Nanoscale materials have many potential advantages because of their quantum confinement, cost and producibility by low-temperature chemical methods. Advancement of theoretical methods as well as the availability of modern high-performance supercomputers allow us to control and exploit their microscopic properties at the atomic scale, hence making it possible to design novel nanoscale molecular devices with interesting features (e.g switches, rectifiers, negative differential conductance, and high magnetoresistance). In this thesis, state-of-the-art theoretical calculations have been performed for the quantum transport properties of nano-structured materials within the framework of Density Functional Theory (DFT) and the Nonequilibrium Green's Function (NEGF) formalism. The switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes is investigated. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond, and thus lowers the conductance. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation. Also examined is current-induced migration of atoms in nanoscale devices that plays an important role for device operation and breakdown. We studied the migration of adatoms and defects in graphene and carbon nanotubes under finite bias. We demonstrate that current-induced forces within DFT are non-conservative, which so far has only been shown for model systems, and can lower migration barrier heights. Further, we investigated the quantum transport behavior of an experimentally observed diblock molecule by varying the amounts of phenyl (donor) and pyrimidinyl (acceptor) rings under finite bias. We show that a tandem configuration of two dipyrimidinyl-diphenyl molecules improves the rectification ratio, and tuning the asymmetry of the tandem set-up by rearranging the molecular blocks greatly enhances it. It has been recently demonstrated that the large band gap of boronitrene can be significantly reduced by carbon functionalization. We show that specific defect configurations can result in metallicity, raising interest in the material for electronic applications. In particular, we demonstrate negative differential conductance with high peak-to-valley ratios, depending on the details of the material, and identify the finite bias effects that are responsible for this behavior. Also, we studied the spin polarized transport through Mn-decorated topological line defects in graphene. Strong preferential bonding is found, which overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced π magnetism. Finally, the results obtained herein advance the field of quantum electronic transport and provide significant insight on switches, rectification, negative differential conductance, magnetoresistance, and current-induced forces of novel nanoscale materials.

Many of the most pressing global challenges today and in the future center around the scarcity of sustainable energy and water sources. The innovative microbial fuel cell (MFC) technology addresses both as it utilizes bacteria to convert wastewaters into electricity. Advancing this technology requires a better understanding of the optimal materials, designs and conditions involved. The micro-sized MFC was recently developed to serve this need by providing a rapid testing device requiring only a fraction of the materials. Further, development of micro-liter scale MFCs has expanded into potential applications such as remote and self-sustained power sources as well as on-chip energy generators. By using microfabrication, the fabrication and assembly of microsized MFCs is potentially inexpensive and mass produced. The objective of the work within this dissertation was to explore and optimize the micro-sized MFC to maximize power and current generation towards the goal of a usable and application-oriented device. Micro-sized MFCs were examined and developed using four parameters/themes considered most important in producing a high power generating, yet usable device: Anode- The use of nano-engineered carbon nanomaterials, carbon nanotubes and graphene, as anode as well as testing semiconductor industry standard anode contact area materials for enhanced current production. 5 Cathode- The introduction of a membrane-less air cathode to eliminate the need for continuous chemical refills and making the entire device mobile. Reactor design- The testing of four different reactor designs (1-75 μLs) with various features intended to increase sustainability, cost-effectiveness, and usability of the microsized MFC. Fuels- The utilization of real-world fuels, such as industrial wastewaters and saliva, to power micro-sized MFCs. The micro-sized MFC can be tailored to fit a variety of applications by varying these parameters. The device with the highest power production here was designed to be an inexpensive and robust power source in applications like point-of-care diagnostics in developing countries. This 25 μL graphene nanomaterial anode, air cathode device in an inexpensive flexible rubber architecture was powered by saliva and achieved 3.55 μW/cm2 and 35.2 W/m3. The continued optimization of MFC technology promises many interesting and innovative applications.

No description available.

