Characterization Of Structural Defects Research Articles

Characterization of defect structures in nanoscaled W-doped $${\text {TiO}_2}$$ tested as supercapacitor electrode materials

Characterization of defect structures in nanoscaled W-doped $${\text {TiO}_2}$$ tested as supercapacitor electrode materials

Impact of different structural defects on fundamental properties of blue phosphorene nanotubes

Using density functional theory, we present the effect of different structural defects on electronic and optical properties of blue phosphorene nanotubes of both armchair and zigzag chirality. In addition, we have considered the influence of an applied electric field on the electronic states of either pristine and defect-laden structures. The main defective features considered are double vacancies and Stone–Wales defects, although results with these imperfections are, as well, compared with those arising when single vacancies of two types are regarded. The possible transition from semiconducting to metal-like behavior induced by the applied field for large enough zigzag nanotubes is predicted. Deviations of the optical response of defective systems compared to the pristine case are mainly revealed for the visible range and above, with an evident quantitative anisotropy related to the specific polarization of the incident light: parallel or perpendicular to the nanotube growth direction. This characterization of structural defects and their effects on the optoelectronic properties of blue phosphorene nanotubes is required to define how the surface of the nanotubes could be utilized to develop new optoelectronic devices.

Characterization of Defect Structure in Epilayer Grown on On-Axis SiC by Synchrotron X-ray Topography

Characterization of Defect Structure in Epilayer Grown on On-Axis SiC by Synchrotron X-ray Topography

Characterization of Structural Defects in (Cd,Zn)Te Crystals Grown by the Travelling Heater Method

Structural defects and compositional uniformity remain the major problems affecting the performance of (Cd, Zn)Te (CZT) based detector devices. Understanding the mechanism of growth and defect formation is therefore fundamental to improving the crystal quality. In this frame, space experiments for the growth of CZT by the Travelling Heater Method (THM) under microgravity are scheduled. A detailed ground-based program was performed to determine experimental parameters and three CZT crystals were grown by the THM. The structural defects, compositional homogeneity and resistivity of these ground-based crystals were investigated. A ZnTe content variation was observed at the growth interface and a high degree of stress associated with extensive dislocation networks was induced, which propagated into the grown crystal region according to the birefringence and X-ray White Beam Topography (XWBT) results. By adjusting the growth parameters, the ZnTe variations and the resulting stress were efficiently reduced. In addition, it was revealed that large inclusions and grain boundaries can generate a high degree of stress, leading to the formation of dislocation slip bands and subgrain boundaries. The dominant defects, including grain boundaries, dislocation networks and cracks in the interior of crystals, led to the resistivity variation in the crystals. The bulk resistivity of the as-grown crystals ranged from 109 Ωcm to 1010 Ωcm.

Nondestructive Damage Testing of Beam Structure Based on Vibration Response Signal Analysis

Nondestructive damage-testing technology based on vibration signal analysis makes full use of the response characteristics of wave and energy. With the advantages of wide bandwidths of response frequency and high sensitivity, the nondestructive testing technology based on vibration signal analysis has a superiority in the application for the detection and characterization of structural defects, and has become one of the important methods for the nondestructive testing of structural material defects and damage. This paper presents a novel method of detection localization and quantitative analysis for local damage in beam structures, based on the response analysis of vibration signals. A damage-detection and -identification algorithm based on a unscented Kalman filter (UKF) was designed, which greatly reduces the computational workload in the process of damage identification over that in conventional methods. The method presented in this paper has significances to widen the application scope of the nondestructive testing method, and increase the recognition efficiency and effectiveness of this kind of method in engineering.

Recent advances in defect electrocatalysts: Preparation and characterization

This review outlines the progress achieved in the production of various defects, synthesis strategies and characterization techniques for defect electrocatalysts. This review briefly summarizes recent progress in defect electrocatalysts, and the synthesis strategies and characterization techniques for defects are systematically discussed. Although challenges in the characterization of defect structures in the electrocatalytic reaction process remain, the dynamic evolution of defect sites is predicted to be helpful for designing and preparing high-performance electrocatalysts for commercial applications. Furthermore, due to an insufficient understanding of the defect-structure-property relationships, future possibilities for the reasonable design of defect electrocatalysts to obtain desirable performance are suggested.

