Publications

Rutile-TiO2/hybrid halide perovskite CH3NH3PbI3-xClx interfaces are investigated by ab initio density functional theory calculations. The role of chlorine in achieving enhanced solar cell power conversion efficiencies is in the focus of recent studies, which point to increased carrier mobilities, reduced recombination rates, a driven morphology evolution of the perovskite layer and improved carrier transport across the interface. As it was recently established that chlorine is preferentially localized in the vicinity of the interface and not in the bulk of the perovskite layer, we analyze the changes introduced in the electronic properties by varying the chlorine concentration near the interface. In particular, we discuss the effects introduced in the electronic band structure and show the role of chlorine in the enhanced electron injection into the rutile-TiO2 layer. Taking into account these implications, we discuss the conditions for optimizing the solar cell efficiency in terms of interfacial chlorine concentration.

Using electrodeposition, we have grown nanowires of ZnCoO with Cu codoping concentrations varying from 4-10 at. %, controlled only by the deposition potential. We demonstrate control over magnetic Co oxide nano-precipitate formation in the nanowires via the Cu concentration. The different magnetic behavior of the Co oxide nano-precipitates indicates the potential of ZnCoO for magnetic sensor applications. (C) 2014 AIP Publishing LLC.

Single and double layer phase change memory structures based on GeTe and GaSb thin films were deposited by pulsed laser deposition (PLD). Their crystallization behavior was studied using in-situ synchrotron techniques. Electrical resistance vs. temperature investigations, using the four points probe method, showed transition temperatures of 138 degrees C and 198 degrees C for GeTe and GaSb single films, respectively. It was found that after GeTe crystallization in the stacked films, Ga atoms from the GaSb layer diffused in the vacancies of the GeTe crystalline structure. Therefore, the crystallization temperature of the Sb-rich GaSb layer is decreased by more than 30 degrees C. Furthermore, at 210 degrees C, the antimony excess from GaSb films crystallizes as a secondary phase. At higher annealing temperatures, the crystalline Sb phase increased on the expense of GaSb crystalline phase which was reduced. Extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges revealed changes in their local atomic environments as a function of the annealing temperature. Simulations unveil a tetrahedral configuration in the amorphous state and octahedral configuration in the crystalline state for Ge atoms, while Ga is four-fold coordinated in both as-deposited and annealed samples. (C) 2014 AIP Publishing LLC.

By combining two types of ligands, phenylpyridine and quinoline, a new type of organometallic IrQ(ppy)(2) compound has been synthesized, which exhibits two phosphorescences: green and red. Using an appropriate catalyst, the final IrQ(ppy)(2) compound has a good chemical yield up to 60% and becomes a stable dual emitter at room temperature. This compound is important because it exhibits stable red emission which is limited by the quantum yield due to the low energy band gap. As a result, an overlap between the ground state and the excited state occurs due to the vibrations that increase the nonradiative transitions, destroying the red emissions. Structural characteristics of the IrQ(ppy)(2) powder reveal a triclinic structure confirmed by x-ray diffraction and scanning electron microscopy images. Thermal analysis of the final compound confirms a good stability against decomposition and structural changes up to 350 degrees C. X-ray photoelectron spectroscopy reveals Ir-O chemical bonds and several differences between the intermediate and final compounds, such as Ir-Cl bonds. Cathodoluminescence patterns show a phosphorescent triclinic structure with a higher efficiency for the red color. Backscattering electron images prove that there is a uniform distribution of iridium ions in the IrQ(ppy)(2) nanocrystals.

PdCu nanoparticles were synthesized by the alkaline polyol method and further supported on alumina or titania. The nanoparticles show a crystalline Pd core and a shell rich in amorphous copper as was put in evidence by complex characterization methods (X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and CO chemisorption). The performances of as-prepared catalysts in the water phase reduction of nitrate were assessed in comparison with catalysts obtained by impregnation. Supported nanoparticle catalysts show a high activity in the reduction of nitrates, better than the impregnated catalysts. The importance of the support choice was discussed. The catalyst based on PdCu nanoparticles supported on titania (PCT-np) is the most active, selective, and stable among those investigated. Also, the influence of pH conditions on the PCT-np catalyst performances was emphasized.

