For many years, scientists have argued about the existence of a depletion gap between water and hydrophobic surfaces. Several recent reports based on high-resolution synchrotron X-ray reflectivity seemed to give a positive conclusion, but they were not in good agreement quantitatively, mainly because the amount being discussed was at experimental resolution. A research group led by Professor P. Dutta (Northwestern University , Illinois , USA
Recently in Surface and Interface Category
Soft X-ray resonant diffraction and reflectivity have become one of the most promising tools with which to study magnetic materials. At Diamond Light Source, Oxfordshire , UK
Professor P. Dutta (
A Dutch neutron research group at Delft University of Technology, Netherlands, recently published a paper describing the extension of their coherence theory on neutron scattering to X-ray reflectivity. For more information, see the paper, "Coherence approach in neutron, x-ray, and neutron spin-echo reflectometry", V. O. de Haan et al., Phys. Rev. B81, 094112 (2010).
Rubrene (5,6,11,12-tetraphenylnaphthacene, C42H28) is a red colored polycyclic aromatic hydrocarbon. As an organic semiconductor, the most promising application is in organic light-emitting diodes (OLEDs) and organic field-effect transistors, which are the core elements of flexible displays. Recently, Professor Y. Wakabayashi (Osaka University, Japan) and his colleagues have studied the near surface structure of Rubrene single crystal by crystal truncation rod (CTR) scattering, which gives a modulated profile in the tail of a series of Bragg peaks (0 0 z). The research group employed coherent Bragg rod analysis (COBRA) rather than conventional curve fitting analysis to determine the electron density profile along the depth. The analysis has shown that the molecules at the surface are slightly expanded along the surface normal direction, while the second or deeper molecular layers are not affected by the existence of the surface. Their research can be extended to applications of other similar organic semiconductors. For more information, see the paper, "Sub-Å Resolution Electron Density Analysis of the Surface of Organic Rubrene Crystals", Y. Wakabayashi et al., Phys. Rev. Lett. 104, 066103 (2010). For information on COBRA, see, for example, "Direct determination of epitaxial interface structure in Gd2O3 passivation of GaAs", Y. Yacoby et al., Nature Materials 1, 99 (2002).
Foamlike, cellular structures of the monolayer of organic capped nanoparticles can sometimes be observed on liquid surfaces. Professor M. K. Sanyal (Saha Institute of Nuclear Physics,
It is well known that the physical properties of semiconductor nanostructures, which have been grown in most cases by the Stranski-Krastanow (SK) mechanism, depend on their size, shape, strain and composition. In the case of the growth of Ge on Si(001), where the 2D-3D transition is driven by the 4.16% lattice mismatch between Ge and Si, the increase of Ge coverage above a critical thickness of around 4 ML can make coherent islands. First, square pyramids appear, and then dome-shaped islands are formed. At about 9 ML, the misfit strain can no longer be accommodated coherently and larger islands called superdomes are present. This raises detailed questions as to dependence on the growth rate, temperature etc. To provide answers to such questions, in-situ X-ray studies are extremely important. Professor G. Bauer (
Dr. A. von Bohlen (Institute for Analytical Sciences, Germany
Professor T. Rayment (
It is well known that nanoparticles often enhance catalytic activity. However, it is still an open question as to whether the metallic or the oxidized state of the particle is the catalytically more active phase. It is therefore significant to study the oxidation/reduction process of metallic nanoparticles. A group led by Professor H. Dosh (Max-Planck-Institut für Metallforschung , Germany
Some of the most well known self-assembled monolayers (SAMs) are alkyl sulfides on gold surfaces. They have many potential applications in molecular electronics, biosensors, and nanopatterning. However, there have still been unsolved problems in basic research regarding Au-S interaction. Recently, Professor A. Morgante (Universita' di Trieste, Italy) and his colleagues published the results of grazing incidence X-ray diffraction and density functional theory-based molecular dynamics simulations for hexanethiol and methylthiol. The research group demonstrated surface complexes wherein two S atoms are joined by an intermediate Au adatom (RS-Au-SR) for longer chain cases. It was found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first surface layer contains vacancies as well as gold adatoms that are laterally bound to two sulfur atoms. Competition between SAM ordering and disordering of interfacial Au atoms takes an important role in the system. For more information, see the paper, "X-ray Diffraction and Computation Yield the Structure of Alkanethiols on Gold(111)", A. Cossaro et al., Science, 321, 943-946 (2008).
