Finite-element simulation for X-ray volume diffractive optics based on the wave optical theory.

We developed a novel numerical simulation method for volume diffractive optics based on the Takagi-Taupin (TT) dynamical theory of diffraction. A general integral system of equations with a powerful and convenient distortion function was developed for finite-element analysis (FEA). The proposed framework is promising with regard to flexibility, robustness, and stability and has potential for solving dynamical X-ray diffraction problems related to diffractive optical elements of arbitrary shape and deformation. This FEA method was used for evaluating laterally graded multilayer (LGML) mirrors, and a general coordinate system was introduced to make the geometric optimization simple and effective. Moreover, the easily implemented boundary conditions inherent in FEA, combined with the analysis of the energy resolution derived from the TT theory, can make the simulation of volume diffractive optics, including LGML mirrors, more accurate. Thus, a comprehensive and highly efficient computation of LGML mirror diffraction problems was performed. The evaluation of the effects of the figure errors can provide practical guidance for the fabrication of X-ray optical elements.

Fabrication and ferromagnetism of Si-SiGe/MnGe core-shell nanopillars.

Si-Si0.5Ge0.5/Mn0.08Ge0.92 core-shell nanopillar samples were fabricated on ordered Si nanopillar patterned substrates by molecular beam epitaxy at low temperatures. The magnetic properties of the samples are found to depend heavily on the growth temperature of the MnGe layer. The sample grown at a moderate temperature of 300 °C has the highest Curie temperature of 240 K as well as the strongest ferromagnetic signals. On the basis of the microstructural results, the ferromagnetic properties of the samples are believed to come from the intrinsic Mn-doped amorphous or crystalline Ge ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. After being annealed at a temperature of 500 °C, all the samples exhibit the same Curie temperature of 220 K, which is in sharp contrast to the different Curie temperature for the as-grown samples, and the ferromagnetism for the annealed samples comes from Mn5GeSi2 compounds which are formed during the annealing.

Improvement of depth resolution in depth-resolved wavenumber-scanning interferometry using wavenumber-domain least-squares algorithm: comparison and experiment.

It is important to improve the depth resolution in depth-resolved wavenumber-scanning interferometry (DRWSI) owing to the limited range of wavenumber scanning. In this work, a new nonlinear iterative least-squares algorithm called the wavenumber-domain least-squares algorithm (WLSA) is proposed for evaluating the phase of DRWSI. The simulated and experimental results of the Fourier transform (FT), complex-number least-squares algorithm (CNLSA), eigenvalue-decomposition and least-squares algorithm (EDLSA), and WLSA were compared and analyzed. According to the results, the WLSA is less dependent on the initial values, and the depth resolution δz is approximately changed from δz to δz/6. Thus, the WLSA exhibits a better performance than the FT, CNLSA, and EDLSA.

Nuclear resonant inelastic X-ray scattering at high pressure and low temperature.

A new synchrotron radiation experimental capability of coupling nuclear resonant inelastic X-ray scattering with the cryogenically cooled high-pressure diamond anvil cell technique is presented. The new technique permits measurements of phonon density of states at low temperature and high pressure simultaneously, and can be applied to studies of phonon contribution to pressure- and temperature-induced magnetic, superconducting and metal-insulator transitions in resonant isotope-bearing materials. In this report, a pnictide sample, EuFe2As2, is used as an example to demonstrate this new capability at beamline 3-ID of the Advanced Photon Source, Argonne National Laboratory. A detailed description of the technical development is given. The Fe-specific phonon density of states and magnetism from the Fe sublattice in Eu(57)Fe2As2 at high pressure and low temperature were derived by using this new capability.

1,8-Diiodooctane as the processing additive to improve the efficiency of P3HT:PC61BM solar cells.

Controlling the blend morphology is critical for achieving high power conversion efficiency in polymer/fullerene bulk heterojunction (BHJ) photovoltaic devices. As a simple and effective method to control morphology, adding processing additives has been widely applied in the organic BHJ solar cells. In this paper, we demonstrate that adding 1,8-diiodooctane as a processing additives is an effective method to improve the morphology and the efficiency of bulk heterojunctions (BHJ) solar cells based on the regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PC61BM). We investigated the unique way in which the 1,8-diiodooctane plays the rule to enhance the performance of solar cells according to different morphology and crystallinity of active layers prepared with and without the additive. The morphology is studied with atomic force microscopy (AFM) and Grazing Incidence X-ray Diffraction (GIXRD). We also find a balance between a large interfacial area for exciton dissociation and continuous pathways for carrier transportation when the additive is used.

Enhanced performance of inverted organic photovoltaic cells using CNTs-TiO(X) nanocomposites as electron injection layer.