Pure titanium dioxide (TiO2) and graphene oxide (GO) as well as TiO2/GO composite structures were grown by matrix-assisted pulsed laser evaporation (MAPLE) in a controlled oxygen atmosphere. The MAPLE target dispersions were prepared using distilled water as solvent matrix, with TiO2 nanoparticles (NPs) and GO platelets serving as host materials. Two laser sources, a free-running IR Er:YAG (lambda = 2940 nm, tau(fwhm) congruent to 350 mu s, nu = 10 Hz) and a UV KrF* excimer (lambda = 248 nm, tau(fwhm) congruent to 25 mu s, nu = 10 Hz) laser, were used for the transfer and immobilization experiments by infrared (IR)- and ultraviolet (UV)-MAPLE, respectively. The potential physical mechanisms implied in both the IR- and UV-MAPLE processes are discussed, based on numerical simulations of temperature evolution of the distilled water matrix, TiO2 NPs, and GO platelets. Our results demonstrate the effectiveness of IR- and UV-MAPLE processes for the immobilization of nanoentities onto solid substrates. During IR-MAPLE, the laser radiation is primarily absorbed by the water matrix. The materials transferred to the substrate surface resemble the initial starting materials used for the preparation of the MAPLE target dispersions. Conversely, during UV-MAPLE the UV radiation is mainly absorbed by the nanoentities dispersed in the water matrix. The structural transformation of the nanoentities deposited by UV-MAPLE is significant as compared to the starting materials.

A comparative study of two different biocatalytic models, e.g. enzyme immobilized on magnetic particles (EIMP) and cross-linking enzyme aggregates onto magnetic particles (CLEMPA) was performed. The first model was designed as enzyme-immobilized on the magnetic particles surface (EIMP). The second model was constructed as a network structure with the enzyme aggregates and magnetic particles placed into the nodes and polyglutaraldehyde cross-linker as the network ledges. The design was called cross-linking enzyme aggregates onto magnetic particles (CLEMPA). The biocatalysts were prepared using lipase enzyme from Aspergillus niger for catalyzing the glycerol (Gly) conversion to glycerol carbonate (GlyC). The biocatalyst characteristics for both designs (EIMP and CLEMPA) were evaluated using scanning electron microscopy (SEM), laser light scattering (LLS) and UV-Vis techniques. The EIMP model was strongly influenced by the composition of the polymeric layer covering the particles surface, while the size of the magnetic particles affected mostly the CLEMPA design. Also, the biocatalytic capacity of the tested models was evaluated as maximum 52% Gly conversion with 90% GlyC selectivity for EIMP, and 73% Gly conversion with 77% GlyC selectivity for CLEMPA. Both biocatalytic models were successfully used to prepare GlyC from "crude" glycerol collected directly from the biodiesel process (e.g. 49% Gly conversion with 91% GlyC selectivity for EIMP and 70% Gly conversion with 80% GlyC selectivity for CLEMPA).

We investigate the structure of epitaxially grown hexagonal boron nitride (h-BN) on Ir(111) by chemical vapor deposition of borazine. Using photoelectron diffraction spectroscopy, we unambiguously show that a single-domain h-BN monolayer can be synthesized by a cyclic dose of high-purity borazine onto the metal substrate at room temperature followed by annealing at T = 1270 K, this method giving rise to a diffraction pattern with 3-fold symmetry. In contrast, high-temperature borazine deposition (T = 1070 K) results in a h-BN monolayer formed by domains with opposite orientation and characterized by a 6-fold symmetric diffraction pattern. We identify the thermal energy and the binding energy difference between fcc and hcp seeds as key parameters in controlling the alignment of the growing h-BN clusters during the first stage of the growth, and we further propose structural models for the h-BN monolayer on the Ir(111) surface.