Average Positron Lifetime Research Articles

High concentration quenching, improved PLQY and high color purity of novel Gd2TiO5:Eu3+ triggered by complete solubility of dopant

In the surge of new inorganic luminescent host for lanthanide ion for efficient solid-state lighting, Ln2TiO5 comes as a new class of materials with all optically suited properties. Triggered by very high solubility of Eu3+ ion in Gd2TiO5 with minimal lattice strain, negligible doping induced structural distortion and no charge compensating defects; Gd2TiO5:Eu3+ (GTOE) delivered an extraordinary credentials as novel phosphor material. GTOE demonstrated no concentration quenching up to 10 mol % doping; monotonic rise in emission intensity, higher excited state lifetime, high red color purity∼99% and PLQY∼ 14%. Luminescence lifetime spectroscopy suggested stabilization of Eu3+ in both GdO7 with distorted pentagonal bipyramid (DPB) and distorted hexagonal bipyramid (DHB) geometry having similar coordination and symmetry and is well supported by two stark components in 5D0→7F0 transitions. Positron annhilation lifetime spectroscopy (PALS) suggested that average positron lifetime remained constant in all the samples indicating no charge compensating defects formation further justifying occupancy of Eu3+ in isovalent Gd3+ site. Judd-Ofelt analysis suggested that local symmetry around Eu3+ ion and polarizability of Eu–O bonds in GTO host doesn't change throughout the concentration range. This work proposed an excellent candidate for red emitting materials towards phosphor converted white light emitting diodes (pcLEDs) having high dopant solubility, PLQY and color purity.

Microstructural dependence of defect formation in iron-oxide thin films

Passivating iron-oxide films are grown atop iron films simulating the corrosion process in a nuclear reactor environment. Two oxide films grown via physical vapor deposition at 600 °C and room temperature exhibited dense-epitactic and columnar-polycrystalline, microstructures respectively. A third oxide film grown in open air at 600 °C exhibited an eqiuaxed, porous morphology. Cubic maghemite and magnetite phases in each oxide film were identified via grazing incidence X-ray diffraction. Positron annihilation spectroscopy was used to characterize point defects and measure their depth and size distributions in each oxide layer and showed a range of average positron lifetimes from 0.23 ns in the high temperature, vapor deposited film, 0.35 ns in the room temperature-grown film, and 0.31 ns in the thermally grown oxide. These data indicate that the film morphology, which varies greatly in these films, leads to very different defect content. Finally, four-dimensional scanning transmission electron microscopy was used to measure the internal stress of each film and was correlated to the strain state presented in the X-ray diffraction spectra. The defect formation in each film is reasoned through using a thin film growth model.

Effects of neutron flux on irradiation-induced hardening and defects in RPV steels studied by positron annihilation spectroscopy

Neutron-flux effects on irradiation-induced hardening and microstructures in a reactor pressure vessel steel were studied. An A533B-type steel containing no Cu was neutron-irradiated with fluxes of 1 × 1014 n/cm2/s (high-flux) or 1 × 1012 n/cm2/s (low-flux) to the same fluence of approximately 3 × 1019 n/cm2, and the same temperature of approximately 290 °C. The recovery behavior of irradiation-induced defects and irradiation-hardening, ΔHv, was investigated by post-irradiation isochronal annealing from 275 to 450 °C. In both the high- and low-flux cases, the recovery behavior of ΔHv and the average positron lifetime, τave, corresponded well to the annealing, suggesting that defects in which positrons are trapped are the origin of irradiation-hardening. The values of ΔHv and τave in the high-flux sample started to recover at around 350 °C, while those in the low-flux sample started to recover at around 400 °C. Thus, in the high-flux sample, unstable defects transiently existing at low temperature but annealed out at around 350 °C, are indicated. Such defects are suggested to be defect-(Mn, Ni, Si) complexes, where the nature of the defect is that of a mono-vacancy and/or dislocation loops.

