Scientific publications

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.

This paper provides a selective description of the development of nanostructured materials and the fabrication of the devices for optical applications. Examples are interference coatings, refractive and diffractive lenses, and macro- and micro-GRIN (graded refractive index) optical elements. Hybrid materials containing nanoparticles are of particular interest for the production of optical elements because, by exploiting the intrinsic solid state properties of the nanoparticles, nanocomposites can be tailored to exhibit the desired properties. A particular advantage of wet chemical processing lies in its great flexibility for depositing functional coatings.

Die Arbeit beschreibt die Entwicklung eines neuen Materials, das auf eine mechanische Belastung zehnmal empfindlicher reagiert als die heute genutzten metallischen DMS-Materialen. In diesem Kompositmaterial liegen mit Kohlenstoff umhüllte Nickelcluster mit einem Durchmesser zwischen 10 nm und 20 nm vor. Die Umhüllungen bestehen aus einigen gebogen Graphenlagen, die die Nickelcluster wie Zwiebelschalen umgeben. Elektronenmikroskopische Aufnahmen zeigten, dass die Metallcluster isoliert voneinander vorliegen, sich also nicht berühren, so dass der Stromtransport durch die Hüllen aus Kohlenstoff beeinträchtigt ist. Wird nun eine Dehnung oder Verzerrung dieses komplexen Materials zB durch einen äußeren Druck hervorgerufen, so beeinflusst dies den Stromtransport durch das Material, also seinen elektrischen Widerstand, sehr stark. Dies ermöglicht eine sehr empfindliche elektrische Messung des Drucks oder der Kraft. Neben einer um den Faktor elf erhöhten Dehnungsempfindlichkeit (k-Faktor bis zu 22) im Gegensatz zu herkömmlichen DMS-Materialen weisen diese Dünnschichten einen auf null einstellbaren Temperaturkoeffizienten des elektrischen Widerstands auf (TKR kleiner als ±25 ppm/K). Erste Prototypen in Form von Stahlmembran-Drucksensoren zeigen eindrucksvoll das Potenzial dieser Dünnschichten für Sensoranwendungen auf. Die hohe Dehnungsempfindlichkeit konnte bestätigt werden, auch viele weitere Sensorparameter wie z. B. das Kriechen, die Linearität der Kennlinie und das Signalrauschen konnten untersucht werden und liegen in für diese Sensoren typischen Bereichen.

The aim of our present study was the development of a drug multilayer-based carrier system for delivery of water-insoluble drugs. As drug, we applied the anticancer drug 5,10,1 5,20-tetrakis(3-hydroxyphenyl)porphyrin, mTHPP. which is a model photosensitizer or photodynamic therapy. Gold nanoparticles (AuNP) with a diameter of 14.5 +/- 0.9 am were prepared and used as template for the layer-by-layer approach. The drug and the negatively charged polyelectrolyte (PC) poly(styrene sulfonate) sodium salt (PSS) were complexed with a new developed method using freeze-drying. The complexation efficiency was determined to be similar to 11-12 monomers PSS per mTHPP molecule by CHNS analysis and UV/vis measurement. Molecular (locking simulations revealed pi-pi interactions and H-bonding to be the responsible mechanisms. A drug multilayer system based on the layer-by-layer (LbL) technique utilized the water-soluble complex is anionic layer material and poly(allylamine hydrochloride) (PAH) as cationic layer. The modified AuNP were characterized by different physicochemical techniques such as UV/vis. zeta-potential, ICP-OES, and TEM. To the best of our knowledge. We could demonstrate for the first time the adsorption of three drug layers to a nanoparticulate system. Furthermore, the adaptation of the LbL-technique resulted in drastically increased drug deposition efficiency (factor of 100). Furthermore, we developed a new and comfortable way to solubilize water-insoluble drugs in water.