Presence Of Point Defects Research Articles

Investigation of the effect of point defects on the Li-ion conductivity of Li3InCl6.

The effect of point defects on the performance of the Li3InCl6 solid electrolyte was investigated using first-principles simulations. The Li-ion conductivity of Li3InCl6 was improved by the presence of point defects, such as a 4g site occupation by In or a Cl vacancy. The diffusion of Li ions was activated by the movement of some Li ions to vacant sites in the In layer, and the point defects reduced the energy barrier of this step, resulting in an overall increase in Li-ion diffusivity. The underlying mechanism of this effect was examined in terms of the structural and chemical properties of Li-Cl and In-Cl bonds, which indicated the energy barrier was closely related to changes in the bond distance and covalent interaction between the migrating Li and nearby Cl ions and nonuniformity among In-Cl bonds. Thermodynamic analysis showed that defect formation was unfavorable during conventional high-temperature synthesis; therefore, it is suggested that heating is applied at low temperatures only as a supplementary process to modify the crystallinity after preparing defective Li3InCl6 crystals by mechanical or solution-based routes to practically implement the effect of point defects and improve the solid electrolyte performance of Li3InCl6.

Role of Chemistry and Crystal Structure on the Electronic Defect States in Cs-Based Halide Perovskites.

The electronic structure of a series perovskites ABX3 (A = Cs; B = Ca, Sr, and Ba; X = F, Cl, Br, and I) in the presence and absence of antisite defect XB were systematically investigated based on density-functional-theory calculations. Both cubic and orthorhombic perovskites were considered. It was observed that for certain perovskite compositions and crystal structure, presence of antisite point defect leads to the formation of electronic defect state(s) within the band gap. We showed that both the type of electronic defect states and their individual energy level location within the bandgap can be predicted based on easily available intrinsic properties of the constituent elements, such as the bond-dissociation energy of the B–X and X–X bond, the X–X covalent bond length, and the atomic size of halide (X) as well as structural characteristic such as B–X–B bond angle. Overall, this work provides a science-based generic principle to design the electronic states within the band structure in Cs-based perovskites in presence of point defects such as antisite defect.

Soliton Scattering on Impurities with Modified Exchange Interactions in Anisotropic Ferromagnetic Chains

The soliton propagation in an anisotropic ferromagnetic chain where the exchange interactions of an impurity spin with its neighbors are modified is investigated. We considered easy-axis anisotropy of the chain which leads to the formation of bright soliton solutions. The character of the soliton-impurity interaction is different when the exchange coupling of the spin is modified in the x,y-plane or in the z-direction. Our results show that the action of the defect depends on the soliton wave number in a complicated manner and for large velocities becomes significant. A comparison with the soliton dynamics in the presence of point defects is made.

Defects in ferroelectric HfO2.

The discovery of ferroelectricity in polycrystalline thin films of doped HfO2 has reignited the expectations of developing competitive ferroelectric non-volatile memory devices. To date, it is widely accepted that the performance of HfO2-based ferroelectric devices during their life cycle is critically dependent on the presence of point defects as well as structural phase polymorphism, which mainly originates from defects either. The purpose of this review article is to overview the impact of defects in ferroelectric HfO2 on its functional properties and the resulting performance of memory devices. Starting from the brief summary of defects in classical perovskite ferroelectrics, we then introduce the known types of point defects in dielectric HfO2 thin films. Further, we discuss main analytical techniques used to characterize the concentration and distribution of defects in doped ferroelectric HfO2 thin films as well as at their interfaces with electrodes. The main part of the review is devoted to the recent experimental studies reporting the impact of defects in ferroelectric HfO2 structures on the performance of different memory devices. We end up with the summary and perspectives of HfO2-based ferroelectric competitive non-volatile memory devices.

Role of threading dislocations and point defects in the performance of GaN-based metal-semiconductor-metal ultraviolet photodetectors

