Scientific publications

Nanoscale zinc oxide (ZnO) powder with Brunauer-Emmelt-Teller surface area of 43 m2 g-1 has been synthesized by soft chemistry at low temperature via reaction of zinc acetate dehydrate (Zn(CH3COO)2.2H2O) and sodium hydroxide (NaOH). The influence of the pH value of the sol on the structure and morphology of ZnO powder have been investigated by X-Ray Diffraction, Scanning Electron Microscopy and High Resolution Transmission Electron Microcopy. Their thermal properties have been determined by simultaneous Differential Thermal Analysis and Thermogravimetry coupled to Mass Spectroscopy analysis. The nanoparticles are single crystals with (101) preferred orientation but agglomerated. Their crystallite size can be adjusted from 15 nm to 35 nm by controlling the pH value between 7 and 13. Thick porous crystalline coatings have been obtained by doctor blade coating on conducting SnO2:F glass substrates using pastes prepared by wetting the crystalline powders with polyethylene glycol and water. After sintering at 400 °C and Ruthenium 535 dye sensitization, the coatings have been tested in a three electrode electrochemical cell containing an appropriate electrolyte in the dark and under 450 W Xenon lamp illumination. The influence of the electrolyte iodine concentration, the film thickness and the light intensity on the current density are presented and discussed. Such coatings appeared promising for the realization of dye sensitized solar cells.

As ferroelectric ceramic/polymer composites may offer the possibility to lower the operating voltage of state of the art polymer ferroelectric memories, the retention behavior of these composites needs to be studied. This work presents for the first time the polarization decay of a ceramic/polymer composite memory and discusses some influencing parameters. The existence of a depolarization field in such particulate structures, which is responsible for the polarization loss is demonstrated by finite element simulations, and first theoretical predictions are generated how to reduce this depolarizing field. Interestingly, first results from the experimental approach of varying the ionic concentration in the surrounding medium of the particles contradict these theoretical assumptions.

Different kinds of ZnO nanowires were synthesized by vapor-solid and vapor-liquid-solid methods via a chemical vapor transport and condensation process. The samples were characterized by scanning electron microscopy, X-ray diffraction (XRD) and photoluminescence analyses. The control on the growth morphologies can be achieved by the source materials and by using a thin gold layer as a catalyst. 15-80 nm nanowires were obtained and XRD patterns show two different growth directions. High intensity green light from photoluminescence spectroscopy was observed which shows that the experimental results could be useful for light-emitting materials and other optoelectronic device applications.

Using direct oxidation of Zn powder as a source material, zinc oxide comb and saw structures were fabricated by vapor phase transport and condensation. X-ray diffraction pattern showed that the samples had a wurtzite crystal structure. Different structures such as comb-like nano- and microstructures and saw-like structures have been studied by scanning electron microscopy. ZnO comb-like nanostructures with an average size of 15–20 nm and 50 nm were observed, as well as comb-like and saw-like microstructures. Photoluminescence (PL) spectra of these ZnO samples at room temperature were discussed. PL spectra showed strong UV and green emissions that indicate their possible application in optoelectronic devices. Also, the I–V behavior of a single nanowire was investigated. Approximately linear current versus voltage was observed for this nanowire, which showed good ohmic contacts to structure.

In this work, Cu(In,Ga)Se2 (CIGS) nanoparticles were synthesized using a wet chemical method. The method is based on a non-vacuum thermal process that does not use selenization. The effects of temperature, source materials, and growth conditions on the phase and particle size were investigated. X-ray diffraction results confirm the formation of a tetragonal CIGS structure as the main phase with the purity more than 99% obtained by energy-dispersive X-ray spectroscopy (EDX). The morphology and size of the samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Using these methods, 20-80 nm particles were obtained. Through measurements of the absorption spectra of CIGS nanoparticles, the band gap of the synthesized material was determined to be about 1.44 eV, which corresponds to an acceptable wavelength region for absorber layers in solar cells.

We exploit the strong optical anisotropy of metal nanorods to measure their mobility in a complex fluid. Gold rods in hot agarose solutions cause dynamic, depolarized scattering with an exponentially decaying autocorrelation. As the solution cools down, its decay constant increases. At a certain temperature, the autocorrelation drastically changes its shape and the dynamic contrast drops. We show that, at this temperature, the gelling liquid confines the rods and dampens their motion almost entirely. Depolarized scattering proves to be extraordinarily sensitive to the transition from Brownian to confined motion. We calculate true mobilities for the particles using Pusey and van Megen's correction of the Siegert relation for nonergodic systems. Multipoint measurements show that the rods are immobilized throughout the gel.

Crystalline and amorphous materials composed of the same atoms exhibit strikingly different properties. Likewise, the behavior of materials composed of mesoscale particles depends on the arrangement of their constituent particles. Here, we demonstrate control over particle arrangement during agglomeration. We obtain disordered and ordered agglomerates of the same alkyl thiol-coated gold nanoparticles depending on temperature and solvent. We find that ordered agglomeration occurs exclusively above the melting temperature of the ligand shells. Many-particle simulations show that the contact mechanics of the ligand shells dominate the order-disorder transition: Purely spherical particle-particle interactions yield order, whereas localized "stiction" between the ligand shells leads to disorder. This indicates that the "stickiness" and the packing of the agglomerates can be switched by the state of the ligand shells. It suggests that contact mechanics govern ordering in a wide range of nanoparticles.

Noble gas and metal atoms form minimum-energy clusters. Here, we present analogous agglomerates of gold nanoparticles formed in oil-in-water emulsions. We exclude interfacial templating and nucleation-and-growth as formation mechanisms of these supraparticles. Similar to atomic clusters, the supraparticles form when a mobile precursor state can reconfigure until the nanoparticles' interactions with each other and with the liquid-liquid interface are maximized. This formation mechanism is in striking contrast to that previously reported for microparticle clusters.

In this paper the inhibitive performance of cerium molybdate nanowires prepared by a novel methodology is reported. The methodology is based on a low-temperature, controlled-rate mixing process. Structurally, cerium molybdate nanowires were found to be amorphous-like materials whose inhibiting action towards aluminium alloy 2024-T3 was demonstrated by DC polarization and electrochemical impedance spectroscopy (EIS), corroborated by microstructural surface analysis of the metallic substrate. The observed inhibiting action is attributed to the higher solubility of the cerium molybdate nanomaterials with respect to crystalline sodium cerium molybdate obtained at room temperature.