Accumulation Of Vacancies Research Articles

Bandgap tuning in samarium-modified bismuth titanate by site engineering using iron and cobalt co-doping for photovoltaic application

Multiferroic materials have faced great interest for solar energy conversion applications due to the charge separation caused by the effective ferroelectric polarization and the photovoltage generated across the bandgap, which in principle can lead to highly efficient energy conversion. However, ferroelectric materials may show poor visible light absorption, mainly due to the large bandgap. In this work, a novel approach is adopted to effectively reduce the bandgap of a multiferroic material by doping transition metal (TM) ions, namely (Co/Fe) into the Ti site of the Bi3.25Sm0.75Ti3O12 (BSmT) lattice. The structural optimization results show that there is a difference in the lengths of the Δ(Ti(1)-O1) and Δ(Ti(2)-O5) bonds and that their value increases with increasing Co/Fe concentration, resulting in structural distortions. The optical studies showed that the BSmT band gap was successfully reduced from 3.22 eV to 2.02 eV with Co/Fe co-doping. The structural distortion explains this reduction in bandgap due to the modulation of orbital overlap and compensation mechanism. In trivalent (Fe3+/Co3+) co-doping at the titanium (Ti4+) site, oxygen vacancies are formed to maintain electroneutrality. To prove the multiferroelectricity behavior in this study, the ferromagnetism and ferroelectricity ordering of the sintered BSmT:FC2 sample was studied at room temperature. The results show unsaturated leaky P-E hysteresis loops due to domain pinning caused by the accumulation of oxygen vacancies near the domain boundaries. At the same time, distinct S-type M-H hysteresis loops were obtained, indicating typical ferromagnetic ordering. The results obtained in this work can be considered promising to promote the building of intrinsic multiferroics to develop photovoltaic devices for next-generation applications.

Room-temperature multiferroic behaviour in Co/Fe co-substituted layer-structured Aurivillius phase ceramics

Room-temperature single-phase multiferroic materials have attracted the considerable attention of may scientists due to their technological applications in the next-generation electronic devices. Here, we report the structural evolution and its relationship with the ferroelectric and magnetic properties of Aurivillius Bi3.25Sm0.75Ti(3-x) (Fe,Co)xO12 (BSmT:FCx); for (0≤x ≤ 0.4) ceramics to elucidate the room-temperature multiferroics induced by Fe/Co co-substitution. The XRD analysis and structural refinements reveal that incorporating of Co, and Fe ions into Ti sites of BSmT host lattices gives rise to a single orthorhombic phase with no evidence of secondary phases. The X-ray photoelectron spectroscopy (XPS) spectra indicate the existence of oxygen vacancies in the investigated samples. Pure BiT samples exhibit diamagnetic behavior, while BSmT:FCx; for (0≤x ≤ 0.4) exhibits ferromagnetic behavior at room temperature. As a result, the sample with x = 0.4 displayed the highest remnant and saturation magnetization values. It is believed that the occurrence of magnetism causes by the magnetic double exchange interaction between the non-equivalent magnetic ions and oxygen. The reversible polarization induced by an electric field indicates the ferroelectric character of the ceramics at room temperature. The co-substituting samples show unsaturated P-E hysteresis loops, which could be caused by the domain pinning induced by the accumulation of oxygen vacancies near the domain boundaries. We expect these compounds are promising for developing electronic devices for future applications.

Direct correlation between void formation and lithium dendrite growth in solid-state electrolytes with interlayers.

