Ce manuscrit présente une étude expérimentale sur les propriétés optiques des nanorubans de graphène (acronyme anglais : GNRs) et des boites quantiques de graphène (acronyme anglais : GQDs) synthétisés par la chimie ascendante.Pour la partie sur les GNRs, les spectres d'absorption et de photoluminescence ainsi que les mesures de la durée de vie sur la dispersion impliquent la formation d'états excimères résultant de l'agrégation des GNRs. Au moyen de la microscopie confocale et de la microscopie à force atomique, nous observons l'émission de petits agrégats de GNRs confirmant leur capacité à émettre de la lumière à l'état solide. D'autre part, les caractérisations optiques des GNRs synthétisés sur une surface d’or présentent des caractéristiques de Raman remarquables, impliquant les propriétés vibrationnelles spécifiques des GNRs par rapport au graphène et aux nanotubes de carbone. La PL observée est spectralement large avec une énergie plus élevée que celle de la bande interdite des GNRs. Cela pourrait être lié aux défauts créés lors de la préparation de l'échantillon.Pour la partie sur les GQDs, les résultats de spectroscopie optique indiquent que les GQDs sont individualisées en dispersion plutôt que sous la forme d’agrégats. Ensuite, grâce à la microphotoluminescence, nous abordons directement les propriétés intrinsèques des GQDs uniques. Des mesures de corrélation de photons de second ordre révèlent que les GQDs présentent une émission de photons uniques avec une grande pureté. De plus, l'émission de GQD présente une bonne photo-stabilité avec une brillance élevée. Comme premier exemple de l'accordabilité optique des GQDs via le contrôle de la structure, nous observons que l'émission de GQDs fonctionalisés avec des atomes de chlore est décalée de près de 100 nm tout en maintenant une émission de photons uniques. / This manuscript presents an experimental study on the optical properties of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs) synthesized by bottom-up chemistry.For the part on GNRs, the optical absorption and photoluminescence spectra as well as the life-time measurements on the dispersion of solution-mediated synthesized GNRs implies the formation of excimer states as a result of aggregation of GNRs. By means of confocal fluorescence microscopy and atomic force microscopy, we observe the emission of small GNR aggregates confirming the ability of GNRs to emit light in the solid state. On the other hand, the optical characterizations of on-surface synthesized GNRs shows remarkable Raman features, implying the distinct vibrational properties of GNRs compared to graphene and carbon nanotubes. The observed PL is spectrally broad with higher energy instead of a bright bandgap emission, which might be related to the defects created during the sample preparation.For the part on GQDs, the optical spectroscopy results indicate that GQDs are individualized in dispersions rather than in the form of aggregates. Then by means of microphotoluminescence, we directly address the intrinsic properties of single GQDs. Second-order photon correlation measurements reveal that GQDs exhibit single-photon emission with a high purity. Notably, the emission of GQDs has good photo-stability with high brightness. As a first example of the optical tunability of GQDs through the control of their structure, we observe that the emission of single edge-chlorinated GQDs is redshifted by almost 100 nm while maintaining the single-photon emission.

