Showing papers on "Wurtzite crystal structure published in 2015"

Distributions of phonon lifetimes in Brillouin zones

Abstract: A collaboration of researchers from Japan and France present a comprehensive study of phonon lifetimes and thermal conductivity for 33 zincblende- and wurtzite compounds using linearized phonon Boltzmann equation and first-principles anharmonic phonon calculations. The software that the authors created for this study will be released as an open source package and should be of help in the search of new materials for thermoelectric applications.

Photogenerated excitons in plain core CdSe nanocrystals with unity radiative decay in single channel: the effects of surface and ligands.

Abstract: A systematic and reproducible method was developed to study the decay dynamics of an exciton, a photogenerated electron–hole pair, in semiconductor nanocrystals in solution. Results revealed that the excitons in plain core CdSe nanocrystals in either zinc-blende or wurtzite or mixed lattice structures could be reproducibly prepared to decay radiatively in unity quantum yield and in single channel. The single-channel lifetime was found to increase monotonically by increasing size of the CdSe nanocrystals, with zinc-blende ones increasing in a relatively slow pace. Surface inorganic stoichiometry was found to be a sensitive parameter to affect the exciton decay dynamics for all crystal structures with different sizes. Excess Se (Cd) sites on the surface were found to induce short (long) lifetime channels for the excitons. Both types of traps reduced the quantum yield of the radiative decay of the excitons, and the hole traps associated with Se sites were nearly not emissive. With optimal surface inorganic s...

Defect Engineering and Phase Junction Architecture of Wide-Bandgap ZnS for Conflicting Visible Light Activity in Photocatalytic H2 Evolution

Abstract: ZnS is among the superior photocatalysts for H2 evolution, whereas the wide bandgap restricts its performance to only UV region. Herein, defect engineering and phase junction architecture from a controllable phase transformation enable ZnS to achieve the conflicting visible-light-driven activities for H2 evolution. On the basis of first-principle density functional theory calculations, electron spin resonance and photoluminescence results, etc., it is initially proposed that the regulated sulfur vacancies in wurtzite phase of ZnS play the key role of photosensitization units for charge generation in visible light and active sites for effective electron utilization. The symbiotic sphalerite-wurtzite phase junctions that dominate the charge-transfer kinetics for photoexciton separation are the indispensable configuration in the present systems. Neither ZnS samples without phase junction nor those without enough sulfur vacancies conduct visible-light photocatalytic H2 evolution, while the one with optimized ...

Novel ZnO/Fe2O3 Core–Shell Nanowires for Photoelectrochemical Water Splitting

Abstract: A facile and simple fabrication of Fe2O3 as a shell layer on the surface of ZnO nanowires (NW) as a core–shell nanoelectrode is applied for the photoelectrochemical (PEC) splitting of water. An ZnO NW array of core diameter ∼80 nm was grown on a fluorine-doped tin-oxide (FTO) substrate with a hydrothermal method; subsequent deposition and annealing achieved a shell structure of the Fe2O3 layer of thickness a few nm. Fe2O3 in the α phase and ZnO in the wurtzite phase were identified as the structures of the shell and core, respectively, through analysis with X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The ZnO/Fe2O3 core–shell NW showed an excellent PEC response to the oxidation of water, and also benefited from a negative shift of onset potential because of an n/n heterojunction structure. A detailed energy diagram of the ZnO/Fe2O3 core–shell NW was investigated with a Mott–Schottky analysis. This novel core–shell nanostructure can hence not only exhibit a great p...

Controlled growth of ZnO nanorod arrays via wet chemical route for NO2 gas sensor applications

Abstract: Metal oxide gas sensors are promising devices that are widely used to detect various gases at moderate temperatures. In this study, nitrogen di-oxide (NO 2 ) sensors were fabricated using zinc oxide (ZnO) nanorod arrays. ZnO nanorod arrays (ZNAs) with various rod lengths were deposited using a wet chemical route with zinc acetate as a precursor. The structural and surface morphological properties of the ZNAs were investigated using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The XRD patterns showed ZNAs with wurtzite crystal structures that were preferentially oriented in the (0 0 2) direction. The intensity of the (0 0 2) plane was found to vary with the length of the nanorods. FESEM micrographs show that the ZNAs had a vertical alignment perpendicular to the substrate, and the diameter and length of the nanorods increased as the nanorod deposition time was increased. The gas sensing performance was studied as a function of the nanorod length, operating temperature, time and gas concentration. The length and inter-rod space was observed to play a crucial role in determining the gas sensing performance of the devices. ZNA gas sensors deposited for 9 h and operating at a temperature of 175 °C were able to detect NO 2 at a concentration of 100 ppm with a high sensitivity of 3100%.

Efficient water reduction with gallium phosphide nanowires

Abstract: Photoelectrochemical hydrogen production from solar energy and water offers a clean and sustainable fuel option for the future. Planar III/V material systems have shown the highest efficiencies, but are expensive. By moving to the nanowire regime the demand on material quantity is reduced, and new materials can be uncovered, such as wurtzite gallium phosphide, featuring a direct bandgap. This is one of the few materials combining large solar light absorption and (close to) ideal band-edge positions for full water splitting. Here we report the photoelectrochemical reduction of water, on a p-type wurtzite gallium phosphide nanowire photocathode. By modifying geometry to reduce electrical resistance and enhance optical absorption, and modifying the surface with a multistep platinum deposition, high current densities and open circuit potentials were achieved. Our results demonstrate the capabilities of this material, even when used in such low quantities, as in nanowires.

