In agriculture nitrogen fixation by rhizobial inocula is an environmentally and economically beneficial alternative to synthetic fertilization. The effectiveness of rhizobial inocula can be limited by the inability of inoculum strains to compete with indigenous strains for nodule occupancy. Sinorhizobium meliloti fixes nitrogen in a complex symbiotic relationship with legume hosts including the agriculturally important forage Medicago sativa and the model legume Medicago truncatula. The ability to utilize organic compounds has emerged as an important trait for competitiveness for nodule occupancy in S. meliloti and other rhizobia. This thesis describes the use of bacterial genetics to characterize two carbon metabolism loci in S. meliloti. A genetic locus for erythritol catabolism was characterized and shown to encode an ABC transporter that is required for the catabolism of erythritol, adonitol and L-arabitol, as well as the genes for the catabolism of these three polyols. The ability to utilize erythritol was not necessary for the ability to compete for nodule occupancy in S. meliloti, in contrast to Rhizobium leguminosarum. A genetic locus that encodes components of the De Ley-Doudoroff pathway of galactose catabolism was identified and also characterized. The inability to catabolize galactose resulted in an increased ability to compete for nodule occupancy in S. meliloti. Evidence is presented that is consistent with the hypothesis that increased competitiveness resulted from enhanced production of the symbiotic exopolysaccharide succinoglycan. Inferences are drawn that contribute to the broader understanding of rhizobium-legume symbiosis. / October 2014

No description available.

There were two purposes of this investigation: the first was to develop a method for studying spore germination, hyphal growth and mycelial development of the aquatic actinomycetes under the microscope so that the life cycle of this organism could be continuously followed. The second purpose was to determine the rate of carbon utilization from various types of nutritional sources, and to correlate this with the colony development and spore formation.

The major purpose of this investigation was to determine the effect of carbon monoxide on the development of various morphological phases of a single isolate of the aquatic streptomycetes and to elucidate the physiological differences between terminal respiration in the primary and secondary stages. A secondary purpose was to repeat the work of Michael Lee Higgins involving the effect of anaerobiosis on the growth of the organism.

Planting trees on agricultural land either as farm woodlands or agroforestry (trees integrated with farming) is one option for reducing the level of atmospheric carbon dioxide. Trees store carbon as biomass, and may increase carbon storage in the ground. A review of the literature outlined uncertainty relating to changes in carbon storage after planting trees on agricultural land. The aim of this thesis is to deter¬mine the impact of tree planting on arable and pasture land in terms of above and belowground carbon storage and thereby address these uncertainties, and assess the implications for the Woodland Carbon Code: a voluntary standard for carbon storage in UK woodlands. Measurements of soil organic carbon to a depth of 1.5 m were taken at two field sites in Bedfordshire in the UK: a 19 year old silvoarable trial, and a 14 year old silvopasture and farm woodland. On average 60% and 40% of the soil carbon (rel¬ative to 1.5 m) was found beneath 0.2 and 0.4 m in depth respectively. Whilst tree planting in the arable system showed gains in soil organic carbon (12.4 t C ha−1 at 0–40 cm), tree planting in the pasture was associated with losses of soil organic carbon (6.1–13.4 t C ha−1 at 0–10 cm). Evidence from a nearby mature grazed woodland indicate that these losses may be recovered. No differences associated with tree planting were found to the full 1.5 m, though this may be due to a lack of statistical power. Measurements of above and belowground biomass, and the root distribution of 19 year old poplar (Populus spp.) trees (at the silvoarable trial) and ash (Fraxinus excelsior) trees ranging from 7 to 21 years (at several field sites across Bedfordshire) were made, involving the destructive harvest of 48 trees. These measurements suggest that Forestry Commission yield tables overestimate yield for poplar trees grown in a silvoarable system. An allometric relationship for determining ash tree biomass from diameter measurements was established. The biophysical model Yield-SAFE was updated to take into account root growth, and was parameterised using field measurements. It was successfully used to describe existing tree growth at two sites, and was then used to predict future biomass carbon storage at the silvoarable trial. Measurements indicate that losses in soil carbon at relatively shallow depths can offset a large proportion of the carbon stored in tree biomass, but assessing changes on a site by site basis may be prohibitively expensive for schemes such as the Woodland Carbon Code.

