Defect Topology Research Articles

Topology of boron substitutional defects in single-walled carbon nanotubes: A first-principles study

This is a theoretical study of boron-doped single-walled carbon nanotubes. The same topology of primitive nanodomains, located at different positions on single-walled carbon nanotubes, leads to different electronic band structures. We propose a ϕ term. Density functional theory was corrected for van der Waals interactions and used to carry out the periodic boundary condition geometry optimization, where boron formed the topologies of primitive nanodomains. The calculated bulk structure and local structure spectroscopic parameters can be used for comparison with experimental results to confirm the theoretical models.

Crucial Role of Ni Point Defects and Sb Doping for Tailoring the Thermoelectric Properties of ZrNiSn Half-Heusler Alloy: An Ab Initio Study.

In the wide group of thermoelectric compounds, the half-Heusler ZrNiSn alloy is one of the most promising materials thanks to its thermal stability and narrow band gap, which open it to the possibility of mid-temperature applications. A large variety of defects and doping can be introduced in the ZrNiSn crystalline structure, thus allowing researchers to tune the electronic band structure and enhance the thermoelectric performance. Within this picture, theoretical studies of the electronic properties of perfect and defective ZrNiSn structures can help with the comprehension of the relation between the topology of defects and the thermoelectric features. In this work, a half-Heusler ZrNiSn alloy is studied using different defective models by means of an accurate Density Functional Theory supercell approach. In particular, we decided to model the most common defects related to Ni, which are certainly present in the experimental samples, i.e., interstitial and antisite Ni and a substitutional defect consisting of the replacement of Sn with Sb atoms using concentrations of 3% and 6%. First of all, a comprehensive characterization of the one-electron properties is performed in order to gain deeper insight into the relationship between structural, topological and electronic properties. Then, the effects of the modeled defects on the band structure are analyzed, with particular attention paid to the region between the valence and the conduction bands, where the defective models introduce in-gap states with respect to the perfect ZrNiSn crystal. Finally, the electronic transport properties of perfect and defective structures are computed using semi-classical approximation in the framework of the Boltzmann transport theory as implemented in the Crystal code. The dependence obtained of the Seebeck coefficient and the power factor on the temperature and the carrier concentration shows reasonable agreement with respect to the experimental counterpart, allowing possible rationalization of the effect of the modeled defects on the thermoelectric performance of the synthesized samples. As a general conclusion, defect-free ZrNiSn crystal appears to be the best candidate for thermoelectric applications when compared to interstitial and antisite Ni defective models, and substitutional defects of Sn with Sb atoms (using concentrations of 3% and 6%) do not appreciably improve electronic transport properties.

Rotational V-Y Fasciocutaneous Advancement Island Flap to Cover Deep Sacrococcygeal Defects due to Pilonidal Disease, Pressure Ulcers, and Other Inflammatory Conditions.

The sacrococcygeal area supports the lower body and endures mechanical forces during movement. However, current treatment methods for deep caudal sacrococcygeal defects have limitations, resulting in insufficient tissue for deep pocket obliteration and considering only the two-dimensional advancement plane in a three-dimensional defect topology. Our study proposes using a rotational V-Y fasciocutaneous advancement island flap to reconstruct deep caudal sacrococcygeal defects. By considering the three-dimensional nature of the defect, we distinguish a coccygeal plane of the V-Y flap from a sacral plane and set different directions and depths of movement for each plane. From March 2016 to July 2022, 12 patients underwent successful treatment with this surgery, and no complications or recurrences were observed in the study group. Our research found that patients in our study exhibited a smaller intercoccygeal angle than the average angle of the general Korean population, as previously reported. This implies a more pronounced curvature between the sacral and coccygeal planes. Therefore, our methods, which consider the three-dimensional structures of sacrococcygeal pathology, are significant. This technique provides a mechanically robust reconstruction after resecting deep sacrococcygeal pathology, with well-padded tissue to prevent dead space and wound disruption.

Application of level-set type topology optimization for defect topology identification analysis using hammering response data

In this study, we presented the defect identification analysis based on the level-set type topology optimization using hammering testing data. To simulate the oscillation behavior of concrete structure, the equation of motion in three dimensions is introduced as the governing equation. The performance function is defied by the time integral of the squared residual between the computed and observed displacements. The problem is to find the defect topologies so as to minimize the performance function. In the model with a small number of observation points, it was found that nonexistent defects disappeared by using the weighted sensitivity method. In addition, it was confirmed that the model with a large number of observation points did not show the same tendency as the model with a small number of observation points because the accuracy of defect identification increased as the number of observation points increased.

