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Cholesterol plays a key role in the molecular and mesoscopic organisation of lipid membranes and it is expected that changes in its molecular structure (e.g., through environmental factors such as oxidative stress) may affect adversely membrane properties and function. In this study, we present evidence that oxidation of cholesterol has significant effects on the mechanical properties, molecular and mesoscopic organisation and lipid–sterol interactions in condensed monolayers composed of the main species found in the inner leaflet of the erythrocyte membrane. Using a combination of experimental methods (static area compressibility, surface dilatational rheology, fluorescence microscopy, and surface sensitive X-ray techniques) and atomistic molecular dynamics simulations, we show that oxidation of cholesterol to 7-ketocholesterol leads to stiffening of the monolayer (under both static and dynamic conditions), significant changes in the monolayer microdomain organisation, disruption in the van der Waals, electrostatic and hydrophobic interactions between the sterol and the other lipid species, and the lipid membrane hydration. Surface sensitive X-ray techniques reveal that, whilst the molecular packing mode is not significantly affected by cholesterol oxidation in these condensed phases, there are subtle changes in membrane thickness and a significant decrease in the coherence length in monolayers containing 7-ketocholesterol.

Multilayers consisting of six GaN quantum dot (QD) layers separated by AlN spacer layers were grown by molecular beam epitaxy. Structural properties and the effects of thermal annealing were analyzed using X-ray diffraction (XRD), X-ray reflection (XRR) and Rutherford backscattering spectrometry/Channeling (RBS/C). Annealing was performed in nitrogen atmosphere for 20 min at temperatures between 1000 and 1200 °C. The GaN QDs on the surface were destroyed at temperatures ≥ 1100 °C while the AlN spacer layers protected the deeper lying QD layers. The multilayer structure was well preserved for all studied temperatures. Indications of interdiffusion in the interfaces were found. Thickness fluctuations were evidenced by XRR. Copyright © 2010 John Wiley & Sons, Ltd.

We applied neutron reflectivity measurements to hydrophilic and hydrophobic self-assembled monolayers (SAMs) to probe water density at these interfaces. The measurements were motivated by a previous theoretical study which reported a reduced water density at a hydrophobic surface (Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 4570) and by our own computer simulations on the hydration force and water density between two methoxy tri(ethylene glycol) terminated undecylthiolate SAMs adsorbed on gold substrates used to study protein adsorption. These simulations predicted that the surfaces are slightly hydrophobic and are characterized by a reduced water density at the interface (Pertsin, A. J.; Hayashi, T.; Grunze, M. J. Phys. Chem. B 2002, 106, 12274). In disagreement with the marginal reduction in water density derived in the simulations, the neutron reflectivity measurements reported here indicate an unexpectedly extended (∼4 nm) water layer at the SAM surface with a noticeably reduced density (85−90% of the density of bulk water). The reproducibility of the experimental results with the methoxy tri(ethylene glycol) terminated undecylthiolate SAMs was confirmed with four different samples and by one measurement using a contrast-matched D2O/H2O water mixture. We also used neutron reflectivity measurements to study the water density at the water/SAM interface of hydroxy hexa(ethylene glycol) and hydroxy tri(ethylene glycol) terminated undecylthiolate SAMs. Except for one of the hydroxy hexa(ethylene glycol) samples studied, the results are consistent with the presence of bulk water in direct contact with the interface. We also discuss possible artifacts and problems in the analysis. Our results on nonfunctionalized hydrophobic octadecanethiolate and hydrophilic hydroxy-terminated undecylthiolate SAMs give physically unreasonable and nonconclusive models for the water interface on these surfaces, respectively. The best fit of the data for the hydrophobic surface gives an unreasonably low water density, possibly due to the presence of air inclusions in the film and/or adsorbed air “nanobubbles”. The results obtained on the hydrophilic hydroxy-terminated surface can be fitted equally well with a model assuming an interface water density that is higher or lower than that of bulk water, demonstrating the ambiguities associated with describing the organic/liquid interface with a box model for the Q range accessible in our experiment.