Solid-state Li-ion batteries with lithium anodes offer higher energy densities and are safer than conventional liquid electrolyte-based Li-ion batteries. However, the growth of lithium dendrites across the solid-state electrolyte layer leads to the premature shorting of cells and limits their practical viability. Here, using solid-state Li half-cells with metallic interlayers between a garnet-based lithium-ion conductor and lithium, we show that interfacial void growth precedes dendrite nucleation and growth. Specifically, void growth was observed at a current density of around two-thirds of the critical current density for dendrite growth. Computational calculations reveal that interlayer materials with higher critical current densities for dendrite growth also have the largest thermodynamic and kinetic barriers for lithium vacancy accumulation at their interfaces with lithium. Our results suggest that interfacial modification with suitable metallic interlayers decreases the tendency for void growth and improves dendrite growth tolerance in solid-state electrolytes, even in the absence of high stack pressures.

Synergistic Effect of Electrical Stress and Neutron Irradiation on Silicon Carbide Power MOSFETs

The synergistic effect of radiation and bias voltage for the commercial silicon carbide (SiC) power MOSFETs was investigated by 14-MeV neutron irradiation in this article. The types of C3M0065090D and C2M0080120D commercial SiC power MOSFETs from CREE studied in this article show consistent changes after the 14-MeV neutron irradiation. The results show that the devices biased during irradiation experienced more degradation than that only irradiated and biased devices. Compared with the devices only irradiated and biased, the devices biased during irradiation experienced an additional decrease in saturation current and a further decrease in peak transconductance. The defects in the devices were characterized by the low-frequency noise (LFN) and deep-level transient spectrum (DLTS) methods. The results show that the synergistic effect increased the defect concentration of the device, resulting in further degradation of the electrical performance of the device compared to the control group without a bias voltage. The defect characterization shows that the drain–source voltage leads to the accumulation of carbon and silicon vacancies induced by irradiation at the interface, which results in intensified radiation damage and significant changes in the performance of the device at low neutron fluence.

Modelling dynamic precipitation in pre-aged aluminium alloys under warm forming conditions

This work presents a model for deformation enhanced precipitate growth and coarsening kinetics in pre-aged 7xxx aluminium alloys under warm forming conditions (100 to 200 °C). A multi-class Kampmann–Wagner framework is used to describe the evolution of precipitate size distribution, where the effect of deformation is incorporated through enhanced solute diffusivity resulting from deformation induced excess vacancies. A classical phenomenological model is used to describe the accumulation of excess vacancies. The model is validated with experimental warm deformation data from the literature, and applied to investigate a wide range of deformation conditions to predict the effect of strain, strain rate and temperature on precipitate growth and coarsening kinetics. It is demonstrated that the interaction of solute supersaturation and excess vacancy concentration can lead to complex non-monotonic precipitate growth rate variation for certain regimes of strain rate and temperature.

The effect of temperature and bias on the energy storage of a Ru/YSZ/Ru thin-film device

The next generation of all-solid-state thin-film energy storage devices, such as supercapacitors and pseudocapacitors, requires a wide operating temperature range to work under demanding conditions. We have conducted an electrical study of the Ru/YSZ/Ru thin film device to better understand the nature of the ionic conduction processes during the transition from the super-to pseudocapacitive regime as a function of the temperature. Also, it was correlated the storage characteristics of the device to its electrical properties. The complex modulus analysis indicates increased ion mobility with temperature. A DC bias increased the mobility by three orders of magnitude. The activation energy for the ionic mobility without and with DC bias was 1.06 eV and 1.86 eV, respectively. The difference in energy is attributed to the electrostatic repulsion originated by the decomposition of YSZ, the accumulation of ions and vacancies at interfaces, and reactions at the electrode interfaces. Galvanostatic charge-discharge showed a maximum energy density of 200 mWh/cm3 @170 °C. The transition from a super-to a pseudocapacitive behavior occurs at the temperature where redox reactions initiate at electrodes interfaces, as determined by the equivalent circuits for impedance curves. At this point, the increase of energy density becomes steeper.

