O. Chaix-Pluchery

A phase transition close to room temperature in BiFeO3 thin films

BiFeO3 (BFO) multiferroic oxide has a complex phase diagram that can be mapped by using appropriately substrate-induced strain in epitaxial films. By using Raman spectroscopy, we conclusively show that films of the so-called supertetragonal T-BFO phase, stabilized under compressive strain, display a reversible temperature-induced phase transition at about 100 °C, and thus close to room temperature.

Physical Properties of Annealed ZnO Nanowire/CuSCN Heterojunctions for Self-Powered UV Photodetectors

The low-cost fabrication of ZnO nanowire/CuSCN heterojunctions is demonstrated by combining chemical bath deposition with impregnation techniques. The ZnO nanowire arrays are completely filled by the CuSCN layer from their bottoms to their tops. The CuSCN layer is formed of columnar grains that are strongly oriented along the [003] direction owing to the polymeric form of the β-rhombohedral crystalline phase. Importantly, an annealing step is found essential in a fairly narrow range of low temperatures, not only for outgassing the solvent from the CuSCN layer, but also for reducing the density of interfacial defects. The resulting electrical properties of annealed ZnO nanowire/CuSCN heterojunctions are strongly improved: a maximum rectification ratio of 2644 at ±2 V is achieved following annealing at 150 °C under air atmosphere, which is related to a strong decrease in the reverse current density. Interestingly, the corresponding self-powered UV photodetectors exhibit a responsivity of 0.02 A/W at zero bias and at 370 nm with a UV-to-visible (370-500 nm) rejection ratio of 100 under an irradiance of 100 mW/cm(2). The UV selectivity at 370 nm can also be readily modulated by tuning the length of ZnO nanowires. Eventually, a significant photovoltaic effect is revealed for this type of heterojunctions, leading to an open circuit voltage of 37 mV and a short circuit current density of 51 μA/cm(2), which may be useful for the self-powering of the complete device. These findings show the underlying physical mechanisms at work in ZnO nanowire/CuSCN heterojunctions and reveal their high potential as self-powered UV photodetectors.

Investigation of thickness-dependent stress in PbTiO3 thin films

X-ray diffraction (XRD) and Raman spectroscopy were used to investigate stress dependence on thickness in PbTiO3 (PTO) films grown by pulsed liquid injection metalorganic chemical vapor deposition on a LaAlO3 (001) substrate (LAO). Films on sapphire substrate (R plane) were used as the polycrystalline film reference. Epitaxial PTO films with a dominant c domain structure are grown on LAO substrate, whereas the films on sapphire are polycrystalline. A detailed investigation of the PTO/LAO film microstructure by XRD gives evidence of PTO twinning. Both techniques reveal that PTO films are under tensile in-plane stresses. The study of the film thickness dependence of microstrains, grain size, volume fraction of a domains, as well as surface morphology of PTO/LAO films indicates that these parameters are clearly correlated. A change in the relaxation mechanism between 65 and 125 nm of film thickness has been evidenced. A c parameter gradient occurs throughout the film depth; it originates in stress relaxation...

Raman scattering from Ti3SiC2 single crystals

The lack of single crystalline Ti3SiC2 samples is currently limiting the accurate measurement of its basic properties as its layered crystalline structure presents a very strong anisotropy. In this letter, we report the growth of pure Ti3SiC2 single crystals after a careful study of the Ti3SiC2 liquidus surface extent through thermodynamical calculations. From a Raman scattering study on those single crystals, an unambiguous assignment of most of the phonon modes has been established, giving an answer to the discrepancies existing in the literature.

Anisotropy of thermal expansion in YAlO3 and NdGaO3

YAlO3 and NdGaO3 thermal expansion coefficients were measured using in situ powder x-ray diffraction in the temperature range of 28–650 °C. They exhibit a clear anisotropy: The expansion, quite similar along the [100] and [001] directions, is much lower along the [010] direction. The formation of cracks observed in YBa2Cu3O7−δ thin films deposited on YAlO3 and NdGaO3 substrates is likely related to the anisotropy. Stress value calculations have been performed in both systems. They indicate that the intrinsic components are specifically high in YAlO3.

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.

Silicon carbide nanotubes growth: an original approach

Because of their unique properties, silicon carbide nanotubes (SiC-NTs) have aroused particular research interest. In this letter, a new approach to fabricate SiC-NTs via the carburization of Si-NWs is presented. By controlling the pressure during the carburization process, out-diffusion of Si through the SiC layer can be monitored. Finally, 3C-SiC-NTs with faceted {2?0?0} sidewall planes are obtained with an excellent crystalline quality. The external diameter is about 300?nm (nearly the same than the etched Si-NWs used) and the thickness of the sidewalls is about 40?100?nm. The crystalline quality as well as the good reproducibility of the process may lead to various applications in physics, chemistry, energy storage and biology.

Thermal annealing of amorphous Ti-Si-O thin films

Ti-Si-O thin films were deposited using an aerosol chemical vapor deposition process at atmospheric pressure. The film structure and microstructure were analysed using several techniques before and after thermal annealing. Diffraction results indicate that the films remain X-ray amorphous after annealing whereas Fourier transform infrared spectroscopy gives evidence of a phase segregation between amorphous SiO2 and well crystallized anatase TiO2. Crystallization of anatase TiO2 is also clearly shown in the Raman spectra. Transmission electron microscopy analysis indicates that anatase TiO2 nanograins are embedded in a SiO2 matrix in an alternated SiO2/TiO2 multilayer structure.

Raman spectroscopy and X-ray diffraction studies of stress effects in PbTiO/sub 3/ thin films

The evolution of domain structure and in-plane and out-of-plane lattice parameters in PbTiO3 (PTO) films on SrTiO3 (STO), LaAlO3 (LAO) and MgO substrates was studied as a function of film thickness and temperature by X-ray diffraction (XRD). Microstrains and average grain sizes were determined in several crystallographic directions for films of different thicknesses using a Williamson-Hall and Langford analysis. Stress relaxation with film thickness was observed in different domains by Raman spectroscopy and greatly influences lattice parameters. Specific contributions of a-and c-domains on Raman spectra were analyzed and correlated to domains fraction. The high temperature structural phase transition was followed by Raman spectroscopy, showing that the transition depends on the nature of stress.

Raman Study of CeO2 Texture as a Buffer Layer in the CeO2/La2Zr2O7/Ni Architecture for Coated Conductors

The CeO2/La2Zr2O7/Ni piled-up structure is a very promising architecture for YBa2Cu3O7 (YBCO) coated conductors. We have grown YBCO/CeO2/LZO/Ni epitaxial structures by metalorganic decomposition (MOD) and metalorganic chemical vapor deposition (MOCVD) methods. The crystallographic quality of the CeO2 layer is not well determined by conventional X-ray diffraction (XRD) due to the superposition of LZO and CeO2 reflections. An alternative simple Raman spectroscopy analysis of the crystalline quality of the CeO2 films is proposed. The F2g Raman mode of CeO2 can be quantified either by using two polarization configurations (crossed or parallel) or at two different rotation angles around the normal axis (0° and 45°) to obtain information about the sample texture. The sample texture can be determined via a quality factor (referred to as the Raman intensity ratio, RIR) consisting of calculating the ratio of the integrated intensity of the CeO2 F2g mode at 0° and 45° in parallel polarization. This factor correlates with superconducting performance and the technique can be used as an on-line nondestructive characterization method.