PLD-assisted VLS growth of aligned ferrite nanorods, nanowires, and nanobelts-synthesis, and properties

We report here a systematic synthesis and characterization of aligned alpha-Fe2O3 (hematite), epsilon-Fe2O3, and Fe3O4 (magnetite) nanorods, nanobelts, and nanowires on alumina substrates using a pulsed laser deposition (PLD) method. The presence of spherical gold catalyst particles at the tips of the nanostructures indicates selective growth via the vapor-liquid-solid (VLS) mechanism. Through a series of experiments, we have produced a primitive "phase diagram" for growing these structures based on several designed pressure and temperature parameters. Transmission electron microscopy (TEM) analysis has shown that the rods, wires, and belts are single-crystalline and grow along <111>m or <110>h directions. X-ray diffraction (XRD) measurements confirm phase and structural analysis. Superconducting quantum interference device (SQUID) measurements show that the iron oxide structures exhibit interesting magnetic behavior, particularly at room temperature. This work is the first known report of magnetite 1D nanostructure growth via the vapor-liquid-solid (VLS) mechanism without using a template, as well as the first known synthesis of long epsilon-Fe2O3 nanobelts and nanowires.     

Kinetics of membrane raft formation: fatty acid domains in stratum corneum lipid models

The major barrier to permeability in skin resides in the outermost layer of the epidermis, the stratum corneum (SC). The major SC lipid components are ceramides, free fatty acids, and cholesterol. Ternary mixtures containing these constituents are widely used for physicochemical characterization of the barrier. Prior X-ray diffraction and IR spectroscopy studies have revealed the existence of ordered lipid chains packed in orthorhombic subcells. To monitor the kinetics of formation of regions rich in fatty acids, the current study utilizes a modification of the method (J. Phys. Chem. 1992, 96, 10008) developed to monitor component demixing in n-alkane mixtures. The approach is based on changes in the scissoring or rocking mode contours in the IR spectra of (orthorhombically packed) ordered chains. In the current study, equimolar mixtures of ceramides (either non-hydroxy fatty acid sphingosine ceramide or alpha-hydroxy fatty acid sphingosine ceramide) with chain perdeuterated fatty acids (either palmitic or stearic acid) and cholesterol reveal a time evolution of the scissoring contour of the deuterated fatty acid chains following quenching from relatively high temperatures where random mixing occurs. Segregation of domains enriched in the fatty acid component is observed. The kinetics of segregation are sensitive to the quenching temperature and to the chemical composition of the mixture. The kinetic regimes are conveniently catalogued with a power law of the form P=Ktalpha where P is a (measured) property related to domain composition. The time scales for demixing in these experiments are similar to times observed in several studies that have tracked the restoration of the in vivo permeability barrier following nonthermal challenges to SC integrity. Further evidence for the physiological importance of the current measurements is the detection of these phases in native SC. The current work constitutes the first direct, structure-based determination of the kinetics of barrier formation in relevant skin lipid barrier models.     

DESIGN,CALIBRATION, AND INITIAL RESULTS ON A COLOR/TEMPERATURE-MAPPING GOLD-FILM HEATER

The goal of this article is to present a new heater design which combines a thin-film resistometry technique with a double-incident liquid-crystal film in the form of a coated (printed) sheet to provide both a spatial mean surface temperature and a color/ temperature map. The heater is demonstrated to yield spatial temperature results consistent with those of other boiling studies in the literature while providing the additional functionality of a mean temperature measurement. The heater design and calibration are outlined and four boiling tests are presented to show the applicability of such a neater for fundamental boiling research, including some information in the conduction and convection regimes, electric-field-enhanced boiling research, and microgravity boiling research.     

Initial applications of the molecular model to compute defect vibrations of oxygen in silicon

Abstract

We have employed the molecular model introduced first by Jaswal to compute the vibrational spectra of oxygen bearing defects in a silicon crystal. This was done in the context of a silicon molecular cluster with outer valencies terminated by hydrogen. We employ the MINDO/3 semi-empirical electronic structure method to compute the total energy of the molecular cluster. We examine the conditions in applications of the molecular model required for accurate predictions of oxygen local-mode vibrational frequencies. We find that the oxygen atom and its nearest neighbor silicon atoms must be allowed to vibrate. The nearest-neighbor and next nearest-neighbor shells of silicon atoms must be allowed to relax from their lattice positions. The outermost relaxed shell of silicon atoms should be bonded to silicon atoms in their lattice positions. We apply the molecular model to three defects of crystalline silicon; interstitial oxygen, oxygen in a vacancy (the A-center), and two oxygen atoms in a vacancy. Comparison of our computed local-mode oxygen vibration frequencies with experiment shows the computed oxygen local-mode frequencies to be almost uniformly 10% greater than those observed. Isotope shifts fit experiment equally well. We conclude that the molecular model represents an accurate and efficient approach for the computation of defect local mode vibrational frequencies for oxygen and other defects in crystalline silicon.     

