Graphene is a two dimensional honeycomb lattice of carbon atoms which has recently attracted considerable attention because of rapid experimental progress, and because of its novel physical properties. In this work, we will discuss recent theoretical work in which we have proposed new types of ordered electronic states in graphene bilayers, including pseudospin magnets which show spontaneous charge transfer between two layers, and excitonic superfluids which could have remarkably high transition temperatures. This work will conclude with some speculations on the possibility of radically new types of electronic devices in these systems whose operation is based on collective electronic behavior. / text

Engineered nanomaterials (NMs), defined as artificially made particles possessing at least one dimension within 1 – 100 nm range, have different physical and biological properties from bulk materials of the same chemistry due to their increased surface areas. Their novel properties have facilitated the prolific growth of commercial NM-incorporating products. NMs may be leached into the aquatic environment during the product life-cycle, but their ecological impacts on marine ecosystems are currently largely unknown. Therefore, this study primarily aimed to investigate the physicochemical characteristics (particle and aggregate sizes, dissolution rate) and in vivo toxicities of commonly-used metallic NMs to marine organisms under various environmental scenarios. First, in vivo ecotoxicity screening tests, using the marine diatom Skeletonema costatum and the rotifer Brachionus sp., were conducted for nine common nano metal oxides which had been applied in various commercial products. Among them, nano zinc oxide (nZnO) and nano magnesia (nMgO) were found to be the most toxic NMs to both organisms, as they induced oxidative stress by increasing reactive oxygen species (ROS) production in the organism. The endocrine disrupting potential of nZnO was revealed by its modulation of transcriptions of the genes for retinoid X receptor (RXR) in the rotifer. Conversely, nano alumina (nAl2O3) and nano indium oxide (nIn2O3) were the least toxic NMs. Due to its high potency, toxicity of nZnO was further evaluated using five marine organisms (i.e., microalgae Thalassiosira pseudonana and S. costatum, copepod Tigriopus japonicus, amphipod Elasmopus rapax, and medaka Oryzias melastigma). Crustaceans were generally more sensitive to nZnO. Toxicity of nZnO was mainly attributed to metal ion dissolution, although nanoparticulate effects such as aggregation and adhesion of nZnO onto the animal’s exoskeleton as well as physical disruption of cell structures could not be discounted. Due to the fact that the dissolution of nZnO decreased from 16 mg Zn L-1 at 4°C to 1.4 mg Zn L-1 at 35°C, and Zn ion was the main contributing factor for nZnO toxicity at 25°C, it was postulated that nZnO toxicity would increase with decreasing temperature. This hypothesis was tested with S. costatum, O. melastigma and the amphipod Melita longidactyla through a factorial design experiment (i.e., 2 concentrations x 4 temperatures). In agreement with the hypothesis, the growth of S. costatum was significantly inhibited by nZnO at the lowest test temperature (15°C). However, contradictive results were observed in the two animal species. For instance, the amphipod could reduce the nZnO uptake and its toxicity by undergoing metabolic depression and dormancy at lower temperatures. As the morphology and coating of NMs utilized in commercial products may differ from those employed in toxicity studies, T. japonicus was exposed to nZnO-containing sunscreens to assess the effects of nZnO and Zn2+ released in seawater during epidermal applications. Based on their genetic biomarker responses, the results suggested that other components in sunscreens could react synergistically or antagonistically on nZnO toxicity. Clearly, there is a need for further study of the combined effects of NMs and other common chemical contaminants to marine organisms. / published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy

