A research group led by Professors Y. Takanishi (Kyoto University, Japan) and A. Iida (Photon Factory, KEK, Japan) has recently published its successful investigation into the local layer structure of bent-core liquid crystal, 4-Br-14-O-PIMB, which includes Br atoms. The group employed a monochromatic X-ray microbeam (3 μm × 4 μm), and observed X-ray scattering from the cell near the Br K absorption edge. They were able to discover some satellite peaks reflecting the superlattices. For more information, see the paper, "Microbeam resonant x-ray scattering from bromine-substituted bent-core liquid crystals", Y. Takanishi et al., Phys. Rev. E81, 011701 (2010).
Recently in Structure of Crystalline and Non-Crystalline Materials Category
X-ray Photon Correlation Spectroscopy (XPCS) is a novel technique which reveals the slow dynamics of equilibrium and non-equilibrium processes in condensed matter systems. A group led by Professor N. P. Balsara (
So far, X-ray microscopy with many types of lens has achieved great success in the observation of biological cells. In order to extend the limits of spatial resolution and efficiency, X-ray diffraction microscopy (also called coherent X-ray diffraction imaging), which uses coherent X-rays and some image reconstruction algorithms instead of an optical lens system, is now considered as a promising procedure to see whole cells at once and pick out much smaller features, down to around 10 nm or even less. A research group led by Professor C. Jacobsen (
Nanometer scale dipole moments in the polarization clusters in BaTiO3 are believed to be thermally excited and thermally relaxed within a picosecond time scale. However, so far, there have been no reports on the direct observation of the dynamics of these dipole moments in such a very short time scale. The limitation here is mainly due to the low spatial coherence of the X-ray beam, in particular when synchrotron radiation is used as a light source. Professor K. Namikawa (Tokyo Gakugei Univ, Japan
So far, diffusion in solids has been investigated by profiling the depth dependence of tracer atoms diffused into the sample. Although one can obtain the diffusion constant from this, the question is how diffusion takes place on the atomic scale, rather than on the micron scale. Sometimes quasielastic neutron scattering as well as Mobauer spectroscopy can be used in a very limited number of fortunate cases. A research group led by Professor G. Vogl (
In X-ray diffraction experiments, one measures the intensity (amplitude) of the diffracted X-rays as a function of position in the reciprocal space, and the information on the phase is always missing. For many years, this so-called phase problem has been thought as one of the biggest problems in X-ray crystallography. Professor E. Wolf (
Professor H. Dosch (Director of Deutsches Elektronen-Synchrotron (DESY), Germany
The use of short pulses of extremely bright synchrotron X-rays has opened up a new world. In
In classical metallurgy, there exists a very famous rule known as Hume-Rothery's rule, which describes the conditions necessary for the formation of a solid solution from two independent metals. In order to have a substitutional crystalline solid solution in which the atoms of one element randomly substitute for atoms of another element in a crystal structure, the components must have an atomic size within 15% and electronegativity within 0.4 of each other. According to this rule, a Ce-Al solid solution cannot be obtained. Recently, a research team led by Professor H.K. Mao (Carnegie Institution of Washington) and Professor R. Ahuja (
A method for realizing sub-angstrom spatial resolution in diffractive imaging of single nanocrystals
Diffractive imaging is a technique for so-called lens-less microscopy, and uses diffraction intensity (image) and phase retrieval calculations rather than focusing systems such as lenses, which are not free from aberrations. The spatial resolution is basically limited only by the amount of high-angle scattering. Therefore, the technique has been considered as having the potential to achieve atomic resolution for hard X-rays or other short-wavelength particle beams. However, so far, the reported results have been still at the level of several nanometers. Recently, a research group at the
Professor A. Cupane (University of Palermo , Italy
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).
