Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO₂ junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO₂ network (Au "in" TiO₂, Au_IN-TiO₂ aerogel) or deposited onto preformed TiO₂ aerogel (Au "on" TiO₂, Au_ON/TiO₂ aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but Au_IN-TiO₂ aerogels impose a greater degree of Au contact with TiO₂ particles than does the Au_ON/TiO₂ form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO₂ network to Au||TiO₂ junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO₂ network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO₂, induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO₂ junctions in Au_IN-TiO₂ aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O₂ to O^2- at the interface. Solid-state ¹H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO₂ interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO₂ network. Taken together, our results show that Au||TiO₂ interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.

In Situ Hydrogen Plasma Exposure for Varying the Stoichiometry of Atomic Layer Deposited Niobium Oxide Films for Use in Neuromorphic Computing Applications

Niobium oxide (NbO_x) materials of various compositions are of interest for neuromorphic systems that rely on memristive device behavior. In this study, we vary the composition of NbO_x thin films deposited via atomic layer deposition (ALD) by incorporating one or more in situ hydrogen plasma exposure steps during the ALD supercycle. Films with compositions ranging from Nb₂O₅ to NbO₂ were deposited, with film composition dependent on the duration of the plasma exposure step, the number of plasma exposure steps per ALD supercycle, and the hydrogen content of the plasma. The chemical and optical properties of the ALD NbO_x films were probed using spectral ellipsometry, X-ray photoelectron spectroscopy, and optical transmission spectroscopy. Two-terminal electrical devices fabricated from ALD Nb₂O₅ and NbO₂ thin films exhibited memristive switching behavior, with switching in the NbO₂ devices achieved without a high-field electroforming step. The ability to controllably tune the composition of ALD-grown NbO_x films opens new opportunities for realizing a variety of device structures relevant for neuromorphic computing and other emerging electronic and optoelectronic applications.

Phenomenological Model for Defect Interactions in Irradiated Functional Materials

The ability to tailor the performance of functional materials, such as semiconductors, via careful manipulation of defects has led to extraordinary advances in microelectronics. Functional metal oxides are no exception – protonic-defect-conducting oxides find use in solid oxide fuel cells (SOFCs) and oxygen-deficient high-temperature superconductors are poised for power transmission and magnetic imaging applications. Similarly, the advantageous functional responses in ferroelectric materials that make them attractive for use in microelectromechanical systems (MEMS), logic elements, and environmental energy harvesting, are derived from interactions of defects with other defects (such as domain walls) and with the lattice. Chemical doping has traditionally been employed to study the effects of defects in functional materials, but complications arising from compositional heterogeneity often make interpretation of results difficult. Alternatively, irradiation is a versatile means of evaluating defect interactions while avoiding the complexities of doping. Here, a generalized phenomenological model is developed to quantify defect interactions and compare material performance in functional oxides as a function of radiation dose. The model is demonstrated with historical data from literature on ferroelectrics, and expanded to functional materials for SOFCs, mixed ionic-electronic conductors (MIECs), He-ion implantation, and superconductors. Experimental data is used to study microstructural effects on defect interactions in ferroelectrics.

Sb@Ni12@Sb20-/+ and Sb@Pd12@Sb20n Cluster Anions, Where n = +1, -1, -3, -4: Multi-Oxidation-State Clusters of Interpenetrating Platonic Solids