M.Tech. / Among the materials being used for gas sensors, metal oxides are the most important materials because of their potential to detect many gases at low concentrations. Nevertheless, sensors made of metal oxide need to be operated at high temperatures (above 200°C) and have a weak sel ectivity. In order to overcome this difficulty, the materials are being investigated for gas sensing applications. Carbon nanotubes (CNTs) are promising materials with unique properties, such as high electrical conductivity, mechanical strength, nanometer–scale sizes, and high aspect ratio. Their adsorption ability and high surface area make them attractive as gas sensing materials, which have been intensively studied. CNTs can be used solely or combined with metals and oxides materials in order to constitute efficient gas sensors. In the present research, multi–walled CNTs (MWCNTs) were coated with tin dioxide (SnO2) and incorporated into two epoxy resins with widely different mechanical properties in order to study the effect of CNTs on the morphology, mechanical, electrical, and sensing properties of the composites. In the MWCNT/polymer composite study, Epon 828 was used as the polymer matrix and D–2000 (which gives rubbery composites) and T–403 (which gives glassy composites) as the hardeners. Composite were prepared with 0.1 wt.% MWCNTs in an epoxy matrix. Pristine MWCNTs (MWCNTs not treated with any acid and therefore used as received) and SnO2–MWCNTs were used for comparison and a two–step curing procedure was used with initial temperature set at 75°C for 3 hours, followed by additional 3 hours at 125°C. The sample s were characterized for morphology, mechanical, thermo–mechanical and electrical properties using scanning electron microscopy (SEM), an Instron tensile tester, dynamic mechanical analysis (DMA) and Cascade Microtech four–point probe, respectively. In both cases, strong covalent bonds were created as a bridge between the CNTs and matrix, but due to differences in viscosity, the nanotubes dispersion was much better in the rubbery epoxy resin than in the glassy epoxy resin. A 77% increase in tensile modulus was observed in the rubbery system using 0.1 wt.% SnO2–MWCNTs compared to the neat rubbery epoxy. As for the glassy epoxy based composite, only a 3% improvement in tensile modulus could be observed. In addition to the mechanical properties, the presence of CNTs has demonstrated a material with high vi electrical conductivity. But for the surface measurements during the gas sensing analysis, the conductivity was very low for the composites to be used for this application as envisioned. MWCNTs coated with SnO2 nanoparticles used in the present study, were synthesized by a microwave synthesis method. The composite samples were characterized by X–ray diffraction (XRD), Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR) and Brunauer–Emmet–Teller (BET) surface area analysis. These techniques gave evidence for surface and chemical modifications of the synthesized composites. The results showed microwave synthesis to be a very efficient method in producing CNTs that are densely coated and well dispersed with SnO2 nanoparticles in a very short time (total reaction time of 10 minutes). Microwave synthesis is particularly interesting because of the energy used, the higher temperature homogeneity and the shorter reaction times led to nanoparticles with high crystallinity and a narrow particle size distribution. Controlling the morphology by varying synthesis conditions such as temperature, pressure and time is also possible.

Abstract A theoretical study of the Kolbe-Schmitt reaction mechanism, performed using a DFT method, reveals that the reaction between sodium phenoxide and carbon dioxide proceedswith the formation of three transition states and three intermediates. In the first step of the reaction, a polarized ONa bond of sodium phenoxide is attacked by the carbon dioxide molecule, and the intermediate NaPh-CO2 complex is formed. In the next step of the reaction the electrophilic carbon atom attacks the ring primarily at the ortho position, thus forming two new intermediates. The final product, sodium salicylate, is formed by a 1,3-proton shift from C to O atom. The mechanism agrees with the experimental data related to the Kolbe-Schmitt reaction.

