Z. Y. Wu

Computed tomography algorithm based on diffraction-enhanced imaging setup

Recently taking advantage of the novel diffraction-enhanced imaging (DEI) method, one very effective and practical phase contrast imaging technique—a new x-ray computed tomography scheme based on DEI (DEI-CT) showed promising results, really superior to those of conventional CT imaging. In this letter, we show that the application of the conventional CT reconstruction algorithm is not always satisfactory for the new DEI-CT imaging and a new mathematical framework for imaging reconstruction is presented. Experimental data collected at the Beijing synchrotron radiation facility are also discussed using the new algorithm.

Reverse saturable absorption of platinum ter/bipyridyl polyphenylacetylide complexes

Transition metal poly-ynes are very interesting nonlinear optical materials for optical limiting applications because of their low linear absorption at low intensities, but high nonlinear absorption at high intensities in the entire visible spectral range. The reverse saturable absorption (RSA) of seven platinum ter/bipyridyl polyphenylacetylide complexes has been investigated at 532 nm with nanosecond pulses. In an f/214 geometry, with sample linear transmission of 90% in a 2 mm cell, the reverse saturable absorption thresholds for these complexes are 8–30 mJ/cm2. The maximum output fluence of these complexes are limited to 0.45–1.16 J/cm2 for incident fluence as high as 3.6 J/cm2. The RSA is strongly influenced by the nature of the triaryl coordination ligand and the conjugation length of the polyphenylacetylide ligand in these complexes. This RSA is mainly attributed to the triplet excited state absorption, with a ratio of effective excited state to ground state absorption cross sections as high as ∼20...

Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy

To understand and improve the applications of titanium-oxide nanoparticles, it is extremely important to perform a detailed investigation of the surface and the interior structural properties of nanocrystalline materials, such as rutile and anatase with diameter of few nanometers. Here, x-ray absorption spectroscopy has been used to identify the local Ti environment and characterize the related electronic structure. We combine experimental results at the Ti K edge in both bulk and nanocrystal samples to determine the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the x-ray absorption near-edge structure spectra. The correlation between peak intensities and surface-to-volume ratio of nanoparticles is also discussed.

A new iterative algorithm to reconstruct the refractive index

The latest developments in x-ray imaging are associated with techniques based on the phase contrast. However, the image reconstruction procedures demand significant improvements of the traditional methods, and/or new algorithms have to be introduced to take advantage of the high contrast and sensitivity of the new experimental techniques. In this letter, an improved iterative reconstruction algorithm based on the maximum likelihood expectation maximization technique is presented and discussed in order to reconstruct the distribution of the refractive index from data collected by an analyzer-based imaging setup. The technique considered probes the partial derivative of the refractive index with respect to an axis lying in the meridional plane and perpendicular to the propagation direction. Computer simulations confirm the reliability of the proposed algorithm. In addition, the comparison between an analytical reconstruction algorithm and the iterative method has been also discussed together with the convergent characteristic of this latter algorithm. Finally, we will show how the proposed algorithm may be applied to reconstruct the distribution of the refractive index of an epoxy cylinder containing small air bubbles of about 300 micro of diameter.

Phase retrieval using polychromatic illumination for transmission X-ray microscopy.

An alternative method for quantitative phase retrieval in a transmission X-ray microscope system at sub-50-nm resolution is presented. As an alternative to moving the sample in the beam direction in order to analyze the propagation-introduced phase effect, we have illuminated the TXM using X-rays of different energy without any motor movement in the TXM system. Both theoretical analysis and experimental studies have confirmed the feasibility and the advantage of our method, because energy tuning can be performed with very high energy resolution using a double crystal monochromator at a synchrotron beam line, and there is zero motor error in TXM system in our approach. High-spatial-resolution phase retrieval is accomplished using the proposed method.

Electrochemical hydrogen storage of Ti–V-based body-centered-cubic phase alloy surface-modified with AB5 nanoparticles

A composite of Ti–V-based bcc phase alloy surface-modified with AB5 nanoparticles was prepared by ball milling. The composite showed significantly improved electrochemical hydrogen release capacities. For example, the 30 min ball milled Ti–30V–15Mn–15Cr+10wt%AB5 showed a discharge capacity in the first cycle, at 353 K, of 886mAhg−1, corresponding to 3.38 wt % of hydrogen, with a 45mAg−1 discharge current. It is thought that this high capacity is due to the enhanced electrochemical-catalytic activity from the alloy surface covered with AB5 nanoparticles, which not only have better charge-discharge capacity themselves, acting as both an electrocatalyst and a microcurrent collector, but also result in the greatly enhanced hydrogen atomic diffusivities in the nanocrystalline relative to their conventional coarse-grained counterparts. These results provide new insight for use of Ti–V-based bcc phase alloy for high-energy batteries.

Surface structure of α-Fe2O3 nanocrystal observed by O K-edge X-ray absorption spectroscopy

X-ray absorption near edge structure (XANES) spectra is used as a probe of surface structure of alpha-Fe2O3 nanocrystal, prepared by sol-gel method. We present O K-edge XANES of alpha-Fe2O3 in nanocrystal and bulk by total electron yield at the photoemission station of Beijing Synchrotron Radiation Facility. The spectrum of alpha-Fe2O3 shows a splitting of the pre-edge structure, which is interpreted as two subsets of Fe 3d t(2g) and e(g) orbitals in oxygen octahedral (O-h) crystal field, and is also sensitive to long-range order effects. However, no distinguishable splitting of the pre-edge peak of nanocrystal alpha-Fe2O3 is observed. This suggests that there exists the distorted octahedral coordination around Fe sites and also the long-range disorder due to the surface as compared with bulk alpha-Fe2O3

Multiple-scattering approach with complex potential in the interpretation of electron and photon spectroscopies

We present a unitary cluster approach to the calculation of several electron and photon spectroscopies, ranging from core and valence level photoelectron diffraction and absorption to electron, Auger and anomalous diffraction. Electron energy loss and Auger–photoelectron coincidence spectroscopies can also be treated in the same frame. This approach is based on multiple-scattering theory with a complex optical potential of the Hedin–Lundqvist type and is valid for all electron kinetic energies. Similarities and differences between these diffraction techniques are examined and cluster size convergence is discussed in connection with the electron mean free path. Applications to selected problems ar ep resented to illustrate the method, both for structural and electronic analysis.

XANES quantitative structural determination of the sandwich bis(naphthalocyaninato) cerium complex

The molecular structure of the sandwich double-decker bis(naphthalocyaninato) cerium complex was determined for the first time in a quantitative way using a new method of analysis of the Ce L3-edge X-ray absorption near-edge structure (XANES) in the framework of the full multiple scattering theory. An average Ce-N bond length of 2.47 A was determined. In this complex structure the pyrrole rings play the key role in the determination of the XANES spectral features, and the differences in bond lengths between the Ce atom and the eight pyrrole N atoms are 0.14 A, addressing a significant distortion of these rings around cerium. These results may be used to study double-decker imidazole rings, present as the structure model in the photosynthesis center and in sandwich-structured lanthanide chlorophyll molecules.

Identification of metal-cage coupling in a single metallofullerene by inelastic electron tunneling spectroscopy

We report hybrid density functional theory calculations for inelastic electron tunneling spectroscopy (IETS) of a single metallofullerene Gd@C82. It is found that the metal atom inside the carbon cage can have significant impact on the IETS spectral profiles of the system, by modulating both the vibration and electron density. It is demonstrated that the IETS signals are very sensitive to the changes in the metal position and charge states, so that provide a unique tool for identifying the metal-cage coupling in metallofullerenes.