Sodium-ion batteries (SIBs) are rechargeable batteries analogous to lithium-ion batteries but use sodium ions (Na+) as the charge carriers. They are considered a promising alternative for lithium-ion batteries (LIBs) in renewable large-scale energy storage applications due to their similar electrochemical mechanisms and abundant sodium resources. Two-dimensional (2D) materials, with atomic or molecular thickness and large lateral lengths, have emerged as important functional materials due to their unique structures and excellent properties. These 2D nanosheets have been highly studied as sodium-ion battery anodes. They have large interlayer spacing, which can effectively buffer the big volume expansion and prevent electrode collapse during the charge-discharge process. Different strategies such as preparing composites, heterostructures, expanded structures, and chemical functionalization can greatly improve cycling stability and lead to high reversible capacity. In this dissertation, state-of-the-art SIB based on 2D material electrodes will be presented. In particular, Tin-based 2D materials and laser-scribed graphene anodes are discussed. Different strategies involving engineering both synthesis methods, intrinsic properties of materials, and device architecture are used to optimize the battery performance.

>Magister Scientiae - MSc / Anode catalysts are one of the key components of direct alcohol fuel cells (DAFCs). They play a huge role in the alcohol oxidation reaction (AOR) that occurs on the anode side. Palladium (Pd) supported on carbon material has been reported to have good catalytic activity towards alcohol oxidation reactions. Better stability and activity has been reported for catalysts supported on conductive polymers like polypyrrole (PPy) when compared to traditional carbon support material. This study investigated the effect of support materials on Pd and PdCo electro-catalysts while concurrently determining the support material that can improve the activity and stability of Pd and PdCo electro-catalysts used as direct alcohol fuel cells catalysts. All Pd and PdCo catalysts supported onPPy (prepared using oxidative polymerization method), reduced graphene oxide (rGO) and prepared using modified Hammers method and multi-walled carbon nanotubes and pre-treated by acid. All the catalysts were synthesized using the modified polyol method.

The magnetic properties of axially confined, hydrogenated single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24 are systematically explored by density functional theory. Emphasis is placed on the relation between the ground-state magnetic moments of SWCNTs and zigzag graphene nanoribbons (ZGNRs). Comparison between the SWCNTs considered here and ZGNRs of equal length gives rise to two basic questions: 1) how does the nanotube curvature affect the antiferromagnetic order known to prevail for ZGNRs, and 2) to what extent do the magnetic moments localized at the SWCNT edges deviate from the zero-curvature limit of n/3 μB? In response to these questions, it is found that systems with n≥7 display preference for antiferromagnetic order at any length investigated, whereas for n=5, 6 the magnetic phase varies with tube length. Furthermore, elementary patterns are identified that describe the progression of the magnitude of the magnetic moment with n for the longest tubes explored in this work. The spin densities of the considered SWCNTs are analyzed as a function of the tube length L, with L ranging from 3 to 11 transpolyene rings for n≥7 and from 3 to 30 rings for n=5 and 6. Magnetic carbon nanostructures are explored by density functional theory calculations on axially confined, single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24. For SWCNTs with n≥7, antiferromagnetic (AFM) order is favored energetically over ferromagnetic (FM) order for all lengths L investigated, whereas for n=5, 6 the magnetic phase varies with tube length (see picture).

The magnetic properties of axially confined, hydrogenated single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24 are systematically explored by density functional theory. Emphasis is placed on the relation between the ground-state magnetic moments of SWCNTs and zigzag graphene nanoribbons (ZGNRs). Comparison between the SWCNTs considered here and ZGNRs of equal length gives rise to two basic questions: 1) how does the nanotube curvature affect the antiferromagnetic order known to prevail for ZGNRs, and 2) to what extent do the magnetic moments localized at the SWCNT edges deviate from the zero-curvature limit of n/3 μB? In response to these questions, it is found that systems with n≥7 display preference for antiferromagnetic order at any length investigated, whereas for n=5, 6 the magnetic phase varies with tube length. Furthermore, elementary patterns are identified that describe the progression of the magnitude of the magnetic moment with n for the longest tubes explored in this work. The spin densities of the considered SWCNTs are analyzed as a function of the tube length L, with L ranging from 3 to 11 transpolyene rings for n≥7 and from 3 to 30 rings for n=5 and 6. Magnetic carbon nanostructures are explored by density functional theory calculations on axially confined, single-walled carbon nanotubes (SWCNTs) of the (n,0)-type with n=5-24. For SWCNTs with n≥7, antiferromagnetic (AFM) order is favored energetically over ferromagnetic (FM) order for all lengths L investigated, whereas for n=5, 6 the magnetic phase varies with tube length (see picture).

