Salah Habouti

Correlation between structure, dielectric, and ferroelectric properties in BiFeO3–LaMnO3 solid solution thin films

Chemical solution processed BiFeO3 thin films usually show high leakage currents and poor ferroelectric properties. The optimizations of processing conditions and doping with appropriate elements have been among the most promising strategies to enhance the overall properties of BiFeO3 thin films. However, detailed studies on the effects of doping elements on the structure and through it on the properties are still lacking. In this work we investigate the interplay between structure and dielectric-electric-ferroelectric properties of BiFeO3–LaMnO3 [Bi1−xLaxFe1−xMnxO3 (0≤x≤0.1)] solid solution thin films grown on Pt-terminated silicon substrates. The BiFeO3–LaMnO3 system is shown to undergo a structural transition from monoclinic to orthorhombic-tetragonal with various degrees of orthorhombic distortion. All LMO-containing films show a Debye-like dielectric relaxation with the activation energy of 0.90 eV. Both dielectric relaxation and leakage current mechanisms were interpreted in terms of ionized oxygen ...

Multiferroic properties of Bi0.9Gd0.1Fe0.9Mn0.1O3 thin film

Monolithic oxides whose lattice structures exhibit both ferroelectric and magnetic ordering and eventually intrinsic coupling between them are known as multiferroics. The possibility to switch magnetization by an electric field and reciprocally polarization by a magnetic field may allow new applications spanning the fields from new memory devices to medical measuring technology. Until now, the only monolithic oxide that has been shown to exhibit both (weak) magnetism and ferroelectric polarization at room temperature is BiFeO3 (BFO). However, as this material is a line compound, i.e., with sharply defined stoichiometry, its properties largely depend on defects and parasitic phases that could arise during processing. In the present work, we report on a new compound based on the solid solution GdMnO3–BiFeO3 (GdMBFO) that exhibit a fairly high magnetization coupled with moderate ferroelectric polarization at room temperature. Using conductive-tip atomic force microscopy, we show the highly electrically homog...

Macro–meso-porous TiO2, ZnO and ZnO–TiO2-composite thick films. Properties and application to photocatalysis

We report on a new and versatile method for preparing homogenous, crack-free and macro–meso-porous thick films of TiO2, ZnO and ZnO–TiO2 composites using a single step coating procedure and fairly low annealing temperatures of a maximum of 400 °C. The method relies on an oxide nanopowder filler in an optimized precursor solution. The films are well adherent to the substrate, show homogeneously distributed open porosity, and are hydrophilic. The photocatalytic activity of these films was characterized using methyl orange as a model dye. We show that the TiO2 films have better photocatalytic activity than ZnO and ZnO–TiO2 composite films. The advantages of these composite films over particulate photocatalysts lie in their robustness and ease of application as no filtration is needed. Furthermore it is possible to apply them on suitable large area membranes.

LaMnO3 effects on the ferroelectric and magnetic properties of chemical solution deposited BiFeO3 thin films

Thin films of the BiFeO3–LaMnO3 solid solution were processed on (111)‐Pt∕Ti∕SiO2∕Si substrates via spin coating. Microstructure, and leakage current, ferroelectric and magnetic properties are reported. It is shown that the addition of 5mol% LaMnO3 substantially improves the properties. Leakage currents that are several orders of magnitude lower than those of pure BiFeO3 were obtained. The leakage currents follow the Poole-Frenkel mechanism, and a trap depth of 0.94eV was obtained. The addition of LaMnO3 resulted in saturated polarization hysteresis loops with a high remnant polarization 2Pr of 88μC∕cm2. Finally, LaMnO3 also leads to higher coercive fields but smaller saturation magnetization.

Synthesis of silver nano-fir-twigs and application to single molecules detection

We process silver nanostructures via electrodeposition into porous aluminium oxide templates. The degree of pore ordering determines the morphology of the silver nanostructure from smooth to branched nanowires. Nanostructures reminiscent of fir-twigs form in templates with large and low-ordered pores. We demonstrate the usefulness of these nano-fir-twigs as SERS substrates using a 10−15 M R6G dye solution. The enhancement factor of 1011 obtained should allow single molecules to be detected.