In this study, we fabricated inverted organic photovoltaic cells with the structure ITO/carbon nanotubes (CNTs)-TiO(X)/P3HT:PCBM/MoO3/Al by spin casting CNTs-TiO(X) nanocomposite (CNTs-TiO(X)) as the electron injection layer onto ITO/glass substrates. The power conversion efficiency (PCE) of the 0.1 wt% single-walled nanotubes (SWNTs)-TiO(X) nanocomposite device was almost doubled compared with the TiO(X) device, but with increasing concentration of the incorporated SWNTs in the TiO(X) film, the performance of the devices appeared to decrease rapidly. Devices with multi-walled NTs in the TiO(X) film have a similar trend. This phenomenon mainly depends on the inherent physical and chemical characteristics of CNTs such as their high surface area, their electron-accepting properties and their excellent carrier mobility. However, with increasing concentration of CNTs, CNTs-TiO(X) current leakage pathways emerged and also a recombination of charges at the interfaces. In addition, there was a significant discovery. The incorporated CNTs were highly conducive to enhancing the degree of crystallinity and the ordered arrangement of the P3HT in the active layers, due to the intermolecular π-π stacking interactions between CNTs and P3HT.

A von Hamos full-cylindrical spectrometer based on striped Si/Ge crystal for advanced x-ray spectroscopy.

High-energy resolution core-level spectroscopies, including a group of different techniques to obtain element-specific information of the electronic structure around an absorption site, have become powerful tools for studying the chemical state, local geometric structure, and the nature of chemical bonding. High-resolution x-ray absorption and x-ray emission spectroscopies are well-established experimental techniques but have always been limited by the number of emitted photons and the limited acceptance of solid angles, as well as requiring high energy stability and repeatability for the whole experimental setup. A full-cylindrical x-ray spectrometer based on flexible HAPG (highly annealed pyrolitic graphite) mosaic crystals is an effective solution for the above issues. However, large-area HAPG remains expensive and is often not easy to access. Here, we present an alternative approach by using segmented single crystals (Si and Ge) with different orientations instead of the HAPG as a dispersive element. The proposed method drastically improved the energy resolution up to 0.2-2 eV in the range of 2-10 keV. High-pressure x-ray emission and resonant x-ray emission spectra are presented to demonstrate the capabilities of the instrument. The new design is particularly suitable for high-resolution spectroscopy applications at fourth-generation synchrotron radiation sources or free-electron lasers.

GexSi1-x virtual-layer enhanced ferromagnetism in self-assembled Mn0.06Ge0.94 quantum dots grown on Si wafers by molecular beam epitaxy.

Self-assembled Mn0.06Ge0.94 quantum dots (QDs) on a Si substrate or GexSi1-x virtual substrate (VS) were grown by molecular beam epitaxy. The GexSi1-x VS of different thicknesses and Ge compositions x were utilized to modulate the ferromagnetic properties of the above QDs. The MnGe QDs on GexSi1-x VS show a significantly enhanced ferromagnetism with a Curie temperature above 220 K. On the basis of the microstructural and magnetization results, the ferromagnetic properties of the QDs on GexSi1-x VS are believed to come from the intrinsic MnGe ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. At the same time, we found that by increasing the Ge composition x of GexSi1-x VS, the ferromagnetism of QDs grown on VS will markedly increase due to the improvements of hole concentration and Ge composition inside the QDs. These results are fundamentally important in the understanding and especially in the realization of high Curie temperature MnGe diluted magnetic semiconductors.

[Research on AlInGaN quaternary alloys as MQW barriers in GaN-based laser diodes].

InGaN/GaN, InGaN/InGaN and InGaN/AlInGaN multi-quantum-well (MQW) laser diodes (LDs) were grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The GaN (0002) synchrotron X-ray diffraction (XRD), electroluminescence (EL) and optical power-current (L-I) measurement reveal that AlInGaN quaternary alloys as barriers in MQWs can improve the crystal quality, optical emission performance, threshold current and slope efficiency of the laser diode structure to a large extent compared with other barriers. The relevant mechanisms are that: 1. The Al component increases the barrier height of the MQWs so that more current carriers will be caught in. 2. The In component counteracts the strain in the MQWs that decreases the dislocations and defects, thereby the nonradiative recombination centers are decreased. 3. The In component decreases the piezoelectric electric field that makes the electrons and the holes recombine more easily.

Local insight into the La-induced structural phase transition in multiferroic BiFeO3 ceramics by x-ray absorption fine structure spectroscopy.

