Vacancy Trapping Research Articles

Stability of X-C-vacancy complexes (X=H, He) in vanadium from first principles investigations

We investigate interstitial C interactions with H/He, stability of Cn-vacancy/Hn-C-vacancy/Hen-C-vacancy complexes (n = 1–5) and trapping of H/He in C-vacancy/C2-vacancy/H-He-C-vacancy in bcc vanadium using first-principles calculations. Interstitial C-H/C-He interactions are very weak and H/He preferential site keeps unchanged. A vacancy can accumulate two C atoms and the C2-vacancy cluster is more stable than the C-vacancy cluster. The electron localization function analysis shows that the C atoms form strong C-C bonds in vacancy and weaken the C-vanadium bonds. The stable configurations of Hn/Hen-C-vacancy clusters are partially different and the dissolution of H/He from Hn-C-vacancy/Hen-C-vacancy complexes is easier than Hn-vacancy/Hen-vacancy complexes with n ≥ 2. The synergetic interactions of H and He in C-vacancy complex are also investigated and H/He trapping strength decreases in H-He-C-vacancy complex. The presence of C weakens vacancy trapping for more H/He atoms due to large distance of C-H/C-He.

An investigation of the sites occupied by atomic barium in solid xenon-A 2D-EE luminescence spectroscopy and molecular dynamics study.

A detailed characterisation of the luminescence recorded for the 6p 1P1-6s 1S0 transition of atomic barium isolated in annealed solid xenon has been undertaken using two-dimensional excitation-emission (2D-EE) spectroscopy. In the excitation spectra extracted from the 2D-EE scans, two dominant thermally stable sites were identified, consisting of a classic, three-fold split Jahn-Teller band, labeled the blue site, and an unusual asymmetric 2 + 1 split band, the violet site. A much weaker band has also been identified, whose emission is strongly overlapped by the violet site. The temperature dependence of the luminescence for these sites was monitored revealing that the blue site has a non-radiative channel competing effectively with the fluorescence even at 9.8 K. By contrast, the fluorescence decay time of the violet site was recorded to be 4.3 ns and independent of temperature up to 24 K. The nature of the dominant thermally stable trapping sites was investigated theoretically with Diatomics-in-Molecule (DIM) molecular dynamics simulations. The DIM model was parameterized with ab initio multi-reference configuration interaction calculations for the lowest energy excited states of the Ba⋅Xe pair. The simulated absorption spectra are compared with the experimental results obtained from site-resolved excitation spectroscopy. The simulations allow us to assign the experimental blue feature spectrum to a tetra-vacancy trapping site in the bulk xenon fcc crystal-a site often observed when trapping other metal atoms in rare gas matrices. By contrast, the violet site is assigned to a specific 5-atom vacancy trapping site located at a grain boundary.

Intrinsic defects and spectral characteristics of SrZrO3 perovskite

First-principles calculations and experiment analysis were performed to study the internal relation between seven types of intrinsic defects and the persistent luminescence in SrZrO3 host material. The calculation shows that rich zirconium defects have the low energy cost and thus are easy to form. Zr vacancies are too high energy to play any role in defect which is related luminescence phenomenon of SrZrO3 phosphor. However, oxygen vacancies stand out as a likely candidate, because it can yield two carrier reservoirs: a fully-occupied singlet electron's reservoir which lies above the valence band maximum, and an empty triply degenerate hole's reservoir which is just below the conduction band minimum. Sr vacancies are not directly relevant to the persistent luminescence due to its too shallow electron trap level. The characteristics of these defects are fully explained by the equilibrium properties of SrZrO3. An experimental study of the thermoluminescence glow for these defects is conducted and the calculation is consistent with the experimental results. A mechanism of the persistent luminescence for SrZrO3:Pr3+, Eu3+ is explained according to oxygen vacancies trap center. Findings of this study may serve as theoretical references for controlling intrinsic traps by more refined experiments.