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
Progress in nano sciences requires further development of local structural probes, particularly for the study of non-uniform materials. As material functions are often concerned with heterogeneity and some hierarchical orders of the structures, some kind of zooming from low to high resolution will become crucial in the future. Furthermore, in addition to two-dimensional (2D) imaging of an object with a lateral resolution determined by the beam size, some depth resolution is important for a better understanding of materials. So far, X-ray techniques have had several limitations with respect to such points. Recently, French scientists led by Professor J-L. Hodeau (CNRS, Grenoble, France) have reported an interesting development. They are trying to combine pencil-beam tomography with X-ray diffraction to examine unidentified phases in nanomaterials and polycrystalline materials. The experiments were for a high-pressure pellet containing several carbon phases and a heterogeneous powder containing chalcedony and iron pigments. For more information, see the paper, "Probing the structure of heterogeneous diluted materials by diffraction tomography", P. Bleuet et al., Nature Materials, 7, 468 (2008).
Recently, some very interesting research on magnetic noise from antiferromagnets has been published. Unlike ferromagnets, the characteristics of which have been studied for many years, antiferromagnets have remained a mystery because their internal structure was too fine to be measured. Their internal order is on the same scale as the wavelength of X-rays, and therefore, X-ray photon correlation spectroscopy, which measures 'speckle' patterns, can give a unique 'fingerprint' of a particular magnetic domain configuration. It was found that the domain wall motion is thermally activated at temperatures above 100 K, but not so at lower temperatures. For more information, see the paper, "Direct measurement of antiferromagnetic domain fluctuations", O. G. Shpyrko, et al., Nature 447, 68 (2007).
For many years, the existence of magnetic carbon has remained an enigma. Previous claims to have solved the mystery were subsequently disproved when it was found that magnetic metals like iron, nickel, etc, were probably present in the carbon samples. Recently, Dr. Ohldag (Stanford Synchrotron Radiation Laboratory) and his colleagues have shown that pure carbon can be made permanently magnetic at room temperature after carrying out a series of careful measurements including scanning transmission X-ray microscopy, X-ray magnetic circular dichroism (XMCD), PIXE analysis (to check for contamination by magnetic metals), AFM, and MFM etc. The team found that the magnetic order originates only from the carbon p-electron system. For more information, see the paper, p-Electron Ferromagnetism in Metal-Free Carbon Probed by Soft X-Ray Dichroism", H. Ohldag et al., Phys. Rev. Lett., 98,187204 (2007).
The Sub-Picosecond Pulse Source (SPPS) is a prototype X-ray free electron laser built using the 2-mile-long linear accelerator at Stanford Linear Accelerator Center (SLAC), California, United States. To date, ultrafast phenomena have been mainly studied with femtosecond lasers operating at ultraviolet to infrared wavelengths; however, these wavelengths are not short enough for structural studies on atomic distances. Therefore, the emergence of short pulse laser in the hard X-ray region represents a significant challenge. Recently, at Stanford, an international collaborative team from 20 different institutions succeeded in observing the atomic motion of Bismuth crystal, which, although cubic, has a slight elongation along the diagonal called a Peierls distortion. The measurements have brought new fundamental insights into the dynamics of the material, which shows very strong coupling between the electronic and ionic structures. The results could also be used to screen many theoretical calculations made so far. For more information, see the paper, "Ultrafast Bond Softening in Bismuth: Mapping a Solid's Interatomic Potential with X-rays ", D. M. Fritz et al., Science 315, 633 (2007).
