An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.
Iron is indispensable to the growth and metabolism of all marine organisms, including bacteria. In this work, we investigated and compared the influence of iron(III) concentration on the growth of and siderophore production by two heterotrophic bacteria--Micrococcus luteus and Bacillus silvestris. Our results showed that the iron concentration strongly influences the growth of both species. The growth curves were different for each iron concentration and each strain. M. luteus grew more rapidly than B. silvestris, but produced a roughly four times smaller quantity of siderophores. Both M. luteus and B. silvestris secreted hydroxamate-type siderophores and alpha-keto/alpha-hydroxy acids, but did not produce catecholates. This paper is probably the first to report on siderophore production by B. silvestris and M. luteus isolated from seawater. Moreover, the influence of different iron concentrations on the growth of and siderophore production in these bacteria has been documented. This provides further evidence indicating iron bioavailability as the actual reason for siderophore release by biota.
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