Structural and optical properties of Mg doped ZnS quantum dots and biological applications

Zn1−xMgxS (x = 0, 0.2 and 0.4) quantum dots (QDs) were prepared by co-precipitation method. The Mg dopant did not modify the cubic blende structure of ZnS QDs. The Mg related secondary phase was not detected even for 40% of Mg doping. The size mismatch between host Zn ion and dopant Mg ion created distortion around the dopant. The creation of distortion centres produced small changes in the lattice parameters and diffraction peak position. All the QDs showed small sulfur deficiency and the deficiency level were increased by Mg doping. Band gap of the QD was decreased due to the dominated quantum confinement effect over compositional effect at initial doping of Mg. But at higher doping the band gap was increased due to compositional effect, since there was no change in average crystallite size. The prepared QDs had three emission bands in the UV and Visible regions corresponding to near band edge emission and defect related emissions. The electron transport reaction chain which forms free radicals was broken by sulfur vacancy trap sites. Therefore, the ZnS QDs had better antioxidant activity and the antioxidant behaviour was enhanced by Mg doping. The enhanced UV absorption and emission of 20% of Mg doped ZnS QDs let to maximize the zone of inhibition against E. Coli bacterial strain.

Defect formation, ordering, and transport in SrFe1–x Si x O3–δ (x = 0.05–0.20)

Oxygen nonstoichiometry of perovskite-like SrFe1–x Si x O3–δ (x = 0.05–0.20), studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 10−20–0.5 atm at 700–950 °C, decreases with Si4+ additions. The equilibrium $$ {p}_{{\mathrm{O}}_2} $$ –T−δ diagrams can be adequately described by a model accounting for anion site-exclusion effects near highly stable SiO4 tetrahedra and energetic favorability of the defect clusters formed by two tetrahedra sharing one oxygen vacancy. This model was validated by atomistic computer simulations. The standard thermodynamic functions for oxygen incorporation and iron disproportionation reactions are essentially independent of silicon concentration, as for the migration activation energies of the p- and n-type electronic charge carriers. On the contrary, at low temperatures, Si-doping leads to a higher oxygen deficiency, simultaneously suppressing long-range vacancy ordering and increasing oxygen coordination of iron cations as estimated from the Mossbauer spectra. These phenomena are associated, again, with vacancy trapping near randomly distributed Si4+. The Mossbauer spectroscopy, transmission electron microscopy, and electron diffraction studies showed that Si4+ substitution progressively reduces the content of brownmillerite-like nanodomains typical for SrFeO3-based materials.

Clustering and segregation of small vacancy clusters near tungsten (0 0 1) surface

Abstract Nanoporous metals have been shown to exhibit radiation-tolerance due to the trapping of the defects by the surface. However, the behavior of vacancy clusters near the surface is not clear which involves the competition between the self-trapping and segregation of small vacancy clusters (Vn) nearby the surface. In this study, we investigated the energetic and kinetic properties of small vacancy clusters near tungsten (0 0 1) surface by combining molecular statics (MS) calculations and object Kinetic Monte Carlo (OKMC) simulations. Results show that vacancies could be clustered with the reduced formation energy and migration energy of the single vacancy around a cluster as the respective energetic and kinetic driving forces. The small cluster has a migration energy barrier comparable to that for the single vacancy; the migration energy barriers for V1–5 and V7 are 1.80, 1.94, 2.17, 2.78, 3.12 and 3.11 eV, respectively. Clusters and become unstable near surface (0 0 1) and tend to dissociate into the surface. At the operation temperature of 1000 K, the single vacancy, V2, 2 V 3 V3 and V4 were observed to segregate to the surface within a time of one hour. Meanwhile, larger clusters survived near the surface, which could serve as nucleating center for voids near the surface. Our results suggest that under a low radiation dose, surface (0 0 1) could act as a sink for small vacancy clusters, alleviating defect accumulation in the material under a low radiation dose. We also obtained several empirical expressions for the vacancy cluster formation energy, binding energy, and trapping radius as a function of the number of vacancies in the cluster.

