Positron Annihilation Spectra Research Articles

Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy.

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique to investigate defects in solids, such as vacancy clusters and grain boundaries in metals and alloys, as well as lattice imperfections in semiconductors. Positron spectroscopy is able to reveal the size, structure and concentration of vacancies with a sensitivity of 10-7. In the field of porous and amorphous systems, PALS can probe cavities in the range from a few tenths up to several tens of nm. In the case of polymers, PALS is one of the few techniques able to give information on the holes forming the free volume. This quantity, which cannot be measured with macroscopic techniques, is correlated to important mechanical, thermal, and transport properties of polymers. It can be deduced theoretically by applying suitable equations of state derived by cell models, and PALS supplies a quantitative measure of the free volume by probing the corresponding sub-nanometric holes. The system used is positronium (Ps), an unstable atom formed by a positron and an electron, whose lifetime can be related to the typical size of the holes. When analyzed in terms of continuous lifetimes, the positron annihilation spectrum allows one to gain insight into the distribution of the free volume holes, an almost unique feature of this technique. The present paper is an overview of PALS, addressed in particular to readers not familiar with this technique, with emphasis on the experimental aspects. After a general introduction on free volume, positronium, and the experimental apparatus needed to acquire the corresponding lifetime, some of the recent results obtained by various groups will be shown, highlighting the connections between the free volume as probed by PALS and structural properties of the investigated materials.

The evolution of structure and magnetic properties of GdCr1-Fe O3 (0 ≤ x ≤ 0.5) polycrystalline samples

In this paper, the evolution of crystal structures and magnetic properties of GdCr 1- x Fe x O 3 (0 ≤ x ≤ 0.5) samples with different doping levels was systematically investigated by powder X-ray diffraction, Fourier transform infrared spectra, positron annihilation spectra, and temperature-dependent and field-dependent magnetization measurements. The lattice constants and Cr(Fe)O 6 octahedral deformation increase with Fe doping showing an anomaly at x = 0.4 which is further confirmed by positron annihilation and Fourier transform spectra. Fe doping in GdCrO 3 can suppress the magnetization reversal in the parent phase as well as its antiferromagnetic ordering temperature T N at low doping levels but enhance T N at high doping levels. A possible spin flip or flop transition under field-cooled-cooling process occurs for the samples of x = 0.3 and 0.4 at low temperature. The coercive fields of weak ferromagnetic state with different doping levels at 10 and 100 K also exhibit a local maximum at around x = 0.4 which is accordingly correlated to the concentrations of the vacancy defects in the sample. • The information of cationic vacancy defects is analyzed by positron annihilation technique. • The correlation between lattice parameters and vacancy defects is unveiled. • Defects in GdCr 1- x Fe x O 3 samples have a great impact on their magnetic properties. • Chemical doping and defect engineering are effective tools to manipulate R CrO 3 materials.

Vacancy-Modulated of CuS for Highly Antibacterial Efficiency via Photothermal/Photodynamic Synergetic Therapy.

Cu-based nanomaterials have been developed to alleviate the problem of antibiotic resistance due to their superior properties and good biocompatibility. Defects in nanomaterials have a major role in improving photocatalytic performance. Herein, two CuS nanospheres with predominant VCuSCu and VCuSS vacancy (abbreviated as CuS and CuS-T150, respectively) characterized by positron annihilation spectra are synthesized. The combination of experimental and theoretical calculation results demonstrates that CuS-T150 exhibits excellent antibacterial, achieving bactericidal rates of 99.9% against to Escherichia coli (E. coli) under 808nm laser irradiation. Compared with CuS, the superior antimicrobial activity of CuS-T150 is mainly attributed to its stronger ability to adsorb oxygen molecules, more easily bind with surface of E. coli, and higher photothermal conversion efficiency (PTCE). This work provides a deeper understanding of nanomaterials with vacancy modulated the antibacterial efficiency by synergistic effect of photodynamic and photothermal therapy.

