Position Of Vacancy Research Articles

A composite model of memristors based on barrier and dopant drift mechanisms

This paper presents a hybrid model for TiO2-based memristors, integrating the dopant drift mechanism with the Schottky barrier theory. We introduce the movement of oxygen vacancies as a dynamic variable to modulate changes in memristors. Furthermore, the variation of the dominate mechanism of the TiO2 memristors under different operating conditions is studied, which is related to the position of the internal oxygen vacancy. The proposed model accurately captures the rectification linearity, and effectively elucidates the dominant current mechanisms manifested in six distinct regions of the I-V curves. Our model exhibits better predication with reduced errors when applied to Pt/TiO2/Pt memristors. The proposed model can well describe the dual-mechanism memristor phenomenon, and provides a reference for the subsequent study of multi-mechanism behavior in memristors.

Study of Formation and Properties of F4TNCQ/MoS2 Heterostructures with point defects driven by DFT and Neural Network Potential

State-of-the-art computational techniques allow us to perform the comprehensive investigation of organic/inorganic heterostructures based on F4TCNQ molecule and MoS2 monolayer. The features of electronic and optical properties with the presence of point defects at MoS2 surface were studied. We found that halogen impurities lead to strengthening of organic/inorganic interface. Based on variation of the binding energy between F4TCNQ and MoS2 surface the neural network potential was trained with the aim to study the possibility of formation of monomolecular layer. Molecular dynamics calculation shows the anchoring behavior of F4TCNQ molecule in a position of sulfur vacancy with the same binding energy as in case of halogen doping. Present results show advantages of defect creation in MoS2 surface with the aim to control the ordering of monomolecular layer and the properties of the whole heterostructure. We show the first fundamental step of using organic/inorganic heterostructures for application in optoelectronic sensors and detectors.

Effects of framework dopants on luminescent properties of red nitride phosphors via band structure modelling

Currently, researchers have been able to manipulate the luminescent properties and thermal stability of nitride red phosphor Sr2Si5N8:Eu2+ through rare earth doping. However, there is limited research on the doping of its framework. This work investigated the crystal structure, band structure, and luminescence properties of Sr2Si5N8: Eu2+ and its framework doped systems. We selected B, C, and O as doping elements because the corresponding formation energies of these elements doped structures are relatively low. Then, we calculated the band structures of various doped systems in both the ground and excited states. This analysis aimed to investigate their luminescent properties using the configuration coordinate diagram. The results show that doping with B, C, or O has minimal effect on the emission peak width. The thermal quenching resistance of the B- or C-doped system was enhanced due to the increased energy differences between the 5d energy level of the electron-filled state in the excited state and the bottom of the conduction band, compared to the undoped system. However, the thermal quenching resistance of the O-doped system varies depending on the position of the silicon vacancy. The work indicates that framework doping can also improve the thermal quenching resistance of phosphors besides rare earth ions doping.

Engineering oxygen vacancy to accelerate proton conduction in Y-doped BaZrO3

Proton conducting oxides have drawn great interest as electrolytes for proton-conducting reversible solid oxide cells (P-RSOCs), but suffered from the inferior ionic conductivity. To accelerate proton conduction, oxygen vacancy engineering via calcium-doping is proposed and validated, which generates more oxygen vacancies to increase proton concentration and importantly tailors the position of oxygen vacancy to accelerate proton diffusion. TG and EIS results show that calcium-doped BaZr0.8Y0.2O3-δ (BZY2), BaZr0.8Ca0.1Y0.2O3-δ (BZCa1Y2), owns higher proton concentration, and demonstrates ionic conductivity of 0.008 S cm−2 at 700 °C, 2.7 times higher than BZY2. The lower activation energy of conductivity in BZCa1Y2 confirms the faster proton conduction behavior. DFT calculation concludes that oxygen vacancies prefer to cluster with Ca site and proton diffusion barrier is decreased most when vacancy is tailored to generate nearing Ca. When considering the concentration of proton and oxygen vacancy, proton diffusion coefficient obtained from Ab-initio Molecular Dynamics simulation is 1.1×10−5 cm2s−1 for BZCa1Y2 in 200 °C, larger than BZY2 (9.0×10−6 cm2s−1) and verifying the accelerated proton diffusion due to the tailored oxygen vacancy. The oxygen vacancy engineering provides a further understanding of proton diffusion in proton conducting oxides and a new promising opportunity to improve conductivity.