The role of threading dislocations and point defects is investigated by comparing the performance of metal-semiconductor-metal ultraviolet photodetectors (PDs) fabricated on GaN epilayers grown by hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) techniques. It is found that the density of threading dislocations is higher in HVPE GaN epilayers, however, the devices fabricated on them show a higher photo response, lower leakage current and faster transient response when compared to those fabricated on MOVPE GaN epilayers. It is noticed that a high density of threading dislocations of HVPE grown GaN epilayers doesn't always restrict their usefulness in the development of specific devices. On the other hand, an inferior performance of PDs fabricated on MOVPE GaN epilayers is observed despite their low dislocation density, which is explained by considering the presence of point defects. Further, the room temperature electronic transport is found to be dominated by thermionic emission (thermionic field emission) mechanism in devices fabricated on GaN epilayers grown by HVPE (MOVPE) technique. In case of HVPE based devices, a switching of dominant transport mechanism is seen at ~200 K during cooling down whereas no such behaviour is observed for MOVPE based devices. Key factors affecting the performance of ultraviolet PDs fabricated on GaN epilayers grown by the two techniques and associated charge transport mechanisms are discussed. • Crystalline quality of MOVPE GaN templates is found to be better than HVPE GaN. • HVPE GaN based MSM devices turn out to be better than those fabricated on MOVPE GaN. • A high dislocation density of HVPE GaN doesn't always restrict their usefulness. • Fundamental physical reasons for a better performance of MSM devices are discussed. • Transport mechanism switches at ~200 K for HVPE based GaN MSM devices.

A DFT+U study on the thermodynamic properties of defective Gd2Zr2O7 pyrochlore

With many technologies and applications downscaling to nanometer dimensions, single point defects can modify the physical properties of compounds significantly, and advancing the fundamental understanding is critical to defect engineering and control of material properties. In the present study, first-principles calculations based on density functional theory (DFT) are carried out to study how the presence of point defects (vacancy, interstitial and antisite) affects the mechanical and thermal properties of Gd2Zr2O7 pyrochlore. As compared with the pure Gd2Zr2O7, the defective states exhibit smaller elastic moduli and better ductility. The Debye temperatures for the defective Gd2Zr2O7 are generally lower than those for pristine compound, indicating that the defective states have larger thermal expansion coefficient. This study suggests that under irradiation the thermomechanical properties of pyrochlores are influenced significantly, and intentional controlling of the point defects may benefit to improve their mechanical stability.

Oxygen-vacancy induced magnetic phase transitions in multiferroic thin films

Multiferroics in which giant ferroelectric polarization and magnetism coexist are of tremendous potential for engineering disruptive applications in information storage and energy conversion. Yet the functional properties of multiferroics are thought to be affected detrimentally by the presence of point defects, which may be abundant due to the volatile nature of some constituent atoms and the high temperatures involved in the synthesis of materials. Here, we demonstrate with theoretical methods that oxygen vacancies may enhance the functionality of multiferroics by radically changing their magnetic interactions in thin films. Specifically, oxygen vacancies may restore missing magnetic super-exchange interactions in large axial ratio phases, leading to full antiferromagnetic spin ordering, and induce the stabilization of ferrimagnetic states with considerable net magnetizations. Our theoretical study should help to clarify the origins of long-standing controversies in bismuth ferrite and improve the design of technological applications based on multiferroics.

Ordered Ti-Fe-O nanotubes as additive-free anodes for lithium ion batteries

Manufacturing of binder-free mixed oxide electrodes with an unique 1D tubular morphology, high lithium storage ability and long durability represents a real challenge to assemble efficient lithium ion batteries for safe operation. Herein, the successful fabrication of mixed Ti-Fe-O nanotubes (NTs) by a single potentiostatic anodization step of a TiFe12 alloy at room temperature is reported. With the help of suitable characterization tools, the mixed oxide tubes in the different preparation stages were investigated for crystallinity and phase formation including an analysis on the structure defects which influence the material´s performance. Their morphology and element distribution were evaluated in the bulk and on the surface as well as oxidation state changes of the transition metal elements and the solid-electrolyte interphase formation were investigated up to the operando mode. The high anodization voltage along with the substrate composition induced the formation of a thin membrane layer covering the top of Ti-Fe-O NTs. The electrochemical performance of NTs as potential anodes in Li-ion batteries was evaluated vs. Li/Li+ without any binder or conductive additives. The effect of the annealing temperature on crystallinity and Li-ion storage ability of NTs was investigated as well. The Ti-Fe-O NTs electrodes demonstrated an initial discharge capacity of 291 mAh g−1 at a current rate of 0.15C (1C = 335 mA g−1). The annealed crystalline NTs showed a higher reversible capacity of 155 mAh g−1 than the amorphous nanotubes after direct fabrication at room temperature (98 mAh g−1) after 50 charging/discharging cycles with a Coulombic efficiency close to 100% without a provable decomposition of the tubular structure but slight visible changes in their morphology. The noticed increase of the capacity of the Ti-Fe-O NT arrays treated at 600 °C is attributed to the enhanced ionic conductivity originated from short pathways for the Li+ ion transport across the grain boundaries of the crystalline domains. In contrast, the presence of point defects, atomic displacements and planes sliding in the amorphous matrixes surrounding the crystalline entities hinder Li+ ion transport through scattered diffusion pathways, and hence a lower lithium storage is demonstrated.