Design of Olefin-Phobic Zeolites for Efficient Ethane and Ethylene Separation

Selective adsorption of ethane from a mixture of ethane (C2H6) and ethylene (C2H4) has emerged as an energy-efficient process for obtaining high-purity ethylene. Recently, it has been reported that some novel microporous organometallic and organic adsorbents can preferentially adsorb C2H6 over C2H4. Conversely, zeolite adsorbents, which are widely used in industry due to their high stability and low material cost, generally prefer C2H4 adsorption (“olefin-philic”). In this study, we carefully investigated the effects of chemical composition (Si/Al ratio), silanol defect, and pore topology (BEA, CHA, and MFI) of zeolites to develop efficient “olefin-phobic” adsorbents. The results showed that the Si–O–(Na+)–Al (weak Lewis acid) and isolated Si–OH (weak Brønsted acid) groups in the zeolite frameworks undesirably increased the affinity for C2H4 (Lewis base). Consequently, defect-free pure silica zeolites exhibited promising C2H6/C2H4 selectivities (1.98–2.25), which were superior to those of Al-containing zeolites (0.26) and pure silica zeolites with abundant silanols (1.50), at 298 K and 1 bar. In particular, defect-free pure silica BEA exhibited the highest C2H6/C2H4 selectivity (2.25), large C2H6 uptake (2.27 mmol g–1), and facile regeneration at 298 K. The adsorbent enabled the efficient separation of high-purity C2H4 (>99.95%) from a mixture of C2H6/C2H4 with high productivity (22.98 L L–1). The productivity was comparable or even superior to that of recently reported C2H6-selective adsorbents. The present results provide hope for the development of efficient, stable, and scalable inorganic adsorbents for the selective C2H6 adsorption.

Numerical studies for defect topology identification based on the adjoint variable and the finite element methods

This paper describes the visualization of defect regions in a structure via hammering test that employs the level set type topology optimization based on the concept of the phase-field method. In this study, the residual between the calculated displacement and the observed displacement is defined as the performance function, with the identification problem of defect regions being formulated based on the adjoint variable and finite element methods. Numerical experiments were performed to investigate the effect of the regularization parameter, the number of observation point, the evaluation equation of convergence and the application of the weighted gradient. Consequently, it was found that the number, position and size of defect can be adjusted using the regularization parameter τ. In addition, it was seen that if the number of observation points and the evaluation equation in the iterative computation of topology update are appropriately given, the identified defect topology is close to the exact defect topology. Furthermore, it was found that defect topologies can be more accurately identified by applying weighted sensitivities.

Identification of non-dimensional density distribution of concrete structures based on self-attention using hammering response data

The aging of concrete structures has become a serious problem in Japan, and periodic maintenance is essential for preventing accidents caused by structural aging. In this study, a method for estimating defects in concrete using data from hammering tests on a concrete plate using machine learning was developed. A neural network based on self-attention (SAN) to estimate the three-dimensional position and size of the defects was constructed. Moreover, a dataset was created from the topology of the internal defects and the acceleration response waveform when a concrete plate was struck. The entire plate was represented as a non-dimensional density matrix. The SAN used scalograms of the acceleration responses as the input. Furthermore, two types of data augmentation (‘Flip’ and ‘Rotate’) were proposed. The use of both data augmentation techniques achieved the highest accuracy. By setting an appropriate number of rotations, the model was able to estimate all defects in the dataset.

Dislocation imaging in fcc colloidal single crystals

Abstract We describe the imaging of dislocations in crystals that are composed of micron-size colloidal particles. The dislocations form in an fcc crystalline film close to the interface with a template that is used to grow these crystals. We use a simple laser diffraction microscope (LDM) to image the strain field of the dislocations on a medium-range length scale. This setup is inspired by the transmission electron microscopy technique used to image dislocations in atomic crystals. Confocal microscopy allows us to reconstruct the defect topology in the colloidal crystal in three dimensions and on the particle scale. We take advantage of the three-dimensional particle coordinates determined by confocal microscopy to simulate LDM image contrast for specific imaging conditions.

Defect topology identification analysis using hammering response data based on level-set type topology optimization

In this study, we present numerical results for the defect topology identification based on the adjoint variable and the finite element methods. The performance function is defined by the time integration of square value of difference between the computed and the observed displacements, and we solve the problem that unknown defect topology is identified so as to minimize the performance function. As for the constraint condition for the performance function, the equation of motion is introduced, and the performance function is extended by the adjoint variable method. We can obtain the Lagrange function, the first variation of the Lagrange function is calculated. Consequently, the gradient of the Lagrange function with respect to the level-set function is obtained, and the defect topology is finally obtained by solving the reaction diffusion equation using this gradient. Some numerical results are shown in this study.

