Laetitia Rapenne

Morphological changes and electrochemical response of mixed nickel manganese oxides as charge storage electrodes

Nickel manganese (Ni–Mn) oxide films were prepared by potentiostatic electrodeposition over stainless steel and post-thermal annealing at 250 °C. Morphological and structural changes were observed depending on the Ni to Mn ratio in the electrolyte. Single-phase Ni1−xMnxO oxide was formed at low Mn content, whereas two phases composed of Ni1−xMnxO and NixMn3−xO4 were formed at high Mn content. Electrochemical studies revealed an increase of the specific capacitance of the mixed Ni–Mn oxide electrodes compared to the single metal oxides, thus pin-pointing a synergistic effect. The Ni–Mn oxide film displayed a specific capacitance of about 300 F g−1 in a potential window of 0.5 V at an applied current density of 1 A g−1. The rate capability of the oxides was 58% when the applied current was increased from 1 A g−1 to 10 A g−1. The Ni–Mn oxide film exhibits excellent cycling stability with 100% capacitance retention after 1500 cycles.

Structural evolution, magnetic properties and electrochemical response of MnCo2O4 nanosheet films

MnCo2O4 spinel oxide nanosheets were prepared via electrodeposition and post thermal annealing on stainless steel substrates. The structural transformation of an electrodeposited hydroxide phase into a spinel phase was achieved by thermal annealing at different temperatures (250 °C, 350 °C, 450 °C and 650 °C). The surface morphology of the films revealed the presence of a nanosheet percolation network that was converted into nanoplatelets after annealing at 650 °C. The nanosheets are composed of nanocrystals and the crystal size of the MnCo2O4 spinel oxide increased from 10 nm after 250 °C annealing to 100 nm after 650 °C annealing, in which a twinning was observed. The magnetic transition temperature also increased from 101 K to 176 K for the films annealed at 250 °C and 650 °C, respectively. The spinel films displayed specific capacitance values above 400 Fg−1 at 1 Ag−1, making these spinel oxides promising pseudocapacitive materials.

High-resolution transmission electron microscopy observations of La2Zr2O7 thin layers on LaAlO3 obtained by chemical methods

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. HRTEM observations of La2Zr2O7 thin layers on LaAlO3 obtained by chemical methods L. Rapenne, Carmen Jiménez, Tristan Caroff, C. Millon, Stéphanie Morlens, Pascale Bayle-Guillemaud, François Weiss

Characterization of Bulk 3C-SiC Single Crystals Grown on 4H-SiC by the CF-PVT Method

Because of the formation of DPB (Double Positioning Boundary) when starting from a hexagonal <0001> seed, DPB-free 3C-SiC single crystals have never been reported up to now. In a recent work we showed that, using adapted nucleation conditions, one could grow thick 3C-SiC single crystal almost free of DPB [1]. In this work we present the results of a multi-scale investigation of such crystals. Using birefringence microscopy, EBSD and HR-TEM, we find evidence of a continuous improvement of the crystal quality with increasing thickness in the most defected area, at the sample periphery. On the contrary, in the large DPB-free area, the SF density remains rather constant from the interface to the surface. The LTPL spectra collected at 5K on the upper part of samples present a nice resolution of multiple bound exciton features (up to m=5) which clearly shows the high (electronic) quality of our 3C-SiC material.

One-step process to form a nickel-based/carbon nanofoam composite supercapacitor electrode using Na2SO4 as an eco-friendly electrolyte

In this work, NiOx is anodically electrodeposited onto carbon nanofoam (CNF) to form a composite electrode devoted to supercapacitor applications. The use of NiSO4 as precursor in electrodeposition results in the formation of NiO and NiOOH species, as confirmed by XPS analysis, by means of a one-step anodic process. The presence of both NiO and NiOOH suggests the existence of pseudocapacitance, as observed in MnO2 and RuO2 materials. By employing Na2SO4, an eco-friendly electrolyte, the resulting composite delivers a specific capacitance of 150 F g−1 at 1 A g−1 considering the total mass of the electrode (deposit plus substrate). In addition, this composite electrode can operate in a very broad potential window, as high as 2.2 V, suggesting its application in high energy density electrochemical supercapacitors.

