Search

Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8. ispartof: ADVANCED MATERIALS vol:33 issue:17 ispartof: location:Germany status: published

ispartof: ACS ES&T ENGINEERING vol:2 issue:10 pages:1857-1870 status: published

Rare-earth (RE) oxyhydride thin films show a color-neutral, reversible photochromic effect at ambient conditions. The origin of the photochromism is the topic of current investigations. Here, we investigated the lattice defects, electronic structure, and crystal structure of photochromic YHxOy and GdHxOy thin films deposited by magnetron sputtering using positron annihilation techniques and X-ray diffraction, in comparison with Y, YH∼1.9, Y2O3, Gd, GdH∼1.8, and Gd2O3 films. Positron annihilation lifetime spectroscopy (PALS) reveals the presence of cation monovacancies in the as-deposited Y and YH∼1.9 films at concentrations of ∼10-5 per cation. In addition, vacancy clusters and nanopores are found in the as-prepared YHxOy and Y2O3 films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y- A nd Gd-based films reflects the transition from a metallic to an insulating nature of the RE metal, metal hydride, semiconducting oxyhydride and insulating oxide films. In-situ illumination DB-PAS shows the irreversible formation predominantly of di-vacancies, as PALS showed that cation mono-vacancies are already abundantly present in the as-prepared films. The formation of di-vacancies supports conjectures that H-(and/or O2-) ions become mobile upon illumination, as these will leave anion vacancies behind, some of which may subsequently cluster with cation vacancies present. In addition, in RE oxyhydride films, partially reversible shifts in the Doppler parameters are observed that correlate with the photochromic effect and point to the formation of metallic domains in the semiconducting films. Two processes are discussed that may explain the formation of these metallic domains and the changes in optical properties associated with the photochromic effect. The first process considers the reversible formation of metallic nanodomains with reduced O: H composition by transport of light-induced mobile hydrogen and local oxygen displacements. The second process considers metallic nanodomains resulting from the trapping of photoexcited electrons in an eg orbital at the yttrium ions surrounding positively charged hydrogen vacancies that are formed by light-induced removal of hydrogen atoms from octahedral sites. When a sufficiently large concentration, on the order of ∼10%, is reached in a certain domain of the film, band formation of the eg electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

Energy‐efficient indoors temperature and humidity control can be realised by using the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium‐based metal–organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium‐based MOFs: (1) in aqueous solutions using a continuous‐flow reactor and (2) through the vapour‐assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers to obtain polymorph materials with tuneable water sorption properties. The synthesis conditions determine the crystal structure and either the MIL‐53 or MIL‐68 type structure with square‐grid or kagome‐grid topology, respectively, is formed. Fine‐tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation and room climatisation applications., Going with the flow: A collection of microporous aluminium fumarate‐mesaconate metal–organic frameworks (MOFs) are prepared by using an aqueous flow reactor and vapour‐assisted synthesis. The materials have a variable and, for some, previously unreported crystal structure, and their tuneable water sorption properties depend on the preparation method and linker content.

ispartof: ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS vol:8 issue:9 pages:818-824 status: published

© 2019 Elsevier B.V. Membrane technology is currently still excluded from separations in more aggressive feeds due to limited chemical robustness. To extent its applicability, a novel thin-film composite (TFC)membrane was synthesized via the homopolymerization of epoxide monomers, resulting in robust poly(epoxyether)top-layers with >90% rose bengal (MW = 1017 Da)and 70% methyl orange (MW = 327 Da)retention with reasonable water fluxes (>2 L m -2 h −1 bar −1 ). The superior chemical stability of this novel nanofiltration membrane type was proven via treatments in pH 1 and 500 ppm NaOCl (pH 4)for, respectively, 48 h and 2.5 h, after which an unchanged or even improved membrane performance was observed. Additionally, the synthesis of the thin top-layer occurred via an interfacial initiation of the polymerization (IIP), rather than via state-of-the-art interfacial polymerization (IP). This IIP approach allowed to convert well-known monophasic bulk epoxide polymerization (commonly used in e.g. the automotive and coating industry), into the synthesis of thin, yet cross-linked top-layers. ispartof: JOURNAL OF MEMBRANE SCIENCE vol:582 pages:442-453 status: published

