Long-range Defects Research Articles

Anomalously strong size effect on thermal conductivity of diamond microparticles

Diamond has the known highest thermal conductivity of around 2000 W m−1 K−1 and is, therefore, widely used for heat dissipation. In practical applications, synthetic diamond microparticles are usually assumed to have similar thermal conductivity to that of bulk diamond because the particle size is larger than the theoretical phonon mean free path, so that boundary scattering of heat-carrying phonons is absent. In this report, we find that the thermal conductivity of diamond microparticles anomalously depends on their sizes. The thermal conductivity of diamond microparticles increases from 400 to 2000 W m−1 K−1 with the size growing from 20 to 300 μm. We attribute the abnormally strong size effect to the long-range defects during the growth process based on analysis of point defects, dislocations, and thermal penetration depth dependence of thermal conductivity. Our results play a vital role in the design of diamond composites and in the improvement of the thermal conductivity of synthetic diamonds.

Coupled local residual shear and compressive strain in NaNbO3 ceramics under cooling

Stabilizing lead-free antiferroelectrics at room temperature is key for advancing greener and more efficient energy storage devices. While NaNbO3 solid solutions hold great promise for high energy density applications, its pure form displays structural instabilities arising from irreversible electric-field induced phase transitions and/or an undesired coexistence with its ferroelectric polymorph. To unravel how mechanical constraints imposed by residual stresses, structural defects, and microstructure disrupt the stability of the NaNbO3 antiferroelectric state, we used in situ Dark-Field X-ray Microscopy to map local microstructural deformations in a single embedded {100}pc grain. By replicating typical heat treatment conditions, we show that the ferroelectric phase nucleates as a result of the coupled interplay between residual shear and compressive strain distributions that manifest during cooling towards ambient temperature. In addition, the microstrain relaxation behavior indicates that long-range defects preferentially nucleate at the expense of the antiferroelectric phase in regions at sub-micrometer distances from the grain center. Our findings illustrate that adequate temperature control during low temperature sintering, heat treatments, or in operando conditions may be vital in dictating the structure-property relationships of NaNbO3 ceramics, ensuring their suitability for efficient energy storage applications.

Mechanistic Insight into the Defect-Engineered White Light Emission from the Single-Phase Orthovanadate Phosphor Synthesized by a Facile Rapid Microwave-Assisted Synthesis.

The synthesis of single-phase barium orthovanadate phosphors by a one-pot microwave-assisted hydrothermal approach has been reported, wherein the homogeneous thermal zone generated at the molecular level by microwave radiation gives rise to tunable distortion in the tetrahedral VO4-3 and oxygen vacancies, eventually enabling intrinsic white light emission with CIE of 0.31,0.38, high photoluminescence internal quantum efficiency of 35%, and external quantum efficiency of 28% whereas phosphor synthesized by the hydrothermal route exhibits only bluish-green emission (PLQE: 0.5%). The Rietveld refinement confirms the formation of a single trigonal phase having dissimilar V-O bond lengths and bond angles, implying the formation of a distorted phosphor under optimized conditions, and corroborates with Raman and Fourier transform infrared analyses. The X-ray photoelectron spectroscopy and electron paramagnetic resonance analysis reveal that the origin of white light emission is due to short- and long-range defects, in particular the oxygen vacancies, which eventually form an intermediate energy level in the forbidden region between the valence and conduction bands. Lifetime spectra show triexponential fitting, corresponding to two charge transfer blue and green emission bands (3T2, 3T1 to 1A1) and one oxygen vacancy-related red emission at RT. Furthermore, these phosphors are thermally stable, as no change in the structure or emission characteristics are observed. A prototype fabricated using a 365 nm chip exhibits white-light-emission CIE of 0.353,0.392, correlated color temperature of 4867 K, color rendering index of 85, and high luminous efficacy of 102 lm/W at 140 mA operating current, portentous for practical applications.

