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Co-integration of nanomaterials into microdevices poses several technological challenges and presents numerous scientific opportunities that have been addressed in this paper by integrating zinc oxide nanowires (ZnO-NWs) into a microfluidic chamber. In addition to the applications of these combined materials, this work focuses on the study of the growth dynamics and uniformity of nanomaterials in a tiny microfluidic reactor environment. A unique experimental platform was built through the integration of a noninvasive optical characterization technique with the microfluidic reactor. This platform allowed the unprecedented demonstration of time-resolved and spatially resolved monitoring of the in situ growth of NWs, in which the chemicals were continuously fed into the microfluidic reactor. The platform was also used to assess the uniformity of NWs grown quickly in a 10-mm-wide microchamber, which was intentionally chosen to be 20 times wider than those used in previous attempts because it can accommodate applications requiring a large surface of interaction while still taking advantage of submillimeter height. Further observations included the effects of varying the flow rate on the NW diameter and length in addition to a synergetic effect of continuous renewal of the growth solution and the confined environment of the chemical reaction.

In an effort to scale-up nanomaterial growth over large surface areas, we aim to effectively study the structural non-homogeneities within the arrays of zinc oxide nanowires (ZnO-NWs). The assessment of the lateral gradient of the nanowires' characteristics is presented including their height and surface density. To this end, spectroscopic ellipsometry and the rather recently reported technique of spectral domain attenuated reflectometry are used as two fast, simple and non-invasive characterization methods with further capabilities of scanning over the sample surface. Simple models are proposed by considering ZnO-NWs as the equivalent of thin stratified layers based on the effective medium approach. The methodology not only reveals the presence of gradients, but also enables quantitative analysis for all the samples grown using the hydrothermal method with different growth times ranging from 0.5 h up to 4 h. The gradients are confirmed using scanning electron microscopy (SEM) observations taken as a reference. The results also suggest that the sample orientation during the growth influences the NW growth besides the other parameters already known to affect the growth mechanisms.

Structured and functional microfluidic channels make microfluidic chips and devices more powerful in a variety of applications. In situ decoration of nanostructures on the channel wall is a low-cos...

Atmospheric pollution is a major issue affecting environment and health. Therefore, semi-conductor-nanoparticles-based photocatalytic infrastructures appeared as a promising solution to improve urban air quality due to their abilities to mineralize toxic organic compounds. To avoid the direct use of free nanoparticles, as nanomaterials are suspected to be carcinogenic, this work intends to develop civil engineering materials functionalized by direct growth of ZnO nanostructures as a more environmentally friendly and biocompatible approach to reduce the air pollution. By scaling-up an innovative and low-cost hydrothermal direct growth synthesis, a few square meters paved with tiling and bitumen road were easily produced in order to evaluate their photocatalytic activity at large scale under solar lamp in a climatic chamber (Sense-City, 3200 m3) to reflect real atmospheric air purification situations. Observations provide insights into their ability to simultaneously remove various pollutants from a real car exhaust (O3, COX, NOX, VOCs) and their durability, as well as understandings of the weather conditions impact on the photocatalytic degradation of these pollutants and of the relationships between pollutants trend evolution. This study also seems to prove that, when exposed to sunlight, bitumen roads are responsible of pollutants emission and causes modifications in the Chapman cycle.

ZnO nanowire (NW) arrays were successfully grown on pre-seeded indium Tin oxide (ITO)-coated glass substrate using hydrothermal synthesis. Herein, the effects of ZnO seed layer density on the performance of ZnO NWs were investigated in details. The orientation and the dimension of ZnO NWs were found to depend on seeded substrate density as shown by scanning electron microscopy (SEM) micrographs which revealed that the typical morphology with the most uniform size can be obtained. From the X-ray diffraction (XRD) measurement, it can be seen that hexagonal c -axis oriented NWs were grown. The resonant Raman scattering was also investigated in details. It confirmed the wurtzite structure of the NW arrays and expected for good optical properties. The optical band gaps of synthesized ZnO NWs were found to decrease comparing to bulk ZnO and as function of seed layer. The photoluminescence (PL) spectra at room temperature have shown strong UV excitonic emission and weak deep-level emissions which reveal that the as-grown NW arrays have good optical properties with limited deep-level defects.