Tuning the cohesive and swelling forces in porous membranes by post-synthesis solvent-treatment was already proven to be a successful way to increase the membrane selectivity in aqueous redox flow battery application without sacrificing too much permeability of the membranes. This method was now extended to non-aqueous applications and the mechanism behind the tuning of the pore sizes upon solvent-treatment was studied in more detail. Porous polyvinylidene fluoride (PVDF) and polysulfone (PSF) membranes were thus prepared via phase inversion and subsequently treated with different solvents followed by the evaporation of this solvent to adjust the structural porosities of these asymmetric membranes and better understand the underlying mechanism. For this purpose, water and acetonitrile permeances were determined before and after the solvent-treatment and these permeances were linked to the (solvent)-(membrane polymer) Hansen solubility parameters. The membrane density changes were characterized by positron annihilation lifetime spectroscopy (PALS). It was proven that the pore adjustment that was created by the solvent-treatment was reversible when the membrane was later on applied in a feed solution with high affinity for the membrane polymer.

© 2019 Elsevier Ltd A set of new stiff and contorted polyimides were prepared to systematically link polymer structure with gas separation properties. Seven novel bridged binaphthalene polyimides were synthesized for this purpose using commercial dianhydrides pyromellitic dianhydride (PMDA), biphenyl-tetracarboxylic acid dianhydride (BPDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). From these polyimides, freestanding membranes were prepared and tested on mixed gas CO 2 /CH 4 and CO 2 /N 2 separation performance. They were further characterized using WAXS and PALS and through the simulation of their free volume elements (FVEs) and gas separation properties. The binaphthalene polyimides had CO 2 /CH 4 and CO 2 /N 2 mixed gas selectivities of respectively 40 and 29 and mixed gas permeabilities of about 20 barrer for both gas mixtures. ispartof: EUROPEAN POLYMER JOURNAL vol:114 pages:134-143 status: published

This report presents a study about the morphology of a cross-linked aromatic polyamide (PA) film in its dry and wet state using small-angle neutron scattering (SANS), positron annihilation lifetime spectroscopy (PALS), and scanning electron microscopy (SEM). PA is used as active skin layer for reverse osmosis membranes and determines the selectivity of the membrane with respect to water and salt molecules. This selectivity of PA is largely determined from its morphology. The PA film scatters the neutrons much stronger and shows a different profile as expected from randomly distributed nanoscopic large pores. SANS contrast variation, using supercritical CO2 as contrast medium, confirms that pores are the only scattering centers. The analysis shows that interconnected pores of an average radius of about 16 A determine the scattering. The pores are formed as a network of channels showing a fractal structure. PALS determines pores of ∼6 A diameter representing the diameter of such channels. The volume fraction of the pores is estimated as ∼10% which after water up-take increases by ∼30% due to slightly swelling of the pores.

Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Here we identify lead monovacancy (VPb) defects in MAPbI3 (MA = CH3NH3+) using positron annihilation lifetime spectroscopy with the aid of density functional theory. Experiments on thin film and single crystal samples all exhibited dominant positron trapping to lead vacancy defects, and a minimum defect density of ~3 × 1015 cm−3 was determined. There was also evidence of trapping at the vacancy complex \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$({{{{{\rm{V}}}}}}_{{{{{\rm{Pb}}}}}}{{{{{\rm{V}}}}}}_{{{{{\rm{I}}}}}})^{-}$$\end{document}(VPbVI)− in a minority of samples, but no trapping to MA-ion vacancies was observed. Our experimental results support the predictions of other first-principles studies that deep level, hole trapping, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{{{{{\rm{V}}}}}}}_{{{{{{\rm{Pb}}}}}}}^{2-}$$\end{document}VPb2−, point defects are one of the most stable defects in MAPbI3. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms., Point defects compromise the electronic performance of hybrid perovskites, yet no experimental identifications have been reported. Here, the authors, for the first time, identify lead monovacancy defect in MAPbI3 using positron annihilation lifetime spectroscopy with the aid of density functional theory.

The landscape of possible polymorphs for some metal-organic frameworks (MOFs) can pose a challenge for controlling the outcome of their syntheses. Demonstrated here is the use of a template to control in the vapor-assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films. Introducing a small amount of either ethanol or dimethylformamide vapor during the reaction between ZnO and 4,5-dichloroimidazole vapor results in the formation of the porous ZIF-71 phase, whereas other conditions lead to the formation of the dense ZIF-72 phase or amorphous materials. Time-resolved in situ small-angle X-ray scattering reveals that the porous phase is metastable and can be transformed into its dense polymorph. This transformation is avoided through the introduction of template vapor. The porosity of the resulting ZIF powders and films was studied by N2 and Kr physisorption, as well as positron annihilation lifetime spectroscopy. The templating principle was demonstrated for other members of the ZIF family as well, including the ZIF-7 series, ZIF-8_Cl, and ZIF-8_Br. ispartof: ANGEWANDTE CHEMIE-INTERNATIONAL EDITION vol:60 issue:14 pages:7553-7558 ispartof: location:Germany status: published