Correlation studies between physical properties and concentration of voids entrenched in V2O5 mixed lead bismuth silicate glass system by means of positron annihilation spectroscopy

In this investigation we have estimated the fraction of free volume defects and also their size (to the nanoscale) entrenched in V2O5 doped PbO added Bi2O3–SiO2 glass system (by means of positron annihilation spectroscopy (PAS) studies) with a view to understand the influence of such defects on several physical properties of this glass system. The glasses of the chemical composition viz., 30PbO–5Bi2O3‒ (65- x) SiO2:x V2O5 (0 ≤ x ≤ 0.6, insteps of 0.1) are used in this study. Glasses were prepared by traditional melt quenching technique. PAS studies were perormed using 22Na (0.1 MBq) positron radioactive source. Positron annihilation lifetime is found to be significantly affected by the concentrations of V2O5 dopant. The third component I3 of positrons is found to be feeble in all the glasses. For this reason, initially, we have used a simple two component positron trapping model (ignoring the third component) to calculate annihilation parameters viz., the bulk average positron lifetime (τb), positron trapping rate in defect (κd), fraction of trapped positrons (η), the average lifetime for all the positrons (τavg) and defect-related time (τd). The calculations were repeated with three component positron model. The results of positron annihilation studies have indicated that concentration of free volume nano sized defects, the fraction (fv) of volume defects and their radius (R) are the lowest for the glass V3. Such minimal value of defect concentration is ascribed to dominant existence of vanadium ions in V5+ valence state that involves in glass network forming with VO5 structural units. Whereas, the observed increased value of fv with increase of V2O5 from 0.3 to 0.6 mol% is attributed to growing concentration of vanadyl complexes (VO)2+ that have acted as modifiers. Quantitative information obtained on dependence of concentration of free volume imperfections on V2O5 content in the studied glass is observed to be in good agreement with the inferences drawn from several macroscopic properties that include electrical properties reported earlier.

Size effect in nonstoichiometric titanium monoxide and vanadium carbide nanocrystals measured by positron lifetime spectroscopy

A size effect in nonstoichiometric titanium monoxide TiOy and vanadium carbide VCy nanocrystals containing high concentrations of structural vacancies was detected by electron-positron annihilation. The effect consists of a considerable increase in the positron lifetime with the specific surface area. An analysis of the intensity of a long lifetime surface component in the positron annihilation spectra and the average positron lifetime for different nonstoichiometric compounds indicated a universal character of the observed size effect.

Vacancy complexes in nonequilibrium germanium-tin semiconductors

Depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to identify vacancy-related complexes and probe their evolution as a function of Sn content in GeSn epitaxial layers. Regardless of the Sn content in the 6.5–13.0 at. % range, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, relative to that of epitaxial and bulk Ge references, thus confirming the formation of open volume defects during growth. The measured average positron lifetimes were found to be the highest (380–395 ps) in the region near the surface and monotonically decrease across the analyzed thickness but remain above 350 ps. All GeSn layers exhibit average lifetimes that are 20–160 ps higher than those recorded for the Ge reference. Surprisingly, these lifetimes were found to decrease as the Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge, thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were also found to contain a small fraction of vacancy clusters, which become less important as the Sn concentration increases. The interaction and possible pairing between Sn and vacancies have been proposed to explain the reduced formation of larger vacancy clusters in GeSn when the Sn content increases.

Effect of dopant concentration and annealing of Yttrium doped CuO nanocrystallites studied by positron annihilation spectroscopy