Matériaux à base de graphène et d’oxyde de graphène ont attiré une grande attention depuis sa découverte. Cependant, comme la feuille de graphène a une surface spécifique élevée, il tend à former un agglomérat irréversible ou même empiler pour former le graphite par π-π empilage et Van-der Waals interactions. Les modifications doivent être faites pour séparer les feuilles de graphène sans apporter trop de dégâts dans sa structure aromatique. Dans cette thèse, nous avons lancé deux méthodes pour faire la modification du graphène, réaction de substitution nucléophile pour l’oxyde de graphène avec un C/O ~ 2 (FGS2), tandis que la demande électronique inverse réaction de Diels-Alder pour l’oxyde de graphène avec un très faible teneur en oxygène C/O ~ 20 (FGS20). Comme dans le second cas, FGS20 fonctionnalisés par tetrazine possède une excellente conductivité, il a été en outre combiné avec un polypyrrole pour fabriquer un matériau de supercondensateur.Dans le chapitre 2, nous avons greffé de manière covalente des dérivés de tétrazine à l'oxyde de graphène par substitution nucléophile. Comme l'unité de tétrazine est électroactif et riche en azote, avec un potentiel de réduction sensible du type de substituant et degré de substitution, nous avons utilisé l'électrochimie et la spectroscopie de photoélectrons X pour démontrer des preuves claires pour le greffage par liaison covalente. La modification chimique a été soutenue par spectroscopie infrarouge à transformée de Fourier et analyse thermique. Tétrazines greffé sur l'oxyde de graphène affichent différentes pertes de masse par rapport à graphène non modifiée et sont plus stables que les précurseurs moléculaires. Enfin, un dérivé de pontage tétrazine a été greffée entre des feuilles d'oxyde de graphène pour démontrer que la distance de séparation entre les feuilles peut être maintenue lors de la conception de nouveaux matériaux à base de graphène, y compris les structures d'oxydo-réduction chimiquement liés, les structures d'oxydoréduction.Dans le chapitre 3, des molécules modèles de graphène ont été sélectionnés afin de déterminer les conditions optimales de réaction entre graphène et tétrazine dérivés. Toutes les molécules de tétrazine ont d'abord été étudiés par électrochimie et ensuite mis à réagir avec le graphène par la demande électronique inverse Diels-Alder (DAinv) réaction dans un réacteur à micro-ondes, la XPS a été réalisée pour étudier sa composition chimique et de prouver la modification avec succès du graphène. Ensuite, le matériau de graphène tétrazine fonctionnalisé a été appliqué sur une électrode en acier inoxydable et ses performances électrochimiques ont été évaluées par voltamétrie cyclique et les tests de charge-décharge. La plupart des tétrazine modifié matériaux de graphène a montré de très bonnes performances électrochimiques et une faible résistance due à une bonne accessibilité des ions, ce qui en fait l'un des matériaux d'électrodes les plus prometteuses pour les supercondensateurs jusqu'à présent. Dans le chapitre 4, polypyrrole (PPy)-graphène nanocomposites ont été synthétisés par polymérisation de PPy sur les feuilles de graphène fonctionnalisés par tétrazine. Le matériau de graphène modifié contient des unités pyridazine tel que démontré par XPS. Puis PPy a été déposé sur ce matériau de graphène fonctionnalisé soit par polymérisation chimique ou électrochimique. Cellules de pièces symétriques ont été faites pour mesurer la capacité dans une configuration à deux électrodes. Les nanocomposites de polypyrrole-graphène avec 40% PPy présentent les meilleures performances électrochimiques et une faible résistance en raison d'une bonne accessibilité des ions, ce qui en fait l'un des meilleurs matériaux d'électrodes pour supercapacitor jusqu'à présent. / Graphene and graphene oxide based materials have attracted great attention since its discovery. However, as graphene sheet has a high specific surface area, it tends to form an irreversible agglomerates or even restack to form graphite through π–π stacking and van-der Waals interactions. Modifications need to be done to separate graphene sheets without bringing too much damage in its aromatic structure.In this thesis, two methods have been introduced to do the modification of graphene, nucleophilic substitution reaction for graphene oxide with a C/O~2 (FGS2), while inverse electron demand Diels-Alder reaction for graphene oxide with a very low oxygen content C/O~20 (FGS20). As in the latter case, tetrazine functionalized FGS20 has excellent conductivity, it has been further combined with polypyrrole to fabricate supercapacitor material.In chapter 2, we have covalently grafted tetrazine derivatives to graphene oxide through nucleophilic substitution. Since the tetrazine unit is electroactive and nitrogen-rich, with a reduction potential sensitive to the type of substituent and degree of substitution, we used electrochemistry and X-ray photoelectron spectroscopy to demonstrate clear evidence for grafting through covalent bonding. Chemical modification was supported by Fourier transform infrared spectroscopy and thermal analysis. Tetrazines grafted onto graphene oxide displayed different mass losses compared to unmodified graphene and were more stable than the molecular precursors. Finally, a bridging