Alkali-assisted engineering of ultrathin graphite phase carbon nitride nanosheets with carbon vacancy and cyano group for significantly promoting photocatalytic hydrogen peroxide generation under visible light: Fast electron transfer channel

Exfoliating bulk graphite phase carbon nitride (g-C3N4) into 2D nanosheets is considered to be an effective method to enhance its photocatalytic activity. However, optical absorption capacity of the exfoliated g-C3N4 nanosheets are lower than that of the original bulk g-C3N4 due to the quantum size effect. Here, the ultrathin graphite phase carbon nitride nanosheets containing both carbon vacancy and cyano group (UCNS580) were prepared by two-step calcination in air with the assistance of KOH. The formation and position of carbon vacancy and cyano group were first investigated and determined. The simultaneous introduction of carbon vacancy and cyano group not only improved light absorption range and intensity of g-C3N4 nanosheets, but also more importantly constructed a fast transfer channel for photogenerated electrons, further enhancing the separation efficiency and migration ability of photogenerated carriers. The cyano group as the accumulation center of photogenerated electrons and the oxygen adsorption center increased the proportion of one-step two-electrons reaction path to efficiently generate H2O2. As a result, UCNS580 exhibited highly boosted H2O2 generation activity, its H2O2 production yield for 6 h reached 939 µmol/L and the formation rate was up to 4167 µM h−1 g−1, which was in priority in the reported literature under the same conditions.

Dynamic analysis of boron nitride nanotube using different boundary conditions under influence of vacancy defect: Insights from finite element method

In this paper, the effect of defects such as atomic vacancies (single atom vacancies like boron atom(Vb) or nitrogen atom vacancy(Vn) and di-atomic vacancies(VB-N) corresponds to vacancy of one boron and adjacent one nitrogen atom) on the vibrational behaviour of a single-walled boron nitride nanotube (SWBNNT) are investigated using finite element modelling in context of their applicability as mass sensors. The change in resonance frequency produced by vacancy defects at various points throughout the span when a mass is connected to the tip has also been examined. Space frames with three-dimensional components and point masses have been used to explain the cantilevered armchair (5, 5) and zigzag (10, 0) SW-BNNT with changing vacancy defects. The position of atomic vacancy has been varied (i.e., at 25 %, 50 % and 90 % of length from fixed end) to investigate the effect of location of defect along the length on the resonance behaviour of SW-BNNT. Further defective BNNTs have been analysed in such a way that the same can be used as nanomechanical resonator for detecting small mass of the order of femtogram level. For this,a small mass (varying from 10-20 to 10−25kg) is attached at the tip of cantilevered SW-BNNT. Substantial drop in frequency shift is observed when the position of defect moves toward the free end. the excitation frequency of defective BNNT is larger near the free end as compared to pristine one.

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+.

First-Principles Studies of the Surface and Interface between/Among Metal Oxides Used in SOFCs

On the surface and interface, all materials change the property such as stability and electronic states from them on bulk. Human being continue to apply the phenomena to the key technologies from early times. For example, in solid oxide fuel cells (SOFCs), important phenomena such as fuel decomposition, charge transfer etc., occur at surface or triple phase boundary (TPB) that consists of catalyst, electrolyte, and gas phase. To solve remaining issues of SOFCs such as degradation or efficiency improvement, important is to know the detail of property changes of materials used in SOFCs on the surface and interface. Theoretical study is efficient approach to investigate material property, especially electronic states. Therefore, in this study, we have focused on surface and interface of metal oxides used as electrolytes in SOFCs and investigated theoretically their property using two calculation models with first-principles calculation. In concrete, one is density functional theory (DFT) calculations with the TPB model that consists of Ni catalyst cluster, YSZ electrolyte, and gas phase. In this case, we have tried to decide the reliable structure to discuss properties on TPB and evaluated adsorption reactivity etc. The other is DFT calculations with the hetero-structure interface model that consists of zirconia (ZrO2) and ceria (CeO2). In this case, we have tried to evaluated electronic states on that interface and discussed in terms of conductivity control. Here, only the results of optimized YSZ (111) surface model and density of state (DOS) change in ZrO2/CeO2 hetero-structure are shown as examples for word limitation. The CASTEP software was used for all calculations.Geometry optimization of YSZ (111) surface model have been performed with some arrangements of yttrium (Y) atoms and oxygen vacancy in order to identify the most stable structure. Our model consists of (111) three layers includes 2×2 or 2×4 ZrO2 unit cells, 15 Å vacuum layer, and 9.1 % substituted Y atoms. In the most stable YSZ 2×2 model, one Y atom is on the top Zr atoms layer and another Y atom is on the middle Zr atoms layer nearest to the top Y atom, and oxygen vacancy is on the 2nd O atoms layer nearest to the top Y atom. We have found the position of oxygen vacancy against Y atoms and surface is sensitive to the YSZ surface stability. Oxygen vacancy does not prefer to stick around two Y atoms. It indicates that improvement for the geometry unstability of cubic ZrO2 is more important than keeping local electroneutrality even near on the surface. By applying this finding, we have performed geometry optimization of YSZ 2×4 unit cell model with changing Y atom arrangements in the same layer and keeping the stable structure in 2×2 unit cell model. The most stable YSZ 2×4 surface model finally-obtained is shown in attached figure (A).Our 2nd topic is for the electronic states change on the interface of metal oxides. We have calculated DOS with pure ZrO2, pure CeO2, and several ZrO2/CeO2 models hetero-connected with (111) surfaces. DOS changes by hetero-connection are shown in attached figure (B). A new small peak not appeared in the cases of pure ZrO2 and pure CeO2 is observed in 01 eV near Fermi level. It is considered that the peak is caused by the interaction between 4f orbital shown in 13 eV of Ce atoms and 2p orbital shown in -50 eV of O atoms in ZrO2 on the interface. In addition, the peak height is influenced only by the thickness of CeO2 layers, not that of ZrO2 layers. Our results suggest that we can control electronic conductivity of these metal oxides with hetero-structuration in atomic level especially at low temperature. Other results and detailed discussion will be reported at our meeting publications (ECS Transactions) and presentation in this conference. Figure 1