In this paper, we report the solution combustion synthesized Yttrium doped CuO nanocrystallites studied by positron annihilation lifetime (PAL) and Doppler broadening (DB) spectroscopy. The lattice defect in Y doped CuO nanocrystallites is presented by the effect of dopant concentration and annealing temperature. X-ray diffraction measurements indicated monoclinic CuO with impurity phase (Y2O3) of Y at elevated temperature. Scanning Electron Microscopy observations showed the particles are more agglomerated upon annealing. The PAL study exhibited three-lifetime components (τ1, τ2, and τ3) wherein, as-prepared CuO exhibited an increased lifetime (τ1) of 212 ps compared to the reported bulk lifetime of 169 ps. The analysis confirmed the mixed contribution of the annihilations of trapped positrons in CuO bulk, point, and extended defects. The τ2 and τ3 are ascribed to the vacancy clusters and voids. Defects modification with doping concentration and annealing temperature are explained in detail. The average positron lifetime (τav) of all the samples increases with doping concentration and decreases with annealing temperature. Grain growth hindrance due to Y ion migration is presented effectively, signifying the higher average lifetime. Comparative analysis of the average lifetime with the extracted from the valance electron annihilations associated S parameter from DB and on the crystallite size variation with annealing is presented on the basis of defects formation and its curing mechanism. The defect structure change owing to charge compensation of Y in CuO is explained. The defect modification of doped and annealed CuO is explained well from the change in crystallite growth and surface defects.

Defect Study in CoCrFeMnNi High Entropy Alloy by Positron Annihilation Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy is applied to study the crystal defects in CoCrFeMnNi high entropy alloy. The material is measured in the as‐received (electro eroded; non‐etched) state; it shows a defect‐related positron lifetime of 220 ps, corresponding most likely to divacancies, with a relatively high concentration. Furthermore, this sample is annealed in the temperature range up to 1000 K and positron lifetime is measured after each annealing step. The average positron lifetime is found to decrease with annealing and reaches 121 ps after annealing at 1000 K, indicating the existence of some remaining defects in the material. To show whether the observed defects are present in the bulk sample or only in the surface area, another sample is deeply etched and shows only one lifetime component of 112 ps, which is assigned to the defect‐free bulk lifetime in this alloy. This study indicated that the investigates high entropy alloy (in etched state) does not contain open‐volume defects within the sensitivity range of positron annihilation. Here, the defects observed in the non‐etched sample are present in the subsurface region, which are assumed to be generated most probably during wire‐erosion of the samples. In addition, X‐ray diffraction results indicate that the crystal structure is FCC with a lattice constant of (3.597 ± 0.002) Å, which is not affected by annealing.

Synthesis, defect characterization and photocatalytic degradation efficiency of Tb doped CuO nanoparticles

Monoclinic undoped and Tb doped CuO are prepared by solution combustion method and annealed at different temperatures. The effect of annealing and doping on their structural and optical properties of CuO are examined using XRD, FTIR and DRS. The surface and lattice defects in CuO and Tb doped CuO is analyzed qualitatively and quantitatively using positron lifetime and Doppler broadening spectroscopy. The average positron lifetime and electron momentum (energy) S parameter increases owing to the number of vacancies in the CuO lattice upon doping and decreases with increasing temperature. The migration of vacancies from grain to grain boundary region is observed at 600°C annealed samples. At 800 °C, the overall behavior of lifetime value denotes that the vacancy type defect is recovered, cluster vacancy and microvoids exists with reducing size. The photocatalytic performance of undoped and Tb doped CuO on degradation of methylene blue (MB) and methyl orange (MO) is investigated under visible light for two different lamp power and dye concentration. The influence of annealing temperature and dopant ion on the efficiency is also elaborated. Enhanced photocatalytic efficiency in Tb doped CuO is observed upon annealing. X-ray photoelectron spectroscopy (XPS) result indicates that the valence states of Cu, O and Tb ions exist at the surface of the particles. Brunauer–Emmett–Teller N2 adsorption–desorption analyses were employed to characterize specific surface area and porosity of Tb doped CuO. The doped CuO with pore size of about ∼34nm have a surface area of 16–28m2/g. The surface area effect plays an important role in the enhanced catalytic performance on Tb doped catalysts.

