Scientific publications

Small ceramic particles with implied high ferroelectric properties are highly demanded for achieving fine-grained ceramics as well as in the preparation of ceramic/polymer composites. While a variety of synthetic methods offers ferroelectric particles with different sizes and crystallographic phases, a simple possibility to directly compare the ferroelectric properties of such particles is lacking. Besides the industrial importance, accessing the intrinsic ferroelectric properties of small particles will certainly help to enlighten the discussion about the existence of a size effect in ferroelectricity. We report on a setup to correctly determine ferroelectricity of BaTiO3 particles, well dispersed in an organic polymer matrix.

Transparent and scratch-resistant zirconium oxide thin films were deposited in radio-frequency plasma-enhanced chemical vapor deposition process on glass and polycarbonate substrates using zirconium-tetra-tert-butoxide, [Zr(OtBu)4], as the precursor. Investigations on film morphology (AFM), thickness (cross-sectional SEM), phase structure (X-ray diffraction), chemical composition (X-ray photoelectron spectroscopy), and optical properties (UV/Vis, ellipsometry) revealed the interplay of process parameters (plasma power, gas composition, deposition time, and precursor flux) on the composition and properties of the coatings. High-quality transparent coatings (>90%) with a tunable refractive index (n=1.7-2.1) and abrasion-resistant properties were deposited on both polymer and glass substrates. Despite low deposition temperatures (<100°C), the coatings showed good adherence to the substrate and extraordinary barrier properties that were further improved by depositing intermediary SiOx-layers. Phase analysis of the predominantly amorphous films showed the incipient crystallization of ZrO2.

Two-step chemical vapor deposition (CVD) in combination with metal-catalyzed vapor-liquid-solid (VLS) growth mechanism was used as a new approach to synthesize and modify the morphology of SnO2-TiO2 core-shell nanowires. SnO2 nanowires were grown by decomposition of Sn(OtBu)4 precursor on Au-coated Al2O3 substrates followed by the growth of a TiO2 overlayer by the CVD of Ti(OiPr)4. Structural characterization of SnO2-TiO2 heterostructures by X-ray diffraction and high-resolution TEM revealed the formation of mixed SnxTi1–xO2 compositions (x = 0.857-1.0) at the SnO2-TiO2 phase boundaries, where x depended on the annealing temperatures. The observed oriented attachment of TiO2 nanoclusters on single crystal SnO2 core was possibly due to low lattice mismatch. The photocatalytic studies suggested an enhanced activity in SnO2-TiO2 heterostructures due to higher surface area and change in the electronic structures caused by Sn4+ doping in TiO2.

Oriented SnO2 nanowire (NW) arrays were grown on TiO2(001) substrates by molecule-based chemical vapor deposition of Sn(OtBu)4. Preferred growth directions ([101], [-101], [011], and [0-11]) of SnO2 NWs were correlated to surface energy consideration and crystallographic relationship between the substrate and NW material by comprehensive electron microscopy and bulk tomographic studies. The proposed growth incubation model (tilting of catalyst/NW interface due to driving force of surface energy) explains the formation mechanism of oriented SnO2 NWs on TiO2 substrates. These results allowed us to also study the I-V characteristics and gas-sensing properties of individual SnO2 NWs grown with a perfectly defined crystal orientation ([101]).

We report the synthesis of well-dispersed core-shell Au@SiO2 nanoparticles with minimal extraneous silica particle growth. Agglomeration was suppressed through consecutive exchange of the stabilizing ligands on the gold cores from citrate to l-arginine and finally (3-mercaptopropyl)triethoxysilane. The result was a vitreophilic, stable gold suspension that could be coated with silica in a biphasic mixture through controlled hydrolysis of tetraethoxysilane under L-arginine catalysis. Unwanted condensation of silica particles without gold cores was limited by slowing the transfer across the liquid-liquid interface and reducing the concentration of the l-arginine catalyst. In-situ dynamic light scattering and optical transmission spectroscopy revealed the growth and dispersion states during synthesis. The resulting core-shell particles were characterized via dynamic light scattering, optical spectroscopy, and electron microscopy. Their cores were typically 19 nm in diameter, with a narrow size distribution, and could be coated with a silica shell in multiple steps to yield core-shell particles with diameters up to 40 nm. The approach was sufficiently controllable to allow us to target a shell thickness by choosing appropriate precursor concentrations.