Several conflicting theories of the adsorption of aurocyanide onto activated carbon presently exist. To resolve the mechanism, adsorption and elution of aurocyanide are examined by several techniques, including Mossoauer spectroscopy, X-ray photoelectron spectroscopy, X-ray diffractometry, Fourier Transform Infrared spectrophotometry, ultraviolet-visible spectrophotometry and scanning electron microscopy. The evidence gathered indicates that, under normal plant conditions, aurocyanide is extracted onto activated carbon in the form of an ion pair M n+ [Au(CN) 2 ] n, and eluted by hydroxide or cyanide. The hydroxide or cyanide ions react with the carbon surface, rendering it relatively hydrophilic with a decreased affinity for neutral species. Additional adsorption mechanisms are shown to operate under other conditions of ionic strength, pH, and temperature. The poor agreement in the literature regarding the mechanism of adsorption of aurocyanide onto activated carbon is shown to be due to the fact that different mechanisms operate under different experimental conditions. The AuCN produced on the carbon surface by acid treatment is shown to react with hydroxide ion via the reduction of AuCN to metallic gold with formation of Au (CN) 2 , and the oxidation of cyanide to cyanate. Other species, such as An(CN)5 and Ag(CN)g adsorb onto activated carbon by a similar mechanism to that postulated for Au(CN)2 . Ion association of MAu(CN) 2 salts in aqueous solution is demonstrated by means of potentiometric titration and conductivity measurements, and various associated species of KAu(CN), salts are shown to occur in organic solvents by means of infrared spectrophoteaietric and distribution measurements. A kinetic model was developed for elution of aurocyanide from activated carbon and was found to predict gold elution performance successfully using the Zadra procedure. The influence of the surface chemistry and structure of activated carbon on adsorption of aurocyanide was investigated by characterization of activated carbons that were synthesized or oxidized under various conditions. Synthetic polymeric adsorbents with characteristics similar to activated carbons were also studied. The evidence suggests that a large micropore volume is important in providing suitable active sites for adsorption. Another important factor is the presence of basic functional groups within the micropore, which act as solvating agents for the ion pair. The aim is to provide a self-consistent adsorption mechanism that accounts for all observations presented in the literature. Interpretation of results in terms of preconceived ideas, and neglect of observations of other authors has greatly contributed to current disagreement in the literature.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of PhD. August 9, 2017 / Nano-structured carbon lms, long studied due to the promise of exceptional quantum transport properties, present a signi cant problem in condensed matter due to the disorder which inherently forms in these materials. This work addresses the role of structural disorder in low dimensional carbon systems. The in uence of structural disorder on resonant transmission is studied in diamond-like carbon superlattices. Having established a model for disorder, this model for the structural changes is then applied to interpret experimental measurements of diamond-like carbon superlattices. The role of phonons on resonant transmission under a high frequency gate potential was also studied. This model for structural disorder in heterogeneous carbon lms was then applied to disordered superconductors close to the Anderson-Mott transition using the inhomogeneous Bogoliubov-de Gennes theory. This analysis is then used in support of experimental work to understand the superconductor-insulator transition in boron doped nano-crystalline diamond lms. Coherent quantum transport e ects were demonstrated in structurally-disordered diamondlike carbon (DLC) superlattices through distinct current modulation (step-like features) with negative differential resistance in the current-voltage (I-V) measurements. A model for these structurally disordered superlattices was developed using tight-binding calculations within the Landauer-B uttiker formalism assuming a random variation of the hopping integral following a Gaussian distribution. Calculations of the I-V characteristics for different con gurations of superlattices compliment the interpretation of the measured I-V characteristics and illustrate that while these DLC superlattice structures do not behave like conventional superlattices, the present model can be used to tailor the properties of future devices. Furthermore this tandem theoretical and experimental analysis establishes the validity of the model for structural disorder. The same model for the variation of disorder was then applied to interpret the electronic transport properties of disordered graphene-like carbon thin films. The influence of