Recently in Surface and interface Category

A team led by Professor Harald Ade (North Carolina State University, USA) has reported that grazing resonant soft X-ray scattering (GRSoXS), a technique measuring diffusely scattered soft X-rays from grazing incidence, can reveal the statistical topography of buried thin-film interfaces. So far, in wide variety of material systems, the internal structures of layered systems, particularly interfaces between different materials, have been critical to their functions. However, the analysis of buried interfaces has always presented some difficulties. It is known that X-ray electric field intensity distribution along the depth can be controlled by a change of either the incidence angle or the X-ray energy. The research team was able to manipulate it by scanning the X-ray energy, and succeeded in identifying the microstructure at different interfaces of a model polymer bilayer system such as PMMA/PEG. The authors attempted to gauge the feasibility of the technique for further practical systems like an organic thin-film transistor, PS[100nm]/PBTTT[50nm]/Si. For more information, see the paper, "Topographic measurement of buried thin-film interfaces using a grazing resonant soft x-ray scattering technique", E. Gann et al., Phys. Rev. B90, 245421 (2014).

Professor P. S. Pershan (Harvard University, USA) has recently published an interesting review paper on X-ray studies of the interface between liquid metals and their coexisting vapor. For more information, see the paper, "Review of the highlights of X-ray studies of liquid metal surfaces", P. S. Pershan, J. Appl. Phys., 116, 222201 (2014).

A group led by Prof. Julie V. Macpherson (Warwick University, England) has reported electrochemical X-ray fluorescence, which can quantitatively detect heavy metals in solution. In this technique, electrochemical preconcentration of a species of interest onto the target electrode is achieved by cathodic electro-deposition. X-ray fluorescence can then help unambiguous elemental identification and quantification of metal concentration. The key is that the electrochemical preconcentration step improves the detection limit by over 4 orders of magnitude, and it can reach the sub-ppb level. For more information, see the paper, "Electrochemical X-ray Fluorescence Spectroscopy for Trace Heavy Metal Analysis: Enhancing X-ray Fluorescence Detection Capabilities by Four Orders of Magnitude", L. A. Hutton et al., Anal. Chem., 86, 4566 (2014).

Dr. B. Beckhoff (Physikalisch-Technische Bundesanstalt, Germany) and his colleagues have recently published some successful applications of grazing-incidence X-ray fluorescence and near-edge X-ray absorption fine structure to nano-scale thin layers of chemically vapor deposited B_xC_yN_z on metallic Ni. For more information, see the paper, "Nondestructive and Nonpreparative Chemical Nanometrology of Internal Material Interfaces at Tunable High Information Depths", B. Pollakowski et al., Anal. Chem., 85, 193 (2013).

Resonant X-ray scattering is powerful technique for the study of electronic structure at the nanoscale. However, the optical properties of the constituent components of a material must be known prior to modeling of the scattered intensity. Professor J. B. Kortright (Lawrence Berkeley National Laboratory, USA) and his collaborator have recently proposed a method of refining electronic structure, in the form of optical properties, simultaneously with physical structure, in a Kramers-Kronig (K-K) consistent manner. This technique has been applied to specular reflectivity from a SrTiO₃ single crystal, and both a nonresonant surface contaminant layer and a modified SrTiO₃ surface region have been evidenced. For more information, see the paper, "Kramers-Kronig constrained modeling of soft x-ray reflectivity spectra: Obtaining depth resolution of electronic and chemical structure", K. H Stone et al., Phys. Rev. B86, 024102 (2012).

The 2012 workshop on buried interface science with X-rays and neutrons was held at KEK, Tsukuba, Japan, on June 26-28, 2012. This was the latest in a series of 18 workshops held since 2001. There are increasing demands for sophisticated metrology in order to observe multilayered materials with nano-structures (dots, wires, etc), which are finding applications in electronic, magnetic, optical and other devices. X-ray and neutron analysis is known for its ability to observe in a nondestructive manner even 'buried' function interfaces as well as the surface. In addition to such inherent advantages, recent remarkable advances in micro analysis and quick time-resolved analysis in X-ray reflectometry are extremely important. The latest progress in novel quantum beam technologies, such as XFELs, ERLs, as well as many other table-top laser-like machines could push such techniques towards further sophisticated applications. The present workshop gathered together those with different research backgrounds, i.e., from semiconductor electronics to chemical bio materials, and even theoretical groups were invited to give insights into unsolved problems on buried interfaces.

