Bruno Masenelli

ZnO dense nanowire array on a film structure in a single crystal domain texture for optical and photoelectrochemical applications

A single crystal domain texture quality (a unique in-plane and out-of-plane crystalline orientation over a large area) ZnO nanostructure of a dense nanowire array on a thick film has been homogeneously synthesized on a-plane sapphire substrates over large areas through a one-step chemical vapor deposition (CVD) process. The growth mechanism is clarified: a single crystal [02(-)1] oriented ZnAl(2)O(4) buffer layer was formed at the ZnO film and the a-plane sapphire substrate interface via a diffusion reaction process during the CVD process, providing improved epitaxial conditions that enable the synthesis of the high crystalline quality ZnO nanowire array on a film structure. The high optoelectronic quality of the ZnO nanowire array on a film sample is evidenced by the free exitonic emissions in the low-temperature photoluminescence spectroscopy. A carrier density of ~10(17) cm(-3) with an n-type conductivity of the ZnO nanowire array on a film sample is obtained by electrochemical impedance analysis. Finally, the ZnO nanowire array on a film sample is demonstrated to be an ideal template for a further synthesis of a single crystal quality ZnO-ZnGa(2)O(4) core-shell nanowire array on a film structure. The fabricated ZnO-ZnGa(2)O(4) sample revealed an enhanced anticorrosive ability and photoelectrochemical performance when used as a photoanode in a photoelectrochemical water splitting application.

Structural transition in rare earth oxide clusters

Size effects, such as structure transition, have been reported in small clusters of alkali halide compounds. We extend the study to rare earth sesquioxide (Gd(2)O(3)) clusters which are as ionic as the alkali halide compounds, but have a more complicated structure. In a clean and controlled environment (ultra high vacuum), such particles are well crystallized, facetted and tend to adopt a rhombic dodecahedron shape. This indicates the major role of highly ionic bonds in preserving the crystal lattice even at small sizes (a few lattice parameter). Based on both cathodo-luminescence and transmission electron microscopy, we report the existence of a structural transition from bcc to monoclinic at small sizes.

YAG:Ce nanoparticle lightsources

We investigate the luminescence properties of 10 nm yttrium aluminum garnet (YAG) nanoparticles doped with Ce ions at 0.2%, 4% and 13% that are designed as active probes for scanning near-field optical microscopy. They are produced by a physical method without any subsequent treatment, which is imposed by the desired application. The structural analysis reveals the amorphous nature of the particles, which we relate to some compositional defects as indicated by the elemental analysis. The optimum emission is obtained with a doping level of 4%. The emission of the YAG nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder whereas the 13% doped particles hardly exhibit any luminescence. In the latter case, the presence of Ce(4+) ions is confirmed, indicating that the Ce concentration is too high to be incorporated efficiently in YAG nanoparticles in the trivalent state. By a unique procedure combining cathodoluminescence and Rutherford backscattering spectrometry, we demonstrate that the enhancement of the particle luminescence yield is not proportional to the doping concentration, the emission enhancement being larger than the Ce concentration increase. Time-resolved photoluminescence reveals the presence of quenching centres likely related to the crystalline disorder as well as the presence of two distinct Ce ion populations. Eventually, nano-cathodoluminescence indicates that the emission and therefore the distribution of the doping Ce ions and of the defects are homogeneous.We investigate the luminescence properties of 10 nm YAG nanoparticles doped with Ce ions at 0.2%, 4% and 13% that are designed as active probes for Scanning Near field Optical Microscopy. They are produced by a physical method without any subsequent treatment, which is imposed by the desired application. The structural analysis reveals the amorphous nature of the particles, which we relate to some compositional defect as indicated by the elemental analysis. The optimum emission is obtained with a doping level of 4%. The emission of the YAG nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder whereas the 13% doped particles hardly exhibit any luminescence. In the latter case, the presence of Ce4+ ions is confirmed, indicating that the Ce concentration is too high to be incorporated efficiently in YAG nanoparticles in the trivalent state. By a unique procedure combining cathodoluminescence and Rutherford backscattering spectrometry, we demonstrate that the enhancement of the particles luminescence yield is not proportional to the doping concentration, the emission enhancement being larger than the Ce concentration increase. Time-resolved photoluminescence reveals the presence of quenching centres likely related to the crystalline disorder as well as the presence of two distinct Ce ions populations. Eventually, nano-cathodoluminescence indicates that the emission and therefore the distribution of the doping Ce ions and of the defects are homogeneous.

