Jean-René Duclere

Optical properties of an epitaxial Na0.5Bi0.5TiO3 thin film grown by laser ablation: Experimental approach and density functional theory calculations

This study reports on the determination of the optical constants of a lead-free ferroelectric material, Na0.5Bi0.5TiO3. The optical transmission measurements were carried out in the 300–3000 nm wavelength range, on a (100)-oriented Na0.5Bi0.5TiO3 thin film, epitaxially grown by laser ablation on a (100)MgO single crystal substrate. Applying the “method of the envelopes,” developed by Manifacier et al. [J. Phys. E 9, 1002 (1976)] and by Swanepoel [J. Phys. E 16, 1214 (1983)], the analysis of the observed interference fringes allowed extracting some optical data for Na0.5Bi0.5TiO3, namely the linear refractive and extinction indices n and k, the absorption coefficient α, and as well the nature (direct or indirect transition) and value of the optical band gap. It was found that n∞=2.321 and the transmission data suggested a 3.30 eV indirect transition. Our experimental results are thus in opposition to the few data reported so far for Na0.5Bi0.5TiO3, where a direct transition was evoked. Therefore, we have c...

Electrical properties of (110) epitaxial lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition: Macroscopic and nanoscale data

We report the electrical properties, measured both at the macroscopic and nanometric scales, of epitaxial (110)-Na0.5Bi0.5TiO3 (NBT) thin films grown on (110)Pt/(110)SrTiO3 by pulsed laser deposition (PLD). The influence of the A-site composition (Na and/or Bi excess) on both the structural/microstructural characteristics and the electrical properties is discussed. Whatever the composition of the NBT target, the final layers are systematically epitaxially grown, with NBT crystallites mainly (110)-oriented, and as well (100)-oriented for some minor proportion. Atomic force microscopy (AFM) images reveal the coexistence of two kinds of grains presenting different shapes: namely flat and elongated grains, corresponding to (100)- and (110)-oriented NBT crystallites, respectively. The macroscopic ferroelectric properties were measured at room temperature. A rather well-defined shape of the hysteresis loops was obtained: the incorporation of a Bi excess in the target clearly improves the saturation of the loops...

Growth of ZnO nanostructures on Au-coated Si: Influence of growth temperature on growth mechanism and morphology

ZnO nanostructures were grown on Au-catalyzed Si silicon substrates using vapor phase transport at growth temperatures from 800 to 1150 °C. The sample location ensured a low Zn vapor supersaturation during growth. Nanostructures grown at 800 and 850 °C showed a faceted rodlike morphology with mainly one-dimensional (1D) growth along the nanorod axis. Samples grown at intermediate temperatures (900, 950, and 1050 °C) in all cases showed significant three dimensional (3D) growth at the base of 1D nanostructures. At higher growth temperatures (1100 and 1150 °C) 3D growth tended to dominate resulting in the formation of a porous, nanostructured morphology. In all cases growth was seen only on the Au-coated region. Our results show that the majority of the nanostructures grow via a vapor-solid mechanism at low growth temperatures with no evidence of Au nanoparticles at their tip, in sharp contrast to the morphology expected for the vapor-liquid-solid (VLS) process often reported as the growth mechanism on Au-c...

A new oxyfluorotellurate(IV), InTe2O5F.

A new oxyfluorotellurate(IV), indium fluoridopentaoxidotellurate(IV), InTe2O5F, has been synthesized by solid-state reaction and structurally characterized. The crystal structure consists of a three-dimensional framework formed by InO4F2 octahedra and Te2O5 units. The InO4F2 octahedra are linked through the F atoms, which lie on twofold axes, giving rise to helical chains. These helical chains are connected via the Te2O5 units. The helical chains of indium octahedra surround cavities, into which the lone pairs of electrons of the Te atoms point.

Thermal conductivity of SrBi2Nb2O9 ferroelectric thin films

Ferroelectric Aurivillius SrBi2Nb2O9 films of different thicknesses were prepared on Si(100) substrates by pulsed laser deposition. The films are polycrystalline and exhibit an increased grain size with film thickness. The cross-plane thermal conductivity of such SrBi2Nb2O9 films was measured in the 100–300K temperature range by the 3ω method and it was observed that the thermal properties of this family of ferroelectric materials show a glasslike behavior. The measurements performed on a polycrystalline bulk SrBi2Nb2O9 sample, in the 10–300K temperature range, led to identical conclusions. The origin of this unexpected glasslike behavior is discussed.

Epitaxial growth and properties of lead-free ferroelectric Na 0.5 Bi 0.5 TiO 3 thin films grown by pulsed laser deposition on various single crystal substrates

The epitaxial growth of lead-free ferroelectric Na_0.5Bi_0.5TiO₃ (NBT) thin films on various single crystal substrates was successfully achieved, using the pulsed laser deposition technique (PLD). The present work is divided in two parts, focused on: (i) the growth of NBT layers on c- and r-sapphire (Al₂O₃) substrates, with and without introducing a CeO₂ buffer layer, and (ii) the growth of NBT layers on bare (001)SrTiO₃ substrates, with and without introducing a LaNiO₃ layer, that could be used as a bottom electrode. In the first part, it was shown that the introduction of a CeO₂ buffer layer completely modifies the out-of-plane growth orientation of the NBT films, as well as their microstructure. Indeed, (001)NBT films epitaxially grow only on r-Al₂O₃ substrates buffered with epitaxial (001)CeO₂ layers, while, growing simply NBT on top of bare c or r-Al₂O₃ substrates, or on top of CeO₂/c-Al₂O₃ heterostructures leads to polycrystalline or textured films. In the second part, we demonstrate that (001)-oriented NBT layers deposited on either bare (001)SrTiO₃ or (001)SrTiO₃ substrates (STO) covered with (001)LaNiO₃ (LNO) are systematically epitaxially grown. Furthermore, the microstructure of the samples is strongly affected by the introduction of the LaNiO₃ layer.