SPM: Scanning positron microscope

The Munich scanning positron microscope, operated by the Universität der Bundeswehr München and the Technische Universität München, located at NEPOMUC, permits positron lifetime measurements with a lateral resolution in the µm range and within an energy range of 1 – 20 keV

Identification of lead vacancy defects in lead halide perovskites

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 7 1015 cm−3 was determined. There was also evidence of trapping at the vacancy complex (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, 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

Non-steady diffusion and adsorption of organic micropollutants in ion-exchange membranes : effect of the membrane thickness

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 theOMPtransport 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-exchangemembranes, and between their thicknesses. The dynamic diffusion-adsorption model was used to calculate the adsorption and diffusion coefficients ofOMPs. 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

Nature of the positron state in CdSe quantum dots

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 figure

Elucidating the Roles of Polyamide Layer Structural Properties in the Permeability–Selectivity Tradeoff Governing Aqueous Separations

A tradeoff relationship between permeability and selectivity of reverse osmosis and nanofiltration aqueous separation membranes is increasingly being documented. However, there is currently no comprehensive mechanistic framework to describe the roles of membrane structural properties in the transport tradeoff. This study investigates two key structural properties of the widely used thin-film composite polyamide (TFC-PA) membranes, namely, the free volume element (FVE) size and the effective transport pathway, and examines their influence on the tradeoff behavior. Permeability and selectivity performance were characterized by challenging chemically modified TFC-PA membranes with two neutral organic tracers. Positron annihilation lifetime spectroscopy (PALS) determined that the FVE diameters slightly increased for more permeable membranes, but the marginal size enlargement cannot fully account for the permeability trend. Instead, analysis using the hindered transport model showed that shortening of the effective transport pathway is identified as having a more significant effect on raising the water permeability. On the other hand, membrane selectivity is found to be dominated by FVE size and is essentially independent of the transport pathway. Lastly, a framework reconciling experimental evidence with transport theory is proposed to relate the influence of membrane structural properties on the permeability–selectivity tradeoff. Findings of this study provide fundamental insights for understanding the transport phenomena in aqueous separation membranes

Annealing behavior of open spaces in AlON films studied by monoenergetic positron beams

The impact of nitridation on open spaces in thin AlONx films deposited by a reactive sputtering technique was studied by using monoenergetic positron beams. For AlONx films with x = 0%–15%, positrons were found to annihilate from trapped states in open spaces, which coexist intrinsically in an amorphous structure with three different sizes. Nitrogen incorporation into the Al2O3 film increased the size of the open spaces, and their density increased as the post-deposition annealing temperature increased. The effect of nitrogen incorporation, however, diminished at x = 25%. The observed change in the network structure was associated with the formation of a stable amorphous structure, which we could relate to the electrical properties of AlONx/SiO2/Si gate stacks

Vacancy-type defects in Al2O3/GaN structure probed by monoenergetic positron beams

Defects in the Al2O3(25 nm)/GaN structure were probed by using monoenergetic positron beams. Al2O3 films were deposited on GaN by atomic layer deposition at 300 °C. Temperature treatment above 800 °C leads to the introduction of vacancy-type defects in GaN due to outdiffusion of atoms from GaN into Al2O3. The width of the damaged region was determined to be 40–50 nm from the Al2O3/GaN interface, and some of the vacancies were identified to act as electron trapping centers. In the Al2O3 film before and after annealing treatment at 300–900 °C, open spaces with three different sizes were found to coexist. The density of medium-sized open spaces started to decrease above 800 °C, which was associated with the interaction between GaN and Al2O3. Effects of the electron trapping/detrapping processes of interface states on the flat band voltage and the defects in GaN were also discussed

Aqueous Flow Reactor and Vapour-Assisted Synthesis of Aluminium Dicarboxylate Metal-Organic Frameworks with Tuneable Water Sorption Properties

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