Bending and reverse bending during the fabrication of novel GaAs/(In,Ga)As/GaAs core-shell nanowires monitored byin situx-ray diffraction.

We report on the fabrication of a novel design of GaAs/(In,Ga)As/GaAs radial nanowire heterostructures on a Si 111 substrate, where, for the first time, the growth of inhomogeneous shells on a lattice mismatched core results in straight nanowires instead of bent. Nanowire bending caused by axial tensile strain induced by the (In,Ga)As shell on the GaAs core is reversed by axial compressive strain caused by the GaAs outer shell on the (In,Ga)As shell. Progressive nanowire bending and reverse bending in addition to the axial strain evolution during the two processes are accessed byin situby x-ray diffraction. The diameter of the core, thicknesses of the shells, as well as the indium concentration and distribution within the (In,Ga)As quantum well are revealed by 2D energy dispersive x-ray spectroscopy using a transmission electron microscope. Shell(s) growth on one side of the core without substrate rotation results in planar-like radial heterostructures in the form of free standing straight nanowires.

Chemical Vapor Deposition and High-Resolution Patterning of a Highly Conductive Two-Dimensional Coordination Polymer Film.

Crystalline coordination polymers with high electrical conductivities and charge carrier mobilities might open new opportunities for electronic devices. However, current solvent-based synthesis methods hinder compatibility with microfabrication standards. Here, we describe a solvent-free chemical vapor deposition method to prepare high-quality films of the two-dimensional conjugated coordination polymer Cu-BHT (BHT = benzenehexanothiolate). This approach involves the conversion of a metal oxide precursor into Cu-BHT nanofilms with a controllable thickness (20-85 nm) and low roughness (<10 nm) through exposure to the vaporized organic linker. Moreover, the restricted metal ion mobility during the vapor-solid reaction enables high-resolution patterning via both bottom-up lithography, including the fabrication of micron-sized Hall bar and electrode patterns to accurately evaluate the conductivity and mobility values of the Cu-BHT films.

Sample chamber for synchrotron based in-situ X-ray diffraction experiments under electric fields and temperatures between 100†K and 1250†K.

Many scientific questions require X-ray experiments conducted at varying temperatures, sometimes combined with the application of electric fields. Here, a customized sample chamber developed for beamlines P23 and P24 of PETRA III at DESY to suit these demands is presented. The chamber body consists mainly of standard vacuum parts housing the heater/cooler assembly supplying a temperature range of 100 K to 1250 K and an xyz manipulator holding an electric contact needle for electric measurements at both high voltage and low current. The chamber is closed by an exchangeable hemispherical dome offering all degrees of freedom for single-crystal experiments within one hemisphere of solid angle. The currently available dome materials (PC, PS, PEEK polymers) differ in their absorption and scattering characteristics, with PEEK providing the best overall performance. The article further describes heating and cooling capabilities, electric characteristics, and plans for future upgrades of the chamber. Examples of applications are discussed.

A Scandium MOF with an Unprecedented Inorganic Building Unit, Delimiting the Micropore Windows.

A new scandium metal-organic framework (Sc-MOF) with the composition of [Sc(OH)(OBA)], denoted as Sc-CAU-21, was prepared under solvothermal reaction conditions using 4,4'-oxidibenzoic acid (H2OBA) as the ligand. Single-crystal structure determination revealed the presence of the new inorganic building unit (IBU) {Sc8(µ-OH)8(O2C)16}. It is composed of cis-connected ScO6 polyhedra forming an eight-membered ring through bridging µ-OH groups. The connection of the IBUs leads to a 3D framework, containing 1D pores with a diameter between 4.2 and 5.6 Å. Pore access is limited by the size of the IBU, and in contrast to the isoreticular aluminum compound Al-CAU-21 [Al(OH)(OBA)], which is nonporous toward nitrogen at 77 K, Sc-CAU-21 exhibits a specific surface area of 610 m2 g-1. The title compound is thermally stable in air up to 350 °C and can be employed as a host for photoluminescent ions. Sc-CAU-21 exhibits a ligand-based blue emission, and (co)substituting Sc3+ ions with Ln3+ ions (Eu3+, Tb3+, and Dy3+) allows the tuning of the emitting color of the phosphor from red to green. Single-phase white-light emission with CIE color coordinates close to the ideal for white-light emission was also achieved. The luminescence property was utilized in combination with powder X-ray diffraction to study in situ the crystallization process of Sc-CAU-21:Tb and Sc-CAU-21:Eu. Both studies indicate a two-step crystallization process, with a crystalline intermediate, prior to the formation of Sc-CAU-21:Ln.

