Intracellular urease activity in the lichen Cladonia verticillaris, and its implication for toxicity.

Urea is currently used as a nitrogen fertilizer in many plant cultures, such as sugar cane. Several lichen species grow in the edges of the fields fertilized with urea. This implies that the hydrolysis of an excess of urea by soil bacteria or by the lichens themselves would increase the concentration of ammonia in the lichen thallus to a level that may be toxic to the photobiont. However, Cladonia verticillaris produces urease through positive feedback by urea supplied from the medium. This urease is partially secreted to the media or retained on the external surface of algal cells, as demonstrated herein by an adequate cytochemical reaction. This implies that ammonia produced by urea hydrolysis will be immediately dissolved in the water filling the intercellular spaces on the thallus. A possible protection mechanism against eventual ammonia toxicity, derived from the results described here, is also discussed.

Interaction between lamellar twinning and catalyst dynamics in spontaneous core-shell InGaP nanowires.

Semiconductor nanowires oriented along the [211] direction usually present twins parallel to their axis. For group IV nanowires this kind of twin allows the formation of a catalyst-nanowire interface composed of two equivalent {111} facets. For III-V nanowires, however, the twin will generate two facets with different polarities. In order to keep the <211> orientation stable, a balance in growth rates for these different facets must be reached. We report here the observation of stable, micron-long <211>-oriented InGaP nanowires with a spontaneous core-shell structure. We show that stacking fault formation in the crystal region corresponding to the {111}A facet termination provides a stable NW/NP interface for growth along the <211> direction. During sample cool down, however, the catalyst migrates to a lateral {111}B facet, allowing the growth of branches perpendicular to the initial orientation. In addition to that, we show that the core-shell structure is non-concentric, most likely due to the asymmetry between the facets formed in the NW sidewall; this effect generates stress along the nanowire, which can be relieved through bending.

Heat Dissipation Interfaces Based on Vertically Aligned Diamond/Graphite Nanoplatelets.

Crystalline carbon-based materials are intrinsically chemically inert and good heat conductors, allowing their applications in a great variety of devices. A technological step forward in heat dissipators production can be given by tailoring the carbon phase microstructure, tuning the CVD synthesis conditions. In this work, a rapid bottom-up synthesis of vertically aligned hybrid material comprising diamond thin platelets covered by a crystalline graphite layer was developed. A single run was designed in order to produce a high aspect ratio nanostructured carbon material favoring the thermal dissipation under convection-governed conditions. The produced material was characterized by multiwavelength Raman spectroscopy and electron microscopy (scanning and transmission), and the macroscopic heat flux was evaluated. The results obtained confirm the enhancement of heat dissipation rate in the developed hybrid structures, when compared to smooth nanocrystalline diamond films.