Modeling of fuel retention in the pre-damaged tungsten with MeV W ions after exposure to D plasma

Modeling of high-Z ion irradiated-induced damages on fuel retention inside tungsten (W) material has been performed in this work. The upgraded Hydrogen Isotope Inventory Processes Code (HIIPC) is applied to model the deuterium (D) retention inside pre-damaged W during exposed to low-energy D flux, and the W is pre-irradiated by 20 MeV W-ion before exposed to D flux. Three types of trap, i.e. mono-vacancies, dislocations and grain boundary vacancies, are considered in the present model. The mono-vacancy defects induced by energetic W ions are calculated by SRIM code. First, the model is validated against the available experimental data under the same D flux exposure conditions, showing the reasonable agreement. Then, the effect of radiation-induced defects produced by pre-exposed energetic W-ion with different energy and fluence on the fuel retention are studied, confirming that the irradiation-induced traps play a dominated role on the fuel retention in the surface of the material (∼ micrometer). Finally, the effects of different type of defect, D fluence, and wall temperature on the fuel retention are discussed systemically, and these modeling results are in well agreement with the previous studies.

Effect of germanium doping on the formation kinetics of vacancy-dioxygen complexes in high dose neutron irradiated crystalline silicon

The effect of germanium (Ge) doping on the formation kinetics of vacancy-dioxygen (VO2) complexes in high dose neutron irradiated crystalline silicon (c-Si) has been quantitatively investigated using infrared spectroscopy at 10 K. It is observed that Ge doping of 1019 cm−3 enhances the formation of vacancy-oxygen (VO) complexes by ∼15% during neutron irradiation and slightly suppresses the conversion of VO into VO2 complexes. By studying the generation kinetics of VO2 complexes in the temperature range of 300–345 °C, it is found that the activation energies of VO2 generation are determined to be 1.52 and 1.71 eV in the reference and Ge-doped c-Si, respectively. According to the theory for diffusion limited reactions, it is suggested that Ge doping can retard the VO diffusion in c-Si and therefore reduce the capture probability of Oi for VO complexes. This may be attributed to the temporary trapping of vacancies by Ge atoms. Hence, the formation of VO2 complexes in c-Si is slightly suppressed by Ge doping.

The Molecular Dynamics Study of Vacancy Formation During Solidification of Pure Metals

In order to understand the defect trapping during solidification in pure elements, we have performed molecular dynamics simulations on both aluminum and nickel. We find that vacancies are the dominant defects in the product crystals for both metals. For slight undercooling, the vacancy concentration strongly depends on the growth velocity, rather than the growth orientations, and there is an approximately linear relationship between the growth velocity and vacancy concentration. However, for deep undercooling, the vacancy concentration shows a remarkable anisotropy between (100) and (110) orientations. Based on the competition between atomic diffusion and growth, a possible mechanism for vacancy trapping is suggested.

AC stress and electronic effects on SET switching of HfO2 RRAM

An AC stress test was performed to investigate the accompanying electronic effects in a HfO2 resistive random access memory during the SET transition, which featured a sudden decrease in resistance. Comparing the DC and AC measurement results indicated the pronounced influence of interrupted stress on both the mean values and variations of time to SET. First-principles calculations suggested that the charge states (+2, +1, or neutral) of oxygen vacancies affect the migration barrier for forming oxygen vacancy clusters. Therefore, a charge-state-dependent SET model is proposed to include the additional electronic effects induced by the dynamics of electron trapping and detrapping in oxygen vacancies during AC stress. A trimodal Weibull fitting based on the proposed model reproduced the experimental time to SET distributions obtained in a wide range of AC stress conditions.