Structures of the two-dimensional atomic nuclei on ZnO (0001)-Zn and ( 000 1 ¯ )-O polar surfaces were studied by first principles density functional theory. The polarity-dependent nucleation dynamics was investigated by simulating two-dimensional (2D) nuclei consisting of 1-8 ZnO monomers on both polar surfaces. According to total energy calculations, average binding energy per ZnO monomer of the surface nuclei was analyzed to investigate if the nucleation and growth will proceed reasonably in physics. We found nucleation on (0001)-Zn surface was easier than that on ( 000 1 ¯ )-O surface. By using atomistic thermodynamics analysis, we calculated the Gibbs free energy of formation of these nuclei and made a comparison between the two polar surfaces. On (0001)-Zn surface, the critical Gibbs free energy of formation is much lower than that on ( 000 1 ¯ )-O surface under the same supersaturation, which leads to a much larger ZnO growth rate and rougher morphology, in accordance with experimental results. In addition, energetic analysis of nucleation at real thermodynamic conditions was achieved by introducing the temperature- and pressure-dependent chemical potentials of ZnO precursors.

We present online monitoring of nanowires growth within a microfluidic chamber. This achievement is introduced for the first time thanks to simple and noninvasive real-time optical method, where the results were confirmed later by scanning electron microscopy (SEM). Nanowire length and diameter could be monitored versus growth time thanks to remote optical measurement. In this work, we also report kinetics study and online monitoring of in-situ growth of Zinc-Oxide nanowires (ZnO-NWs) within a microfluidic chamber showing the impact of different growth parameters on the NWs characteristics. It appeared that growth within a microfluidic reactor provides 4-times faster growth rate compared to macroscopic growth due to the confined environment of the chemical reaction. It also appears that varying the flow rate has more influence on the NWs diameter rather than the NWs length.

Environmental chemical pollution in water and air is induced by increasing human activity such as industry, transportation, intensive agriculture and growing urbanization. These global environmental issues are now listed among the sustainable development goals (SDGs), hence the increasing scientific and technological interests on water depollution and air purification, besides the needs for selective measurement of pollutants. ZnO nanostructures have been widely studied as a very promising material to rise such challenge of depollution/purification. Online monitoring of air purification is presented, where purification is made by photo-catalysis using ZnO nanowires, while monitoring is achieved by a MEMS-FTIR spectrometer operating in the mid-infrared MIR spectral range. The experiment was conducted on toluene, acetone as well as tobacco smoke. The whole system appears as an efficient and cheap solution for indoor and outdoor purification and monitoring of air.

Photocatalysis is one of the most promising processes for treating air and water pollution. Innovative civil engineering materials for environmental depollution by photocatalysis have already been synthesized by incorporating TiO2 or ZnO nanoparticles in cement. This method suffers from two flaws: first, most of the NPs are incorporated into the cement and useless for photocatalysis; second, rain and wind could spread the potentially carcinogenic nanoparticles from the cement surface into nature. Thus, we propose the efficient synthesis of nontoxic and biocompatible ZnO nanostructures solely onto the surface of commercially available concrete and tiling pavements by a low-cost and low-temperature hydrothermal method. Our samples exhibited enhanced photocatalytic activity for degrading organic dyes in aqueous media, and dye molecules are commonly used in the pharmaceutical, food, and textile industries. Durability studies showed no loss of efficiency after four photocatalysis experiments. Such supported structures, which are easy to implement onto the varying surfaces of commercially available materials, are promising for integration into civil engineering surfaces for environmental depollution in our daily life., Photocatalysis: Synthesizing safe zinc oxide nanowires for civil engineering materials An easy, low-cost method has been developed for successfully synthesizing zinc oxide (ZnO) nanostructures onto commercially available materials for environmentally safe use in civil engineering. Photocatalysis is the process whereby light accelerates a chemical reaction, and it holds promise as a means of tackling pollution caused by civil engineering materials. Previous efforts to produce such materials by incorporating ZnO and titanium oxide (TiO2) nanoparticles for photocatalysis have suffered drawbacks, notably the nanoparticles being incorporated within the cement and thus useless for photocatalysis. A team headed by Yamin Leprince-Wang at Université Paris-Est, France developed a low-cost, low-temperature hydrothermal method for efficiently synthesizing nontoxic, biocompatible ZnO nanowires solely onto the surface of commercially available concrete, which allowed for excellent photocatalytic efficiency. The team believes that its method has good potential for environmentally safe application on road surfaces.

