Aqueous CSD approach for the growth of novel, lattice-tuned La x Ce 1Àx O d epitaxial layers †

Lanthanum-cerium oxide (LCO) films were deposited on Ni-5%W substrates by chemical solution deposition (CSD) from water-based precursors. LCO films containing different ratios of lanthanum and cerium ions (from CeO 2 to La 2 Ce 2 O 7 ) were prepared. The composition of the layers was optimized towards the formation of LCO buffer layers, lattice-matched with the superconducting YBa 2 Cu 3 O y layer, useful for the development of coated conductors. Single, crack-free LCO layers with a thickness of up to 140 nm could be obtained in a single deposition step. The crystallinity and microstructure of these lattice-matched LCO layers were studied by X-ray diffraction techniques, RHEED and SEM. We find that only layers with thickness below 100 nm show a crystalline top surface although both thick and thin layers show good biaxial texture in XRD. On the most promising layers, AFM and (S)TEM were performed to further evaluate their morphology. The overall surface roughness varies between 3.9 and 7.5 nm, while the layers appear much more dense than the frequently used La 2 Zr 2 O 7 (LZO) systems, showing much smaller nanovoids (1-2 nm) than the latter system. Their effective buffer layer action was studied using XPS. The thin LCO layers supported the growth of superconducting YBCO deposited using PLD methods. Introduction Among the current second generation of coated conductors, YBa 2 Cu 3 O 7Àx (YBCO) is of world-wide interest because of its excellent superconducting properties and its possibility to be deposited as thin film, overcoming the intrinsic brittle nature of this ceramic material. Recently, numerous efforts were undertaken for scaling up the fabrication of coated YBCO conductors to commercial lengths using various fabrication methods. 1,2 To obtain high quality, biaxially textured c-axis growth of YBCO on top of Ni-tapes, one or more intermediate buffer layers are always necessary. They prevent oxidation of the metal substrate during annealing of the YBCO at higher temperatures and diffusion of Ni-ions into the superconducting layer. Furthermore, the buffer layers need to assure well-textured growth of the YBCO top layer. Therefore, good texture alignment, both in-plane and out-ofplane, together with a close lattice match with YBCO thin film and substrate are principal requirements for good buffer layers. Moreover, the layers need to be dense, free of cracks and homogeneous. In practice, these high requirements translate into multiple buffer layer systems, 15-21 All these works report a possibility of growing single buffer layer on the given substrate with optimized properties. These single buffer layers showed good superconducting properties. shows that oxygen vacancies in LCO clearly prefer a Ce environment over a La one, resulting in a reduction of the Ce ions neighboring the oxygen vacancy. 26 Related to this, a similar work reported that the creation of vacancies and defects in the structure of CeO 2 , changing the oxidation state of the Ce ion, influences the stability and thus the reactivity of the layer. 27 Hence, La 1Àx Ce x O 2Àx/2 is a promising oxide material for use in solid oxide fuel cells as electrolyte layer or in catalysts 22-28 and could be envisaged as an alternative buffer layer for YBCO coated conductors, related to appropriate crystal structure and possible improved stability compared to CeO 2 . La 3+ is reported to be incorporated in the CeO 2 crystal lattice up to doping percentages of 55%. Since La 3+ has a larger radius than Ce 4+ (1.30 vs. 1.11 A), 29 it increases the cubic lattice parameter of ceria upon doping (from $5.42 to$5.6 A). 32-35 By varying the composition of the LCO material from pure CeO 2 to La 2 Ce 2 O 7 , the lattice mismatch with the Ni-5%W tape increases from 8 to 11%. At the same time, with increasing La content, an almost perfect lattice match with the YBCO 7Àx a-axis, YBCO 6+x or the YBCO 7Àx b-axis can be obtained. We studied the influence of the elemental composition of the material on its lattice parameter and the quality of the epitaxial growth of LCO layers directly on buffered tape. The research focused on the investigation of solid solution compositions with optimal lattice match toward YBCO, the growth of single buffer layers on Ni-5%W substrates exhibiting dense, crack-free, uniform and textured morphology and structure and the study of the buffer capacity of these coatings. Experimental Precursor solution synthesis Aqueous LCO precursor solutions (0.4 M) are prepared by dissolving cerium acetate (Ce(CH 3 COO) 3 xH 2 O, Sigma-Aldrich, ) was slowly added to the EDTA solution at room temperature and stirred for 10 minutes. The pH of the mixed solution was adjusted from 3 to 6 by addition of ammonia. After heating to 60 C, ethylene glycol (EG, Sigma-Aldrich, >99%) was added (molar ratio EDTA : EG ¼ 1 : 6). Finally, excess solvent was evaporated while stirring at 60 C, until the desired concentration of 0.4 mol L À1 was obtained. This precursor solution has a final pH of 6 and a viscosity of 5.60 mPa s. Preparation of powders A series of powders with varying compositions (La x Ce 1Àx O d with 0 # x # 1) were prepared from the precursor solutions discussed above. The powders were obtained by pouring 15 mL of the precursor solution into an Al 2 O 3 crucible that was placed inside a muffle furnace. The conversion into solid matter was carried out under air by reaching 1000 C from room temperature in 4 hours and let to stabilize for 2 hours. They were then cooled down to rt within 3 hours time. Solution deposition and heat treatment Ni-5%W tapes of 1 cm width and 80 mm thickness (manufactured by EVICO GmbH) were supplied by Zenergy Power GmbH. The tapes were cut into strips of 2.5 cm length for dip-coating. These substrates were dip-coated at a withdrawal