Asymmetric Diffraction Peak Research Articles

Study of the structure of fibrous carbon materials using X-ray diffractometry

New possibilities for diagnosing the structure of carbon materials by X-ray diffraction analysis are described. The studies were performed on an X-ray diffractometer D8 ADVANCE (filtered Cu K α radiation) using carbon fibers (CF) differed in the mode of heat treatment (~2300 to ~3000°C). It is shown that when studying the structure of fibrous carbon materials characterized by a large depth of X-rays penetration, it is necessary to limit the thickness of the object under study to ~0.1 mm to provide the self-focusing condition for the entire sample volume engaged in formation of the diffracted beam. The material of high modulus carbon fibers can be heterogeneous and contain coherent scattering regions wherein the crystal structure parameters are somewhat different and experimentally observed diffraction peaks 002, 004, and 006 become asymmetric and are not described by the Gauss, Lorentz, or Voigt functions. Thus, the standard method used for determination of the average value of the interplanar spacing and the size of coherent scattering regions does provide reliable information on the real structure of the material. The profile of the asymmetric diffraction peak 002 was analyzed using the Origin program thus providing information about fine (heterogeneous) structure of the carbon fiber material which largely determines the physicomechanical properties. The data on 002 diffraction peak profiles of carbon fibers before and after removal of the Cu K α 2 component are presented. The data on decomposition of the experimentally observed asymmetric 002 diffraction peaks into symmetric peaks described by the Gaussian, Lorentz, and Voigt functions are compared. The most preferable is the decomposition into the components described by the Gaussian or Voigt functions.

Ultrafast X-Ray Diffraction Studies of the Phase Transitions and Equation of State of Scandium Shock Compressed to 82 GPa

Using x-ray diffraction at the Linac Coherent Light Source x-ray free-electron laser, we have determined simultaneously and self-consistently the phase transitions and equation of state (EOS) of the lightest transition metal, scandium, under shock compression. On compression scandium undergoes a structural phase transition between 32 and 35GPa to the same bcc structure seen at high temperatures at ambient pressures, and then a further transition at 46GPa to the incommensurate host-guest polymorph found above 21GPa in static compression at room temperature. Shock melting of the host-guest phase is observed between 53 and 72GPa with the disappearance of Bragg scattering and the growth of a broad asymmetric diffraction peak from the high-density liquid.

The effect of specimen surface curvature on x-ray diffraction peak profiles

The effect of specimen surface curvature on profiles of asymmetric X-ray diffraction peaks obtained using the Bragg-Brentano geometry with a position sensitive detector (PSD) was studied. Asymmetric diffraction peaks were experimentally obtained from cylindrical surfaces with controlled curvature. Peaks were collected for a set of curvature radii, diffraction angles, and materials. A mathematical approach and a computer model for calculations of peak profiles were developed: a general method for computation of peak profiles is consideration of diffraction cones intersection with the PSD surface. Effects of axial and radial divergence, finite sample size, and local surface tilt were included in the model. Calculated peak profiles agree with reflections obtained experimentally at a wide range of curvature radii and diffraction angle values. Computation of important characteristics such as the peak centroid and change in position of the maximum and full-width-half-maximum is provided.

Reduction of the threading edge dislocation density in AlGaN epilayers by GaN nucleation for efficient 350 nm light emitting diodes

We report on the reduction of the threading edge dislocation density of Al 0.15 Ga 0.85 N buffer layers for efficient 350nm light-emitting diodes (LEDs). Structures were grown by metalorganic vapor phase epitaxy (MOVPE) on (0001) sapphire substrates using three-dimensional (3D) facetted GaN nucleation islands. The flattening of the overgrowing AlGaN buffer layers could be controlled by choosing appropriate growth conditions resulting in smooth surfaces. High-resolution X-ray diffraction (HRXRD) ω-scans show that a prolonged 3D growth phase leads to a narrowing of the asymmetric diffraction peaks and hence to an effective reduction of the density of edge-type threading dislocations. Photoluminescence (PL) and electroluminescence (EL) measurements show directly the beneficial effect of the improved crystal quality on the optical emission properties. The output power of LED structures grown on an optimized buffer was increased by a factor of 6 compared to structures grown on a two-dimensional (2D) low Al content AlGaN nucleation layer.

Investigation of characteristics of laterally overgrown GaN on striped sapphire substrates patterned by wet chemical etching

The new developed maskless lateral-epitaxial-overgrowth technique, in which the striped substrates are patterned by wet chemical etching, is systematically investigated using scanning electron microscopy, X-ray diffraction, and atomic force microscopy (AFM). Wing tilt is measured for the GaN films on patterned substrates with a range of “fill factor” (ratio of groove width to stripe period) and for the GaN in different growth time. It is found that changes in these parameters have a significant effect on the extent and distribution of wing tilt in the laterally overgrown regions relative to the GaN directly on the sapphire substrate. Increasing the thickness of GaN films is benefit to reduce wing tilt. The tilt is avoided in the GaN films with 4.5 μm thickness and fill factor for 0.46. The full-width at half-maximum of X-ray rocking curves of the asymmetric diffraction peaks and the image of AFM both show that the threading dislocations in the developed lateral epitaxial overgrowth of GaN films are reduced sharply. The GaN template could be used as an excellent substrate to fabricate high-performance optoelectronic devices.