ZnO nanoparticles as a luminescent down-shifting layer for photosensitive devices

The optical properties of ZnO nanoparticles (NPs) fabricated by three different methods were studied by the UV-excited continuous wave photoluminescence in order to estimate their down-shifting (DS) efficiency. Such a luminescent layer modifies the incident solar radiation via emitting wavelengths better matching the spectral response of the underlying photosensitive device (photodiode), thereby increasing its efficiency. Some of the studied ZnO NPs were subsequently deposited on the front side of commercial silicon photodiodes and the external quantum efficiency (EQE) characteristics of the final devices were measured. Through comparison of the photodiodes EQE characteristics before and after the deposition of the ZnO NPs layer, it was concluded that for the photodiode with a low UV sensitivity (about 8%), the ZnO luminescent layer produces a down-shifting effect and the EQE in the UV and blue range improves by 16.6%, while for the photodiodes with a higher initial UV sensitivity (about 50%), the EQE in this range decreases with the ZnO layer thickness, due to the effects competing with DS, like the diminution of the ZnO layer transmittance and an increasing diffusion.

EFFICIENT ULTRAVIOLET LIGHT FREQUENCY DOWN-SHIFTING BY A THIN FILM OF ZnO NANOPARTICLES

The maximal efficiency of a single junction solar cell (SC) is defined as the Shockley–Queisser limit, which determines the maximal output power which can be furnished by a SC as a function of the bandgap of the semiconductor constituting the cell. The short wavelength spectral response of a SC can be improved if a luminescent down-converting layer is added to the SC structure. We propose the use of a layer containing ZnO nanoparticles (NPs) as a luminescent down-shifter. ZnO is able to absorb efficiently the ultraviolet light (λ < 400 nm), where the SC spectral response is low and to re-emit lower energy photons (longer wavelength photons) for which the SC spectral response is enhanced, thus increasing the total photocurrent. The stoichiometry and crystallinity of ZnO NPs can be controlled and adjusted to obtain the highest visible photoluminescent emission, indicator of an efficient down-shifting. The ratio between the ZnO UV absorption and visible emission is also estimated and from these results we expect the increase of the SC efficiency using ZnO NPs as a down-shifting layer placed on the front side of the SC.

Thermodynamics of Nanoparticles: Experimental Protocol Based on a Comprehensive Ginzburg-Landau Interpretation

The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. On the basis of experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Ginzburg-Landau theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on the discrepancies in the literature as it identifies several possible parameters that should be taken into account to properly describe the transformations. For the sake of clarity and standardization, we propose an experimental protocol that must be followed during high-pressure investigations of nanoparticles in order to obtain coherent, reliable data that can be used by the scientific community.

Extended-Defect-Related Photoluminescence Line at 3.33 eV in Nanostructured ZnO Thin Films

The 3.33 eV photoluminescence line is investigated in ZnO thin films deposited by dip coating. These films are oriented along the c-axis and exhibit basal-plane stacking faults and random grain boundaries. It is found that the relative intensity of the free exciton peak to the 3.33 eV line decreases as the nanoparticle size is reduced and that the corresponding Huang–Rhys factor is about 0.5. This reveals that excitons bound to extended defects at grain boundaries are involved. Also, post growth annealing strongly affects the photoluminescence spectra. In particular, the 3.31 eV line coming from stacking faults is enhanced at high annealing temperature.