Effect of Cationic/Anionic Diffusion Dominated Passive Film Growth on Tribocorrosion

Tribocorrosion behaviours of nickel (Ni) and niobium (Nb) in sodium sulfate (Na2SO4) solution under potentiodynamic and potentiostatic conditions were studied. Under the potentiodynamic condition, the passivation was early broken, accompanied by a sharp increase in frictional coefficient on Nb. The current was more fluctuant, and larger material loss appeared at the higher potential in the potentiostatic condition. However, these phenomena did not occur for Ni, and it even showed lower material loss at the higher potential in the potentiostatic tribocorrosion test. The differences in tribocorrosion behaviour had a close relationship to the passive film growth mechanism, which decided the passive film/metal interface structure. Nb with anionic diffusion dominated mechanism in passive growth would cause the accumulation of oxygen vacancies at the passive film/metal interface. This may weaken the adhesion between the metal and the passive film. However, with the cationic diffusion dominated passive film growth on Ni, cation vacancies concentrated at the passive film/tribo-film interface, and this did not affect the adhesion between metal and passive film. Ni or other passive elements with the cationic diffusion-dominated mechanism in passive film growth were recommended as the alloying element for improving the tribocorrosion resistance of alloys.

Room-temperature co-regulation of resistive and magnetic states in Fe3O4/PZT/ZCO multiferroic heterostructure with diluted magnetic semiconductor

A novel multiferroic heterostructure composed of Fe3O4/PbZr0.2Ti0.8O3(PZT)/Zn0.95Co0.05O(ZCO) is prepared on the Pt(111)-Ti-SiO2-Si substrate in this work to achieve the synchronous electric regulation on magnetic and resistive states at room temperature. When the polarization direction of the PZT layer is reversed with the write voltage, the depletion or accumulation of oxygen vacancies is formed at the PZT/ZCO interface, exhibiting the resistance switching between high and low resistive states with a ratio up to 1.19 × 104. Meanwhile, based on the interfacial charge effect and bound magnetopolaron (BMP) theory, magnetic modulation with a magnetoelectric (ME) coupling coefficient up to 133.2 Oe/V is realized at room temperature. More interestingly, by measuring the magnetoresistance, four distinct resistance states are displayed in this multiferroic structure with the ferroelectric reversal. Our results show that the multiferroic system containing both ferromagnetic and diluted magnetic semiconductor can simultaneously realize magnetoelectric coupling and resistance switching at room temperature. The magnetic signal can also be converted into an electrical signal based on the magnetoresistance effect.

Artificial Synapses Based on WSe2 Homojunction via Vacancy Migration.

Artificial synapses based on two-dimensional (2D) transition metal dichalcogenides (TMDs) materials have attracted wide attention to boost the development of neuromorphic computing in recent years. Various structures have been adopted to build 2D-material-based artificial synapses. In lateral- and vertical-structures, the realization of synaptic function mainly results from the migration of the defects and vacancies, which requires the strong ion diffusion ability. Here, we successfully demonstrate an artificial synapse based on lateral WSe2 homojunction. The migration of Se vacancies from the thin region to the thick region has been promoted by applying negative gate voltage, resulting in n-type doping in the thick region due to the accumulation of Se vacancies, which would diminish the barrier width of the metal-semiconductor junctions in the thick region. Consequently, the transformation from a high-resistance state (HRS) to a low-resistance state (LRS) is achieved. Significantly, our device can efficiently emulate the biological synaptic functions with a large synaptic weight change. Additionally, the transition from short-term memory (STM) to long-term memory (LTM) can be accomplished with a simpler structure, which would be beneficial to realizing the large-scale integration of transistor-based artificial synapses.

