Defect Configurations Research Articles

Behavior of Hydrogarnet‐Type Defects in Hydrous Stishovite at Various Temperatures and Pressures

AbstractDense polymorphs of silica have been demonstrated experimentally to incorporate from 1.5 wt% to as much as 11.6 wt% H2O as OH groups, with implications for the hydrogen budgets of Earth and other planets. This OH is thought to enter the SiO2 structure via a charge‐balanced substitution in which silicon vacancies (VSi) are compensated by protonating four of the surrounding six oxygen atoms, often referred to as a hydrogarnet‐type defect. There are many possible configurations for this defect structure in dense silica, but the nature of these configurations and whether they can be distinguished experimentally is unknown. We present here density functional theory calculations that systematically assess the possible configurations of a hydrogarnet‐type defect in stishovite (rutile‐type SiO2), with direct comparisons to experimental vibrational spectroscopy data. We predict that stishovite synthesized at 450 K and 10 GPa quenched to room temperature is dominated by a single defect type with tetrahedral geometry. This leads to OH stretching modes (2,500–3,000 cm−1) and SiOH bending modes (∼1,400–1,450 cm−1) largely consistent with experimentally observed modes. One remaining issue is that our calculations produce results compatible with experimental data on H to D exchange, but do not explain why a considerable fraction of the 1,420 cm−1 mode shifts by only 40 cm−1 in deuterated samples. At elevated pressures and temperatures, we find that a second square planar defect configuration also becomes favorable, leading to modes that should allow differentiation from the tetrahedral configuration.

Regulation of the electronic structure and ferromagnetism by implanting topological defects in a bismuth monolayer

Bismuth (Bi) monolayers have attracted much attention due to their intriguing physical and chemical applications, which are strongly related to the existence of topological defects. However, the intrinsic correlation between the defect types and particular electronic structures of Bi monolayers still needs to be understood. Herein, we intentionally implanted various typical topological defects into Bi monolayers to reveal the differences in the electronic structure, in which the monovacancies (MVs) can lead to a transformation from a semiconductor to a metal. More interestingly, a particular defect configuration can give rise to a spin polarization and room-temperature ferromagnetism, demonstrating the spintronic applications. This work reveals that the physical property differences of Bi monolayers are strongly dependent on their defect types.

Formation of Amorphous Carbon Multi‐Walled Nanotubes from Random Initial Configurations

Amorphous carbon nanotubes (a‐CNT) with up to four walls and sizes ranging from 200 to 3200 atoms have been simulated, starting from initial random configurations and using the Gaussian Approximation Potential. The important variables (like density, height, and diameter) required to successfully simulate a‐CNTs were predicted with the machine learning random forest technique. The width of the a‐CNT models ranged between 0.55–2 nm with an average inter‐wall spacing of 0.31 nm. The topological defects in a‐CNTs were analyzed and new defect configurations were observed. The electronic density of states and localization in these phases were discussed and delocalized electrons in the π subspace were identified as an important factor for inter‐layer cohesion. Spatial projection of the electronic conductivity favors axial transport along connecting hexagons, while non‐hexagonal parts of the network either hinder or bifurcate the electronic transport. A vibrational density of states was calculated and is potentially an experimentally comparable fingerprint of the material. The appearance of a low‐frequency radial breathing mode was discussed and the thermal conductivity at 300 K was estimated using the Green‐Kubo formula.

The role of native point defects and donor impurities in the electrical properties of ZnSb2O4: a hybrid density-functional study.