Dr. D. Babonneau (PhyMat, CNRS UMR 6630, Université de Poitiers, France) and his colleagues have recently analyzed morphological characteristics of nanoripple patterns prepared by broad beam-ion sputtering of Al₂O₃ and Si₃N₄ amorphous thin films as well as 2D arrays of Ag nanoparticles obtained by glancing angle deposition on Al₂O₃ nanorippled buffer layers. They employed 3D reciprocal space mapping in the grazing incidence small-angle X-ray scattering geometry. For more information, see the paper, "Quantitative analysis of nanoripple and nanoparticle patterns by grazing incidence small-angle x-ray scattering 3D mapping", D. Babonneau et al., Phys. Rev. B85, 235415 (2012).

A German group led by Dr. B. Beckhoff (Physikalisch-Technische Bundesanstalt (PTB), Berlin) recently analyzed quantitatively the buried B₄C nanolayer on a silicon substrate by using synchrotron radiation at BESSY II. The thickness and elemental composition were successfully determined by reference-free X-ray fluorescence spectrometry under conventional and grazing-incidence conditions. For more information, see the paper, "Complementary Characterization of Buried Nanolayers by Quantitative X-ray Fluorescence Spectrometry under Conventional and Grazing Incidence Conditions", R. Unterumsberger et al., Anal. Chem., 83, 8623 (2011).

Professor J. R. Engstrom (Cornell University) and his colleagues have recently published a detailed comparative study on surface morphology obtained from in-situ, time-resolved X-ray reflectivity, which is extremely feasible as a tool for investigating surface and interfaces during thin film growth, but requires some modeling of the growth process for the interpretation. The research group prepared two sets of organic thin films, pentacene/SiO₂ and diindenoperylene SiO₂; for each system, giving a total of four films, grown to different thicknesses, under nominally identical conditions. The X-ray reflectivity data were analyzed based on three different models, and the obtained parameters were directly compared with AFM data. It was found that all models employed can give good agreement between the surface morphology obtained from fits with the actual morphology at early times. On the other hand, this agreement deteriorates at later times, once the root-mean squared (rms) film roughness exceeds about 1 monolayer. It was also found that the best fits to reflectivity data, corresponding to the lowest values of χ², do not necessarily yield the best agreement between simulated and measured surface morphologies, simply because the model reproduces all local extrema in the data. For more information, see the paper, "Quantitative modeling of in situ x-ray reflectivity during organic molecule thin film growth", A. R. Woll et al., Phys. Rev. B84, 075479 (2011).

Inelastic X-ray scattering is a powerful modern tool to study lattice dynamics of condensed matter. Recently an international team led by Dr. J. Serrano (Polytechnic University of Catalonia, Spain) has tried to extend the technique to several micron-thick systems by introducing grazing-incidence geometry. Their sample is indium nitride grown on a sapphire substrate with a gallium nitride buffer layer inbetween, but X-rays only probe the surface, and not the substrate underneath. The analysis was combined with ab initio calculations to determine the complete elastic stiffness tensor, the acoustic and low-energy optic phonon dispersion relations. This finding could be a help in developing new types of solar cells. For more information, see the paper, "InN Thin Film Lattice Dynamics by Grazing Incidence Inelastic X-Ray Scattering", J. Serrano et al., Phys. Rev. Lett. 106, 205501 (2011).

A research team led by Professor J. Larsson (Lund University, Sweden) has recently performed time-resolved X-ray reflectivity measurements with 100 picosecond resolution at ID09B, at the European Synchrotron Radiation Facility (ESRF). The experiment is a so-called pump-probe measurement, i.e., the repetition of the measurement with systematic change of the delay time of the pump (laser light) and probe (X-ray) pulses. In their research, amorphous carbon films with a thickness of 46 nm were excited with laser pulses (100 fs duration, 800 nm wavelength, and 70 mJ/cm² fluence). Here, the laser-induced stress caused a rapid expansion of the thin film followed by a relaxation of the film thickness as heat diffused into the silicon substrate. The researchers succeeded in measuring changes in film thickness by X-ray reflectivity with a short X-ray pulse (100 ps duration). It was observed that thermal stress generated by laser excitation causes the film to rapidly expand and increases the surface roughness substantially. The subsequent relaxation of film thickness is governed by heat diffusion into the substrate. For more information, see the paper, "Picosecond time-resolved x-ray reflectivity of a laser-heated amorphous carbon film", R. Nuske et al., Appl. Phys. Lett. 98, 101909 (2011).