Separation performance of alcohol-induced silk fibroin membranes with homogeneous and heterogeneous microstructures

Biological membranes were prepared from silk fibroin (SF) to investigate the mechanism SF microstructure that induced by different alcohols (C1–C4OH). SF membranes which were induced by less carbon of alcohols had a more hydrophilic surface (∼44.4°) than that induced by multi-carbon alcohol (∼80.5°). Positron annihilation spectrum showed that treating SF membrane using highly polar methanol/water generates heterogeneous structures. The internal hydrophobic segment had more β-sheets and smaller free volume than the internal hydrophilic segments because the latter flipped over to the surface. Consequently, the methanol/water-induced SF membrane showed excellent isopropyl alcohol separation efficiency. The low polar butanol/water-induced SF membranes formed homogeneous structures with a more hydrophobic surface and a larger total free volume than the methanol/water-induced SF membranes. Therefore, the crystallinity of the SF membranes could be controlled through different alcohols/water treatment, where the homogeneous phase could transform into the heterogeneous phase.

The influence of Zr doping on the structure, vacancy defects, and magnetic properties of GdMnO3 ceramics

The influences of the Zr doping on the structure, vacancy defects, and magnetic properties of GdMn1-xZrxO3 (x = 0.00–0.20) ceramic samples synthesized using a solid-state reaction method were investigated. All the GdMn1-xZrxO3 samples formed a single-phase structure, and no phase transformation occurred within the experimental doping range; however, Zr doping caused lattice distortion. The microstructure of the GdMn1-xZrxO3 samples was compact, and Zr doping reduced the grain size. The positron annihilation spectrum (PAS) measurements showed that Zr doping had a great influence on the cation vacancy concentration of GdMn1-xZrxO3 samples. The vacancy concentration increased with increasing Zr doping content from 0.00 to 0.10, and then decreased with the further increase in Zr doping content. XPS results showed that the charge compensation induced by Zr doping was achieved mainly by the decrease of cation valence (forming Mn2+) in the x = 0.00–0.10 ceramics. Appropriate Zr doping obviously improved the transition temperatures and magnetization of the GdMn1-xZrxO3 samples. This investigation demonstrated that the cation vacancy concentration is one of the important factors that could tailor the magnetic properties of GdMn1-xZrxO3ceramics.

Influence of Ti ions valence states on the optical and magnetic properties of GdCr1-xTixO3 (0≤x≤0.05)

The vacancy defects, optical and magnetic properties of GdCr1-xTixO3 (0≤x ≤ 0.05) polycrystalline samples were investigated in this research. The crystal structural analyses show that the orthorhombic perovskite structure exists in all the samples. The positron annihilation spectra reveal that the vacancy defects and local electron density vary with the change of Ti ions valence states. The magnetic measurements show that the antiferromagnetic transition is shifted to lower temperature with the replacement of Ti ions in Cr sites, and the value of spin reorientation caused by the exchange interaction between Gd3+ ion and Cr3+ ion can be influenced by the change of Ti ions valence states, which rises from x = 0 to 0.04 and then decreases at x = 0.05 sample. Simultaneously, the optical band gap decreases from 2.7 eV to 0.8 eV with the introduction of Ti ions into the lattice, the result can provide a reference for improving optical activity of rare-earth chromium oxides.

Vacancy-Enhanced Photothermal Killing of Bacteria Mediated by Graphene Oxide.

With the emergence of antibiotic resistance, the development of efficient antimicrobial agents has become increasingly important. Graphene oxide (GO) has been used as an antibacterial agent, but how to realize and improve the antibacterial properties of GO is still required critically. Herein, we prepared two GO samples, abbreviated as GO-V11 and GO-V9. Positron annihilation spectra showed that they possessed predominantly VCCCCCCCCCCC (V11C) and VCCCCCCCCC (V9C) carbon vacancies, respectively. Their photothermal antibacterial properties were measured against Gram-negative Escherichia coli (E. coli) and Gram-positive Bacillus subtilis (B. subtilis) by using colony-forming unit and liquid optical density assays. GO-V9 displayed a higher photothermal antibacterial efficiency toward the two bacteria than GO-V11 because GO-V9 had a higher photothermal conversion efficiency (PTCE) (57.3%) than GO-V11 (42.5%). To reveal the difference in their PTCEs and antibacterial efficiencies, their energy band structures were tested with density functional theory calculations. The different vacancies changed the energy band structure from the indirect band gap of GO-V11 to the quasi-metallic band gap of GO-V9. The quasi-metallic band gap showed the higher PTCE, so we revealed the importance of the band gap of GO for its antibacterial mechanism. Tuning the vacancy properties is promising for improving the photothermal antibacterial efficiency.