Theoretical study on near UV and visible optical absorption characteristics of Ti-doped α-Al2O3 single crystals

A first principles method is used to study the origin of the near-ultraviolet (300∼400 nm) and visible absorption band (400∼600 nm) in Ti-doped α-Al2O3 crystals. The electronic and optical properties of Al2O3 supercells containing substitutional Ti3+, interstitial Ti3+ and various Ti ion pairs were calculated. The results show that the absorption peaks at 388 (reported) and 325 nm (unreported) are both caused by the charge transfer transition from the substitutional Ti3+ 3d orbital to the adjacent Al3+ 3s3p orbital. When interstitial Ti3+ and Al vacancy coexist, the interstitial Ti3+ will relax to the position of the adjacent Al vacancy, forming a defect like substitutional Ti3+. The line-contact Ti3+-Ti3+, face-contact Ti3+-Ti3+ and point-contact Ti4+-Ti3+ ion pairs are the main causes for the multi-peak configuration and small bumps in the visible absorption band. Moreover, two other possible causes for this phenomenon are supplied from the perspective of ligand field theory and thermal activation.

Stability and interaction of cation Frenkel pair in wurtzite semiconductor materials

The understanding of defects is an important issue in the development of semiconductor materials. Using ab initio calculations and ab initio molecular dynamics, the stability and interaction of cation Frenkel pairs were investigated in four wurtzite semiconductor materials, GaN, AlN, InN and ZnO. With regard to GaN material, an interesting phenomenon was found that the recombination path of Ga Frenkel pair is a kick-out mechanism which is similar to the migration of Ga interstitial. Based on the obtained path, the recombination energy barriers of different positions of Frenkel pairs were calculated respectively. The calculation results demonstrated that the recombination behavior of Ga Frenkel pair is not only determined by the distance but also the relative position of vacancy and interstitial. Meanwhile, the value of the projected interaction radius was observed to be about 7.5 Å. Compared with the small interaction range of N Frenkel pair, Ga Frenkel pair was found to be unstable, which is consistent with previous radiation experiments. Furthermore, the same phenomenon in GaN material was also found in the other three wurtzite materials and the projected interaction radius of wurtzite materials was concluded as 7/3 lattice parameter. In general, this theoretical study provides an insight of the stability of cation Frenkel pair and improves our understanding of defects in wurtzite semiconductors.

Effect of the single vacancy in particle pile on heat transfer performance of particle pile

This paper reports the effect of a single vacancy on heat transfer of particle pile. A three-dimensional steady-state heat transfer model of particle pile with a single vacancy was established. The effects on the temperature distribution, heat transfer mode and heat flux characteristics of the particle pile were studied. In the area close to the vacancy, the particle temperature distribution is significantly affected, and the average temperature line is significantly deformed. The heat conduction between particles along the Y-direction is the dominant heat transfer mode around the vacancy. Compared with a particle pile without vacancy, the total heat flux at the hot wall decreases by 0.63% and the apparent thermal resistance of the particle pile increases by 0.71%. The size of the region affected by a vacancy on heat transfer was determined. At the X-direction, the size of the region affected by a vacancy is 1.79 times of particle size. Upstream position of vacancy at the Y-direction, the size of the region is 4.03 times of particle size. Downstream position of vacancy at the Y-direction, the size of the region is 3.81 times of particle size. The research results can be used to improve the thermal resistance network theory.