Summary There is no efficient wastewater treatment solution for removing organic micropollutants (OMPs), which, therefore, are continuously introduced to the Earth's surface waters. This creates a severe risk to aquatic ecosystems and human health. In emerging water treatment processes based on ion-exchange membranes (IEM), transport of OMPs through membranes remains unknown. We performed a comprehensive investigation of the OMP transport through a single IEM under non-steady-state conditions. For the first time, positron annihilation lifetime spectroscopy was used to study differences in the free volume element radius between anion- and cation-exchange membranes, and between their thicknesses. The dynamic diffusion-adsorption model was used to calculate the adsorption and diffusion coefficients of OMPs. Remarkably, diffusion coefficients increased with the membrane thickness, where its surface resistance was more evident in thinner membranes. Presented results will contribute to the improved design of next-generation IEMs with higher selectivity toward multiple types of organic compounds., Graphical Abstract, Highlights • Electroneutrality overcomes diffusive potential of charged OMPs • The interactions between OMPs and the IEMs determine OMP transport • Adsorption influences diffusion lag time of neutral OMPs • Diffusion coefficients of neutral OMPs increased with IEMs thickness, Chemistry; environmental chemical engineering; environmental science; chemical engineering

Vacancy-type defects in Mg-implanted GaN were probed using monoenergetic positron beams. Mg+ ions were implanted to provide box profiles with Mg concentrations [Mg] of 1017-1019 cm-3. For as-implanted samples, the major defect species was determined to be Ga-vacancy (VGa) related defects such as divacancy (VGaVN) and/or their complexes with impurities. For Mg-implanted samples, an agglomeration of vacancies started at 800-1000°C annealing, leading to the formation of vacancy clusters such as (VGaVN)3. For the sample with [Mg]=1019 cm-3, the trapping rate of positrons to the vacancies decreased with increasing annealing temperature (≥1100°C), which was attributed to the change in the charge state of vacancy-type defects from neutral to positive (or negative to neutral) due to the activation of Mg. For Mg- and H-implanted samples, the hydrogenation of vacancy-type defects started after 800°C annealing. Comparing with the annealing behavior of defects for the samples without H-implantation, the clustering of vacancy-type defects was suppressed, which can be attributed to the interaction between Mg, H, and vacancies.

Polymer membranes are critical to many sustainability applications that require the size-based separation of gas mixtures. Despite their ubiquity, there is a continuing need to selectively affect the transport of different mixture components while enhancing mechanical strength and hindering aging. Polymer-grafted nanoparticles (GNPs) have recently been explored in the context of gas separations. Membranes made from pure GNPs have higher gas permeability and lower selectivity relative to the neat polymer because they have increased mean free volume. Going beyond this ability to manipulate the mean free volume by grafting chains to a nanoparticle, the conceptual advance of the present work is our finding that GNPs are spatially heterogeneous transport media, with this free volume distribution being easily manipulated by the addition of free polymer. In particular, adding a small amount of appropriately chosen free polymer can increase the membrane gas selectivity by up to two orders of magnitude while only moderately reducing small gas permeability. Added short free chains, which are homogeneously distributed in the polymer layer of the GNP, reduce the permeability of all gases but yield no dramatic increases in selectivity. In contrast, free chains with length comparable to the grafts, which populate the interstitial pockets between GNPs, preferentially hinder the transport of the larger gas and thus result in large selectivity increases. This work thus establishes that we can favorably manipulate the selective gas transport properties of GNP membranes through the entropic effects associated with the addition of free chains.

The APEX (A Positron Electron eXperiment) collaboration aims to magnetically confine a low-temperature electron-positron pair plasma. By using a pair of ExB plates, positrons generated by the NEPOMUC facility are drift-injected into the confinement field created by a supported permanent magnet. Fine-tuning the fields generated by electrodes and magnetic coils increased the injection efficiency to 100% and positron confinement times to more than 1s. A newly installed electron gun has been used to inject electrons, guided alongside the positron beam, into the confinement volume. This contribution describes the recent upgrades required for the first dual species experiments.