Is crystal plasticity non-conservative? Lessons from large deformation continuum dislocation theory

Crystal plasticity for large plastic deformations is usually modeled as volume conserving. However, there is experimental evidence that huge amounts of vacancies are produced during plastic deformation. The only regularly considered mechanism of non-conservative dislocation motion is climb of edge dislocations, but this is supposed to be enabled by high vacancy concentrations rather than producing them. In the current contribution we show that large deformation kinematics applied to evolving fields of geometrically necessary dislocations accounts for kinking and jogging of dislocations when cutting through one another. Therefore, dislocation fields in multiple slip situations will not be confined to slip planes and their motion will necessarily involve non-conservative motion of jogs. Because inelastic volume changes in crystals are only possible in the presence of point defects, we find that large deformation continuum dislocation theory must be coupled to an evolving vacancy concentration. We suggest to treat vacancy diffusion as an independent inelastic deformation mechanism yielding a three-way multiplicative decomposition of the deformation gradient. Small numerical examples are used to illustrate the kinematics of emerging and moving kinks and jogs, and the ensuing production of vacancy concentration. Continuum thermodynamics is used to derive coupled evolution equations for geometrically necessary dislocation density vectors and vacancy concentration. We conclude that large deformation crystal plasticity as an averaged theory of dislocations may not be conservative and that the usual kinematic assumption Lp=∑sγ˙sms⊗ns does not hold in multiple slip deformation.

Low energy argon ion irradiation effect on magnetic behavior of polycrystalline Cu substituted ZnO

Zn1-xCuxO (0 ≤ x ≤ 1) polycrystalline samples were prepared by solid state reaction route and then irradiated with 1.2 MeV argon ions at a fluence value of 1.1 × 1016 ions/cm2. The samples were characterized by X-ray diffraction, Raman spectroscopy and VSM measurements. The X-ray diffraction analysis confirms that the structural properties of Zn1-xCuxO films do not change substantially with ion beam irradiation. However, Raman studies clearly show the lattice distortion and the presence of point defects. All the samples exhibit room temperature ferromagnetic behavior. The saturation magnetization value decreases with increasing Cu concentration due to the enhancement of diamagnetic contribution and the improvement of amorphous nature of the samples.

Polarity Inversion of Aluminum Nitride Thin Films by using Si and MgSi Dopants

Polarity is among the critical characteristics that could governs the functionality of piezoelectric materials. In this study, the polarity of aluminum nitride (AlN) thin films was inverted from Al-polar to N-polar by doping Si into AlN in the range of 1–15 at.%. Polarity inversion from Al-polar to N-polar also occurred when MgSi was codoped into AlN with Mg to Si ratio was less than 1. However, the polarity can be reversed from N-polar to Al-polar when the ratio of Mg and Si was greater than 1. The effect of Si and MgSi addition was investigated with regards to their crystal structure, lattice parameters, polarity distribution and the oxidation state of each elements. Furthermore, the effect of intermediate layer as well as the presence of point defect (i.e. aluminum vacancy) were investigated and how these factors influence the polarity of the thin films are discussed in this report.

Electronic Properties of Defective MoS2 Monolayers Subject to Mechanical Deformations: A First‐Principles Approach

Monolayers (MLs) of group‐6 transition‐metal dichalcogenides (TMDs) are semiconducting 2D materials with direct bandgap, showing promising applications in various fields of science and technology, such as nanoelectronics and optoelectronics. These MLs can undergo strong elastic deformations, up to about 10%, without any bond breaking. Moreover, the electronic structure and transport properties, which define the performance of these TMD MLs in nanoelectronic devices, can be strongly affected by the presence of point defects, which are often present in the synthetic samples. Thus, it is important to understand both effects on the electronic properties of such MLs. Herein, the electronic structure and energetic properties of defective MoS2 MLs are investigated as subject to various strains, using density functional theory simulations. The results indicate that strain leads to strong modifications of the defect levels inside the bandgap and their orbital characteristics. Strain also splits the degenerate defect levels up to an amount of 450 meV, proposing novel applications.

