A novel method of evaluating SANS curves is presented which for the first time allows the simultaneous determination of the internal geometry and hydration of single bilayers in unilamellar vesicles at high water excess. A multistrip model unambiguously defines the water-free hydrophobic core of the membrane as a region where the neutron scattering length density equals that of a typical hydrocarbon chain. Further, the outer water-accessible parts of the membrane are defined as regions where the neutron scattering length density differs from those of pure hydrocarbons and from water. Three independent structural parameters (thickness c of the water-free core, thickness h of the water-accessible coat of the bilayer membrane, and the relationship between the scattering length densities of these membrane regions) are directly obtained by fitting the scattering curve in the q range from 0.06 nm -1 to 3.82 nm -1 . The surface requirements of the amphiphiles and the number of water molecules located in the membrane can be calculated from these parameters. If there are multilamellar vesicles, one obtains additionally the repeat distance and the percentage of these vesicles. The potential of the method is demonstrated by elucidating structural and hydration parameters of mixed unilamellar POPC/C 12 E 4 vesicles.
The authors describe a comprehensive secondary ion mass spectrometry (SIMS) calibration procedure for the quantification of matrix and impurity elements of epitaxially grown AlxGa1−xN layers over the full compositional range of 0 ≤ x ≤ 1. For that a set of eight samples was grown by metalorganic vapor phase epitaxy, characterized with respect to AlN mole fraction and implanted with impurity and dopant elements (H, C, O, and Si). The compositional analysis using various techniques yielded consistent Al contents x with an accuracy of ±1%. For the quantitative characterization of impurities by SIMS, calibration curves were generated using a 14.5 keV Cs+ primary beam at an angle of incidence of 25°. Measured sputter rates decrease with a nearly linear slope as a function of Al content in the range of 0 ≤ x < 0.48. At higher Al concentrations the sputter rates show only a weak dependence on AlN mole fraction. Matrix ion intensity ratios of AlCs+/GaCs+ change linearly with direct and inverse proportionality as a function of x/(1−x). The absolute sensitivity factors for H, C, and Si follow an exponential reduction with increasing AlN mole fraction only for lower Al concentrations (0 ≤ x < 0.48). The calculated relative sensitivity factors are determined by the respective reference intensities depending on the AlN mole fraction.
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