Yves Dumont

Large room temperature magnetoresistance of transparent Fe and Ni doped ZnO thin films

The electrical, optical, and magnetic properties of pure, 1%Fe, and 1%Ni doped ZnO thin films grown by spray pyrolysis technique were studied. All samples are transparent (T ≈ 85%) in VIS and near infrared region of wavelength. Ni and Fe doped ZnO layers are paramagnetic. Resistivity versus temperature has semiconducting behavior. Large value of magnetoresistance at 300u2009K at 1.3u2009T: MRu2009=u200956%/T for 1%Fe doped ZnO and MRu2009=u200928%/T for 1%Ni doped samples have been observed. These exceptional values of MR at room temperature originate probably from hopping conductivity in polycrystalline diluted magnetic semiconductor in paramagnetic high dilution limit.

Impurity-to-band activation energy in phosphorus doped diamond

The value of the impurity-to-band activation energy EA of a dopant is a basic feature of the electrical conductivity of semiconductors. Various techniques were used to determine EA in n-type diamond doped with phosphorus, giving values of EA varying from 0.43u2009eV to 0.63u2009eV, the value EA of 0.6u2009eV being commonly accepted for the ionization energy of phosphorus donors in diamond. Nevertheless, up to now, the dispersion of the experimental values of EA remains unexplained. In this work, we investigate the electrical properties of a set of n-type diamond homoepitaxial films with different phosphorus concentrations by Hall effect measurements in order to deduce EA and to understand the evolution of this energy with the dopant concentration. We show that, below 2 ×u20091019 cm−3 phosphorus, the decrease of EA is mainly controlled by the concentration of ionized defects resulting from the donor compensation. The role of ionized defects in the decrease of EA is analyzed on the basis of existing models adapted to the case of diamond. The proposed model provides a correct description of the experimental data. It can be used to quantitatively predict the activation energy of phosphorus in n-type diamond for given donor and compensating acceptor concentrations.

Conductivity type inversion in wide band gap antiferromagnetic FeTiO3

By quasi-chemical reaction method for FeTiO3 material in oxygen atmosphere, electron and hole concentrations for broad range of oxygen partial pressure have been calculated. Oxygen pressures, at which electrical and hole conductivity can be achieved, were estimated. Thin layers of FeTiO3 have been grown on transparent Al2O3 (0001) substrates by pulsed laser deposition technique at different oxygen partial pressures. Structural, optical, and electrical properties have been studied for these samples. By special post-annealing treatment, the conductivity type inversion from p to n has been achieved as predicted by our thermodynamic analysis.

Bismuth iron garnet Bi3Fe5O12: A room temperature magnetoelectric material

The possibility to control the magnetic properties of a material with an electric field (or conversely, electric properties with magnetic field) at room temperature is highly desirable for modern applications.

Tuning the conductivity type in a room temperature magnetic oxide: Ni-doped Ga0.6Fe1.4O3 thin films

Ni-Doped thin films of the room temperature ferrimagnetic oxide Ga0.6Fe1.4O3 were deposited by pulsed laser deposition and their electronic transport and structural and magnetic properties were studied. The actual insertion of the Ni cations within the Ga0.6Fe1.4O3 structure has been checked by resonant X-ray scattering. A clear extremum is noticed for all properties for the 2% Ni doping: extrema in the crystallographic cell parameters of the films, maximum in the Curie temperature, and maximum in the electric resistivity. We also observed a change of conductivity type for this dopant concentration, from n-type for Ni contents below 2% to p-type for Ni contents above 2%. We explain this behavior by the existence of oxygen vacancies in the pulsed laser deposited Ga0.6Fe1.4O3 thin films, which results in the reduction of some of the Fe3+ into Fe2+ cations, and n-type conduction via a hopping mechanism. The insertion of Ni2+ cations first deals with the presence of oxygen vacancies and reduces the number of n-type carriers in the films, in a compensation-like mechanism. When the number of introduced Ni2+ cations dominates the number of oxygen vacancies, conductivity becomes p-type and starts to increase again. We believe that the tunability of the conduction type and magnitude in thin films of a room temperature ferrimagnetic material paves the way towards new all oxide electronic devices.

Control of p-type conduction in Mg doped monophase CuCrO2 thin layers

This work aims to clarify the origin of hole conduction in undoped and Mg-doped CuCrO2 oxide in order to have the possibility of controlling it by corresponding growth parameters. A chemical spray pyrolysis procedure for the deposition of p-type semiconductor thin films is described. The as-deposited films were amorphous. The formation of highly crystalline CuCrO2 and Mg-doped CuCrO2 films with a single phase delafossite structure was realized by annealing between 600 °C and 960 °C in a nitrogen atmosphere. The carrier concentration and the point defects of the samples are calculated by using the developed Kroger method of quasi-chemical reactions. p-type conductivity was predicted and observed in the undoped and Mg doped CuCrO2 sample, and with n ~ 1018 cm−3 carrier concentrations for 4%Mg doping. The electrical resistivity for a 4% Mg doped sample was 1.4 Ωcm with a Seebeck coefficient of +130 μV K−1 at 40 °C. By electroparamagnetic resonance spectroscopy Cr3+ and Cu2+ related defects were studied.

Synthesis and characterization of multifunctional superparamagnetic iron oxide nanoparticles (SPION)/C60 nanocomposites assembled by fullerene–amine click chemistry

The fabrication of multifunctional nanostructures constitutes an interesting area of research. Here we used a simple fullerene–amine coupling chemistry to produce a photofunctional and antioxidant magnetic material composed of multiple C60 molecules covalently bound to core monodispersed superparamagnetic iron oxide nanoparticles (SPIONs). For this, core SPIONs were synthesized by thermal decomposition, using iron(III) acetylacetonate as the iron precursor, and the resulting hydrophobic SPIONs were coated with a thin amino-functionalized silica layer. C60 molecules were covalently linked to the core SPIONs by a simple fullerene–amine coupling reaction, a method promoting the assembly of the pursued nanocomposite. Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy were used all along the fabrication process to demonstrate the efficient assembly of the SPION/C60 nanocomposite. We confirm that the latter brings together the disparate physico-chemical properties of its components. Using vibrating sample magnetometry we demonstrate that the core SPION confers a superparamagnetic character to the nanocomposite. We also demonstrate that fullerenes confer photosensitizing properties to the latter, as illustrated by the photo-oxidation of 1,5-dihydroxynaphthalene. Interestingly, the hydrophobic SPION/C60 nanocomposite was easily solubilized in water using polyvinylpyrrolidone, offering the possibility of evaluating its properties in physiological conditions and paving the way to further biomedical applications. Therefore, we also demonstrate that water-soluble SPION/C60 has a good T2 relaxivity, conferred by its core superparamagnetic component, and could potentially be used as a contrast agent in magnetic resonance imaging. Furthermore, we show that the hydrophilic nanocomposite has a remarkable antioxidant capacity which is provided by its fullerene component. In summary, our work provides a facile method to produce composite nanostructures that bring together the attractive properties of monodispersed SPIONs and those of fullerene C60 molecules, which can be exploited in a broad range of applications, such as photodynamic therapy, photocatalytic oxidation of organic pollutants and radical scavenging, to name only a few.

Interplay between epitaxial strain and low dimensionality effects in a ferrimagnetic oxide

Thin film properties are strongly influenced by strain and low dimensionality effects, especially when the film thickness is about a few unit cells, which corresponds to the thicknesses targeted in most of contemporary studies.