speed ranging between 20 and 60 mm min À1 in a clean room (class 10 000) to prevent contamination with dust particles. The as-deposited layers were transformed into a gel by placing them in a drying furnace at 60 C for one hour. The amorphous gel layers were transformed into crystalline layers by applying a suitable heat treatment. The as-deposited layers were first heated from room temperature to 450 C (ramp rate of 1 C min À1 ) and let to dwell for 1 hour. Secondly, a heating rate of 3 C min À1 was applied from 450 C until 900 C (dwell time ¼ 1 h). Finally, the films were heated to 1050 C at a 10 C min À1 ramp with a dwell time of one hour. After the heat treatment, the furnace was switched off and the samples were left to cool inside the furnace. The entire heating and cooling process was carried out in an Ar-5%H 2 atmosphere (gas flow range: 0.1 L min À1 ). Characterization The thickness of the crystallized layers was analyzed by spec- View Online films on Ni-5%W substrates (l ¼ 638.3 nm, n ¼ 1.998). The crystallinity of the film was analyzed using a standard X-ray diffractometer for q-2q analysis (CuK a , Siemens D 5000). f-Scans were measured using a Bruker, AXS Discoverer diffractometer. To verify the crystallinity/texturation of the top surface (5 nm) we used Reflection High Energy Electron Diffraction (RHEED, Staib Instruments) at an energy of 30 keV under an incidence angle of 1-2 . The microstructure of the surface was analyzed with a scanning electron microscope (FEG-SEM, FEI). The buffer layer action was evaluated by monitoring the Ni penetration depth using X-ray photo-electron spectroscopy (S-Probe Monochromatized XPS spectrometer from Surface Science Instruments (VG) with an Al-source (monochromatized Al-radiation: 1486 eV)). The measured surface was 250 mm by 1000 mm. Experimental data were processed using the software package CasaXPS (Casa Software Ltd., UK), by including a Shirley background and Scofield sensitivity factors. Finally, a THEVA Cryoscan was used to measure the critical current density of the YBCO layers deposited on top of the LCO buffer layer. LCO specimens for (S)TEM investigations were prepared by conventional mechanical polishing and grinding followed by ion milling using a Res 100 Baltec ion milling machine operating at 4.5 kV and 3.5 mA for several hours. Plan-view specimens were ion milled from the substrate side only at an angle of 12 . A final polishing stage was done at 6 . High Angle Annular Dark Field (HAADF)-Scanning Transmission electron microscopy (STEM) imaging was carried out at EMAT on a double aberration-corrected FEI Titan 80-300 microscope operating in STEM mode. For HAADF-STEM imaging at 300 kV an inner collection semi-angle b of 50 mrad was used. Results and discussion Powders and cell parameters , these different precursor solutions were converted into solid matter by heating to 1000 C under air and studied by X-ray diffraction 26 They find that La 3+ addition in CeO 2 is more energetically favored when assuming such a structure. More specifically, it is shown that the introduced oxygen vacancies prefer a Ce tetrahedral surrounding. For a fluorite lattice with randomly distributed La and Ce cations, Vanpoucke et al. also calculate that the number of charge compensating oxygen vacancies will outnumber the number of Ce tetrahedra for a La concentration of about 43 at%, indicating the driving force of the pyrochlore transformation. In our case, all experimental XRD patterns obtained from powders are indicative of the typical pyrochlore-fluorite lattice for the full doping range, and no peaks related to the formation of secondary phases such as La 2 O 3 can be found. We can see a clear shift of the reflections towards lower 2q values as a function of La 3+ content. Using the (CeO 2 , fluorite) reflections indexed in 19,43 , we continued our research on thin films deposited on Ni-5%W tape. Again, a series of samples with varying La 3+ content, ranging from pure CeO 2 to La 2 Ce 2 O 7 , were prepared. The withdrawal speed during dip coating was set at 20 mm min À1 . In this way, layers with a thickness of 60 to 70 nm, as determined from ellipsometry, can be obtained. In line with the results for powder diffraction, no trace of any secondary phase can be found, and all layers exhibit the expected fluorite type structure. Also for the layers, the reflections shift to lower 2q values as a function of La 3+ content, related to its larger ionic radius compared to Ce 4+ . The lattice parameters extracted from these indexed XRD reflections, using Bragg's law, are plotted as a function of La 3+ content in A. Since LCO grows 45 rotated versus the Ni-5%W cubic lattice for lattice matching, this means that the substrate (sqrt(2a 2 ) ¼ 5.01 A) has a negative mismatch compared to the LCO layers and thus compressive strain will occur in the thin films. On the other hand, the LCO powders were synthesized under air while thin films on Ni-5%W tape have to be heat treated under reducing conditions (Ar-5%H 2 ). It is well known that LCO is very sensitive to the formation of oxygen vacancies and these can influence the lattice parameters. Vanpoucke et al. have published, based on DFT simulations, that in the case where no oxygen vacancies exist and thus similar to when processing under air, the lattice parameter changes 2.75% when moving from CeO 2 to La 2 Ce 2 O 8 . 26 From our experimental values for powders we find a value of 2.95%. In the case of LCO samples containing oxygen vacancies, the theoretical lattice expansion by La 3+ doping is expected to be smaller, somewhere between 2.7 and 1.7%, depending on the lattice site being occupied by the vacancies. These numbers are in line with a number of experimental reports. 38-41,43-45 Here, we find an experimental expansion of the lattice parameter of $2.4%, showing good agreement with the theoretical expectations. Fro