Two processes for crosslinking polyvinyl alcohol (PVA) with sulfosuccinic acid (SSA) and thermal crosslinking were used to fabricate a proton exchange membrane (PEM). Such PEMs are used in different fields involving fuel cell applications. The crosslinking reaction between PVA and SSA was confirmed using Fourier-transform infrared (FTIR) spectroscopy. The characterization of the prepared membranes, namely, ion exchange capacity (IEC), thermal analyses, water uptake, and ionic conductivity, was carried out. The IEC of the prepared membranes was found to be between 0.084 and 2.086 mmol g-1, resulting in an essential increase in the ionic conductivity. It was observed that the ionic conductivity was in the range of 0.003-0.023 S cm-1, depending on both temperature and SSA content. From the thermogravimetric analysis (TGA) results, it was revealed that the thermal stability of the crosslinked membranes improved. Moreover, water uptake decreased with increasing SSA content. Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of the PVA/SSA membranes and their distribution at different ambient temperatures and relative humidity (RH) values. At room temperature, no significant change was observed in the free-volume holes up to 15 wt% SSA; thereafter, the size of the free-volume holes increased with the SSA content. The PALS results show that at different humidity values, the size of the free-volume holes for crosslinked PVA/SSA membranes is lower than those for Nafion membranes, i.e., the gas permeability for the prepared PVA/SSA membranes is less than that for the Nafion membrane. In addition, a strong correlation between the water uptake, ionic conductivity, tensile strength, and free-volume holes was observed.

A simple solvent- and catalyst-free method is presented for the synthesis of the mesoporous metal-organic framework (MOF) MAF-6 (RHO-Zn(eIm)2) based on the reaction of ZnO with 2-ethylimidazole vapor at temperatures ≤ 100 °C. By translating this method to a chemical vapor deposition (CVD) protocol, mesoporous crystalline films could be deposited for the first time entirely from the vapor phase. A combination of PALS and Kr physisorption measurements confirmed the porosity of these MOF-CVD films and the size of the MAF-6 supercages (diam. ~2 nm), in close agreement with powder data and calculations. MAF-6 powders and films were further characterized by XRD, TGA, SEM, FTIR, PDF and EXAFS. The exceptional uptake capacity of the mesoporous MAF-6 in comparison to the microporous ZIF-8 is demonstrated by vapor-phase loading of a molecule larger than the ZIF-8 windows.

Energy-efficient indoors temperature control can be realised through adsorption chillers or adsorption heat pumps based on the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium-based metal-organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium-based MOFs: (1) in aqueous solutions using a continuous flow reactor and (2) through the vapour-assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers. The synthesis conditions determine the crystal structure (symmetry and topology), either the MIL-53 or MIL-68 type with square-grid or kagome-grid topology, respectively. Fine-tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation applications.

© 2019 Elsevier B.V. Bipyridine-based UiO-67 Metal-Organic Frameworks (MOFs) are for the first time successfully applied as filler material in Matrimid mixed-matrix membranes (MMMs). Extensive characterization of fillers (via XRD, NMR, SEM, ATR-FTIR, CO 2 and N 2 physisorption) and membranes (via XRD, SEM, ATR-FTIR, DSC, TGA and PALS) were performed to gain insight in the MMM separation behavior. Lewis basic sites in the MMMs function as CO 2 -carriers, doubling CO 2 /CH 4 selectivity (from 38 for pristine Matrimid to 75 for the MMM with 10 wt% UiO-67-33), while the CO 2 permeability was simultaneously improved from 16 Barrer (Matrimid) up till 26 Barrer for the MMM with 10 wt% UiO-67-33 (+63%). Gas permeation experiments suggested a facilitated transport mechanism to be responsible for these high mixed-gas selectivities. The concentration of the filler inside the membrane could be increased to 30 wt%, resulting in a permeability increase of 350% without losing selectivity compared to the Matrimid membrane. Use of N-heterocyclic ligands proved to be an interesting strategy and good alternative for functionalization of MOF surfaces to obtain improved membrane performance. ispartof: JOURNAL OF MEMBRANE SCIENCE vol:576 pages:78-87 status: published

