Cédric Leuvrey

Structural and photoluminescence properties of ZnO thin films prepared by sol-gel process

The present study focuses on the structural and optical properties of ZnO thin films fabricated by sol-gel process and spin coated onto Si (100) and quartz substrates. The ZnO films have a hexagonal wurtzite structure with a grain size of about 50 nm. The x-ray photoelectron spectroscopy measurements reveal the presence of Zn2+ and of zinc hydroxyl groups at the film. Optical properties were studied by photoluminescence (PL) and absorption spectroscopy at low and room temperatures. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, PL observations show two transitions: one near the absorption edge in the ultraviolet (UV) region and the second centered at 640 nm, characteristic of the deep electronic levels in the bandgap. The spectrum at 6 K is dominated by donor bound exciton lines and donor-acceptor pair transitions. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of g...

Optical properties of ZnO thin films prepared by sol-gel process

The present study focused on ZnO thin films fabricated by sol-gel process and spin coated onto Si (100) and quartz substrates. ZnO thin films have a hexagonal wurtzite structure with a grain diameter about 50nm. Optical properties were determined by photoluminescence (PL) and absorption spectroscopy. The absorption spectrum is dominated by a sharp excitonic peak at room and low temperatures. At room temperature, two transitions were observed by PL. One near to the prohibited energy band in ultraviolet (UV) region and the other centered at 640nm, characteristic of the electronic defects in the band-gap. The spectrum at 6K is dominated by donor-bound exciton lines and donor-acceptor pair transition. LO-phonon replica and two-electron satellite transitions are also observed. These optical characteristics are a signature of high-quality thin films.

Spacing-dependent dipolar interactions in dendronized magnetic iron oxide nanoparticle 2D arrays and powders

Self-assembly of nanoparticles (NPs) into tailored structures is a promising strategy for the production and design of materials with new functions. In this work, 2D arrays of iron oxide NPs with interparticle distances tuned by grafting fatty acids and dendritic molecules at the NPs surface have been obtained over large areas with high density using the Langmuir-Blodgett technique. The anchoring agent of molecules and the Janus structure of NPs are shown to be key parameters driving the deposition. Finally the influence of interparticle distance on the collective magnetic properties in powders and in monolayers is clearly demonstrated by DC and AC SQUID measurements. The blocking temperature T(B) increases as the interparticle distance decreases, which is consistent with the fact that dipolar interactions are responsible for this increase. Dipolar interactions are found to be stronger for particles assembled in thin films compared to powdered samples and may be described by using the Vogel Fulcher model.

Monolayer and multilayer assemblies of spherically and cubic-shaped iron oxide nanoparticles

Nowadays, nanoparticles are considered as the building blocks of the future nanotechnological devices and the development of strategies for processing nanoparticles into thin films has become a strategic challenge. In this context, the assembling of spherically shaped iron oxide nanoparticles displaying various sizes and of cubic-shaped nanoparticles has been investigated using the Langmuir–Blodgett technique. Homogeneous and dense monolayer and multilayer films have been obtained on large areas. The organisation in films has been studied by combining GISAXS and image analysis of SEM micrographs. The quality of the film has been determined to be mainly dependent on the chemical nature of the substrate and the amount of surfactant molecules at the surface of the nanoparticles (i.e. the organic coating).

New layered organic-inorganic magnets incorporating azo dyes

New hybrid organic/inorganic magnets have been synthesized by insertion of stilbazolium-carboxylate and/or -sulfonate dyes into cobalt and copper layered hydroxides. Co2(OH)3.57(MR)0.43·1.88 H2O (MR⊂Co (1)) (MRNa: Methyl Red sodium salt: sodium 2-[4-(Dimethylamino)phenylazo]benzoate), Co2(OH)3.27(MO)0.73·2.94 H2O (MO⊂Co (2)) (MONa: Methyl Orange sodium salt: sodium 4-[4-(Dimethylamino)phenylazo]benzenesulfonate), Co2(OH)3.45(OrangeIV)0.55·2.92 H2O (OrangeIV⊂Co (3)) (OrangeIVNa: OrangeIV sodium salt: sodium 4-[4-(Anilino)phenylazo]benzenesulfonate), and Co2(OH)3.46(MY10)0.27·2.34 H2O (MY10⊂Co (4)) (MY10Na2: Mordant Yellow 10 disodium salt: sodium 3-[4-(sodium sulfonate)phenylazo]-6-hydroxybenzoate) have been synthesized by anionic exchange in alkaline media starting from Co2(OH)3(OAc)·H2O. Cu2(OH)3.23(MR)0.77·2.92 H2O (MR⊂Cu (5)), Cu2(OH)3.29(MO)0.71·2.31 H2O (MO⊂Cu (6)), and Cu2(OH)3.39(OrangeIV)0.61·1.80 H2O (OrangeIV⊂Cu (7)) were obtained starting from the pre-intercalated copper hydroxide Cu2(OH)3(DS) (DS−: dodecylsulfate). This is the first time pre-intercalation is used for the insertion of molecules within the interlamellar space of simple layered hydroxides. The hybrid copper layered hydroxides show ferrimagnetic behaviour with ordering at very low temperature (below 1.8 K). The cobalt analogues are ferrimagnets with ordering temperatures ranging from 6.9 K to 18 K.

