Homoepitaxial n-core: p-shell gallium nitride nanowires: HVPE overgrowth on MBE nanowires

We present the homoepitaxial growth of p-type, magnesium doped gallium nitride shells by use of halide vapor phase epitaxy (HVPE) on n-type gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy (MBE). Scanning electron microscopy shows clear dopant contrast between the core and shell of the nanowire. The growth of magnesium doped nanowire shells shows little or no effect on the lattice parameters of the underlying nanowires, as measured by x-ray diffraction (XRD). Photoluminescence measurements of the nanowires show the appearance of sub-bandgap features in the blue and the ultraviolet, indicating the presence of acceptors. Finally, electrical measurements confirm the presence of electrically active holes in the nanowires.     

Highly selective GaN-nanowire/TiO2-nanocluster hybrid sensors for detection of benzene and related environment pollutants

Nanowire-nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO(2)) nanoclusters for selectively sensing benzene and other related aromatic compounds. Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) nanoclusters using RF magnetron sputtering. The sensor fabrication process employed standard microfabrication techniques. X-ray diffraction and high-resolution analytical transmission electron microscopy using energy-dispersive x-ray and electron energy-loss spectroscopies confirmed the presence of the anatase phase in TiO(2) clusters after post-deposition anneal at 700?°C. A change of current was observed for these hybrid sensors when exposed to the vapors of aromatic compounds (benzene, toluene, ethylbenzene, xylene and chlorobenzene mixed with air) under UV excitation, while they had no response to non-aromatic organic compounds such as methanol, ethanol, isopropanol, chloroform, acetone and 1,3-hexadiene. The sensitivity range for the noted aromatic compounds except chlorobenzene were from 1% down to 50 parts per billion (ppb) at room temperature. By combining the enhanced catalytic properties of the TiO(2) nanoclusters with the sensitive transduction capability of the nanowires, an ultra-sensitive and selective chemical sensing architecture is demonstrated. We have proposed a mechanism that could qualitatively explain the observed sensing behavior.     

Role of FSH, numbers of FSH receptors and testosterone in the regulation of inhibin secretion during the seasonal testicular cycle of adult rams

The regulation of inhibin secretion has not been elucidated fully in male ruminants. The aim of this study was to determine the relative importance of FSH and testosterone concentrations, and FSH receptors, in the control of secretion of immunoactive inhibin in rams. In Expt 1, temporal changes in hormone concentrations and testicular FSH binding were determined for two groups of rams (n = 4) kept under opposite, alternating 4 month periods of long (16 h light:8 h dark) and short (8 h light:16 h dark) days. Testicular biopsies (1-2 g) were collected when the testes were regressed, redeveloping, redeveloped and regressing. In Expt 2, separate groups of rams (n = 4) kept under natural photoperiod (latitude 45 degrees 48 minutes N) were designated as controls or passively immunized (for 3 weeks) with sufficient oestradiol antiserum to increase testosterone secretion without altering LH and FSH; this was done when the testes were regressed (non-breeding season) and redeveloped (breeding season). In both groups of rams (Expt 1), 'seasonal' increases in FSH concentrations began a few weeks earlier than did increases in inhibin concentrations. FSH reached maximum concentrations during testicular recrudescence, whereas numbers of FSH receptors in the testis and circulatory inhibin concentrations did not reach peak values until the testes were fully developed. Numbers of FSH receptors per testis, but not FSH concentration, were positively correlated (r = 0.65) with inhibin concentrations across the four stages of the testicular cycle. Near the end of testicular recrudescence early in the breeding season (Expt 2), relatively high FSH concentration was associated with increased abundance of FSH receptor mRNA (90%) and number of receptors (45%) in the testis and increased inhibin concentrations (50%), compared with when the testes were regressed. Moderate, physiological increases in testosterone secretion in immunized rams did not affect inhibin in either season. These results indicate that: (i) FSH stimulation of immunoactive inhibin secretion by Sertoli cells as testes recrudesce is via increases in secretion (early) and cognate receptors (late); (ii) FSH upregulates the synthesis of its own receptor late in recrudescence; and (iii) the positive correlation (r = 0.70) observed between circulatory testosterone and immunoactive inhibin does not reflect a causal relationship.