To investigate inhomogeneous defect distributions, e.g. close to fatigue cracks or dispersive alloys with positron annihilation lifetime spectroscopy a pulsed positron beam with a diameter in the range of 1 µm and with a time resolution in the order of 250 ps is needed. The Scanning Positron Microscope (SPM) fulfills the requirements for position resolved positron lifetime measurements and was operated by the Universitat der Bundeswehr Munchen. To increase the countrate, the SPM is currently transferred to the intense positron source NEPOMUC. The SPM Interface connects the SPM to the NEPOMUC source and en-hances the beam quality to the needs of the SPM.In this contribution we will give an overview of the SPM Interface and its performance. The finale stage of the SPM Interface is the positron elevator which has been further developed. We will discuss the apparatus in detail and show measurements of the beam quality with a beam elevation of 8 keV.From our measurements we are able to predict the performance of the SPM at NEPOMUC. In the future position resolved measurements will be possible with an improved spatial resolution of about 0.3 µm and an event rate of about 3.7 kHz.To investigate inhomogeneous defect distributions, e.g. close to fatigue cracks or dispersive alloys with positron annihilation lifetime spectroscopy a pulsed positron beam with a diameter in the range of 1 µm and with a time resolution in the order of 250 ps is needed. The Scanning Positron Microscope (SPM) fulfills the requirements for position resolved positron lifetime measurements and was operated by the Universitat der Bundeswehr Munchen. To increase the countrate, the SPM is currently transferred to the intense positron source NEPOMUC. The SPM Interface connects the SPM to the NEPOMUC source and en-hances the beam quality to the needs of the SPM.In this contribution we will give an overview of the SPM Interface and its performance. The finale stage of the SPM Interface is the positron elevator which has been further developed. We will discuss the apparatus in detail and show measurements of the beam quality with a beam elevation of 8 keV.From our measurements we are able to predict the performance ...

Metal-organic frameworks (MOFs) enable the design of host-guest systems with specific properties. In this work, we show how the confinement of anthracene in a well-chosen MOF host leads to reversible yellow-to-purple photoswitching of the fluorescence emission. This behavior has not been observed before for anthracene, either in pure form or adsorbed in other porous hosts. The photoresponse of the host-guest system is caused by the photodimerization of anthracene, which is greatly facilitated by the pore geometry, connectivity, and volume as well as the structural flexibility of the MOF host. The photoswitching behavior was used to fabricate photopatternable and erasable surfaces that, in combination with data encryption and decryption, hold promise in product authentication and secure communication applications. ispartof: ANGEWANDTE CHEMIE-INTERNATIONAL EDITION vol:58 issue:8 pages:2423-2427 ispartof: location:Germany status: published

© 2018 American Chemical Society. It is generally accepted in membrane technology that crystalline zones in polymeric membranes do not contribute to transport of liquids or gases. In the current study, poly(3-alkylthiophene)s (P3ATs), i.e., homopolymers and random copolymers, were synthesized to study the influence of the supramolecular organization on membrane gas separations. The monomers were polymerized via KCTCP, and GPC analysis shows that the polymers have a narrow dispersity. DSC analysis of the polymers reveals that the homopolymers, in contrast to the copolymers, crystallized, confirming their higher degree of supramolecular organization. This was supported by UV-vis absorption spectra of the polymer films, where a red-shift and a characteristic shoulder absorption peak around 600 nm were observed for the homopolymers, while absent for the copolymers. More surprisingly, the homopolymers were found to be 2 orders of magnitude more permeable to CO2 than the copolymers and also more selective. ispartof: MACROMOLECULES vol:51 issue:23 pages:9943-9950 status: published

The high-efficiency injection of a low-energy positron beam into the confinement volume of a magnetic dipole has been demonstrated experimentally. This was accomplished by tailoring the three-dimensional guiding-center drift orbits of positrons via optimization of electrostatic potentials applied to electrodes at the edge of the trap, thereby producing localized and essentially lossless cross-field particle transport by means of the E×B drift. The experimental findings are reproduced and elucidated by numerical simulations, enabling a comprehensive understanding of the process. These results answer key questions and establish methods for use in upcoming experiments to create an electron-positron plasma in a levitated dipole device.

Positron annihilation depth-profiling is used as an innovative tool to monitor the evolution of vacancy defects in two series of ZnO:Al transparent conductive oxide (TCO) layers for Cu(In,Ga)Se2 (CIGS) solar cells under accelerated degradation at 85 °C/85% relative humidity. The first series of ZnO:Al layers are deposited directly on flat glass substrates, leading to low densities of (extended) grain boundaries in the ZnO:Al. These ZnO:Al layers only show an increase in open volume upon degradation in the near-surface range. The second series of ZnO:Al layers are deposited on the more corrugated surface of CdS/CIGS/Mo solar cells, and show, on the other hand, a pronounced formation of open volume throughout the layer. Its depth-dependence is consistent with in-diffusion of molecules such as H2O and CO2 into the ZnO:Al layer via the grain boundaries, as primary driver for the degradation. The detected time-dependence of the growth of open volume at the grain boundaries in the ZnO:Al TCO layer matches the time scale of the observed reduction in solar cell efficiency and series resistance, suggesting that the generated open volume induces a significant barrier against charge carrier transport.