The ceramic material zinc antimony oxide ZnSb2O4 has promising electrical and magnetic properties, making it suitable for various applications such as electrochemical and energy storage. However, the effects of point defects and impurities on its electrical properties have never been revealed. Here, we employ hybrid density-functional calculations to investigate the energetics and electronic properties of native point defects and donor impurities in ZnSb2O4. The energetically favorable configurations of native point defects under selected growth conditions (O-rich and O-poor) are identified based on the calculated formation energies. The study finds no shallow donor and shallow acceptor defects with low formation energies. Still, the oxygen vacancy (VO) has the lowest formation energy among the donor-type defects under O-rich and O-poor conditions. However, it acts as a very deep acceptor, making it unlikely to provide free electron carriers to the conduction band. Moreover, electron carriers are likely to be compensated by the formation of zinc vacancies (VZn) and the zinc substituted for antimony (ZnSb), which behave as dominant acceptors. Our analysis of the charge neutrality conditions estimates that the Fermi level of undoped ZnSb2O4 would be pinned in a range that is 2.60 eV to 3.12 eV above the VBM for O-rich to O-poor growth conditions, respectively, suggesting that this material is semi-insulating. The possibility of enhancing free electron carriers by doping with Al, Ga, In, and F impurities is also investigated. Our results, however, indicate that high n-type conductivity is hindered by self-compensation in which the impurities additionally act as electron killers. Our results suggest that other impurity candidates and approaches may need to be considered to efficiently dope this material into n-type. Overall, this work paves the way for point defect engineering in this class of ternary oxides.

Imperfections are not 0 K: free energy of point defects in crystals.

Defects determine many important properties and applications of materials, ranging from doping in semiconductors, to conductivity in mixed ionic-electronic conductors used in batteries, to active sites in catalysts. The theoretical description of defect formation in crystals has evolved substantially over the past century. Advances in supercomputing hardware, and the integration of new computational techniques such as machine learning, provide an opportunity to model longer length and time-scales than previously possible. In this Tutorial Review, we cover the description of free energies for defect formation at finite temperatures, including configurational (structural, electronic, spin) and vibrational terms. We discuss challenges in accounting for metastable defect configurations, progress such as machine learning force fields and thermodynamic integration to directly access entropic contributions, and bottlenecks in going beyond the dilute limit of defect formation. Such developments are necessary to support a new era of accurate defect predictions in computational materials chemistry.

New insights into the process of intrinsic point defects in PuO2.

Intrinsic point defects are known to play a crucial role in determining the physical properties of solid-state materials. In this study, we systematically investigate the intrinsic point defects, including vacancies (VPu and VO), interstitials (Pui and Oi), and antisite atoms (PuO and OPu) in PuO2 using the first-principles plane wave pseudopotential method. Our calculations consider the whole charge state of these point defects, as well as the effect of oxygen partial pressure. This leads to a new perspective on the process of intrinsic point defects in PuO2. We find that the antisite atoms OPu and PuO are more likely to appear in O-rich and O-deficient environments, respectively. Interestingly, the most energetically favorable type of Schottky defect is {2VPu3-: 3VO2+} in an O-rich environment, while {4VO1+: VPu4-} is preferred in an O-deficient environment. These results differ from the commonly known {VPu4-: 2VO2+} type of Schottky defect. Moreover, under O-deficient conditions, we predict that the stable cation Frenkel defect is {VPu4+: Pui4+}, while the most stable anion Frenkel defect is {VO2+: Oi2-} under O-rich conditions. Lastly, we find that the only two types of antisite pairs that can appear are {OPu5-: PuO5+} and {OPu6-: PuO6+}, with the latter being the more stable configuration. These unconventional defect configurations provide a new viewpoint on the process of intrinsic point defects in PuO2 and lay theoretical foundations for future experiments.

Defect configuration of ceria for Pt anchoring toward efficient methanol oxidation

As a potential next-generation power source for portable electronic devices, commercialization process of direct methanol fuel cell (DMFC) technology is hindered by the high dependence of anode methanol oxidation reaction (MOR) on precious Pt catalyst. In order to improve the efficiency of Pt toward MOR catalysis, a Ni doping strategy is proposed for defect engineering on ceria substrate to achieve uniform dispersion of Pt nanoparticles. Besides, Ni could also act as electron donor for Pt and hence favor the removal of CO intermediate on Pt and act as a co-catalyst toward MOR. Superior MOR activity and great stability is therefore achieved for the as-prepared Pt/CeO2@Ni catalyst with 3 times higher peak MOR current density compared with Pt/C catalyst. Due to the evenly anchored Pt and enhanced CO oxidation ability caused from Ni doped ceria substrate, Pt utilization of the Pt/CeO2@Ni catalyst is calculated to be 3.24 times higher than that of the commercial Pt/C catalyst. By considering the significantly improved stability, the Pt/CeO2@Ni catalyst has the potential for application in DMFC devices.