Positron Annihilation Spectroscopic Studies of Cu<sub>2</sub>Te Thermoelectric Material

Positron annihilation lifetime spectroscopy (PALS) and coincident Doppler-broadening spectroscopy (CDBS) have been used for investigating the evolution of vacancy-type defects in the thermoelectric material Cu2Te which annealed at different temperatures. The results of PALS show that a fraction of positrons has got annihilated at the surfaces and the sample which annealed at 450 °C has the highest concentration of surface defects. The average positron lifetime and the S parameter have the same trends which gradually increase with the increase of the annealing temperature. This change implies that the total concentration of the defects has been changed with the change of the annealed temperatures. The results of the CDBS ratio spectrum and S-W plot indicate that the defect species have no change after annealing at different temperatures.

Effect of FLiBe Infiltration Pressure on Microstructure of Matrix for TMSR Fuel Elements (FEs)

The matrix graphite of fuel elements (FEs) with infiltration of 2LiF-BeF2(FLiBe) at different pressures varying from 0.4 MPa to 1.0 MPa, has been studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and positron annihilation lifetime (PAL) measurement. The result of XRD reveals that diffraction patterns of FLiBe appear in matrix graphite infiltrated with FLiBe at a pressure of 0.8 MPa and 1.0 MPa. The surface morphology from SEM shows that FLiBe mainly distributes within macro-pores of matrix graphite. PAL measurement indicates that there are mainly two positron lifetime components in all specimens:τ1~0.21 ns and τ2 ­~0.47 ns, ascribed to annihilation of positrons in bulk and trapped-positrons at surface, respectively. The average positron lifetime decreases with infiltration pressure, due to the decrease in annihilation fraction of positrons with surface after infiltration of FLiBe into macro-pores.

Microstructure and Defects of MgO/ZrO<sub>2</sub> Nanocrystals Investigated by Positron Annihilation Spectroscopy

Pure MgO, ZrO2 and mixture MgO/ZrO2 nanocrystals were annealed in air from 100 to 1200°C. Variation of the microstructure and defects was investigated by positron lifetime spectroscopy and X-ray diffraction. The experiment results showed that the average positron lifetime of mixture MgO/ZrO2 was more larger than that of single phase MgO and ZrO2, and decreased with the increasing annealing temperature. Thermal annealing below 600°C, the movement of grain boundaries mainly led a reduce of the number of microvoids, and vacancy defects began to recover due to the growth of MgO nanoparticles after annealing between 600 to 900°C. Furthermore, ZrO2 nanoparticles began to grow above 900°C, meanwhile the recovery of vacancy and vacancy clusters in MgO/ZrO2 nanoparticles are restrained because of synergic effect between MgO and ZrO2 nanoparticles.

Positron Spectroscopy of Defects in Hydrogen-Saturated Zirconium

Zirconium alloys, such as Zr-1Nb are widely used as cladding materials for nuclear fuel elements of light water reactors. Hydrogen embrittlement problem causes degradation of these parts of nuclear reactors. It is known, that hydrogen uptake causes changes in microstructure and defect structure of metals. The aim of this work is study of Zr-1Nb alloys’ defect structure after hydrogen saturation up to the concentration of 600 ppm. Saturation of hydrogen was carried out from the gas phase under high temperature and pressure. This study reveals the increase of average positron lifetime with the increase of hydrogen concentration. Value of the average positron lifetime achieves plateau when the concentration of hydrogen is about 600 ppm. Also the following effects were detected in the material after hydrogen uptake up to different concentrations: crystal lattice expansion, dislocations and vacancy-like defects formation, as well as the defect-hydrogen complexes formation.

Positron Annihilation Study of Ternary Sb2Te3−x Se x for Its Tuning Electrical and Thermal Properties

Atomic scale point defects play important roles in tuning the carrier concentration and ultimately influencing electrical and thermal properties. Herein, we fabricated the ternary Sb2Te3−xSex alloys to study the intimate relationship of internal point defects and thermoelectric performance. The Se substitution of Te atoms in the Sb2Te3 lattice decreased the electrical conductivity from 2.2 × 105 S/m to 6.4 × 104 S/m owing to the reduced holes concentration. The declined point defects, including antisite defects and vacancies in materials, gave rise to the decrease in carrier concentration. The Seebeck coefficient of the ternary Sb2Te3−xSex exhibited an increase with doping of Se atoms. Simultaneously, the thermal conductivity behaved a fallihg trend as well as increasing Se content. As a result, the ZT value reached the maximum from the corresponding Sb2Te2.9Se0.1 pellet. Positron annihilation measurement revealed that the average positron lifetime showed a monotonic decrease with Se addition, demonstrating the reduced point defects, which was in agreement with the thermoelectric performance.