Probing the role of grain boundaries in single Cu nanoparticle oxidation by in situ plasmonic scattering

Grain boundaries determine physical properties of bulk materials including ductility, diffusivity, and electrical conductivity. However, the role of grain boundaries in nanostructures and nanoparticles is much less understood, despite the wide application of nanoparticles in nanophotonics, nanoelectronics, and heterogeneous catalysis. Here, we investigate the role of high-angle grain boundaries in the oxidation of Cu nanoparticles, using a combination of in situ single particle plasmonic nanoimaging and $postmortem$ transmission electron microscopy image analysis, together with ab initio and classical electromagnetic calculations. We find an initial growth of a 5-nm-thick ${\mathrm{Cu}}_{2}\mathrm{O}$ shell on all nanoparticles, irrespective of different grain morphologies. This insensitivity of the ${\mathrm{Cu}}_{2}\mathrm{O}$ shell on the grain morphology is rationalized by extraction of Cu atoms from the metal lattice being the rate limiting step, as proposed by density functional theory calculations. Furthermore, we find that the change in optical scattering intensity measured from the individual particles can be deconvoluted into one contribution from the oxide layer growth and one contribution that is directly proportional to the grain boundary density. The latter contribution signals accumulation of Cu vacancies at the grain boundaries, which, as corroborated by calculations of the optical scattering, leads to increased absorption losses and thus a decrease of the scattering, thereby manifesting the role of grain boundaries as vacancy sinks and nuclei for Kirkendall void formation at a later stage of the oxidation process.

Dislocation reaction-based formation mechanism of stacking fault tetrahedra in FCC high-entropy alloy

In conventional FCC metals and alloys, the formation mechanisms of stacking fault tetrahedron (SFT) based on the vacancy cluster and Frank loop have been widely accepted; however, the dislocation reaction-based formation mechanism of SFT without Frank loop and vacancy cluster remains controversial. In this work, we studied the formation of the SFTs in CoCrFeNiCu high-entropy alloy (HEA) bicrystals and their single-crystal counterparts subjected to shear deformation using molecular dynamics (MD) simulations. The migration and thickening of grain boundaries, and the nucleation and glide of dislocations from the grain boundaries are the predominant carriers of deformation, which are the prerequisite for the formation of SFT. We observed two types of dislocation reaction-based formation mechanisms, where there is no vacancy aggregation and Frank loop participation. The formation of SFT originates from the growth of the bottom face of the pyramid in the bicrystal HEA, while in the single-crystal HEA, SFT nucleates at its vertex, and extends towards its bottom.

Relationship between ion vacancy mobility and hysteresis of perovskite solar cells

The relation between ion vacancy mobility and hysteresis of perovskite solar cells (PSCs) is investigated by computer simulation. Due to the ion vacancy migration, the hysteresis in PSCs strongly depends on the scan rate of J-V sweeps. The hysteresis only occurs in certain regime of scan rate. We calculate the evolution of ionic charge density stored within the Debye layers to discuss the process of ion vacancy migration and the variation of ion vacancy accumulation during the J-V sweeps. Based on this discussion, we give the physical picture of hysteresis. We discover that the regime of scan rate where hysteresis shows up moves towards higher value with increasing mobility of ion vacancy.

CH3NH3+ and Pb immobilization through PbI2 binding by organic molecule doping for homogeneous organometal halide perovskite films

PCBM and PMMA doping prevent Pb migration and hinder Pb-assisted aggregation of MA vacancies in MAPbI3 films, achieving structural and chemical homogeneity with enhancement of performance and stability in MAPbI3-based solar cells.

Effects of Tungsten on Vacancy Aggregation Behavior and its Induction for Interstitial and Vacancy Migration in Tantalum–Tungsten Alloys: A First-Principles Study

Effects of Tungsten on Vacancy Aggregation Behavior and its Induction for Interstitial and Vacancy Migration in Tantalum–Tungsten Alloys: A First-Principles Study

