Density-controlled growth of vertical InP nanowires on Si(111) substrates.

A procedure to achieve the density-controlled growth of gold-catalyzed InP nanowires (NWs) on (111) silicon substrates using the vapor-liquid-solid method by molecular beam epitaxy is reported. We develop an effective and mask-free method based on controlling the number and the size of the Au-In catalyst droplets in addition to the conditions for the NW nucleation. We show that the NW density can be tuned with values in the range of 18 µm-2 to <0.1 µm-2 by the suitable choice of the In/Au catalyst beam equivalent pressure (BEP) ratio, by the phosphorous BEP and the growth temperature. The same degree of control is transferred to InAs/InP quantum dot-nanowires, taking advantage of the ultra-low density to study by micro-photoluminescence the optical properties of a single quantum dot-nanowires emitting in the telecom band monolithically grown on silicon. Optical spectroscopy at cryogenic temperature successfully confirmed the relevance of our method to excite single InAs quantum dots on the as-grown sample, which opens the path for large-scale applications based on single quantum dot-nanowire devices integrated on silicon. .

Controlling the shape of a tapered nanowire: lessons from the Burton-Cabrera-Frank model.

The propagation of sidewall steps during the growth of nanowires is calculated in the frame of the Burton-Cabrera-Frank model. The stable shape of the nanowire comprises a cylinder section on top of a cone section: their characteristics are obtained as a function of the radius of the catalyst-nanowire area, the desorption-limited diffusion length of adatoms on the terraces, and the sticking of adatoms at step edges. The comparison with experimental data allows us to evaluate these last two parameters for InP and ZnTe nanowires; it reveals a different behavior for the two materials, related to a difference by an order of magnitude of the desorption-limited diffusion length.

Effects of aluminium and bacterial lipopolysaccharide on oxidative stress and immune parameters in roach, Rutilus rutilus L.

Aluminium is used in diverse anthropogenic processes at the origin of pollution events in aquatic ecosystems. In the Champagne region (France), high concentrations of aluminium (Al) are detected due to vine-growing practices. In fish, little is known about the possible immune-related effects at relevant environmental concentrations. The present study analyzes the simultaneous effects of aluminium and bacterial lipopolysaccharide (LPS), alone and in combination, on toxicological biomarkers in the freshwater fish species Rutilus rutilus. For this purpose, roach treated or not with LPS were exposed to environmental concentrations of aluminium (100 µg/L) under laboratory-controlled conditions for 2, 7, 14 and 21 days. After each exposure time, we assessed hepatic lipoperoxidation, catalase activity, glutathione reductase activity and total glutathione content. We also analyzed cellular components related to the LPS-induced inflammatory response in possible target tissues, i.e. head kidney and spleen. Our results revealed a significant prooxidant effect in the liver cells and head kidney leukocytes of roach exposed to 100 µg of Al/L for 2 days. In liver, we observed more lipoperoxidation products and lower endogenous antioxidant activity levels such as glutathione reductase activity and total glutathione content. These prooxidant effects were associated with a higher oxidative burst in head kidney leukocytes, and they were all the more important in fish stimulated by LPS injection. These findings demonstrate that environmental concentrations of Al induce oxidative and immunotoxic effects in fish and are associated to an immunomodulatory process related to the inflammatory response.

Polychlorinated biphenyls in freshwater salmonids from the Kerguelen Islands in the Southern Ocean.

The Subantarctic Kerguelen Islands (49°S, 70°E) contain freshwater ecosystems among the most isolated in the world. Concentrations of polychlorinated biphenyls (PCBs) were assessed in the muscle of 48 brook trout and 38 brown trout caught during summer and spring 2006 in the rivers, lakes and ponds of Kerguelen. The sum of 29 PCBs averaged 404 and 358 ng g(-1) lipid, and dioxin-like PCB was 19 and 69 ng g(-1) lipid, in brook and brown trout, respectively. The values showed a high variability and some fish accumulated PCBs at levels similar to those of fish from impacted areas. While inter-sex differences were limited, the season and the morphotype appeared to have the most influence. Fish captured in summer had muscle PCB concentrations about three times higher than those caught in spring and the 'river' morphotype of brook trout showed the highest PCB levels.