Giant defect emission enhancement from ZnO nanowires through desulfurization process.

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments

Transcatheter tricuspid valve implantation: A multicentre French study

SummaryBackgroundTranscatheter valve-in-valve (VIV) implantation in failing bioprosthesis is an emerging field in cardiology.AimTo report on a French multicentre experience and a literature review of tricuspid VIV implantation.MethodsWe approached different institutions and collected 10 unpublished cases; a literature review identified 71 patients, including our 10 cases. Clinical aspects and haemodynamic data are discussed.ResultsAmong our 10 unpublished cases, the reason for implantation was significant tricuspid stenosis (n=4), significant tricuspid regurgitation (n=1) or mixed lesion (n=5). Implantation was performed under general anaesthesia at mean age 28±17 years. The 22mm Melody valve was implanted in seven patients; the Edwards SAPIEN valve was implanted in three patients. The procedure succeeded in all cases, despite two embolizations in the right cardiac chambers; in both cases, the valve was stabilized close to the tricuspid annulus using a self-expandable stent, before implantation of a second Edwards SAPIEN valve. Functional class improved in all but one case. Mean diastolic gradient decreased from 9±2.45mmHg to 3.65±0.7mmHg (p=0.007); no more than trivial regurgitation was noticed. Among the published cases, the Melody valve was implanted in 41 patients, the Edwards SAPIEN valve in 29 patients and the Braile valve in one patient. Short-term results were similar for our 10 cases, but mid-term results are not yet available.ConclusionsTricuspid VIV implantation using the Melody or Edwards SAPIEN valves is a feasible and effective procedure for selected patients with failing bioprosthesis

Preparation par epitaxie en phase liquide et etude physique des solutions solides Cd_xHg_1_-_xTe 0,5 < x <1

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Characterization of sol gel Zn1-xCaxO thin layers deposited on p-Si substrate by spin-coating method

International audienceThin films Zn1-xCaxO (0≤x≤6%) on a p-Si substrate are elaborated by sol-gel process and spin coating. X-ray diffraction displays a hexagonal wurtzite structure with an increase of lattice parameters confirming the substitution of Zn 2+ by Ca 2+. The estimation of crystallite sizes along the three main crystallographic planes is practically constant (22 nm) suggesting a spherical symmetry shape of the crystallites which is confirmed by SEM. UV-visible reflectance spectra attest a band gap tuning from 3.144 to 3.262 eV which is confirmed by the PL measurement at 2 K for Zn0.94Ca0.06O sample by the appearance of a broad band around 3.508 eV. Although the luminescence of the samples is weak, we well distinguish the free exciton emission (FXA) positioned at 3.376 eV and the bound exciton recombination (D 0 X) around 3.362 eV

Effect of the plasma experimental parameters on the dose and profiles of hydrogen in-diffused into the GaN/AlGaN/GaN/Si high electron mobility transistor

International audienceThe structures of electronic devices based on GaN and its AlGaN alloys grown on foreign substrates, display a high threading dislocations concentration (TDs); 107–1011 cm-2, introducing deffects diminishing the devices predicted performances. The High Electron Mobility Transistor (HEMT) core is constituted by an AlGaN/GaN heterojunction, where the AlGaN layer, of nanometric dimensions, results on the formation of the high mobility-two-dimensional electron gas (TEG). The TEG performance might be improved by its hydrogenation, although the process can, as well, catastrophically degrade the device. Here is presented a detailed study of the effect of the hydrogenation experimental parameters: plasma power, sample temperature and duration, on its dose and distribution throughout the structure. This study found that the dose depends on the square of the plasma power, increases linearly on the hydrogenation time and slightly proportional to the sample temperature. These results shed light on how to improve the AlGaN/GaN HEMT performance