Metastability of Constant-Density Flocks

We study numerically the Toner-Tu field theory where the density field is maintained constant, a limit case of "Malthusian" flocks for which the asymptotic scaling of correlation functions in the ordered phase is known exactly. While we confirm these scaling laws, we also show that such constant-density flocks are metastable to the nucleation of a specific defect configuration, and are replaced by a globally disordered phase consisting of asters surrounded by shock lines that constantly evolves and remodels itself. We demonstrate that the main source of disorder lies along shock lines, rendering this active foam fundamentally different from the corresponding equilibrium system. We thus show that in the context of active matter also, a result obtained at all orders of perturbation theory can be superseded by nonperturbative effects, calling for a different approach.

Predicting defect stability and annealing kinetics in two-dimensional PtSe2 using steepest entropy ascent quantum thermodynamics

The steepest-entropy-ascent quantum thermodynamic (SEAQT) framework was used to calculate the stability of a collection of point defects in 2D PtSe2 and predict the kinetics with which defects rearrange during thermal annealing. The framework provides a non-equilibrium, ensemble-based framework with a self-consistent link between mechanics (both quantum and classical) and thermodynamics. It employs an equation of motion derived from the principle of steepest entropy ascent (maximum entropy production) to predict the time evolution of a set of occupation probabilities that define the states of a system undergoing a non-equilibrium process. The system is described by a degenerate energy landscape of eigenvalues, and the entropy is found from the occupation probabilities and the eigenlevel degeneracies. Scanning tunneling microscopy was used to identify the structure and distribution of point defects observed experimentally in a 2D PtSe2 film. A catalog of observed defects includes six unique point defects (vacancies and anti-site defects on Pt and Se sublattices) and twenty combinations of multiple point defects in close proximity. The defect energies were estimated with density functional theory, while the degeneracies, or density of states, for the 2D film with all possible combinations or arrangements of cataloged defects was constructed using a non-Markovian Monte-Carlo approach (i.e. the Replica–Exchange–Wang–Landau algorithm (Vogel et al 2013 Phys. Rev. Lett. 110 210603)) with a q-state Potts model. The energy landscape and associated degeneracies were determined for a 2D PtSe2 film two molecules thick and unit cells in area (total of 5400 atoms). The SEAQT equation of motion was applied to the energy landscape to determine how an arbitrary density and arrangement of the six defect types evolve during annealing. Two annealing processes were modeled: heating from 77 K (−196 C) to 523 K (250 ∘C) and isothermal annealing at 523 K. The SEAQT framework predicted defect configurations, which were consistent with experimental STM images.

Nanosphere Lithography-Based Fabrication of Spherical Nanostructures and Verification of Their Hexagonal Symmetries by Image Analysis

Nanosphere lithography (NSL) is a cost- and time-effective technique for the fabrication of well-ordered large-area arrays of nanostructures. This paper reviews technological challenges in NSL mask preparation, its modification, and quality control. Spin coating with various process parameters (substrate wettability, solution properties, spin coating operating parameters) are discussed to create a uniform monolayer from monodisperse polystyrene (PS) nanospheres with a diameter of 0.2–1.5 μm. Scanning electron microscopy images show that the PS nanospheres are ordered into a hexagonal close-packed monolayer. Verification of sphere ordering and symmetry is obtained using our open-source software HEXI, which can recognize and detect circles, and distinguish between hexagonal ordering and defect configurations. The created template is used to obtain a wide variety of tailor-made periodic structures by applying additional treatments, such as plasma etching (isotropic and anisotropic), deposition, evaporation, and lift-off. The prepared highly ordered nanopatterned arrays (from circular, triangular, pillar-shaped structures) are applicable in many different fields (plasmonics, photonics, sensorics, biomimetic surfaces, life science, etc.).

