Fraction Of Vacancies Research Articles

Improved cation valence state in molybdenum oxides by ultraviolet-ozone treatments and its applications in organic light-emitting diodes

The authors demonstrate a thick MoO3 layer (60 nm) as a good short reduction layer in organic light emitting diodes (OLEDs), which is especially useful for large-area and flexible OLEDs to prevent short circuit issues. The crystallization of organic material and the increase of driving voltage induced by a thick MoO3 layer in OLEDs were resolved by a simple ultraviolet-ozone (UV-ozone) treatment. Ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscope analyses revealed that a longer UV-ozone treatment resulted in an optimized fraction of oxygen vacancies in MoO3, which is responsible for the improved device performance.

Heat treated electrolytic manganese dioxide for primary Li/MnO2 batteries: Effect of manganese dioxide properties on electrochemical performance

The primary capacity of heat treated manganese dioxide in Li/MnO2 batteries is directly related to the numerous physical properties which characterize this material. A statistical model is employed to isolate the influence of material structure, composition and morphology on the electrochemical performance at the discharge rates 2, 5, 10 and 20mAg−1. Among the most influential parameters are the pyrolusite content, Mn(IV) percentage, cation vacancy fraction and surface area. Some of the materials investigated show higher specific capacities than literature materials, with further improvement to the performance of these materials anticipated through the intelligent selection of heat treated manganese dioxide which exhibit the ideal properties outlined in this study.

Molecular dynamics simulations of radiation damage in D019 Ti3Al intermetallic compound

Molecular dynamics (MD) has been applied to simulate the radiation damage created in displacement cascades in D019 Ti3Al structural intermetallics. Collision cascades formed by the recoil of either Al or Ti primary knock-on atoms (PKA) with energy EPKA=5, 10, 15 or 20keV were considered in Ti3Al single crystals at T=100, 300, 600 and 900K. At least 24 different cascades for each (EPKA, T, PKA type) set were simulated. A comprehensive treatment of the modelling results has been carried out. We have evaluated the number of Frenkel pairs, fraction of Al and Ti vacancies, self-interstitial atoms and anti-sites as a function of (EPKA,T, PKA type). Preferred formation of both Al vacancies and self-interstitial atoms in D019 Ti3Al exposed to irradiation has been detected.

Self- and Solute Diffusion, Interdiffusion and Thermal Vacancies in the System Iron-Aluminium

Starting from fundamental aspects of thermal vacancies and solid-state self-and solute diffusion, this paper reviews procedures for tracer-and interdiffusion studies and of the major techniques for vacancy studies by dilatometry and positron annihilation in metals. Equilibrium vacancy and diffusion studies performed on pure iron and aluminium are mentioned at first. We also comment some peculiarities of solute diffusion in aluminium. Positron annihilation and differential dilatometry studies for Fe-Al alloys with various compositions are summarized and new experimental studies by the authors are reported for vacancy migration in Fe61Al39. All these studies indicate a relatively high fraction of thermal vacancies with relatively low mobility in this type of iron-aluminides as compared to pure metals. Tracer diffusion of iron and of several substitutional solutes such as Co, Ni, Cr, Mn, Zn, and In in Fe-Al from the Münster laboratory are summarized. The diffusion studies of Fe-Al cover various alloy composition between Fe3Al and FeAl and several structures such as A2, B2 and D03. Interdiffusion coefficients obtained from diffusion couples between Fe-Al alloys are discussed together with Fe tracer diffusion data. The Darken-Manning equation is used to deduce Al diffusivities therefrom. The latter are hardly accessible to radiotracer experiments due to a lack of a suitable Al tracer. Diffusion of Al is slightly faster than diffusion of Fe indicating diffusion mechanisms with coupled jumps of Fe and Al atoms.