A model to predict image formation in the three-dimensional field ion microscope

Field ion microscopy (FIM) was the first technique to image individual atoms on the surface of a material. By a careful control of the field evaporation of surface atoms, the bulk of the material is exposed, and, through digital processing of a sequence of micrographs, an atomically-resolved three-dimensional reconstruction can be achieved. 3DFIM is particularly suited to the direct observation of crystalline defects that underpin the physical properties of materials: vacancies and vacancy clusters, interstitials, dislocations, or grain boundaries. Yet, further developments of 3DFIM are necessary to turn it into a routines technique. Here, we introduce first a protocol for 3DFIM image processing and subsequent tomographic reconstruction. Second, we propose a numerical model enabling simulation of the FIM imaging process. The model combines the meshless algorithm for field evaporation proposed by Rolland et al. (Robin–Rolland Model, or RRM) with fundamental aspects of the field ionization process of the gas image involved in FIM. The proposed model enables the simulation of imaging artefacts that are induced by non-regular field evaporation and by the disturbed electric field distribution near atomic defects. Our model enables more precise interpretation of 3DFIM characterization of structural defects.

A comparative characterization of defect structure in NiCo and NiFe equimolar solid solution alloys under in situ electron irradiation

Dislocation loops in NiCo and NiFe, both being single-phase, equimolar solid solution alloys, are generated by in situ high-voltage electron irradiation and characterized inside a transmission electron microscope. In addition to the different defect growth rates, the two alloys present dislocation loops with distinct shape evolution and element segregation, which reflect the element-sensitive, intrinsic properties (e.g., defect energetics, lattice distortion, and local ordering) of the alloys. These results provide evidence that the structure and dynamics of radiation-induced defects in concentrated alloys depend on not only the number but also the species of alloying elements.

Machine learning characterization of structural defects in amorphous packings of dimers and ellipses.

Structural defects within amorphous packings of symmetric particles can be characterized using a machine learning approach that incorporates structure functions of radial distances and angular arrangement. This yields a scalar field, softness, that correlates with the probability that a particle is about to be rearranged. However, when particle shapes are elongated, as in the case of dimers and ellipses, we find that the standard structure functions produce imprecise softness measurements. Moreover, ellipses exhibit deformation profiles in stark contrast to circular particles. In order to account for the effects of orientation and alignment, we introduce structure functions to recover the predictive performance of softness, as well as provide physical insight into local and extended dynamics. We study a model disordered solid, a bidisperse two-dimensional granular pillar, driven by uniaxial compression and composed entirely of monomers, dimers, or ellipses. We demonstrate how the computation of softness via a support vector machine extends to dimers and ellipses with the introduction of orientational structure functions. Then we highlight the spatial extent of rearrangements and defects, as well as their cross correlation, for each particle shape. Finally, we demonstrate how an additional machine learning algorithm, recursive feature elimination, provides an avenue to better understand how softness arises from particular structural aspects. We identify the most crucial structure functions in determining softness and discuss their physical implications.

Superdislocations and point defects in pyrochlore Yb2Ti2O7 single crystals and implication on magnetic ground states

This study reports atomic-scale characterization of structural defects in Yb2Ti2O7, a pyrochlore oxide whose subtle magnetic interactions is prone to small perturbations. Due to discrepancies in the reported magnetic ground states, it has become a pressing issue to determine the nature of defects in this system. In the present study, we use atomic resolution scanning transmission electron microscopy techniques to identify the type of defects in the ytterbium titanate single crystals grown by the conventional optical floating zone (FZ) method. In addition to the known point defects of substitution Yb on Ti B-sites, extended defects such as dissociated superdislocations and anti-phase boundaries were discovered for the first time in this material. Such defects were prevalently observed in the FZ grown single crystals (of a darker color), in contrast to the stoichiometric white polycrystalline powders or high quality colorless single crystals grown by the traveling solvent floating zone technique. The lattice strains from these extended defects result in distortions of Yb-tetrahedron. A change of Ti valance was not detected at the defects. Our findings provide new insights into understanding the nature of defects that are of great importance for the physical property studies of geometrically frustrated compounds. Furthermore, this work sheds light on the complicated core structure of superdislocations that have large Burgers vectors in oxides with complex unit cells.

ESI-TOF mass spectrometry of cationic carbosilane dendrimers: A potent tool for characterization of structural defects.

