Bridget R. Rogers

Role of Nitrogen-Doped Graphene for Improved High-Capacity Potassium Ion Battery Anodes

Potassium is an earth abundant alternative to lithium for rechargeable batteries, but a critical limitation in potassium ion battery anodes is the low capacity of KC8 graphite intercalation compounds in comparison to conventional LiC6. Here we demonstrate that nitrogen doping of few-layered graphene can increase the storage capacity of potassium from a theoretical maximum of 278 mAh/g in graphite to over 350 mAh/g, competitive with anode capacity in commercial lithium ion batteries and the highest reported anode capacity so far for potassium ion batteries. Control studies distinguish the importance of nitrogen dopant sites as opposed to sp3 carbon defect sites to achieve the improved performance, which also enables >6× increase in rate performance of doped vs undoped materials. Finally, in situ Raman spectroscopy studies elucidate the staging sequence for doped and undoped materials and demonstrate the mechanism of the observed capacity enhancement to be correlated with distributed storage at local nitrogen sites in a staged KC8 compound. This study demonstrates a pathway to overcome the limitations of graphitic carbons for anodes in potassium ion batteries by atomically precise engineering of nanomaterials.

Catalytic Atom Recombination on ZrB2/SiC and HfB2/SiC Ultrahigh-Temperature Ceramic Composites

Results are presented of an experimental investigation into the efficiency of zirconium diboride/silicon carbide and hafnium diboride/silicon carbide ultrahigh-temperature ceramic composites for catalyzing the surface recombination of dissociated oxygen and nitrogen at moderate surface temperatures. Experiments were conducted with a diffusion-tube side-arm reactor, together with laser-induced fluorescence species detection diagnostics. Experiments reveal recombination coefficients in the range between silica glasses and oxidized metals, as well as evidence of environment-induced surface modification.

Morphological modulation of bimetallic nanostructures for accelerated catalysis

Bimetallic nanostructures are of significant technological interest due to their ability to uniformly combine properties of two distinct metals, giving rise to multimodal characteristics. In this work, we have synthesized Au/Ag core/shell bimetallic nanostructures and investigated the role of temperature in controlling the morphological evolution and the corresponding impact on catalytic transformation. By increasing the reaction temperature from 35 °C to 80 °C, the edge morphologies of Au/Ag nanostructures evolved from rounded to sharp corners, which directly impact the catalytic properties. The size of the bimetallic nanostructures also increased when the temperature was raised due to faster Ag+ reduction along specific crystallographic planes, giving rise to red shifts in the plasmon resonance. The catalytic activity of Au/Ag nanostructures was compared to commercially purchased Ag nanospheres for the reduction of 4-nitrophenol to 4-aminophenol with NaBH4. The reaction rate for 4-nitrophenol reduction was significantly higher on Au/Ag-NSs relative to the Ag nanospheres, while the induction time was lowest on the Ag nanospheres. These observations were attributed to the simultaneous effects of (i) surface area available for catalytic reaction, (ii) crystallographic facets supporting the nanostructures, (iii) surface ligands, and (iv) composition of the metal nanostructures.

Characterization of boron charge traps at the interface of Si/SiO2 using second harmonic generation

We report results from optical second harmonic generation studies of boron charge traps near the interface of Si/SiO2. Our data suggest that a static electric field at the interface is formed during the oxide growth process due to the presence of negative boron ions (B−) in the silicon substrate and positive boron ions (B+) in the oxide. We demonstrated that the B+ state traps could be filled through the creation of neutral boron states created by internal photoelectron emission. By fitting our data, we found that the effective interface susceptibility |χ(2)| depends on doping concentration.

Amplification of Surface-Initiated Ring-Opening Metathesis Polymerization of 5-(Perfluoro-n-alkyl)norbornenes by Macroinitiation

This article reports the enhanced rate of the surface-initiated polymerization (SIP) of 5-(perfluoro-n-alkyl)norbornenes (NBFn) by combining two SIP techniques, namely surface-initiated atom-transfer polymerization (SI-ATRP) to grow a macroinitiator and surface-initiated ring-opening metathesis polymerization (SI-ROMP) to produce the final coating. This polymerization approach promotes the rapid growth of dense partially fluorinated coatings that are highly hydrophobic and oleophobic and yield thicknesses from 4-12 μm. Specifically, the growth rate and the limiting thickness of pNBFn with different side chain lengths (n = 4, 6, 8, and 10) at various monomer concentrations and temperatures are evaluated through two approaches: growing the polymer from an initiator-terminated monolayer (control) or from a modified poly(2-hydroxyethyl methacrylate) (PHEMA) macroinitiator. X-ray photoelectron spectroscopy (XPS) analysis shows that 38% of the hydroxyl termini in the macroinitiator react with a norbornenyl diacid chloride (NBDAC) molecule, and 7% of such anchored norbornenyl groups react with a catalyst molecule. The kinetic data have been modeled to determine the propagation velocity and the termination rate constant. The PHEMA macroinitiator provides thicker films and faster growth as compared to the monolayer, achieving a 12 μm thick coating of pNBF8 in 15 min. Increasing the monomer side chain length, n, from 4 to 10 improves the growth rate and the limiting polymer thickness. Performing the polymerization process at higher temperature increases the growth rate and the limiting thickness as evidenced by an increase in the film growth rate constant. Arrhenius plots show that the reactions involved in the macroinitiation process exhibit lower activation energies than those formed from a monolayer. Electrochemical impedance spectroscopy reveals that the films exhibit resistance against ion transport in excess of 1 × 10(10) Ω·cm(2).