A series of experiments on x-ray reflection from thin multilayers and films on a silicon wafer were carried out. The x-ray source and detector were fixed and the glancing angle was changed by sample stage rotation. The use of a Geiger-Muller counter allowed bizarre symmetric patterns to be obtained in the reflected x-ray intensity. The origin of these patterns is not clear at the present stage of the investigation. To perform a preliminary analysis of the x-ray reflection from multilayers, the Microsoft Access database has been developed.

We present 3-50 keV NuSTAR observations of the active galactic nuclei Mrk 335 in a very low flux state. The spectrum is dominated by very strong features at the energies of the iron line at 5-7 keV and Compton hump from 10-30 keV. The source is variable during the observation, with the variability concentrated at low energies, which suggesting either a relativistic reflection or a variable absorption scenario. In this work, we focus on the reflection interpretation, making use of new relativistic reflection models that self consistently calculate the reflection fraction, relativistic blurring and angle-dependent reflection spectrum for different coronal heights to model the spectra. We find that the spectra can be well fitted with relativistic reflection, and that the lowest flux state spectrum is described by reflection alone, suggesting the effects of extreme light-bending occurring within similar to 2 gravitational radii (R-G) of the event horizon. The reflection fraction decreases sharply with increasing flux, consistent with a point source moving up to above 10 R-G as the source brightens. We constrain the spin parameter to greater than 0.9 at the 3 sigma confidence level. By adding a spin-dependent upper limit on the reflection fraction to our models, we demonstrate that this can be a powerful way of constraining the spin parameter, particularly in reflection dominated states. We also calculate a detailed emissivity profile for the iron line, and find that it closely matches theoretical predictions for a compact source within a few R-G of the black hole.

We investigate substrate roughness-induced fluctuations on liquid films in the presence of polar (exponential) and apolar (van der Waals) interactions in the complete wetting regime. The liquid/vapor interface roughness amplitude sigma(w) increases rapidly with film thickness epsilon above a critical thickness epsilon(c) for which the film is stable (or it does not rupture due to presence of polar interactions), and it reaches a maximum at a thickness epsilon(m) slightly larger than epsilon(c) if polar and apolar components are of comparable strength and for small polar potential ranges. As the strength of the polar interaction decreases with respect to the apolar; behavior characteristic of that of apolar interactions within the Derjaguin approximation is recovered for moderate film thicknesses (epsilon > epsilon(m)); sigma(w)proportional to zeta(-2) with zeta the healing length. [S1063-651X(99)11701-X].

The purpose of the designed reactor is (i) to obtain polycrystalline and/or amorphous thin films by controlled deposition induced by a reactive sputtering magnetron and (ii) to perform a parallel in situ structural study of the deposited thin films by X-ray diffraction, in real time, during the whole growth process. The designed reactor allows for the control and precise variation of the relevant processing parameters, namely, magnetron target-to-sample distance, dc magnetron voltage, and nature of the gas mixture, gas pressure and temperature of the substrate. On the other hand, the chamber can be used in different X-ray diffraction scanning modes, namely, θ-2θ scanning, fixed α-2θ scanning, and also low angle techniques such as grazing incidence small angle X-ray scattering and X-ray reflectivity. The chamber was mounted on a standard four-circle diffractometer located in a synchrotron beam line and first used for a preliminary X-ray diffraction analysis of AlN thin films during their growth on the surface of a (100) silicon wafer. © 2013 American Institute of Physics Fil: Burgi, Juan Mauel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Rosario. Instituto de Fisica de Rosario (i); Argentina Fil: Newenschwander, R.. Laboratorio Nacional de Luz Sincrotron Campinas; Fil: Kellermann, G.. Universidade Federal Do Parana; Fil: García Molleja, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Rosario. Instituto de Fisica de Rosario (i); Argentina Fil: Craievich, A.. Instituto de Fi­sica Universidad de San Pablo; Fil: Feugeas J.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Rosario. Instituto de Fisica de Rosario (i); Argentina