Aerogel is a form of nanofoam, an engineered material designed for its high strength-to-weight ratio for application wherever lightness and strength are needed. Now, the internal structure is within the scope of X-ray analysis. Lawrence Livermore and Lawrence Berkeley scientists have successfully applied the coherent X-ray diffraction technique to Ta2O5 nanofoam, the density of which is 1.2 % to the bulk, and have reconstructed 3D images to determine its strength and potential new applications. Combining the obtained structural information with detailed simulations, the research team showed that the blob-and-beam network structure explains why the materials are weaker than expected. For more information, see the paper, "Three-Dimensional Coherent X-Ray Diffraction Imaging of a Ceramic Nanofoam: Determination of Structural Deformation Mechanisms", A. Barty et al., Phys. Rev. Lett., 101, 055501 (2008).
Zeolites are microporous crystalline materials, and in the unit cell, the tetrahedrally coordinated Si and Al atoms occupy the so-called crystallographic T-sites. In addition to their pore size, Al's occupancy in the specific T-sites is extremely important in catalytic activity. So far, however, the distribution of Al has remained an unresolved problem. Recently, Professor J. A. van Bokhoven (ETH Zurich , Switzerland
Analysis of X-ray and neutron reflectivity is usually done by modeling the scattering length density profile (such as multilayers) of the sample and performing a least square fit to the measured, phaseless reflectivity data. Professor T. Salditt (Institute for X-ray Physics, Universitat Gottingen) and his colleague recently attempted to extend the inversion technique. The research group discussed conditions for uniqueness, which are applicable in the kinematic limit (Born approximation), and for the most relevant case of box model profiles with Gaussian roughness. They also demonstrated that an iterative method to reconstruct the profile based on regularization works well. For more information, see the paper, "Iterative reconstruction of a refractive-index profile from x-ray or neutron reflectivity measurements", T. Hohage et al., Phys. Rev. E77, 051604 (2008).
A group at the Max Planck Institute led by Professor H. Dosch recently performed detailed studies on the gap between water and a water-repelling surface. Silicon wafers, functionalized by a self-assembled monolayer of octadecyl-trichlorosilane (OTS), provide strongly hydrophobic substrates. The main interest here is what happens when water comes onto the OTS layer. The experiment was not easy, because the liquid water-solid interface is deeply buried in this case. In this research, X-ray reflectivity measurements using an unusually high-energy beam (72.5 keV) were carried out. The results indicate the existence of a hydrophobic gap on a molecular scale at the solid-water interface. For more information, see the paper, "High-resolution in situ X-ray study of the hydrophobic gap at the water-octadecyl-trichlorosilane interface", M. Mezger et al., Proc. Natl. Acad. Sci. USA, published online before print November 20, 2006
Argonne National Laboratory researchers in collaboration with Xradia, Inc. have developed a novel X-ray surface topography technique by combining X-ray reflection, which is sensitive to height or depth on a sub nanometer scale, and full-field X-ray microscopy with condenser and objective Fresnel zone plates. Recent rapid progress in X-ray microscopy now allows scientists to obtain X-ray images with ca. 10 nm spatial resolution. However, so far, almost all full-filed imaging has employed transmission geometry. The present research has extended the technique to reflection geometry. It has become possible to image the distribution of molecular-scale interfacial features directly and non-invasively with full-field imaging. Interfacial phase contrast from elementary defect structures allows direct observation of 0.6-nm-high monomolecular steps at a solid surface. For more information, see the paper, "Observation of subnanometre-high surface topography with X-ray reflection phase-contrast microscopy", P. Fenter et al., Nature Physics, 2, 700-704 (2006).
Professor P. Pershan (Harvard University, USA) and his colleagues recently found a crystalline monolayer at the surface of the eutectic liquid Au82Si18, at temperatures above the alloy's melting point. This is unusual for a liquid surface, as the atomic arrangements are ordinarily strongly disordered. In addition, they found that the gold-silicon eutectic alloy has 7-8 layers near its surface, whereas many metallic liquids typically show only 2-3 distinct atomic layers. The phenomena are considered as indicative of surface freezing. The research group employed X-ray reflectivity and grazing incidence X-ray diffraction techniques for the analysis. For more information, see the paper, "Surface Crystallization in a Liquid AuSi Alloy", Oleg G. Shpyrko et al., Science 313, 77 (2006).