Icosahedral quasicrystals (i-QCs) are long-range ordered solids that show non-crystallographic symmetries such as five-fold rotations. Their detailed atomic structures are still far from completely understood, because most stable i-QCs form as ternary alloys suffering from chemical disorder. Recently, a French-Japanese collaborative team led by Professor A. P. Tsai (Tohoku University, Japan) has succeeded for the first time in obtaining a detailed structure solution for i-YbCd5.7. Similar to normal crystals, i-QCs exhibit beautiful diffraction patterns, but their lack of periodicity prevents conventional analysis. However, mathematically, i-QCs can be seen as the projection in 3D of a structure that is periodic in a virtual space of higher dimension. This resolves the situation because it allows conventional crystallography to be used in the higher-dimensional space. The obtained result represents an essential starting point for finding the atomic structure of more complex i-QCs. The team's X-ray experiments were done with synchrotron X-rays at D2AM beamline, ESRF in Grenoble, France. For more information about the analysis, see the paper, "Atomic structure of the binary icosahedral Yb-Cd quasicrystal", H. Takakura et al., Nature Materials, 6, 58-63 (2007).
In January 2006, the Stardust spacecraft brought back a number of tiny particles from comet Wild 2, which is believed to have originated within a cloud of comets just beyond the orbit of Neptune called the Kuiper Belt. The particles have been analyzed by X-rays at six synchrotron radiation facilities around the world, ESRF (France), APS (Argonne, USA), SSRL(Stanford, USA), ALS (Berkeley, USA), NSLS (Brookhaven, USA) and SPring-8 (Japan). The particles from this comet are important because they are believed to be close to the starting material of the solar system, which is now about 4.5 billion years old. The particles were found to contain a wide variety of minerals and organic materials that look similar to those seen in primitive meteorites found on earth, but the samples also revealed the presence of new materials not previously found in meteorites. It was also discovered that the samples contained minerals similar to Calcium Aluminum-rich inclusions (CAIs), which can be formed at high temperatures, i.e., in the innermost part of the solar nebula, well inside the orbit of Mercury. For more information on the Stardust mission, visit http://stardust.jpl.nasa.gov/home/index.html. Some interesting results have been published as part of a special series of papers in the Dec. 15, 2006, edition of the journal Science.
By combining coherent X-ray scattering with a method of direct phase recovery called over-sampling, lens-free microscopy in the X-ray region becomes a realistic technique. The latest hot topic is the extension of the technique from two to three dimensions. One of the most promising ways of applying this technique is the recently reported combination of (i) ab initio phase retrieval of 2D coherent diffraction patterns with a guided hybrid input-output algorithm and (ii) 3D image reconstruction with equally sloped tomography. The scheme was applied to quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to 17×17×17 nm3. The internal GaN-Ga2O3 core shell structure was successfully captured in three dimensions. For more information about the analysis, see the paper, "Three-Dimensional GaN-Ga2O3 Core Shell Structure Revealed by X-Ray Diffraction Microscopy", J. Miao et al., Phys. Rev. Lett. 97, 215503 (2006).
Sodium saccharinate, NaC7H4NO3S•xH2O, listed in most catalogues as a dihydrate (x = 2), has been extensively used as a food additive and has constituted the basic component of the diabetics' diet for about 125 years. However, due to such factors as the instability of the crystal, the large unit cell and a very complex and heavily disordered structure, scientists have been unable to establish its composition with any certainty, until now. Dr. P. Naumov (Nat'l Inst for Mater. Sci., Japan) and his collaborators recently succeeded in the first determination of the crystal structure, by using special techniques for preserving unstable crystals during X-ray data collection. This crystal structure, which has as many as 16 formula units in the asymmetric unit (Z' = 16) as well as one of the largest unit cells, represents one of the most difficult cases for a small molecular species such as the saccharinate ion. It was found that, instead of being a dehydrate, the crystal is in fact a 1.875 hydrate, because of a structural misfit and the lack of two water molecules per asymmetric unit. The composition can be best described as Na64(C7H4NO3S)64•120H2O. At a meeting of the Asian Crystallographic Association held in Tsukuba, Japan, Dr. Naumov received the Best Presentation Award. For more information, see the paper, "Solid-state structure and temperature/evacuation-induced dehydration of sodium saccharinate 1.875 hydrate", P. Naumov et al., Angewandte Chemie, International Edition in English, 44, 1251 (2005).