Air pollution is used to refer to the release of pollutants into the air, where these pollutants are harmful to the human health and our planet. The main source of these pollutants comes from energy production and consumption that release Volatile Organic Compounds (VOCs) such as BTEX and Aldehydes group. Real time monitoring of these VOCs in factories, stations, homes and in the street is important for analysis of the pollution sources fingerprint and for alerting, when exceeding the harmful limits. In this work we report the use of a MEMS FTIR spectrometer in the mid-infrared for this purpose. The spectrometer works in the wavelength range of 1.6 μm - 4.9 μm with a resolution down to 33 cm-1. This covers the absorption spectrum of water vapour, BTEX, Aldehydes and CO2 around 2.65 μm, 3.27 μm, 3.6 μm and 4.3 μm, respectively. The spectra of Toluene with different concentrations are measured, using a multipass gas cell with a physical length of 50 cm and an optical path length of 20 m, showing excellent sensor linearity. The minimum concentration measured is 350 ppb limited by the interference of the side lobes of the strong absorption of water vapour, which can be overcome in the future by humidity compensation. The SNR is measured and found to be 5000:1, corresponding to a detection limit of about 90 ppb. The achieved results open the door for a compact and low-cost solution targeting air pollution monitoring.

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In this work, we propose a simple and time-saving method for the characterization of the ZnO-NWs. The method is based on the measurement of the spectral reflection of the ZnO-NWs in the UV-VIS-NIR ranges. Then the ZnO-NWs effective refractive index, and subsequently the density, and the length are obtained making use of the interference pattern contrast and periodicity in the reflection response versus wavelength. The extracted NWs length and density using the proposed method show good agreement with the SEM results. This characterization method opens the door for easy and cheap monitoring of the growth within microfluidic environment.

In this paper, different contents of molybdenum oxide/tungsten oxide (MoO3/WO3) composite nanostructures were synthesized by hydrothermal method. Field emission scanning electron microscopy images revealed that the morphologies of WO3 nanostructures were significantly influenced by the Mo amount. Furthermore, the introduction strategy of MoO3 into WO3 could effectively improve the gas sensing properties. Especially, the sensor based on the 4 mol% MoO3/WO3 composite nanostructures exhibited enhanced gas sensing performance, giving a low limit of detection (500 ppb). It shows high responses of 28.5 and 18.2–100 ppm ethanol and acetone at the operating temperature of 320 °C, which were about 2.3 and 1.7 times higher than those of the pure WO3, respectively. The enhanced sensing properties of MoO3/WO3 gas sensor can be attributed to the addition of MoO3, which has been discussed in relation to the gas sensing mechanism.

Two types of ZnO nanostructure have been fabricated to make a comparative study on their gas sensing performance: the conventional ZnO nanowire arrays were synthesized by hydrothermal method and the hierarchical ZnO nanowires/nanofibers nanostructures were prepared through a combination of the hydrothermal and electrospinning methods. Field emission scanning electron microscopy study showed a quiet homogeneous morphology both for both nanostructures. Three kinds of commonly used gases, such as ethanol, acetone and ammonia were chosen for ZnO nanostructure gas sensing property study. The UV-Visible spectroscopy measurements showed a higher detection sensitivity of ZnO NWs for ammonia compared to ethanol and acetone, and an enhanced sensing performance for the hierarchical nano- structure, which has a higher surface to volume ratio. On the other hand, the enhancement was more obviously in the case of ethanol sensing. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A study of the ZnO nanowire array photocatalysis performance has been carried out under UV radiation for the degradation of the toxic organic compounds such as methylene blue (MB), methyl orange (MO), and acid red 14 (AR14) dyes, which are commonly used in textile and pharmaceutics industries. UV–Vis spectrometry has been used to follow the dye degradation characterization. The degradation mechanism has been proposed for each dye, which will give a reasonable explanation about different degradation degree under same experimental conditions. MB and AR14 showed a better degradation performance than MO.