Corrosion detracts some 3% from global GDP. From a positive point of view, however, chemical attack of metal surfaces may result in surface nano-structures with interesting technological applications such as catalysts and sensors. Professor H. Dosch (Max Planck Institute) and his colleagues have recently clarified a self-organization process on the surface of Cu3Au(111) single crystal alloy in a sulphuric acid solution, by means of a sophisticated X-ray diffraction technique with the aid of a brilliant synchrotron beam at ESRF, Grenoble, France. They observed many interesting phenomena. In the initial moments of corrosion, an extremely thin gold-rich layer, which had an unexpected crystalline and well-ordered structure, was formed. As the corrosion proceeded, this alloy layer was transformed into gold nano-islands of 20 to 1.5 nm. These islands eventually developed into a porous gold metal layer. For more information, see the paper, "Initial corrosion observed on the atomic scale", F. U. Renner et al., Nature, 439, 707-710 (2006).
At SPring-8, Harima Japan, Dr. M. Takahasi (Japan Atomic Energy Agency) and his coworkers have recently established a powerful surface X-ray diffraction tool for observing the growth process of semiconductor-like GaAs. The main feature of the method is the use of multi-energy X-rays, and because of this, it is possible to identify both the atomic arrangements and the type of atoms. Another significant advantage is the capability of real-time monitoring due to the employment of a brilliant undulator beam. It was demonstrated that the surface structure called c(4x4), which is observed under certain growth conditions, has dimmers that consist of gallium and arsenic atoms in the top surface layer. For more information, see the paper, "Element-Specific Surface X-Ray Diffraction Study of GaAs(001)-c(4×4)", M. Takahasi et al., Phys. Rev. Lett. 96, 055506 (2006).
The distribution of ions in solution at an interface is key to the fundamental understanding of electrochemistry as well as to the design of materials and devices such as biomembranes. So far, classical descriptions of ion distributions, such as the Guoy-Chapman theory (see, G. Gouy, C. R. Acad. Sci. 149, 654 (1910) and D. L. Chapman, Phil. Mag. Ser. 6 25, 475 (1913)), which ignores the details of molecular structure, have been widely used. Professor M. Schlossman (University of Illinois at Chicago) and his colleagues recently performed very precise X-ray reflectivity measurements to obtain experimentally ion distributions at the interface between solutions (0.01 ~0.08M) of tetrabuytlammonium (TBA) tetraphenylborate (TPB) in nitrobenzene and aqueous TBA bromide. They found significant deviations from the Guoy-Chapman theory in describing their data. However, on the other hand, molecular dynamics calculations produced potentials that could be used to predict distributions with the Poisson-Boltzmann equation without adjustable parameters. The experiments were done at the Chemistry and Materials section of the Consortium for Advanced Radiation Sources (ChemMatCARS) beamline 15-ID at the Advanced Photon Source (APS, at Argonne National Laboratory). For more information, see the paper, "Ion Distributions near a Liquid-Liquid Interface", L. Guangming et al., Science, 311, 216-218 (2006).
Control of nano-structures with molecular precision is a key problem in nano sciences and technologies. While the surface can be readily imaged by scanning probe microscopes, it is not easy to observe buried structures nondestructively. Dr. O. Sakata and his colleagues recently reported on their success in fabricating Bi nanowires on a Si(001) substrate and their encapsulation in an epitaxially grown crystalline silicon layer. To explore the buried nanowires, they employed X-ray diffraction (reciprocal-lattice space mapping) with 25.3 keV photons at grazing-incidence geometry (~0.1 deg) using an image plate as a 2D detector. The results indicate that the nanolines maintain their one-dimensional character and Bi dimerization. The experiments were carried out at beamline BL13XU, SPring-8, Harima, Japan. For more information, see the paper, "Encapsulation of atomic-scale Bi wires in epitaxial silicon without loss of structure", O. Sakata et al., Phys. Rev. B 72, 121407(R) (2005).
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