Circumventing the miscibility gap in InGaN nanowires emitting from blue to red.

Widegap III-nitride alloys have enabled new classes of optoelectronic devices including light emitting diodes, lasers and solar cells, but it is admittedly challenging to extend their operating wavelength to the yellow-red band. This requires an increased In content x in In x Ga1-x N, prevented by the indium segregation within the miscibility gap. Beyond the known advantage of dislocation-free growth on dissimilar substrates, nanowires may help to extend the compositional range of InGaN. However, the necessary control over the material homogeneity is still lacking. Here, we present In x Ga1-x N nanowires grown by hydride vapor phase epitaxy on silicon substrates, showing rather homogeneous compositions and emitting from blue to red. The InN fraction in nanowires is tuned from x = 0.17 up to x = 0.7 by changing the growth temperature between 630 °C and 680 °C and adjusting some additional parameters. A dedicated model is presented, which attributes the wide compositional range of nanowires to the purely kinetic growth regime of self-catalyzed InGaN nanowires without macroscopic nucleation. These results may pave a new way for the controlled synthesis of indium-rich InGaN structures for optoelectronic applications in the extended spectral range.

LIDAR developments at Clermont-Ferrand--France for atmospheric observation.

We present a Rayleigh-Mie-Raman LIDAR system in operation at Clermont-Ferrand (France) since 2008. The system provides continuous vertical tropospheric profiles of aerosols, cirrus optical properties and water vapour mixing ratio. Located in proximity to the high altitude Puy de Dôme station, labelled as the GAW global station PUY since August 2014, it is a useful tool to describe the boundary layer dynamics and hence interpret in situ measurements. This LIDAR has been upgraded with specific hardware/software developments and laboratory calibrations in order to improve the quality of the profiles, calibrate the depolarization ratio, and increase the automation of operation. As a result, we provide a climatological water vapour profile analysis for the 2009-2013 period, showing an annual cycle with a winter minimum and a summer maximum, consistent with in-situ observations at the PUY station. An overview of a preliminary climatology of cirrus clouds frequency shows that in 2014, more than 30% of days present cirrus events. Finally, the backscatter coefficient profile observed on 27 September 2014 shows the capacity of the system to detect cirrus clouds at 13 km altitude, in presence of aerosols below the 5 km altitude.

Ultralong and defect-free GaN nanowires grown by the HVPE process.

GaN nanowires with exceptional lengths are synthesized by vapor-liquid-solid coupled with near-equilibrium hydride vapor phase epitaxy technique on c-plane sapphire substrates. Because of the high decomposition frequency of GaCl precursors and a direct supply of Ga through the catalyst particle, the growth of GaN nanowires with constant diameters takes place at an exceptional growth rate of 130 µm/h. The chemical composition of the catalyst droplet is analyzed by energy dispersive X-ray spectroscopy. High-resolution transmission electron microscopy and selective area diffraction show that the GaN nanowires crystallize in the hexagonal wurzite structure and are defect-free. GaN nanowires exhibit bare top facets without any droplet. Microphotoluminescence displays a narrow and intense emission line (1 meV line width) associated to the neutral-donor bound exciton revealing excellent optical properties of GaN nanowires.

Encapsulation of InP/ZnS Quantum Dots into MOF-5 Matrices for Solid-State Luminescence: Ship in the Bottle and Bottle around the Ship Methodologies.

The utilization of InP-based quantum dots (QDs) as alternative luminescent nanoparticles to cadmium-based QDs is actively pursued. However, leveraging their luminescence for solid-state applications presents challenges due to the sensitivity of InP QDs to oxidation and aggregation-caused quenching. Hence, an appealing strategy is to protect and disperse InP QDs within hybrid materials. Metal-organic frameworks (MOFs) offer a promising solution as readily available crystalline porous materials. Among these, MOF-5 (composed of {Zn4O}6+ nodes and terephthalate struts) can be synthesized under mild conditions (at room temperature and basic pH), making it compatible with InP QDs. In the present work, luminescent InP/ZnS QDs are successfully incorporated within MOF-5 by two distinct methods. In the bottle around the ship (BAS) approach, the MOF was synthesized around the QDs. Alternatively, in the ship in the bottle (SIB) strategy, the QDs were embedded via capillarity into a specially engineered, more porous variant of MOF-5. Comparative analysis of the BAS and SIB approaches, evaluating factors such as operational simplicity, photoluminescence properties, and the resistance of the final materials to leaching were carried out. This comparative study provides insights into the efficacy of these strategies for the integration of InP/ZnS QDs within MOF-5 for potential solid-state applications in materials chemistry.

