X-ray rocking curve analysis of superlattices

We present detailed analyses of x-ray double-crystal rocking curve measurements of superlattices. The technique measures depth profiles of structure factor, and profiles of perpendicular and parallel strains relative to the underlying substrate. In addition to providing a detailed picture of the state of stress, the profiles are a direct measure of the composition modulation. The thickness of the period of modulation and the average strain are determined with a precision of ~1%. The detailed structure of the period is determined to ~5%. We obtain an expression relating the structure of the rocking curve to the structure of the period. This expression allows analytic determination of the structure without Fourier transformation or computer fitting. We show the influence of small random fluctuations in layer thicknesses and strains. The technique is applied to a 15-period GaAlAs/GaAs and a ten-period AlSb/GaSb superlattice grown on GaAs and GaSb substrates, respectively. In the former, the thickness of the period was 676 Å and the perpendicular strain varied between zero for the GaAs layer and 0.249% for the layer with peak (93%) Al concentration. Transition regions, ~100 Å thick, with continuously varying composition, were found between the GaAs and the Ga0.07 Al0.93As layers. Fluctuations in structural properties were less than 5% of the average. The AlSb/GaSb superlattice had a period of 610 Å with sharp transition regions between the layers and negligible fluctuations from period to period. The perpendicular strains were −0.03% and 1.25%, respectively, for the GaSb and AlSb layers. A uniform parallel strain of 0.03% was found throughout the superlattice. Nonzero parallel strain indicates that a small fraction of the misfit between the superlattice and the substrate is plastically accommodated by net edge dislocations lying in a narrow region (a few hundred Å thick) at the interface with the substrate. The net number of edge dislocations was calculated to be ~1×10^4/cm^2. The measured perpendicular strains were in excellent agreement with the values calculated from bulk lattice parameters, elastic properties, and the parallel strain. For both superlattices, the standard deviation of random atomic displacements away from perfect crystal sites was below 0.1 Å, in agreement with reported ion channeling and electron diffraction measurements of superlattices. The rocking curve method is a major tool for quantitative analysis of superlattices

Optimizing end-labeled free-solution electrophoresis by increasing the hydrodynamic friction of the drag-tag

We study the electrophoretic separation of polyelectrolytes of varying lengths by means of end-labeled free-solution electrophoresis (ELFSE). A coarse-grained molecular dynamics simulation model, using full electrostatic interactions and a mesoscopic Lattice Boltzmann fluid to account for hydrodynamic interactions, is used to characterize the drag coefficients of different label types: linear and branched polymeric labels, as well as transiently bound micelles. It is specifically shown that the label's drag coefficient is determined by its hydrodynamic size, and that the drag per label monomer is largest for linear labels. However, the addition of side chains to a linear label offers the possibility to increase the hydrodynamic size, and therefore the label efficiency, without having to increase the linear length of the label, thereby simplifying synthesis. The third class of labels investigated, transiently bound micelles, seems very promising for the usage in ELFSE, as they provide a significant higher hydrodynamic drag than the other label types. The results are compared to theoretical predictions, and we investigate how the efficiency of the ELFSE method can be improved by using smartly designed drag-tags.Comment: 32 pages, 11 figures, submitted to Macromolecule

Damage and strain in epitaxial GexSi1–x films irradiated with Si

The damage and strain induced by irradiation of both relaxed and pseudomorphic GexSi1–x films on Si(100) with 100 keV 28Si ions at room temperature have been studied by MeV 4He channeling spectrometry and x-ray double-crystal diffractometry. The ion energy was chosen to confine the major damage to the films. The results are compared with experiments for room temprature Si irradiation of Si(100) and Ge(100). The maximum relative damage created in low-Ge content films studied here (x=10%, 13%, 15%, 20%, and 22%) is considerably higher than the values obtained by interpolating between the results for relative damage in Si-irradiated single crystal Si and Ge. This, together with other facts, indicates that a relatively small fraction of Ge in Si has a significant stabilizing effect on the retained damage generated by room-temperature irradiation with Si ions. The damage induced by irradiation produces positive perpendicular strain in GexSi1–x, which superimposes on the intrinsic positive perpendicular strain of the pseudomorphic or partially relaxed films. In all of the cases studied here, the induced maximum perpendicular strain and the maximum relative damage initially increase slowly with the dose, but start to rise at an accelerated rate above a threshold value of ~0.15% and 15%, respectively, until the samples are amorphized. The pre-existing pseudomorphic strain in the GexSi1–x film does not significantly influence the maximum relative damage created by Si ion irradiation for all doses and x values. The relationship between the induced maximum perpendicular strain and the maximum relative damage differs from that found in bulk Si(100) and Ge(100)

Weight management in a cohort of Irish inpatients with serious mental illness (SMI) using a modular behavioural programme. A preliminary service evaluation

<p>Abstract</p> <p>Background</p> <p>Weight gain is commonly observed during psychotropic treatments for chronic forms of severe mental illness and is most rapid during the early treatment phases. All formats of behavioural weight intervention programmes have suggested that weight gain can be prevented or reversed in some patients. There is no data on these programmes in acutely unwell inpatients whom may be the major beneficiaries.</p> <p>Methods</p> <p>A modular behavioural intervention programme (Solutions for Wellness) used in SMI outpatients since 2002 in Ireland has been adapted for inpatient use. Preliminary data is reported from 5 centres in Ireland.</p> <p>Results</p> <p>In 47 inpatients the mean weight change was +0.26 kg (SD 2.02) with a median change of 0 kg. Mean follow-up was 23.7 (SD 21.6) days, and median 14 days (range 6–98 days). There was no difference in mean weight change in those patients involved for > 35 days compared with < 35 days (+0.26 kg; 0.25 kg; p = 0.5). Weight loss or maintenance was seen in 70% of patients.</p> <p>Conclusion</p> <p>These preliminary data are supportive of the concept that acutely unwell inpatients with SMI may engage with a behavioural weight programme. Weight change observed contrasts with the significant weight gain often seen in most subjects. Further clinical trials are warranted.</p

Monoclonal antibodies directed to fucoidan preparations from brown algae

Cell walls of the brown algae contain a diverse range of polysaccharides with useful bioactivities. The precise structures of the sulfated fucan/fucoidan group of polysaccharides and their roles in generating cell wall architectures and cell properties are not known in detail. Four rat monoclonal antibodies, BAM1 to BAM4, directed to sulfated fucan preparations, have been generated and used to dissect the heterogeneity of brown algal cell wall polysaccharides. BAM1 and BAM4, respectively, bind to a non-sulfated epitope and a sulfated epitope present in the sulfated fucan preparations. BAM2 and BAM3 identified additional distinct epitopes present in the fucoidan preparations. All four epitopes, not yet fully characterised, occur widely within the major brown algal taxonomic groups and show divergent distribution patterns in tissues. The analysis of cell wall extractions and fluorescence imaging reveal differences in the occurrence of the BAM1 to BAM4 epitopes in various tissues of Fucus vesiculosus. In Ectocarpus subulatus, a species closely related to the brown algal model Ectocarpus siliculosus, the BAM4 sulfated epitope was modulated in relation to salinity levels. This new set of monoclonal antibodies will be useful for the dissection of the highly complex and yet poorly resolved sulfated polysaccharides in the brown algae in relation to their ecological and economic significance