Magnesium-Vacancy Optical Centers in Diamond.

We provide the first systematic characterization of the structural and photoluminescence properties of optically active centers fabricated upon implantation of 30-100 keV Mg+ ions in synthetic diamond. The structural configurations of Mg-related defects were studied by the electron emission channeling technique for short-lived, radioactive 27Mg implantations at the CERN-ISOLDE facility, performed both at room temperature and 800 °C, which allowed the identification of a major fraction of Mg atoms (∼30 to 42%) in sites which are compatible with the split-vacancy structure of the MgV complex. A smaller fraction of Mg atoms (∼13 to 17%) was found on substitutional sites. The photoluminescence emission was investigated both at the ensemble and individual defect level in the 5-300 K temperature range, offering a detailed picture of the MgV-related emission properties and revealing the occurrence of previously unreported spectral features. The optical excitability of the MgV center was also studied as a function of the optical excitation wavelength to identify the optimal conditions for photostable and intense emission. The results are discussed in the context of the preliminary experimental data and the theoretical models available in the literature, with appealing perspectives for the utilization of the tunable properties of the MgV center for quantum information processing applications.

Estimating bulk and edge topological indices in finite open chiral chains

We develop a formalism to estimate, simultaneously, the usual bulk and edge indices from topological insulators in the case of a finite sample with open boundary conditions and provide a physical interpretation of these quantities. We then show that they converge exponentially fast to an integer value when we increase the system size and also show that bulk and edge index estimates coincide at finite size. The theorem applies to any non-homogeneous system, such as disordered or defect configurations. We focus on one-dimensional chains with chiral symmetry, such as the Su–Schrieffer–Heeger model, but the proof actually only requires the Hamiltonian to be of short range and with a spectral gap in the bulk. The definition of bulk and edge index estimates relies on a finite-size version of the switch-function formalism where the Fermi projector is smoothed in energy using a carefully chosen regularization parameter.

Phase Transition Dynamics in a Complex Oxide Heterostructure.

Understanding the behavior of defects in the complex oxides is key to controlling myriad ionic and electronic properties in these multifunctional materials. The observation of defect dynamics, however, requires a unique probe-one sensitive to the configuration of defects as well as its time evolution. Here, we present measurements of oxygen vacancy ordering in epitaxial thin films of SrCoO_{x} and the brownmillerite-perovskite phase transition employing x-ray photon correlation spectroscopy. These and associated synchrotron measurements and theory calculations reveal the close interaction between the kinetics and the dynamics of the phase transition, showing how spatial and temporal fluctuations of heterointerface evolve during the transformation process. The energetics of the transition are correlated with the behavior of oxygen vacancies, and the dimensionality of the transformation is shown to depend strongly on whether the phase is undergoing oxidation or reduction. The experimental and theoretical methods described here are broadly applicable to insitu measurements of dynamic phase behavior and demonstrate how coherence may be employed for novel studies of the complex oxides as enabled by the arrival of fourth-generation hard x-ray coherent light sources.

Fingerprints of carbon defects in vibrational spectra of GaN considering the isotope effect