Enzyme-like antibacterial activities of Cu9S5 nanoflowers with vacancy-type dependence

Due to the increasing drug resistance of bacteria and the limitations of antibacterial nanomaterials through photodynamical and photothermal routes, it is necessary to develop novel nanomaterials with outstanding enzyme-like activities and bacteria-binding capability to produce reactive oxygen species (ROS) to kill bacteria. Herein, we report the synthesis of two kinds of Cu 9 S 5 nanoflowers composed of assemblies of nanosheets, abbreviated as Cu 9 S 5 -1 and Cu 9 S 5 -2, which possess predominantly V CuSCu and V CuSS vacancies, respectively, as confirmed by the positron annihilation spectra. Both experimental and theoretical calculation results reveal that Cu 9 S 5 -2 exhibits excellent enzyme-like activities because of stronger bacteria-binding ability and higher electron affinity that endow it with a superior antibacterial activity compared to Cu 9 S 5 -1. This study provides a deeper understanding of nanomaterials with vacancy-type-dependent enzyme-like activities and insights for the rational design of high-performance antibacterial agents in the future. Cu 9 S 5 -1 and Cu 9 S 5 -2 nanoflowers possess V CuSCu and V CuSS vacancies, respectively Cu 9 S 5 -2 displays excellent bacteria-binding ability and higher electron affinity Cu 9 S 5 -2 exhibits better enzyme-like antibacterial properties compared with Cu 9 S 5 -1 Chen et al. synthesize two kinds of Cu 9 S 5 nanoflowers with different vacancies and report that Cu 9 S 5 -2 (with V CuSS vacancies) possesses excellent enzyme-like antibacterial properties due to stronger electron affinity and adsorption compared to Cu 9 S 5 -1 (with V CuSCu vacancy). This finding may provide new insights for the design of high-performance antibacterial materials.

Oxygen Vacancy-Enhanced Ultrathin Bi2O3-Bi2WO6 Nanosheets' Photocatalytic Performances under Visible Light Irradiation.

The oxygen vacancy caused by ultrathin structures would be introduced into the semiconductor photocatalyst to boost its photocatalytic activity. Herein, ultrathin Bi2O3-Bi2WO6 nanosheet composites have been successfully synthesized via a facile hydrothermal method. Compared to pure Bi2WO6 nanosheets, the Bi2O3-Bi2WO6 nanosheet composites possess abundant oxygen vacancies, which was confirmed by the positron annihilation spectra. The ultrathin Bi2O3-Bi2WO6 nanosheet composites exhibited remarkable photocatalytic degradation performance for oxytetracycline compared with that of pure Bi2WO6 nanosheets. The excellent photocatalytic activities of Bi2O3-Bi2WO6 composites could be attributed to the heterojunction structure and the oxygen vacancies caused by ultrathin structures.

Proton irradiation induced phase transformation in Ni-Mn-Ga thin films

This paper reports the martensitic transformation of NiMnGa film induced by proton irradiation, which is of great significance for evaluating the reliability of NiMnGa film. Ni-Mn-Ga thin films are spotlighted as next-generation alternatives to currently used micro-actuators in the MEMS system for aircraft. Nonetheless, their potential applications are hindered by the stability under proton irradiation in the space. In the present manuscript, the microstructure evolutions of Ni50Mn25Ga25 thin films are investigated under various proton irradiation doses. A large number of point defects are introduced into the thin films at the irradiation dose over 1 × 1016 p/cm2 as indicated by the positron annihilation spectrum. And the type of defects remains the same after irradiation Consequently, 7 M martensite is formed on the surface of austenitic Ni50Mn25Ga25 thin films driven by the internal stress filed based on the GI-XRD and TEM observations.

The microstructure and magnetic properties of Ca2+ ion doped GdCrO3

In this paper, the crystal structure, vacancy defect, local electron density and magnetic properties of Gd1-xCaxCrO3 (0 ≤ x ≤ 0.3) polycrystalline samples were investigated systematically. The crystal structural analyses show that all the samples are orthorhombic phase and a structural distortion happens around x = 0.3. Due to the formation of Cr4+ ions, both the lattice constant and the Cr–O bond length decrease. The results of positron annihilation spectrum reveals that the vacancy defect concentration increases and the local electron structure changes with the introduction of Ca2+ ions. The field-cooled (FC) and zero-field cooled (ZFC) curves of Gd1-xCaxCrO3 samples measured under H = 100 Oe exhibits negative magnetization characteristics due to the interaction between Gd3+ and Cr3+ ions, and the magnetism can be affected by the structural distortion.