Many readers of this news column are familiar with total-reflection X-ray fluorescence (TXRF). They also know that experiments can be done with a wavelength-dispersive mode, besides ordinary measurement with a silicon drift detector or a Si(Li) detector. If the spectrometer is optimized to see inelastic X-ray scattering spectra, what happens? Very recently, a research team led by Dr. P. H. Fuoss (Argonne National Laboratory, USA) published a very interesting report. The experiment used soft X-rays to observe the electronic structure of a 10-nm-thick La_0.6Sr_0.4CoO₃ epitaxial layer grown on a SrTiO₃ substrate. By comparing data acquired under total X-ray reflection and penetrating conditions, it was found that the O K-edge spectra from a 10 nm thin film and that from the underlying substrate can be separated successfully. For more information, see the papers, "Total-Reflection Inelastic X-Ray Scattering from a 10-nm Thick La0.6Sr0.4CoO3 Thin Film", T. T. Fister et al., Phys. Rev. Lett. 106, 037401 (2011).

In Issue 47, vol. 22 (2010) of Journal of Physics: Condensed Matter, a special section features a compilation of articles on exploring surfaces and buried interfaces of functional materials by advanced X-ray and neutron techniques. Many of the authors are members of a group established in the Japan Applied Physics Society, and this is their 9^th collection of articles since 2001. Unlike many other surface-sensitive methods, these techniques do not require ultra high vacuum, and therefore, a variety of real and complicated surfaces fall within the scope of analysis. It must be particularly emphasized that the techniques are capable of seeing even buried function interfaces as well as the surface. Furthermore, the information, which ranges from the atomic to mesoscopic scale, is highly quantitative and reproducible. Such features are fairly attractive when exploring multilayered materials with nanostructures (dots, tubes, wires, etc), which are finding applications in electronic, magnetic, optical and other devices. Visit the Web page to download the papers in this collection, http://iopscience.iop.org/0953-8984/22/47

A research group led by Professor H. Zabel (Ruhr-Universität Bochum, Germany) has recently published an interesting paper discussing the solution to a well-known problem in X-ray reflectivity. The technique is for layered thin films, and can give the layer thickness, surface/interface roughness and correlations of the interface roughness parallel and perpendicular to the interface. Due to the finite size of the receiving detector slit, it will always collect not only pure specular reflection but also diffusely scattered radiation. For many years, the separation of the diffuse contribution to the intensity of specular reflection has been an important topic for reliable data analysis. The researchers propose several measurements using different slit openings for specular scans, and show some applications to realistic systems, such as periodic V/Fe multilayers prepared on MgO substrate, with V and Pd capping layers. For more information, see the paper, "Separation of the diffuse contribution to the specular x-ray scattering of multilayer films", V. P. Romanov et al., Phys. Rev. B 82, 165416 (2010).

X-ray reflectivity is one of the most power analytical tools for observing the layered structures of thin films. So far, many calculations have been done by combining Parratt's recursive formalism with Nevot-Croce corrections on the Fresnel coefficients. The technique basically provides detailed information on the roughness of the surface and interfaces, in addition to the precise thickness values of each layer. However, the analysis of the roughness has not been always straightforward, because it is also necessary to consider multiple diffuse scattering. Recently, Dr. A. M. Polyakov (National University of Science and Technology 'MISiS', Russia) and his colleague published an interesting paper describing a novel approach to the calculation of X-ray reflectivity. Their method is based on the Green function formalism using Kirchhoff's integral equation for describing the X-ray wavefield propagation through a random rough surface separating vacuum and medium. Readers would find it interesting that the influence of multiple diffuse scattering effects upon grazing X-ray specular scattering is essential for the correlation lengths that are of the order of, and/or less than, the X-ray absorption length. Although the present calculation is only valid for the random surface heights described in the frame of Gaussian statistics, the present approach can be further extended in the future. For more information, see the papers, "X-ray specular scattering from statistically rough surfaces: a novel theoretical approach based on the Green function formalism", F. N. Chukhovskii et al., Acta Cryst., A66, 640 (2010).

