Publikationen

Dissipated power in metal oxide nanowires (rNW<45 nm) often causes important self-heating effects and as a result, undesired aging and failure of the devices. Nevertheless, this effect can be used to optimize the sensing conditions for the detection of various gaseous species, avoiding the requirement of external heaters. In this letter, the sensing capabilities of self-heated individual SnO2 nanowires toward NO2 are presented. These proof-of-concept systems exhibited responses nearly identical to those obtained with integrated microheaters, demonstrating the feasibility of taking advantage of self-heating in nanowires to develop ultralow power consumption integrated devices.

Deposition of thin films through vaccum processes plays an important role in industrial processing of decorative and functional coatings. Many metal oxides have been prepared as thin films using different techniques, however obtaining compositionally uniform phases with a control over grain size and distribution remains an enduring challenge. The difficulties are largely related to complex compositions of functional oxide materials, which makes a control over kinetics of nucleation and growth processes rather difficult to control thus resulting in non-uniform material and inhomogeneous grain size distribution. Application of tailor-made molecular precursors in low pressure or plasma-enhanced chemical vapor deposition (CVD) techniques offers a viable solution for overcoming thermodynamic impediments involved in thin film growth. In this paper molecule-based CVD of functional coatings is demonstrated for iron oxide (Fe2O3, Fe3O4), vanadium oxide (V2O5, VO2) and hafnium oxide (HfO2) phases followed by the characterization of their microstructural, compositional and functional properties which support the advantages of chemical design in simplifying deposition processes and optimizing functional behavior.

One-dimensional nanoscopic Fe3O4 architectures are grown by molecule-based chemical vapor deposition CVD using [Fe(OBut)3]2 precursor and Au particles as growth templates. Growth of magnetite shell on tin oxide nanowires is also achieved, obtaining magnetic-semiconductor heterostructures. Structural characterization of the different nanostructures and the first electrical measurements on an individual magnetite nanowire are reported.

One dimensional (1D) inorganic materials are gaining increasing attention because of their unique structural features and interesting functional properties. Given the structural stability, they show promising application potential in vacuum as well as in oxidizing atmospheres, which provides them a competitive edge over their carbon-based counterparts. A number of synthetic procedures have been developed and demonstrated for 1D nanostructures that have led to intriguing morphological variations (wires, tubes, belts, rods, etc.), however the control over radial and axial dimensions remains a continuing challenge. In addition, the choice of material is rather limited. We have developed a generic approach for the size-selective and site-specific growth of nanowires by combining vapor-liquid-solid (VLS) approach with molecule-based chemical vapor deposition. The synthesis of nanowires (NWs) is based on the decomposition of discrete molecular species, which allows growing nanowires at low temperatures with a precise control over their diameter and length. The precursor chemistry can be tuned to facilitate the stripping of organic ligands and to achieve complete decomposition that is critical for maintaining the gas phase super-saturation necessary for 1D growth. High-yield synthesis of elemental (Ge) and compound semiconductors (SnO2, Fe3O4, V2O5, In2O3) was performed by the chemical vapor deposition of appropriate metal-organic precursors. Axial and radial dimensions of the NWs were varied by adjusting the precursor feedstock, deposition temperature, and catalyst size. Finally, the device potential of these building blocks as photo- and gas sensors was investigated by integrating individual nanowires in electrical circuits using focussed ion beam (FIB) assisted nano-lithography.

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO3 sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M(H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field (HE=5590 kOe), anisotropy field (HA=0.5 kOe) and Dzyaloshinsky-Moriya antisymmetric field (HD=149 kOe) are in good agreement with previous reports on this system.

We investigate the formation of ferrihydrite nanoparticles (NPs) by hydrolysis of the Fe(III) alkoxide Fe(OtBu)3. Controlled amounts of water, up to 3.0 vol%, were added to the precursor solution yielding a series of hydrolyzed samples ranging from P0.0 (the unreacted precursor) to P3.0. X-ray diffraction (XRD) analysis evidenced the formation of high-crystalline ferrihydrite NP in sample P3.0, with grain size estimate of about 3.2 nm. The transition from the molecular precursor to the formation of crystalline magnetic NPs was followed through magnetization measurements M(T) and M(H), as well as Mössbauer spectroscopy (MS). M(T) measurements indicate a paramagnetic (PM) behavior for sample P0.0, characteristic of binuclear Fe-O-Fe units, which evolves to a superparamagnetic (SPM) behavior, with an energy barrier for the blocking process estimated for sample P3.0 as Ea=4.9×10-21 J (Ea/kB=355 K), resulting in a high effective anisotropy constant Keff=290 kJ/m3. Magnetization loops at 5 K progressively change from PM-like to ferromagnetic-like shape upon increasing the hydrolysis process, although hysteresis (Hc≈500 Oe) only is apparent for P2.0 and higher. MS spectra at room temperature are PM/SPM doublets for all samples, while the MS spectra at T=4.2 K reveal increasingly well-defined magnetic ordering as hydrolysis of the precursor stepwise progresses until well-crystallized ferrihydrite particles are formed.

