanowires, a significant reduction (10-15 times) was observed with a strong dependence on the size of the nanowires.While size effects reduce the thermal conductivity by enhanced boundary scattering, doping these nanowires can incorporate mass-difference scattering at atomic length scales. The temperature dependence of thermal conductivity was determined for doped nanowires of silicon to observe a reduction in thermal conductivity to a value of 4.6 W.m-1K-1 in highly n-doped silicon nanowires with 38 nm diameter. Taking into account the electrical conductivity and calculated Seebeck coefficient, a ZT of 0.5 was observed. With these significant increase in the efficiency of silicon as a thermoelectric material, a real practical application to devices is not far from reality.

Doctor of Philosophy / Department of Chemistry / Takashi Ito / Self-organized nanoporous materials have drawn a lot of attention because the uniform, highly dense, and ordered cylindrical nanopores in these materials provide a unique platform for chemical separations and chemical sensing applications. Here, we explore self-organized nanopores of PS-b-PMMA diblock copolymer thin films and anodic gallium oxide for chemical separations and sensing applications. In the first study, cyclic voltammograms of cytochrome c on recessed nanodisk-array electrodes (RNEs) based on nanoporous films (11, 14 or 24 nm in average pore diameter; 30 nm thick) derived from polystyrene-poly(methylmethacrylate) diblock copolymers were measured. The faradic current of cytochrome c was observed on RNEs, indicating the penetration of cytochrome c (hydrodynamic diameter ≈ 4 nm) through the nanopores to the underlying electrodes. Compared to the 24-nm pores, the diffusion of cytochrome c molecules through the 11- and 14-nm pores suffered significantly larger hindrance. The results reported in this study will provide guidance in designing RNEs for size-based chemical sensing and also for controlled immobilization of biomolecules within nanoporous media for biosensors and bioreactors. In another study, conditions for the formation of self-organized nanopores of a metal oxide film were investigated. Self-organized nanopores aligned perpendicular to the film surface were obtained upon anodization of gallium films in ice-cooled 4 and 6 M aqueous H2SO4 at 10 V and 15 V. The average pore diameter was in the range of 18 ~ 40 nm, and the anodic gallium oxide was ca. 2 µm thick. In addition, anodic formation of self-organized nanopores was demonstrated for a solid gallium monolith incorporated at the end of a glass capillary. Nanoporous anodic oxide monoliths formed from a fusible metal will lead to future development of unique devices for chemical sensing and catalysis. In the final study, surface chemical property of self-organized nanoporous anodic gallium oxide is explored through potentiometric measurements. The nanoporous anodic and barrier layer gallium oxide structures showed slow potentiometric response only at acidic pH (≤ 4), in contrast to metallic gallium substrates that exhibited a positive potentiometric response to H⁺ over the pH range examined (3-10). The potentiometric response at acidic pH probably reflects some chemical processes between gallium oxide and HCl.

<p>In this doctoral project, a relatively new form of carbon material, with unique narrow pore size distribution around 7 Å and with uniform structure, has been electrochemically characterised using the single particle microelectrode technique. The carbon has been used as electrode material for supercapacitors. This type of capacitors is used as high power energy buffers in hybrid vehicles and for stationary power backup. The principle for the microelectrode technique consists of connecting a carbon particle with a carbon fibre by means of a micromanipulator. The single particle and carbon fibre together form a microelectrode. Combination of this technique with electroanalytical methods such as cyclic voltammetry and potential step measurements allows for the survey of electrochemical phenomena and for the determination of ion transport parameters inside the nanopores.</p><p>A mathematical model based on Fick’s second law, for diffusion of ions inside the nanopores at non steady state, was used for the determination of effective diffusion coefficients (Deff). The coefficients were calculated from an asymptotic solution of Fick’s equation, applied for a thin layer adjacent to the external surface of the carbon particles and valid for the current response in a short time region. Another asymptotic solution was obtained, using spherical geometry and valid for the current response in a long time region.</p><p>In this doctoral work, the carbon particles have been exposed to potential cycling, which mimics that of large electrodes during operation of a double layer capacitor. The potential-current response, E-I, for the nanoporous carbon, shows a pure capacitive behaviour between –0.5 V and 0.1 V vs. the Hg|HgO reference electrode. The detection of the faradaic processes beyond these potentials was possible by lowering of the voltammometric sweep rate. The electrochemical processes occurring at positive and at negative potential were investigated separately.</p><p>Cyclic voltammometric measurements showed that the chemisorption of hydroxyl groups, occurring between 0.1 and 0.3 V, leads to a mild oxidation of the carbon structure, resulting in surface groups containing an oxygen atom at a specific carbon site (e.g., phenolic or quinine type). These oxygen-containing surface groups caused an increase of the specific capacitance, which remained constant throughout a number of voltammometric cycles. The Deff decreased on the other hand with the number of cycles. The Deff decreases also with the positive potential. The evaluation of Deff indicates adsorption of hydroxyl groups and an increase of the effective tortuosity of the pore system.</p><p>The oxidation of the carbon particles, between 0 and 0.5 V, leads to more extensive oxidation and to surface groups containing two oxygen atoms at a single carbon site, followed by formation of carbonate ions. The oxygen-containing surface groups and carbonate ions formed at these potentials do not contribute to the specific capacitance and drastically retard or obstruct the ion transport inside the nanopores.</p><p>At negative potentials the carbon particles show a dominantly capacitive behaviour. The faradaic processes taking place below –0.5 V vs. Hg|HgO reference electrode are generation and adsorption of hydrogen. These processes do not perturb significantly the electrochemical and ion transport properties of the nanoporous carbon particles. It was found that hydrogen generation occurs at –0.5 V vs. Hg|HgO and that two hydrogen oxidation processes take place at positive potentials. The results indicate that the weakly adsorbed hydrogen undergoes oxidation between 0 and 0.1 V and that the strongly adsorbed hydrogen is oxidised at more positive potentials.</p><p>The single particle technique was adapted for the determination of diffusion coefficients of an organic electrolyte. The different size of the anions and cations caused different transport characteristics at negative and positive potentials. Slow cycling was found important for ion penetration inside the nanopores and for the evaluation of the effective diffusion coefficients.</p><p>The effective diffusion coefficients for the nanoporous carbon using aqueous 6M KOH and 0.1M TEABF4 in acetonitrile were estimated to 1.4 (±0.8).10-9 cm2 s-1 and 1.3 (±0.4) 10-8 cm2 s-1, respectively.</p>