High-heat flux Cu-0.8Y alloys investigated by positron annihilation spectroscopy

This work studies the thermal stability of the microstructure and the evolution of the defects of two high-heat flux Cu-0.8 wt%Y alloys fabricated following two alternative powder metallurgy routes. One batch was produced by direct hot isostatic pressing (HIP) consolidation of Cu-0.8 wt%Y pre-alloyed atomized powders while an additional ball milling processing step was introduced before HIP sintering for the second alloy. The stability and recovery characteristics of the vacancy type defects in these alloys in the as-produced state and after processing by severe equal channel angular pressing to achieve a refine microstructure have been investigated by positron lifetime and coincidence Doppler broadening measurements in samples subjected to isochronal annealing from room temperature to 900 °C. Microhardness measurements and electron transmission microscopy analysis have also been performed to support the results obtained from the positron annihilation spectroscopy analysis techniques. The recovery curves of the positron lifetime and S-W plots show a recovery stage in agreement with the recovery stage V for Cu. However, a full recovery is not accomplished, and a stage that reverts the previous recovery takes place after annealing above ~600 °C, that leads to the formation of very stable defects at temperatures up to 900 °C, identified as vacancy aggregates and nanocavities. The characteristic shape of the coincidence Doppler broadening indicates that the dispersed Y-O particles in the Cu matrix appear to be responsible for stabilizing the vacancy aggregates and nanocavities for temperatures above 600–700 °C.

Radiation stability of nanostructured hydroxyapatite Ca10(PO4)6(OH)2 under ion irradiations

Radiation stability of nanostructured hydroxyapatite with different interfacial structure is studied. Hydroxyapatite nanoparticle (HAp-np) is fabricated by calcination in muffle furnace and densified nanocrystalline hydroxyapatite (HAp-nc) is synthesized by spark plasma sintering . X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations show the hexagonal structure for both HAp-np and HAp-nc. Irradiation responses of HAp-np and HAp-nc are studied via in situ TEM techniques. The samples are irradiated with 1 MeV Kr 2+ ions over the temperature range of 150 K to 523 K. In situ TEM observations reveal a higher critical amorphization dose of HAp-nc (0.075 dpa) compared to 0.05 dpa of HAp-np at room temperature, indicating enhanced amorphization resistance. The radiation tolerance shows a great dependence on temperature, and a higher critical dose can be observed at elevated temperatures. A lower critical amorphization temperature ( T c ) of HAp-nc (∼437 K) than HAp-np (∼545 K) indicates better radiation tolerance since the lower T c shows better defect annealing ability. This better radiation tolerance of HAp-nc is attributed to the higher sink strength and lower interface energy carried by densified grain boundaries as free-standing open surfaces of HAp-np can carry more isolated dangling bonds that results in a higher interface energy. This excess interface energy can lower the energy difference between crystalline phase and amorphous state, leading to HAp-np to be less radiation resistant. Radiation-induced amorphization is attributed to the accumulation of oxygen vacancies and the glass-like structure rearranged by displaced oxygen and phosphorus atoms.

Molecular dynamics studies of lattice defect effects on tritium diffusion in zirconium

Tritium diffusion in α-Zr containing point defects such as vacancies or self-interstitial atoms (SIAs) is simulated using molecular dynamics. Point defects rapidly aggregate to form extended defects, such as 3D nanoclusters and Frank loops. The geometry of extended defects is affected by the presence of tritium. At low temperature and in the absence of tritium, vacancies aggregate to form stacking fault pyramids. Addition of tritium at these temperatures promotes aggregation of vacancies to form 3D nanoclusters, within which the tritium concentration can be sufficiently high to suggest that these defects may serve as nucleation sites for hydride precipitation. Trapping of tritium in vacancy nanocluster reduces the calculated bulk diffusivity by an amount proportional to the vacancy concentration. At high temperature, vacancy clusters change shape to form planar basal dislocation loops, which bind tritium less strongly, leading to a sharp reduction in the fraction of trapped tritium and a corresponding increase in tritium diffusivity at high temperature. In contrast, SIAs increase tritium diffusion through α-Zr. Analysis of atomic trajectories shows that tritium does not interact directly with SIAs. Diffusion enhancement is instead related to expansion of the lattice.