Pressure-Dependent Photoluminescence Study of Wurtzite InP Nanowires

The elastic properties of InP nanowires are investigated by photoluminescence measurements under hydrostatic pressure at room temperature and experimentally deduced values of the linear pressure coefficients are obtained. The pressure-induced energy shift of the A and B transitions yields a linear pressure coefficient of αA = 88.2 ± 0.5 meV/GPa and αB = 89.3 ± 0.5 meV/GPa with a small sublinear term of βA = βB = -2.7 ± 0.2 meV/GPa(2). Effective hydrostatic deformation potentials of -6.12 ± 0.04 and -6.2 ± 0.04 eV are derived from the results for the A and B transitions, respectively. A decrease of the integrated intensity is observed above 0.5 GPa and is interpreted as a carrier transfer from the first to the second conduction band of the wurtzite InP.

Intense visible emission from ZnO/PAAX (X = H or Na) nanocomposite synthesized via a simple and scalable sol-gel method.

Intense visible nano-emitters are key objects for many technologies such as single photon source, bio-labels or energy convertors. Chalcogenide nanocrystals have ruled this domain for several decades. However, there is a demand for cheaper and less toxic materials. In this scheme, ZnO nanoparticles have appeared as potential candidates. At the nanoscale, they exhibit crystalline defects which can generate intense visible emission. However, even though photoluminescence quantum yields as high as 60% have been reported, it still remains to get quantum yield of that order of magnitude which remains stable over a long period. In this purpose, we present hybrid ZnO/polyacrylic acid (PAAH) nanocomposites, obtained from the hydrolysis of diethylzinc in presence of PAAH, exhibiting quantum yield systematically larger than 20%. By optimizing the nature and properties of the polymeric acid, the quantum yield is increased up to 70% and remains stable over months. This enhancement is explained by a model based on the hybrid type II heterostructure formed by ZnO/PAAH. The addition of PAAX (X = H or Na) during the hydrolysis of ZnEt2 represents a cost effective method to synthesize scalable amounts of highly luminescent ZnO/PAAX nanocomposites.

Zinc oxide nanoparticles as luminescent down-shifting layer for solar cells

The manufacturing of photovoltaic (PV) devices has developed considerably in recent decades, spurred by continuous growth in the demand for renewable energy sources. About 90% of currently fabricated solar arrays are made of crystalline silicon (Si). Considerable research effort has been applied to increasing the efficiency of Si PV devices. However, the spectral response of a Si PV device does not match the solar emission spectrum owing to the limited absorption of the Si constituting the PV solar cell active layer (see Figure 1). A single-junction Si solar cell is transparent to photons with energies below the bandgap energy, and additional sunlight is lost because of thermalization induced by higher-energy (UV) photons. This is the origin of the largest proportion of losses in commercially available Si solar cells. One way to increase solar cell efficiency is to transform the solar emission spectrum so that it overlaps better with the Si absorption spectrum. Up-conversion2 and down-conversion techniques3 are among those used to convert the incident solar light into a spectrum that matches the absorption of the active layer in solar cells. Up-conversion permits the conversion of IR light to visible light by simultaneous absorption of two photons. This is a nonlinear process, so the probability of such a transition is quite low. In down-conversion, one photon with higher energy (UV or blue) can be converted into two identical photons of equal energy, two times lower than the initial one. This concept is interesting but limited, as it requires the existence of an intermediate energy level exactly in the middle of the bandgap of the downconverting material. Our research focuses on the down-shifting technique.4, 5 Like down-conversion, down-shifting also permits the conversion Figure 1. Global solar spectrum at air mass 1.5 showing the fraction that is currently absorbed by a thick silicon (Si) device and the additional regions of the spectrum that can contribute toward upand down-conversion (UC and DC, respectively). : Wavelength. (Reprinted with permission from Elsevier.1)