MD study of primary damage in L10 TiAl structural intermetallics

Computer modelling by molecular dynamics has been applied to study the radiation damage created in collision cascades in L10 TiAl intermetallic compound. Either Al or Ti primary knock-on atoms (PKA) with energy 5keV⩽EPKA⩽20keV were introduced in the intermetallic crystals at temperatures ranging from 100K to 900K. At least 24 different cascade for each (EPKA, T, PKA type) set were modelled in order to simulate a random spatial and temporal distribution of PKAs and provide statistical reliability of the results. The total yield of more than 760 simulated cascades is the largest yet reported for this binary intermetallic material. A comprehensive treatment of the modelling results has been carried out. The number of Frenkel pairs, fraction of Al and Ti vacancies, self-interstitial atoms and anti-sites as a function of (EPKA, T, PKA type) has been established. Preferred formation of Al self-interstitial atoms has been detected in L10 TiAl structural intermetallics exposed to irradiation.

Colloidal crystals full of invisible vacancies

All solids at finite temperatures contain point defects such as vacancies and interstitials. Even at very low concentrations, such defects have a pronounced effect on the transport properties (e.g., electronic) of otherwise pure crystals. However, the effect of point defects on the thermodynamic properties of crystals is small, simply because the mole-fraction of defects is low—at least, in one-component crystals. In fact, in most simple metallic and ionic crystals, but also in colloidal hard-sphere crystals, the vacancy fraction (i.e., the fraction of empty lattice sites) close to the melting transition is typically substantially less than one part in a thousand. The article by Smallenburg et al. (1) in PNAS presents an astounding simulation result: some colloidal crystals may have vacancy concentrations that are almost three orders of magnitude higher than what has been observed before in simple, one-component crystals.

Ferromagnetism induced by oxygen-vacancy complex in (Mn, in) codoped ZnO

Mn doped Zinc oxide (ZnO) thin films were prepared by metal organic chemical vapor deposition (MOCVD) technique. Structural characterizations by X-ray diffraction technique (XRD) and photoluminescence (PL) indicate the crystal quality of ZnO films. PL and Raman show a large fraction of oxygen vacancies (VO2+) are generated by vacuum annealed the film. The enhancement of ferromagnetism in post-annealed (Mn, In) codoped ZnO could result from VO2+ incorporation. The effect of VO2+ on the magnetic properties of (Mn, In) codoped ZnO has been studied by first-principles calculations. It is found that only In donor cannot induce ferromagnetism (FM) in Mn-doped ZnO. Besides, the presence of VO2+ makes the Mn empty 3d-t2g minority state broadened, and a t2g-VO2+ hybrid level at the conduction band minimum forms. The presence of VO2+ can lead to strong ferromagnetic coupling with the nearest neighboring Mn cation by BMP model based on defects reveal that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Mn 3d electrons trapped in oxygen vacancies.

Molecular dynamics simulation on mechanical properties of crystalline CoSb3 with vacancy defect

The present work has investigated the tensile mechanical behavior of the skutterudite CoSb3 single-crystal in the presence of antimony vacancies, since the antimony atoms in CoSb3 are active and are usually easy to lose in practice. The molecular dynamics simulation method is employed. The vacancy atoms, whose fraction is limited up to 5%, are chosen randomly. The virtual uniaxial tension is carried out by strain controlling along a principal crystallographic direction at 300K. The specimens with vacancies show similar stress–strain response features to there of the perfect crystal. However, the effective Young's modulus decreases linearly with the increase of the vacancy content, and the ultimate strength drops substantially from no vacancy to even a small vacancy fraction. Temperature dependence of the simulation results is also considered. Both Young's modulus and the ultimate strength exhibit an approximately linear reduction with increasing temperature for a specific vacancy fraction, and moreover, the reduction rate is comparable for different vacancy fractions. The Vacancy distribution effect is briefly discussed as well. As the vacancy concentration becomes uniform, the ultimate strength of the material would be promoted significantly.

