Crystalline Defects Research Articles

Non-destructive identification of edge-component burgers vector of threading dislocations in SiC wafers by birefringence imaging

Non-destructive characterization of crystalline defects in SiC wafers is important for manufacturing high-performance SiC power devices with high productivity. The present study shows that the edge component of the Burgers vector can be identified by birefringence imaging under conditions deviating slightly from the crossed-Nicols condition and by calculation of the in-plane shear stress distribution. It is also found that the combination of birefringence observation and X-ray topography allows the discrimination of the family of threading screw dislocations. The present results will further the understanding of the relationship between defect structure and the properties of SiC power devices.

Construction of 2D-2D S-scheme heterojunction based graphdiyne (g-CnH2n−2) coupling with highly crystalline nitrogen defect g-C3N4 for efficient photocatalytic overall water splitting

Graphdiyne (GDY) is an up-and-coming two-dimensional all-carbon nanostructured material, which fills the long-standing gap in carbon material science and opens up a whole new way for the research of electronic, optical and semiconductor materials. In this work, a 2D-2D S-scheme heterojunction was constructed by 2D GDY coupling with highly crystalline nitrogen defect g-C3N4 nanosheets (GDY/g-C3N4-VN) for efficient photocatalytic overall water splitting. GDY was fabricated by the cross-coupling reaction of hexaethynylbenzene with CuI as catalyst and substrate. And the highly crystalline nitrogen defect porous g-C3N4-VN was fabricated by the alkali-molten salt-assisted method and the existence of N defects was testified by Transmission Electron Microscopy (TEM) and Electron Paramagnetic Resonance (EPR). In addition, the successful construction of S-scheme heterojunction between GDY and crystalline g-C3N4-VN was strongly demonstrated by in situ XPS, Electron Spin Resonance (ESR) and Density Function Theory (DFT) calculations. The construction of GDY/crystalline g-C3N4-VN (GNG-X) heterojunctions enhances the charge density and raise detaching efficiency of photogenerated carriers. The excellent and photostable photocatalytic hydrogen evolution activity of 17.87 μmol h−1 was acquired on CNG-25, which increased of 25.23 folds than that of GDY alone. Most importantly, photocatalytic overall water splitting experiments were carried out by loading 3 wt% Co3O4 and 1 wt% Pt as cocatalysts on CNG-25, and the yields of H2 and O2 were obtained as 0.48 μmol h−1 and 0.24 μmol h−1, respectively, which further demonstrated the promising application value of CNG-25 photocatalyst. This work presents a simple strategy for the design and manufacture of novel 2D GDY-based S-scheme heterojunction to enhance photocatalytic activity and achieve overall water splitting.

Deducing the internal interfaces of twisted multilayer graphene via moiré-regulated surface conductivity.

The stacking state of atomic layers critically determines the physical properties of twisted van der Waals materials. Unfortunately, precise characterization of the stacked interfaces remains a great challenge as they are buried internally. With conductive atomic force microscopy, we show that the moiré superlattice structure formed at the embedded interfaces of small-angle twisted multilayer graphene (tMLG) can noticeably regulate surface conductivity even when the twisted interfaces are 10 atomic layers beneath the surface. Assisted by molecular dynamics (MD) simulations, a theoretical model is proposed to correlate surface conductivity with the sequential stacking state of the graphene layers of tMLG. The theoretical model is then employed to extract the complex structure of a tMLG sample with crystalline defects. Probing and visualizing the internal stacking structures of twisted layered materials is essential for understanding their unique physical properties, and our work offers a powerful tool for this via simple surface conductivity mapping.

