Positron annihilation and constant photocurrent method measurements on a-Si:H films: A comparative approach to defect identification

Defect structure of hydrogenated amorphous silicon thin-films was studied by positron annihilation spectroscopy (PAS), whereas the density of states below the Fermi level was measured by constant photocurrent method (CPM). Divacancies and large vacancy clusters were identified as the main defects present in these films, with relative concentrations strongly dependent on the rf-power. Correlation between PAS, CPM results and I(V) characteristics of solar cells suggests the creation of energy levels above the Fermi energy, not observable by CPM, related to large vacancy clusters.http://www.sciencedirect.com/science/article/B6TVT-4JX373P-2/1/af7c69b0fdf17cefb2432fb357d5d86

On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range

Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of ~1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids.http://www.sciencedirect.com/science/article/B6THY-4SHF49N-1S/1/3eb43650299e0466e76cbbbfdaca9fa

Positron and positronium annihilation patterns in zeolites and bulk ceramics

Positron lifetime (LT) measurements have revealed that in porous media the annihilation pattern is probably dominated by a near-saturation level trapping both for the positron and the positronium as well, which masks the contribution from the ldquofree annihilationrdquo, i.e., annihilation from delocalised positrons. In this work we compare positron lifetime spectra of crystalline zeolites with bulk ceramic materials fabricated by sintering from the same constituents as the zeolites and analyse common features and differences between them. For the dense ceramic samples the contribution from long-living components to the annihilation pattern is substantially lower than that for the zeolites. The results are analysed with the aim to extract new knowledge for the zeolites and for the sensitivity limits for free volume studies in ceramics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Free-Volume Studies in Polycaprolactone/Poly(propylene oxide) Urethane/Urea Membranes by Positron Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy was used to study the free volume parameters in polypropylene oxide-based tri-isocyanate terminated prepolymer/polycaprolactone diol bi-soft urethane/urea membranes (PU/PCL) with PCL content from 5 up to 25 wt%. Positron annihilation lifetime spectroscopy measurements carried out in the temperature range 298-324 K mirrored the phase separation of the various soft and hard segments present in the membranes. The size and amount of free volume holes determined by positron annihilation lifetime spectroscopy appear to be correlated with the CO 2 gas permeability through the membranes

Positron annihilation - ICPA-9

Proceedings of the 9th International Conference on Positron Annihilation (ICPA-9), Szombathely, Hungary, 1991

Performance Analysis in Positron Annihilation Methods

Since the applicability of many experimental methods is often limited not by the physical processes involved but by the long measuring times encounted, the shortening of the recording time required to achieve sufficient statistical accuracy of the estimations of the parameters of interest is of essential importance. Analysis of the statistical utility rate u initially introduced for characterizing Mössbauer measurements allows one to determine the optimized experimental settings and may lead to a reduction of the measuring time for a given experiment. It is pointed out that the strategy for improving the experimental performance might vary depending on the particular applications of the same positron annihilation method

Ion-exchange with Pd/Pt and Fe and their reduction to metallic state in zeolites : positron annihilation and Mössbauer studies

Positron annihilation and Mössbauer-effect measurements were performed on fajausite X-, Y- and ZSM-5 zeolites in as-produced and also in ion-exchanged (with Fe and Pd or Pt) and subsequently reduced states. A correlation of the positron data with the filling-up of the cages of the zeolites due to the formation of metallic particles was found. The Mössbauer results revealed that the metallic particles are bimetallic