Macromolecular polyelectrolytes are gaining considerable attention for the application in medicine that implies their detailed characterization. We have successfully applied electrospray ionization mass spectrometry (ESI MS) to the analysis of defects in the structure of three generations of polycationic carbosilane dendrimers bearing series of quarternary phosphonium groups at their periphery. Besides expected defects caused by incomplete conversion of particular reaction steps during the synthesis of dendritic scaffold and subsequent peripheral functionalization, also, several products of side reactions were observed together with defects created in the course of measurement (particularly ion exchange products). Defective molecules can be to some extent separated by means of gel permeation chromatography that proves that they are not products of in source fragmentation processes. Within the reaction sequence used for the synthesis of dendrimers under study, hydrosilylation was the source of most defects; the effectivity of quarternization depends on the type of phosphine. Results confirm high sensitivity of ESI MS towards defects, stability of the carbosilane skeleton towards fragmentation under the conditions of ESI ionization, and capability to detect both lower- and higher-molecular weight impurities arising from the synthetic sequence in the same m/z range as the target dendrimer, thus providing valuable view of the polydispersity.

Multimode Electron Tomography as a Tool to Characterize the Internal Structure and Morphology of Gold Nanoparticles.

Three dimensional (3D) characterization of structural defects in nanoparticles by transmission electron microscopy is far from straightforward. We propose the use of a dose-efficient approach, so-called multimode tomography, during which tilt series of low and high angle annular dark field scanning transmission electron microscopy projection images are acquired simultaneously. In this manner, not only reliable information can be obtained concerning the shape of the nanoparticles, but also the twin planes can be clearly visualized in 3D. As an example, we demonstrate the application of this approach to identify the position of the seeds with respect to the twinning planes in anisotropic gold nanoparticles synthesized using a seed mediated growth approach.

Low temperature solid-state wetting and formation of nanowelds in silver nanowires

This article focuses on the microscopic mechanism of thermally induced nanoweld formation between silver nanowires (AgNWs) which is a key process for improving electrical conductivity in NW networks employed for transparent electrodes. Focused ion beam sectioning and transmission electron microscopy were applied in order to elucidate the atomic structure of a welded NW including measurement of the wetting contact angle and characterization of defect structure with atomic accuracy, which provides fundamental information on the welding mechanism. Crystal lattice strain, obtained by direct evaluation of atomic column displacements in high resolution scanning transmission electron microscopy images, was shown to be non-uniform among the five twin segments of the AgNW pentagonal structure. It was found that the pentagonal cross-sectional morphology of AgNWs has a dominant effect on the formation of nanowelds by controlling initial wetting as well as diffusion of Ag atoms between the NWs. Due to complete solid-state wetting, at an angle of ∼4.8°, the welding process starts with homoepitaxial nucleation of an initial Ag layer on (100) surface facets, considered to have an infinitely large radius of curvature. However, the strong driving force for this process due to the Gibbs–Thomson effect, requires the NW contact to occur through the corner of the pentagonal cross-section of the second NW providing a small radius of curvature. After the initial layer is formed, the welded zone continues to grow and extends out epitaxially to the neighboring twin segments.

31P MAS NMR spectroscopy with 93Nb decoupling and DFT calculations: A structural characterization of defects in a niobium-phosphate phase

The present study has investigated the structure of four niobium phosphates compounds using 31P MAS NMR spectroscopy. Niobium 93Nb decoupling, applied during 31P NMR acquisition led to a resolution enhancement by a factor of 2–3, which allowed distinguish phosphorous sites separated by 1 ppm or less. The assignment of 31P spectra has been completed by use of first-principles calculations derived from the original XRD structures. One of the compounds, Na3.04Nb7P4O29, contains a sodium vacancy, clearly revealed in the 31P MAS spectrum obtained with 93Nb decoupling. A series of structural models were proposed to account for the sodium distribution in the structure of Na3.04Nb7P4O29 and the assignment was possible using DFT calculation and a statistical analysis of the NMR parameters.

Dynamical X‐ray diffraction theory: Characterization of defects and strains in as‐grown and ion‐implanted garnet structures

The generalized dynamical theory of X‐ray diffraction by imperfect single crystals is extended to characterize structure imperfections of real single crystals with complex basis and similar crystalline films with inhomogeneous strain fields. The influence of various defects (intrinsic and extrinsic point defects, nanoclusters, and microdefects), simultaneously presented in such structures, on changing both average and fluctuating strain fields as well as structure factors are taken into account. The analytical expressions connecting immediately the coherent and diffuse components of the scattering intensity with statistical characteristics of these defects are obtained. Thus, the self‐consistent description of the coherent and diffuse dynamical diffraction intensity components is provided. Some examples of the application of the developed theoretical model to treat rocking curves measured from various garnet structures by using high‐resolution double‐crystal X‐ray diffractometer are reviewed. Possibilities for the quantitative characterization of structural defects and strain profiles in the as‐grown and ion‐implanted garnet single crystals and yttrium iron garnet films are demonstrated.The self‐consistent dynamical description of coherent and diffuse scattering intensities from imperfect crystal structures with inhomogeneous strain fields and randomly distributed defects make it possible to determine the parameters of strain profiles and statistical characteristics of defects by analytical treating the coherent and diffuse components of rocking curves measured by the high‐resolution double‐crystal X‐ray diffractometer with widely open detector window.