Positron binding in chloroethenes: Modeling positron-electron correlation-polarization potentials for molecular calculations

Positron binding energies and annihilation spectra are calculated using the positron-electron correlation-polarization potential model for a series of six chloroethene molecules. From detailed analysis of the positron densities and positron-molecule interactions, it was found that the positron is attracted by both the potential well around the negative end of the dipole moment and the $\ensuremath{\pi}$ electrons of the C=C double bond. The positron annihilation spectra were simulated using the Feshbach resonance widths, which can be calculated from the couplings between the positron motion and vibrational motion. The importance of $\ensuremath{\pi}$ electrons in positron binding is discussed in terms of the calculated resonance widths.

Unconventional dual-vacancies in nickel diselenide-graphene nanocomposite for high-efficiency oxygen evolution catalysis

Although nickel-based catalysts display good catalytic capability and excellent corrosion resistance under alkaline electrolytes for water splitting, it is still imperative to enhance their activity for real device applications. Herein, we decorated Ni0.85Se hollow nanospheres onto reduced graphene oxide (RGO) through a hydrothermal route, then annealed this composite at different temperatures (400 °C, NiSe2-400 and 450 °C, NiSe2-450) under argon atmosphere, yielding a kind of NiSe2/RGO composite catalysts. Positron annihilation spectra revealed two types of vacancies formed in this composite catalyst. We found that the NiSe2-400 catalyst with dual Ni-Se vacancies is able to catalyze the oxygen evolution reaction (OER) efficiently, needing a mere 241 mV overpotential at 10 mA·cm−2. In addition, this catalyst exhibits outstanding stability. Computational studies show favorable energy barrier on NiSe2-400, enabling moderate OH− adsorption and O2 desorption, which leads to the enhanced energetics for OER.

Influence of Ga3+ ion dopant on the structure and magnetic properties of YCrO3

In this work, Ga3+ doped YCr1-xGaxO3 (x = 0, 0.1, 0.2 and 0.3) polycrystalline samples were prepared by solid state reaction method and their crystal structure, morphology, internal defect, specific heat and magnetic properties were studied. The results show that the lattice constants, lattice volume and particle size increase, while Y/Cr–O bond constant decreases with replacing Cr3+ by Ga3+. The positron annihilation spectrum (PAS) measurement shows that cation vacancy defect exists in all samples and the internal defect concentration fluctuates slightly with increasing of Ga3+ ions doping concentration. For low doping content, an additional magnetization anomaly emerges in the zero-field-cooling curves measured in H = 1000Oe. With the increase of magnetic field, the anomaly can be well suppressed. Simultaneously, for the specific heat curve peak cannot be observed at the corresponding temperature. These results imply the establishment of short-range magnetic order above the antiferromagnetic transition temperature (TN) due to the dilution effect of nonmagnetic Ga3+ doping.

Vacancy-induced toxicity of CoSe2 nanomaterials in rat lung macrophages

Rich vacancies of semiconductor nanomaterials (NMs) give rise to great enhancement of their physical and chemical properties such as magnetic, catalytic, optical, etc. These NMs possessing extensive applications could inevitably enter into the environment and increase the toxic effects on organisms, so it is imperative to investigate the cytotoxicity of NMs with different types of vacancies. Here, one-dimensional cobalt selenide (CoSe2) NMs with different vacancies were synthesized through the same precursor while calcined at different temperatures (P-CoSe2 which calcined at 200 °C and N-CoSe2 which calcined at 230 °C). According to the positron annihilation spectrum, the VSeSe vacancy associate in P-CoSe2 was endowed with two positive charges, while the VCoCoCoSeSe vacancy associate in N-CoSe2 possessed four negative charges. Cell viability assays revealed that N-CoSe2 had higher toxicity to macrophages than P-CoSe2, which was attributed to higher levels of intracellular reactive oxygen species induced by N-CoSe2. Further investigation showed that N-CoSe2 had higher affinity to the mitochondrion-targeting peptide, leading to its preferential distribution in the mitochondria and consequent induction of mitochondrial superoxide production. In contrast, P-CoSe2 exhibited higher affinity to the endoplasmic reticulum (ER)-targeting peptide, facilitating its preferential distribution in the ER and the nuclei and causing higher damage to both organelles as compared to N-CoSe2. These results demonstrated that type of surface vacancies significantly affected biodistribution of NMs in subcellular organelles, which contributed to differential biological behaviors of the NMs.