Langmuir and Gibbs magnetite NP layers at the air/water interface.

The interfacial properties of Fe(3)O(4)@MEO(2)MA(90)-co-OEGMA(10) NPs, recently developed and described as promising nanotools for biomedical applications, have been investigated at the air/water interface. These Fe(3)O(4) NPs, capped with catechol-terminated random copolymer brushes of 2-(2-methoxyethoxy) ethyl methacrylate (MEO(2)MA) and oligo(ethylene glycol) methacrylate (OEGMA), with molar fractions of 90% and 10%, respectively, proved to be surface active. Surface tension measurements of aqueous dispersions of the NPs showed that the adsorption of the NPs at the air/water interface is time- and concentration-dependent. These NPs do not behave as classical amphiphiles. Once adsorbed at the air/water interface, they do not exchange with NPs in bulk, but they are trapped at the interface. This means that all NPs from the bulk adsorb to the interface until reaching maximum coverage of the interface, which corresponds to values between 6 × 10(-4) and 8 × 10(-4) mg/cm(2) and a critical equilibrium surface tension of ∼47 mN/m. Moreover, Langmuir layers of Fe(3)O(4)@MEO(2)MA(90)-co-OEGMA(10) NPs have been investigated by measuring surface pressure-area compression-expansion isotherms and in situ X-ray fluorescence spectra. The compression-expansion isotherms showed a plateau region above a critical surface pressure of ∼25 mN/m and a pronounced hysteresis. By using a special one-barrier Langmuir trough equipped with two surface pressure microbalances, we have shown that the NPs are squeezed out from the interface into the aqueous subphase, and they readsorb on the other side of the barrier. The results have been supported by TEM as well as AFM experiments of transferred Langmuir-Schaefer films on solid supports. This study shows the ability of Fe(3)O(4)@MEO(2)MA(90)-co-OEGMA(10) NPs to transfer from hydrophilic media (an aqueous solution) to the hydrophobic/hydrophilic interface (air/water interface) and back to the hydrophilic media. This behavior is very promising, opening studies of their ability to cross biological membranes.

Epitaxy and Shape Heterogeneity of a Nanoparticle Ensemble during Redox Cycles.

The role of metal-support epitaxy on shape and size heterogeneity of nanoparticles and their response to gas atmospheres is not very well explored. Here we show that an ensemble of Pd nanoparticles, grown on MgO(001) by deposition under ultrahigh vacuum, mostly consists of two distinctly epitaxially oriented particles, each having a different structural response to redox cycles. X-ray reciprocal space patterns were acquired in situ under oxidizing and reducing environments. Each type of nanoparticle has a truncated octahedral shape, whereby the majority grows with a cube-on-cube epitaxy on the substrate. Less frequently occurring and larger particles have their principal crystal axes rotated ±3.7° with respect to the substrate's. Upon oxidation, the top (001) facets of both types of particles shrink. The relative change of the rotated particles' top facets is much more pronounced. This finding indicates that a larger mass transfer is involved for the rotated particles and that a larger portion of high-index facets forms. On the main facets of the cube-on-cube particles, the oxidation process results in a considerable strain, as concluded from the evolution to largely asymmetric facet scattering signals. The shape and strain responses are reversible upon reduction, either by annealing to 973 K in vacuum or by reducing with hydrogen. The presented results are important for unraveling different elements of heterogeneity and their effect on the performance of real polycrystalline catalysts. It is shown that a correlation can exist between the particle-support epitaxy and redox-cycling-induced shape changes.

