Christof W. Schneider

Microfabrication of polystyrene microbead arrays by laser induced forward transfer

In this study we describe a simple method to fabricate microarrays of polystyrene microbeads (PS-μbeads) on Thermanox coverslip surfaces using laser induced forward transfer (LIFT). A triazene polymer layer which acts as a dynamic release layer and propels the closely packed microspheres on the receiving substrate was used for this approach. The deposited features were characterized by optical microscopy, scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. Ultrasonication was used to test the adherence of the transferred beads. In addition, the laser ejection of the PS-μbead pixels was investigated by time resolved shadowgraphy. It was found that stable PS-μbeads micropatterns without any specific immobilization process could be realized by LIFT. These results highlight the increasing role of LIFT in the development of biomaterials, drug delivery, and tissue engineering.

Tuning the multiferroic mechanisms of TbMnO3 by epitaxial strain

A current challenge in the field of magnetoelectric multiferroics is to identify systems that allow a controlled tuning of states displaying distinct magnetoelectric responses. Here we show that the multiferroic ground state of the archetypal multiferroic TbMnO3 is dramatically modified by epitaxial strain. Neutron diffraction reveals that in highly strained films the magnetic order changes from the bulk-like incommensurate bc-cycloidal structure to commensurate magnetic order. Concomitant with the modification of the magnetic ground state, optical second-harmonic generation (SHG) and electric measurements show an enormous increase of the ferroelectric polarization, and a change in its direction from along the c- to the a-axis. Our results suggest that the drastic change of multiferroic properties results from a switch of the spin-current magnetoelectric coupling in bulk TbMnO3 to symmetric magnetostriction in epitaxially-strained TbMnO3. These findings experimentally demonstrate that epitaxial strain can be used to control single-phase spin-driven multiferroic states.

Langmuir probe measurements and mass spectrometry of plasma plumes generated by laser ablation of La0.4Ca0.6MnO3

The plasma formed in vacuum by UV nanosecond laser ablation of La0.4Ca0.6MnO3 in the fluence range of 0.8 to 1.9u2009J cm−2 using both Langmuir probe analysis and energy-resolved mass spectrometry has been studied. Mass spectrometry shows that the main positive ion species are Ca+, Mn+, La+, and LaO+. The Ca+ and Mn+ energy distributions are quite broad and lie in the 0–100u2009eV region, with the average energies increasing with laser fluence. In contrast, the La+ and LaO+ distributions are strongly peaked around 10u2009eV. The net time-of-arrival signal derived from the measured positive ion energy distributions is broadly consistent with the positive ion signal measured by the Langmuir probe. We also detected a significant number of O− ions with energies in the range of 0 to 10u2009eV. The Langmuir probe was also used to measure the temporal variation of the electron density and temperature at 6u2009cm from the ablation target. In the period when O− ions are found at this position, the plasma conditions are consistent wit...

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.

Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering

Abstract Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO3 and SrTiO3 grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO2 and 8 mol% Y2O3 stabilized ZrO2 are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity.

Tracing the plasma interactions for pulsed reactive crossed-beam laser ablation

Pulsed reactive crossed-beam laser ablation is an effective technique to govern the chemical activity of plasma species and background molecules during pulsed laser deposition. Instead of using a constant background pressure, a gas pulse with a reactive gas, synchronized with the laser beam, is injected into vacuum or a low background pressure near the ablated area of the target. It intercepts the initially generated plasma plume, thereby enhancing the physicochemical interactions between the gaseous environment and the plasma species. For this study, kinetic energy resolved mass-spectrometry and time-resolved plasma imaging were used to study the physicochemical processes occurring during the reactive crossed beam laser ablation of a partially 18O substituted La0.6Sr0.4MnO3 target using oxygen as gas pulse. The characteristics of the ablated plasma are compared with those observed during pulsed laser deposition in different oxygen background pressures.

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.

Dense zig-zag microstructures in YSZ thin films by pulsed laser deposition

The very brittle oxygen ion conductor yttria stabilized zirconia (YSZ) is a typical solid electrolyte for miniaturized thin film fuel cells. In order to decrease the fuel cell operating temperature, the thickness of yttria stabilized zirconia thin films is reduced. Often, these thin membranes suffer from mechanical failure and gas permeability. To improve these mechanical issues, a glancing angle deposition approach is used to grow yttria stabilized zirconia thin films with tilted columnar structures. Changes of the material flux direction during the deposition result in a dense, zigzag-like structure with columnar crystallites. This structure reduces the elastic modulus of these membranes as compared to columnar yttria stabilized zirconia thin films as monitored by nano-indentation which makes them more adaptable to applied stress.