Positron annihilation lifetime spectroscopy (PALS) studies of gamma irradiated As2Se3 films used in MIR integrated photonics

Chalcogenide glass (ChG) materials are increasingly being utilized in infrared planar photonic devices due to their tailorable refractive indices and compatibility with other on-chip materials and manufacturing methodologies. The performance and reliability of such integrated photonic components in land or space-based environments where radiation impact structure and properties is important to their ultimate deployment. Bulk glasses and thermally evaporated thin films of As2Se3 were subjected to a gamma exposure of 229Mrad to compare bulk/film material response and the impact of film thickness on such response. Positron annihilation lifetime (PAL) spectroscopy was employed to compare the effect of gamma irradiation on the different forms of the glass. The average positron lifetime was found to be longer for the films than the bulk glass in unirradiated specimens, and the films showed more significant increases in lifetimes with gamma exposure. This effect was more pronounced for thinner films, which showed, for example, an increase in positron lifetime. Positron lifetime was measured over the course of 20days after exposure, over which the effect of the gamma radiation was found to relax at room temperature. Long-term or incomplete relaxation of the gamma-induced changes were observed in the films, while the bulk glass positron lifetime returned to pre-exposed values. A proposed mechanism for these observations based on coordination topological defects (CTD) is presented to explain the initial and longer term response, and the possible consequence when such films are integrated in device geometries.

Positron annihilation lifetime in float‐zone n‐type silicon irradiated by fast electrons: a thermally stable vacancy defect

AbstractTemperature dependency of the average positron lifetime has been investigated for n‐type float‐zone silicon, n‐FZ‐Si(P), subjected to irradiation with 0.9 MeV electrons at RT. In the course of the isochronal annealing a new defect‐related temperature‐dependent pattern of the positron lifetime spectra has been revealed. Beyond the well known intervals of isochronal annealing of acceptor‐like defects such as E‐centers, divacancies and A‐centers, the positron annihilation at the vacancy defects has been observed in the course of the isochronal annealing from ∼ 320 °C up to the limit of reliable detecting of the defect‐related positron annihilation lifetime at ≥ 500 °C. These data correlate with the ones of recovery of the concentration of the charge carriers and their mobility which is found to continue in the course of annealing to ∼ 570 °C; the annealing is accomplished at ∼650 °C. A thermally stable complex consisting of the open vacancy volume and the phosphorus impurity atom, Vop‐P, is suggested as a possible candidate for interpreting the data obtained by the positron annihilation lifetime spectroscopy. An extended couple of semi‐vacancies, 2Vs‐ext, as well as a relaxed inwards a couple of vacancies, 2Vinw, are suggested as the open vacancy volume Vop to be probed with the positron. It is argued that a high thermal stability of the Vs‐ext PVs‐ext (or VinwPVinw.) configuration is contributed by the efficiency of PSi5 bonding. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ordering processes in Fe-Ga alloys studied by positron annihilation lifetime spectroscopy

To investigate the nature of defects in Galfenols (Fe100−xGax) with content near the local minimum (x=20.5 and x=22.4) and the local maximum of magnetostriction (x=26.3), the positron annihilation lifetime spectroscopy (PALS) and the X-ray diffraction analysis (XRD) are applied. It is found that the behavior of vacancies during ordering processes in the alloy with content higher than in stoichiometric composition of Fe3Ga (x>25) differs significantly from the other two alloys (x<25). All the studied alloys quenched from 1000°C exhibit a positron lifetime of about 175ps, which corresponds to high concentration of single vacancies (>2·10−4at−1). The growth of the bcc-born D03 ordered phase with relatively low concentration of vacancies in Fe79.5Ga20.5 and Fe77.6Ga22.4 samples causes a decrease of the average positron lifetime. The formation of the D03 phase in Fe73.7Ga26.3 does not reduce average positron lifetime, which indicates a high concentration of constitutional vacancies in the ordered D03 phase of Fe73.7Ga26.3. The D03→L12 phase transition is always accompanied by additional vacancy formation.