Topology of Orientational Defects in Confined Smectic Liquid Crystals.

We propose a general formalism to characterize orientational frustration of smectic liquid crystals in confinement by interpreting the emerging networks of grain boundaries as objects with a topological charge. In a formal idealization, this charge is distributed in pointlike units of quarter-integer magnitude, which we identify with tetratic disclinations located at the end points and nodes. This coexisting nematic and tetratic order is analyzed with the help of extensive MonteCarlo simulations for a broad range of two-dimensional confining geometries as well as colloidal experiments, showing how the observed defect networks can be universally reconstructed from simple building blocks. We further find that the curvature of the confining wall determines the anchoring behavior of grain boundaries, such that the number of nodes in the emerging networks and the location of their end points can be tuned by changing the number and smoothness of corners, respectively.

Defect topology identification in concrete plates by machine learning based on self-attention using hammering response data

Defect topology identification in concrete plates by machine learning based on self-attention using hammering response data

Identification of three-dimensional defect topology in concrete structures based on self-attention network using hammering response data

Identification of three-dimensional defect topology in concrete structures based on self-attention network using hammering response data

Defect topology and annihilation by cooperative movement of atoms in neutron-irradiated graphite

Graphite has been used as neutron moderator or reflector in many nuclear reactors. The irradiation of graphite in a nuclear reactor results in a complex population of defects. Heating of the irradiated graphite at high temperatures results in annihilation of the defects with release of an unusually large energy, called the Wigner energy. From various experiments on highly irradiated graphite samples from CIRUS reactor at Trombay and ab-initio simulations, we have for the first time identified various 2-, 3- and 4-coordinated topological structures in defected graphite, and provided microscopic mechanism of defect annihilation on heating and release of the Wigner energy. The annihilation process involves cascading cooperative movement of atoms in two steps involving an intermediate structure. Our work provides new insights in understanding of the defect topologies and annihilation in graphite which is of considerable importance to wider areas of graphitic materials including graphene and carbon nanotubes.

A robust effect of the defect on the switching behavior in carbon-based molecular device.

In this paper, we investigate the effects of spin-dependent electron and defect in the carbon-based molecular device. Our proposed molecular device is designed by two carbon chains, which is bonded to a defect. The defect topology includes pentagonal and octagonal carbon rings, which is put between two zigzag-edged graphene nanoribbon (ZGNR). The spin effect and switching symbiosis are shown in this carbon-based device. By switching of the orientation of the defect in two states (S1/S2 states) relative to the two electrodes, the full spin effect is shown. Also, we report the obvious negative differential resistance (NDR) behavior in our proposed molecular device. The results suggest that the proposed composition significantly affects the ratio of current and voltage, which the maximum peak of current (S2 state) is lower than 0.0022μA and could have a potential application in the next generation of molecular circuits.

Observation of Protected Photonic Edge States Induced by Real-Space Topological Lattice Defects.

Topological defects (TDs) in crystal lattices are elementary lattice imperfections that cannot be removed by local perturbations, due to their real-space topology. In the emerging field of topological photonics, photonic topological edge states arise from the nontrivial topology of the band structure defined in momentum space and are generally protected against defects. Here we show that adding TDs into a valley photonic crystal generates a lattice disclination that acts like a domain wall and hosts photonic topological edge states. Unlike previous topological waveguides, the disclination forms an open arc and functions as a free-form waveguide connecting a pair of TDs of opposite topological charge. This interplay between the real-space topology of lattice defects and momentum-space band topology provides a novel scheme to implement large-scale photonic structures with complex arrangements of robust topological waveguides and resonators.

In Situ Raman Spectroscopy as a Tool for Discerning Subtle Structural Differences between Commercial (Ce,Zr)O2-Based OSC Materials of Identical Composition

In situ Raman spectroscopy was used at temperatures in the 50–480 °C range under oxidizing (20% O2/He) and reducing (5% H2/He) flowing gas atmospheres to compare the spectra obtained for a series of industrial rare earth doped CexZr1−xO2−δ oxygen storage capacity (OSC) mixed metal oxide materials of identical at % composition, which were prepared by the same chemical synthesis route, in which one synthesis parameter of the aqueous chemistry was slightly varied. The Raman fingerprint of the anionic sublattice is very sensitive to O atom relocations within the bulk of the material matrix and to the pertinent defect topology in each case. A protocol of sequential Raman measurements and analysis was proposed to discern subtle differences between the oxygen vacancy and defect topologies of the examined materials. It can be concluded that for two materials under comparison for their structures, identical Raman spectra are obtained only if the procedures followed for their preparation are identical; a slight variation of one single parameter (e.g., in the aqueous chemistry stage) results in discernible differences in the Raman spectra. The proposed procedure can serve as a tool for proving or disproving infringement of IPR (Intellectual Property Rights) protected preparation methods of ceria-based mixed metal oxide materials.