Among various electrochemical devices that have been developed for energy storage and conversion, electric double layer capacitors (EDLCs) and direct methanol fuel cells (DMFC) have received much research attention. Nanostructured carbon materials have been playing an important role in the development of these devices, due to such characteristics as good electrical conductivity, high chemical stability, high surface area and large pore volumes and etc. In an EDLC, nanostructured carbon electrodes, possessing pores of varied length scales, can deliver electric energy at high current loadings. This kind of pore structure also benefits the deposition of metal catalysts and the transport of reactants and products in the methanol oxidation reaction. In order to systematically study the structural effects on the electrochemical capacitance and ionic transport, a series of three-dimensional hierarchical carbons with hollow core-mesoporous shell (HCMS) structure were template-synthesized. Periodically ordered macroscopic hollow cores of 330 nm in diameter were surrounded by a mesoporous shell containing uniform pores of 3.9 nm. The shell thickness was stepwise increased from 0, 25, 50 to 100 nm. The HCMS structure was modeled by a 5-level transmission line model to study the capacitance contribution from the pores at different length scale. Results revealed that the HCMS carbon with thicker mesoporous shells can provide high capacitance, while thinner shells could deliver high power output. A series of HCMS carbon sphere supported Pt nanoparticles were synthesized via the “Carbonization over Protected and Dispersed Metal” (CPDM) method. Contrary to the conventional “polyol” synthetic method, whereas most of Pt nanoparticles were deposited on the external surface of carbon spheres; the Pt nanoparticles synthesized via the CPDM method were found encapsulated in the mesoporous carbon shells and highly dispersed throughout the carbon texture. „Accelerated stress tests‟ (ASTs) were conducted to investigate the nanopores confinement effect toward the electrochemical stability of these Pt catalysts. Results revealed that (1) the nanopores confined Pt nanoparticles on HCMS carbon spheres exhibited a stable electrochemical active surface area (ECSA) and catalytic activity; and (2) thick mesoporous carbon shells could provide better protection over the Pt nanoparticles. This “CPDM” method was further extended to synthesize highly alloyed PtRu nanoparticles supported electrocatalysts. It is expected that this CPDM method can also be applied to synthesize other metal/metal oxide supported catalysts with stable electrochemical performance. WO3 has been demonstrated as a promsing co-catalyst for Pt in the methanol oxidation reaction (MOR). The synthesis of Pt-WO3/C catalyst with well-controlled nanoparticle size (2.5 nm) and composition was achieved via a microwave-assisted water-oil microemulsion reaction. Hydrogen adsorption, CO-stripping and Cu- stripping methods were used to estimate the ECSA of Pt in the Pt-WO3/C catalysts. Among these, Cu-stripping method was relatively more reliable due to the overlapping involvement of the WO3 component in the other methods. The methanol oxidation measurement shows that a 1:1 Pt:W ratio catalyst exhibits the highest Pt-mass current density of 271 mA mg-1-Pt, 1.4 times higher than that of commercial E-TEK catalyst. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

In biotechnology, microelectronics, and materials science, many products require intimate attention to microscopic and sub-microscopic construction. Bulk properties of interest often depend on the system microstructure, leading researchers to strive to tailor custom microstructures and predict properties from microstructure—increasingly difficult tasks as component sizes shrink. A promising paradigm for engineering small systems is the idea of designing components which self-assemble into the structures desired, similar to the way that biological systems routinely build themselves from the molecular level up to the macroscopic. In this thesis, I use numerical simulation to study the structural evolution of colloidal and nanoscopic particulate systems. I focus on problems in rheology and adsorption. In the rheological study, I use Stokesian dynamics to investigate a transition where the shear rate qualitatively changes the trajectories of a lattice of particles and imparts a discontinuous, hysteretic viscosity jump. My model shows that a particular face-centered cubic crystal configuration is necessary to reproduce experimental findings. The adsorption studies are approached with two different models. First is a two-dimensional model for the random sequential adsorption of tethered nanoparticles. Tethers provide robust physical and/or electrical connections between particles and a substrate, but they also frustrate order. Hexatic and crystal structures form with surprisingly short tethers of one and four particle radii, respectively. Polydispersities of less than 5–7% (and sufficient tether length) are necessary to form crystal phases, and polydispersities of less than 7–8% are necessary to create hexatic phases. The second set of adsorption studies employs full three-dimensional Brownian dynamics simulations to model electrostatically-repulsive particles that are attracted to a substrate. The zeta-potential of the wall is the primary control of order formation on the surface, and the particle potentials are the primary control of surface coverage. Mixtures of particles that are bidisperse in surface zeta-potential can disrupt order for significant ratio of zeta-potentials, and at large ratios the process creates interesting patterns including dots, clusters, chains, and doped crystals. In each study, system history has a significant effect on the final state of the system; careful attention must be paid to the non-equilibrium process of assembling small systems. / text

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