Lead titanate, lead zirconate, and lead zirconate titanate (PZT) films in the sub-[small mu ]m-range were produced at temperatures around 400 °C using novel single-source precursors in a classical thermal CVD process. The design of two bimetallic alkoxide compounds, a lead titanate and a lead zirconate source with almost identical physical properties and complement miscibility, resulted in a new quasi-single-source PZT precursor, an azeotropic mixture that evaporates at 30 °C and at a pressure of 4 x 10-1 mbar. After thermal treatment at 650 °C, transparent (100)-oriented PZT films with remnant polarization of 20 µC cm-2 and a coercive field strength of 20 V µm-1 were achieved. An additional lead source is not required.

The molecular alumosiloxane (Ph2SiO)8(AlO(OH))4·4Et2O (1) reacts with 4,4′-bipyridine and azobipyridines (4,4′-azobipyridine and 3,3′-azobipyridine) to form polymer adducts. The acidic hydroxygroups of 1 interfere with the pyridine-nitrogen atoms to form O···H···N hydrogen bridges which are the source of one- and two-dimensional networks. As reaction products we have characterized (Ph2SiO)8[AlO(OH)]4·2p-(C5H4N)2 (2), (Ph2SiO)8[AlO(OH)]4·1.5p-(C5H4N)2N2·C4H8O (3) and (Ph2SiO)8[AlO(OH)]4·2m-(C5H4N)2N2 (4). In the crystal structure of 2, the supramolecular adduct forms a one-dimensional connection. In 3 and 4 two-dimensional connections are formed. The azobipyridine ligands in solution show cis-trans-isomerisation by application of light even in the supramolecular state (as base adducts of (Ph2SiO)8[AlO(OH)]4).

Sodium (Na) is an important doping element for Cu(In,Ga)Se2 (CIGS) solar cells. However, when using Na-free flexible substrates like steel foil or polyimide film, it is necessary to ensure an efficient supply of sodium to achieve high cell efficiencies. The common incorporation methods for Na on these Na-free substrates are either to deposit a Na-containing precursor layer (e.g. NaF) onto the molybdenum (Mo) back contact prior to CIGS growth or to coevaporate a Na compound during CIGS growth. Another way is to incorporate sodium after CIGS growth by a post-deposition treatment with NaF. In this work, we tested two alternative Na doping methods which are well suited for a production line due to their easy controllability. One approach is to dope the molybdenum target with Na. With Na-doped Mo layers (Mo:Na) as the back contact, we could achieve efficiencies of 13.1% both on titanium (Ti) and stainless Cr steel foil using a single-stage inline CIGS process. With a low-temperature single-stage CIGS process on polyimide (PI) we reached an efficiency of 11.2% using a Mo:Na back contact. Another doping method involves sol-gel-deposited silicon oxide layers which contain Na (SiOx:Na). We have successfully deposited these sol-gel layers onto stainless steel foil by a roll-to-roll (R2R) method with short annealing times as needed in production. With these SiOx:Na layers we could achieve efficiencies of 13.7% on stainless steel foil and 11.5% on mild steel sheet using a single-stage inline CIGS process.

The contact mechanics of a fibrillar micro-fabricated surface structure made of poly(dimethyl siloxane) (PDMS) is studied. The attachment and detachment of individual fibrils to and from a spherical indenter upon approach and retraction are detected as jumps in force and stiffness. A quantitative model describes the stiffness values by taking into account the deformation of the fibrils and the backing layer. The results emphasize the importance of long-range interactions in the contact mechanics of elastic materials and confirm some of the important concepts underlying the development of fibrillar adhesive materials.