Positron annihilation lifetime study of polyvinylpyrrolidone for nanoparticle-stabilizing pharmaceuticals

Positron annihilation lifetime spectroscopy was applied to characterize free-volume structure of polyvinylpyrrolidone used as nonionic stabilizer in the production of many nanocomposite pharmaceuticals. The polymer samples with an average molecular weight of 40,000gmol−1 were pelletized in a single-punch tableting machine under an applied pressure of 0.7GPa. Strong mixing in channels of positron and positronium trapping were revealed in the polyvinylpyrrolidone pellets. The positron lifetime spectra accumulated under normal measuring statistics were analysed in terms of unconstrained three- and four-term decomposition, the latter being also realized under fixed 0.125ns lifetime proper to para-positronium self-annihilation in a vacuum. It was shown that average positron lifetime extracted from each decomposition was primary defined by long-lived ortho-positronium component. The positron lifetime spectra treated within unconstrained three-term fitting were in obvious preference, giving third positron lifetime dominated by ortho-positronium pick-off annihilation in a polymer matrix. This fitting procedure was most meaningful, when analysing expected positron trapping sites in polyvinylpyrrolidone-stabilized nanocomposite pharmaceuticals.

Formation and annealing of vacancy-P complexes in proton-irradiated germanium

Abstract We have used positron annihilation spectroscopy to study the formation and annealing of defects in germanium irradiated with 15 MeV protons under different doses at 300 K. The samples were n-type Ge ([P] = 2 × 1017 cm−3). Deep and shallow positron states were observed, which were attributed to the vacancy complexes as well as to the negative ion defects. The complex defect is established to involve a vacancy and a phosphorous atom. Isochronal annealing of the irradiated material was carried out over the temperature range 300–740 K. The positron lifetime in the temperature interval 35–300 K was measured after each isochronal annealing step. Two salient temperature stages of annealing were found. The first stage ranging from 400 to 500 K was attributed to the annealing of the vacancy complexes. Vacancy clusters involving two vacancies begin to appear at 420 K in the course of the isochronal annealing, whereas further increase of the annealing temperature up to 520 K gives rise to the formation of clusters of about 3 vacancies. The second annealing stage is over the temperature range of 550–650 K, which is ascribed to annealing of vacancy clusters. The model of negative-ion-type positron traps was used for describing the results of the positron lifetime measurements related to the shallow positron traps. These defects disappear within the annealing temperatures range 500–600 K. After annealing of the irradiated material at 740 K, the average positron lifetime is almost the same as that of the non-irradiated Ge material.

Positron annihilation measurements in high-energy alpha-irradiated n-type Gallium Arsenide

Positron annihilation lifetime spectroscopy and Doppler broadening annihilation line-shape measurements have been carried out in 40-MeV alpha-irradiated n-type GaAs. After irradiation, the sample has been subjected to an isochronal annealing over temperature region of 25–800 °C with an annealing time of 30 min at each set temperature. After each annealing, the positron measurements are taken at room temperature. Formation of radiation-induced defects and their recovery with annealing temperature are investigated. The lifetime spectra of the irradiated sample have been fitted with two lifetimes. The average positron lifetime τavg = 244 ps at room temperature after irradiation indicates the presence of defects, and the value of τ2 (262 ps) at room temperature suggests that the probable defects are mono-vacancies. Two distinct annealing stages in τavg at 400–600 °C and at 650–800 °C are observed. The variations in line-shape parameter (S) and defect-specific parameter (R) during annealing in the temperature region 25–800 °C resemble the behaviour of τavg indicating the migration of vacancies, formation of vacancy clusters and the disappearance of defects between 400 and 800 °C.