In this paper, we examine the carbon defects associated with peaks of infrared (IR) absorption and Raman scattering appearing in GaN crystals at carbon ($^{12}\mathrm{C}$) doping in the range of concentrations from $3.2\ifmmode\times\else\texttimes\fi{}{10}^{17}$ to $3.5\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Here, 14 unique vibrational modes of defects are observed in GaN samples grown by HVPE (hydride vapor phase epitaxy) and then compared with defect properties predicted from first-principles calculations. The vibrational frequency shift in two $^{13}\mathrm{C}$-enriched samples related to the effect of the isotope mass indicates six distinct configurations of the carbon-containing point defects. The effect of the isotope replacement is well reproduced by the density functional theory (DFT) calculations. Specific attention is paid to the most pronounced defects, namely, tricarbon complexes (${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$) and carbon substituting for nitrogen (${\mathrm{C}}_{\mathrm{N}}$). The position of the transition level $(+/0)$ in the bandgap found for ${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$ defects by DFT at 1.1 eV above the valence band maximum suggests that ${({\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}})}^{+}$ provides compensation of ${\mathrm{C}}_{\mathrm{N}}^{\ensuremath{-}}$. Here, ${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$ defects are observed to be prominent yet have high formation energies in DFT calculations. Regarding ${\mathrm{C}}_{\mathrm{N}}$ defects, it is shown that the host Ga and N atoms are involved in the delocalized vibrations of the defect and significantly affect the isotopic frequency shift. Much more faint vibrational modes are found from di-atomic carbon-carbon and carbon-hydrogen (C--H) complexes. Also, we note changes of vibrational mode intensities of ${\mathrm{C}}_{\mathrm{N}}, {\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$, C--H, and ${\mathrm{C}}_{\mathrm{N}}--{\mathrm{C}}_{i}$ defects in the IR absorption spectra upon irradiation in the defect-related ultraviolet/visible absorption range. Finally, it is demonstrated that the resonant enhancement of the Raman process in the range of defect absorption $>2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ enables the detection of defects at carbon doping concentrations as low as $3.2\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$.

Effect of trivalent rare earth doping in cadmium silicates hosts: a theoretical study

The objective of the present work is to investigate the possibilities for extrinsic defects in the three cadmium silicate matrices, CdSiO3, Cd2SiO4 and Cd3SiO5 and understand the final geometry of the most energetically favourable defects, which are generators of the luminescence centres in the material. This analysis will be performed out through classical and static computer modelling, using the code GULP. A number different mechanisms were modelled and their related solid state reactions were devised allowing the energetic calculations for the incorporation of trivalent rare Earth ions (R3+) in these matrixes. The results demonstrated that the trivalent rare Earth ions are preferentially incorporated substitutional at Cd sites with cadmium vacancy being created in order to compensate charge unbalance. Careful investigations of the different distances and angles between the ions involved in the defects formation for CdSiO3 found that all investigated R3+ ions have the same lower energetic defect configuration. The lowest energetic costs were found for a configuration where the rare Earth ions and the accompanying vacancy are almost aligned with a distance of around 7 Å and an angle closer to 180°. These results predict a possible set of two dipoles caused by the position of the cadmium vacancy in defect. Cadmium vacancies were expected to be present in the cadmium silicate due to the low vapor pressure of CdO; however, no previous work was able to pre-establish the final geometric structure of the cadmium silicate matrix in the defective region that can modify the luminescent centres in the material. The study of a wide range of possibilities for the insertion of defects and the proposal for the final configuration of these defects is presented, in order to assist in the identification of the sources and the specific chracteristics of the luminescent emissions presented in cadmium silicates doped with R3+.

Freestyle perforator flap in earlobe reconstruction

BackgroundIn earlobe reconstruction, it is a challenging task to find a simple one-stage technique that is applicable to the variable configurations of defects and that results in a longstanding naturally appearing earlobe.MethodsIn this article, we present a perforator-based flap design for earlobe reconstruction with preoperative perforator detection by pencil doppler to ensure the maximum vascularity of the used flap. Thirty cases of earlobe reconstruction were performed in the Department of Plastic and Reconstructive Surgery at Tanta University in the period from July 2015 to July 2019 using the technique described in this article.ResultsNone of our cases developed necrosis of the flaps. The mean result for the aesthetic evaluation of our cases was 4.7 and the mean patient satisfaction was 4.8 where the maximum score was 5.Conclusionfreestyle perforator flap with preoperative doppler perforator detection is a safe and reliable method of earlobe reconstruction.

Influence of the coordination defects on the dynamics and the potential energy landscape of two-dimensional silica.