Relative Occurrence of Oxygen-Vacancy Pairs in Yttrium-Containing Environments of Y2O3-Doped ZrO2 Can Be Crucial to Ionic Conductivity

Yttria-stabilized zirconia (YSZ) is widely used as an electrolyte in solid oxide fuel cells. Much of current research on ionic conductivity in YSZ has focused on understanding migration barriers for O2– ion movement and vacancy trapping behavior in an attempt to understand why dopant cation edges result in fewer vacancy hopping transitions. We show that the free energy of finding O2–-vacancy (O2–-vac) pairs in a local environment can also be crucial to the hopping transitions. Higher probability of O2–-vac pairs can result in a greater number of transitions. O2– ion movement in bulk YSZ is studied here using multiple independent short molecular dynamics (MD) trajectories. Analysis of the MD trajectories yields free energy of O2–-vac pairs in 42 different local cation (Y3+/Zr4+) environments, to our knowledge calculated for the first time, as well as coarse-grained O2– hopping rates and Arrhenius parameters. On the basis of the free energies we conclude that it is possible that ionic movement is hindered i...

The role of molybdenum in suppressing cold dwell fatigue in titanium alloys.

We test a hypothesis to explain why Ti-6242 is susceptible to cold dwell fatigue (CDF), whereas Ti-6246 is not. The hypothesis is that, in Ti-6246, substitutional Mo-atoms in α-Ti grains trap vacancies, thereby limiting creep relaxation. In Ti-6242, this creep relaxation enhances the loading of grains unfavourably oriented for slip and they subsequently fracture. Using density functional theory to calculate formation and binding energies between Mo-atoms and vacancies, we find no support for the hypothesis. In the light of this result, and experimental observations of the microstructures in these alloys, we agree with the recent suggestion (Qiu et al. 2014 Metall. Mater. Trans. A45, 6075-6087. (doi:10.1007/s11661-014-2541-5)) that Ti-6246 has a much smaller susceptibility to CDF because it has a smaller grain size and a more homogeneous distribution of grain orientations. We propose that the reduction of the susceptibility to CDF of Ti-6242 at temperatures above about 200°C is due to the activation of 〈c+a〉 slip in 'hard' grains, which reduces the loading of grain boundaries.

Structure and conductivity in tungsten doped δ-Bi3YO6

Solid solution formation in the system Bi3Y1−xWxO6+3x/2 has been studied using a combination of X-ray and neutron powder diffraction and a.c. impedance spectroscopy. Compositions in the solid solution adopt the δ-Bi2O3 type structure, with single phases evident from x=0.00 to x=0.20. Evidence for dopant clustering is presented and discussed. Models for the defect structure derived from diffraction studies are presented. Tungsten is proposed to adopt a tetrahedral coordination geometry, with a distorted octahedral geometry adopted by yttrium. Calculated coordination numbers for bismuth of around five are consistent with stereochemical activity of the Bi 6s2 lone pairs of electrons. Despite a significant lowering of the nominal vacancy concentration with respect to δ-Bi3YO6, as well as enhanced vacancy trapping by W6+, tungsten doping is found to have very little influence on the total conductivity of δ-Bi3YO6. This is attributed to the compensating effect of enhanced oxide ion mobility caused by lattice expansion.

Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of SrTiO3 crystals and at the interface of the corresponding LaAlO3/SrTiO3 heterostructures

With infrared (IR) ellipsometry and DC resistance measurements we investigated the photo-doping at the (001) and (110) surfaces of SrTiO$_3$ (STO) single crystals and at the corresponding interfaces of LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures. In the bare STO crystals we find that the photo-generated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet carrier concentration as the confined electrons that were previously observed in LAO/STO (001) heterostructures. A large fraction of these photo-generated charge carriers persist at low temperature at the STO (001) surface even after the UV light has been switched off again. These persistent charge carriers seem to originate from oxygen vacancies that are trapped at the structural domain boundaries which develop below the so-called antiferrodistortive transition at T* = 105 K. This is most evident from a corresponding photo-doping study of the DC transport in STO (110) crystals for which the concentration of these domain boundaries can be modified by applying a weak uniaxial stress. The oxygen vacancies and their trapping by defects are also the source of the electrons that are confined to the interface of LAO/STO (110) heterostructures which likely do not have a polar discontinuity as in LAO/STO (001). In the former, the trapping and clustering of the oxygen vacancies also has a strong influence on the anisotropy of the charge carrier mobility. We show that this anisotropy can be readily varied and even inverted by various means, such as a gentle thermal treatment, UV irradiation, or even a weak uniaxial stress. Our experiments suggest that extended defects, which develop over long time periods (of weeks to months), can strongly influence the response of the confined charge carriers at the LAO/STO (110) interface.