Lipid droplets have gained strong interest in recent years to comprehend how they function and coordinate with other parts of the cell. However, it remains challenging to study the regulation of lipid droplets in live preadipocytes using conventional microscopic techniques. In this paper, we study the effects of fatty acid stimulation and cell starvation on lipid droplets using optical diffraction tomography and Raman spectroscopy by measuring size, refractive index, volume, dry mass and degree of unsaturation. The increase of fatty acids causes an increase in the number and dry mass of lipid droplets. During starvation, the number of lipid droplets increases drastically, which are released to mitochondria to release energy. Studying lipid droplets under different chemical stimulations could help us understand the regulation of lipid droplets for metabolic disorders, such as obesity and diabetes. NRF (Natl Research Foundation, S’pore) Published version

A chiral metasurface, which obtains chirality through a subwavelength artificial structure, is essential for realizing asymmetric transmission in the application of enantioselective sensing, spin-dependent light emission, and other polarization control systems. Here, we studied a split Archimedean spiral metasurface, which can control the propagating wave from asymmetric transmission to symmetric transmission for linear polarized light. As a proof of concept, a dual-band asymmetric transmission is demonstrated in the GHz region using the coupling of the split spiral structures. The maximum asymmetric transmission parameter reaches 53%. By manipulating the height of the split spiral structures using microfluidic technology, a broadband asymmetric transmission is obtained with the bandwidth of 25.9%. Meanwhile, the asymmetric transmission can be controlled from 50% to 0%, enabling the propagation wave from asymmetric transmission to symmetric transmission. Furthermore, the asymmetric transmission is maintained when the metasurface is bent into different curvatures, promising high potential applications for optical isolation, one-way glass, and optical interconnects. NRF (Natl Research Foundation, S’pore) Published version

This paper reports a paper-based electrostatic kinetic energy harvester (e-KEH) implementing multilayered electret films based on electrospun nanofibrous material. It is the first time that a fully flexible electret-based e-KEH is reported. The proposed electret, PVDF-PTFD nanofibrous covered by Parylene C, has a faster stabilization of surface potential than a planar thin film of Parylene C, and a higher stability of charge storage. With a maximum force of 0.5 N and a 3-layer electret, the device capacitance increases from 25 pF to 100 pF during a pressing operation. Working with the optimal resistive load of 16 MΩ, the device pressed manually delivers a peak instantaneous power up to 45.6 μW and an average energy of 54 nJ/stroke, corresponding to a peak instantaneous power density of 7.3 μWcm-2 and an average energy density of 8.6 nJcm-2/stroke. Within 450 manual strokes, a 10-nF capacitor is charged up to 8.5 V by the prototype through a full-wave diode bridge. On a 1-μF capacitor, the energy delivery of 9.9 nJ/stroke has been obtained with a 10 Hz pressing movement excited by a vibrator with a maximum force of 0.5 N.

This paper reports fast and efficient chemical decontamination of water within a tree-branched centimeter-scale microfluidic reactor. The microreactor integrates Zinc oxide nanowires (ZnO NWs) in situ grown acting as an efficient photocatalytic nanomaterial layer. Direct growth of ZnO NWs within the microfluidic chamber brings this photocatalytic medium at the very close vicinity of the water flow path, hence minimizing the required interaction time to produce efficient purification performance. We demonstrate a degradation efficiency of 95% in

