Scientific Highlights 2016

1 December 2016

Pressure and temperature dependence of the laser-induced plasma plume dynamics

The influence of different background gases and substrate heating on the plasma plume dynamics from silver ablation is investigated by species selected time and space resolved imaging. The results provide a time-resolved understanding on how those process parameters affect the expansion: from a free expansion in vacuum with velocities exceeding 20'000 m/s to a very slow expansion in Ar at 1 × 10−1 mbar with arrival velocities of 280 m/s. In addition, we observe a rebound of the ablated material on the substrate holder leading to a re-coating of the ablated target. At 1 × 10−1 mbar, it seems that the expansion of the plasma plume displaces a considerable portion of the background gas and traps it against the frontal area of the substrate holder. This leads to a transient high local pressure just above the substrate. In the case of Ar, the rebound is enhanced due to inelastic scattering, whereas for an O2 background, an area of high reactivity/emission in addition to the rebound is created. Imaging of selected species shows that the light emission in this area is mainly due to excited Ag and metal oxygen species. There is a clear influence of substrate heating on the plasma expansion due to the background gas density gradients, reducing the stopping ability of the background gas and already detectable 2 cm away from the substrate. Both rebound and excitation effects are reduced in intensity due to the substrate heating.

This video clip shows the rebound of Ag at room temperature and 1x10-1 mbar Ar background pressure. The target-substrate distance is 4cm and the total recorded time approx. 60µsec.
Keywords: pulsed laser deposition; laser induced plasma; time-resolved optical spectroscopy; rebound effect;

Facility: ENE, LMX, Thin Films and Interfaces

Reference: A. Ojeda et al, J. Appl. Phys. 120, 225301 (2016)

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1 November 2016

Structure and Conductivity of Epitaxial Thin Films of In-Doped BaZrO3‑Based Proton Conductors

Epitaxial thin films of the proton-conducting perovskite BaZr0.53In0.47O3−δH0.47−2δ, grown by pulsed laser deposition, were investigated in their hydrated and dehydrated conditions through a multitechnique approach with the aim to study the structure and proton concentration depth profile and their relationship to proton conductivity. The techniques used were X-ray diffraction, X-ray and neutron reflectivity, nuclear reaction analysis, and Rutherford backscattering, together with impedance spectroscopy. The obtained proton conductivity and activation energy are comparable to literature values for the bulk conductivity of similar materials, thus showing that grain-boundary conductivity is negligible due to the high crystallinity of the film. The results reveal an uneven proton concentration depth profile, with the presence of a 3−4 nm thick, proton-rich layer with altered composition, likely characterized by cationic deficiency. While this surface layer either retains or reobtains protons after desorption and cooling to room temperature, the bulk of the film absorbs and desorbs protons in the expected manner. It is suggested that the protons in the near-surface, protonrich region are located in proton sites characterized by relatively strong O−H bonds due to weak hydrogen-bond interactions to neighboring oxygen atoms and that the mobility of protons in these sites is generally lower than in proton sites associated with stronger hydrogen bonds. It follows that strongly hydrogen-bonding configurations are important for high proton mobility.
Keywords: pulsed laser deposition; proton conductors; neutron reflectometry; proton uptake;

Facility: ENE, LMX, Thin Films and Interfaces

Reference: L. Mazzei et al, J. Phys. Chem. C 120, 28415−28422 (2016)

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27 September 2016

Investigating the Role of Strain toward the Oxygen Reduction Activity on Model Thin Film Pt Catalysts

Environmentally friendly energy conversion devices such as fuel cells are becoming more and more attractive. However, major impediments to large-scale application still arise on the material side, related to the cost and poor performance of the cathode catalyst. State-of-the-art electrocatalysts are all Pt-based materials, suffering from poor electrochemical oxygen reduction kinetics. Tuning the interatomic distance of Pt atoms represents a promising strategy for reducing the strength of adsorption of oxygenated species to the Pt surface and thus improving the kinetics. In this context, model Pt electrocatalysts of strain-induced varied interatomic spacing were fabricated and tested. Strained Pt films with high crystalline quality can be obtained via epitaxial growth on appropriate single-crystal substrates like (111) SrTiO3, which have lattice parameters different from those of Pt using pulsed laser deposition. Through a proper selection of deposition parameters, the extent of strain in the Pt films can be controlled. This study shows that strain significantly modifies the electrochemical surface properties. In particular, cyclic voltammetry and CO oxidation experiments provide valuable insights into the effect of strain on the adsorption properties of spectator species (e.g., OHad and bisulfates) relevant for oxygen reduction reaction (ORR) kinetics. Furthermore, the strained Pt films exhibit a remarkably higher oxidation reduction reaction activity compared to that of the fully relaxed bulk structure as obtained from ORR polarization curves. This research highlights the importance of proper model systems with defined physical properties to establish design principles for better-performing catalysts.
Keywords: electrocatalysis; model catalyst; oxygen reduction reaction; platinum; pulsed laser deposition; strain; thin films

