Herbert Hutter

Relationship between Cation Segregation and the Electrochemical Oxygen Reduction Kinetics of La0.6Sr0.4CoO3−δ Thin Film Electrodes

Pulsed laser deposited La 0.6 Sr 0.4 CoO 3―δ (LSC) thin film electrodes on yttria stabilized zirconia (YSZ) single crystals were investigated by impedance spectroscopy, time of flight secondary ion mass spectrometry (ToF-SIMS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Effects caused by different film deposition temperatures, thermal annealing and chemical etching were studied. Correlations between changes in electrode polarization resistance of oxygen reduction and surface composition were found. At high deposition temperatures and after thermal annealing an inhomogeneous cation distribution was detected in the surface-near region, most manifest in a significant Sr enrichment at the surface. An activating effect of chemical etching of LSC is described, which can lower the polarization resistance by orders of magnitude. Chemistry behind this activation and thermal degradation was analyzed by ToF-SIMS and ICP-OES measurements of in-situ etched LSC films. The latter allow quantitative depth resolved compositional analysis with nominally sub nm resolution. High resolution scanning electron microscopy images illustrate the accompanying changes in surface morphology. All measurements suggest that stoichiometric LSC surfaces intrinsically exhibit very high activity towards oxygen reduction.

Tensile Lattice Strain Accelerates Oxygen Surface Exchange and Diffusion in La1–xSrxCoO3−δ Thin Films

The influence of lattice strain on the oxygen exchange kinetics and diffusion in oxides was investigated on (100) epitaxial La1–xSrxCoO3−δ (LSC) thin films grown by pulsed laser deposition. Planar tensile and compressively strained LSC films were obtained on single-crystalline SrTiO3 and LaAlO3. 18O isotope exchange depth profiling with ToF-SIMS was employed to simultaneously measure the tracer surface exchange coefficient k* and the tracer diffusion coefficient D* in the temperature range 280–475 °C. In accordance with recent theoretical findings, much faster surface exchange (∼4 times) and diffusion (∼10 times) were observed for the tensile strained films compared to the compressively strained films in the entire temperature range. The same strain effect—tensile strain leading to higher k* and D*—was found for different LSC compositions (x = 0.2 and x = 0.4) and for surface-etched films. The temperature dependence of k* and D* is discussed with respect to the contributions of strain states, formation enthalpy of oxygen vacancies, and vacancy mobility at different temperatures. Our findings point toward the control of oxygen surface exchange and diffusion kinetics by means of lattice strain in existing mixed conducting oxides for energy conversion applications.

Wavelet denoising of Gaussian peaks: A comparative study

Abstract In this paper we apply some recent results on non-linear wavelet analysis to simulated noisy signals of chemical interest. In particular, we compare the wavelet soft universal thresholding algorithm described by Donoho, to Fourier filters and to polynomial smoothers such as the Savitzky-Golay filters (SG). All reconstruction filters were evaluated on the basis of three different criteria: the mean squared error (MSE) both for the whole signal and for an interval centred around the peak, the signal-to-noise ratio (SNR) improvement and the change in the peak area. The simulated data consists of narrow Gaussian peaks with white noise. Signals with low SNR were investigated, since this is a challenging problem for each reconstruction filter. Four common wavelets (Haar, Daubechies, Symmlets and Coiflets) were selected for the wavelet denoising. Our results show that under the chosen conditions wavelet denoising (WD) gives in most cases superior performance over classical filter techniques.

Cation diffusion in La0.6Sr0.4CoO3−δ below 800 °C and its relevance for Sr segregation

Cation diffusion was investigated in La0.6Sr0.4CoO3-δ (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 °C. Isotopic ((86)Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi- and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains - diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size. A- and B-site cations showed very similar temperature dependencies with activation energies of ∼3.5 eV for bulk and ∼4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.

DE-NOISING OF SIMS IMAGES VIA WAVELET SHRINKAGE

Abstract Two-dimensional element distributions generated by scanning secondary ion mass spectrometry (SIMS) are characterised by Poisson statistics of small integer values, specially when the concentration of the measured element is in the sub-ppm range. To achieve high signal-to-noise ratios extremely long measurement time is needed. Because of the signal fluctuations from one measurement point to the next, structures even larger than the resolution of the instrument may not be detectable if the differences in concentration are small. This paper reports the application of a wavelet shrinkage algorithm for de-noising of images following Poisson distribution. In reconstructions of SIMS images resulting from this algorithm the noise is significantly suppressed without great loss of lateral resolution.