Enhanced flux pinning by refined structural/magnetic domains in nickel-doped BaFe2As2 single crystals

The critical current density at high magnetic fields of high-temperature superconducting single crystals and films is mainly determined by the flux pinning force arising from interactions between fluxons and crystalline defects or secondary phases. However, iron-based superconductors, which are acknowledged as promising candidates for high-field applications, suffer from a lack of strong flux pinning centers. It leads to a critical current density (Jc) inferior to that of iron-based superconducting films. In this work, we applied a simple annealing method to tailor flux pinning structures in slightly underdoped BaFe2-xNixAs2 single crystals without significantly compromising superconductivity. We propose that the probable strengthened orthorhombic distortion by post-annealing could generate twin domain boundaries or short-range antiferromagnetic clusters with sizes comparable to the in-plane coherence length. The inclusion of the structural/magnetic structures enhances electron scattering but also provides potent flux pinning centers. Ultimately, the Jc is enhanced by 2.5 times in the samples with optimized pinning landscapes. Our studies highlight the importance of further exploring underdoped iron-based superconductors, in which magnetic/orthorhombic phases that compete and coexist with superconductivity may also act as effective pinning centers.

Electrostimulation and Nanomanipulation of Two-Dimensional MoO3-x Layers Grown on Graphite

Molybdenum trioxide shows many attractive properties, such as a wide electronic band gap and a high relative permittivity. Monolayers of this material are particularly important, as they offer new avenues in optoelectronic devices, e.g., to alter the properties of graphene electrodes. Nanoscale electrical characterization is essential for potential applications of monolayer molybdenum trioxide. We present a conductive atomic force microscopy study of an epitaxially grown 2D molybdenum oxide layer on a graphene-like substrate, such as highly oriented pyrolytic graphite (HOPG). Monolayers were also investigated using X-ray photoelectron spectroscopy, atomic force microscopy (semi-contact and contact mode), Kelvin probe force microscopy, and lateral force microscopy. We demonstrate mobility of the unpinned island under slight mechanical stress as well as shaping and detachment of the material with applied electrical stimulation. Non-stoichiometric MoO3-x monolayers show heterogeneous behavior in terms of electrical conductivity, which can be related to the crystalline domains and defects in the structure. Different regions show various I–V characteristics, which are correlated with their susceptibility to electrodegradation. In this work, we cover the existing gap regarding nanomanipulation and electrical nanocharacterization of the MoO3 monolayer.

Synthesis of Chemically Sharp Interface in NdNiO3/SrTiO3 Heterostructures

The nickel-based superconductivity provides a fascinating new platform to explore high-T c superconductivity. As the infinite-layer nickelates are obtained by removing the apical oxygens from the precursor perovskite phase, the crystalline quality of the perovskite phase is crucial in synthesizing high quality superconducting nickelates. Especially, cation-related defects, such as the Ruddlesden–Popper-type (RP-type) faults, are unlikely to disappear after the topotactic reduction process and should be avoided during the growth of the perovskite phase. Herein, using reactive molecular beam epitaxy, we report the atomic-scale engineering of the interface structure and demonstrate its impact in reducing crystalline defects in Nd-based nickelate/SrTiO3 heterostructures. A simultaneous deposition of stoichiometric Nd and Ni directly on SrTiO3 substrates results in prominent Nd vacancies and Ti diffusion at the interface and RP-type defects in nickelate films. In contrast, inserting an extra [NdO] monolayer before the simultaneous deposition of Nd and Ni forms a sharp interface and greatly eliminates RP-type defects in nickelate films. A possible explanation related to the polar discontinuity is also discussed. Our results provide an effective method to synthesize high-quality precursor perovskite phase for the investigation of the novel superconductivity in nickelates.

Synthesis of Chemically Sharp Interface in NdNiO3/SrTiO3 Heterostructures

Abstract The nickel-based superconductivity provides a fascinating new platform to explore the high-T c superconductivity. As the infinite-layer nickelates are obtained by removing the apical oxygens from the precursor perovskite phase, the crystalline quality of the perovskite phase is crucial in synthesizing high quality superconducting nickelates. Especially, cation-related defects, such as the Ruddlesden-Popper-type (RP-type) faults, are unlikely to disappear after the topotactic reduction process and should be avoided during the growth of the perovskite phase. Herein, using reactive molecular beam epitaxy, we report the atomic-scale engineering of the interface structure and demonstrate its impact in reducing crystalline defects in Nd-based nickelate/SrTiO3 heterostructures. A simultaneous deposition of stoichiometric Nd and Ni directly on SrTiO3 substrate results in prominent Nd vacancies and Ti diffusion at the interface and RP-type defects in nickelate films. In contrast, inserting an extra [NdO] monolayer before the simultaneous deposition of Nd and Ni forms a sharp interface and greatly eliminates RP-type defects in nickelate films. A possible explanation related to the polar discontinuity is also discussed. Our results provide an effective method to synthesize high-quality precursor perovskite phase for the investigation of the novel superconductivity in nickelates.