tetrazine derivative was grafted between sheets of graphene oxide to demonstrate that the separation distance between sheets can be maintained while designing new graphene-based materials, including chemically bound, redox structures.In chapter 3, model molecules of graphene were selected to determine the optimal reaction conditions between graphene and tetrazine derivatives. All tetrazine molecules were firstly studied by electrochemistry and then reacted with graphene through inverse electron demand Diels-Alder (DAinv) reaction in microwave reactor, X-ray photoelectron spectroscopy was carried out to study its chemical composition and prove the successfully modification of graphene. Then the tetrazine functionalized graphene material was coated on a Stainless Steel electrode and its electrochemical performances were assessed by cyclic voltammetry and charge-discharge experiments. Most of the tetrazine modified graphene materials showed very good electrochemical performance and a small resistance due to a good ion accessibility, which makes it one of the most promising electrode materials for supercapacitors so far.In chapter 4, polypyrrole (PPy)-graphene sheet nanocomposites have been synthesized by both chemical and in situ electrochemical polymerization of PPy on tetrazine derivatives functionalized graphene sheets. The modified graphene material contains pyridazine units as demonstrated by XPS. Then PPy was deposited on this functionalized graphene material either by chemical or electrochemical polymerization. Symmetrical coin cells were made to measure the capacitance in a two-electrode configuration. Polypyrrole-graphene nanocomposites with 40% PPy show the best electrochemical performances, with a very large capacitance per weight (326 F g-1 at 0.5 A g-1 and 250 F g-1 at 2 A g-1) and a small resistance due to a good ion accessibility, which makes it one of the best electrode materials for supercapacitors so far.

The need for accessible, safe and reliable energy storage solutions has been accentuated, in recent years, due to the shift from fossil to renewable energy sources. In this context, aluminum-based electrochemical systems have emerged as strong candidates for energy storage devices. Despite that, the successful translation from the laboratory and the commercialization of the technology faces critical challenges that must be overcome. This Dissertation explores carbon and carbon-inorganic cathodes for Al-based electrochemical energy storage devices. We start by understanding carbon cathodes in the presence of acidic ionic liquid electrolytes and draw relevant conclusions on how transition metal catalysts affect different facets of the cell's electrochemical performance. Then, we introduce sulfur and draw insights on the origin of poor cycling stability of carbon/sulfur cathodes as well as on how to extend their cycle life. Next, we focus on maximizing the pseudocapacitive contribution of carbons, and thus cathode capacity, through pore size engineering. Finally, we translate our findings to aqueous electrolytes and fabricate, for the first time, a superior rechargeable aluminum-carbon battery cathode by setting forward a hypothesis of a unique charge-storage mechanism.

Energy Dispersive X-Ray Spectroscopy (EDS) is a technique used to analyze materials to determine their elemental makeup. This technique is used extensively in the semiconductor industry, metallurgical industry, biology, chemistry, materials science, and other fields. EDS detectors are often attached to scanning electron microscopes (SEM) or transmission electron microscopes (TEM) and are actively cooled by liquid nitrogen or Peltier devices. A thin membrane, or window, is fitted to the front of the detector which allows for an airtight seal as well as transmission of x-rays. The challenge for these windows is maximizing the transmission of x-rays while maintaining mechanical integrity. Carbon is an element with a low atomic number and has several allotropes that have attributes desirable for an x-ray window. Amorphous carbon has good chemical resistance as well as being able to be sputtered, a low temperature process. Sputtered amorphous carbon is characterized in this work, including sputtered amorphous carbon that is used as part of x-ray windows. Part of this characterization involved using bulge testing. A bulge testing device was created at BYU and this is presented here. Additionally, this device was used to characterize thin films as part of this work. Graphene is a single layer of sp2 bonded carbon atoms in a plane. It is one of the stiffest materials known, as well as having an extremely high tensile strength (> 200 times steel). Single layer graphene has not been able to span the dimensions needed for use in a detector window, but many-layer graphene (MLG, a film with > 10 stacked layers of graphene) has been shown to span mm size openings. Many-layer graphene films were grown using chemical vapor deposition (CVD) on nickel substrates and suspended over different sized openings as well as on a silicon support structure. A description of synthesis and characterization of these films are presented here. Also presented is additional work to improve the fabrication of these MLG films by developing improved nickel substrate surfaces.