The 2010 workshop on buried interface science with X-rays and neutrons was held at Nagoya University, Japan, on July 25-27, 2010. This was the latest in a series of 15 workshops held since 2001; Tsukuba (December 2001), Niigata (September 2002), Nagoya (July 2003), Tsukuba (July 2004), Saitama (March 2005), Yokohama (July 2006), Kusatsu (August 2006), Kanda-Tokyo (December 2006), Sendai (July 2007), Sapporo (September 2007), Kanda-Tokyo (December 2007), Tsukuba (March 2009), Akihabara-Tokyo (July 2009) and Hiratsuka (March 2010). There are increasing demands for sophisticated metrology in order to observe multilayered materials with nano-structures (dots, wires, etc), which are finding applications in electronic, magnetic, optical and other devices. Solid-liquid interfaces are another very important research subject. X-ray and neutron analysis is known for its ability to observe in a non-destructive manner even buried function interfaces as well as the surface. In addition to such inherent advantages, recent remarkable advances in micro analysis and quick time-resolved analysis in X-ray reflectometry are extremely important. The present workshop gathered together those with different research backgrounds, i.e., from semiconductor electronics to chemical bio materials, and even theoretical groups were invited to give insights into unsolved problems on buried interfaces. The workshop proceedings will be published in IOP conference series: Materials Science and Engineering, no later than the end of 2010.

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) has recently reported some synchrotron X-ray reflectivity results on the interface between water and self-assembled monolayers. To enlarge the depletion gap (if any) as much as possible, they chose hydrophobic fluoroalkylsilane, CF₃(CF₂)₅(CH₂)₂SiCl₃ and CF₃(CF₂)₁₁(CH₂)₂SiCl₃, of which the contact angles were 111 deg and 120 deg, respectively. It was found that the depleted region width increased with contact angle and exceeded the resolution. They also concluded that the contribution of its fluctuation to the interface roughness was substantially smaller than has been considered so far. For more information, see the paper, "How Water Meets a Very Hydrophobic Surface", S. Chattopadhyay et al., Phys. Rev. Lett. 105, 037803 (2010).

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, a novel instrument for single crystal diffraction and thin film reflectivity experiments in the soft X-ray regime has been designed and constructed. It is basically a limited three circle (q, 2q, and c) diffractometer with an additional removable rotation (f), and is equipped with a liquid helium cryostat, and post-scatter polarization analysis. For more information, see the paper, "RASOR: An advanced instrument for soft x-ray reflectivity and diffraction", T. A. W. Beale et al., Rev. Sci. Instrum. 81, 073904 (2010).

Professor P. Dutta (University of Chicago) and his colleagues recently clarified that the surface density profile acquires layered structures at 0.2 T_c; T_c is the liquid-gas critical temperature. The present research was for dielectric liquids, pentaphenyl trimethyl trisiloxane, and pentavinyl pentamethyl cyclopentasiloxane. The X-ray reflectivity technique was employed to determine the surface profile experimentally. The research group had previously found similar phenomena for other liquid dielectric liquids as well as liquid metals. The present studies could strengthen their series of work. For more information, see the paper, "Surface order in cold liquids: X-ray reflectivity studies of dielectric liquids and comparison to liquid metals", S. Chattopadhyay et al., Phys. Rev, B81, 184206 (2010).

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).

Demand for learning analytical techniques for surfaces and interfaces appears to be on the increase. In Tokyo, Japan, the 4th tutorial course on the analysis of thin films and multilayers by X-ray reflectivity was held on March 16. The first Japanese textbook that serves as an introduction to X-ray reflectivity was published in 2009 (also translated into Korean in 2010), and the 8 authors gave lectures as part of the course. Further information is available at http://www.nims.go.jp/xray/ref/ (in Japanese only).

