Scientific publications

Phase-selective synthesis of crystalline vanadium oxides is achieved by the CVD of vanadium oxo-tri-isopropoxide [VO(OiPr)3]. Preformed V-O and V=O units and their differential thermal stability present in the precursor enabled precise modulation of the vanadium/oxygen stoichiometry in the products of the CVD. No additional oxygen carrier is employed during the deposition processes. Subject to deposition temperature (400-700 °C), different phase compositions (V2O5, V7O13, and VO2) are directly obtained and are investigated regarding structural and electrical properties. Hall measurements on VO2 films show a sharp metal-to-semiconductor transition at 78 °C (single crystal, 68 °C) depending on their microstructure formed by an oriented aggregation of individual crystallites.

Single-molecular magnesium-boron precursors [Mg(C3H6-BC8H14)2] and [Mg{C3H6-B(C6H11)2}2] were applied in chemical vapour deposition processes and characterized using SEM, XRD, XPS, AFM and MS methods towards their fragmentation behaviour, film morphology, phase formation and film composition. Depositon procedures in a temperature range from 500 to 1000 °C produced smooth and thin films with Mg and B in the ration 1:2. It could be shown that the contamination of the deposits by a considerably fraction of residual carbon could be reduced by post-deposition in-situ tempering or additionally sputter processes which may lead to improved deposition of MgB2-films.

A simple and useful experimental alternative to field-effect transistors for measuring electrical properties (free electron concentration nd, electrical mobility µ, and conductivity σ) in individual nanowires has been developed. A combined model involving thermionic emission and tunneling through interface states is proposed to describe the electrical conduction through the platinum-nanowire contacts, fabricated by focused ion beam techniques. Current-voltage (I-V) plots of single nanowires measured in both two- and four-probe configurations revealed high contact resistances and rectifying characteristics. The observed electrical behavior was modeled using an equivalent circuit constituted by a resistance placed between two back-to-back Schottky barriers, arising from the metal-semiconductor-metal (M-S-M) junctions. Temperature-dependent I-V measurements revealed effective Schottky barrier heights up to ΦBE=0.4 eV.

Individual tin oxide nanowires (NWs), contacted to platinum electrodes using focused ion beam assisted nanolithography, were used for detecting water vapor (1500-32 000 ppm) in different gaseous environments. Responses obtained in synthetic air (SA) and nitrogen atmospheres suggested differences in the sensing mechanism, which were related to changes in surface density of the adsorbed oxygen species in the two cases. A model describing the different behaviors has been proposed together with comparative evaluation of NW responses against sensors based on bulk tin oxide. The results obtained on ten individual devices (tested > 6 times) revealed the interfering effect of water in the detection of carbon monoxide and illustrated the intrinsic potential of nanowire-based devices as humidity sensors. Investigations were made on sensitivity, recovery time and device stability as well as surface-humidity interactions. This is the first step towards fundamental understanding of single-crystalline one-dimensional (1D) tin oxide nanostructures for sensor applications, which could lead to integration in real devices.

In order to improve the conductivity of Sb:SnO2 (ATO) films, very thin multi-walled carbon nanotubes (MWNTs) functionalized with -NH2 and -COOH groups have been investigated. The new nanocomposites were prepared by the addition of such MWNTs pre-dispersed in a cationic surfactant (Hexadecyltrimethyl ammonium chloride) in the ATO sol at concentrations of 0.05 and 0.1 wt%. Coatings of pure ATO and nanocomposites were prepared by spin coating on borofloat glass substrates. Experiments showed that all coatings containing nanotubes present resistivities of the order of 10-1 to 10-2 Ω cm while pure ATO films prepared under the same conditions show resistivities which are about 16 times higher. Optical transmission in the visible range of all systems studied was about 90%. The results indicate the suitability of MWNTs as additives for enhancing electrical properties of transparent conductive coatings.

Polymers were grafted on aluminum surfaces in order to modify the chemical and physical properties of the interface. The properly cleaned and activated surface of the aluminum substrate was first "silanized" either with 3-(trimethoxysilyl)propylamine or allyltrimethoxysilane. The grafting was carried out following two methods: (i) by the reaction of preformed poly(methyl vinyl ketone) with the aminosilane-modified surface; and (ii) by polymerization of methyl vinyl ketone with the vinylsilane-modified surface. The modified aluminum surfaces were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The new surfaces were examined by contact-angle measurements, and determinations of the Lewis basicity.

Iron oxide nanotubes of 50-150 nm outer diameter and 2-20 nm wall thickness are prepared in ordered arrays. Atomic layer deposition (ALD) of Fe2O3 from the precursor iron(III) tert-butoxide at 130-180 °C yields very smooth coverage of the pore walls of anodic alumina templates, with thickness growth of 0.26(+/- 0.04) A per cycle. The reduced Fe3O4 tubes are hard ferromagnets, and variations of the wall thickness d(w) have marked consequences on the magnetic response of the tube arrays. For 50 nm outer diameter, tubes of d(w) = 13 nm yield the largest coercive field (H-c > 750 Oe), whereas lower coercivities are observed on both the thinner and thicker sides of this optimum.

A gelatin-based electrolyte has been developed and characterized by impedance spectroscopy, X-ray diffraction, UV-vis-NIR spectroscopy and atomic force microscopy (AFM). The heat treatment temperature was found the key factor affecting its ionic conductivity that increases from 1.5 × 10-5 S/cm to 4.9 × 10-4 S/cm by heating from room temperature up to 80 °C. The temperature dependence of the ionic conductivity exhibits an Arrhenius behavior. EC-devices with the configuration K-glass/Nb2O5:Mo EC-layer/gelatin-based electrolyte/(CeO2)x(TiO2)1-x ion-storage (IS) layer/K-glass, have been assembled and characterized. They show a good long time cyclic stability, but the change of the optical density measured at 550 nm after 25 000 cycles was only 0.13.

An electrochemical quartz crystal microbalance was used as a sensitive detector to analyse the mass changes occurring during the coloration/bleaching processes of sol-gel NiO-TiO2 electrochromic layers. Double layers were deposited on gold-coated quartz crystal electrode and sintered at 300°C in air. The electrochemical process was studied in KOH electrolyte in the potential range -0.4 to + 0.57 V vs. SCE during 650 CV cycles. The current density, charge and mass were found to increase with cycling. The shape of the mass spectrum is rather complex and changes continuously by cycling. The mass of the layer increases after each cycle slowly up to about the 150th cycle then it increases strongly after about the 250th cycle. It passes through a maximum around the 570th cycle with high amplitude variation within each cycle and then decreases fast without drastic change of the cathodic charge. Finally, a complete breakdown occurs around the 650th CV cycle impeding to record any further mass variation. The study is divided in two typical regions where the mechanism of coloration is found to change by cycling. During the first 150 CV cycles, the reversible change of the mass with the charge (increase in the anodic range for V > 0.35 V vs. SCE and decrease in the cathodic range for V < -0.2 V) was related to exchange of OH- groups that involves a change of the oxidation state of Ni from 2+ to 3+ and vice versa leading to coloration and bleaching processes, respectively. For further cycling a model is proposed taking into account the incorporation of K(H2O)n+ ions in the hydrated structure of the layer. The irreversible increase of both the mass and charge exchanged during each cycle is interpreted as due to an increase of the amount of Ni(OH)2.