Ion distribution in polyelectrolyte multilayers with standing-wave X-ray fluorescence.

Absolute ion concentration and its profile across polyelectrolyte multilayer films were studied. The films were prepared by alternating adsorption of polyanions and polycations from aqueous solution. Standing-wave X-ray fluorescence was used to map the ion profile. The well-studied multilayer system PSS/PAH was investigated, and bromide ions were used as probe entities. The results show that the sign of the charge of the outermost layer and the washing procedure after finishing the preparation have a decisive effect on the ion concentration and the ion profile. Multilayers with PSS as the outermost layer contain fewer bromide ions than the PAH-terminated multilayers. Exposure to water washes the ions out, but even after 6 h of washing, not all of the bromide ions had been removed.

High-pressure treatment of polytene chromosomes improves structural resolution.

The exceptional cytology provided by polytene chromosomes has made Drosophila melanogaster a premier model for chromosome studies, but full exploitation of polytene cytology is impeded by the difficulty in preparing high-quality chromosome spreads. Here we describe use of high pressure to produce formaldehyde-fixed chromosome spreads, which upon light-microscopy examination reveal structural detail previously observed only in electron microscopy preparations. We demonstrate applications to immunofluorescence and in situ hybridization.

Assessment of hemagglutinin sequence heterogeneity during influenza virus transmission in families.

To examine the molecular epidemiology of influenza virus transmission, the nucleotide sequences of the HA1 domain of the hemagglutinin (HA) gene of 57 influenza A and 24 influenza B viruses recovered in a single season were analyzed. No nucleotide sequence differences were found among the 10 viruses that were recovered twice from the same patient. The nucleotide sequences of influenza A viruses were identical within each family but varied among the 14 families included in the study. The sequences of influenza A viruses recovered from 18 residents of the same community showed that 83% of the viruses differed from the others by at least 1 nucleotide residue. These findings indicate that most cases of influenza in households in which 1 family member has been infected are the result of secondary transmission from the index patient and not of acquisition from other community sources. Substantial genetic conservation during virus transmission within households is indicated.

A release-competent influenza A virus mutant lacking the coding capacity for the neuraminidase active site.

Both influenza A virus surface glycoproteins, the haemagglutinin (HA) and neuraminidase (NA), interact with neuraminic acid-containing receptors. The influenza virus A/Charlottesville/31/95 (H1N1) has shown a substantially reduced sensitivity to NA inhibitor compared with the A/WSN/33 (H1N1) isolate by plaque-reduction assays in Madin-Darby canine kidney (MDCK) cells. However, there was no difference in drug sensitivity in an NA inhibition assay. The replacement of the HA gene of A/WSN/33 with the HA gene of A/Charlottesville/31/95 led to a drastic reduction in sensitivity of A/WSN/33 to NA inhibitor in MDCK cells. Passage of A/Charlottesville/31/95 in cell culture in the presence of an NA inhibitor resulted in the emergence of mutant viruses (delNA) whose genomes lacked the coding capacity for the NA active site. The delNA mutants were plaque-to-plaque purified and further characterized. The delNA-31 mutant produced appreciable yields ( approximately 10(6) p.f.u./ml) in MDCK cell culture supernatants in the absence of viral or bacterial NA activity. Sequence analysis of the delNA mutant genome revealed no compensatory substitutions in the HA or other genes compared with the wild-type. Our data indicate that sialylation of the oligosaccharide chains in the vicinity of the HA receptor-binding site of A/Charlottesville/31/95 virus reduces the HA binding efficiency and thus serves as a compensatory mechanism for the loss of NA activity. Hyperglycosylation of HA is common in influenza A viruses circulating in humans and has the potential to reduce virus sensitivity to NA inhibitors.