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, India) and his lab members studied the time evolution in the structure and morphology of transferred monolayers of gold-thiol nanoparticles, formed at the air-water interface at different surface pressure, on to a silicon surface. The research group employed two complementary techniques, X-ray reflectivity and atomic force microscopy (AFM), to see the whole drying-mediated self-assembly of nanoparticles. For more information, see the paper, "Nanopattern formation in self-assembled monolayers of thiol-capped Au nanocrystals", R. Banerjee et al., Phys. Rev. E80, 056204 (2009).

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 (Institute of Semiconductor and Solid State Physics, Johannes Kepler University in Linz, Austria) and his colleagues recently performed grazing-incidence small angle X-ray scattering (GISAXS) and diffraction (GID) experiments with a UHV-MBE chamber. They clarified the kinetics of the growth of Ge superdomes and their facets on Si(001) surfaces, as a function of deposited Ge thickness for different growth temperatures at a low growth rate, by in situ grazing-incidence small-angle x-ray scattering in combination with in situ grazing-incidence x-ray diffraction. At a low growth rate, intermixing is found to be enhanced and superdomes are formed already at lower coverages than previously reported. In addition, the research team observed that at the dome-to-superdome transition, a large amount of material is transferred into dislocated islands, either by dome coalescence or by anomalous coarsening. Once dislocated islands are formed, island coalescence is a rare event and introduction of dislocations is preferred. The superdome growth is thus stabilized by the insertion of dislocations during growth. For more information, see the paper, "In situ X-ray scattering study on the evolution of Ge island morphology and relaxation for low growth rate: Advanced transition to superdomes", M.-I. Richard et al., Phys. Rev. B 80, 045313 (2009).

The 2009 workshop on 'buried' interface science with X-rays and neutrons was held at Akihabara campus, Tsukuba University, Japan, on July 13-14, 2009. The workshop was the latest in a series held since 2001; Tsukuba (December 2001), Niigata (September 2002), Nagoya (July 2003), Tsukuba (July 2004), Saitama (March 2005), Yokohama (July 2006), Kusatsu (August 2006), Tokyo (December 2006), Sendai (July 2007), Sapporo (September 2007), Tokyo (December 2007) and Tsukuba (March 2009). There are increasing demands for sophisticated metrology in order to observe multilayered materials with nano-structures (dots, wires, etc), which are finding applications in electronic, magnetic, optical and other devices. X-ray and neutron analysis is known for its ability to observe in a non-destructive manner even 'buried' function interfaces as well as the surface. In addition to such inherent advantages, recent remarkable advances in micro analysis and quick time-resolved analysis in X-ray reflectometry are extremely important. The present workshop gathered together those with different research backgrounds, i.e., from semiconductor electronics to chemical bio materials, and even theoretical groups were invited to give insights into unsolved problems on buried interfaces. The workshop proceedings will be published in Transactions of the Materials Research Society of Japan, no later than the end of 2009.

Demand for learning analytical techniques for surfaces and interfaces appears to be on the increase. At Tsukuba in Japan, the 3rd tutorial course on the analysis of thin films and multilayers by X-ray reflectivity was held on May 22. The first Japanese textbook that serves as an introduction to X-ray reflectivity was published in February, and the 7 authors gave lectures as part of the course. Further information is available at http://www.nims.go.jp/xray/ref/ (in Japanese only).

Dr. A. von Bohlen (Institute for Analytical Sciences, Germany) and his colleagues recently published an interesting paper on the analysis of nanoparticles prepared on the substrate by grazing incidence X-ray Standing Waves (XSW) and Total Reflection X-ray Fluorescence (TXRF). The influence of coherence length of X-rays from different X-ray sources, the particle form, particle size and distribution are discussed. For more information, see the paper, "The influence of X-ray coherence length on TXRF and XSW and the characterization of nanoparticles observed under grazing incidence of X-rays", A. von Bohlen et al., J. Anal. At. Spectrom., 2009 (advance article) DOI: 10.1039/b811178b

Professor T. Rayment (School of Chemistry, University of Birmingham, UK) and his colleagues have developed a channel-flow cell to study electrochemical reactions on electrodes by time-resolved X-ray absorption spectroscopy. During the studies with the model system, it was found that a flowing solution is essential to remove any products of beam damage. For more information, see the paper, "Channel-Flow Cell for X-ray Absorption Spectroelectrochemistry", R. J. K. Wiltshire et al., J. Phys. Chem., C 113, 308 (2009)

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) recently reported on some very interesting XRD and GISAXS studies on the oxygen-induced shape transformation of Rh nanoparticles. The experiments were done in-situ, during the oxidation/reduction cycle at high temperature. The group found that shape transformation is driven by the formation of a surface oxide O-Rh-O trilayer, which can stabilize Rh nanoparticles with low-index facets. For more information, see the paper, "Shape Changes of Supported Rh Nanoparticles During Oxidation and Reduction Cycles", P. Nolte et al., Science, 321, 1654-1658 (2008).