Tuning the properties of F:SnO2 (FTO) nanocomposites with S:TiO2 nanoparticles – promising hazy transparent electrodes for photovoltaics applications

The appropriate choice of nanoparticles is proved to be essential in tuning the properties of F:SnO2 (FTO) nanocomposites. With the use of more conductive sulphur-doped TiO2 (S:TiO2) nanoparticles, the sheet resistance of S:TiO2–FTO nanocomposites is successfully reduced down to 38% as compared to the standard flat FTO (11.7 Ω sq−1), while the haze factor of the S:TiO2–FTO nanocomposites can be varied from almost zero (reference flat FTO) up to 60%; moreover the majority of 〈110〉 oriented S:TiO2 nanoparticles leads to a strong (110) texture in the resulting S:TiO2–FTO nanocomposites by local epitaxy. Careful morphology analyses and angle-resolved measurements reveal that the haze factor is proportional to the total surface coverage of the S:TiO2 nanoparticle agglomerates, while the feature size of the agglomerates determines the angular distribution of the scattered light – this is confirmed by an angle-resolved Mueller matrix polarimeter which allows obtaining the optical microscopic and angle-resolved images of the exact same textured region. Our work establishes the guidelines to fabricate FTO and other transparent conductive oxide (TCO) nanocomposites as promising electrodes in solar cells with tunable structural, electrical, and optical properties.

Improvement of the physical properties of ZnO/CdTe core-shell nanowire arrays by CdCl2 heat treatment for solar cells

CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl2 heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl2 heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl2 heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl2 heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells.

Reaction of Ni film with In0.53Ga0.47As: Phase formation and texture

The solid-state reaction between Ni and In0.53Ga0.47As on an InP substrate was studied by X-ray diffraction (XRD) and scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy techniques. Due to the monocrystalline structural aspect of the so-formed intermetallic, it was necessary to measure by XRD a full 3D reciprocal space mapping in order to have a complete overlook over the crystalline structure and texture of the intermetallic. The formation of the intermetallic was studied upon several different Rapid Thermal Annealings on the as-deposited samples. Pole figures analysis shows that the intermetallic features a hexagonal structure (P63/mmc) with an NiAs-type (B8) structure. Although only one hexagonal structure is highlighted, the intermetallic exhibits two different domains characterized by different azimuthal orientations, axiotaxial relationship, and lattice parameters. The intermetallic phases seem to present a rather wide range of stoichiometry according to annealing temperat...

Growth and structural characterization of YMnO3 thin films grown by pulsed liquid injection MOCVD on Si and SrTiO3 substrates

Optimum parameters for the growing of YMnO3 films by pulsed liquid injection metalorganic chemical vapor deposition have been studied. Si substrates were used for the optimization of the deposition process. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that polycrystalline single phase YMnO3 films can be obtained for an optimal ratio of Y and Mn on the injected solution and either amorphous, metastable orthorhombic, and/or hexagonal YMnO3 phases can be obtained depending on the deposition temperature and precursors ratio. In a second stage, YMnO3 films were grown on SrTiO3 substrates. Pure epitaxial orthorhombic YMnO3 phase was confirmed by XRD. The films microstructure, characterized by scanning electron microscopy and TEM, shows a columnar growth. Each columnar grain grows epitaxially with three possible orientations.

Identification of extended defect and interface related luminescence lines in polycrystalline ZnO thin films grown by sol–gel process

The luminescence lines related to extended defects and interfaces in polycrystalline ZnO thin films grown by sol–gel process are deeply investigated by combining temperature-dependent photoluminescence and cathodoluminescence imaging with high-resolution transmission electron microscopy. A typical broad emission band is shown in the range of 3.316 to 3.333 eV and mainly consists of two distinct contributions. At high energy, a 3.333 eV line is associated with interfaces (i.e., free surfaces and grain boundaries) and predominates for small ZnO nanoparticles owing to their high density. The intensity ratio of the excitonic to interface-related transitions is low in this first configuration and the 3.333 eV line is characterized by an activation energy of 12.0 ± 1.2 meV and a Huang-Rhys factor of 0.54 ± 0.05 at 12 K. At low energy, a 3.316 eV line is attributed to basal plane stacking faults that are mostly of I1-type and prevail for large ZnO nanoparticles. The 3.316 eV line is characterized by an activation energy of 6.7 ± 0.8 meV and a Huang Rhys constant of 0.87 ± 0.03 at 12 K. Basal plane stacking faults are most likely formed as the coalescence process proceeds with the decomposition and crystallization processes during annealing. As shown by low-temperature monochromatic cathodoluminescence imaging, the luminescence corresponding to the 3.316 eV line is, in this second configuration, limited to some specific area (i.e., large nanoparticles), and the relative intensity ratio of the excitonic to interface-related transitions is increased due to the smaller free surface area and density of grain boundaries.