July 2010 Archives

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.

Dr. W. Yashiro (University of Tokyo, Japan) and his colleagues recently reported an interesting application of X-ray Talbot interferometry, which usually gives absorption and differential-phase images. As micro-structures of the sample distort X-ray wave fronts, the research group quantitatively discusses how visibility reduction is caused and influenced. They also experimentally demonstrate that this new type of experimental method using visibility contrast is feasible for imaging micro-structures, which have been studied by ultra small angle X-ray scattering so far. For more information, see the paper, "On the origin of visibility contrast in x-ray Talbot interferometry", W. Yashiro et al., Optics Express, 18, 16890 (2010). For more information on visibility contrast, see the paper, "Hard x-ray dark-field imaging using a grating interferometer", F. Pfeiffer et al., Nature Materials, 7, 134 (2008).

The recent advent of coherent soft and hard X-ray sources has facilitated the development of imaging techniques that are capable of being inverted to the real space information extremely quickly. A research group at the SLAC National Accelerator Laboratory, USA, recently developed a novel technique for soft X-rays, based on differential holographic encoding, termed holography with extended reference by autocorrelation linear differential operation (HERALDO). The technique has achieved superior resolution over other similar lensless techniques, such as X-ray Fourier transform holography, while maintaining the signal-to-noise ratio and algorithmic simplicity. The spatial resolution was 16 nm, and this was obtained by synthesizing images in the Fourier domain from a single diffraction pattern, which allows resolution improvement beyond the reference fabrication limit. In addition to the capability of instant high-resolution reconstruction, the technique is found to be robust against data imperfections. It reduces artifacts arising from the commonly-missing central low-q data. For more information, see the paper, "High-Resolution X-Ray Lensless Imaging by Differential Holographic Encoding", D. Zhu et al., Phys. Rev. Lett. 105, 043901 (2010). For more information on the original idea of HERALDO, see the paper, "Holography with extended reference by autocorrelation linear differential operation", M. Guizar-Sicairos and J. R. Fienup, Optics Express, 15, 17592 (2007).

NASA's Mars Exploration Rover Spirit has obtained some significant data on the detailed chemical composition of the rock exposed on the ground surface of the Columbia Hills of the Gusev crater. It was found that the rock is a Mg-Fe carbonate (Mc_0.62Sd_0.25Cc_0.11Rh_0.02, where Mc = magnesite, Sd = siderite, Cc = calcite, and Rh = rhodochrosite) and a forsteritic olivine (Fo_0.72Fa_0.28, where Fo = forsterite and Fa = fayalite). This could suggest extensive aqueous activity under near-neutral pH conditions that would be conducive to habitable environments on early Mars. On this occasion, in addition to a X-ray spectrometer, a Mossbauer (MB) spectrometer and Miniature Thermal Emission Spectrometer (Mini-TES) greatly contributed to the findings. For more information, see the paper, "Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover", R. V. Morris et al., Science 329, 421 (2010).

Scientists led by Dr. N. Awaji (Fujitsu Laboratories, Japan) have recently reported successful large area imaging by Fourier transform holography in both soft and hard X-ray regions. The key was the separation of a holographic mask from the sample, enabling the sample to then be scanned to increase the observation area (~10μm or even more), though the beam size was fairly limited (~1μm) to maintain good coherence. They demonstrated some magnetic domain images of perpendicular magnetized film with soft X-rays (778 eV), and also some patterned samples and the cross-section of the Cu-interconnect-line with hard X-rays (5,500 eV). The spatial resolution for the above energies was 42 and 75 nm, respectively. The experiments were performed at SPring-8 (Harima, Japan). For more information, see the paper, "Large Area Imaging by Fourier Transform Holography Using Soft and Hard X-rays", N. Awaji et al., Appl. Phys. Express 3, 085201 (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).

X-ray fluorescence has provided new information on the technique known as "sfumato", which Da Vinci and other Renaissance painters used to produce delicate gradations in tones or colors across the canvas. Dr. P. Walter (Laboratoire du Centre de Recherche et de Restauration des Musees de France, CNRS, France) and his colleagues recently performed quantitative chemical analysis on seven paintings from the Louvre Museum (including the Mona Lisa), by synchrotron X-ray fluorescence at the European Synchrotron Radiation Facility (ESRF). They were able to clarify how the painter made shadows on faces by the use of layers of glaze or a very thin paint, and by means of the nature of the pigments or additives. For more information, see the paper, "Revealing the sfumato Technique of Leonardo da Vinci by X-Ray Fluorescence Spectroscopy", L. de Viguerie et al., Angewandte Chemie International Edition (Published Online: Jul 14 2010, DOI: 10.1002/anie.201001116).

Short period, high field undulators can enable short wavelength free electron lasers (FELs) at low beam energy. A research group led by Professor J. Rosenzweig (University of California, Los Angeles, USA) has recently unveiled a new design based on an approach that utilizes cryogenic materials. For more information, see the paper, "Short period, high field cryogenic undulator for extreme performance x-ray free electron lasers", F. H. O'Shea et al., Phys. Rev. ST Accel. Beams 13, 070702 (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).

A Brazilian research group recently discussed the thermal influence of soft X-ray free-electron-laser (FEL) pulses on silicon substrate. Such analysis is important, because the peak power of a single FEL pulse is roughly four orders of magnitude higher than that in conventional synchrotron light facilities. Their detailed time-evolution analysis indicates that in a worst case scenario, the second pulse could be adversely affected by dynamic thermal distortion induced by the preceding pulse. For more information, see the paper, "Thermoelastic analysis of a silicon surface under x-ray free-electron-laser irradiation", A. R. B. de Castro et al., Rev. Sci. Instrum. 81, 073102 (2010).

What happens when an atom is excited by extremely strong X-ray photons such as an X-ray laser? A Stanford research group recently published a very exciting report on the ionization of neon (Z=10) by X-ray laser at the Linac Coherent Light Source (LCLS) housed at the SLAC National Accelerator Laboratory in California, USA. The laser used in this experiment is extremely powerful (10¹⁸ W/cm², 10⁵ X-ray photons/Ų), and the research group scanned the X-ray photon energy from 800 eV to 2,000 eV, as well as the pulse width from 80 fs to 230 fs. As the K absorption edge of neon is around 867 eV, below this energy, X-rays can strip some of the eight weakly bound electrons from the outer L shell of the neon atom. Such a process of peeling electrons from atoms would come as no surprise for readers of X-ray spectrometry. Above the absorption edge, K shell electrons are preferentially ejected, creating 1s vacancies that are refilled by electrons from the L shell. Before the relaxation occurs, the remaining K shell electron is even more tightly bound to the neon nucleus than in the ground state. Therefore, the K absorption edge for the system with a 1s vacancy is higher than usual. When the research team raised the X-ray photon energy to 993 eV, both electrons from the inner K shell were knocked out, ionizing the atom from the inside out - in other words, coring the atom. With this "hollow" neon then, a completely empty K shell has been created for the first time by X-ray photons, though similar phenomena may be possible by means of ultra-high temperature plasma, extremely high-energy collision processes etc. For more information, see the paper, "Femtosecond electronic response of atoms to ultra-intense X-rays", L. Young et al., Nature 466, 56 (2010). In the same issue, there is an interesting account by Justin Wark, "X-ray laser peels and cores atoms", Nature 466, 35 (2010).