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Register a new userNanoparticle size distribution estimation by full-pattern powder diffraction analysis
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The increasing scientific and technological interest in nanoparticles has raised the need for fast, efficient and precise characterization techniques. Powder diffraction is a very efficient experimental method, as it is straightforward and non-destructive. However, its use for extracting information regarding very small particles brings some common crystallographic approximations to and beyond their limits of validity. Powder pattern diffraction calculation methods are critically discussed, with special focus on spherical particles with log-normal distribution, with the target of determining size distribution parameters. A 20-nm CeO$_{2}$ sample is analyzed as example.
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Diffraction experiments with holographic gratings recorded in SiO$_2$ nanoparticle-polymer composites have been carried out with slow neutrons. The influence of parameters such as nanoparticle concentration, grating thickness and grating spacing on the neutron-optical properties of such materials has been tested. Decay of the grating structure along the sample depth due to disturbance of the recording process becomes an issue at grating thicknesses of about 100 microns and larger. This limits the achievable diffraction efficiency for neutrons. As a solution to this problem, the Pendell{o}sung interference effect in holographic gratings has been exploited to reach a diffraction efficiency of 83% for very cold neutrons.
In the current extensive studies of transition metal dichalcogenides (TMDCs), compared to hexagonal layered materials, like graphene, hBN and MoS2, low symmetry layered 2D crystals showed great potential for applications in anisotropic devices. Rhenium diselenide (ReSe2) has the bulk space group P1 and belongs to triclinic crystal system with a deformed cadmium iodide type structure. Here we unambiguously determined monolayer and its vertical orientation of rhenium diselenide membrane with an individual electron diffraction pattern, which could be applicable to low symmetry crystal systems, including both triclinic and monoclinic lattices, as long as their third unit-cell basis vector is not perpendicular to basal plane. Atomically resolved image from probe corrected annular dark field scanning transmission electron microscope (ADF-STEM) was employed to validate layer number. Finally, experimental results were well explained by kinematical electron diffraction theory and corresponding simulations.
The structure of bundles of single-walled carbon nanotubes (SWNT) has been refined by Rietveld analysis using neutron and X-ray powder diffraction data. Based on previous simulation studies of powder diffraction data of SWNT and standard Rietveld analyses, we have developed a pattern fit technique for SWNT which provides precise structure parameters. We also show that the present technique can be used with the maximum entropy method (MEM), which is complementary to the Rietveld analysis. Using the neutron diffraction data of pristine SWNT, we have successfully reconstructed the density of carbon nuclei and zero density in the inner cavity of SWNT by MEM.
During past years, a number of reports have been published on synthesis of tetragonal allotrope of boron, t-B52 phase. However, no unambiguous characterization of the crystal structure has been performed to the present time, while remarkable variation of the a/c lattice-parameter ratio raises strong doubts about its uniqueness. Here the Rietveld refinement of the crystal structure of the high pressure - high temperature boron phase synthesized by a direct solid-state transformation of rhombohedral beta-B106 at 20 GPa and 2500 K has been reported for the first time. Although this boron allotrope belongs to the t-B52 type, its structure can be considered as pseudo-cubic with the a/c ratio of sqr(2).
PDFgetX3 is a new software application for converting X-ray powder diffraction data to atomic pair distribution function (PDF). PDFgetX3 has been designed for ease of use, speed and automated operation. The software can readily process hundreds of X-ray patterns within few seconds and is thus useful for high-throughput PDF studies, that measure numerous datasets as a function of time, temperature or other environment parameters. In comparison to the preceding programs, PDFgetX3 requires fewer inputs, less user experience and can be readily adopted by novice users. The live-plotting interactive feature allows to assess the effects of calculation parameters and select their optimum values. PDFgetX3 uses an ad-hoc data correction method, where the slowly-changing structure independent signal is filtered out to obtain coherent X-ray intensities that contain structure information. The outputs from PDFgetX3 have been verified by processing experimental PDFs from inorganic, organic and nanosized samples and comparing them to their counterparts from previous established software. In spite of different algorithm, the obtained PDFs were nearly identical and yielded highly similar results when used in structure refinement. PDFgetX3 is written in Python language and features well documented, reusable codebase. The software can be used either as standalone application or as a library of PDF-processing functions that can be called on from other Python scripts. The software is free for open academic research, but requires paid license for commercial use.
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