Graphene is a 2D material well known for its high intrinsic strength of 100 GPa and Young’s modulus of 1 TPa. Because of its 2D nature, the most promising avenues to utilize graphene as a mechanical material include incorporating it as reinforcement in a nanocomposite and creating free-standing foams and aerogels. However, the current techniques are not well-controlled – the reinforcing graphene particles are often discontinuous and randomly dispersed – making it difficult to accurately model and predict the resulting material properties. Here we aim to develop a framework for a new class of nanocomposites reinforced not by discrete nanoparticles, but by a continuous 3D graphene network. These 3D graphene networks were formed by chemical vapor deposition of graphene on periodic metallic microlattices, thereby providing mechanical reinforcement for the lattices. To assist in the lattice design, analytical models were derived for the mechanical properties of core/shell composite lattices and experimentally validated through compression testing of polymer lattices coated with electroless Ni-P. The models and experiments showed good agreement at lower shell thicknesses, while there was divergence at higher thicknesses, likely due to fabrication imperfections. The analytical models were also applied to hollow metallic lattices coated with graphene and compared to experimental data. The results showed that the models are plausible and suggest that graphene has a significant strengthening effect on the microlattices. These studies represent a paradigm shift in the design and fabrication of nanocomposites as one may now precisely prescribe the placement of the reinforcing nanomaterials. On a broader scale, this work also lays the framework for using a 2D material to span 3D space, enabling further exploration of 2D material properties and applications. One potential application area for a graphene-reinforced polymer composite is in prosthetic heart valves. The tissue of a human heart valve leaflet is heavily reinforced with networks of collagen and elastin fibers. One could similarly incorporate a graphene network as reinforcement within the polymeric leaflets of a prosthetic valve. One promising application of polymeric valves is in growth-accommodating implants for pediatric patients. Here we aim to develop a polymeric valved conduit that can be expanded by transcatheter balloon dilation to match a child’s growth. We designed the valve, characterized and selected materials, fabricated the devices and performed benchtop in vitro testing. The first generation of an expandable biostable valved conduit displayed excellent hydrodynamic performance before and after permanent balloon dilation from 22 to 25 mm. The second generation has shown the potential for a greater dilation from 12 to 24 mm. These results demonstrate concept feasibility and motivate further development of a polymeric balloon-expandable device to replace valves in children and avoid reoperations.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第20728号 / エネ博第356号 / 新制||エネ||70(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 坂口 浩司, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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Thesis advisor: Michelle M. Meyer / Thesis advisor: Jeffery A. Byers / Riboswitches are a class of regulatory structures located in the 5’ untranslated region of many bacterial mRNAs. Validating riboswitch-ligand interactions has historically been costly and low-throughput. Recently, graphene field-effect transistors (G-FETs) have emerged as effective biosensors in detecting interactions of such regulators with charged, high molecular weight analytes. However, a bottleneck still exists in detecting relatively neutral small molecules. The Bacillus subtilis guanine riboswitch (Xpt) within the xpt-pbuX operon contains a purine-responsive aptamer region with affinity for guanine, hypoxanthine, and other purine analogs. The G-FET sensor revealed successful detection of Xpt-hypoxanthine interactions at saturating concentrations. The specificity of Xpt was also demonstrated by a lack of signal detection when incubated with adenine. Therefore, such G-FET devices are effective in detecting aptamer binding to small, electrically-neutral molecules, which will allow for rapid screening of potential therapeutic ligands. Further, different electrical observations of n-doping upon aptamer functionalization and p-doping upon ligand binding reveal unique interactions at the graphene surface. Molecular dynamics simulations were carried out to interpret experimental results and to determine if another well characterized aptamer (FMN) is a suitable candidate for G-FET studies. Trajectory data from the Xpt aptamer domain complexed with hypoxanthine (PDBID: 4FE5) and guanine (PDBID: 1Y27) showed significant differences in root mean square deviation (RMSD) and radius of gyration (Rg) from their respective non-binding mutants. These findings provide evidence that compaction of the RNA phosphodiester backbone is responsible for graphene detection. RMSD and Rg differences from FMN (PDBID: 3F4E) indicate that this aptamer may not show a significant change in G-FET signal. These findings suggest that G-FET biosensors can provide an avenue for the discovery of novel antibiotics for aptamer targets to combat burgeoning antibiotic resistance. / Thesis (BS) — Boston College, 2021. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Biochemistry.