Professor Weckhuysen (Utrecht University, Netherlands) and his colleagues have recently solved the molecular mechanism for the organic-base-mediated synthesis of zeolites. AlPO4-5 is a typical zeolite, which can be constructed from aluminium-based tetrahedra (AlO4) and phosphorus-based tetrahedra (PO4). The research group compared the formation of the chargeless AlPO4-5 framework with the negatively charged framework (known as ZnAPO-34) that is formed by replacing Al3+ in AlPO4-5 with Zn2+. The former contains one-dimensional channels, but the latter spherical cavities rather than channels. By employing not only small and wide angle X-ray scattering (SAXS and WAXS), but also X-ray absorption spectroscopy, it was possible to observe in real time both the structural changes in the aluminophosphate gel and the conformational features of the organic base (tetraethylammonium hydroxide) used as a template for the crystallization of zeolite. The tetraethylammonium ion was found to form a complex with developing zeolite subunits in the gel, adopting a molecular structure close to that found in the final crystal. This molecular recognition process determines which type of crystal lattice is formed. The principal point here is that molecular organization takes place before crystallization. The experiments were done at BM26A, ESRF (Grenoble, France). For more information, see the paper, "A Combined SAXS/WAXS/XAFS Setup Capable of Observing Concurrent Changes Across the Nano-to-Micrometer Size Range in Inorganic Solid Crystallization Processes", A. M. Beale et al., J. Am. Chem. Soc., 128, 12386 (2006). Another interesting account can also be found in "Physical chemistry: Porous solids get organized", R. A. van Santen1, Nature, 444, 46 (2006).
At the FLASH free-electron laser facility at DESY in Hamburg, an international team of scientists recently published the first data on diffraction imaging of a non-crystalline sample. Theoretically, a single X-ray pulse, if it is extremely bright and perfectly coherent, can produce a diffraction pattern from a large macromolecule, a virus or a cell (for example, see, "Potential for biomolecular imaging with femtosecond X-ray pulses", R. Neutze et al., Nature, 406, 752-757 (2000)). In the present experiment, the team tested a laser pulse with 25 fs, 41013 W/cm2/pulse, containing 1012 photons at 32 nm wavelength, and obtained a coherent diffraction pattern from a nanostructured non-periodic object before this exploded into a plasma at ca. 60,000 K. They employed a novel X-ray camera assured of single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. For more information, see the paper, "Femtosecond diffractive imaging with a soft-X-ray free-electron laser", H. N. Chapman et al., Nature Physics, published online 12 November 2006.
Professor Hutchings (Cardiff University, UK) and his colleagues recently published some interesting results on vanadium phosphates (VPOs). VPOs are catalysts used in industry to spur the partial oxidation of n-butane to maleic anhydride, which is then used as a starting material for products such as resins and lubricants. The research group utilized in-situ powder X-ray diffraction, in addition to laser Raman and electron paramagnetic resonance spectroscopies. They determined the transformation of VPO phases as a function of temperature and with various reactants and products present over the catalyst. They concluded that the presence of the reactants rapidly converts w-VOPO4 to d-VOPO4, but that the initial formation of the phase may create V+5 sites associated with increased catalytic activity. For more information, see the paper, "Chemically Induced Fast Solid-State Transitions of w-VOPO4 in Vanadium Phosphate Catalyst ", M. Conte et al., Science. 313, 1270 (2006).