Synthesis and properties of SiO2 photonic crystals modified by DNA

Background: Photonic crystals are structures characterized by periodic modulations of the refractive index with a period commensurate with the wavelength. This periodicity is associated with the existence of a complete band gap in the spectrum of the electromagnetic states of the crystal. The stop zone is called the band gap for the highlighted direction in the crystal. Globular photonic crystals are called three-dimensional photonic crystals, which consist of the same diameter globules. The pores between the globules in the opal allow one to change the refractive index and optical contrast of the material. The task of controlling the stop-zone frequency limits of a globular photonic crystal without changing its physical structure is of practical interest. The easiest way to control the stop-zone parameters is to fill the pores of the photonic crystals with materials with different refractive indices, for example, DNA. Control of the optical parameters of a globular photonic crystal can be used for the creation of optical detectors, sensors, test systems, a quantum biocomputer as well as analyzing and studying a conformational state of DNA. Objectives: the creation of SiO2 globular photonic crystals modified by DNA and studying of the influence of DNA on their optical properties. Materials and Methods: Ethyl alcohol, distilled water, ammonium hydroxide, tetraethoxysilane and DNA were used to synthesize SiO2 photonic crystals. Aqueous DNA solution was used to infiltrate the photonic crystals. We used a visible range spectroscopy for optical experiments and a finite-difference time-domain (FDTD) method for numerical calculations. Results: SiO2 globular photonic crystals modified by DNA were synthesized with 195 nm globules. The reflection spectra of the obtained photonic crystals were measured. A red-shift of the stop-zone maximum after the infiltration of photonic crystals with DNA molecules was found. The electric field distribution was calculated for the photonic crystal with 200 nm globules. Conclusions: FDTD calculations in the linear mode show that the presence of point defects in the structure of the photonic crystal influences the amplification of the local electric field in the interglobular space of the photonic crystal, which houses the DNA molecule at infiltration. The DNA infiltration into the pores of a photonic crystal changes the effective refractive index of the system by 5.99%. Synthesis SiO2 photonic crystals with DNA leads to the formation of a more ordered structure at the macro levels. Thus, DNA serves as a template-like structure for photonic crystals to be assembled on. In this case, the effective refractive index of the system increases by 6.01%.

Electronic and transport properties of blue phosphorene in presence of point defects: A first-principles study

Abstract Using the density functional theory (DFT) and non-equilibrium Green's function (NEGF) method, we present a systematic investigation of the structural, magnetic, electronic, and transport properties of pristine and defective blue phosphorene (BlueP). The single vacancy (SV), four double vacancies (DV), and two Stone–Wales (SW) defect types are studied. The BlueP with SV (5|9) defects is a semiconductor with the magnetic moment of 0.997 μB, while DV and SW defects are non-magnetic semiconductors. The SW (55|77)-2 defect has the lowest formation energy among all of other defects under investigation. The current-voltage (I–V) characteristics of the defective BlueP indicate higher current for the both zigzag and armchair directions. The electronic, magnetic, and transport properties of defective BlueP, deviate from those of their pristine counter parts. The results may help future development of nanoelectronic and spintronic devices.

Large influence of vacancies on the elastic constants of cubic epitaxial tantalum nitride layers grown by reactive magnetron sputtering

We report a comprehensive study on the structure and elastic constants of epitaxial cubic δ-TaxN (x ≈ 1) layers deposited by reactive magnetron sputtering at 570 °C on MgO(001), MgO(110) and MgO(111) substrates. X-ray diffraction shows that all layers are single-phase and have a single-crystal rocksalt-structure, growing with an epitaxial cube-on-cube relationship with respect to their MgO substrates. For (001)-orientation, Brillouin light scattering provides the measurement of c44 (137 ± 3 GPa) and c12 (130 ± 5 GPa) single-crystal elastic constants, while picosecond laser ultrasonics (PLU) provides the third independent one, c11 (530 ± 10 GPa). For (110)- and (111)-orientations, PLU provides selectively the following combinations, (c11+c12+2c44)/2 and (c11+2c12+4c44)/3, respectively. Electrical resistivity measurements show evidence of point defects in the films despite a composition close to 1:1 stoichiometry. Point defects (metal and nitrogen vacancies) are considered in our first-principles calculations to find closer agreement between experimental and theoretical lattice parameter, mass density, sound velocities and elastic constants. Sound velocities and elastic constants are found to be particularly sensitive to the presence of vacancies, resulting into a more compliant and less anisotropic material. While only ∼ − 1% mismatch is found between predicted defect-free and experimental lattice parameters, huge differences of −27% and +180% are observed for c11 and c44, respectively. These discrepancies can be rationalized by considering the presence of point defects (metal vacancy and/or Schottky defect) in concentration ∼11% in the as-deposited TaN films, a compositional value that corresponds to the lowest formation energy of δ-TaNx structure.