Structural Changes in Ceramic Carbonized Hydroxyapatite as a Result of Long-Term Storage at Room Temperature

Carbonated hydroxyapatite (CHA) is the basic mineral component of animal and human bone. Therefore, it is widely used in medicine to repair bone defects. In orthopedic surgeries, ceramic implants are usually used as a biologically active defect filler. In the lattice of CHA carbonate ions can occupy two non-equivalent positions - A and B. A position corresponds to the position of OH- anions in the lattice of hydroxyapatite (HA), and B - PO43-. It is well known that substitution of B-positions with carbonate groups leads to significant distortions of HA lattice, which causes microstresses and crystalline defects in it. Therefore, CHA ceramics as a result of sintering is characterized by significant internal stresses whose relaxation at room temperature can lead to a change in both its phase composition and biological activity. By methods of chemical and X-ray structural analysis, infrared spectroscopy and electron scanning microscopy the ageing process of pressed CHA at room temperature, sintered in an atmosphere of dry carbon dioxide at temperatures 800÷1200 °C was studied. The phase composition and structure of freshly prepared and aged for two years ceramic samples were compared. It is shown that relaxation of internal stresses arising during sintering of presses causes plastic deformation of crystallites accompanied by redistribution of carbonate ions from B to A-position. As a result, displacement of OH- ions from channel (A) positions and decomposition of B-type CHA on CaO and A-type CHA becomes energetically advantageous.

Anomalous elasticity and emergent dipole screening in three-dimensional amorphous solids.

In recent work, we developed a screening theory for describing the effect of plastic events in amorphous solids on its emergent mechanics. The suggested theory uncovered an anomalous mechanical response of amorphous solids where plastic events collectively induce distributed dipoles that are analogous to dislocations in crystalline solids. The theory was tested against various models of amorphous solids in two dimensions, including frictional and frictionless granular media and numerical models of amorphous glass. Here we extend our theory to screening in three-dimensional amorphous solids and predict the existence of anomalous mechanics similar to the one observed in two-dimensional systems. We conclude by interpreting the mechanical response as the formation of nontopological distributed dipoles that have no analog in the crystalline defects literature. Having in mind that the onset of dipole screening is reminiscent of Kosterlitz-Thouless and hexatic transitions, the finding of dipole screening in three dimensions is surprising.

Atomic and Electronic Reconstruction in Defective 0D Molybdenum Carbide Heterostructure for Regulating Lower‐Frequency Microwaves

Abstract0D nanomaterials with high efficiency of atom utilization possess extraordinary tunability over bulk materials. Precise reconstruction of atoms in a 0D nanoparticle toward tuning of crystalline phases and defects is highly desirable but remains a grand challenge. In this study, a crystallization rate‐controlled strategy is reported to achieve controllable reconstruction of atoms in situ, which inducts a series of monodisperse 0D molybdenum carbide nanoparticles (MoxC NP) that anchor on a carbon matrix with adjustable crystalline phases and atom vacancies. Aberration‐corrected transmission electron microscopy, electron paramagnetic resonance technique, density functional theory calculation, and electron holography jointly reveal the atomic reconstruction process and confirm its remarkable effects of optimizing the local electronic states and enhancing the heterointerface interactions. As a result, the optimized MoC/Mo2C heterostructure on the carbon matrix is shown to enable the promoted dielectric response and generate more than 90% absorption of lower‐frequency microwaves (the current 5th‐generation communication band). The control of atomic reconstruction may provide an effective pathway for unlocking tunable dielectric properties of 0D nanomaterials toward various technological applications.

