Thomas Lippert

Polymers designed for laser microstructuring

Abstract Several polymers were tested for an application of laser ablation as an alternative to photolithography. Tailored specialty polymers revealed the highest sensitivity to laser ablation. The most sensitive polymers are based on the photolabile triazene chromophore (NNN) which is unfortunately also sensitive to other processing steps, e.g. wet etching. Polymers with a cinnamylidenemalonic acid ester group, showed not only a high sensitivity but also stability to wet etching, high quality film-formation properties and high resolution ablation structures. This proves that it is possible to apply laser ablation as alternative technique to photolithography, if the polymers are especially designed for laser ablation.

LaTiOxNy Thin Film Model Systems for Photocatalytic Water Splitting: Physicochemical Evolution of the Solid-Liquid Interface and the Role of the Crystallographic Orientation

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. Intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention: One is the critical issue of compositional/structural surface modifications that occur during operation and how these affect photoelectrochemical performance. The second concerns the relation between electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance, since it is exactly at the surface where the visible-light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art oxynitride thin film fabrication and characterization, before focusing on LaTiO2N, selected as a representative photocatalyst. An investigation of the initial physicochemical evolution of the surface is reported. Then, it is shown that after stabilization the absorbed photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations, and be up to 5 times larger than for polycrystalline samples.

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 selec...

Tracing the origin of oxygen for La0.6Sr0.4MnO3 thin film growth by pulsed laser deposition

We report on the semi-quantitative analysis of pulsed laser induced plasma species as well as thin film compositions of La0.6Sr0.4MnO3 grown on SrTiO3 substrates under various background pressure regimes using an 18O isotope labelled La0.6Sr0.4MnO3 target. The importance of negative metal–oxygen or positive metal–oxygen ions to influence the final oxygen composition of the thin film is illustrated through the use of mass spectrometry, where the chemical reactions between the laser ablated target species with the oxygen background molecules are directly characterized. We find that the influence of metal–oxygen negative ions is not as important as the metal–oxygen positive ions to the final oxygen composition of the LSMO film, due to their low stability in high background partial pressures. Furthermore, we observe that the oxygen incorporated in La0.6Sr0.4MnO3 thin films coming from the target is ~44%, 29% and 1% at 2 × 10−3 mbar, 1 × 10−2 mbar and 2 × 10−1 mbar, respectively. When growing films at 10−1 mbar on 18O2 exchanged substrates, almost all oxygen originates from the background and almost none from the substrate or target.

Cation ratio and ferroelectric properties of TbMnO3 epitaxial films grown by pulsed laser deposition

The cation ratio, crystal structure, and ferroelectric properties of TbMnO3 films on (010) oriented YAlO3 substrates grown by pulsed laser deposition were investigated. We found that the cation ratio is altered by the background gas pressure, and that stoichiometric films have larger critical thicknesses than Tb-rich films. The ferroelectric order was severely disturbed in Tb-rich films showing a lower transition temperature and a smaller polarization. In addition, frequency dispersion was observed in the dielectric response of relaxed stoichiometric TbMnO3 films, which may be introduced by defects due to strain relaxation.

Interface Effects on the Ionic Conductivity of Doped Ceria-Yttria-Stabilized Zirconia Heterostructures

Multilayered heterostructures of Ce0.85Sm0.15O2-δ and Y0.16Zr0.92O2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.

Determination of Conduction and Valence Band Electronic Structure of LaTiOxNy Thin Film

The nitrogen substitution into the oxygen sites of several oxide materials leads to a reduction of the band gap to the visible-light energy range, which makes these oxynitride semiconductors potential photocatalysts for efficient solar water splitting. Oxynitrides typically show a different crystal structure compared to the pristine oxide material. As the band gap is correlated to both the chemical composition and the crystal structure, it is not trivial to distinguish which modifications of the electronic structure induced by the nitrogen substitution are related to compositional and/or structural effects. Here, X-ray emission and absorption spectroscopy are used to investigate the electronic structures of orthorhombic perovskite LaTiOx Ny thin films in comparison with films of the pristine oxide LaTiOx with similar orthorhombic structure and cationic oxidation state. Experiment and theory show the expected upward shift in energy of the valence band maximum that reduces the band gap as a consequence of the nitrogen incorporation. This study also shows that the conduction band minimum, typically considered almost unaffected by nitrogen substitution, undergoes a significant downward shift in energy. For a rational design of oxynitride photocatalysts, the observed changes of both the unoccupied and occupied electronic states have to be taken into account to justify the total band-gap narrowing induced by the nitrogen incorporation.

Direct laser deposition of nanostructured tungsten oxide for sensing applications

Nanostructured tungsten trioxide (WO 3 ) thin films are deposited by pulsed laser deposition (PLD) and radio-frequency (RF) assisted PLD onto interdigitated sensor structures. Structural characterization by x-ray diffraction and Raman spectroscopy shows the WO 3 films are polycrystalline, with a pure monoclinic phase for the PLD grown films. The as-fabricated WO 3 sensors are tested for ammonia (NH 3 ) detection, by measuring the electrical response to NH 3 at different temperatures. Sensors based on WO 3 deposited by RF-PLD do not show any response to NH 3 . In contrast, sensors fabricated by PLD operating at 100 °C and 200 °C show a slow recovery time whilst at 300 °C, these sensors are highly sensitive in the low ppm range with a recovery time in the range of a few seconds. The microstructure of the films is suggested to explain their excellent electrical response. Columnar WO 3 thin films are obtained by both deposition methods. However, the WO 3 films grown by PLD are porous, (which may allow NH 3 molecules to diffuse through the film) whereas RF-PLD films are dense. Our results highlight that WO 3 thin films deposited by PLD can be applied for the fabrication of gas sensors with a performance level required for industrial applications.

Polymers with Triazene Units in the Main Chain

Several monomeric and polymeric triazenes have been synthesized and characterized by common methods such as 1H NMR-, IR-, Raman-, UV-spectroscopy, DSC, and GPC. Their thermolysis, photosensitivity, and the effect of H3O+ on their stability have been studied in detail, since those characteristics are very important for an application of these compounds in microlithography.