Spatial and temporal discrimination of silica particles functionalised with luminescent lanthanide markers using time-resolved luminescence microscopy

Luminescent lanthanide complexes of europium and terbium grafted on amino-functionalised silica particles can easily be discriminated from conventional fluorescent markers such as fluorescein thanks to a new and simple time-resolved luminescence microscopy set-up.

Tunable Magnetic Properties of Nanoparticle Two-Dimensional Assemblies Addressed by Mixed Self-Assembled Monolayers

Assemblies of magnetic nanoparticles (NPs) are intensively studied due to their high potential applications in spintronic, magnetic and magneto-electronic. The fine control over NP density, interdistance, and spatial arrangement onto substrates is of key importance to govern the magnetic properties through dipolar interactions. In this study, magnetic iron oxide NPs have been assembled on surfaces patterned with self-assembled monolayers (SAMs) of mixed organic molecules. The modification of the molar ratio between coadsorbed 11-mercaptoundecanoic acid (MUA) and mercaptododecane (MDD) on gold substrates is shown to control the size of NPs domains and thus to modulate the characteristic magnetic properties of the assemblies. Moreover, NPs can be used to indirectly probe the structure of SAMs in domains at the nanometer scale.

Structural defects in Sr2FeMoO6 double perovskite: Experimental versus theoretical approach

The lower than expected magnetization of imperfect Sr2FeMoO6 (SFMO) double perovskites is usually attributed to the presence of Fe at antisite positions that would be antiferromagnetically coupled to their regular neighbors. However, ab initio calculations suggest strongly that such defective Fe sites would be ferromagnetically coupled and, consequently, the magnetization reduction would originate from other kinds of defects. The magnetic, hyperfine, and structural properties of SFMO perovskites prepared by solid-state reaction under a variety of conditions are reported and correlated with ab initio calculations of the magnetic moments and hyperfine fields of Mo and Fe ions in different local environments (antisites, antisite neighbors, and neighbors of an oxygen vacancy). When plotted against the order parameter the experimental magnetization is found to decrease at a rate of about −7.6μB per Mo–Fe antisite pair as in other previous experiments, where the theoretical calculation predicts −6.56μB per anti...

Annealing treatment for restoring and controlling the interface morphology of organic photovoltaic cells with interfacial sputtered ZnO films on P3HT:PCBM active layers

In this paper, we report on the photovoltaic properties of conventional organic photovoltaic solar cells integrating a sputtered ZnO interfacial film deposited on the absorber P3HT:PCBM layer. An emphasis has been put on the influence of the annealing temperature and time for restoring and controlling the P3HT:PCBM/ZnO interface morphology, which can be damaged by the sputtering process. We show a significant improvement in the current–voltage (J–V) characteristics upon annealing up to 160 °C. This is evidenced by the reduction of the S-shape of these curves systematically observed for the cells integrating thick (100 nm) sputtered ZnO films. This approach was also highlighted on cells containing thinner (20 nm) ZnO films using a longer annealing process at 140 °C, which led to a significant improvement of the power conversion efficiency compared with the value recorded in as-prepared cells or in cells with no interfacial ZnO layer. These photovoltaic performances have been related to the change of the morphology of the absorber layer and to the vertical phase segregation of P3HT:PCBM at the interface with ZnO. Optical microscopy, scanning electron microscopy and atomic force microscopy have been performed in order to confirm this approach.

Effect of La doping on the properties of Sr2−xLaxFeMoO6 double perovskite

We report on the carrier doping effect (achieved through La doping) on the electronic structure, crystalline structure, grain morphology, and electric and magnetic properties of the ferromagnetic perovskite Sr2FeMoO6. X-ray diffraction on Sr2−xLaxFeMoO6 (x=0, 0.25, 0.5, and 1) ceramic powders shows that all compounds have the tetragonal crystal structure with I4∕mmm symmetry. The cell parameter a and the volume of the tetragonal structure increase with the La doping and the grain size is halved with respect to that of undoped Sr2FeMoO6. Mossbauer spectroscopy shows that the presence of La reduces the degree of ordering on the Fe and Mo sites which causes a reduction of the saturation magnetization MS. However, the addition of electrons in the system results in an increase of the Curie temperature of the Sr2FeMoO6. All these results are supported by ab initio calculations which indicate that the half-metallic character is preserved upon doping and that the magnetization decreases for perfect samples by 1μB...