The main cause of the fragile-to-strong crossover of 3D silica was previously attributed to the presence of a low-energy cutoff in the potential energy landscape. An important question emerges about the microscopic origin of this crossover and its generalizibility to other glass-formers. In this work, the fragile-to-strong crossover of a model two-dimensional (2D) glassy system is analyzed via molecular dynamics simulation, which represents 2D-silica. By separating the sampled defect and defect-free inherent structures, we are able to identify their respective density of state distributions with respect to energy. A low energy cutoff is found in both distributions. It is shown that the fragile-to-strong crossover can be quantitatively related to the parameters of the energy landscape, involving, in particular, the low-energy cutoff of the energy distribution. It is also shown that the low-energy cutoff of the defect-states is determined by the formation energy of a specific defect configuration, involving two silicon and no oxygen defects. The low-temperature behavior of 2D silica is quantitatively compared with that of 3D silica, showing surprisingly similar behavior.

Atomic composition/configuration dependent bulk moduli of Al–C composites

Embedding carbon in metals has long been known to enhance the mechanical properties of metal carbon composites. We report the possibility of growing Al–C composites by the hot isostatic pressing method, with carbon embedded into an Al lattice in graphitic form without the formation of Al4C3. Raman spectroscopy confirms the formation of sp2-hybridized carbon clusters in the aluminum lattice. The bulk moduli of the samples were measured to be between 60 and 100 GPa. From the results of first principles density functional theory calculations, we show that the formation of sp2-hybridized carbon clusters is more stable than having isolated C scatterers in aluminum. Our results show that the extended network of C clusters shows a higher bulk modulus while isolated scattering centers could lower the bulk modulus. We explain this behavior with the analysis of total charge distribution. Localization of charge density decreases materials’ ability to respond to external stress, thus showing a reduced bulk modulus. Some defect configuration may reduce the symmetry while others keep the symmetry of the host configuration even for the same chemical composition of Al–C composites.

Properties of Oxygen Vacancy and Hydrogen Interstitial Defects in Strontium Titanate: DFT + Ud,p Calculations

This work presents extensive theoretical studies focused on the mixed ion-electron transport in cubic strontium titanate (STO). A new approach to the description of this difficult system was developed within the framework of linear-scaling Kohn–Sham density functional theory, as realized in the ONETEP program. The description we present is free of any empirical parameters and relies on the Hubbard U and Hund’s J corrections applied to both Ti and O atoms. The proposed methodology was validated by considering perfect STO. Its calculated properties were found to be in close agreement with experiments and calculations at higher levels of theory. The validated approach was subsequently employed to study the oxygen vacancy (VO) and the hydrogen interstitial (IH), using very large supercells (625 ± 1 atoms). The relaxed configurations of defects were obtained through fastidious energy minimization and later analyzed from a number of perspectives. The calculated defect formation energies and charge transition levels (CTLs) were found to be in close agreement with the experiment. With the exception of the charge-neutral VO, all considered defects were found to introduce shallow states, located down to 0.2 eV below the conduction band. Our calculations revealed a large 1 eV difference in the thermodynamic and optical CTLs of the neutral VO, explaining the inconsistencies observed─till now─between conduction and spectroscopic measurements. The influence of defects on the bonding characteristics and the crystalline structure of STO was quantified, revealing that both VO and IH defects lead to a significant polarization and strong tilting of the TiO6 octahedra.

Influence of Er substitution on the properties of ZnO: A comprehensive study

Er-doped ZnO (Zn(1-x)Er(x)O) films with (002) preferential orientated wurtzite structure were fabricated on a c-plane sapphire substrate by pulsed laser deposition (PLD). The effects of Er composition, oxygen pressure during growth, and post-growth annealing on the films' structural, electrical, and optical properties were studied by employing a comprehensive spectroscopic approach. Er-doped ZnO films with a resistivity of 4 × 10−4 Ωcm, electron conductivity n+∼1021 cm−3, and high optical transmittance (>90%) were disclosed to be appropriate candidates for transparent conducting electrode applications. A shallow donor is reported to be associated with the ErZn defect, which is optically inactive. Er-doped ZnO samples grown with high Er composition and oxygen pressure don't show n+ conductivity because of the formation of an O-rich ErZn-related compensating defect. Annealing at 750 °C converts the ErZn shallow donor to another defect configuration, which is optically active for intra-4f-shell transition emissions, but no longer a shallow donor. The growth condition for maximizing the intra-4f-shell transition emission was also obtained.