No description available.

Several attempts have been made to combine the unique characteristics of graphene oxide (GO) and commercial polymers for successfully designing and fabricating next-generation membranes in filtration and separation technologies. The first part of the work develops a high flux polyethersulfone ultrafiltration membranes, by embedding GO sheets, starting from the polymer/GO solutions in ionic liquid and N, N dimethylformamide as co-solvents and promoting the pore formation via non-solvent induced phase separation. In the second part of the work, a protic ionic liquid was introduced as a solvent to disperse GO sheets and fabricate GO liquid crystal membranes for nanofiltration. The third part addresses the stability enhancement. GO membranes frequently disintegrate in aqueous environments due to swelling. Ethylenediamine was then used as a crosslinker, and the membranes were tested for organic solvent nanofiltration. Additionally, overcoming the permeation-rejection trade-off is challenging. Hence, the fourth work involved the intercalation of silica nanoparticles to form dual-sized nanochannels. In the final work, GO membranes were fabricated on the surface of hollow fibers to overcome scalability issues, by using a feasible spray coating method for efficient nanofiltration. Hollow fibers were crosslinked with hexamethylene diamine and GO was spray-coated on the crosslinked polymeric fibers for organic solvent nanofiltration. Overall, this study demonstrates the potential of GO in developing high-performance membranes for liquid separations relevant for industrial applications, such as wastewater treatment, food, chemical, petrochemical, and pharmaceutical processing.

La modification des propriétés du graphène, notamment l’ouverture d’une bande interdite par la nanostructuration, est un véritable enjeu pour la physique et pour les applications du graphène. La nanostructuration peut se faire suivant l’approche « top-down » ou « bottom-up ». Au cours de cette thèse nous nous sommes intéressés à la seconde approche. L’approche « bottom-up » permet de contrôler à l’atome près la structure des matériaux. L’objectif de cette thèse est de fabriquer par synthèse chimique des boites quantiques de graphène et des motifs graphéniques contenant un réseau périodique de trous (nanomesh) et d’en étudier les propriétés physiques. Dans une première partie, une « famille » de nanoparticules de graphène a été préparée par synthèse organique via des réactions de Diels-Alder et de Scholl et les propriétés optiques ont été étudiées sur des solutions et à l’échelle de la molécule unique. Dans une deuxième partie, un nouveau type de structures graphéniques intermédiaires entre les boites quantiques et les nanorubans, des nano-bâtonnets de graphène (nanorods) ont été synthétisés. Enfin, plusieurs précurseurs ont été synthétisés pour la réalisation de nanomeshs de graphène. Ces précurseurs permettront d’obtenir, en utilisant le dépôt chimique en phase vapeur dans la chambre d’un microscope à effet tunnel, des nanomesh de graphène présentant des structures différentes. / The manipulation of the electronic properties of graphene, and in particular the bandgap opening by nano-patterning, is a crucial issue for both physics and applications. The nanostructuration can be done either through the top-down approach or the bottom-up approach. This bottom-up approach allows controlling at the atomic level the structure of the materials. The aim of this thesis is to prepare graphene quantum dots and graphene nanomeshes (regular arrays of holes in a graphene sheet) by chemical synthesis, and to study their physical properties. In the first part, a “family” of graphene quantum dots was prepared with organic chemistry via Diels-Alder and Scholl reactions and the optical properties were studied both in solution and at the single molecule scale. In the second part, a new type of graphenic structures intermediate between quantum dots and nanoribbons were synthesized and we named them “graphene nanorods”. These objects are one dimensional but have a controlled length compared to nanoribbons prepared via polymerization. Finally, various precursors were synthesized to create graphene nanomeshes. These precursors will allow the formation, using chemical vapor deposition in a scanning tunneling microscope chamber, of nanomeshes exhibiting different structures and morphology.