Defect Engineering in Photocatalysts and Photoelectrodes: From Small to Big

ConspectusGreen hydrogen as a clean energy carrier plays a critical role in tackling climate change. Solar-driven water splitting is regarded as one of the most promising strategies for green hydrogen production, but the solar-to-hydrogen (STH) conversion efficiency is still far from the industrial requirement. Defect engineering is an effective strategy to enhance the performance of photocatalysts and photoelectrodes. In a crystal, defects range from micro- to macro-levels, such as atomic-scale vacancies and dopants, distortion along the crystal, and defective surface, which all influence the photoresponse, electrical conductivity, and surface properties of semiconductor materials. The methodologies of creating defects have been vigorously explored, while accurate identification and characterization of defects have been focused on much less. Moreover, the widely reported benefits of defects in the photocatalytic system may not necessarily cancel out the negative spillover effect of charge recombination. As a Janus-role structure with different influences, the defect is an old, yet complex, notion, and in-depth understanding of defects is vital toward efficient solar-driven hydrogen generation.In this Account, we provide an overview of the engineering of defects with some typical examples, including atomic vacancies of anion and metal, long-range lattice distortion, and macro-range surface defects. We start with our effort to explore the controllable generation of vacancies, including oxygen vacancies, nitrogen vacancies, and metal vacancies via post-treatment and in situ synthesis. A comprehensive understanding of anion vacancies, especially the oxygen vacancies, has been discussed. The identification of metal vacancies and their role in the photoelectrochemical (PEC) process are further discussed. We then move on to the long-range defects of lattice distortion to understand their formation mechanism and contributions to PEC water splitting. Finally, the defective surface is discussed based on the case studies of two-dimensional (2D) mesoporous crystalline photocatalysts and photoelectrodes.Although significant advances have been achieved in solar-driven hydrogen production, there is still a long way ahead before reaching the industrial production target. To inspire innovative defect engineering for further improving the STH conversion efficiency, we provide the following perspectives. First, precise synthesis strategies should be developed to control the type and concentration of defects. Second, reliable techniques and measurements should be established to make accurate identification and characterization of defects. Last but not least, the interaction of different types of defects should be further explored to guide the rational design of material systems, which may lead to unique synergistic mechanisms in further promoting solar energy conversion efficiency.

Biomimicry in 3D printing design: implications for peripheral nerve regeneration.

Nerve guide conduits (NGCs) connect dissected nerve stumps and effectively repair short-range peripheral nerve defects. However, for long-range defects, autograftsshowbetter therapeutic effects, despite intrinsic limitations. Recent evidenceshows that biomimetic design is essential for high-performance NGCs, and 3D printing is a promising fabricating technique. The current work includes a brief review ofthechallenges for peripheral nerve regeneration. The authors propose a potential solution using biomimetic 3D-printed NGCsas alternative therapies. The assessment of biomimetic designs includes microarchitecture, mechanical property, electrical conductivity and biologics inclusion. The applications of 3D printing in preparing NGCsand present strategies to improve therapeutic effects are also discussed.

Bioinspired Multichannel Nerve Guidance Conduit Based on Shape Memory Nanofibers for Potential Application in Peripheral Nerve Repair.

Repairing peripheral nerve injury, especially long-range defects of thick nerves, is a great challenge in the clinic due to their limited regeneration capability. Most FDA-approved nerve guidance conduits with large hollow lumen are only suitable for short lesions, and their effects are unsatisfactory in repairing long gaps of thick nerves. Multichannel nerve guidance conduits have been shown to offer better regeneration of long nerve defects. However, existing approaches of fabricating multichannel nerve conduits are usually complicated and time-consuming. Inspired by the intelligent responsive shaping process of shape memory polymers, in this study, a self-forming multichannel nerve guidance conduit with topographical cues was constructed based on a degradable shape memory PLATMC polymer. With an initial tubular shape obtained by a high-temperature molding process, the electrospun shape memory nanofibrous mat could be temporarily formed into a planar shape for cell loading to realize the uniform distribution of cells. Then triggered by a physical temperature around 37 °C, it could automatically restore its permanent tubular shape to form the multichannel conduit. This multichannel conduit exhibits better performance in terms of cell growth and the repair of rat sciatic nerve defects. These results reveal that self-forming nerve conduits can be realized based on shape memory polymers; thus, the fabricated bioinspired multichannel nerve guidance conduit has great potential in peripheral nerve regeneration.