Facility: ENE, Electrochemistry; LMX, Thin Films and Interfaces

Reference: Sandra E. Temmel, Emiliana Fabbri, Daniele Pergolesi, Thomas Lippert, and Thomas J. Schmidt, ACS Catalysis 6, 7566 (2016)

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22 August 2016

Coexisting multiple order parameters in single-layer LuMnO3 films

Magnetoelectric multiferroics hold great promise for electrical control of magnetism or magnetic control of ferroelectricity. However, single phase ferroelectric materials with a sizeable ferromagnetic magnetization are rare. Here, we demonstrate that a single-phase orthorhombic LuMnO3 thin film features coexisting magnetic and ferroelectric orders. The temperature dependence of the different order parameters are presented with ferromagnetic order appearing below 100 K and thus at much higher temperatures than ferroelectricity or antiferromagnetism (TN, TFE ≤ 40K).
Keywords: magnetism; ferroelectricity; multiferroics; neutron scattering; pulsed laser deposition; strain; thin films

Facility: SINQ; LNS; LDM; LMU; LMX, Thin Films and Interfaces

Reference: C.W. Schneider et al, Phys. Rev. B 94, 054423 (2016)

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27 April 2016

Coexisting multiple order parameters in single-layer LuMnO3 films

Gas sensors based on tin oxide (SnO2) and palladium doped SnO2 (Pd:SnO2) active materials are fabricated by a laser printing method, i.e. reactive laser-induced forward transfer (rLIFT). Thin films from tin based metal-complex precursors are prepared by spin coating and then laser transferred with high resolution onto sensor structures. The devices fabricated by rLIFT exhibit low ppm sensitivity towards ethanol and methane as well as good stability with respect to air, moisture, and time. Promising results are obtained by applying rLIFT to transfer metal-complex precursors onto uncoated commercial gas sensors. We could show that rLIFT onto commercial sensors is possible if the sensor structures are reinforced prior to printing. The rLIFT fabricated sensors show up to 4 times higher sensitivities then the commercial sensors (with inkjet printed SnO2). In addition, the selectivity towards CH4 of the Pd:SnO2 sensors is significantly enhanced compared to the pure SnO2 sensors. Our results indicate that the reactive laser transfer technique applied here represents an important technical step for the realization of improved gas detection systems with wide-ranging applications in environmental and health monitoring control.
Keywords: gas sensors; laser printing; reactive laser-induced forward transfer; environmental and health monitoring control;

Facility: ENE; LMX, Thin Films and Interfaces

Reference: Alexandra Palla Papavlu, Thomas Mattle, Sandra Temmel, Ulrike Lehmann, Andreas Hintennach, Alain Grisel, Alexander Wokaun & Thomas Lippert, Scientific Reports 6, Article number: 25144 (2016)

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25 February 2016

In situ stress observation in oxide films and how tensile stress influences oxygen ion conduction

Many properties of materials can be changed by varying the interatomic distances in the crystal lattice by applying stress. Ideal model systems for investigations are heteroepitaxial thin films where lattice distortions can be induced by the crystallographic mismatch with the substrate. Here we describe an in situ simultaneous diagnostic of growth mode and stress during pulsed laser deposition of oxide thin films. The stress state and evolution up to the relaxation onset are monitored during the growth of oxygen ion conducting Ce0.85Sm0.15O2-δ; thin films via optical wafer curvature measurements. Increasing tensile stress lowers the activation energy for charge transport and a thorough characterization of stress and morphology allows quantifying this effect using samples with the conductive properties of single crystals. The combined in situ application of optical deflectometry and electron diffraction provides an invaluable tool for strain engineering in Materials Science to fabricate novel devices with intriguing functionalities.
Keywords: strain; thin films; pulsed laser deposition; optical wafer curvature measurements; oxygen ion conductors;

Facility: ENE; LMX, Thin Films and Interfaces

Reference: Aline Fluri, Daniele Pergolesi, Vladimir Roddatis, Alexander Wokaun & Thomas Lippert, Nature Communications 7, 064421 (2016)

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