Characterization of structural defects in SnSe2 thin films grown by molecular beam epitaxy on GaAs (111)B substrates

Tin selenide thin films have been grown by molecular beam epitaxy on GaAs (111)B substrates at a growth temperature of 150°C, and a microstructural study has been carried out, primarily using the technique of transmission electron microscopy. The Se:Sn flux ratio during growth was systematically varied and found to have a strong impact on the resultant crystal structure and quality. Low flux ratios (Se:Sn=3:1) led to defective films consisting primarily of SnSe, whereas high flux ratios (Se:Sn>10:1) gave higher quality, single-phase SnSe2. The structure of the monoselenide films was found to be consistent with the Space Group Pnma with the epitaxial growth relationship of [011]SnSe//[11¯0]GaAs, while the diselenide films were consistent with the Space Group P3¯m1, and had the epitaxial growth relationship [21¯1¯0]SnSe2//[11¯0]GaAs.

Characterizing the structure of topological insulator thin films

We describe the characterization of structural defects that occur during molecular beam epitaxy of topological insulator thin films on commonly used substrates. Twinned domains are ubiquitous but can be reduced by growth on smooth InP (111)A substrates, depending on details of the oxide desorption. Even with a low density of twins, the lattice mismatch between (Bi, Sb)2Te3 and InP can cause tilts in the film with respect to the substrate. We also briefly discuss transport in simultaneously top and back electrically gated devices using SrTiO3 and the use of capping layers to protect topological insulator films from oxidation and exposure.

Atomic Defects in Two Dimensional Materials.

Atomic defects in crystalline structures have pronounced affects on their bulk properties. Aberration-corrected transmission electron microscopy has proved to be a powerful characterization tool for understanding the bonding structure of defects in materials. In this article, recent results on the characterization of defect structures in two dimensional materials are discussed. The dynamic behavior of defects in graphene shows the stability of zigzag edges of the material and gives insights into the dislocation motion. Polycrystalline graphene is characterized using advanced electron microscopy techniques, revealing the global crystal structure of the material, as well as atomic-resolution observation of the carbon atom positions between neighboring crystal grains. Studies of hexagonal boron nitride (hBN) are also visited, highlighting the interlayer bonding, which occurs upon defect formation, and characterization of grain boundary structures. Lastly, defect structures in monolayer polycrystalline transition metal dichalcogenides grown by CVD are discussed.

Characterization of structural defects in the silicon crystal grown with various seeds

To see the effect of the seeds on the crystallization of the silicon ingot, two types of seed were prepared and applied to the mono-like multi-crystalline silicon crystallization. One of them was conical shaped seed divided vertically into 4 pieces and the other one was chipped silicon seeds. Both of the seeds were placed at the bottom of the crucible before the growth of the multi-crystalline silicon material with a conventional directional solidification process. Compared to the conventional thin and flat seeds, conical shaped seed result in the larger single crystalline portion in the ingot although the gap of the seed division might act as source of dislocation generation during the crystal growth. This was expected to be due to the lateral growth of the silicon single crystalline grown from the conical seed which is taller than normal flat seeds. And also, comparison of the Minority Carrier Life Time (MCLT) between the silicon crystal grown from the chipped silicon seeds and the silicon crystal grown without any seed was conducted. It was found that the silicon crystal grown with chipped silicon seeds shows higher MCLT than the silicon crystal grown without seeds, although both crystals look similar as multi-crystalline silicon.

Characterization of structural defects in semipolar GaN layers grown on patterned sapphire substrates

We present the transmission electron microscopy characterization of GaN layers grown by metal–organic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) on patterned sapphire substrates (PSSs). We determined that the predominant I1-type basal stacking faults (BSFs) propagated toward the surface without an annihilation reaction, even when the layer thickness was increased by HVPE, whereas some threading dislocations were annihilated through their coalescence. It is revealed that BSFs were generated owing to the growth on the −c plane facet at the initial stage of MOVPE growth. One way to improve the crystalline quality is to use PSSs with wide terraces to expand the defect-free region.