disorder on the activation energy in few layer graphitic lms was modelled and compared with experimental observations through collaboration. The lms, grown by laser ablation, allowed the speci c e ects of structural disorder in the sp2 - C phase to be probed. Defects acted as effective barriers resulting in localization of charge carriers. Electron transmission spectra, calculated with a tight-binding model, accounted for the change of localization length as a result of disorder in the sp2 - C phase. This theoretical study showed that the localization length of the thin graphitic lms can be tuned with the level of disorder and was shown to be consistent with experimental studies. The in uence of nitrogen incorporation on resonant transmission in DLC superlattices was then studied theoretically. This study illuminated the speci c role of the nitrogen potential in relation to the Fermi level (EF ) in nitrogen incorporated amorphous carbon (a- CN) superlattice structures. In a-CN systems, the variation of conductivity with nitrogen percentage has been found to be strongly non-linear due to the change of disorder level. The e ect of correlated carbon and nitrogen disorder was investigated in conjunction with the nitrogen potential through analysis of transmission spectra, calculated using a tight binding model, which showed two broad peaks related to these species. It was shown that the characteristic transmission time through nitrogen centres can be controlled through a combination of the nitrogen potential and correlated disorder. In particular, by controlling the arrangement of the nitrogen sites within the sp2 - C clusters as well as their energetic position relative to EF , a crossover of the pronounced transmission peaks of nitrogen and carbon sites can be achieved. Furthermore, it was shown that nitrogen incorporated as a potential barrier can also enhance the transmission in the a-CN superlattice structures. The strong non-linear variation of resistance and the characteristic time of the structures can explain the transport features observed experimentally in a-CN fi lms. This analysis was then partnered with measurements performed on nitrogen-incorporated carbon superlattices (N-DLC QSL) by Neeraj Dwivedi (National University of Singapore). The electrical characteristics of these nitrogen incorporated superlattice devices revealed prominent negative di erential resistance (NDR) behavior. The interpretation of these measurements was supported by 1D tight binding calculations of disordered superlattice structures (chains), which included signi cant bond alternation in sp3-hybridized regions. This analysis showed improved resonant transmission, which can be ascribed to nitrogendriven structural modi cation of the N-DLC QSL structures, especially the increased sp2-C clustering that provides additional conduction paths throughout the network. In order to determine the in uence of additional factors on coherent quantum states in molecular systems as an extension to the analysis on superlattices, a theoretical study of the electron-phonon interaction in double barrier structures under the in uence of a timedependent gate potential was undertaken. The Floquet theory was employed along with expansion in a polaron eigenbasis to render a multi-dimensional single body problem. An essentially exact solution was found using the Riccati matrix technique. It was demonstrated that optimal transmission can be achieved by varying the frequency of the gate potential. In addition, it was shown that the gate potential can be used to control the energy of the resonant states very precisely while maintaining optimal transmission. Having gained a deep understanding of the structural changes induced in carbon systems through the incorporation of nitrogen, a similar structural model was then applied to study the changes induced in diamond and nanocrystalline fi lms by boron incorpora- tion. Boron doped diamond provides an interesting superconductor with ongoing debate surrounding the nature of the impurity band and the effect on the superconducting phase transition of structural changes induced by boron incorporation. The in uence of disorder, both structural (non-diagonal) and on-site (diagonal), was studied through the inhomogeneous Bogoliubov-de Gennes (BdG) theory in narrow-band disordered superconductors with a view towards understanding superconductivity in boron doped diamond (BDD) and boron-doped nanocrystalline diamond (B-NCD) lms. We employed the attractive Hubbard model within the mean eld approximation, including a short range Coulomb interaction between holes in the narrow acceptor band. We studied substitutional boron incorporation in a triangular lattice, with disorder in the form of random potential uctuations at the boron sites. The role of structural disorder was investigated through non-uniform variation of the tight-binding coupling parameter where, following experimental ndings in BDD and B-NCD lms, we incorporated the