Ion-exchange membrane (IEM)-based processes are used in the industry or in the drinking water production to achieve selective separation. The transport mechanisms of organic solutes/micropollutants (i.e., paracetamol, clofibric acid, and atenolol) at a single-membrane level in diffusion cells were similar to that of salts (i.e., diffusion, convection, and electromigration). The presence of an equal concentration of salts at both sides of the membrane slightly decreased the transport of organics due to lower diffusion coefficients of organics in salts and the increase of hindrance and/or decrease of partitioning in the membrane phase. In the presence of a salt gradient, diffusion was the main transport mechanism for non-charged organics, while the counter-transport of salts promoted the transport of charged organics through electromigration (electroneutrality). Conversely, the co-transport of salts hindered the transport of charged organics, where diffusion was the main transport mechanism of the latter. Although convection played a role in the transport of non-charged organics, its influence on the charged solutes was minimal due to the dominant electromigration. Positron annihilation lifetime spectroscopy showed a bimodal size distribution of free-volume elements of IEMs, with both classes of free-volume elements contributing to salt transport, while larger organics can only transport through the larger class. ispartof: WATER RESEARCH vol:190 ispartof: location:England status: published

An elevator increases the potential energy of a particle beam with respect to ground potential without any alteration of kinetic energy and other beam parameters. This elevator is necessary for the implementation of the Munich Scanning Positron Microscope (SPM) at the intense positron source NEPOMUC at the research reactor FRM II in Munich. The principles of the rf elevator for pure electrostatically guided positrons are described. Measurements of beam quality behind the elevator are reported, which confirm that after the implementation of elevator and SPM at NEPOMUC the SPM can be operated at a considerably improved resolution ( ~ 0.3 μ m ) and event rate (~3.7 kHz) compared to the laboratory based β + -source.

Membrane deformation under an applied hydraulic pressure, often termed compaction, is observed in almost all pressure-driven membrane processes. Most notably, compaction decreases water permeability in conventional reverse osmosis (RO) and is expected to critically hinder high-pressure reverse osmosis (HPRO) for hypersaline brine desalination. In this work, we demonstrated that compaction decreases the water permeability of commercial RO membranes from 2.0 L m−2 h−1 bar−1 at 70 bar applied hydraulic pressure to 1.3 L m−2 h−1 bar−1 at 150 bar. The morphological effects of compaction were primarily associated with changes in the support layer, where a ~60% decrease in cross-sectional thickness is observed following compaction at 150 bar hydraulic pressure. In contrast, positron annihilation lifetime spectroscopy demonstrates that the selective layer does not compact irreversibly. The mechanism that drives compaction was found to be the difference in hydraulic pressure across the interface of the selective and support layers. We further found that compaction can reduce the support layer surface porosity by up to ~95%. This decreased porosity is identified as the cause for compaction-induced water permeability decline, while the intrinsic permeability of the selective layer is not influenced by compaction. As such, we conclude that compaction of the support layer has an inextricable impact on composite membrane performance. Finally, we propose recommendations for developing compaction-resistant membranes that can maintain high water permeability, and thus good desalination performance, in high-pressure membrane applications, such as HPRO.

Vacancy-type defects in AlN films were probed by using monoenergetic positron beams. The AlN films were deposited on sapphire substrates by using a radio-frequency sputtering technique. Epitaxial films grown by metalorganic vapor phase epitaxy on the sputtered AlN films were also characterized. For the sputtered AlN, the major defect species was identified to be complexes between Al-vacancy and oxygen atoms located at nitrogen sites. Vacancy clusters were introduced after annealing at 1300 °C in the N2 atmosphere but their concentration decreased with a higher annealing temperature. The vacancy–oxygen complexes, however, still existed in the AlN film after annealing at 1700 °C. For the AlN epitaxial films, the concentration of vacancy clusters increased as the growth temperature increased up to 1300 °C but it decreased with the post-growth annealing at 1700 °C.

Voids in SiO2 films deposited on GaN were probed by using monoenergetic positron beams. The films were fabricated on GaN substrates by using plasma-enhanced chemical vapor deposition. The size and density of the voids in the films increased up to an annealing temperature of 800 °C and then decreased at 1000 °C. The observed annealing behaviors of the voids were attributed to the desorption of impurities incorporated during the deposition process and the shrinkage of the Si–O matrix by high-temperature annealing. Vacancy-type defects were introduced into the GaN substrate after 1000 °C annealing in O2 atmosphere due to the diffusion of Ga from the substrate to the SiO2 film. No out-diffusion of Ga into the SiO2 film was observed for the annealing in N2 atmosphere. Thus, the observed out-diffusion of Ga was attributed to the enhanced oxidation of GaN during the annealing in O2 atmosphere. The diffusion of positrons implanted into the GaN substrate toward the SiO2 film was suppressed by annealing, suggesting a decrease in the negative charges in the SiO2 film or near the SiO2/GaN interface.

Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In, Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85 ◦C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35 nm2/hour characteristic for in-diffusion of molecules such as H2O andCO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells. PhySH: Solar Cells, Positron Annihilation Spectroscopy, Grain Boundaries, Vacancies, Thin Films, Diffusion, Electrical Properties, Solid State Chemistry, Optoelectronics

Chlorination remains a big hurdle in membrane technology as the most commonly used membranes for water purification consist of a polyamide top-layer, which is not fully resistant towards chlorine-induced oxidation. In this work, DOW FILMTECTM BW30 membrane elements were systematically chlorinated with NaOCl at pilot-scale under acidic conditions (pH4) at 10 bar for 2.5 h. Variations in membrane performance and their physico- chemical properties were determined by ATR-FTIR, XPS, WD-XRF, SEM, AFM and zeta-potential measurements. With increasing bleaching concentration, both membrane roughness and chlorine incorporation via N- and ring- chlorination increased, while surface charge remained quasi unaltered. Both water flux and salt passage de- creased proportionally over the whole concentration range. Accordingly, positron annihilation lifetime spec- troscopy (PALS) revealed a decrease in the size of the top-layer free-volume elements as chlorine concentration increased, confirming, for the first time in a quantitative manner, the so-called tightening effect. The obtained results also show that thin-film composite (TFC) membranes are altered differently when chlorinated under pressure than via simple immersion, as conventionally performed in literature. ispartof: Journal of Membrane Science vol:551 pages:123-135 status: published

Incorporation of MOFs in interfacially polymerized Thin-Film Nanocomposite (TFN) membranes has widely been shown to result in increased membrane performance. However, the exact functioning of these membranes is poorly understood as large variability in permeance increase, filler incorporation and rejection changes can be observed in literature. The synthesis and functioning of TFN membranes (herein exemplified by ZIF-8 filled polyamide (PA) membranes prepared via the EFP method) was investigated via targeted membrane synthesis and thorough characterization via STEM-EDX, XRD and PALS. It is hypothesized that the acid generated during the interfacial polymerization (IP) at least partially degrades the crystalline, acid-sensitive ZIF-8 and that this influences the membrane formation (through so-called secondary effects, i.e. not strictly linked to the pore morphology of the MOF). Nanoscale HAADF-STEM imaging and STEM-EDX Zn-mapping revealed no ZIF-8 particles but rather the presence of randomly shaped regions with elevated Zn-content. Also XRD failed to show the presence of crystalline areas in the composite PA films. As the addition of the acid-quenching TEA led to an increase in the diffraction signal observed in XRD, the role of the acid was confirmed. The separate addition of dissolved Zn2+ to the synthesis of regular TFC membranes showed an increase in permeance while losing some salt retention, similar to observations regularly made for TFN membranes. While the addition of a porous material to a TFC membrane is a straightforward concept, all obtained results indicate that the synthesis and performance of such composite membranes is often more complex than commonly accepted.

We report the manipulation of positron orbits in a toroidal dipole magnetic field configuration realized with electric fields generated by segmented electrodes. When the toroidal circulation motion of positrons in the dipole field is coupled with time-varying electric fields generated by azimuthally segmented outer electrodes, positrons undergo oscillations of their radial positions. This enables quick manipulation of the spatial profiles of positrons in a dipole field trap by choosing appropriate frequency, amplitude, phase, and gating time of the electric fields. According to numerical orbit analysis, we applied these electric fields to positrons injected from the NEPOMUC slow positron facility into a prototype dipole field trap experiment with a permanent magnet. Measurements with annihilation γ-rays clearly demonstrated the efficient compression effects of positrons into the strong magnetic field region of the dipole field configuration. This positron manipulation technique can be used as one of essential tools for future experiments on the formation of electron-positron plasmas.We report the manipulation of positron orbits in a toroidal dipole magnetic field configuration realized with electric fields generated by segmented electrodes. When the toroidal circulation motion of positrons in the dipole field is coupled with time-varying electric fields generated by azimuthally segmented outer electrodes, positrons undergo oscillations of their radial positions. This enables quick manipulation of the spatial profiles of positrons in a dipole field trap by choosing appropriate frequency, amplitude, phase, and gating time of the electric fields. According to numerical orbit analysis, we applied these electric fields to positrons injected from the NEPOMUC slow positron facility into a prototype dipole field trap experiment with a permanent magnet. Measurements with annihilation γ-rays clearly demonstrated the efficient compression effects of positrons into the strong magnetic field region of the dipole field configuration. This positron manipulation technique can be used as one of essen...