Morphology control of 1D noble metal nano/heterostructures towards multi-functionality

We synthesize Pt, Pd, Au and layered multimaterial nanotubes via electrodeposition in anodic aluminium oxide (AAO) templates. We show that time controlled growth at constant electrolyte concentration allows the morphology and chemistry of the nanostructure to be controlled to a high degree. In particular the wall thickness of the nanotubes can be tuned from thin wall nanotubes to nanorods. The applications targeted are electrocatalysis, plasmon resonance and molecular detection via surface enhanced Raman spectroscopy (SERS). The electro-oxidation of methanol on Pt and that of formic acid on Pd are investigated and compared with state of the art Pt/C and Pd/C catalysts and some of the recent similar work from the literature. Nanotubes of both metals (Pt, Pd) show very high electrochemical surface area (ECSA) and enhanced electrocatalytic activity. Our electrocatalysis results show great promise towards direct liquid fuel cells. In terms of molecular detection using SERS we show that the SERS activity of Au-nanostructures is strongly dependent on morphology with very high SERS enhancements being related to partly filled Au-nanotubes.

La0.8Sr0.2MnO3 buffer layer effects on microstructure, leakage current, polarization, and magnetic properties of BiFeO3 thin films

BiFeO3 thin films were processed on La0.8Sr0.2MnO3 (LSMO) buffered silicon substrate via chemical solution deposition. Short-wave ultraviolet assisted pyrolysis was conducted in oxygen atmosphere in order to improve microstructure. A fine, homogeneous, and phase-pure grain structure with smooth topography was obtained. X-ray photoelectron spectroscopy analysis indicates diffusion of lanthanum and manganese from LSMO into the BiFeO3 film leading to a diffuse interface. Nevertheless, improved polarization and leakage resistance properties were obtained. For the first time polarization switching was obtained in sol-gel BiFeO3 films using capacitance-voltage (C−V) curves. This has been imputed to homogeneous local electrical properties of the film, corroborated by surface scanning potential microscopy investigations. The leakage current mechanisms were investigated as function of temperature, and could be interpreted in terms of the Frenkel-Poole mechanism with a trap depth of 1.6 eV. Finally, the magnetic pr...

Ordered Nanomaterial Thin Films via Supported Anodized Alumina Templates

Supported anodized alumina template films with highly ordered porosity are best suited for fabricating large area ordered nanostructures with tunable dimensions and aspect ratios. In this paper we first discuss important issues for the generation of such templates, including required properties of the Al/Ti/Au/Ti thin film heterostructure on a substrate for high quality templates. We then show examples of anisotropic nanostructure films consisting of noble metals using these templates, discuss briefly their optical properties and their applications to molecular detection using surface enhanced Raman spectroscopy. Finally we briefly address the possibility to make nanocomposite films, exemplary shown on a plasmonic-thermochromic nanocomposite of VO2-capped Au-nanorods.

A template-free synthesis and structural characterization of hierarchically nano-structured lithium-titanium-oxide films

A template-free sol–gel method for the synthesis of hierarchically nano-structured lithium-titanium-oxide films is proposed involving a p-type doping of titanium isopropoxide sols with lithium. The structures of the films are analyzed by scanning electron microscopy, X-ray powder diffraction and Raman spectroscopy. Different phases can be accessed by controlling the annealing temperature. Supersaturated anatase (Ti, Li)O2 is formed at 400 °C, whereas anatase and/or rutile interspersed with Li2TiO3 and Li4Ti5O12 are observed between 450 °C and 800 °C. The nanostructure formation is explained by a dissolution–crystallization–self-assembly mechanism.

Au-NR/VO2-NP nanocomposites supported on glass substrates: microstructure and optical properties

On-glass-substrate gold nanorods (Au-NRs) are processed via electrodeposition into anodized alumina template films that were fabricated using an Al/Au/Ti film stack on glass. The Au-NRs are capped with VO2 nanoparticles obtained from sputtered vanadium caps. The nanostructure arrays show intense plasmon resonance and are thermochromic with VO2 size dependent transition temperature.