The UbiI (VisC) aerobic ubiquinone synthase is required for expression of type 1 pili, biofilm formation, and pathogenesis in uropathogenic Escherichia coli

UNLABELLED Uropathogenic Escherichia coli (UPEC), which causes the majority of urinary tract infections (UTI), uses pilus-mediated adherence to initiate biofilm formation in the urinary tract. Oxygen gradients within E. coli biofilms regulate expression and localization of adhesive type 1 pili. A transposon mutant screen for strains defective in biofilm formation identified the ubiI (formerly visC) aerobic ubiquinone synthase gene as critical for UPEC biofilm formation. In this study, we characterized a nonpolar ubiI deletion mutant and compared its behavior to that of wild-type bacteria grown under aerobic and anoxic conditions. Consistent with its function as an aerobic ubiquinone-8 synthase, deletion of ubiI in UPEC resulted in reduced membrane potential, diminished motility, and reduced expression of chaperone-usher pathway pili. Loss of aerobic respiration was previously shown to negatively impact expression of type 1 pili. To determine whether this reduction in type 1 pili was due to an energy deficit, wild-type UPEC and the ubiI mutant were compared for energy-dependent phenotypes under anoxic conditions, in which quinone synthesis is undertaken by anaerobic quinone synthases. Under anoxic conditions, the two strains exhibited wild-type levels of motility but produced diminished numbers of type 1 pili, suggesting that the reduction of type 1 pilus expression in the absence of oxygen is not due to a cellular energy deficit. Acute- and chronic-infection studies in a mouse model of UTI revealed a significant virulence deficit in the ubiI mutant, indicating that UPEC encounters enough oxygen in the bladder to induce aerobic ubiquinone synthesis during infection. IMPORTANCE The majority of urinary tract infections are caused by uropathogenic E. coli, a bacterium that can respire in the presence and absence of oxygen. The bladder environment is hypoxic, with oxygen concentrations ranging from 4% to 7%, compared to 21% atmospheric oxygen. This work provides evidence that aerobic ubiquinone synthesis must be engaged during bladder infection, indicating that UPEC bacteria sense and use oxygen as a terminal electron acceptor in the bladder and that this ability drives infection potential despite the fact that UPEC is a facultative anaerobe.

Underlayer work function effect on nucleation and film morphology of chemical vapor deposited aluminum

The dependence of early stage of dimethylaluminum hydride (DMAH)-sourced aluminum chemical vapor deposition on underlayer material was investigated. Identical process conditions were used to deposit the aluminum on TiN, TaN and Ti-W surfaces. Surface coverage and particle densities of aluminum deposited on TiN were much greater than those deposited on Ti-W or TaN. Work function measurements performed on the three metal surfaces suggest that the difference in nucleation rate on TiN compared to TaN and Ti-W is due its increased ability to donate electrons to the DMAH decomposition process.

Vanderbilt Pelletron - Low Energy Protons and Other Ions for Radiation Effects on Electronics

Vanderbilt University School of Engineerings Pelletron has been used to conduct single event effects, displacement damage and total ionizing dose testing on electronics and materials. A custom vacuum chamber with adjustable stage and various feedthroughs facilitate testing. Beam scattering using thin gold foils provide good beam uniformity over approximately 1 inch diameter. Over 25 peer reviewed papers have been published that include results from tests performed. External users are invited to use this facility.

Radiation Effects on the Photoluminescence of Rare-Earth Doped Pyrochlore Powders

This work examined the sensitivity of the luminescence spectra of europium-doped lanthanum zirconate to different types and amounts of radiation exposure. For the samples and radiation sources used in this work (X-rays and protons), changes in the photoluminescence of europium-doped lanthanum zirconate were minimal. However, when the phosphor was paired with an alternate luminescent material, which had a differing radiation response, relative changes in the photoluminescence of the two materials were correlated to the radiation exposure.

Dual process dielectric formation for decoupling capacitors on flexible substrates

Large area, high density integrated capacitors within printed wiring boards can provide a substantial decoupling capacitance with very low parasitic inductance. Tantalum pentoxide (Ta2O5) is an excellent dielectric for this application due to the relatively high dielectric constant (~ 22-24), however the difficulty of fabricating large, defect-free capacitors has thus far prevented the realization of practical applications. This work demonstrates high performance capacitors with Ta2O5 dielectric developed with a two step oxidation scheme consisting of reactive sputtering followed by anodization. Thin films of Ta2O5 were deposited by reactive sputtering on silicon and also on Upilexreg covered glass wafers using dc magnetron sputtering with a gas flow ratio of 10/90 O2/Ar. In the two-step oxidation scheme, anodization is performed after reactively sputtering tantalum oxide films to obtain a densifled oxide structure. The electrical and physical properties of these two step sputtered/ anodized tantalum oxide films are shown to be superior to those of tantalum oxide films prepared by either anodization or sputtering alone. This work has shown that Ta2O5 is a potential dielectric for integrated capacitors that could be used in advanced packaging applications.