Facile synthesis of N-doped TiO2 nanosheets with enhanced photocatalytic activity

Titanium dioxide is a widely used photocatalyst for photodegradation of water pollutants. There has been a growing interest to improve the photocatalytic performance by using nanoscale forms of titania due to increased specific surface area of catalyst. Among the various nanoscale morphologies, nanosheets hold a remarkable distinction due to their higher specific surface area and lower crystalline defects in comparison the spherical particles. The performance of a photocatalyst can be further enhanced by using certain dopants and modifiers. In the present work, titanium dioxide nanosheets were doped with nitrogen using two nitrogen sources (Urea and Ammonia). X-ray diffraction and Field emission scanning electron microscopy were used to characterise the synthesized nanosheets. Dye degradation experiments were conducted to analyse the enhancement of photocatalytic activity.

Beyond surface: Correlating CuI /CuII and O-vacancy surface sites in CuxO nanocubes with their activities as noble metal-free anodes for fuel cells

Nanocrystals' surface area and shape significantly impact their activities in fuel cells. On the other hand, additional parameters that may affect their activities cannot be neglected. Here, we demonstrate that the presence of crystalline defects (such as oxygen vacancies) and the relative concentration of CuI/CuII may also vary with the decrease of CuxO nanocubes sizes, contributing to the observed activities as noble metal-free anodes for methanol fuel cells. However, such differences did not follow the size as expected, showing that this parameter does not play a critical role in determining materials' properties. To this end, CuxO nanocubes having controllable sizes (50, 65, and 85 nm) were synthesized by a simple and similar protocol, leading to nanocrystals enclosed by {100} surfaces. When the catalytic activity of the different-sized CuxO nanocubes was performed toward the electrooxidation of methanol in alkaline media, the observed performances decreased as follows: 50 nm > 85 nm > 65 nm-CuxO nanocubes, in which 50 nm-CuxO nanocubes led to the best electrocatalytic results. Interestingly, our results showed that the differences in the catalytic activities of CuxO nanocubes displaying different sizes could not only be assigned to a gain of surface area with a decrease in particle size. More specifically, XPS results indicated that the reduction of particle size led to an increase in both Os/OL and Cu(I)/Cu(II) ratios, demonstrating the enrichment of oxygen vacancies at the surface of CuxO nanocubes, which also contributed for the observed catalytic activities.

Intrinsic and extrinsic dopings in epitaxial films MnBi2Te4

The intrinsic antiferromagnetic topological insulator MnBi2Te4 and members of its family have been the subject of theoretical and experimental research, which has revealed the presence of a variety of defects and disorders that are crucial in determining the topological and magnetic properties. This also brings about challenges in realizing the quantum states like the quantum anomalous Hall and the axion insulator states. Here, utilizing cryogenic magnetoelectric transport and magnetic measurements, we systematically investigate the effects arising from intrinsic doping by antisite defects and extrinsic doping by Sb in MnBi2Te4 epitaxial films grown by molecular beam epitaxy. We demonstrate that the nonequilibrium condition in epitaxy allows a wide growth window for optimizing the crystalline quality and defect engineering. While the intrinsic antisite defects caused by the intermixing between Bi and Mn can be utilized to tune the Fermi level position as evidenced by a p-to-n conductivity transition, the extrinsic Sb-doping not only compensates for this doping effect but also modifies the magnetism and topology of the film, during which a topological phase transition is developed. Conflicting reports from the theoretical calculations and experimental measurements in bulk crystals versus epitaxial films are addressed, which highlights the intimate correlation between the magnetism and topology as well as the balance between the Fermi-level positioning and defect control. The present study provides an experimental support for the epitaxial growth of the intrinsic topological insulator and underlines that the topology, magnetism, and defect engineering should be revisited for enabling a steady and reliable film production.