Scientists at the Japan Atomic Energy Agency (JAEA) led by Dr W. Utsumi have proved that the formation of bulk metallic glass of elemental Zr and Ti, which was recently reported (see for example, Zhang and Zhao, Nature 430, 332 (2004) and Y. Wang et al., Phys. Rev. Lett. 95, 155501 (2005)) was some sort of phantom. The experiment basically took the form of X-ray diffraction in high-temperature and high-pressure conditions, but in addition to the normal energy-dispersive detector, the research group employed an in situ angular-dispersive X-ray diffractometer equipped with a 2D detector and X-ray transparent anvils. The disappearance of all the Bragg peaks in the one-dimensional energy-dispersive data could be taken as evidence of amorphization. However, the research group found several intense Bragg spots in their angular-dispersive data, even in the exact same conditions where amorphization was reported. This indicates that Zr and Ti do not form glass, but that the grains grow rapidly. The experiments were carried out at BL14B1 and BL22XU, SPring-8, Japan. For more information, see the paper, "Does Bulk Metallic Glass of Elemental Zr and Ti Exist?", T. Hattori et al., Phys. Rev. Lett., 96, 255504 (2006).
The appearance of the ultimate X-ray microscope, with atomic-scale resolution and capable of seeing deep inside objects, has long been awaited. Professor I. Robinson (University College London, UK) and his team recently made a significant step towards realizing this dream, using the technique of coherent X-ray diffraction imaging, the possibility of which was first pointed out by Sayre (Acta Crystallogr. 5, 843 (1952)) but not demonstrated until 1999 by Miao et al (Nature 400, 342 (1999)). They observed the growth of nanometer-sized Pb crystals inside the vacuum chamber. The results showed that asymmetries in the diffraction pattern can be mapped to deformities, providing a detailed 3-D map of their location in the crystal. This new method shows that the interior structure of atomic displacements within single nanocrystals can be obtained by direct inversion of the diffraction pattern. The technique is an attractive alternative to electron microscopy because of the superior penetration of materials of interest by the electromagnetic waves, which are often less damaging to the sample than electrons. The experiments were done at beamline 34-ID-C at the Advanced Photon Source (APS) in the United States. For more information, see the paper, "Three-dimensional mapping of a deformation field inside a nanocrystal", Mark A. Pfeifer et al., Nature 442, 63 (2006).
The traditional tools of nanotechnology - the atomic force microscope and the scanning tunneling microscope - enable scientists to see atoms, but not their response to events, which at that scale occur in the order of nano seconds or shorter. Professor P. Evans (Univ of Wisconsin-Madison) and his colleagues recently succeeded in visualizing domain wall motion during polarization switching of a Pb(Zr,Ti)O3 capacitor using time-resolved x-ray microdiffraction. The work was done using Argonne National Laboratory's Advanced Photon Source, a synchrotron light source capable of generating very tightly focused beams of X-rays. The X-rays are delivered to the sample in fast pulses over an area no larger than hundreds of nm. For more information, see the paper, "Nanosecond Domain Wall Dynamics in Ferroelectric Pb(Zr,Ti)O3 Thin Films", A. Grigoriev et al., Phys. Rev. Lett. 96, 187601 (2006).
It is well known that a piece of metal deforms in an irreversible or plastic manner when it is bent. This property is important from the standpoint of the feasibility of forming various types of metallic products as well as toughness as a structural material. Scientists from Riso National Laboratory, Denmark recently tried taking "snapshots" with hard X-rays. They observed some extremely interesting phenomena, i.e., the emergence and disappearance of the dislocation structure, which takes place during deformation. For more information, see the paper, "Formation and Subdivision of Deformation Structures During Plastic Deformation", B. Jakobsen et al., Science 312, 889 (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).
Professor E. Ma (Johns Hopkins University, USA) and his colleagues recently succeeded in explaining the atomic packing of metallic glasses, which are of great importance due to their distinctive mechanical and magnetic properties. The structure is known as 'amorphous' (non-crystalline) and shows no sharp Bragg peaks in the X-ray diffraction pattern. The research group adopted quite a unique strategy; first, they aimed at obtaining 3D pictures in the short-to-medium range, unlike conventional atomic-level analysis, which looks only at short-range order, and secondly, they did not resort to a predetermined structural model but used reverse Monte Carlo simulations based on experimental X-ray diffraction and absorption data. One of their key findings was that metallic glass atoms do not arrange themselves in a completely random way. Instead, groups of 7-15 atoms tend to arrange themselves around a central atom, forming 3D shapes called Kasper polyhedra, which join together in unique ways as small nanometer-scale clusters. For more information, see the paper, "Atomic packing and short-to-medium-range order in metallic glasses", H. W. Sheng et al., Nature, 439, 419-425 (2006).