concurrent increase in structural disorder with increasing boron concentration. Stark differences between the ffects of structural and on-site disorder were demonstrated and showed that structural disorder has a much greater e ect on the density of states, mean pairing amplitude and super uid density than on-site potential disorder. We showed that structural disorder can increase the mean pairing amplitude while the spectral gap in the density of states decreases, with states eventually appearing within the spectral gap for high levels of disorder. This study illustrated how the effects of structural disorder can explain some of the features found in superconducting BDD and B-NCD lms, such as a tendency towards saturation of the critical temperature (Tc) with boron doping and deviations from the expected Bardeen-Cooper-Shrie er (BCS) theory in the temperature dependence of the pairing amplitude and spectral gap. The variation of the super uid density considering only structural disorder was markedly different from the variation with on-site disorder only and revealed that structural disorder is far more detrimental to superconductivity and accounts for the relatively low Tc of BDD and B-NCD in comparison to the Tc predicted using the conventional BCS theory. This theoretical work was then used to interpret features in the measured transport properties of B-NCD lms with di erent doping concentrations and microstructures. The temperature dependence of a distinct local maximum in eld dependent magnetoresistance measurements showed suppression of the density of states as the system breaks up into superconducting regions separated by grain boundaries. Differential resistance measurements at different temperatures and magnetic fi elds showed a transition from a local minimum at zero applied current, indicative of persisting superconducting regions, to a local maximum. A power law dependence over a certain current range in the measured I-V characteristics at di erent magnetic elds suggests a Berezinski-Kosterlitz-Thouless (BKT) transition. In addition, features in the magnetoresistance clearly indicate additional phases. Together with features in current-voltage measurements, these signatures show the coexistence of superconductivity and additional competing phases close to the Anderson-Mott transition. / LG2018

Thesis advisor: Lawrence T. Scott / The distinctive molecular structure of carbon nanotubes makes them desirable for electronic and chemical materials; however, current production methods are limited with respect to purity and chirality. Geodesic polyarenes serve as superb templates for the bottom up synthesis of carbon nanotube end-caps, setting the chirality and dimensions of the carbon nanotubes. The work herein describes the synthetic efforts towards the rational synthesis of a [6,6] carbon nanotube end-cap. Chapter 1 describes the efforts towards the synthesis of a C60H12 end-cap, in which the synthesis of an advanced intermediate, peri-bis(dibenzo[a,g]corannulene) is complete; however, the insolubility of this material proved to be problematic in a subsequent cycloaddition reaction. This reaction is examined computationally in order to understand the failure of the addition of dienophile, maleic anhydride, to peri-bis(dibenzo[a,g]corannulene). In Chapters 2 and 3, the development of solubility-enhancing methods is described. The development of a solubility-enhancing dienophile is successfully employed to induce the solubility of a formerly insoluble diene, peri-bis(dibenzo[a,g]corannulene), through Diels-Alder addition. Another method, employs the incorporation of tert-butyl groups onto peri-bis(dibenzo[a,g]corannulene) to successfully induce solubility. The enhanced-solubility enables the successful Diels-Alder addition of simple maleimide dienophiles, installing all necessary carbon atoms for the desired end-cap. Pyrolysis of the bis-anhydride derived from the aromatized bis-maleimide adduct afforded the C60H12 end-cap, which is the second carbon nanotube end-cap ever synthesized and the first of these dimensions. Chapter 3 also explores a palladium catalyzed intramolecular arylation reaction to form a pivotal intermediate in the synthesis of the end-cap, dibenzo[a,g]corannulene. The mechanism for the formation of a problematic byproduct resulting from reductive dehalogenation is discussed. Utilizing a deuterium labeled solvent, it is found that deuterium is incorporated onto the hydrocarbon, indicating that the solvent (N,N-dimethylformamide-d7) is the source of hydrogen for the reductive dehalogenation. These conditions are further exploited in Chapter 4 for the convenient perdeuteration of a variety of polycyclic aromatic hydrocarbons. Chapter 5 describes the first synthesis of a nitrogen containing geodesic polyarene, dibenzo[g,m]azacorannulene. This synthesis is completed in seven steps from a commercially available source in a 28% overall yield. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.