Andreas Langner

Nondestructive Replication of Self-Ordered Nanoporous Alumina Membranes via Cross-Linked Polyacrylate Nanofiber Arrays

Ordered nanofiber arrays are a promising material platform for artificial adhesive structures, tissue engineering, wound dressing, sensor arrays, and self-cleaning surfaces. Their production via self-ordered porous alumina hard templates serving as shape-defining molds is well-established. However, their release requires the destruction of the hard templates, the fabrication of which is costly and time-consuming, by wet-chemical etching steps with acids or bases. We report the nondestructive mechanical extraction of arrays of cross-linked polyacrylate nanofibers from thus recyclable self-ordered nanoporous alumina hard templates. Silica replicas of the latter were synthesized using the extricated nanofiber arrays as secondary molds that could be mechanically detached from the molded material. The approach reported here, which can be combined with microstructuring, may pave the way for the high-throughput production of both functional nanofiber arrays and ordered nanoporous membranes consisting of a broad range of material systems.

Facile Large-Scale Fabrication of Proton Conducting Channels

A new approach to the facile large-scale fabrication of robust silicon membranes with artificial proton conducting channels is presented. Ordered two-dimensional macroporous silicon was rendered proton conducting by growing a thick uniform polyelectrolyte brush using surface-initiated atom transfer radical polymerization throughout the porous matrix. The fabricated silicon-poly(sulfopropyl methacrylate) hybrid membranes were evaluated for their proton conductivity, ion exchange capacity, and water uptake. With proton conductivities in the range of 10(-2) S/cm, these proof-of-concept experiments highlight a promising alternative for producing tailorable proton conducting membranes. This approach constitutes a benchmark for the preparation and study of model systems and, in addition, for the large-scale fabrication of membranes suitable for a wide range of technological applications.

Hybrid Polymer−Silicon Proton Conducting Membranes via a Pore-Filling Surface-Initiated Polymerization Approach

An alternative approach for the creation of proton conducting platforms is presented. The methodology is based on the so-called pore-filling concept, which relies on the filling of porous matrices with polyelectrolytes to obtain proton conducting platforms with high dimensional stability. Polymer-silicon composite membranes, with well-defined polyelectrolyte microdomains oriented normal to the plane of the membrane, were prepared using photoelectrochemically etched silicon as a microstructured scaffold. Ordered two-dimensional macroporous silicon structures were rendered proton conducting by filling the micropores via a surface-initiated atom transfer radical polymerization process. The morphological aspects, chemical stability, and performance of the hybrid assemblies were characterized by a set of techniques including scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, nuclear magnetic resonance and impedance spectroscopy, among others. The fabricated silicon-poly(sodium 2-acrylamide-2-methylpropane sulfonate) hybrid membranes displayed proton conductivities in the range of 1x10(-2) S/cm. This work illustrates the potential of hybrid polymer-silicon composite membranes synthesized by pore-filling surface-initiated polymerization to create proton conducting platforms in a simple and straightforward manner. Versatility and relative ease of preparation are two key aspects that make this approach an attractive alternative for the molecular design and preparation of proton conducting systems.

Formation of straight 10 nm diameter silicon nanopores in gold decorated silicon.

We observe pore formation with diameters in the 10 nm range in silicon when it is covered with gold particles. This pore etching occurs when the sample is put in 5 wt % hydrofluoric acid (HF) solution for a few minutes. The pores form along the 100 direction, which is also the preferred direction of macro- and mesopores electrochemically etched into silicon. No etching occurs if the dissolved oxygen is removed from the aqueous HF solution or the gold is removed from the silicon surface. This leads to the assumption that the dissolved oxygen acts as an oxidant as in the case of stain etching with gold as cathodic material. A tentative model is suggested to explain why all of the observed nanopores have roughly the same diameter of about 10 nm. These pores can occur for inhomogeneously gold-covered planar silicon surfaces but also in MBE (molecular beam epitaxy) grown silicon nanowires since these nanowires are covered unintentionally with gold nanoclusters at their cylindrical surface.

Applications of the high-resolution optical reconstruction of digital holograms

Adressable spatial light modulators with as much as possible ideal phase modulation are the precondition for their application in digital holography. An adapted driver electronics for the modulator and a correct knowledge of the modulation behavior can lead to a dynamic phase modulating device with nearly linear characteristic curve and a maximum phase range of 2π. We show a system for recording and reconstruction of digital holograms applying a spatial light modulator for the optical reconstruction and the digital processing of the holograms. The data of a CCD-camera are taken to a PC and sent to a spatial light modulator. In that sense we realised an analog-digital converter for recording and a digital-analog converter for the optical hologram reconstruction. We discuss the resolution of the reconstruction and their applications, especially possibilities for the manipulation with the reconstructed wave field.

Dynamisches System für die Aufnahme und optische Rekonstruktion von digitalen Hologrammen (Dynamical System for Recording and Optical Reconstruction of Digital Holograms)

Abstract Ein System zur Aufnahme und Rekonstruktion von digitalen Hologrammen mit digitaler Bearbeitung der Hologrammdaten wird gezeigt. Daten von einer CCD-Kamera werden in den PC eingelesen und von diesem auf den Lichtmodulator gegeben. Damit wird ein Analog-Digital-Konverter (Hologrammaufnahme) und ein Digital-Analog-Konverter (optische Hologrammrekonstruktion) realisiert.

High resolution coherent optical reconstruction of digital holograms and their applications

Liquid crystal panels, originally designed and fabricated for projection systems, are used as spatial light modulator in optical correlators or in fringe projection systems. An adapted driver electronics and measurements of the phase modulation behavior can lead to a dynamical phase modulating system with an almost linear modulation and a maximum phase shift of 2π. We built up a system with a LCD based spatial light modulator in order to realize the optical the reconstruction of digital holograms. Hologram reconstructions and the use of holographic interferograms for deformation detection are presented and future developments and applications are discussed.