Markus Winterer

Luminescence properties of nanocrystalline Y2O3:Eu3+ in different host materials

In this study, the optical properties of nanocrystalline europium doped yttria, Y2O3:Eu3+ were investigated in dependence on different caging hosts such as porous MCM-41, porous silica, and porous alumina with pore sizes ranging between 2.7 to 80 nm. These results were compared to nanopowders measured in air and aqueous solution whose particle sizes were 5 nm and 8 nm, respectively. All these results were compared to a commercial lamp phosphor powder with a grain size of about 5 μm. The structural properties of the samples were determined by x-ray diffraction and transmission electron microscopy. Investigated optical properties are the photoluminescence emission spectra, the excitation spectra, the lifetimes, and the quantum efficiencies. A heavy dependence of the charge transfer process on the surrounding will be reported and discussed.

Electronic Impurity Doping in CdSe Nanocrystals

We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core-shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping.

Nanocrystalline titania films and particles by chemical vapor synthesis

Thick crystalline titania films with grain sizes below 30 nm, as well as nanocrystalline titania particles with sizes below 10 nm, are prepared by a modified CVD method called chemical vapor synthesis (CVS). Pyrolysis of titanium tetraisopropoxide (TTIP) is performed in a hot-wall reactor, in a mixed helium/oxygen atmosphere, using liquid precursor delivery. Film thicknesses of up to 150 μm on silicon (100) substrate materials are achieved. The influence of process parameters (particularly reactor position) on the microstructure and growth rate is examined. Process temperatures, starting at film-forming (CVD) conditions and going up to exclusive particle formation (CVS) are investigated. Grain size, particle size, and crystallographic phases are determined by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Synthesis and local structure of doped nanocrystalline zinc oxides

Nanocrystalline zinc oxides are produced by chemical vapor synthesis and characterized by x-ray diffraction, high resolution transmission electron microscopy, nitrogen sorption, and extended x-ray absorption fine structure. Dopant elements (aluminum, gallium, and indium) influence the particle size of the powders as well as lattice parameters and local structure. The different effects of the three dopant elements are characterized.

Rietveld analysis of electron powder diffraction data from nanocrystalline anatase, TiO2

The structure of nanocrystalline anatase (TiO2) was successfully refined from electron powder diffraction data using the Rietveld technique. A polycrystalline sample (average crystal size about 70 A) was characterised by selected area electron diffraction in a conventional transmission electron microscope operated at 300 kV. Radially integrated intensities were extracted from digitised photographic films and used in the course of structure refinements by a standard program for Rietveld analysis. The structure was refined in space group I4(1)/amd (#141) with lattice parameters a = 3.7710(9) A and c = 9.430(2) A. The reliability factors of the refinement are Rwp = 5.2% and R(B) = 2.6%. The close agreement of the refined structural parameters with previous results obtained from neutron diffraction on coarse-grained powders proves the applicability of the method for characterising nanocrystalline powders. The present study shows that Rietveld analysis on electron powder data is a good compliment to the existing methods for accurate structural investigations on nanocrystalline materials and thin films.

Reduced-Pressure Chemical Vapor Synthesis of Nanocrystalline Silicon Carbide Powders

Nanocrystalline β-SiC powders with grain and particle sizes well below 10 nm are prepared by thermal decomposition of silicon organic precursors under reduced pressure conditions. This modified CVD process is called chemical vapor synthesis (CVS). The influence of synthesis parameters on powder characteristics is investigated. Small grain and particle sizes together with low agglomeration and high crystallinity are achieved for tetramethylsilane as precursor, total pressures below or equal to 1000 kPa, reaction temperatures above 1400 K, and precursor partial pressures below or equal to 660 Pa.

