July 2012 Archives

The 15th international conference on X-ray absorption fine structure was recently held in Beijing, China, from July 22 to 28, 2012. In addition to many applications of the XAFS technique in a variety of scientific fields, reports and discussions were held on progress in theory and software, as well as some advanced experiments such as time-resolved XAFS. The next conference will take place at Karlsruhe, Germany in summer 2015. For further information, visit the web page, http://www.ixasportal.net/ixas/index.php?option=com_content&view=article&id=90&Itemid=134

Extremely strong pulses from X-ray free electron laser (XFEL) can change the material structure. Recently, scientists at LCLS (Linac Coherent Light Source), Stanford, USA, have reported the amorphous to crystalline phase transition of carbon by femtosecond 830 eV XFEL beam. The research group employed atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy to discuss the change of the sp²/sp³ ratio (graphitization), as well as the change of local order of the irradiated sample area. It was found that the phase transition threshold fluence is 282 ± 11 mJ/cm², and also the transition is mainly due to thermal activation rather than a non-thermal mechanism such as ionization etc. For more information, see the paper, "Amorphous to crystalline phase transition in carbon induced by intense femtosecond x-ray free-electron laser pulses", J. Gaudin et al., Phys. Rev. B86, 024103 (2012).

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).

Several electron-microscopist groups have recently reported that a Si drift detector with a 60~100 mm² effective area can be used to detect characteristic X-rays from a single atom in nanomaterials such as silicon and platinum in monolayer and multilayer grapheme, as well as erbium in a C₈₂ fullerene cage supported in a single-walled carbon nanotube. They employed a tiny electron beam of 0.1 nm in the aberration-corrected scanning transmission electron microscope. As will be clear for readers of X-ray Spectroscopy journal, the discussion is a kind of major and/or minor component analysis of extremely small volume rather than so-called ultra trace element analysis. The signal intensity was apparently very weak, but was in the order of some counts/sec according to the reports. Such high sensitivity points to the significant potential of the energy dispersive detector system. On the other hand, further detailed analysis including the estimation of parasitic background will be necessary. For more information, see the papers, "Single atom identification by energy dispersive x-ray spectroscopy", T. C. Lovejoy et al., Appl. Phys. Lett., 100, 154101 (2012), and "Detection of photons emitted from single erbium atoms in energy-dispersive X-ray spectroscopy", K. Suenaga et al., Nature Photonics, advanced online publication doi:10.1038/nphoton.2012.148.