Publikationen

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.

Innovative Technologien bedürfen einer adäquaten arbeitsmedizinischen Risikobewertung mit Ableitung geeigneter Arbeitsschutzmaßnahmen. Dies ist auch für die Nanotechnologie zu fordern. Basierend auf einer überwiegend langjährigen beruflichen Exposition gegenüber gezielt synthetisierten Nanomaterialien mit Primärpartikelgrößen zwischen <10 und 100 nm erfolgte die Konzeption eines vielschichtigen diagnostischen Programms, welches individuelle Faktoren und potentielle Wirkprinzipien von Nanomaterialien berücksichtigt, und einem Kollektiv von 10 Personen angeboten wurde. Eine Partikelmessung am Arbeitsplatz ergab zum jetzigen Zeitpunkt bei verschiedensten Tätigkeiten keine hohen Spitzenkonzentrationen. Eine Ausnahme stellte die Spritzlackierung mit Nanolacken dar, bei der Konzentrations-Maxima bis 140.000 pt/cm³ dokumentiert werden konnten, der Anwender allerdings durch Einsatz eines effektiven Atemschutzes nicht belastet war. Relevante Inhalationsmöglichkeiten bestanden anamnestisch vor allem in den vergangenen Jahren, zurückzuführen auf im Entwicklungsprozess befindliche Herstellungs- und Verarbeitungsverfahren. An relevanten diagnostischen Befunden fand sich zum einen eine obstruktive Ventilationsstörung bei der Hälfte des Kollektivs, ferner bei 3 Personen ein computertomographisch bestätigtes Lungenemphysem. Zum anderen zeigte die Hälfte der Kleingruppe im LTT eine positive Reaktion auf Zirkonium, ein Metall, welches in Form des nanopartikulären ZrO2 eine zentrale Stellung innerhalb des hier dargestellten speziellen Expositionsspektrums einnimmt. Bei einer Person mit allergischer Rhinopathie (und ohne atopische Diathese) konnte neben der LTT-Reaktion im Prick-Test eine positive Reaktion auf Zirkonium chlorid dokumentiert werden. Trotz aufwändiger und kostenintensiver Methodik lieferte die Analyse zahlreicher Biomarker im Atemexhalat keine sicheren und mit anderen Befunden der Diagnostik eindeutig korrelierenden Hinweise auf frühe entzündliche Veränderungen im Bereich des Respirationstraktes – dem primären Zielorgan potentiell adverser Effekte von Nanopartikeln. Neben einer notwendigen messtechnischen Erfassung von Nanopartikeln am Arbeitsplatz zur Beurteilung einer Gefährdung und Bewertung bzw. Optimierung von Schutzmaßnahmen sollten im Rahmen einer betriebsärztlichen Betreuung potentielle Effekte am Respirationstrakt und kardiovaskulären System sowie die Auslösung von Allergien mitberücksichtigt werden.

In this paper, we propose a new adhesive system of dimpled surfaces. The principle is derived from a contact mechanics model. The material is assumed to be linear elastic and isotropic, and attraction between the surfaces of the half-spaces is modeled via the concept of a specific adhesion energy. It is found that large and small detachments are unstable and will either grow or shrink spontaneously when their sizes are perturbed. It is shown that this phenomenon can lead to a new bi-stable adhesive system in which weak adhesion can be converted to strong adhesion by the application of pressure.

The widespread use of engineered nanomaterials increases the exposure of the materials to humans. Therefore, it is necessary to know how these materials interact with cells. One approach is to trace particles by fluorescent labeling. The aim of the present work was to study the behavior of silica particles in A549 cells. For the first time, we applied stimulated emission depletion (STED) microscopy for this approach. Therefore, SiO2 particles conjugated with Atto647N were prepared by L-arginine-catalyzed hydrolysis of tetraethoxysilane. The Atto647N labeled SiO2 particles exhibit a mean size of 128±7nm and a zeta-potential of -11mV in cell culture medium. STED microscopy enables subdiffraction resolution imaging of single Atto647N labeled SiO2 particles not only in pure solution but also in a cellular environment. To visualize Atto647N labeled SiO2 particles inside A549 cells, the membrane was labeled and image stacks, that give three-dimensional information, were taken after 5, 24, and 48 h exposure of particles to cells. During this incubation period, an increase in particle uptake was observed and STED micrographs allowed us to evaluate the agglomeration of Atto647N labeled SiO2 particles inside A549 cells. Our results show that STED microscopy is a powerful technique to study particles in a cellular environment.

The mechanistic details of the pressure-induced B1-B2 phase transition of rubidium chloride are investigated in a series of transition path sampling molecular dynamics simulations. The B2→B1 transformation proceeds by nucleation and growth involving several, initially separated, nucleation centers. We show how independent and partially correlated nucleation events can function within a global mechanism and explore the evolution of phase domains during the transition. From this, the mechanisms of grain boundary formation are elaborated. The atomic structure of the domain-domain interfaces fully support the concept of Bernal polyhedra. Indeed, the manifold of different grain morphologies obtained from our simulations may be rationalized on the basis of essentially only two different kinds of Bernal polyhedra. The latter also play a crucial role for the B1→B2 transformation and specific grain boundary motifs are identified as preferred nucleation centers for this transition.

In this paper we report on the influence of particle size distribution, particle substrate interaction, and drying behavior on the self-assembly process using ligand stabilized silver particles. Two-dimensional particle arrays were characterized using transmission electron microscopy and extensive image analysis. The formation of such structures was observed in situ using an environmental scanning electron microscope in WET-STEM mode. The results confirm that a small particle size distribution is crucial for the formation of regular particle patterns with long range order, but also the particle substrate interaction and the particle density have an influence on the degree of ordering. Additionally, we find that separated binary particle assemblies keep the orientation of their two-dimensional hexagonal lattices over alternating domains of small and big particles. This is probably enabled due to the formation of dislocations and a small change of the course of the lattice lines within the respective boundary.