July 2009 Archives

A recent edition of Nature News featured the successful application of a carbon nanotube (CNT)-based X-ray source to medical imaging. A group led by Professor O. Zhou (University of North Carolina in Chapel Hill, USA) has developed a micro 3D CT system. The main idea behind such very rapid scanning is simply electronic switching of 3D arrayed CNT X-ray sources, rather than mechanical motion. For more information, see the article, "Nanotubes sharpen X-ray vision", Zeeya Merali, Nature News, doi:10.1038/news.2009.744 The original research papers were published in April 2009 ("A dynamic micro-CT scanner based on a carbon nanotube field emission X-ray source", G Cao et al Phys. Med. Biol. 54, 2323 (2009))

The following awards were presented during the plenary session of the 58th Annual Denver X-Ray Conference:

The 2009 Barrett Award was presented to Robert Von Dreele, Argonne National Laboratory, Argonne, IL.

The 2009 Jenkins Award was presented to Tim Fawcett, International Centre for Diffraction Data, Newtown Square, PA.

There was no winner for the 2009 Jerome B. Cohen Student Award.

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).

Since 1984, laboratory-scale X-ray lasers have been extensively studied. The shortest wavelength achieved so far is 3.6 nm, with a weak intensity. On the other hand, X-ray free-electron lasers (XFEL) based on self-amplified spontaneous emission (SASE) from a long undulator in the linear electron accelerator will be available in near future. The next idea is the use of XFEL to pump a photoionization inner-shell X-ray laser in an atomic gas. Dr. R. London (Lawrence Livermore National Lab) and a colleague have recently published their theoretical calculations. For more information, see the paper, "Atomic inner-shell X-ray laser pumped by an x-ray free-electron laser", N. Rohringer et al., Phys. Rev. A 80, 013809 (2009).

Professor H. Dosch (Director of Deutsches Elektronen-Synchrotron (DESY), Germany) and his colleagues recently published a very interesting paper on the symmetry of disordered systems. They propose a new technique, X-ray cross correlation analysis (XCCA). This measures X-ray speckles and is basically an extension of X-ray photon correlation spectroscopy (XPCS). The samples studied were colloidal glasses, and the research group was able to observe clear symmetries that conventional X-ray diffraction has been unable to extract. The research group recommends using brilliant coherent X-ray sources, such as X-ray free electron lasers for future research. For more information, see the paper, "X-ray cross correlation analysis uncovers hidden local symmetries in disordered matter", P. Wochnera et al., Proc Nat Aca Sci, 106, 11511 (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.

Kyushu University has recently constructed its own new beamline at the SAGA Light Source, which is one of Japan's compact synchrotron facilities. For more information, visit the Web page, http://www.saga-ls.jp/

Imaging individual objects of several nanometer resolution in space and several femtosecond resolution in time, is now one of the most exciting experiments in X-ray physics. Over the past decade, coherent X-ray diffraction has overcome a lot of limits in imaging noncrystalline objects at a resolution in the order of X-ray wavelength. So far, X-ray free electron lasers (or, in the mean time, 3^rd generation synchrotron sources) have been considered as a promising source, but the table-top source is no doubt extremely important for many new sciences. Recently, Dr. H. Merdji (CEA Saclay, France) and his colleagues reported the feasibility of a laser-driven soft X-ray source, which uses the 25^th harmonics (32 nm wavelength, 20 fs pulse width) of a Ti:sapphire laser. They succeeded in observing diffraction patterns from isolated nano-objects with a single 20 fs pulse. Images were reconstructed with a spatial resolution of 119 nm from the single shot and 62 nm from multiple shots. For more information, see the paper, "Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source", A. Ravasio et al., Phys. Rev. Lett. 103, 028104 (2009).

In January 2006, NASA's Stardust spacecraft brought comet coma particles and interstellar grains from Comet 81P/Wild2. Synchrotron facilities all over the world have been used for extensive analysis of the chemical composition and crystal structures of the matter. Recently, Professor L. Vincze (X-ray Microspectroscopy and Imaging Group, Ghent University, Belgium) and his colleagues reported the results of 3D X-ray imaging based on X-ray fluorescence (XRF) tomography. In the present research, a 200 nm beam was employed, because the typical size of the particles from space was 2 microns. The measurement consisted of 2D scanning XRF maps for each rotation angle of the sample. In the XRF spectra, many peaks were found; Ca, Cr, Mn, Fe, Cu, Se etc. For more information, see the paper, "X-ray Fluorescence Nanotomography on Cometary Matter from Comet 81P/Wild2 Returned by Stardust", G. Silversmit et al., Anal. Chem., Article ASAP, DOI: 10.1021/ac900507x For related work on the same 'star dust' by other groups, for example, see "Chondrulelike Objects in Short-Period Comet 81P/Wild 2", Tomoki Nakamura et al., Science, 321, 1664-1667 (2008) and "Mixing Fraction of Inner Solar System Material in Comet 81P/Wild2", A. J. Westphal et al, The Astrophysical Journal, 694, 18-28 (2009).

Dr. P. Glatzel (European Synchrotron Radiation Facility (ESRF), Grenoble, France) and his colleagues recently published an interesting paper reporting systematic studies on both X-ray absorption and Kα emission spectra from sulfur compounds. The compounds' spectra were compared with quantum chemical calculations using density functional, multiple-scattering, and atomic multiplet theory. It was found that the near-edge absorption spectra are mainly determined by the geometry of the first coordination sphere in the case of the sulfates and sulfite, while strong orbital hybridization in the case of sulfides results in a much more complex analysis. On the other hand, the spectral shape of the Kα fluorescence lines shows little influence of the chemical environment, but its energy position is correlated with the valence-shell electron population. The experiments were done at beamline ID26, ESRF. The spectrometer used for Kα emission is a combination of a Johansson Si(111) crystal and a CCD camera. The energy resolution was 0.44 eV for S Kα. For more information, see the paper, "Electronic Structure of Sulfur Studied by X-ray Absorption and Emission Spectroscopy", R. A. Mori et al., Anal. Chem., Article ASAP, DOI: 10.1021/ac900970z