Generally, relaxor ferroelectrics exhibit a strong polarization dependence on the applied electric field, which so far has been explained by the behavior of the polar nano-regions (PNRs). Recently, scientists at the U.S. Department of Energy's Brookhaven National Laboratory investigated the short-range polar order of Pb(Zn1/3Nb2/3)O3 (PZN) under an electric field. X-ray diffuse scattering is very sensitive to local inhomogeneities and the results indicated an unexpected redistribution of PNRs in real space, i.e., the PNR fields preferred to line up perpendicular to the external field instead of aligning with it. The experiments were done at the beamline X22B at the National Synchrotron Light Source (NSLS, at Brookhaven National Laboratory). For more information, see the paper, "Electric-field-induced redistribution of polar nano-regions in a relaxor ferroelectric", G. Xu et al., Nature Materials, in the January 15, 2006, online edition.
Imaging with coherent X-rays at high spatial resolution is a promising technique for obtaining information on the internal structures of non-crystalline specimens. Researchers at Cornell High Energy Synchrotron Source (CHESS, Cornell University, USA) recently succeeded in extending the Fresnel theory to retrieve phase information needed for a full image reconstruction. The algorithm gives 3D full field imaging with X-rays. This new scheme has been developed for coherent X-rays, but the distorted-object concept can be applied to other diffraction and imaging fields such as using visible light, electrons, and neutrons. The method is particularly important with respect to the utilization of future X-ray sources that have fully coherent photon beams. Part of their work was published in Phys. Rev. B 72, 033103 (2005). For more information, visit http://news.chess.cornell.edu/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).
Some very interesting structural studies have been performed recently at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, on photo-chemically generated, short-lived (<10-6 sec) iodo radicals. The research team dissolved a molecule of C2H4I2 in liquid methanol and then subjected it to a short laser pulse. This excited the molecule, which then cooled down while releasing heat into the surrounding liquid. As a consequence, the temperature rose and the liquid started to expand in response to the increase in temperature. The absorption of light triggered a chemical reaction, which the researchers studied with picosecond time resolution. The research team measured the change in shape and composition as early as 100 picoseconds after the initial explosion, then at an interval of 10 nanoseconds, then 1 microsecond and so on. From these measurements, the team obtained direct structural evidence of the bridged radical (CH2ICH2) in a polar solution. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. For more information, see the paper, "Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution", H. Ihee et al., Science, 309, 1223-1227, (2005).
It is known that the colours of many flowers are produced by anthocyanin, which has 6 different types of structure; a cyanidin-type anthocyanin is responsible for the red in roses, while most blue flowers have delphinidin-type anthocyanin. However, the same cyanidin-type anthocyanin makes roses red but cornflowers blue. The phenomenon has so far not been entirely explained. A Japanese group led by Professor K. Takeda (Tokyo Gakugei University, Koganei, Tokyo) recently carried out detailed X-ray analysis and clarified that a complex of six molecules each of anthocyanin and flavone, with one ferric iron, one magnesium and two calcium ions is responsible for the blue in cornflowers. For more information, see the paper, "Phytochemistry: Structure of the blue cornflower pigment", M. Shiono et al., Nature, 436, 791 (2005).
