High-angle Diffraction Patterns Research Articles

Low-voltage coherent electron microscopy based on a highly coherent electron source built from a nanoemitter

It has been a general trend to develop low-voltage electron microscopes due to their high imaging contrast of samples and low radiation damage. Atomic-lattice-resolved transmission electron microscopes with voltages as low as 15–40 kV have been demonstrated. However, achieving an atomic resolution at voltages lower than 10 kV is extremely difficult. An alternative approach is a coherent imaging or phase retrieval imaging, which requires a sufficiently coherent source, an adequately small illumination area on the sample, the detection of high-angle diffraction patterns with a sufficient signal-to-noise ratio, and an appropriate theoretical reconstruction algorithm. This study proposes several transmission-type schemes to achieve coherent imaging of thin materials (less than 5 nm thick) with atomic resolution at voltages lower than 10 kV. Experimental schemes of both lens-less and lens-containing designs and preliminary results based on a highly coherent single-atom electron source are presented. The image plate is designed to be retractable to record the transmission patterns at different positions along the beam propagation direction. In addition, the authors proposed reflection-type coherent electron imaging schemes as novel methods for characterizing surface atomic and electronic structures of materials. The ultimate goal is to achieve high-contrast and high-spatial-resolution imaging of thin materials, such as two-dimensional materials, or molecules, such as organic or biological molecules, under low-dose conditions.

Comparing results of X-ray diffraction, \xb5-Raman spectroscopy and neutron diffraction when identifying chemical phases in seized nuclear material, during a comparative nuclear forensics exercise

This work presents the results for identification of chemical phases obtained by several laboratories as a part of an international nuclear forensic round-robin exercise. In this work powder X-ray diffraction (p-XRD) is regarded as the reference technique. Neutron diffraction produced a superior high-angle diffraction pattern relative to p-XRD. Requiring only small amounts of sample, µ-Raman spectroscopy was used for the first time in this context as a potentially complementary technique to p-XRD. The chemical phases were identified as pure UO2 in two materials, and as a mixture of UO2, U3O8 and an intermediate species U3O7 in the third material.

Evaluation of TiTe2 as a Diffusion Barrier in the Synthesis of (Bi2Te3)5(SnTe)5 Misfit Layer Compound

ABSTRACTThe evolution of modulated reactants designed to form (Bi2Te3)5(TiTe2)5(SnTe)5(TiTe2)5 and (Bi2Te3)5(SnTe)5 are compared and contrasted. The modulated reactant designed to form (Bi2Te3)5(SnTe)5 interdiffused at 220°C forming SnBi2Te4 rather than the desired superlattice structure. The second sample was designed to have TiTe2 as a potential diffusion barrier to prevent the formation of SnBi2Te4. This second sample remained layered after annealing at 220°C. SnTe crystals are observed in the high angle diffraction pattern after this annealing, but there is evidence for the beginning of SnBi2Te4 formation. Annealing this sample at 300°C results in the formation of SnBi2Te4. The interdiffusion of Sn and Bi appears to occur before the formation of the desired TiTe2 structure.

Synchrotron X-ray studies suggest that the core of the transthyretin amyloid fibril is a continuous β-sheet helix