Effect of the calcination temperature on the visible light photocatalytic activity of direct contact Z-scheme g-C3N4-TiO2 heterojunction

Z-scheme g-C3N4-TiO2 heterojunctions containing g-C3N4 nanosheets with different thickness were prepared by sintering the mixture of g-C3N4 and nanotube titanic acid (denoted as NTA) at different temperatures in air. As-prepared Z-scheme g-C3N4-TiO2 heterojunctions were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible light diffuse reflectance spectrometry, electron spin resonance, and photoluminescence spectrometry. Findings indicate that the annealing temperature has crucial effects on the visible-light photocatalytic activity (λ≥420nm) of the as-prepared Z-scheme g-C3N4-TiO2 heterojunctions. The Ti3+ and porous g-C3N4 nanosheets formed upon the calcination at 600°C as well as the low concentration of bulk single-electron trapped oxygen vacancy are favorable to the transport of the photoexcited charge carriers. This, in association with the Z-scheme system, contributes to improving the photocatalytic activity of g-C3N4-TiO2 photocatalysts. As a result, g-C3N4-TiO2 photocatalyst prepared at 600°C exhibits good photocatalytic activity towards the degradation of propylene and hydrogen generation by water-splitting under visible light irradiation.

IR studies of the oxygen and carbon precipitation processes in electron irradiated tin-doped silicon

Oxygen (O) and carbon (C) are key impurities in silicon (Si) and the control of their properties and behavior is an important issue for the microelectronic industry. A number of these properties can be manipulated by isovalent doping. Here we employ Fourier transform infrared (FTIR) spectroscopy to study the evolution of O and C concentration as well as the evolution of the oxygen precipitate bands in electron- irradiated tin (Sn) doped Si, subjected to isochronal anneals up to 950 °C. Special attention was given in connecting infrared absorption bands with certain precipitation morphologies. In this study, bands at 1040, 1060, 1080 and 1170 cm−1 generally attributed to precipitate morphologies were detected. Using arguments from classical theoretical mechanics we have attributed the 1040 cm−1 band to a structure more close to a spherical morphology, although the 1060 and 1080 cm−1 bands were attributed to structures more close to octahedral and polyhedral morphologies, respectively. Additionally the band at 1170 cm−1 was attributed to platelet precipitates. The effect of C and (C, Sn) co-doping Si in the morphologies of the precipitates bands was investigated in detail. It was found that in the irradiated material C suppresses the formation of spheroidal precipitates although it enhances the platelet precipitates, whereas in Si containing C and Sn the opposite behavior was detected. The presence of the two impurities modifies the number of the O precipitates and affects the relative density among the formed morphologies in Si, determining whether the spheroidal or platelet precipitates will prevail. The phenomenon was discussed taking into consideration the effect of the density of the nucleation sites on the interfacial energy of the precipitates. Furthermore, an inverse annealing stage in the evolution curve of C, namely an increase of C concentration prior to its complete disappearance was studied. This recovery stage was determined to be enhanced in (C, Sn) co-doping Si with relatively low Sn content although in Si with high Sn content the increase of C is substantially lower. An explanation was suggested based on the ability of the Sn to temporarily trap vacancies, thus affecting the restoration of the C substitutional atoms in the Si lattice.