In this paper, electrospun piezoelectric PVDF-TrFE nanofibers were used for the fabrication of two types of flexible nanogenerator (NG) devices based on the direct piezoelectric effect, allowing the conversion of mechanical energy into electrical energy. The first one is composed of quite well aligned thin film nanofibers of about 35 μm and the second one is composed of random nanofibers of about 50 μm. The influence of the applied stress and strain rate on the output for both types of NG was studied. It is shown that the pulse peaks generated by NG increase with the applied mechanical strain frequency, the generated output is also proportional to the applied stress amplitude. The first NG loaded in bending mode can generate a maximum voltage of 270 mV. By connecting two devices in series/parallel, the voltage/current value could be multiplied by two. The second NG which was biased in compression mode using a shaker controlled by a force sensor, can generate a potential of about 7 V under 3.6 N applied force.

Terahertz metasurface absorption materials, which absorb terahertz wave through subwavelength artificial structures, play a key role in terahertz wave shielding and stealth technology, etc. However, most of the metasurface absorption materials in terahertz suffer from limited tuning range and narrow incident angle characteristics. Here, we demonstrate a liquid-metal-based metasurface through microfluidic technology, which functions as a terahertz absorption material with broadband tunability and wide-angle features. The proposed terahertz metasurface absorption material exhibits an experimental tuning range from 0.246 THz to 0.415 THz (the tuning range of central frequency reaches 51.1%), and the tuning range maintains at high level with wide-angle response up to 60°. NRF (Natl Research Foundation, S’pore) Published version

Metasurface absorbing material, which obtains near-unity electromagnetic absorption through subwavelength artificial structure, plays an important role in the area of stealth and shielding technology, biological imaging, etc. However, they usually suffer from narrow bandwidth and only work on planar surfaces. Here, for the first time, this study demonstrates a soft water-resonator-based metasurface, which functions as an active absorbing material across an ultrabroadband range of Ku, K, and Ka bands. Distinct from conventional metallic metasurface, the water-resonator-based metasurface absorbs the microwave by dielectric magnetic resonance and periodic grating effect, which has a perfect absorptivity of ≈99% and an absorption bandwidth (absorptivity higher than 90%) that covers 78.9% of the central frequency. Furthermore, near-unity absorption is maintained when the soft metasurface material is bent into different curvatures, promising high potential applications for antennas in reducing side lobe radiation, eliminating wall reflection in anechoic chambers, antiradar detection, and stealth. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) EDB (Economic Devt. Board, S’pore) Accepted version

A new study shows that a moving contact line could be more dangerous for a hydrophobic coating layer on a surface than lamellar flow. The tiny water drops generate a driving force that removes the coating molecules, collecting and depositing them on the substrate as the contact line recedes.

We demonstrate the hybridization of the plasmonic modes in directly coupled whispering gallery cavities fabricated on silver films and present the mode patterns and energy levels using cathodoluminescence spectroscopy. Although the energy of the most antisymmetrically coupled modes is higher than that of the corresponding symmetrically coupled ones, the contrary cases happen for small quantum number modes. We attribute the phenomenon to the different surface plasmon polariton paths between the symmetrically and antisymmetrically coupled modes. These results provide an understanding of the resonant properties in coupled plasmonic cavities, which have potential applications in nanophotonic devices.

Duo to their multifunctional properties, the piezoelectric ZnO nanowires are of great interest in many applications such as the energy harvesting. In this work, a numerical modelling based on the finite element method was performed to study the potential distribution inside the ZnO nanowire and the impact of the geometric parameters on the generated piezopotential both in bending and compression deformation. In this context, a nanowire discharge behavioural study was carried out in order to obtain an analytic expression which connects the piezopotential and nanowire geometrical parameters in the bending deformation. Furthermore, a heuristic algorithm Particle Swarm Optimization (PSO) has been used in order to improve the electromechanical performance of ZnO nanowire based nanogenerator. It is fount that under the same order of force applied on a nanowire, the compression mode results in a piezopotential 9 times larger than that in the bending deformation. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; Solar cells based on one-dimensional nanostructures have recently emerged as one of the most promising candidates to achieve high-efficiency solar energy conversion due to their reduced optical reflection, enhanced light absorption, and enhanced carrier collection. In nature, the rainforest, consisting of several stereo layers of vegetation, is the highest solar-energy-using ecosystem. Herein, we gave an imitation of the rainforest configuration in nanostructure-based solar cell design. Novel multilayer nanorainforest solar cells based on p-Si nanopillar array/n-CdS nanoparticles/n-ZnO nanowire array heterostructures were achieved via a highly accessible, reproducible and controllable fabrication process. By choosing materials with appropriate bandgaps, an efficient light absorption and enhanced light harvesting were achieved due to the wide range of the solar spectrum covered. Si nanopillar arrays were introduced as direct conduction pathways for photon-generated charges' efficient collection and transport. The unique strategy using PMMA as a void-filling material to obtain a continuous, uniform and low resistance front electrode has significantly improved the overall light conversion efficiency by two orders of magnitude. These results demonstrate that nanorainforest solar cells, along with waferscale, low-cost and easily controlled processing, open up substantial opportunities for nanostructure photovoltaic devices.

