Bruno Berini

Correlation between electrical and ellipsometric properties on high-quality epitaxial thin films of the conductive oxide LaNiO3 on STO (001)

Direct current (dc) electrical transport properties (resistivity and Hall) and spectroscopic ellipsometric measurements have been combined to study high-quality epitaxial thin films of LaNiO3 (LNO) on SrTiO3 (001) substrates. Stoichiometry of LaNiO3 films was changed by varying the oxygen partial pressure between 240 and 420 μbar during the growth in a pulsed laser deposition setup. Structural and temperature-dependent transport properties conform to epitaxial LaNiO3. The ellipsometric data, measured in the 350–850 nm spectral range, are adjusted with a dispersion relation of Drude-Lorentz (DL) type. In addition to the Drude term, only one Lorentz oscillator was used in the modeling for consistency with existing density of states calculations. Increasing oxygen pressure does not induce significant variations of the electronic density, the Hall mobility, or the Drude plasma energy. Neither are the pseudo-cubic lattice parameter and the Lorentz oscillator resonance energy significantly modified. Resistivity...

Excitonic recombinations in hBN: from bulk to exfoliated layers

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices is now of intense research focus, and it becomes particularly important to evaluate the role played by crystalline defects on their properties. In this paper, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First, the link between the presence of structural defects and the recombination intensity of trapped excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replicas of exciton recombinations. Second, the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations, indicating extremely compact excitons in h-BN.

Systematic investigation of the growth and structural properties of FeTiO3±δ epitaxial thin films

An understanding of the thin film structure and properties of ilmenite is crucial for the future study of ilmenite-hematite solid solutions for spintronics applications. Here, we report a systematic study of the FeTiO3±δ thin film structure as a function of the deposition parameters during pulsed laser ablation. The films are found to be crystalline and epitaxial on α-Al2O3(0001) substrates for a wide range of substrate temperatures and oxygen pressures. Initial stages of the film growth and in-plane lattice relaxation were investigated. The increase in the a lattice parameter of up to 6% above its bulk value was observed and is most probably due to the buckling of a few ilmenite monolayers at the film/substrate interface because of a significant compressive stress induced on the film by sapphire substrate. In spite of the absence of a diffraction peak corresponding to the ordered R3¯ phase, these films consist of pure ilmenite phase with negligible net magnetization. Film conductivity is tuned over three...

Magnetic properties of the magnetophotonic crystal based on bismuth iron garnet

This article reports on the magnetism of continuous and patterned bismuth iron garnet (Bi3Fe5O12 or BIG) thin films for magnetophotonic crystal (MPC) applications. The exact knowledge of the magnetic properties is crucial for the design of fully functional MPC. BIG thin films were grown on several types of isostructural substrates by pulsed laser deposition. The growth conditions and bismuth transfer were optimized to obtain good quality magneto-optically active films compatible with nanostructuring process. MPC were successfully fabricated from BIG/GGG(001) films with low roughness and high Faraday rotation. Magnetic characteristics (magnetization, anisotropy, magnetic domains, magnetization reversal) of the continuous BIG films and MPC were extensively studied and compared to the results of the micromagnetic simulations performed for MPC with different anisotropy. The present study shows that the fabrication of the MPC structure lowers the magnetocrystalline and uniaxial in-plane anisotropies and induce...

High temperature ellipsometry of the conductive oxide LaNiO3

LaNiO3 thin films have been deposited on SrTiO3 (100) by pulsed laser deposition. Substrate temperature and oxygen pressure have been investigated in the ranges of 640–840°C and 200–600μbars, respectively. The use of LaNiO3 as an electrode requires good electrical, structural, and morphological properties. Optimized deposition parameters were determined to be in the range from 650to700°C for the substrate temperature and in the oxygen pressure range from 300to400μbars. Spectroscopic ellipsometry has been used to study in situ the growth of LaNiO3 at high temperature and variation of the optical constants during the cooling from growth to ambient temperature. The metallic behavior of the LaNiO3 is directly observed from the spectral variation of the ellipsometric parameters, tanΨ and cosΔ. For the initial stages of the film growth a determination of the thickness by ellipsometry is possible until the skin depth is reached. The optical constants, refractive index n and extinction coefficient k, have been ca...