CeO2nanoparticles synthesized by a microwave-assisted hydrothermal method: evolution from nanospheres to nanorods

Ceria (CeO2) plays a vital role in emerging technologies for environmental and energy-related applications. The catalytic efficiency of ceria nanoparticles depends on its morphology. In this study, CeO2 nanoparticles were synthesized by a microwave-assisted hydrothermal method under different synthesis temperatures. The samples were characterized by X-ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, electron paramagnetic resonance spectroscopy and by the Brunauer–Emmett–Teller method. The X-ray diffraction and Raman scattering results indicated that all the synthesized samples had a pure cubic CeO2 structure. Rietveld analysis and Raman scattering also revealed the presence of structural defects due to an associated reduction in the valence of the Ce4+ ions to Ce3+ ions caused by an increasing molar fraction of oxygen vacancies. The morphology of the samples was controlled by varying the synthesis temperature. The TEM images show that samples synthesized at 80 °C consisted of spherical particles of about 5 nm, while those synthesized at 120 °C presented a mix of spherical and rod-like nanoparticles and the sample synthesized at 160 °C consisted of nanorods with 10 nm average diameter and 70 nm length. The microwave-assisted method proved to be highly efficient for the synthesis of CeO2 nanoparticles with different morphologies.

EFFECT OF NANO PARTICLE SIZES ON HIGH PRESSURE RAMAN SCATTERING IN NANOCRYSTALLINE CERIUM DIOXIDE

Nanocrystalline CeO 2 with different particle sizes has been studied under high pressure using Raman spectroscopy techniques and diamond anvil cell at room temperature. The pressure shift of the first-order Raman frequency for each particle sizes was measured. Linear dependence of the first order Raman frequency on pressure for each particle sizes has been observed. We found that the first order Raman frequency decreases with the decreasing particle sizes under ambient condition and the lattice constant increases with the decreasing particle size. The increasing molar fraction of oxygen vacancies with the decreasing particle size is responsible for the lattice expansion.

Heat Treated Electrolytic Manganese Dioxide for Li/MnO2 Batteries: Effect of Precursor Properties

The present study demonstrates that the properties of heat-treated electrolytic manganese dioxide (EMD) materials can be linked to the key properties of the precursor EMD used. The investigation holds particular significance for the informed selection of precursor EMD materials that result in superior performing heat treated materials in Li/MnO2 batteries. Kinetic analysis is used to determine the precise heating regime necessary to, theoretically, completely remove structural water from six different EMD samples. It was found that the oxidation of Mn(III) to Mn(IV) occurred to a greater extent for samples with initially high pyrolusite content and a low fraction of cation vacancies after heat treatment at low temperatures. This same Mn(III) to Mn(IV) conversion was also enhanced after high temperature treatment for those samples with a higher manganese content and low cation vacancy fraction. Additionally, at low heat treatment temperatures the parent γ-MnO2 structure and cation vacancies were better retained in samples with low starting BET surface area. Finally, this work determined that samples with high BET surface area tend to lose proportionally less of this value as a consequence of heat treatment, compared to those with low initial values.

Manganese dioxide structural effects on its thermal decomposition

In this work, the influence of the structure, composition and morphology on the amount of mass lost from γ-MnO2 during heat treatment, and the activation energy for this process (from iso-conversional kinetic analysis), was determined using a statistical approach. It was found that the differential thermogravimetric (DTG) data for electrolytic manganese dioxide (EMD) samples typically show two regions of mass loss; water bound to surface sites and structural water at or near the surface removed between ∼120 and 200 °C, and bulk hydroxyl groups lost in the range ∼200–400 °C. The composition and morphology had the greatest effect on the amount of mass lost; namely, BET surface area and percentage Mn(III) for the first process (r2 = 0.92) and percentage Mn(IV), percentage Mn(III) and cation vacancy fraction for the second (r2 = 0.89). The activation energy for the first processes was most affected by BET surface area, c0 and crystallite size in the (2 2 1) direction (r2 = 0.89). Relatively poor correlations were found between the amount of water lost and the activation energy for the second process and the material properties measured in this work. This likely relates to the slow kinetics of the first mass loss which complicates the relationships existing for the second.