Atomistic simulations to study point defect dynamics in bi-crystalline niobium

Niobium is an important element in Zr–Nb alloy, which is widely employed for structural material in many nuclear reactors. Superior mechanical properties and creep resistance in conjunction with low neutron cross-section, makes Zr–Nb alloy a suitable candidate for the manufacturing of pressure tubes and fuel cladding in nuclear reactors. Molecular dynamics based simulations were performed to study the point defect dynamics in niobium. Atomistic simulations were performed in the environment of classical mechanics to investigate the effect of grain boundary configurations on the point defect dynamics. Symmetric and asymmetric tilt grain boundaries were generated between the two crystals of niobium, and defect formation and migration energies were calculated as a function of distance from the grain boundary plane. A relationship between the grain boundary energies and point defect formation energies was predicted, and it was concluded that higher energy grain boundaries have higher tendency to behave as sink for point defects. Lower offset in vacancy formation energies between the initial and terminating position of vacancy, energetically favours the migration of point defects, whereas higher energy offset between bulk and grain boundaries require more energy to migrate vacancy towards the grain boundary.

The Effect of Jump Attempt Frequencies on the Ionic Conductivity of Doped Ceria

The macroscopic oxygen ion conductivity in doped ceria is determined by the microscopic activation energy barriers and jump attempt frequencies of oxygen ion jumps. While the influence of the local jump environment on the migration energy is widely investigated, its influence on the attempt frequency is rarely investigated. In this work, attempt frequencies in Sm, Yb, and Gd doped ceria are calculated using density functional theory. Moreover, ionic conductivities for varying local jump attempt frequencies in different jump environments are investigated using Kinetic Monte Carlo simulations. For doping along the migration pathway, where the migrating oxygen ion passes between two adjacent cations, large dopants lead to an increase and small dopants to a decrease in the attempt frequency. Sm doping in nearest neighborhood to the start position of the migrating oxygen vacancy also leads to an increase in attempt frequency. Kinetic Monte Carlo simulations show that at intermediate Sm dopant fractions oxygen vacancies frequently jump toward and away from dopants explaining why for Sm doped ceria one of the highest conductivities for a ternary cerium oxide was measured due to its low dopant-oxygen vacancy association in both nearest and next-nearest neighborhood.

Properties of spin polarization and spin transport for zigzag antimonene nanoribbons with single vacancies

Defects are unavoidable in two-dimensional materials which may change their geometric structures and intrinsic physical properties. Based on the spin-polarized density functional theory and the non-equilibrium Green's function method, we have investigated the spin polarization and spin transport properties of zigzag antimonene nanoribbons (Z-SbNRs) with vacancy defect at different positions. It can be found that the ground state of the defective Z-SbNRs transforms from ferromagnetism (FM) to anti-ferromagnetism (AFM) when the position of the vacancy varies and spin polarization is induced by the vacancy defect. The calculated band structures show that all energy bands split and the defective Z-SbNRs exhibit semiconductor characteristics with smaller band gaps when comparing with the perfect Z-SbNRs. From the analysis of current-voltage characteristic, it can be observed that the threshold voltages of the defective Z-SbNRs are smaller and spin-resolved currents are larger than that of the perfect Z-SbNRs. Interestingly, the spin polarization and spin transport properties of defective Z-SbNRs are sensitive to the position of the single vacancy, which means the properties can be tuned by varying the position of the vacancy. The results indicate that the Z-SbNRs with the single vacancy is a promising candidate in spintronic devices.

Origin of the two-dimensional electron gas at the interface of NdGaO3/SrTiO3

Density functional theory within the generalized gradient approximation is employed to investigate the two-dimensional electron gas (2DEG) at the interface of NdGaO3/SrTiO3 (0 0 1) heterostructure. For the superlattice model, the 2DEG originates from the Ti dxy electrons, which are transferred from the NdGaO3 side. However, the interface of the thin film model exhibits insulated properties. In order to investigate the different properties of the two models, it is noted that large polar distortion in the NdGaO3 side occurs in the thin film model comparing to the superlattice model, and the built-in electric field is screened by the large distortion, and result in preventing the electron transfer from the NdGaO3 side to Ti dxy orbitals. For the thin film model, the surface layer of NdGaO3 is the energy favor position of oxygen vacancy, which results in the rotation of the oxygen octahedrons. The 2DEG is induced by the balance between the distortion by the surface and the oxygen octahedrons rotation by the oxygen vacancy.