Strong trapping and slow diffusion of helium in a tungsten grain boundary

We have investigated the segregation, trapping and diffusion of He in a ∑3<110>{111} W grain boundary (GB) using combined techniques of ab initio and classical atomistic simulations. We show that, with an average segregation energy of −3.20 eV, the strong He trapping can be attributed to a GB interstitial trapping or a vacancy trapping mechanism, while an average energy barrier of 1.97 eV leads to a slow diffusion of He in the GB plane. We further reveal by molecular dynamics simulations that the He diffusion will be dictated by GB migration through the motion of GB disconnections. Interestingly, we also observe a He-induced GB structural transition in classical simulations. The present work suggests that the GB does not provide fast transport channel for He, providing useful reference for the possible application of polycrystalline W under He irradiation in advanced nuclear fusion reactors.

Kinetic Monte Carlo Simulations of Helium Cluster Nucleation in Tungsten with Preexisting Vacancies

The object kinetic Monte Carlo code Kinetic Simulations Of Microstructure Evolution (KSOME) was used to study the subsurface helium clustering behavior in tungsten as a function of temperature, helium implantation rate, and vacancy concentration. The simulations evaluated helium implantation fluxes from 1022 to 1026 m−2 · s−1 at temperatures from 473 to 1473 K for 100-eV helium ions implanted below tungsten surfaces and for vacancy concentrations between 1 and 50 parts per million. Such vacancy concentrations far exceed thermodynamic equilibrium values but are consistent with supersaturated concentrations expected during concurrent, or preexisting, neutron irradiation. The thermodynamics and kinetic parameters to describe helium diffusion and clustering are input to KSOME based on values obtained from atomistic simulation results. These kinetic Monte Carlo results clearly delineate two different regimes of helium cluster nucleation, one dominated by helium self-trapping at high implantation rates and lower temperatures and one where helium–vacancy trapping dominates the helium cluster nucleation at lower implantation rates and higher temperatures. The transition between these regimes has been mapped as a function of implantation rate, temperature, and vacancy concentration and can provide guidance to understand the conditions under which neutron irradiation effects may contribute to subsurface gas nucleation in tungsten plasma-facing components.

Dual role of Sb ions as electron traps and hole traps in photorefractive Sn_2P_2S_6 crystals

Doping photorefractive single crystals of Sn2P2S6 with antimony introduces both electron and hole traps. In as-grown crystals, Sb3+ (5s2) ions replace Sn2+ ions. These Sb3+ ions are either isolated (with no nearby perturbing defects) or they have a charge-compensating Sn2+ vacancy at a nearest-neighbor Sn site. When illuminated with 633 nm laser light, isolated Sb3+ ions trap electrons and become Sb2+ (5s25p1) ions. In contrast, Sb3+ ions with an adjacent Sn vacancy trap holes during illumination. The hole is primarily localized on the (P2S6)4− anionic unit next to the Sb3+ ion and Sn2+ vacancy. These trapped electrons and holes are thermally stable below ∼200 K, and they are observed with electron paramagnetic resonance (EPR) at temperatures below 150 K. Resolved hyperfine interactions with 31P, 121Sb, and 123Sb nuclei are used to establish the defect models.

Vacancy relaxation in cuprous oxide (Cu2−xO1−y)

Phonons are produced when an excited vacancy in cuprous oxide (Cu2O) relaxes. Time resolved luminescence was used to find the excited copper vacancy (acceptor) and oxygen vacancy (donor) trap levels and lifetimes. It was also used to determine the typical energy and number of phonons in the phonon pulses emitted by vacancies. The vacancy properties of cuprous oxide are controlled by several synthesis parameters and by the temperature. We directly demonstrate the absorption of light by oxygen vacancies with transient absorption. Copper and oxygen vacancies behave differently, in part because the two kinds of traps capture carriers from different states. For example, the copper vacancy luminescence lifetime is around 25 times greater at low temperature. However, both kinds of vacancy luminescence are consistent with a Poissonian multiple phonon emission model.