Analysis of Crystalline Defects Caused by Growth on Partially Planarized Spalled (100) GaAs Substrates

We analyze the effect of growth on non-(100) surfaces resulting from incomplete planarization of spalled GaAs wafers on the defect structure of GaAs solar cell layers grown by hydride vapor phase epitaxy (HVPE). Controlled spalling of (100)-oriented GaAs has the potential to reduce substrate costs for III-V epitaxy; however, it creates regularly faceted surfaces that may complicate the growth of high-quality III-V optoelectronic devices. We leverage the anisotropic growth rate of HVPE to planarize these faceted GaAs substrates, reducing the surface roughness and degree of faceting. We observe degraded solar cell performance and material quality in sample areas where facets are not completely removed. We used dark lock-in thermography and photoluminescence to identify recombination in areas that were not fully planarized. We used cathodoluminescence to identify the presence of extended defects in these regions, which are correlated with bandgap fluctuations in the material. We hypothesize that these defects were created by strain from compositional fluctuations in ternary alloys grown on the faceted surfaces. This work elucidates the potential issues of solar cells grown on faceted surfaces and builds understanding toward realizing high performance III-V photovoltaics with the cost-reduction potential of controlled spalling.

Coral-shaped Mn-CuS with hierarchical pores and crystalline defects for high-efficiency H2O2 production via electrocatalytic two-electron reduction

We report the development of non-noble transition metal sulfide catalysts Mn-CuS-x for high-efficiency and selective H2O2 production. In this study, we synthesized Mn-CuS-x with coral-shaped hierarchical porous structure through one-pot hydrothermal reaction. The Mn-CuS-x possesses large specific surface area with plenty of crystalline defects via heteroatom doping of Mn. The Mn-CuS-x kinetically favored 2e− pathway over 4e− pathway, resulting in high selectivity (>92%) to H2O2 production and high H2O2 concentration (∼24.5 mM at 0.6 V vs. RHE) within 3 h for the optimized Mn-CuS-2 catalysts (synthesized with a molar ratio of 1:3:8 (MnCl2·4 H2O: CuCl2·2 H2O: C2H5NS)). The Mn-CuS-2 showed excellent stability, indicating that the Mn-CuS electrocatalysts can simultaneously achieve both high stability and enhanced H2O2 production. Our first-principles calculations further confirmed that the heteroatom Mn doping to CuS thermodynamically makes the 2e–-ORR pathway more favorable. This study provides a green, low-cost, and efficient route to produce H2O2, which is very promising for generating chemicals and liquid fuels.

Adaptive multigrid strategy for geometry optimization of large-scale three dimensional molecular mechanics

In this paper, we present an efficient adaptive multigrid strategy for the geometry optimization of large-scale three dimensional molecular mechanics. The resulting method can achieve significantly reduced complexity by exploiting the intrinsic low-rank property of the material configurations and by combining the state-of-the-art adaptive techniques with the hierarchical structure of multigrid algorithms. To be more precise, we develop a oneway multigrid method with adaptive atomistic/continuum (a/c) coupling, e.g., blended ghost force correction (BGFC) [1] approximations with gradient-based a posteriori error estimators on the coarse levels. We utilize state-of-the-art 3D mesh generation techniques to effectively implement the method. For 3D crystalline defects, such as vacancies, micro-cracks and dislocations, compared with brute-force optimization, complexity with superior rates can be observed numerically, and the strategy has a five-fold acceleration in terms of CPU time for systems with 108 atoms.