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) and his colleagues employed the X-ray standing wave technique to study Al distribution in scolecite (CaAl₂Si₃O₁₀-3H₂O, hydrated calcium aluminum silicate). They measured the intensity of X-ray fluorescence, Al K, Si K and Ca Kα near the Bragg conditions of (040), (002) and (-402) reflections. The experiments were done at beamline ID32, ESRF. For more information, see the paper, "Determining the aluminium occupancy on the active T-sites in zeolites using X-ray standing waves", J. A. van Bokhoven et al., Nature Materials, 7, 551-555 (2008).

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).

There appears to be increasing demand for learning analytical techniques for surfaces and interfaces. In Japan, the 2^nd tutorial course on the analysis of thin films and multilayers by X-ray reflectivity was held on March 26. Although a similar school was run only 4 months earlier, an additional 50 young participants came to Tsukuba for the course. In France, the 3^rd school was held at Giens on May 4-8. The organizers were Professors A. Gibaud (Université du. Maine), R. Lazzari (Institut des NanoSciences de Paris) and J. Daillant (Institut Rayonnement Matière de Saclay). Of particular note is that SAXS, GI-SAXS and In-plane XRD have been newly included in the program, in addition to ordinary X-ray reflectivity. Further information is available at http://www.nims.go.jp/xray/ref/ (in Japanese only) and http://www.univ-lemans.fr/~gibaud/ecoledegiens/ (in French only), respectively

The 1^st tutorial course on the analysis of thin films and multilayers by X-ray reflectivity was held in Tsukuba, Japan, on November 29-30. The first and second days were for beginners and experts, respectively, but most of the total of 63 participants attended both of them. The textbook distributed at the school will be published in 2008. The 2^nd course will take place in March 2008. Further information is available at http://www.nims.go.jp/xray/ref/ (in Japanese only).

Since 2005, the APEC "Technological Cooperative Framework of Nano Scale Measurement and Analytical Methods" has been aiming to share the most recent advances in nanometer analytical and measurement methods from representatives of government, the private sector, R&D organizations and academia. To this end, the 2007 APEC nanoscale measurement forum was organized by Industrial Technology Research Institute (ITRI), Taiwan, and was held at Taipei, on September 5-7, 2007. The invited speakers were as follows; Peter Hatto (IonBond Ltd., Chairman of ISO/TC 229), Chanchana Thanachayanont (National Metal and Materials Technology Center, Thailand), Wen-Li Wu (NIST, USA), Michael Garner (Intel, USA), Chih-Ming Ke (Taiwan Semiconductor Manufacturing Company), Aleksandar Stefanovic (PANalytical, Singapore), Sang-Hee Suh (Center for Nanostructured Materials Technology, Korea), Kenji Sakurai (NIMS, Japan), Laura E. Depero (University of Brescia, Italy), Chun Zhang (Nano and Advanced Materials Institute, Hong Kong), Keiji Takahata (AIST, Japan), and Keh-Chyang Leou (National Tsing Hua University, Taiwan). For further information, contact Wei-En Fu, Center for Measurement Standards, ITRI, Taiwan, Phone +886-3-573 2220, WeienFu@itri.org.tw, http://www.nml.org.tw/en/APECNanoForum/