Optical Properties of Ion Accumulation Areas in MAPbX3 Single Crystals

AbstractHybrid organic–inorganic perovskite (HOIP) material‐based devices have attracted much attention in the past few years owing to their superior photoelectric performance. However, ion migration has hindered the commercialization of these devices. The accumulation of migrated ions or defects near electrodes and boundaries is inevitable under a voltage bias. This paper reports a systematic study on the optical properties of ion accumulation areas in HOIP. Quantitative electric fields are applied to MAPbX3 (MA = methylammonium and X = Cl, Br, or I) single crystals, and micro‐optical characterizations, including photoluminescence (PL) spectra, PL lifetimes, and Raman spectra, are performed. Significant differences in the intensity and peak position are observed in the PL spectrum near the anode and cathode regions, respectively. The phenomena observed in the study can be attributed to the migration and accumulation of halide ions, halide vacancies, and MA+ ions. Moreover, the results reveal that halide ions and vacancies play a significant role in carrier recombination. Finally, the Raman spectrum confirms that the migrated ions affect the interaction between the organic MA+ ions and inorganic Pb–X cages.

Atomistic Insights on the Full Operation Cycle of a HfO2-Based Resistive Random Access Memory Cell from Molecular Dynamics.

We characterize the atomic processes that underlie forming, reset, and set in HfO2-based resistive random access memory (RRAM) cells through molecular dynamics (MD) simulations, using an extended charge equilibration method to describe external electric fields. By tracking the migration of oxygen ions and the change in coordination of Hf atoms in the dielectric, we characterize the formation and dissolution of conductive filaments (CFs) during the operation of the device with atomic detail. Simulations of the forming process show that the CFs form through an oxygen exchange mechanism, induced by a cascade of oxygen displacements from the oxide to the active electrode, as opposed to aggregation of pre-existing oxygen vacancies. However, the filament breakup is dominated by lateral, rather than vertical (along the filament), motion of vacancies. In addition, depending on the temperature of the system, the reset can be achieved through a redox effect (bipolar switch), where oxygen diffusion is governed by the applied bias, or by a thermochemical process (unipolar switch), where the diffusion is driven by temperature. Unlike forming and similar to reset, the set process involves lateral oxygen atoms as well. This is driven by field localization associated with conductive paths.

Radiation-induced amorphization and recrystallization of hydroxyapatite nanoparticles

Hydroxyapatite, Ca10(PO4)6(OH)2, is considered as an important apatite-type material for the incorporation and disposal of actinides and fission products. Hydroxyapatite nanoparticles with different size ranging from 20 nm to 280 nm are synthesized via calcining the bovine bones under different temperatures and durations. The samples are irradiated with 1 MeV Kr2+ ions and 200 keV electrons to study the displacive and ionizing effects on the irradiation behaviors. In situ transmission electron microscopy (TEM) observation shows hydroxyapatite nanoparticles can be amorphized by 1 MeV Kr2+ ions and the previously amorphized samples experience a rapid ionizing-radiation-induced recrystallization upon 200 keV electrons irradiations. A strong size effect on the displacive irradiation-induced amorphization and ionizing irradiation-induced recrystallization are observed. Under ion irradiation, a lower critical temperature (Tc) is observed for a larger sized sample, indicating enhanced amorphization tolerance. This result indicates excess interface energy resulting from the smaller sized hydroxyapatite nanoparticle, may lower the energy difference between the crystalline phase and amorphous state, degrading the radiation tolerance. Radiation-induced amorphization is attributed to the accumulation of oxygen vacancies upon ion irradiation and the glass-like structure formed by the rearrangement of displaced oxygen and phosphorus atoms. Whereas under electron irradiation, the recrystallization fluences decrease with the reduction of particle size since a higher density of dangling bonds exist in a smaller sized hydroxyapatite. These unstable bonds are relatively easy to break and reforming under ionizing electron irradiation, which drives the recrystallization process.