Confidence Interval for the Risk-Score Index

Risk-score indices are a simple, applicable, and easy-to-calculate tool for regression models, which can be used when computers are not available. The risk-score index is a partial summation of the rounded model coefficients that maintains essential properties of the model coefficients, such as their weight in the model and the correlation matrix. In a certain sense, the risk score can be viewed as a transformation into a pre-selected scale. The risk score is generally represented as a point estimate. Thus, the risk-score index is a categorical variable that relates each and every category uniquely to risk. The main argument of this paper is that the risk score, which is calculated as a partial summation of rounded beta coefficients, is a statistic, and, therefore, it has its own variance. This variance is divided into three factors—the original β-coefficients variances, the rounding-error variance, and the variance from the relations between these two factors. Since the variance of the score is typically not negligible, it is preferable to consider the confidence interval centered at the point estimate and not just the point estimate itself. By using the confidence interval for the risk score, one can quantify the accuracy of the score and also compare different scores, which otherwise is not always possible.     

Effect of Two‐Dimensional Crystal Orbitals on Fermi Surfaces and Electron Transport in Three‐Dimensional Perovskite Oxides

Perovskite oxides are candidate materials in catalysis, fuel cells, thermoelectrics, and electronics, where electronic transport is vital to their use. While the fundamental transport properties of these materials have been heavily studied, there are still key features that are not well understood, including the temperature‐squared behavior of their resistivities. Standard transport models fail to account for this atypical property because Fermi surfaces of many perovskite oxides are low‐dimensional and distinct from traditional semiconductors. In this work, the low‐dimensional Fermi surfaces of perovskite oxides are chemically interpreted in terms of two‐dimensional crystal orbitals that form the conduction bands. Using SrTiO₃ as a case study, the d/p‐hybridization that creates these low‐dimensional electronic structures is reviewed and connected to its fundamentally different electronic properties. A low‐dimensional band model explains several experimental transport properties, including the temperature and carrier‐density dependence of the effective mass, the carrier‐density dependence of scattering, and the temperature dependence of resistivity. This work highlights how chemical bonding influences semiconductor transport.     

Defect‐Controlled Electronic Properties in AZn2Sb2 Zintl Phases

Experimentally, AZn₂Sb₂ samples (A=Ca, Sr, Eu, Yb) are found to have large charge carrier concentrations that increase with increasing electronegativity of A. Using density functional theory (DFT) calculations, we show that this trend can be explained by stable cation vacancies and the corresponding finite phase width in A_1?xZn₂Sb₂ compounds.     

Library synthesis using 5,6,7,8-tetrahydro-1,6-naphthyridines as scaffolds

The chemistry of 5,6,7,8-tetrahydro-1,6-naphthyridine scaffolds, synthesized by intramolecular cobalt-catalyzed [2 + 2 + 2] cyclizations, has been exploited for library synthesis. Urea, amide, and sulfonamide formations were used in the synthesis of a 101-membered library. Screening of the library for antituberculosis activity revealed three lead compounds.     

Analysis of formaldehyde formation in wastewater using on-fiber derivatization-solid-phase microextraction-gas chromatography-mass spectrometry

A method has been developed for the quantification of the formation of formaldehyde during the advanced oxidation treatment (AOT) of wastewater destined for reuse. This method uses solid-phase microextraction (SPME) with on-fiber derivatization followed by gas chromatography-mass spectrometry (GC-MS) analysis. Based on calculated method detection limits (MDL) and ambient background levels, the method reporting (MRL) limit for formaldehyde was set at 10 microg/L. Precision for formaldehyde using this technique resulted in 23% relative standard deviation (RSD), while the internal standard, acetone-d(6), was only 6%. This method was used to evaluate the formation of formaldehyde in bench scale UV-AOT experiments using natural organic matter (NOM) fortified reagent water and tertiary treated wastewater effluent. Results suggest that the formation of formaldehyde increases in both the reagent water and wastewater matrices with increasing UV exposure and hydrogen peroxide concentrations, with overall higher concentrations of formaldehyde in the wastewater samples. No appreciable amount of formaldehyde formation was observed when UV was applied in the absence of hydrogen peroxide in both matrices tested.