This work focuses on the preparation of novel ternary transparent conducting oxide coatings on glass by the sot-gel method. The coatings were deposited by spin-coating from solutions of appropriate metal precursors and heat-treated at different heat-treatment procedures. An increase in electrical conductivity was achieved by a final forming gas treatment. Best electrical and optical properties have been obtained for coatings of crystalline Zn2SnO4, Zn3In2O6 and Zn5In2O8 and X-ray amorphous ZnSnO3 with resistivities in the order of 10-2-10-1 Ωcm, an average transmission in the visible of 85% and an average surface roughness of similar to 1 nm. ZnGa2O4 and GaSbO4 coatings showed no electrical conductivity. For Zn2SnO4 coatings, a restricted crystallite growth was observed probably due to phase segregation effects. Electrical properties of coatings in the system ZnO-In2O3 were interpreted on the basis of the percolation theory.

The molecular structure of trans-dichlorodioxobis(triphenylphosphate) molybdenum(VI), MoO2Cl2[OP(OPh)3]2 has been determined. Crystal data: Monoclinic, Pn, a = 11.767(2), b = 10.341(2), c = 15.682(3) Å, β = 92.27(3)°, V = 1906.8(7) Å-3, Z = 2. Trans-dichlorodioxobis(triphenylphosphate)molybdenum(VI) was obtained by the reaction of molybdenum oxide, HCl and triphenylphosphate and was characterized by elemental analysis, IR, and 1H-NMR spectroscopy.

The dinuclear gold complexes [{Au(PPh3)}2(µ-dmid)] (1) (dmid) 1,3-dithiole-2-one-4,5-dithiolate) and [{Au(PPh3)}2(µ-dddt)] (2) (dddt) 5,6-dihydro-1,4-dithiine-2,3-dithiolate) were synthesized and characterized by X-ray crystallography. Both complexes exhibit intramolecular aurophilic interactions with Au · · · Au distances of 3.1984(10) Å for 1 and 3.1295(11) Å for 2. A self-assembly reaction between 4,5-bis(2-hydroxyethylthio)-1,3-dithiole-2-thione ((HOCH2CH2)2dmit) and [AuCl(tht)] affords the complex [AuCl{(HOCH2CH2)2dmit}]2 (4), which possesses an antiparallel dimeric arrangement resulting from a short aurophilic contact of 3.078(6) Å. This motif is extended into two dimensions due to intra- and intermolecular hydrogen bonds via the hydroxyethyl groups, giving rise to a supramolecular network. Three compounds were investigated for their rich photophysical properties at 298 and 77 K in 2-MeTHF and in the solid state; [Au2(µ-dmid)(PPh3)2] (1), [Au2(µ-dddt)(PPh3)2] (2), and [AuCl{(HOCH2CH2)2dmit}] (4). 1 exhibits relatively long-lived LMCT (ligand-to-metal charge transfer) emissions at 298 K in solution (370 nm; τe ∼17 ns, where M is a single gold not interacting with the other gold atom; i.e., the fluxional C-SAuPPh3 units are away from each other) and in the solid state (410 nm; τe ∼70 µs). At 77 K, a new emission band is observed at 685 nm (τe = 132 µs) and assigned to a LMCT emission where M is representative for two gold atoms interacting together consistent with the presence of Au · · · Au contacts as found in the crystal structure. In solution at 77 K, the LMCT emission is also red-shifted to 550 nm (τe ∼139 µs). It is believed to be associated to a given rotamer. 2 also exhibits LMCT emissions at 380 nm at 298 K in solution and at 470 nm in the solid state. 4 exhibits X/MLCT emission (halide/metal to ligand charge transfer) where M is a dimer in the solid state with obvious Au · · · Au interactions, resulting in red-shifted emission band, and is a monomer in solution in the 10-5 M concentration (i.e., no Au · · · Au interactions) resulting in blue-shifted luminescence. Both fluorescence and phosphorescence are observed for 4.