Analysis of tow architecture variability in biaxially braided composite tubes

Spatial deviations in composite braid unit cell dimensions limit mechanical performance and are challenging to model. Full-field measurements of the irregular tow placement in biaxially braided composite tubes were obtained by digital image correlation. Analysis of the reconstructed tow paths showed long-range defects along the hoop direction with short-range defects dominating the biased tow direction. A route toward numerical simulation of the composite response is established by developing a 3D model of the composite using the measured tow trajectories. Finally, a comparison to previously-published triaxial braid results indicates axial tows work to constrain unit cells dimensions along the hoop direction.

Full non-contact laser-based Lamb waves phased array inspection of aluminum plate

Lamb waves defect detection technique is suitable to detect long-range defects and multi-defects in plates because of its attractive properties such as multi-modes, low attenuation, and multi-defect sensitivity. As the laser beams have the tiny size and noncontact property, in this paper a full non-contact laser-based Lamb waves phased array inspection technique was proposed for defect detection in an aluminum plate. A Q-switched Nd: YAG pulse laser generating Lamb waves and a continuous laser inspection system was used to inspect the out-of-plane displacements. The laser-generated Lamb waves have the broadband property and low temporal resolution that are not suitable for defects detection directly. So the continuous wavelet transform was used to extract narrowband signals from laser-generated Lamb wave and a linear mapping algorithm was adopted to compensate for the dispersion of the extracted narrowband signals. Two experiments were carried out to verify the applicability and effectiveness of the developed full non-contact laser-based Lamb waves phased array inspection technique. Two compact phased array imaging algorithms, the total focusing method and the sign coherence factor algorithm, were adopted to image the defects with processed signals. The experimental results accurately located the positions of the defects.

Strong Long-Range Relaxations of Structural Defects in Graphene Simulated Using a New Semiempirical Potential

We present a new semiempirical potential for graphene, which includes also an out-of-plane energy term. This novel potential is developed from density functional theory (DFT) calculations for small numbers of atoms and can be used for configurations with millions of atoms. Our simulations show that buckling caused by typical defects such as the Stone–Wales (SW) defect extends to hundreds of nanometers. Surprisingly, this long-range relaxation lowers the defect formation energy dramatically—by a factor of 2 or 3—implying that previously published DFT-calculated defect formation energies suffer from large systematic errors. We also show the applicability of the novel potential to other long-range defects including line dislocations and grain boundaries, all of which exhibit pronounced out-of-plane relaxations. We show that the energy as a function of dislocation separation diverges logarithmically for flat graphene but converges to a constant for free-standing buckled graphene. A potential in which the atoms are attracted to the 2D plane restores the logarithmic behavior of the energy. Future simulations employing this potential will elucidate the influence of the typical long-range buckling and rippling on the physical properties of graphene.

Transport and optical properties of single- and bilayer black phosphorus with defects

We study the electronic and optical properties of single- and bilayer black phosphorus with shortand long-range defects by using the tight-binding propagation method. Both types of defect states are localized and induce a strong scattering of conduction states reducing significantly the charge carrier mobility. In contrast to properties of pristine samples, the anisotropy of defect-induced optical excitations is suppressed due to the isotropic nature of the defects. We also investigate the Landau level spectrum and magneto-optical conductivity, and find that the discrete Landau levels are sublinearly dependent on the magnetic field and energy level index, even at low defect concentrations.

Resonance effects on the Raman spectra of graphene superlattices

In this work, a study of resonance effects in the Raman spectra of twisted bilayer graphene (tBLG) is presented. The analysis takes into account the effect of the mismatch angle $\ensuremath{\theta}$ between the two layers, and also of the excitation laser energy on the frequency, linewidth, and intensity of the main Raman features, namely the rotationally induced $R$ band, the $G$ band, and the second-order ${G}^{\ensuremath{'}}$ (or $2D$) band. The resonance effects are explained based on the $\ensuremath{\theta}$ dependence of the tBLG electronic structure, as calculated by ab initio methodologies. The twist angle $\ensuremath{\theta}$ also defines the observation of a ``$D$-like'' band which obeys the double-resonance process, but relies on the superlattice along with long-range defects in order to fulfill momentum conservation. The study was possible due to the development of a route to produce and identify rotationally stacked bilayer graphene by means of atomic force microscopy (AFM).