K₅Sb₄ and K₃Sb₇ Zintl ion precursors react with Pd(PPh₃)₄ in ethylenediamine/toluene/PBu₄⁺ solutions to give crystals of Sb@Pd₁₂@Sb₂₀^n-/PBu₄⁺ salts, where n = 3, 4. The clusters are structurally identical in the two charge states, with nearly perfect I_h point symmetry, and can be viewed as an Sb@Pd₁₂ icosahedron centered inside of an Sb₂₀ dodecahedron. The metric parameters suggest very weak Sb-Sb and Pd-Pd interactions with strong radial Sb-Pd bonds between the Sb₂₀ and Pd₁₂ shells. All-electron DFT analysis shows the 3- ion to be diamagnetic with I_h symmetry and a 1.33 eV HOMO-LUMO gap, whereas the 4- ion undergoes a Jahn-Teller distortion to an S = 1/2 D_3d structure with a small 0.1 eV gap. The distortion is predicted to be small and is not discernible by crystallography. Laser desorption-ionization time-of-flight mass spectrometry (LDI-TOF MS) studies of the crystalline samples show intense parent Sb@Pd₁₂@Sb₂₀^- ions (negative ion mode) and Sb@Pd₁₂@Sb₂₀⁺ (positive ion mode) along with series of Sb@Pd_12-y@Sb_20-x^-/+ ions. Ni(cyclooctadiene)₂ reacts with K₃Sb₇ in en/tol/Bu₄PBr solvent mixtures to give black precipitates of Sb@Ni₁₂@Sb₂₀^n- salts that give similar Sb@Ni₁₂@Sb₂₀^-/+ parent ions and Sb@Ni_12-y@Sb_20-x^-/+ degradation series in the respective LDI-TOF MS studies. The solid-state and gas-phase studies of the icosahedral Sb@M₁₂@Sb₂₀^n-/n+ ions show that the clusters can exist in the -4, -3, -1, +1 (M = Pd) and +1, -1 (M = Ni) oxidation states. These multiple-charge-state clusters are reminiscent of redox-active fullerenes (e.g., C₆₀ⁿ, where n = +1, 0, -1, -2, -3, -4, -5, -6).

Formation of Thick Dense Yttrium Iron Garnet Films Using Aerosol Deposition

Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% of the bulk. The primary advantage of AD is that the deposition takes place entirely at ambient temperature; thereby enabling film growth in material systems with disparate melting temperatures. This report describes in detail the processing steps for preparing the powder and for performing AD using the custom-built system. Representative characterization results are presented from scanning electron microscopy, profilometry, and ferromagnetic resonance for films grown in this system. As a representative overview of the capabilities of the system, focus is given to a sample produced following the described protocol and system setup. Results indicate that this system can successfully deposit 11 µm thick yttrium iron garnet films that are  > 90% of the bulk density during a single 5 min deposition run. A discussion of methods to afford better control of the aerosol and particle selection for improved thickness and roughness variations in the film is provided.

Vibrio campbellii hmgA-mediated pyomelanization impairs quorum sensing, virulence, and cellular fitness

Melanization due to the inactivation of the homogentisate-1,2-dioxygenase gene (hmgA) has been demonstrated to increase stress resistance, persistence, and virulence in some bacterial species but such pigmented mutants have not been observed in pathogenic members of the Vibrio Harveyi clade. In this study, we used Vibrio campbellii ATCC BAA-1116 as model organism to understand how melanization affected cellular phenotype, metabolism, and virulence. An in-frame deletion of the hmgA gene resulted in the overproduction of a pigment in cell culture supernatants and cellular membranes that was identified as pyomelanin. Unlike previous demonstrations in Vibrio cholerae, Burkholderia cepacia, and Pseudomonas aeruginosa, the pigmented V. campbellii mutant did not show increased UV resistance and was found to be ~2.7 times less virulent than the wild type strain in Penaeus monodon shrimp virulence assays. However, the extracted pyomelanin pigment did confer a higher resistance to oxidative stress when incubated with wild type cells. Microarray-based transcriptomic analyses revealed that the hmgA gene deletion and subsequent pyomelanin production negatively effected the expression of 129 genes primarily involved in energy production, amino acid, and lipid metabolism, and protein translation and turnover. This transcriptional response was mediated in part by an impairment of the quorum sensing regulon as transcripts of the quorum sensing high cell density master regulator LuxR and other operonic members of this regulon were significantly less abundant in the hmgA mutant. Taken together, the results suggest that the pyomelanization of V. campbellii sufficiently impairs the metabolic activities of this organism and renders it less fit and virulent than its isogenic wild type strain.