International audience; Wereport in this work the fabrication of ZnO nanowires and nano-pillars by electrodeposition using three types of micro- and nano-structured templates. First, ZnO nanowires were synthesized in a nanoporous polycarbonate template and their morphology and monocrystalline structure were checked by conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) techniques. The optical properties of the monocrystalline ZnO nanowires were also studied by measuring the photo-fluorescence spectra of the sample. Then, the micro-structured template was made by optical lithography and the electrodeposited ZnO micro-rods were analysed by scanning electron microscopy (SEM) imaging and X-ray diffraction (XRD). Finally, UV-nanoimprint templates were used for the fabrication of regular ZnO nano-pillar array. SEM images showed quite homogenous ZnO nano-pillar array on which the collective I–V characteristic has been carried out in order to determine the nature of the Au/ZnO contact. SEM images also revealed an effect of size increase of the ZnO microrods and nano-pillars with respect to that of the PMMA template. XRD patterns showed the hexagonal Wurtzite crystalline structure with cell parameters slightly larger than the literature ones.

We report a tunable metamaterial for wide-angle and broadband absorption by tuning the height of meta-water-capsule through microfluidics. An 80% absorption is achieved within 67% bandwidth when the incident angle varies from 0° to 45°.

A THz ultra-broadband absorber is realized using tunable metamaterials driven by a microfluidic system. In this work, a new technology is developed to precisely and continuously control the height of micro liquid pillars. Based on this technology, a proof-of-principle demonstration of tunable THz absorber is shown to have absorption frequency tuning from 0.245 THz to 0.415 THz with a tuning range of 51.5%. It creates a new paradigm for active metadevices based on tunable metamaterials with promising applications in detectors, sensors, imaging systems and stealth.

In this paper, a tunable THz lens array based on reconfigurable metamaterials is reported. The metamaterial consists with 40 × 40 liquid metal microdroplets, in which the shape is tuned under different air pressures. The droplets are formed by injecting the liquid mercury into a pre-designed microchannel network and arranged to focus incident THz wave. The focus spot size can be tuned with different droplet geometries. As a result, a tunable THz lens is constructed and the smallest spot size of 0.54λ is achieved. The tunable metamaterial lens is realized through simple fabrication processes, which has potential applications in flat lens and imaging system.

International audience; A new metal-organic chemical vapour deposition (MOCVD) technique was used for fabrication of epitaxial La-manganite thin films La1-xMnO3-delta with both lanthanum and oxygen deficiencies. Three kinds of substrates, i.e. SrTiO3 (001), LaAlO3 (012) and MgO (001), were chosen for comparison. Electrical and magnetic property measurements revealed that the amplitude of the colossal magnetoresistance (CMR) is dependent on the nature of the substrate whereas the electrical and magnetic transition temperatures are not sensitive to the substrate type. In order to correlate the physical properties of the films to their crystallization quality and their growth morphology, scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as high-resolution transmission elect:ron microscopy (HRTEM), were employed. Cross-sectional TEM investigations showed that the La1-xMnO3-delta is generally in good epitaxy with all three kinds of substrate. The orientation relationships between the film and substrate are rather well defined at the vicinity of the interface but not at the surface of the film. Deposition of La1-xMnO3-delta on SrTiO3 produces the best interfacial match since these two have the least lattice misfit. This is consistent with the best magnetoresistive property measured for the investigated systems. (C) 2001 Elsevier Science B.V. All rights reserved.