Modeling of excess vacancy annihilation at different types of sinks

The equilibrium site fraction of vacancies increases with temperature and, thus, annealing and rapid quenching may lead to states with a significant vacancy supersaturation. Excess vacancies can then gradually annihilate at available sinks represented by jogs at dislocations, by grain boundaries or free surfaces. Significant supersaturation by vacancies may also lead to the nucleation and growth of Frank loops acting as additional sinks. Three models corresponding to three different annihilation mechanisms are developed in this paper. They refer to annihilation of excess vacancies at jogs at dislocation with a constant density, at homogeneously distributed Frank loops with a constant density and at grain boundaries. The simulations based on the models are performed for individual annihilation mechanisms under isothermal and non-isothermal conditions as well as for simultaneous annihilation of vacancies at Frank loops and dislocation jogs and grain boundaries using different cooling conditions.

Facile Electrochemical Synthesis of Hexagonal Cu2O Nanotube Arrays and Their Application

Large-scale and highly oriented single-crystalline hexagonal Cu(2)O nanotube arrays have been successfully synthesized using a two-step solution approach, which involves the electrodeposition of oriented Cu(2)O nanorods and a subsequent dissolution technique along the c axis to form a tubular structure. Herein, NH(4)Cl was found to be an effectual additive, and it can successfully realize the dissolution process of Cu(2)O from nanorods to nanotubes. The dissolution mechanism of Cu(2)O from nanorods to nanotubes was illustrated in detail. These prepared Cu(2)O nanotube arrays were characterized by SEM, EDS, XRD, XPS, and TEM. The photoluminescence (PL) spectrum of Cu(2)O nanotube arrays was also measured, and it shows there is a greater fraction of copper or oxygen vacancies in these prepared Cu(2)O nanotubes. Finally, the applications of Cu(2)O nanotube arrays for gas sensors were investigated in this paper.

A reactive force-field (ReaxFF) Monte Carlo study of surface enrichment and step structure on yttria-stabilized zirconia

To investigate surface segregation in yttria-stabilized zirconia (YSZ), DFT energies describing surface energy as a function of yttrium lattice position were used to parameterize a reactive-force field (ReaxFF). We used ReaxFF to perform Monte Carlo (MC) simulated annealing to sample structural configurations of flat YSZ (111) and vicinal YSZ (111) stepped surfaces. We evaluated yttrium surface segregation, oxygen vacancy position, and surface step composition for flat and stepped YSZ surfaces. It is thermodynamically favorable for yttrium atoms to segregate to the surface of YSZ, and specifically to step edge sites. Surface saturation of yttrium occurs at approximately 40% (40:60 Y:Zr ratio) while yttrium concentration at the step edge does not approach a saturation value, suggesting that steps on the YSZ surface are mainly yttria-terminated. We found that it is thermodynamically favorable for oxygen vacancies to occupy positions in the subsurface layer of YSZ, and a higher fraction of vacancies occupy positions NN to Y than NN to Zr. Yttrium segregation to step edges on the YSZ surface does not lower the surface formation energy of the stepped surface below that of the flat (111) termination, suggesting that the stability of YSZ surface steps observed experimentally is due to kinetic barriers for surface re-ordering.

Analysis of Ga coordination environment in novel spinel zinc gallium oxy-nitride photocatalysts

Novel zinc gallium oxy-nitrides, with the spinel structure and visible light band gaps, have been synthesized by nitridation of zinc gallate produced by sol–gel synthesis. These spinel oxy-nitrides have band gaps of 2.5 to 2.7 eV, surface areas of 16 to 36 m2 g−1, and nitrogen content less than 1.5%. They degrade methylene blue dye in visible light. The reduction in band gap is associated with the incorporation of nitrogen in the zinc gallate structure and corresponding changes in the anion position parameter as well as the presence of a small fraction of gallium tetrahedral centers and anion vacancies. While spinel oxy-nitrides are produced under nitridation at 550 °C, at higher temperatures they are consumed to form wurzitic oxy-nitrides. The wurtzite materials also have band gaps less than 3 eV but their surface areas are 2 to 5 m2 g−1. The changes associated with the gallium coordination as the spinel zinc gallate precursor transforms into the spinel oxy-nitride at 550 °C, and further changes into the wurtzite oxy-nitride at 850 °C are studied through X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, neutron powder diffraction, X-ray absorption spectroscopy and other techniques. The protocol developed in this report opens an avenue for the synthesis of semiconductors having the spinel crystal structure and band gaps engineered to the visible region with potential applications for opto-electronic devices and photocatalytic processes.