A highly selective and wide range ammonia sensor—Nanostructured ZnO:Co thin film

Abstract: Ammonia sensing characteristics of undoped and cobalt (Co)-doped nanostructured ZnO thin films were investigated. Polycrystalline nature with hexagonal wurtzite structure and high crystalline quality with dominant (0 0 2) plane orientation of Co-doped ZnO film were confirmed by the X-ray diffractogram. Scanning electron micrographs of the undoped film demonstrated the uniform deposition of sphere-shaped grains. But, smaller particles with no clear grain boundaries were observed for Co-doped ZnO thin film. Band gap values were found to be 3.26 eV and 3.22 eV for undoped and Co-doped ZnO thin films. Ammonia sensing characteristics of Co-doped ZnO film at room temperature were investigated in the concentration range of 15–1000 ppm. Variation in the sensing performances of Co-doped and pure ZnO thin films has been analyzed and compared.

Combinatorial insights into doping control and transport properties of zinc tin nitride

Abstract: ZnSnN2 is an Earth-abundant semiconductor analogous to the III–nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate n-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have explored a growth-temperature-composition space for Zn1+xSn1−xN2 over the ranges 35–340 °C and 0.30–0.75 Zn/(Zn + Sn). In this way, we identified an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 × 1018 cm−3. Films grown at 230 °C with Zn/(Zn + Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 × 1018 cm−3) and high mobility (8.3 cm2 V−1 s−1). Using this approach, we establish the direct bandgap of cation-disordered ZnSnN2 at 1.0 eV. Furthermore, we report tunable carrier density as a function of cation composition, in which lower carrier density is observed for higher Zn content. This relationship manifests as a Burstein–Moss shift widening the apparent bandgap as cation composition moves away from Zn-rich. Collectively, these findings provide important insight into the fundamental properties of the Zn–Sn–N material system and highlight the potential to utilize ZnSnN2 for photovoltaics.

Synthesis of Zinc Oxide Nanoparticles via Sol - Gel Route and Their Characterization

Abstract: National Center for Packing and Packaging, Corporation of research and industrial development, Iraqi Ministry of industry and minerals Abstract In this work, zinc oxide nanoparticles were readily synthesized through sol-gel method using zinc acetate as a precursor. The crystalline structure, morphology of synthesized ZnO nanoparticles were observed using powder X-ray diffraction (XRD), FTIR analysis, scanning electron microscopy (SEM) and their optical properties characterized using UV -visible spectroscopy.XRD results revealed that the prepared ZnO sample is highly crystalline, having wurtzite crystalstructure. FT-IR spectra peak at 417.52 cm -1 indicated characteristic absorption bands OF ZnO nanoparticles. UV-Vis absorption spectrum showed a typical spectrum for ZnO nanoparticles. The SEM image shows that ZnO nanoparticles prepared in this study are spherical in shape with smooth surface.

Effect of solvents on ZnO nanostructures synthesized by solvothermal method assisted by microwave radiation: a photocatalytic study

Abstract: The present work reports the synthesis of zinc oxide (ZnO) nanoparticles with hexagonal wurtzite structure considering a solvothermal method assisted by microwave radiation and using different solvents: water (H2O), 2-ethoxyethanol (ET) and ethylene glycol (EG). The structural characterization of the produced ZnO nanoparticles has been accessed by scanning electron microscopy, X-ray diffraction, room-temperature photoluminescence and Raman spectroscopies. Different morphologies have been obtained with the solvents tested. Both H2O and ET resulted in rods with high aspect ratio, while EG leads to flower-like structure. The UV absorption spectra showed peaks with an orange shift for synthesis with H2O and ET and blue shift for synthesis with EG. The different synthesized nanostructures were tested for photocatalyst applications, revealing that the ZnO nanoparticles produced with ET degrade faster the molecule used as model dye pollutant, i.e. methylene blue.

In Situ Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes

Abstract: ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the in situ formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures.

Facile Synthesis of Quasi Spherical ZnO Nanoparticles with Excellent Photocatalytic Activity

Abstract: In this article, we report on the synthesis, characterization and photocatalytic activity of quasi spherical ZnO nanoparticles obtained by an egg white assisted facile sol–gel type wet method. The material was characterized for its structural, textural and optical properties. The hexagonal wurtzite crystalline structure of ZnO with high phase purity was confirmed by the X-ray diffraction analysis. The mesoporous texture generated from the inter-agglomeration of ZnO nanoparticles was clearly shown in the transmission electron microscopy (TEM) images. The N2 sorption analysis indicated a specific surface area of 18 m2/g, with monomodal mesoporosity. The optical studies had shown the decreased optical band gap (3.28 eV) of the sample with the existence of a number of crystal defects, especially oxygen vacancies in the sample. The aquatic dye pollutants were effectively degraded under UV irradiation over the ZnO nano photocatalysts. They were also found to be reusable up to five consecutive runs without loss in catalytic activity, indicating their high photostability against photocorrosion.

Excellent acetone sensor of La-doped ZnO nanofibers with unique bead-like structures

Abstract: In this work, Lanthanum (La) doped ZnO nanofibers with bead-like structures were facilely produced by electrospinning technique. The obtained La-doped ZnO products were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller method, transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). The results show that La doping changes the structures of ZnO nanofibers markedly. La-doped ZnO nanofibers have unique bead-like nanostructures, in which two phases of hexagonal La2O3 and wurtzite ZnO coexist along with partially incorporation of La into ZnO lattice. The gas sensing performances of the La-doped ZnO nanostructures to acetone were investigated via static gas sensor testing system. The sensing test results indicate that an appropriate amount of La doping greatly improves the gas sensing properties of ZnO nanofibers. The 1.0 wt% La-doped ZnO sensor has the highest selectivity and response (64, to 200 ppm acetone at 340 °C), in addition to its short response time and recovery time. The unique bead-like structure and the gas sensing mechanism of La-doped ZnO nanofibers are discussed. The La-doped ZnO nanostructures we have produced can be used as a promising material for acetone sensors.