Motivated by the need for obtaining low reflectivity silicon surfaces, we report on (sub-) micro-texturing of silicon using a high throughput fabrication process involving SF6/O2 reactive ion etching at cryogenic temperatures, leading to Black Silicon (BS). The corresponding high aspect ratio conical spikes of the microstructured surface give rise to multiple reflections and hence, enhanced absorption under electromagnetic radiation. Aiming a better understanding of this mechanism, we performed a systematic study by varying several plasma process parameters: O2/SF6 gas flow rate ratio, silicon temperature, bias voltage, and etching time. We determined the process window which leads to BS formation and we studied the influence of the process parameters on the surface morphology of the obtained BS samples, through analysis of scanning electron microscopy images. The measured optical reflectance of BS is in the order of 1% in the visible and near infrared ranges (400–950 nm). We noticed that the lowest reflectance is obtained close to the threshold parameters of BS formation. Absorptance spectral response of BS is measured from 1.3 to 17 μm, and we observed a great enhancement of absorptance up to about 75% compared to flat silicon. We also obtained through these experiments, a clear evidence of a correlation between the excellent optical properties and the aspect ratio of the BS conical microstructures in the measured wavelength ranges.

The formation mechanism of experimentally observed epitaxial morphologies on (000 ± 1) polar surfaces of ZnO microwire was studied by density functional theory simulation of atomic step models on both polar surfaces. In situ observations by environmental scanning electron microscopy were employed to control and detect the morphology evolutions during the epitaxial growth process. Edge formation energies for representative step models were calculated as a function of Zn chemical potential. The energetically favorable step structures were determined under different conditions, which could be related to certain surface morphologies regarding the aspects of crystallography and growth kinetics. Our experiments can be well explained by theoretical simulations.

Lattice strain is a useful and economic way to tune the device performance and is commonly present in nanostructures. Here, we investigated for the first time the exciton spectra evolution in bent ZnO microwires along the radial direction via high spatial/energy resolution cathodeluminescence spectroscopy at 5.5 K. Our experiments show that the exciton peak splits into multi fine peaks towards the compressive part while retains one peak in the tensile part and the emission peak displays a continuous blue-shift from tensile to compressive edges. In combination with first-principles calculations, we show that the observed NBE emission splitting is due to the valence band splitting and the absence of peak splitting in the tensile part maybe due to the highly localized holes in the A band and the carrier density distribution across the microwire. Our studies may pave the way to design nanophotonic and electronic devices using bent ZnO nanowires.

7pp; International audience; ZnO thin films were electrodeposited in aqueous solution on gilded p-type Si wafer substrates. Thermal treatments were carried out on different films in Ar atmosphere at different temperatures, between 200 and 600 ◦C. Surface morphology studies using scanning electron microscopy and atomic force microscopy show a smooth surface for an annealing temperature of 400 ◦C with a roughness mean square value of about 15 nm and a precipitation of ZnO microcrystals on the deposit surface at 600 ◦C. X-ray diffraction experiments reveal a decrease in the c-parameter value from 5.223 to 5.206Å after treatment at 600 ◦C, due to the removal of hydrogen from the film. Raman spectroscopy analyses show an improvement in the crystal quality of the film and a decrease in the compressive stress inside the deposit. Photoluminescence observations reveal an important change in the UV emission band after annealing at 200 ◦C. A visible region emission band at 580 nm, ascribed to interstitial oxygen, is observed for the as-grown deposit and decreases as the annealing temperature increases. An emission band appears near 525 nm for samples annealed at 400 and 600 ◦C; this band is ascribed to oxygen vacancies created during annealing treatment. This result is in agreement with the energy dispersive x-ray spectroscopy experiments which revealed loss of oxygen.