One-step flame synthesis of SnO2 / TiO2 composite nanoparticles for photocatalytic applications

SnO2 / TiO2 composite nanoparticles have been synthesized in a single-step by feeding evaporated precursor mixtures into an atmospheric pressure diffusion flame. Particles with controlled Ti: Sn ratios were produced at various flow rates of oxygen, and the resulting powders were characterized by BET surface area analysis, XRD, TEM, EDAX and UV-Vis spectroscopy. For the lowest concentration (3.4 mol %) of SnO2 employed in this study anatase phase of TiO2 is stabilized, while segregation of SnO2 is seen at medium (6.9 to 12.4 mol %) and high concentrations (20.3 mol %). Though the equilibrium phase diagram predicts complete solubility of one oxide in another at all compositions, segregation of SnO2 phase is observed which is explained by the usage of diffusion flame in the present study. The particle formation mechanism of SnO2 / TiO2 composites is proposed basing on the single component aerosol formation. Photocatalytic activity of the composite particles is tested for the degradation of methylene blue and is compared with pure TiO2 synthesized under similar conditions. Improved photocatalytic activity of the composite particles is attributed to the stabilized anatase phase and better charge separation due to the coupling of TiO2 and SnO2 within the composite nanoparticles.

Electrical properties of aluminum-doped zinc oxide (AZO) nanoparticles synthesized by chemical vapor synthesis.

Aluminum-doped zinc oxide nanoparticles have been prepared by chemical vapor synthesis, which facilitates the incorporation of a higher percentage of dopant atoms, far above the thermodynamic solubility limit of aluminum. The electrical properties of aluminum-doped and undoped zinc oxide nanoparticles were investigated by impedance spectroscopy. The impedance is measured under hydrogen and synthetic air between 323 and 673 K. The measurements under hydrogen as well as under synthetic air show transport properties depending on temperature and doping level. Under hydrogen atmosphere, a decreasing conductivity with increasing dopant content is observed, which can be explained by enhanced scattering processes due to an increasing disorder in the nanocrystalline material. The temperature coefficient for the doped samples switches from positive temperature coefficient behavior to negative temperature coefficient behavior with increasing dopant concentration. In the presence of synthetic air, the conductivity firstly increases with increasing dopant content by six orders of magnitude. The origin of the increasing conductivity is the generation of free charge carriers upon dopant incorporation. It reaches its maximum at a concentration of 7.7% of aluminum, and drops for higher doping levels. In all cases, the conductivity under hydrogen is higher than under synthetic air and can be changed reversibly by changing the atmosphere.

Photoluminescence properties of nanocrystalline Y2O3:Eu3+ in different environments

Abstract In this study the optical properties of nanocrystalline europium doped yttria Y2O3:Eu3+ were investigated in dependence on different environments such as a silica based molecular sieve MCM-41, with a pore sizes of 2.7nm, a free nanopowder on air and in aqueous solution. All these results were compared to a commercial lamp phosphor powder with a grain size of about 5μm. Investigated optical properties are the photoluminescence emission spectra, the excitation spectra, the lifetimes and the quantum efficiencies. Heavy differences of the change transfer process will be reported and discussed with respect to the structure. A low optical quantum efficiency of the nanoparticles is explained by changes in the crystal field.

Reverse Monte Carlo analysis of extended x-ray absorption fine structure spectra of monoclinic and amorphous zirconia

Extended x-ray absorption fine structure spectra of monoclinic zirconia (m-ZrO2) and of amorphous zirconia (a-ZrO2) in form of a thin film are analyzed by reverse Monte Carlo simulations. Partial pair distribution functions (PDF, gZr–O and gZr–Zr) are extracted. Coordination numbers and distances determined for m-ZrO2 are identical to results of structural analysis by diffraction. The local structure of a-ZrO2 consists of a widely spread Zr–O shell (from 1.9 to 3.3 A) corresponding to an eightfold oxygen coordination around zirconium ions. The first peak consists of four nearest oxygen neighbors at 2.15 A. The Zr–Zr PDF of a-ZrO2 displays a broad peak between 3.2 and 5.2 A consisting of twelve zirconium next nearest neighbors at an average distance of about 4 A and weak density fluctuations at larger coordination distances.