Micromechanical Model of a Polycrystalline Ferroelectrelastic Material with Consideration of Defects

Constitutive equations for describing the nonlinear behavior of a polycrystalline ferroelectroelastic material are proposed which take into account the dissipative movement of domain walls, the presence of point defects, and their effect on switching processes in the temperature range not accompanied by phase transitions. The method of two-level homogenization is used to describe the behavior of a polycrystalline ferroelectroelastic material at the macro-level. Accounting for defects in the micromechanical model of ferroelectroelastic materials has significantly improved the predictive ability of the model under multiaxial loading. Comparison of the results of computations with experimental data on dielectric hysteresis curves and switching surfaces under nonproportional loading of PZT-4D, PZT-5H, and BaTiO3 polycrystalline piezoelectric ceramics shows that the proposed model has good prediction accuracy.

An Approach to Optimize the Thermoelectric Properties of III-V Ternary InGaSb Crystals by Defect Engineering via Point Defects and Microscale Compositional Segregations.

Thermoelectric power generators require semiconductor materials with controlled phonon and free charge carrier transport properties. This could be achieved by changing their molecular and lattice dynamics through introducing/controlling structural imperfections (defects engineering). The structural imperfections such as point defects and compositional segregations in a multicomponent alloy are observed experimentally, and their impact on electron and phonon transport properties was explained. The thermoelectric properties of a III-V ternary alloy InGaSb was improved by the presence of point defects and compositional segregations. The compositions were segregated randomly, and they had a major impact on the phonon contribution to the thermal conductivity. The point defects affected electrical resistivity, and the Seebeck coefficient was influenced by carrier concentration. The figure of merit (ZT) of In0.95Ga0.05Sb is enhanced to 0.62 at 573 K, and it is the highest among any other reported values of binary/ternary III-V semiconductor alloys. The enhancement in the ZT of InGaSb from the viewpoints of point defects and compositional segregations are explained. This experimental defect engineering study could be helpful to understand and improve the thermoelectric properties of many other crystalline materials.

Geometric scattering of a scalar particle moving on a curved surface in the presence of point defects

A nonrelativistic scalar particle that is constrained to move on an asymptotically flat curved surface undergoes a geometric scattering that is sensitive to the mean and Gaussian curvatures of the surface. A careful study of possible realizations of this phenomenon in typical condensed matter systems requires dealing with the presence of defects. We examine the effect of delta-function point defects residing on a curved surface S. In particular, we solve the scattering problem for a multi-delta-function potential in plane, which requires a proper regularization of divergent terms entering its scattering amplitude, and include the effects of nontrivial geometry of S by treating it as a perturbation of the plane. This allows us to obtain analytic expressions for the geometric scattering amplitude for a surface consisting of one or more Gaussian bumps. In general the presence of the delta-function defects enhances the geometric scattering effects.

Short-range order formation in Fe-Co alloys: NMR study and first-principles calculations

The short/long-range order formation in Fe1-x-Cox (x < 0.3) alloys has been studied by the nuclear magnetic resonance (NMR) technique, as well as ab initio based atomistic simulation. The NMR measurements show up the formation of a certain short-range order (SRO) in dilute limit above the Curie temperature TC and of the D03-type SRO (with the dominance of 3rd Co-Co nearest neighbors) in the concentration range 0.2 < x < 0.3 after quenching and subsequent annealing in the ferromagnetic state. The results of Monte Carlo simulations of binary Fe-Co alloys with ab initio interatomic interactions predict SRO in agreement with the experiment for small concentrations of Co (CCo< 0.1), while the B2-type ordering is preferable in binary alloy in the ferromagnetic state. We demonstrate that the presence of point defects (vacancies, interstitial) can change essentially the ordering in alloys with the Co content 20-30% and result in the D03-type SRO formation in a reasonable agreement with the experiment.

Vibrational Signatures of Carboxylated Graphene: A First-Principles Study

Raman and infrared (IR) spectroscopies are fast, efficient, and nondestructive techniques commonly used for structural characterization of nanoscale materials. The complimentary characters of the Raman and IR spectroscopies make them ideally suited for experimental studies of carbon nanostructures. Using first-principles computational methods based on density functional perturbation theory, we computed nonresonant Raman and IR absorption spectra of carboxylated graphene containing no surface defects, containing Stone–Wales defects, and containing divacancies. Our calculations demonstrated that the presence of point defects near the functionalization sites significantly altered the Raman and IR spectra of carboxylated graphene. In all cases, we observed the emergence of new Raman and IR absorption bands in the range of low and high vibration frequencies. The calculated Raman and IR spectra showed clearly distinguishable spectroscopic signatures associated to different types of structural defects present in...