The 2007 workshop on 'buried' interface science with X-rays and neutrons was held at the Institute of Materials Research, Tohoku University, in Sendai, Japan, on July 22-24, 2007. The workshop was the latest in a series held since 2001; Tsukuba (December 2001), Niigata (September 2002), Nagoya (July 2003), Tsukuba (July 2004), Saitama (March 2005), Yokohama (July 2006), Kusatsu (August 2006) and Tokyo (December 2006). There are increasing demands for sophisticated metrology in order to observe multilayered materials with nano-structures (dots, wires, etc), which are finding applications in electronic, magnetic, optical and other devices. Unlike many other surface-sensitive methods, X-ray and neutron analysis is known for its ability to see even 'buried' function interfaces as well as the surface. It is highly reliable in practice, because the information, which ranges from the atomic to mesoscopic scale, is quantitative and reproducible. However, the method should be upgraded further to cope with more realistic problems in nano sciences and technologies. Current X-ray methods can give atomic-scale information for quite a large area on a scale of mm²-cm². These methods can deliver good statistics for an average, but sometimes it is necessary to analyze a specific part in nano-scale rather than an average structure. In addition, there is a need to see unstable changing structures and related phenomena in order to understand more about the mechanism of the functioning of nano materials. Quick measurements are therefore important. Furthermore, in order to apply the method to a more realistic and complex system, we need some visual understanding to discuss the relationship among the different structures that are present in the same viewing. Therefore, 2D/3D real-space imaging is important. Interpretation of roughness is another significant subject, while combination with grazing-incidence small angle scattering (GISAS) will become much more widespread than before. The use of coherent beams and several other new approaches are also significant. Leading senior academics in this field were invited as commentators, Professors J. Harada (Nagoya University & Rigaku Corporation), S. Kikuta (The University of Tokyo & JASRI) and J. Mizuki (JAEA). The invited speakers from Tohoku University in Sendai, workshop site, Professors K. Takanashi, M. Kawasaki and M. Yanagihara, talked about the hot topic of spintronics, and/or control of 'buried' magnetic interfaces. It was stressed that the use of techniques sensitive to specific interfaces is crucial in analyzing many unsolved problems in this field. The workshop proceedings will be published electronically in Journal of Physics: Conference Series, http://www.iop.org/EJ/journal/1742-6596

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).

A workshop on 'buried' interface science with X-rays and neutrons was held in Yokohama, Japan, on July 3-4. This was one in a series of workshops that have been organized annually since 2001. The precise and non-destructive analysis of nano-structures (dots, wires etc), which are most likely to be 'buried' under several capping layers, has become extremely important from the standpoint of fundamental understanding as well as its application to electronic, magnetic, optical and other devices. Unfortunately, most sophisticated surface-sensitive techniques are not helpful in such cases, but reflectometry and other related methods using X-rays and neutrons are very promising because they are able to explore atomic-scale structures along their depth. Besides the variety of applications, the workshop also featured intensive discussions of several advanced extensions and/or upgrades of the method. One of the most interesting directions is the combination of the method with grazing-incidence small angle scattering (GISAS). Professor Alain Gibaud (Université du Maine, France) was invited to give a lecture on this topic. Another invited speaker from outside Japan was Dr. Burkhard Beckhoff (Physikalisch-Technische Bundesanstalt, Germany). The workshop proceedings are available from Science Information and Library Service Division, High Energy Accelerator Research Organization (KEK), Phone: +81-29-864-5137, Fax: +81-29-864-4604, irdpub@mail.kek.jp. Another workshop contact, particularly with respect to future plans, is Kenji Sakurai, sakurai@yuhgiri.nims.go.jp.

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).

The Japan Society of Applied Physics (JSAP) recently approved the launch of a new professional group for X-ray and neutron analysis on surfaces and 'buried' interfaces. This is significant in that the new group will bring together those who are currently working with X-ray and neutron reflectometry or those who are simply interested in these subjects. So far in Japan, there have been very few meetings to discuss scientific problems in this area despite growing demand. There also exists a strong demand to plan and build beamlines dedicated to reflectometry and related methods at synchrotron radiation (Photon Factory and SPring-8) and neutron facilities (J-PARC to be started in 2008). The group will discuss applications with respect to a variety of materials, i.e., semiconductors, metals, ceramics, polymers, magnetic materials, and multilayers. It is of key importance to extend the technique in order to devise solutions for difficult problems in realistic specimens --. in particular, analysis of specific small areas and/or unstable systems that need to be measured in a very short time. The JSAP has an English-language Web page: http://www.jsap.or.jp/english/index.html

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).