Fabrication and characterization of ZnO:Sb/n-ZnO homojunctions

International audienceAntimony-doped ZnO layers have been grown by metalorganic vapour-phase epitaxy on sapphire and ZnO substrates at high-temperature (950 °C) and low-pressure conditions (50 torr). Nitrous oxide and diethyl-zinc have been used as oxygen and zinc precursors, respectively. The incorporation of antimony has been obtained from the decomposition of triethylantimony doping molecules added in the gas phase. High Sb concentrations were measured from 1019 to 1021 at/cm−3 using secondary ion mass spectroscopy and depend on the nature and the orientation on the substrate. Low-temperature photoluminescence spectra of Sb-doped layers exhibit donor–acceptor pair transitions at 3.253 eV. Unlike Raman spectra of nitrogen-doped ZnO layers which show several local vibrational modes related to nitrogen incorporation, these modes were found to be absent in the antimony-doped ZnO layers. Transmission electron microscopy suggests that the incorporation of Sb is partly related to dislocations and other structural defects. All together, the characterizations suggest the formation of acceptor dopant–defect complexes, such as SbZn-2VZn. Finally, ZnO:Sb/n-ZnO homojunction diodes have been successfully elaborated on ZnO substrate. The current–voltage characteristics of the device exhibit a rectifying behaviour with a turn-on voltage of 3 V

Control of the white-light emission in the mixed two-dimensional hybrid perovskites (C6H11NH3)2[PbBr4−xIx]

International audienceThe control of the composition of mixed organic-inorganic hybrid perovskites by solid-state alloying is a very efficient tool to tune the band gap of the material, leading to enhanced optical performances. At this end, we have elaborated a series of mixed-halide two-dimensional hybrid perovskites (C6H11NH3)(2)[PbBr4-xIx], with 0 2), the photoluminescence spectra showed a sharp peak with a relatively small Stokes shift (less than 0.2 eV) attributed to free or bound excitons emissions. In contrast, when the bromide is majority (x < 2), the photoluminescence response consists in a broadband white-light emission with a very large Stokes shift (between 1.2 eV and 0.3 eV), attributed to self-trapped excitons activated by a strong structural distortion of the inorganic sheets. Confronting optical and structural data highlighted the effect of the structure in monitoring the optical properties, since the intensity of the white-light emission increases with the bromine content and was found to excellently correlate with the change of the angular distortion of inorganic octahedra PbX6 (X = I/Br

Surface effects on exciton diffusion in non polar ZnO/ZnMgO heterostructures

International audienceThe diffusion of excitons injected in ZnO/Zn 0.92 Mg 0.08 O quantum well heterostructures grown by metal-organic-vapor-phase-epitaxy on non-polar ZnO substrates is investigated at room temperature. Cathodoluminescence linescans in a field-emission-gun scanning-electronmicroscope are performed across cleaved cross-sections. A 55 nm diffusion length is assessed for excitons in bulk ZnMgO. When prepared as small angle bevels using focused ion beam (FIB), the effective diffusion length of excitons is shown to decrease down to 8 nm in the thinner part of the slab. This effect is attributed to non-radiative surface recombinations, with a 7 × 10 4 cm s−1 recombination velocity estimated at the FIB-machined ZnMgO surface. The strong reduction of the diffusion extent in such thin lamellae usually used for transmission electron microscopy could be use improve the spatial resolution of cathodoluminescence images, often limited by diffusion processes

Broadband Emission in a New Two-Dimensional Cd-Based Hybrid Perovskite

International audienceOrganic–inorganic hybrid perovskites (OIHP) are developing rapidly as high-performance semiconductors for solid-state solar cells and light emitting devices. Recently, lead-halide two-dimensional (2D) OIHP were found to present bright broadband visible emission, thus, highlighting their potential as single component white-light (WL) emitters. This contribution deals with the preparation of a new Cd-based 2D hybrid perovskite, of the chemical formula (C6H11NH3)2CdBr4 (abbreviated as compound 1), of which structural and optical properties have been studied and analyzed. Room temperature optical absorption (OA) measurements, performed on spin-coated film of compound 1, revealed a sharp excitonic absorption peak at 3.24 eV, and a large exciton binding energy of 377 meV, estimated from low temperature OA spectrum. Upon 325 nm irradiation, compound 1 showed a very broadband WL emission consisting of one peak at 2.94 eV, attributed to exciton confined in the [CdBr4]2– inorganic layers, and a second peak at 2.53 eV resulting from the cyclohexylammonium cations emission. Temperature dependence of PL spectra evidenced anomalous behavior accompanied by singularities around 50 and 150 K in the integrated intensity, the full width at half-maximum and the PL peaks positions. These singularities have been traced back to structural phase transitions, from temperature dependence powder and single crystal X-ray diffraction investigations, from which strong correlations had emerged between the structural distortion of the CdBr6 pseudo-octahedron and the broadening characteristics of the WL emission band. These hitherto unrecognized properties turn this and similar OIHP into perspective candidates for potential applications as WL-emitting diodes