Background Amyloid diseases, which include Alzhemier’s disease and the transmissible spongiform encephalopathies, are characterized by the extracellular deposition of abnormal protein fibrils derived from soluble precursor proteins. Although different precursors seem to generate similar fibrils, no adequate molecular structure of amyloid fibrils has been produced using modern techniques. Knowledge of the fibril structure is essential to understanding the molecular mechanism of amyloid formation and could lead to the development of agents to inhibit or reverse the process. Results The structure of amyloid fibrils from patients with familial amyloidotic polyneuropathy (FAP), which are derived from transthyretin (TTR) variants, has been investigated by fibre diffraction methods using synchrotron radiation. For the first time a significant high-angle diffraction pattern has been observed showing meridional reflections out to 2 å resolution. This pattern was fully consistent with the previously reported cross- β structure for the fibril, but also reveals a new large scale fibre repeat of 115 å. We interpret this pattern as that of a repeating unit of 24 β strands, which form a complete helical turn of β sheet about an axis parallel to the fibre axis. This structure has not been observed previously. We have built a model of the protofilament of the FAP amyloid fibril based on this interpretation, composed of four β sheets related by a single helix axis coincident with the fibre axis, and shown that it is consistent with the observed X-ray data. Conclusions This work suggests that amyloid fibrils have a novel molecular structure consisting of β sheets extended in regular helical twists along the length of the fibre. This implies that the polypeptide chains in the fibres are hydrogen-bonded together along the entire length of the fibres, thereby accounting for their great stability. The proposed structure of the FAP fibril requires a TTR building block that is structurally different from the native tetramer. This is likely to be either a monomer or dimer with reorganized or truncated β sheets, suggesting that amyloid formation may require significant structural change in precursor proteins.

Measurement of the lattice parameters in the individual layers of single-crystal superlattices

A method for determination of the lattice parameters, parallel to the sample surface as well as normal to the sample surface, in the individual layers of single-crystalline superlattices is derived. The method is based on simulations of low angle reflectivity measurements in combination with x-ray diffraction reciprocal space mapping. The number of unknown simulation parameters is reduced from three to one, namely the layer thickness of one of the layers which constitutes the bilayer period, if compared to techniques based only on simulations of the high angle diffraction pattern. The technique is demonstrated by characterizing a single-crystalline Mo/V(001) superlattice grown by dual-target magnetron sputtering onto MgO(001) substrates. The lattice parameters of the tetragonally distorted layers were aMo=0.309 nm, cMo=0.319 nm, aV=0.305 nm, and cV=0.298 nm which correspond to a misfit dislocation density of ∼0.056 dislocations per nm at each interface and a relaxation of ∼1/3 of the coherency strain. The lattice parameters obtained by the method were confirmed by comparing a kinematical calculation of the high angle Mo/V(002) θ–2θ diffraction pattern (using the obtained values as input parameters) with the experimentally determined pattern.

Packing of α-Helical Coiled-Coil Myosin Rods in Vertebrate Muscle Thick Filaments

Muscle myosin filament backbones are aggregates of long coiled-coil α-helical myosin rods, with the myosin heads arranged approximately helically on the filament surface, but the details of the rod packing are not known. Computed Fourier transforms of plausible molecular packing models for the vertebrate striated muscle myosin filament have been compared with observed high-angle X-ray diffraction patterns from plaice fin muscle. Models considered include those in which the coiled-coil rod parts of myosin are packed into various kinds of subfilaments or into a curved molecular crystalline layer. A general conclusion is that if the myosin rods are tilted by less than about 1° or more than about 3° from the filament long axis, very poor agreement is obtained between the computed and observed high-angle diffraction patterns. Qualitative comparison of calculated Fourier transforms, taken together with electron micrograph information, shows that the curved molecular crystal model and a model with hexagonally close-packed 4-nm subfilaments appear to explain the whole set of observations more satisfactorily than the alternatives. It is argued on other grounds that of these two possibilities the curved molecular crystal model is the more plausible.

Ｃｏ／Ａｕ，Ｆｅ／Ａｕ組成変調多層膜のカー効果と磁気異方性

Layered structure and magnetic and magneto-optical properties of Co/Au and Fe/Au compositionally modulated multilayered films (CMF's) prepared by RF sputtering method were investigated. Modulation length (D) varied from about 10 A to more than 100 A. Many satellite peaks were observed in high angle diffraction patterns of the both multilayered films. It was found that the values of magneto-optical Kerr rotation (θK) are low and the spectra is simple if the case of relatively small D, however, a new peak of θK appears at around λ = 500 nm, corresponding to the absorption edge of Au metal, with the increase of D. Moreover, it was also found that perpendicular anisotropy (Ku) is induced gradually with decreasing D in both CMF's. Perticularly, Ku of Co/Au CMF's becomes positive in the range of relatively large D.