10 pages; International audience; Picosecond ultrasonic measurements have been carried out on molybdenum-silicon multilayers in order to study their elastic effective-medium behavior using low-frequency acoustic echoes, and their high-frequency response through localized surface modes. Taking into account the elastic properties of silicon and molybdenum thin films independently measured, we show that the long-wavelength acoustic behavior of the multilayers is well reproduced. Using an effective-medium model that includes only the two constituent layers in the periodic cell without any interfacial zone, sound velocity, echo shape, and acoustical reflection coefficient have been investigated. In contrast, the frequency of the acoustic-phonon surface modes could not be reproduced unless a modification of the surface silicon layer was introduced, in good agreement with x-ray reflectivity measurements. Finally, low-temperature experiments illustrate the potential of such wide stop-band systems for the emission of monochromatic acoustic waves, using the tunneling of a surface-localized mode or a buried nanocavity mode. The filtering and acoustic transduction properties of these artificial layered materials have also been investigated.

5 páginas, 5 figuras, 1 tabla., Compositional, microstructural, and magnetic characterization of [ZnO(30 Å)/Mn(x)]n multilayers prepared by sputtering is presented to study the observed ferromagnetism in the Mn-ZnO system. The nominal Mn layer thickness, x, is varied from 3 to 60 Å, while the number of bilayers, n, is increased to maintain the total amount of Mn constant. Microstructure information was deduced from x-ray reflectivity, Mn oxidation state was determined by x-ray absorption spectroscopy, and magnetic properties were measured over a temperature range of 5–400 K. Magnetic behavior of these samples is found to be related to the Mn layer thickness (x). Multilayers with x ≥ 30 Å exhibit ferromagnetism with a Curie temperature above 400 K, while mostly paramagnetic behavior is obtained for x, We thank the DGI-MEC for financial support under Contracts No. MAT2003-01880 and No. MAT2006-01004. J.G.L. acknowledges the “Ramon y Cajal” program of the Spanish MEC for financial support.

10 pages; International audience; We report on the evolution of the structure and composition of a Pt(3 nm)/Co(0.6 nm)/AlOx(2 nm) trilayer sputtered on Si/SiO2 under oxidation and annealing processes by combined x-ray reflectivity and x-ray absorption studies. We describe the progressive and inhomogeneous oxidation of the layers by increasing the oxidation time. Before annealing, the layers have lower density than bulk samples and noticeable roughness. After thermal annealing, a significant improvement of the quality of the alumina layer goes along with the formation of a CoPt alloy that reduces the number of Co-O bonds. These structural outcomes clarify the evolution of the magnetic and transport properties reported at room temperature in these samples.

We report the design and construction of a novel soft x-ray diffractometer installed at Diamond Light Source. The beamline endstation RASOR is constructed for general users and designed primarily for the study of single crystal diffraction and thin film reflectivity. The instrument is comprised of a limited three circle ({\theta}, 2{\theta}, {\chi}) diffractometer with an additional removable rotation ({\phi}) stage. It is equipped with a liquid helium cryostat, and post-scatter polarization analysis. Motorised motions are provided for the precise positioning of the sample onto the diffractometer centre of rotation, and for positioning the centre of rotation onto the x-ray beam. The functions of the instrument have been tested at Diamond Light Source, and initial test measurements are provided, demonstrating the potential of the instrument., Comment: 10 pages, 9 figures

Al2O3/NbAlO/Al2O3 sandwich dielectric films were grown on InP substrate and annealed. X-ray reflectivity measurements suggested that 1.0 nm interfacial layer existed at InP interface, x-ray diffraction and high resolution transmission electron microscopy indicated the films were crystallized. X-ray photoelectron spectra indicated the oxidization of InP substrate, and the valence-band offset between the dielectric film and InP interface was calculated to be 3.1 eV. The electrical measurements indicated that the leakage current density was 40 mA/cm(2) at gate bias of 1 V, and the equivalent oxide thickness and the dielectric constant were 1.7 and 20 nm, respectively.