<p>För att undersöka skillnad i prestationsförmåga mellan celler sensiterade med växtextraktsbaserad färg, och celler sensiterademed ruteniumkomplex-baserad färg, samt huruvida presskraften påverkar en cells prestationsförmåga, tillverkades icke-slutna fotoelektrokemiska färg-sensiterade solceller med tunnfilmsfotoelektroder av pressad, nanoporös titandioxid.</p><p>Cellerna pressades med tre olika presskrafter och sensiterades med växtextraktsfärg från rödkål, rödbeta, viol och henna, samt en ruteniumkomplex-baserad färg som fick utgöra kontrollbetingelse. För varje cell uppmättes IPCE- och iV-värde och motsvarande fyllnadsgrad (fill factor) och dessa jämfördes.</p><p>Ingen signifikant skillnad kunde fastställas mellan celler pressade med olika presstryck. Bland cellerna sensiterade med växtextraktbaserad färg presterade rödbeta bäst. Cellen med högst effektivitet hade fyllnadsgraden 70%. Emellertid uppvisade de växtfärgade cellerna genomgående sämre effektivitet än de rutenium-sensiterade och fotoströmmarna var mycket låga. IPCE-värdena var allmännt låga: den bäst presterande cellen hade ett IPCE-värde på något över 0,06 i våglängdsintervallet 440-470 nm. En förklaring till detta är de övriga ämnen som förutom pigment återfinns i de växtbaserade färgerna. Dessa hindrar pigmentmättnad och förhindrar att växtfärgen når ruteniumfärgens intensitet. En annan anledning består i svårigheten att passa ihop energinivåerna i cellens elektrolyt-halvledarsystem med energinivåerna hos pigmentet i växtfärgen.</p> / <p>Non-sealed photoelectrochemical dye sensitised solar cells (DSSC) with pressed nanoporous TiO2 thin film photoelectrodes were manufactured for the purposes of finding out whether plant extractbased dye sensitised cells can perform as well as ruthenium complex-based dye sensitised cells and whether the pressing force affects the cell performance.</p><p>The cells were pressed with three different pressing forces and sensitised with plant extracts from red cabbage, beetroot, violet and henna, as well as with a ruthenium complex-based dye for comparison. The IPCE and iV values and the corresponding fill factors of the cells were evaluated and compared.</p><p>No significant difference between the cells pressed with different pressing forces could be established. Among the plant extract-based dye sensitised cells the ones sensitised with beetroot extract performed best. The cell that achieved the highest efficiency had a fill factor of 70%. Compared to the ruthenium-sensitised cells the overall performance of the plant dye sensitised cells were very poor and the produced photocurrents very low. The IPCE values were generally low: one of the best-performing cells had an IPCE value of slightly over 0.06 in the 440-470 nm wavelength ranges. One reason for this is that it is difficult to obtain a plant extract dye as intense and deep in colour as ruthenium complex-based dyes, since pigment saturation is obstructed by the presence of other chemical compounds in the plant extracts. Another is that it is a delicate and difficult matter to match the energy levels in the electrolyte-semiconductor system with the energy levels of the pigments in the plant extract dye.</p>