Interfacing a heteropolytungstate complex and gelatin through a coacervation process: design of bionanocomposite films as novel electrocatalysts

Bionanocomposite films on glassy carbon electrodes (GCEs) have been prepared by a very straightforward and reliable method based on a layer-by-layer deposition of polyoxometalate (POM) (i.e. [BW12O40]5−) and gelatin solutions. The strong immobilisation of [BW12O40]5− on the surface of the GCE results from electrostatic interactions between gelatin and POM, as evidenced in a hybrid hydrogel prepared by a coacervation process. The GCE has also been modified by imidazolium based ionic liquids in order to increase the charge transfer rate. Two modified electrodes with one or two POM layers have been prepared by this strategy. When compared to other POM-based modified electrodes, the novel electrodes with two POM layers exhibit excellent electrocatalytic performance for nitrite detection at pH 3 in terms of sensitivity (868 mA M−1 cm−2) and linear range of nitrite concentrations (50–600 μM).

High temperature phase transitions and critical exponents of Samarium orthoferrite determined by in situ optical ellipsometry

Determining phase transitions has always been a great challenge in material science due to their important fundamental and technological aspects. Recently, iron-based perovskites (RFeO3), exhibiting phase transitions at high temperatures, have attracted much interest for their functional properties at room temperature, such as multiferroicity (BiFeO3) and ultrafast spin dynamics (TmFeO3). In this family of materials, Samarium orthoferrite (SmFeO3) is a weak ferromagnet, ordering at high temperatures and exhibiting an intrinsic spin reorientation transition above room temperature, which is “hidden” in macroscopic magnetization measurements in polycrystalline samples. In the present article, we show that the related magnetic high temperature phase transitions can be studied through their dielectric functions by spectroscopic ellipsometry in situ and without any need for an applied external magnetic field. The presence of this intrinsic spin reorientation transition is demonstrated for textured SmFeO3 films ...

Structural, optical, and magnetic properties of the ferromagnetic semiconductor hematite-ilmenite Fe2-xTixO3-δ thin films on SrTiO3(001) prepared by pulsed laser deposition

Structural, electrical, and magnetic properties of epitaxial thin films of Fe1.5Ti0.5O3−δ grown by pulsed laser deposition on SrTiO3(001) have been studied. For all the investigated range of preparation conditions, the thin films are single phase. The films prepared using oxygen pressure PO2 above 3×10−7 Torr presents a R(3¯)c symmetry structure. A single phase with the R(3¯) symmetry is obtained for the substrate temperature Ts=730 °C and PO2=2.6×10−7 Torr. This sample showed a ferrimagnetic behavior with Curie temperature Tc above room temperature, with a high magnetic moment of 0.6 μB/formula at 300 K. The film containing the disordered R(3¯)c phase is weakly ferromagnetic, with a spin glass-like signature. Optical properties seem to be governed more by the oxygen vacancies induced during the growth, than by the atomic ordering of titanium.

Tuning of oxidation states in the LaNiO3−δ perovskite around the insulator-metal transition

The LaNiO3−δ perovskite has been studied during its reoxygenation process at the reversible insulator-metal transition by spectroscopic ellipsometry for different pressures and temperatures conditions. First, it was demonstrated that the reoxygenation dynamics increases with both increasing pressure and temperature. Considering the temperature dependent experiments, two regimes of kinetics have been identified: a slow reoxygenation dynamics at low temperature (below 523 K) and fast dynamics above 623 K. Second, contrary to our expectations, the reoxygenation process of a preliminary reduced sample is completed after a sufficient time delay even for the smallest investigated temperature of 473 K or oxygen pressure of 0.03 μbar, respectively. Modeling the change in extinction coefficient as a function of time during the reoxygenation, it was found that the oxygen diffusion coefficient varies from 4.2×10−14 to 1.1×10−13 cm2/s depending on the temperature (473 to 523 K). This oxygen diffusion constant is simi...

Epitaxial growth of the high temperature ferromagnetic semiconductor Fe1.5Ti0.5O3 on silicon-compatible substrate

The ilmenite-hematite (IH) solid solution Fe1.5Ti0.5O3 thin films with rhombohedral symmetry were epitaxially grown on silicon-compatible cubic SrTiO3(001) substrates. Ordered (R-3 symmetry) and disordered (R-3c symmetry) cation arrangement along the film c axis was obtained for different oxygen pressure during the IH growth. Drastic changes in magnetic properties of the films were observed as a function of the cation order. The IH thin films with R-3 symmetry have relatively high conductivity and saturation magnetization at 300 K (6 Ω−1 cm−1 and 0.4 μB/Fe respectively), and Curie temperature of 415 K, making IH a promising material for room-temperature spintronics applications.