This paper describes the design of a compact levitated superconducting coil for the production of a magnetic dipole field to eventually trap positron-electron plasma. The closed 300 turn coil is to be constructed from Bi-2223 high-Tc superconducting tape, directly cooled on a cryogenic cold head (with thermal contact enhanced using helium gas), and inductively energized with a second superconducting coil mounted on the same cold head. Levitation will be achieved from above using a water-cooled copper coil outside the vacuum chamber and its current will be feedback controlled using vertical position information from a laser ranger.

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor Quantum Dots (QDs) embedded in thin films. Nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy (PALS) studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab-initio calculations of the positron wave function and lifetime employing a recent formulation of the Weighted Density Approximation (WDA) demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the longstanding question regarding the nature of the positron state in semiconductor QDs, and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques., Comment: 14 pages, 3 figures

In this paper, we present the selective structuring of all three patterns (P1, P2 and P3) of a monolithic interconnection of CIS (Cu(In,Ga)(S,Se)2) thin film solar cells by picosecond laser pulses at a wavelength of 1064 nm. We show results for single pulse ablation threshold values and line scribing of molybdenum films on glass (P1), CIS on molybdenum (P2) and zinc oxide on CIS (P3). The purposes of these processes are the p-type isolation (P1), cell interconnect (P2) and n-type isolation (P3), which are required for complete cell architecture. The half micron thick molybdenum back electrode can be structured with a process speed of more than 15 m/s at about 15 W average power without detectable residues and damage by direct induced laser ablation from the back side (P1). The CIS layer can be structured selectively down to the molybdenum at process speeds up to 1 m/s at about 15 W average power, due to the precision of direct laser ablation in the ultrashort pulse regime (P2). The ZnO front electrode layer is separated by clean trenches with straight side walls at process speeds of up to 15 m/s at about 10 W average power, as a result of indirect induced laser ablation (P3). A validation of functionality of all processes is demonstrated on CIS solar cell modules (30 × 30 cm2). By replacing one state-of-the-art process by a picosecond laser process at a time, solar efficiencies could be increased for P1 and P2 and stayed on a similar level for P3. After an optimization of the patterning processes in the R&D pilot line of AVANCIS, we achieved a new record efficiency for an all-laser-patterned CIS solar module: 14.7% as best value for the aperture area efficiency of a 30 × 30 cm2 sized CIS module was reached. Copyright © 2014 John Wiley & Sons, Ltd.

Recent studies showed that positron annihilation methods can provide key insights into the nanostructure and electronic structure of thin film solar cells. In this study, positron annihilation lifetime spectroscopy (PALS) is applied to investigate CdSe quantum dot (QD) light absorbing layers, providing evidence of positron trapping at the surfaces of the QDs. This enables one to monitor their surface composition and electronic structure. Further, 2D-Angular Correlation of Annihilation Radiation (2D-ACAR) is used to investigate the nanostructure of divacancies in photovoltaic-high-quality a-Si:H films. The collected momentum distributions were converted by Fourier transformation to the direct space representation of the electronpositron autocorrelation function. The evolution of the size of the divacancies as a function of hydrogen dilution during deposition of a-Si:H thin films was examined. Finally, we present a first positron Doppler Broadening of Annihilation Radiation (DBAR) study of the emerging class of highly efficient thin film solar cells based on perovskites.

We have performed first four-dimensional age-momentum correlation (4D-AMOC) measurements at a pulsed high intensity positron micro beam and determined the absolute value of the three-dimensional momentum of the electrons annihilating with the positrons in coincidence with the positron age in the sample material. We operated two position sensitive detectors in coincidence to measure the annihilation radiation: a pixelated HPGe-detector and a microchannel plate image intensifier with a CeBr3 scintillator pixel array. The transversal momentum resolution of the 4D-AMOC setup was measured to be about 17 × 10−3 (FWHM) and was circa 3.5 times larger than the longitudinal momentum resolution. The total time resolution was 540 ps (FWHM). We measured two samples: a gold foil and a carbon tape at a positron implantation energy of 2 keV. For each sample discrete electron momentum states and their respective positron lifetimes were extracted.