Non-Equilibrium Critical Dynamics of Ferromagnets with Long Range Correlated Defects

The simultaneous effect of non-equilibrium initial states and correlation betweendefects of the structure on the evolution of anisotropic disordered systems at the critical pointwas analyzed. The field theory description of the non-equilibrium critical behavior of three-dimensional disordered systems with the long-range correlated defects was given and the dy-namical critical exponent of the short-time evolution was calculated in the two-loop approxima-tion without the use of the "-expansion. The values of the dynamical critical exponent obtainedby using various methods for summing asymptotic series were compared with the results ofthe computer simulation of the non-equilibrium critical behavior of the three-dimensional dis-ordered Ising model in the short-time regime.

Nonequilibrium critical dynamics of structurally disordered systems with long-range correlated defects

The simultaneous effect of nonequilibrium initial states and correlation between defects of the structure on the evolution of anisotropic disordered systems at the critical point has been analyzed. The field theory description of the nonequilibrium critical behavior of three-dimensional disordered systems with the longrange correlated defects has been given and the dynamical critical exponent of the short-time evolution has been calculated in the two-loop approximation without the use of the ɛ expansion. The values of the dynamical critical exponent obtained by using various methods for summing asymptotic series have been compared with the results of the computer simulation of the nonequilibrium critical behavior of the three-dimensional disordered Ising model in the short-time regime.

Possible nodal vortex state inCeRu2

The microscopic property of magnetic vortices in the mixed state of a high-quality ${\mathrm{CeRu}}_{2}$ crystal has been studied by muon spin rotation. We have found that the spatial distribution of magnetic induction $\mathbf{B}(\mathbf{r})$ probed by muons is perfectly described by the London model for the triangular vortex lattice with appropriate modifications to incorporate the high-field cutoff around the vortex core and the effect of long-range defects in the vortex lattice structure at lower fields. The vortex core radius is proportional to ${H}^{(\ensuremath{\beta}\ensuremath{-}1)/2}$ with $\ensuremath{\beta}\ensuremath{\simeq}0.53$ $(H$ being the magnetic field), which is in good agreement with the recently observed nonlinear field dependence of the electronic specific heat coefficient $\ensuremath{\gamma}\ensuremath{\propto}{H}^{\ensuremath{\beta}}.$ In particular, the anomalous increase of magnetic penetration depth in accordance with the peak effect in dc magnetization $(>~{H}^{*}\ensuremath{\simeq}3$ T at 2.0 K) has been confirmed; this cannot be explained by the conventional pair-breaking effect due to magnetic field. In addition, the spontaneous enhancement of flux pinning, which is also associated with the peak effect, has been demonstrated microscopically. These results strongly suggest the onset of collective pinning induced by a new vortex state having an anomalously enhanced quasiparticle density of states for $H>~{H}^{*}.$

Long-range spin-pairing order and spin defects in quantum spin-

Long-range spin-pairing order and spin defects in quantum spin-

Effect of electron irradiation on domain wall pinning defects in 50-50 NiFe

A magnetic measuring technique which sorts out defects according to a distribution function n was used to study the influence of electron irradiation on 50-50 NiFe. The distribution function is determined in terms of the maximum force fm that a defect can exert on a forward moving domain wall, or equivalently, the range z0, which is the distance the mean position of the wall may move past the defect before the wall snaps free from the pinning action of the defect. The range and maximum force are related by a spring constant k, viz., fm=kz0. The quantity n (z0) dz0 gives the number of defects per unit volume having a range between z0 and z0+dz0. Distribution functions were determined before and after electron irradiation. The irradiation was for 100 min with 18-MeV electrons with a dose of 1.1×1017 e/cm2. Following irradiation, there was a substantial decrease in the number of short-range defects (i.e., defects capable of pinning a domain wall only over a relatively short range); there was an increase in the number of long-range defects; and the total number of defects N0 with which a steadily moving wall interacts at a given instant was found to decrease. Although the coercive force decreased, the Barkhausen noise power increased following irradiation. It has been shown by Baldwin and Milstein that the contribution to Barkhausen noise power from defects of a given range z0 is proportional z30; hence the increase in Barkhausen noise is understood in terms of the increase in long-range defects. The experimental results can be understood in terms of the effects of long-range crystallographic ordering which occurs in NiFe alloys which are subjected to electron irradiation.