Insights into the Role of Defects in Fe2(MoO4)3 Catalysts

The catalytic activity and selectivity of Fe2(MoO4)3 obtained from solid-state synthesis protocols is investigated for the oxidative dehydrogenation (ODH) of ethanol to acetaldehyde. While Fe2(MoO4)3 annealed in a MoO3 atmosphere is found to be inactive, ball-milling of such solid-state synthesis precursors leads to an increase in activity, which cannot be attributed purely to the change in the specific surface area nor to a geometric effect of smaller particles. In a systematic study of the synthesis of Fe2O3-free Fe2(MoO4)3 samples, the correlation of ball-milling time, defect concentration, and catalytic activity is presented. In-depth X-ray diffraction studies, magnetic susceptibility, and 57Fe-Mössbauer spectroscopic measurements disclose characteristic signatures of the defect sites in the solid materials introduced by ball-milling and indicate: (i) MoO3-enriched amorphous layers of variable thickness (shell-like) and (ii) crystalline bulk defects corresponding to a reduction in the molar volume of the solid. The onset of recrystallization processes sets in around 300 °C and is accompanied by an enhanced MoO3 mobility in the particles, which adds to an increase in activity and a decreasing selectivity under catalytic conditions. Accordingly, the defects associated with the decreasing amorphous fraction are converted to crystalline bulk defects, which are monitored by the magnetic hyperfine field distribution of the respective Fe sites. Overall, this study shows the importance of the amorphous layer thickness with a sufficient defect concentration and of the bulk defects’ contribution to the chemical gradients, important for the MoO3 mobility and reduction of the MoO3 loss as a volatile species at the solid/gas phase boundary in ODH catalysis.

Influences of lattice strain and SiGe buffer layer thickness on electrical characteristics of strained Si/SiGe/Si(110) heterostructures

In previous studies, we reported on significantly high hole mobility in a strained Si/SiGe/Si(110) heterostructure pMOSFET. In this study, we investigated impact of strain on the hole mobility in this structure. 20-nm thick (110)-oriented strained Si films with different strain levels were formed using solid-source molecular beam epitaxy. A positive correlation between the hole mobility and the strain of the Si layer is verified. It is also shown that formation of growth twins and strain relaxation of SiGe layer in the [1‾10] direction deteriorate the hole mobility. The maximum mobility enhancement factor obtained in this study is about 3. In addition, the influence of the SiGe layer thickness on the off-state leakage current in pMOSFET operation is discussed. It is found that the upper part of the thick SiGe samples includes cause of the leakage current, indicating change of conductivity with the progress of the crystal growth of SiGe layer. This phenomenon is related to the formation process of the crystalline defects. Another finding is the existence of a current path through the substrate, which suggests a defect-assisted current flow across the SiGe/n-Si(110) junction.

Influence of crystalline defects on nitrogen implantation in copper for surface hardening

The design of electrical contacts that combine a high electrical conductivity and excellent wear resistance requires the optimization of surface processing. Surface hardening of copper was achieved by nitrogen implantation and the underlying mechanisms were investigated by electron microscopy. Pure copper and a CuSn alloy were implanted with nitrogen in the recrystallized state and after severe plastic deformation. The larger lattice parameter of CuSn does not promote the implantation of N but the large density of crystalline defects in the as-deformed material gives rise a much thicker implanted layer up to 120±20 nm. In all cases, the strong hardening results from a high density of nanoscaled Cu3N particles (1.2 ± 0.4 1023 m−3). Deformation twins seem to promote both the diffusion and the nucleation of Cu3N.

A direct correlation between structural and morphological defects of TiO2 thin films on FTO substrates and photovoltaic performance of planar perovskite solar cells

Titanium oxide (TiO2) precursor is often deposited on conductive fluorine doped tin oxide (FTO) to convert into crystalline anatase phase after a thermal annealing at 400–550 °C. A higher annealing temperature can increase the crystallinity of TiO2, but it may also induce morphological changes in both TiO2 and FTO. In this work, structural and morphological changes in thermally annealed FTO and TiO2/FTO are identified and semi-quantified by X-ray diffraction, scanning electron and atomic force microscopies. 400 °C annealed TiO2 shows the lowest crystallinity and the highest dislocation density, giving a poor photovoltaic performance of planar perovskite solar cells (p-PSCs). 550 °C annealed TiO2 exhibits the highest crystallinity and lowest dislocation density, but at such temperature the FTO films undergo an increase of surface roughness by more than 25%. As a result, 550 °C annealed TiO2 coatings on FTO substrates show the highest concentration of surface fracture and a larger exposed area of FTO, permitting a direct contact of upcoming perovskite with the FTO substrate. TiO2 thin films annealed at 450 or 500 °C show a compromising between the crystalline structure and defect density, giving a better energy conversion efficiency of 16.3% of p-PSCs prepared under ambient conditions.