Ferromagnetism and half metallicity induced by oxygen vacancies in the double perovskite BaSrNiWO6: DFT study

Using the spin polarized density functional theory (DFT) and exploring the Plane-Wave Self-Consistent Field (PWscf) code implemented in Quantum-ESPRESSO package, we investigate the effect of the Oxygen vacancies (VO) and the Oxygen interstitial (Oi) on the double perovskite BaSrNiWO6. This deals with the magnetic ordering and the electronic structure in such a pure sample exhibiting the insulating anti-ferromagnetic (AFM) state. This study shows that the presence of oxygen deficient defects converts the insulating to half metal with ferromagnetic or anti-ferromagnetic states. The magnetic ordering in BaSrNiWO6−δ depends on the position of the Oxygen vacancy in the unit cell. However, it has been shown that the Oxygen interstitial preserves the anti-ferromagnetic propriety. We have computed the formation energies of different positions of the Oxygen vacancy (VO) and the Oxygen interstitial (Oi) in the BaSrNiWO6 compound. We showed that the formation of VO is easier and vice versa for the Oi formation. The obtained results reveal(VO) and the Oxygen interstitial (Oi) that the anti-ferromagnetic can be converted to ferromagnetic in the double perovskite BaSrNiWO6 induced by Oxygen vacancies VO.

Theoretical Study of Trimethylacetic Acid Adsorption on CeO2(111) Surface

Trimethylacetic acid (TMAA) adsorption on stoichiometric and oxygen-deficient CeO2(111) surfaces was investigated using density functional theory that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U) and long-range dispersion correction. Both the molecular state and dissociative state (TMAA → TMA– + H+) were identified on stoichiometric and oxygen-deficient CeO2(111) surfaces. For the stoichiometric surface, two thermodynamically favorable configurations with adsorption energies of the order of −30 kcal/mol are identified; one is a molecule adsorption state, and the other one is a dissociative state. For the oxygen-deficient surface, dissociative states are more favorable than molecular states. The most favorable configuration is the dissociative adsorption of TMAA with the adsorption energy of the order of −77 kcal/mol. The dissociated TMA moiety takes the position of oxygen vacancy, forming three Ce–O bonds. The signature vibrational frequencies for these thermodynamically stable ...

Effects of Position and Shape of Atomic Defects on the Band Gap of Graphene Nano Ribbon Superlattices

In this work, we study the behavior of introducing atomic size vacancy in a graphene nanoribbon superlattice. Our investigations are based on the density functional theory (DFT) with the Local Density Approximation in Atomistix Toolkit (ATK). We show that, in addition to its shape, the position of vacancy has a major impact on the electrical properties of a graphene nanoribbon superlattice. We show that the band gap of an armchair graphene nanoribbon may be tuned by introducing an appropriate periodic pattern of vacancies. The band gap changes in a zig-zag manner similar to the variation of band gap of a graphene nanoribbon by changing its width. Keywords—Antidot, Atomistix ToolKit, Superlattice, Vacancy.

First principles calculations of the band offset at SrTiO3−TiO2 interfaces

We report on first principles calculations of the properties of the epitaxial SrTiO3−TiO2 (anatase) heterojunction, with an emphasis on the electronic band profile and lineup at the interface. The valence and conduction band offsets are calculated as a function of the number of anatase layers deposited onto the SrTiO3, as well as of the position of an oxygen vacancy with respect to the interface. It is shown that the presence of oxygen vacancies in the SrTiO3 is a way to effectively lower the barrier heights at the interface. Our results are in agreement with recent experiments reporting nearly zero band offset.

Buckling Behaviors of Imperfect Single-Walled Carbon Nanotubes: A Molecular Dynamic Simulation

Carbon nanotube has attracted tremendous scientific and industrial interests due to its exceptional mechanical, electrical and thermal properties. In this paper, classic molecular dynamic simulations are carried out to investigate the buckling behaviors and mechanical properties of single-walled carbon nanotubes under axial compression, both for perfect and imperfect ones introducing atomic vacancies. The effect of chirality, diameter, quantity and position of vacancy are systematically studied. The simulation results reveal that their mechanical properties such as Young’s modulus, critical strain and stress suffering a significant decline as the increasing numbers of vacancies. It is also found that the critical stress and strain are sensitive to position of atomic vacancy. Carbon nanotubes with vacancies located at the center have lower critical strain and are easier to reach the failure stage than those with vacancies at both sides.