A machine perspective of atomic defects in scanning transmission electron microscopy

AbstractEnabled by the advances in aberration‐corrected scanning transmission electron microscopy (STEM), atomic‐resolution real space imaging of materials has allowed a direct structure‐property investigation. Traditional ways of quantitative data analysis suffer from low yield and poor accuracy. New ideas in the field of computer vision and machine learning have provided more momentum to harness the wealth of big data and sophisticated information in STEM data analytics, which has transformed STEM from a localized characterization technique to a macroscopic tool with intelligence. In this review article, we discuss the prime significance of defect topology and density in two‐dimensional (2D) materials, which have proved to be a powerful means to tune a wide range of properties. Subsequently, we systematically review advanced data analysis methods that have demonstrated promising prospects in analyzing STEM data, particularly for identifying structural defects, with high throughput and veracity. A unified framework for atomic structure identification is also summarized.

Npr2 null mutants show initial overshooting followed by reduction of spiral ganglion axon projections combined with near-normal cochleotopic projection.

Npr2 (natriuretic peptide receptor 2) affects bifurcation of neural crest or placode-derived afferents upon entering the brain stem/spinal cord, leading to a lack of either rostral or caudal branches. Previous work has shown that early embryonic growth of cochlear and vestibular afferents is equally affected in this mutant but later work on postnatal Npr2 point mutations suggested some additional effects on the topology of afferent projections and mild functional defects. Using multicolor lipophilic dye tracing, we show that absence of Npr2 has little to no effect on the initial patterning of inner ear afferents with respect to their dorsoventral cochleotopic-specific projections. However, in contrast to control animals, we found a variable degree of embryonic extension of auditory afferents beyond the boundaries of the anterior cochlear nucleus into the cerebellum that emanates only from apical spiral ganglion neurons. Such expansion has previously only been reported for Hox gene mutants and implies an unclear interaction of Hox codes with Npr2-mediated afferent projection patterning to define boundaries. Some vestibular ganglion neurons expand their projections to reach the cochlear apex and the cochlear nuclei, comparable to previous findings in Neurod1 mutant mice. Before birth, such expansions are reduced or lost leading to truncated projections to the anteroventral cochlear nucleus and expansion of low-frequency fibers of the apex to the posteroventral cochlear nucleus.

The Role of Polarity in Nonplanar Semiconductor Nanostructures.

The lack of mirror symmetry in binary semiconductor compounds turns them into polar materials, where two opposite orientations of the same crystallographic direction are possible. Interestingly, their physical properties (e.g., electronic or photonic) and morphological features (e.g., shape, growth direction, and so forth) also strongly depend on the polarity. It has been observed that nanoscale materials tend to grow with a specific polarity, which can eventually be reversed for very specific growth conditions. In addition, polar-directed growth affects the defect density and topology and might induce eventually the formation of undesirable polarity inversion domains in the nanostructure, which in turn will affect the photonic and electronic final device performance. Here, we present a review on the polarity-driven growth mechanism at the nanoscale, combining our latest investigation with an overview of the available literature highlighting suitable future possibilities of polarity engineering of semiconductor nanostructures. The present study has been extended over a wide range of semiconductor compounds, covering the most commonly synthesized III-V (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb) and II-VI (ZnO, ZnTe, CdS, CdSe, CdTe) nanowires and other free-standing nanostructures (tripods, tetrapods, belts, and membranes). This systematic study allowed us to explore the parameters that may induce polarity-dependent and polarity-driven growth mechanisms, as well as the polarity-related consequences on the physical properties of the nanostructures.

Aspects of Defect Topology in Smectic Liquid Crystals

We study the topology of smectic defects in two and three dimensions. We give a topological classification of smectic point defects and disclination lines in three dimensions. In addition we describe the combination rules for smectic point defects in two and three dimensions, showing how the broken translational symmetry of the smectic confers a path dependence on the result of defect addition.