Change in the vacancy concentration in a migrating grain boundary

The previously developed model of migration of incommensurate tilt boundaries is applied to obtain a relationship for the average fraction of grain-boundary vacancies, which is higher than the volume fraction. The concentration of vacancies near a boundary is shown to increase linearly with its migration velocity.

Microscopic observation of vacancy and self-interstitial and the formation of Frenkel pairs in InSb by Mössbauer spectroscopy

Abstract The process of Frenkel pair formation in n-InSb is investigated by application of Mossbauer spectroscopy with correlated Frenkel pair production utilizing the Mossbauer probe 119Sb/119Sn as primary knock-on atom (PKA). This is achieved by starting the Mossbauer decay chain on the precursor 119gTe which feeds the “proper” standard probe 119Sb/119Sn and in this process emits a high energy neutrino which leads to 12 eV monoenergetic recoil energy exerted on the Mossbauer probe. Since the defect production threshold in InSb is about 8 eV, single Frenkel pairs consisting of one vacancy and one self-interstitial can be expected. This simplest possible defect creation mechanism indeed enables identification of two single vacancy-probe configurations and the probe as self-interstitial thereby elucidating the mechanism of defect formation itself. The relative intensities of the defect fractions corrected for the radioactive decay show that about 55–60% of the Mossbauer probes are associated with a defect. The probability of producing a probe associated vacancy does not depend on temperature between 4 and 300 K. First experiments searching for a doping dependence indicate that in p-type material the probe associated vacancy fractions are not formed.

Diffusion jumps of single atoms into vacancies in an iron monolayer studied by nuclear resonant scattering

We study the structure of a model surface, i.e., an Fe monolayer on W(110) by nuclear resonant scattering of x rays. We conclude that at room temperature the structure is nearly perfect, whereas when increasing the temperature up to 770 K an increasing fraction of the Fe atoms feels the appearance of defects manifested by a beat structure due to an electric field gradient. The field gradient is caused by an asymmetry of the neighborhood of part of the atoms and attributable to an increasing fraction of vacancies in the monolayer. With increasing temperature the beat structure gets smeared due to the onset of dynamics in the surface, i.e., the motion of the defects and consequently of the atoms. From the temperature dependence of the amplitude of the beat we deduce the vacancy concentration, thus the vacancy formation energy [0.17(5) eV] and from the broadening with less accuracy we deduce the vacancy migration energy [0.16(6) eV] resulting in 0.33(11) eV for the activation energy of diffusion.

Chemical composition and electrochemical activity of some chemically synthesized γ-MnO2

A series of γ-MnO2 with Mn4+ cation vacancy fraction 1.5 < x < 2.3 was prepared via chemical synthesis. X-ray diffraction (XRD) revealed high degree of microtwinning (T w > 70%) and de Wolff disorder (0.32 < P r < 0.35) in the prepared samples. Thermal analysis showed four-stage decomposition corresponding to the removal of physical water, chemisorbed water, formation of Mn2O3 and Mn3O4, respectively. The chemical composition and physical properties of the prepared samples were expressed in terms of Ruetschi’s cation vacancy model for MnO2. The discharge behavior in 9 M KOH was governed by homogeneous solid-state reduction wherein protons and electrons were simultaneously inserted into MnO2 lattice. Their electrochemical behavior was also studied by performing cyclic voltammetry in 9 M KOH. An attempt was made to correlate obtained energy density as a function of proton transfer rate during the discharge process.