High-resolution X-ray diffraction studies of molecular beam epitaxy-grown HgCdTe heterostructures and CdZnTe substrates

Lattice mismatch between substrates and epitaxial layers of different molefractions can create a variety of distortions and defects in Hg(1−x)Cd(x)Te epilayers, thus degrading the performance of infrared detectors fabricated from this material. X-ray diffraction is a sensitive nondestructive technique, which allows in-depth characterization of the crystal lattice prior to detector fabrication. We present results of triple-axis diffractometry performed on single- and double-layer HgCdTe films grown on (211)B CdZnTe substrates by molecular beam epitaxy (MBE). In this study, both the ω and 2θ diffraction angles have been recorded absolutely so that the diffraction peaks in the RSMs can be positioned directly in reciprocal space, without requiring reference to a substrate peak. The positions of both surface-symmetric and asymmetric diffraction peaks have been used to extract lattice spacings parallel and perpendicular to the (211) growth direction. The relaxed lattice parameter of each epilayer has been calculated assuming that the layers are elastically strained. The low symmetry of the (211) growth direction, coupled with the anisotropic elasticity of zinc-blende semiconductors, results in monoclinic distortion of the lattice, as observed in these samples. In double-layer samples, the mosaicity of both layers is greater than that observed in single epilayers. Layers subjected to a Hg-saturated anneal show greater lattice distortion than as-grown samples.

A new three-dimensional structural model for the CuCl 2 graphite intercalation compound

The in-plane structure of stage-1 to stage-3 CuCl 2 graphite intercalation compounds (GIC) was investigated using X-ray and electron diffraction. A new 3D structural model of these CuCl 2–GIC was defined based on a monoclinic, face-centred pseudo-orthorhombic unit cell and the X-ray and electron diffraction patterns were computed using this model. The modulations of the (00 l) intensities were well reproduced. The asymmetric shape of the CuCl 2–GIC (hk0) reflections may be explained by including additional (hk l) lines into the calculations. This is attributed to the preferred orientation of the domain islands in the GIC. The exact atom positions remain unknown, because defects present in the noncommensurate GIC structure result in broad and asymmetric diffraction peaks. The internal structure of the CuCl 2 layers in the GIC proved to be nearly the same as in the free compound.

An efficient method for calculating asymmetric diffraction peak profiles

An efficient method for evaluating asymmetric diffraction peak profile functions based on the convolution of the Lorentzian or Gaussian function with any asymmetric window function is proposed. When this method is applied to approximate the convolution with the Howard’s window function [J. Appl. Crystallogr. 15, 615 (1982)], only a few terms of numerical integration give satisfactory results, even if the asymmetry is very strong.

Formula for the asymmetric diffraction peak profiles based on double Soller slit geometry

The asymmetric diffraction peak profiles and peak shift of conventional powder diffractometry systems caused by the angular divergence along the vertical axis are reproduced by asymmetrized peak profile functions. The asymmetrization is achieved by convoluting the angular dispersions of both incident and scattered beams along the vertical axis. A general method for mapping vertical window functions to the horizontal direction is proposed, and the formulas of two types of horizontal window functions mapped from symmetric double vertical Bartlett (triangular) and Gaussian window functions are presented. Both formulas incorporate a single asymmetry parameter connected with the open width of the Soller slits along the vertical direction. When experimental diffraction peak profiles are fitted by asymmetrized pseudo-Voigt functions, the asymmetry parameter gives good coincidence with the Soller slit angle, which is clearly specified in a given diffractometer.

Magnetic Nanoparticles Produced in Spontaneous Cationic-Anionic Vesicles: Room Temperature Synthesis and Characterization

Unilamellar vesicles formed spontaneously by forming solutions of single-tailed cationic and anionic surfactants [cetyltrimethylammonium bromide (CTAB) and dodecylbenzenesulfonic acid (HDBS), respectively] have been used as reactors for the direct, room temperature synthesis of nanometer-sized magnetic particles within their internal cores. A micellar anionic surfactant solution was titrated slowly into a micellar cationic surfactant solution containing ferrous chloride, forming defect-free unilamellar vesicles that contained the reactant ferrous ions. CTAB/HDBS molar ratios of 13/7 and 6/4 and ferrous chloride concentrations of 0.1 and 0.05 M were used. Extravesicular ferrous ions were replaced with sodium ions by passing this suspension through a gel permeation chromatography column saturated with isotonic sodium chloride solution. Addition of sodium hydroxide to the extravesicular region caused hydroxyl ions to permeate into the vesicles and react with the available ferrous ions to form the product. Powder X-ray diffraction of the particulate sample showed the intense peaks of magnetite or γ-ferrite. Bragg peak broadening was characteristic of crystallites of roughly the same diameter as particles measured by transmission electron microscopy, indicating that each particle was monocrystalline. All of the intense peaks were distinctly asymmetric. Magnetization measurements showed that the particles were superparamagnetic. Using the Langevin function folded with the particle size distribution to model the magnetization of the sample in response to an applied magnetic field yielded "magnetic" diameter distributions that were consistently smaller than particle size distributions obtained from transmission electron microscopy. The magnetic size is consistent with the existence of a magnetically anomalous surface layer on each particle, as has been reported with other ferrites. The asymmetric diffraction peaks may be explained with a model that permits surface relaxation, and it is possible that the two phenomena are linked.