Study on defect-induced damage behaviors of ADP crystals by 355 nm pulsed laser.

High-quality ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystals were grown in Z direction and in defined crystallographic direction (θ=90°, φ=45°) by the rapid growth method, respectively. Defect-induced damage behavior in 355 nm of three types of ADP samples cutting in type-II matching and third harmonic generation direction from the as-grown crystals were investigated, including the initial laser induced damage (LID) characteristics and the physical and chemical properties of defects which serve as the damage precursors. The evaluations of damage behaviors include the "sampling" laser induced damage threshold (LIDT) by 1-on-1 and R-on-1 methods, bulk damage growth and bulk damage morphology. UV-visible transmittance spectrum, ultraviolet absorption spectrum, fluorescence spectrum, positron annihilation spectrum and the online light scattering measurements were carried out to investigate the defect-induced damage behavior in ADP crystals. The study will provide a reference for the investigations on laser induced damage properties of ADP crystals in short wavelength.

Porous β-Bi2O3 with multiple vacancy associates on highly exposed active {220} facets for enhanced photocatalytic activity

Porous β-Bi2O3 with highly exposed {220} facets was fabricated for the first time and the formation of surface multiple Bi-O vacancy associates was confirmed by the photoluminescence, positron annihilation spectra, and theoretical calculations. Compared with traditional nonporous β-Bi2O3-{201}, the photocatalytic rate of porous β-Bi2O3-{220} was more than twice faster and the photoconversion efficiency increased by 20 times. And various refractory organic pollutants can be effectively degraded and mineralized under visible light. Multiple Bi-O vacancy associates were demonstrated to change electronic structure and enhance the polarization of the related atoms and orbitals. The photogenerated charge separation and transfer was thus improved to produce more surface active species. Simultaneously, the electron transfer from pollutants to the catalyst was found to be accelerated, finally leading to the outstanding photocatalytic performances of porous β-Bi2O3-{220}. These findings provide useful insights into the development of visible-light-driven photocatalysts for water purification.

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.

Effects of the π-electrons in fused polycyclic aromatic hydrocarbons on positron annihilation

The mesoporous phenol-formaldehyde resins (PFR) were synthesized using formaldehyde, phenol, F127 template as raw materials by high temperature hydrothermal method under different temperatures. Nitrogen doped mesoporous PFR were synthesized in the same way with another additional raw material HMTA. Two series of carbon materials PCs and PCNs were obtained after calcinating PFR at 700, 800, 900 °C. N2 adsorption analysis indicates that both the two kind of carbon materials have relatively large specific surface area (325–619 cm2/g for PCs and 431–580 cm2/g for PCNs). TEM and XRD analyses prove highly ordered mesoporous structure of the samples. Both the elemental analysis and IR results suggest a high carbon content and the abundant fused-ring structure of the test materials. The positron annihilation spectra show that the corresponding intensity I3 of the longest lifetime component τ3 is less than 2%, indicating that the π electrons in the fused rings with low ionization energy are capable of trapping positrons and inhibiting the formation of o-Ps.

Magnetic transition and defect characteristics of multiferroic CuFe1-xGaxO2 studied by positron annihilation spectra

The structure, crystal defects and magnetic properties of multiferroic CuFe1-xGaxO2 (CFGO; x = 0–0.07) ceramics are studied systemically. Substitution of Ga3+ for Fe3+ shrinks the CFO lattice, decreases the particle size, and causes small liquid phase formation. Positron annihilation spectroscopy demonstrates that all samples contain a considerable number of vacancy defects. The overall defect environment is virtually unaffected by Ga3+ doping, but the open-volume of the defects is redistributed. Magnetic susceptibility measurements show that Ga doping enhances the strength of the antiferromagnetic interaction between high-spin Fe3+ ions as a result of reduced magnetic correlation length, but decrease the stability of the antiferromagnetic phase. The antiferromagnetic transition temperature, TN2, decreases from 11 K for x = 0 to 8 K for x = 0.07, and this destabilization of the antiferromagnetic phase is closely related to the crystal structure and defects, which are discussed in detail in this work.