Polymeric copper(I)-NHC complexes with bulky bidentate (N

Starting from imidazolium chlorides bearing bulky nitrogen donors, a series of four complexes, mainly [Cu(C^N)Cl]n coordination polymers were obtained directly as luminescent species by simple filtration from the aqueous reaction medium, highlighting a simple, eco-friendly, robust and reproducible synthetic procedure. Additionally, we have shown on the most efficient example that chloride could be exchanged very easily by other halides/pseudohalides (Br-, I-, NCS-, N3-) allowing to slightly modulate the emitted colour while conserving the polymeric structure, except for azide for which a dimer was obtained. The combination of chemical analyses, of photoluminescence studies in the solid state including quantum yield measurement and X-ray diffraction on single crystals and as-synthesized microcrystalline powders highlighted that the polymeric luminescent species was indeed obtained directly by simple filtration and that no major alteration of the structure was observed upon recrystallisation. Samples of all polymeric complexes displayed remarkable stability towards air oxidation remaining unchanged upon storage for several months and partially retaining their photoluminescence properties even after a thermal treatment at 100 °C for 24 h.

Nanostructuration of YAG:Ce Coatings by ZnO Nanowires: A Smart Way to Enhance Light Extraction Efficiency.

In this study, we report on the enhancement of the light extraction efficiency of sol-gel-derived Y3Al5O12:Ce3+ (YAG:Ce) coatings using ZnO nanowire (NW) arrays. The ZnO NWs were grown by hydrothermal synthesis from a ZnO seed layer directly deposited on a YAG:Ce coating. Highly dense and vertically aligned ZnO NW arrays were evidenced on the top of the YAG:Ce coating by electron microscopy. A photoluminescence study showed that this original design leads to a different angular distribution of light together with an increase in emission efficiency of the YAG:Ce coating upon blue excitation, up to 60% more efficient compared to a non-structured YAG:Ce coating (without NWs). These improvements are ascribed to multi-scattering events for photons within the structure, allowing them to escape from the phosphor layer by taking optical paths different from those of the non-structured coating. This strategy of light extraction enhancement appears to be very promising, since it uses soft chemical processes and cheap ZnO NWs and is applicable to any sol-gel-derived luminescent coating.

A First Wide-Open LDH Structure Hosting InP/ZnS QDs: A New Route Toward Efficient and Photostable Red-Emitting Phosphor.

The architecture of Zn-Al layered double hydroxides (LDHs), organo-modified with bola-amphiphiles molecules, is matching its interlayer space to the size of narrow-band red-emitting InP/ZnS core-shell quantum dots (QDs) to form original high-performance functional organic-inorganic QD-bola-LDH hybrids. The success of size-matching interlayer space (SMIS) approach is confirmed by X-ray diffraction, small angle X-ray scattering (SAXS), TEM, STEM-HAADF, and photoluminescence investigations. The QD-Bola-LDH hybrid exhibits a photoluminescence quantum yield three times higher than that of pristine InP/ZnS QDs and provides an easy dispersion into silicone-based resins, what makes the SMIS approach a change of paradigm compared to intercalation chemistry using common host structures. Moreover, this novel hybrid presents low QD-QD energy transfer comparable to that obtained for QDs in suspension. Composite silicone films incorporating InP/ZnS (0.27 wt%) QD-bola-LDH hybrids further show remarkable improved photostability relative to pristine QDs. An LED overlay consisting of a blue LED chip and silicone films loaded with QD-bola-LDH hybrids and YAG:Ce phosphors exhibits a color rendering index close to 94.