Characterization of freestanding photoresist films for biological and MEMS applications

Photoresists are light-sensitive resins used in a variety of technological applications. In most applications, however, photoresists are generally used as sacrificial layers or a structural layer that remains on the fabrication substrate. Thin layers of patterned 1002F photoresist were fabricated and released to form a freestanding film. Films of thickness in the range of 4.5–250 μm were patterned with through-holes to a resolution of 5 μm and an aspect ratio of up to 6:1. Photoresist films could be reliably released from the substrate after a 12-hour immersion in water. The Young’s modulus of a 50 μm-thick film was 1.43 ± 0.20 GPa. Use of the films as stencils for patterning sputtered metal onto a surface was demonstrated. These 1002F stencils were used multiple times without deterioration in feature quality. Furthermore, the films provided biocompatible, transparent surfaces of low autofluorescence on which cells could be grown. Culture of cells on a film with an isolated small pore enabled a single cell to be accessed through the underlying channel and loaded with exogenous molecules independently of nearby cells. Thus 1002F photoresist was patterned into thin, flexible, free-standing films that will have numerous applications in the biological and MEMS fields

Aqueous CSD approach for the growth of novel, lattice-tuned LaxCe1−xOδ epitaxial layers

Lanthanum-cerium oxide (LCO) films were deposited on Ni-5% W substrates by chemical solution deposition (CSD) from water-based precursors. LCO films containing different ratios of lanthanum and cerium ions (from CeO2 to La2Ce2O7) were prepared. The composition of the layers was optimized towards the formation of LCO buffer layers, lattice-matched with the superconducting YBa2Cu3Oy layer, useful for the development of coated conductors. Single, crack-free LCO layers with a thickness of up to 140 nm could be obtained in a single deposition step. The crystallinity and microstructure of these lattice-matched LCO layers were studied by X-ray diffraction techniques, RHEED and SEM. We find that only layers with thickness below 100 nm show a crystalline top surface although both thick and thin layers show good biaxial texture in XRD. On the most promising layers, AFM and (S) TEM were performed to further evaluate their morphology. The overall surface roughness varies between 3.9 and 7.5 nm, while the layers appear much more dense than the frequently used La2Zr2O7 (LZO) systems, showing much smaller nanovoids (1-2 nm) than the latter system. Their effective buffer layer action was studied using XPS. The thin LCO layers supported the growth of superconducting YBCO deposited using PLD methods

Photoluminescence properties of Co2+ doped ZnO nanocrystals

We performed photoluminescence experiments on colloidal, Co2+-doped ZnO nanocrystals in order to study the electronic properties of Co2+ in a ZnO host. Room temperature measurements showed, next to the ZnO exciton and trap emission, an additional emission related to the Co2+ dopant. The spectral position and width of this emission does not depend on particle size or Co2+ concentration. At 8 K, a series of ZnO bulk phonon replicas appear on the Co2+-emission band. We conclude that Co2+ ions are strongly localized in the ZnO host, making the formation of a Co2+ d-band unlikely. Magnetic measurements revealed a paramagnetic behaviour. (c) 2005 Elsevier B.V. All rights reserved