A novel ToF-SIMS operation mode for improved accuracy and lateral resolution of oxygen isotope measurements on oxides

Oxygen isotope exchange with subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a highly valuable tool for determining oxygen diffusion coefficients in oxides. Since ToF-SIMS analysis enables an elemental and chemical mapping, it can also be used to visualize oxygen exchange-active zones by determining the local oxygen isotopic fraction. However, measuring accurate isotopic fractions can be a challenging analytical task owing to secondary ion interaction and signal saturation, particularly when dealing with high secondary ion intensities as commonly found when analyzing oxygen ions from oxides. It is shown that in many cases the calculated 18O− fraction erroneously shifts to higher values and can lead to systematic errors in the determination of diffusion coefficients. A novel ToF-SIMS operation mode, called “Collimated Burst Alignment” (CBA) mode, is therefore introduced to enable a more accurate determination of oxygen isotopic fractions with an optimized lateral resolution of sub-100 nm. Both improvements are rendered possible by a modified beam guidance in the primary ion gun. This modification reduces detector dead time effects and ion interactions to a minimum and secondary ion intensities can be obtained more accurately. The result of this optimization is demonstrated in measurements of the natural isotope abundance of several different oxides including SrTiO3 and Sr-doped LaCoO3.

Investigations of the incorporation of B, P and N into CVD-diamond films by secondary ion mass spectrometry

Abstract B-, P- and N-doped polycrystalline diamond films have been produced using the hot-filament CVD method by the addition of triethylborane, phosphine and molecular nitrogen. The doping level — not aimed at semiconducting applications but to investigate the influence of the addition on diamond crystal growth and the formation of new phases containing the doping elements — was investigated by depth-profiling secondary-ion mass spectrometry (SIMS). The dopant concentrations, which seemed to be linearly related to the dopant gas concentrations, were quantified using ion-implantation standards and compared to nuclear reaction analysis (NRA) results. In the investigated doping region, boron (0.4–2.8 at.%) is enriched during the deposition by a factor of six to nine, phosphorus (10–600 ppma) (ppm atomic) shows a doping efficiency below one, and nitrogen is incorporated below about 100 ppma.

Three dimensional ultra trace analysis of materials

We have developed a new imaging system for secondary ion mass spectrometry, including a new interface to control all functional units of the CAMECA IMS 3f instrument, especially the high voltage channel plate. Use of a 386 PC (HP Vectra RS-25) made a new 20-bit magnetic field control, a new counting board with higher dynamic range and a new sample position unit possible. A double channel plate enables us to detect single ions with a sensitive CCD camera.An Imaging Technology 151 image processor digitizes and accumulates camera data. During summation the image processor detects the brightest and darkest pixel in the channel plate picture, thus channel plate high voltage may be dynamically controlled according to the intensity of the secondary ion signal. This results in fully automatic measurement of unknown samples with large variations in the lateral and depth concentration of elements. A dynamic range for measurement of secondary ion intensities of 108 can be achieved.Software written in C controls the image processor, the channel plate high voltage and all other parts of the instrument, and has a user friendly interactive interface. To visualise multidimensional data (three dimensional distribution of more than one element) a software package was written which allows to correlate elemental distributions.

Bulk and surface characterization of In2O3(001) single crystals

A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 +/- 0.5 x 10(5) Omega cm, which translates into a carrier concentration of approximate to 10(12) cm(-3). The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) angstrom from the previously accepted value of 10.117 angstrom. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 degrees C) surface show a step height of 5 angstrom, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 angstrom height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (E-F). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

Optical Er-doping of Si during sublimational molecular beam epitaxy

We report the first application of sublimation molecular beam epitaxy to grow uniformly and selectively doped Si : Er layers with a high concentration of Er and O (up to ∼ 10 19 cm -3 ) and high quality of crystalline structure. All samples exhibit photoluminescence close to 1.54 μm up to 200 K. New optical active Er center with their main lines at 6502, 6443, 6393, 6342, 6337 and 6268 cm -1 was observed in the PL spectra.