The mineral silica (SiO2) is a common substance that is a constituent of all of the planets in our solar system. At SPring-8, Harima, Japan, Dr. K. Hirose (Tokyo Institute of Technology; Japan Agency for Marine-Earth Science and Technology) and his co-workers recently found that, above 268 GPa and 1800 K, silica exhibits a novel stable high-pressure form with a pyrite-type structure, which is much denser than other known silica phases. This form of silica could be one of the main constituents of the core of a gas-giant planet such as Uranus or Neptune. For more information, see the paper, "The Pyrite-Type High-Pressure Form of Silica", Y. Kuwayama et al., Science, 309, 923-925 (2005).
Projects involving international collaboration are currently under way at the Stanford Linear Accelerator Center, in the U.S., using very bright pulses of X-ray light one thousand times shorter than those typically produced in conventional synchrotron rings. One of the topics studied very recently concerns melting- how solids transform into liquids on ultra fast time scales. In the experiment, laser light was used to melt a crystal of InSb, and then ultra-short X-ray pulses were sent to probe the material. The scattered X-rays provided a glimpse of the first step in the transition from solid to liquid. It was found that the transition state is governed by inertial dynamics, simply stated by Newton's First Law as: an object in motion continues in motion. For more information, see the paper, "Atomic-Scale Visualization of Inertial Dynamics", A. M. Lindenberg et al., Science, 308, 392-395 (2005).
Scientists at German and American synchrotron facilities have recently reported the significance of lensless imaging in achieving extremely high-spatial resolution. Although lenses are generally good at obtaining a magnified image of a sample, they also unfortunately introduce aberrations in the image, which ultimately limit the spatial resolution obtainable. In principle, one can form an image without a lens, by means of a coherent scattering experiment. The challenge is to solve the so-called phase problem. The team recently developed a new approach to X-ray holography, realizing a Fourier transform holography geometry by use of a micro- and nanostructured mask. Special contrast mechanisms can be exploited by resonant soft x-ray scattering and, in the experiment at BESSY, they recorded an image revealing the randomly organized "north" and "south" magnetic regions of a cobalt-platinum film to a spatial resolution of 50 nm, which is 10 times better than that achievable with conventional X-ray focusing optics. In the future, the technique will be used as a method for ultra-fast stroboscopic imaging on a femtosecond time scale using an X-ray free electron laser such as the Linac Coherent Light Source (LCLS), for example, which is expected to open at Stanford in 2009. For more information, see the paper, "Lensless imaging of magnetic nanostructures by X-ray spectro-holography", S. Eisebitt et al., Nature, 432, 885-888 (2004).
The ultra-fast X-ray diffraction technique has now become widely used. Many experiments using this technique are, in principle, a so-called pump-probe measurement, using a Ti:sapphire laser system (wavelength 800 nm, 1-kHz repetition rate with 5-mJ pulse energy and 45-fs duration) and, for example, a moving, 20-mm-thick Cu band to generate characteristic X-ray pulses. Recently, a German group reported the successful imaging of coherent atomic motions in a GaAs/AlGaAs superlattice. The motions are of great interest and are due to the excitation of electron-hole pairs in the GaAs subband. Both expansion of the GaAs layers and contrast of the AlGaAs layers were observed, mainly because bonding in the GaAs layers was affected by the excitation. For more information, see the paper, "Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction", M. Bargheer et al., Science, 306, 1771-1773 (2004).
A whitish cream in a small canister, which was recently discovered during archaeological surveys of the remains of a Roman temple in London, has been found to contain SnO2. Archaeologists think the SnO2 was added intentionally, presumably for use as cosmetic. They believe the unguent was prepared using sophisticated technology: animal fat was heated, possibly with the aim of bleaching it, and the starch was separated by treatment of roots or grains with boiling water, and then white SnO2 , which is readily produced by heating refined tin metal in air, was added. The non-toxic properties of SnO2 would also have been desirable, because by the second century AD, the dangers of lead were becoming recognized. XRF and XRD analysis played an important role in the identification of the ancient cosmetic cream. For more information, see the paper, "Archaeology: Formulation of a Roman cosmetic", R. P. Evershed et al., Nature, 432, 35-36 (2004).
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