Substitution of bismuth by rare earth (RE) ions is of great technological importance to develop room-temperature BiFeO3-based multiferroic materials. Despite this interest, many fundamental properties and the structure-property correlations of RE-doped BiFeO3 remain poorly understood. Here we report a systematical experimental and theoretical exploration on the structural phase transition in Bi1-x La x FeO3 (0 ⩽ x ⩽ 0.2) ceramics. By using x-ray absorption fine structure spectroscopy, we for the first time show that the La3+ dopants in fact substitute the Bi site of secondary nanosized particles with orthorhombic Pbam symmetry instead of the long-believed parental rhombohedral R3c phase at all La3+ doping concentrations (0.001 ⩽ x ⩽ 0.2). This homogeneously mixed two-phase compound cannot be detected by the x-ray diffraction until La content approaching x = 0.1. The finding is further supported by complementary studies of transmission electron microscopy and thermodynamic preference, and it casts serious challenges on the prevailing assumption of La3+ substitution on the Bi3+ site in R3c structure when x ⩽ 0.1 as well as the previously proposed origin of enhanced functional properties based on morphotropic phase boundary. This new insight may ignite a revival on exploring the underlying multiferroic mechanisms in BiFeO3-based materials and facilitate the bottom-up design of novel multifunctional devices.

Pressure-induced phase transitions and structural evolution across the insulator-metal transition in bulk and nanoscale BiFeO3.

The pressure-induced phase-transition sequences and structural evolution across the insulator-metal transition (IMT) in multiferroic BiFeO3 still remain unclear. Here we use a combination of high-pressure XRD, XAFS experiment and first principle calculation to investigate the pressure-derived structural transformations and structure-related properties in bulk and nanoscale BiFeO3 up to 55 GPa. A new Imma structure of BiFeO3 has been discovered in the pressure range of 48-52 GPa, which presents ferromagnetic (FM) metallic properties and therefore plays a key role in the IMT. Local structure study reveals that the Bi3+ cation gradually shifts toward the centrosymmetric position in BiO12 cluster during IMT. Besides, the detailed structural information of post-perovskite Cmcm phase has also been determined and thus the complete phase sequence up to 60 GPa is obtained. Our research provides a structural origin of the IMT and a new way to understand the FM release in BiFeO3 system.

Revisiting local structural changes in GeO2 glass at high pressure.

Despite the great importance in fundamental and industrial fields, understanding structural changes for pressure-induced polyamorphism in network-forming glasses remains a formidable challenge. Here, we revisited the local structural transformations in GeO2 glass up to 54 GPa using x-ray absorption fine structure (XAFS) spectroscopy via a combination diamond anvil cell and polycapillary half-lens. Three polyamorphic transitions can be clearly identified by XAFS structure refinement. First, a progressive increase of the nearest Ge-O distance and bond disorder to a maximum at ~5-16 GPa, in the same pressure region of previously observed tetrahedral-octahedral transformation. Second, a marked decrease of the nearest Ge-O distance at ~16-22.6 GPa but a slight increase at ~22.6-32.7 GPa, with a concomitant decrease of bond disorder. This stage can be related to a second-order-like transition from less dense to dense octahedral glass. Third, another decrease in the nearest Ge-O distance at ~32.7-41.4 GPa but a slight increase up to 54 GPa, synchronized with a gradual increase of bond disorder. This stage provides strong evidence for ultrahigh-pressure polyamorphism with coordination number >6. Furthermore, cooperative modification is observed in more distant shells. Those results provide a unified local structural picture for elucidating the polyamorphic transitions and densification process in GeO2 glass.

Focusing synchrotron radiation using a polycapillary half-focusing X-ray lens for imaging.

An imaging system based on a polycapillary half-focusing X-ray lens (PHFXRL) and synchrotron radiation source has been designed. The focal spot size and the gain in power density of the PHFXRL were 22 microm (FWHM) and 4648, respectively, at 14.0 keV. The spatial resolution of this new imaging system was better than 5 microm when an X-ray charge coupled device with a pixel size of 10.9 x 10.9 microm was used. A fossil of an ancient biological specimen was imaged using this system.

Temperature dependence of interfacial fluctuations of polymerized fatty acid salt multilayers.

X-ray scattering was used to study the temperature dependence of the profile structure of polymerized 10,12-tricosadiynoic acid salt multilayers. The stacking periodicity of the multilayers was found to decrease with increasing temperature due to the conformational changes of the alkyl chains. When the samples were fully hydrated in water, the reflectivity measurement showed that the thermal fluctuations of the interfaces are enhanced with temperature, resulting in reduced ordering. Meanwhile, the diffuse scattering indicated that the thermal fluctuations renormalize the elasticity of the multilayers; both the bending and the compression moduli are reduced. Similar measurements performed in air, however, do not show this thermal enhancement although the stacking periodicity decreases in the same manner. It is implied that water might weaken the interaction between the carboxyl groups and the metal ions so that the polymerized bilayers are softened in water.