Magneto-optical fingerprints of distinct graphene multilayers using the giant infrared Kerr effect

The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC. Moreover, the magnetic field and photon energy dependence of these features enable us to explore the band structure, electron-hole band asymmetries, and mechanisms that activate a CR response in the Kerr effect for various multilayers that coexist in a single sample. Surprisingly, we find that the magnitude of monolayer Kerr effect CRs is not temperature dependent. This unexpected result reveals new questions about the underlying physics that makes such an effect possible.

Comparison of epitaxial graphene on Si-face and C-face 4H SiC formed by ultrahigh vacuum and RF furnace production

We present X-ray photoelectron spectroscopy, van der Pauw Hall mobilities, low-temperature far-infrared magneto transmission (FIR-MT), and atomic force microscopy (AFM) results from graphene films produced by radiative heating in an ultrahigh vacuum (UHV) chamber or produced by radio frequency (RF) furnace annealing in a high vacuum chemical vapor deposition system on Si- and C-face 4H SiC substrates at 1200-1600 degrees C. Although the vacuum level and heating methods are different, graphene films produced by the two methods are chemically similar with the RF furnace annealing typically producing thicker graphene films than UHV. We observe, however, that the formation of graphene on the two faces is different with the thicker graphene films on the C-face RF samples having higher mobility. The FIR-MT showed a 0(-1) --> 1(0) Landau level transition with a square root B dependence and a line width consistent with a Dirac fermion with a mobility >250,000 cm(2) x V(-1) x s(-1) at 4.2 K in a C-face RF sample having a Hall-effect carrier mobility of 425 cm(2) x V(-1) x s(-1) at 300 K. AFM shows that graphene grows continuously over the varying morphology of both Si and C-face substrates.

A comparative study of four methods of evaluating velopharyngeal adequacy

Forty-eight subjects with repaired palatal clefts were divided into three major groups on the basis of speech symptoms and were examined for velopharyngeal valving integrity by means of the R-D nasal manometer, pressure-flow techniques, the Hunter oral manometer, and videofluoroscopy. Videofluoroscopy appeared to provide data that most nearly agreed with predictions of valving made from speech. The Hunter oral manometer was the least useful of the four techniques.

The use and limitations of the barium pharyngogram in the detection of velopharyngeal insufficiency

A barium pharyngogram was incorporated into the multiview videofluoroscopic evaluation of the velopharyngeal portal in 195 subjects in order to assess its usefulness in demonstrating competence or incompetence. Reflux of barium was always associated with a velopharyngeal gap and/or hypernasality; however, the absence of reflux was not similarly diagnostic. The pharyngogram proved to be useful in confirming the presence of a palatal or pharyngeal-flap fistula. While it is helpful in assessing velopharyngeal closure in selected cases, it should not be used in the absence of other information.

The dynamics of Passavant's ridge in subjects with and without velopharyngeal insufficiency--a multi-view videofluoroscopic study

Passavant's ridge was studied in 43 patients via multiview videofluoroscopy incorporating the simultaneous recording of speech. Ratings of the videotapes were made at full speed, in slowmotion, and by stop-framing. The following results were found: (1) Just as there are variable patterns of velopharyngeal closure, there were also variations in the way in which Passavant's ridge is positioned relative to the velum, and in the ridge's subsequent role in velopharyngeal narrowing or closure. (2) The ridge was the primary pharyngeal structure at the level of the velum that closed or locally narrowed the velopharyngeal portal in 37% of patients. (3) Passavant's ridge usually appeared as a structure encompassing both the lateral and posterior pharyngeal walls, and its presence was usually associated with active lateral pharyngeal wall motion. (4) Passavant's ridge was more prominent when the head was in the hyper-extended rather than the neutral position. (5) Passavant's ridge moved in a highly consistent manner, synchronous with velar movement.