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Ulrich Gösele

Publications and source records attributed to Ulrich Gösele.

At least 19 recordsLinked to original sources
PubMed2006

Fast fabrication of long-range ordered porous alumina membranes by hard anodization.

Nanoporous anodic aluminium oxide has been widely used for the development of various functional nanostructures. So far these self-organized pore structures could only be prepared within narrow processing conditions. Here we report a new oxalic-acid-based anodization process for long-range ordered alumina membranes. This process is a new generation of the so-called "hard anodization" approach that has been widely used in industry for high-speed fabrication of mechanically robust, very thick (>100 microm) and low-porosity alumina films since the 1960s. This hard anodization approach establishes a new self-ordering regime with interpore distances, (D(int))=200-300 nm, which have not been achieved by mild anodization processes so far. It offers substantial advantages over conventional anodization processes in terms of processing time, allowing 2,500-3,500% faster oxide growth with improved ordering of the nanopores. Perfectly ordered alumina membranes with high aspect ratios (>1,000) of uniform nanopores with periodically modulated diameters have been realized.

Journal Article↗
PubMed2006

Coherent kinetic control over crystal orientation in macroscopic ensembles of polymer nanorods and nanotubes.

We show that the crystal orientation in polymer nanotubes and nanorods inside porous templates is controlled by the kinetics of nucleation and growth under 2D confinement. Two clear limiting cases are identified: In separated nanostructures, any crystal orientation allowing the growth of lamellar crystals along the pores appears statistically. If a bulklike surface film connects the nanostructures, macroscopic arrays with uniform crystal orientation are obtained, in which the dominant growth direction of the crystals is aligned with the long axes of the pores of the template.

Crystallization↗
PubMed2006

Monocrystalline spinel nanotube fabrication based on the Kirkendall effect.

There is a deep interest in methods to fabricate hollow nanocrystals for potential application as high-efficiency catalysts or drug-delivery agents. Tubular one-dimensional nanocrystals have been prepared for a wide variety of materials, including semiconductors, metals, ferroelectrics and magnetite. They can be produced by rolling up layered materials or via an axial growth in a rolled-up form, coating pores in templates or by eliminating the core of a core-shell nanowire. The Kirkendall effect, a classical phenomenon in metallurgy, was recently applied to explain the formation of hollow spherical nanocrystals. Although the experimental demonstration and theoretical treatment mainly concern binary compounds and planar interfaces or nanoscale spherical interfaces, the fabrication route provided by the Kirkendall effect should be generic, and should also work for high-aspect-ratio hollow cylinders (that is, nanotubes) or even more complex superstructures. In this letter, we report, for the first time, on ultra-long single-crystal ZnAl(2)O(4) spinel nanotubes (total diameter: approximately 40 nm, wall thickness: approximately 10 nm) fabricated through a spinel-forming interfacial solid-state reaction of core-shell ZnO-Al(2)O(3) nanowires involving the Kirkendall effect. Our results simultaneously represent an extension of applying the Kirkendall effect in fabricating hollow nano-objects from zero-dimensional to multidimensional, and from binary to ternary systems.

Journal Article↗
PubMed2006

Template-assisted large-scale ordered arrays of ZnO pillars for optical and piezoelectric applications.

Spatially separated ZnO pillars, typically 300 nm in diameter and 2 microm in height, are fabricated via a template-directed approach that leads to long-range hexagonal order. The templates of Au nanodisk arrays are obtained by using metal membranes as a lithography mask. The growth of ZnO pillars is performed in a double-tube system through vapor diffusion-deposition. The growth mechanism of the pillars is studied in detail and is proposed to be a combination of vapor-liquid-solid and vapor-solid models. The piezoelectric and optical properties of single pillars are characterized using piezoresponse force microscopy and micro-photoluminescence spectroscopy, respectively. The pillars show strong excitonic emissions up to room temperature, which indicate a relatively low defect density and good crystalline quality. The obtained piezoelectric coefficient d(33) is (7.5+/-0.6) pm V(-1), which is to our knowledge the first reported value for a single nanopillar.

Crystallization↗
PubMed2006

Measurement of the bending strength of vapor-liquid-solid grown silicon nanowires.

The fracture strength of silicon nanowires grown on a [111] silicon substrate by the vapor-liquid-solid process was measured. The nanowires, with diameters between 100 and 200 nm and a typical length of 2 microm, were subjected to bending tests using an atomic force microscopy setup inside a scanning electron microscope. The average strength calculated from the maximum nanowire deflection before fracture was around 12 GPa, which is 6% of the Young's modulus of silicon along the nanowire direction. This value is close to the theoretical fracture strength, which indicates that surface or volume defects, if present, play only a minor role in fracture initiation.

Computer Simulation↗
PubMed2006

Atomic layer deposition on biological macromolecules: metal oxide coating of tobacco mosaic virus and ferritin.

Decoration of nanoparticles, in particular biomolecules, gathered high attention in recent years.(1-7) Of special interest is the potential use of biomolecules as templates for the fabrication of semiconducting or metallic nanostructures.(1-7,26) In this work we show the application of atomic layer deposition, a gas-phase thin film deposition process, to biological macromolecules, which are frequently used as templates in nanoscale science, and the possibility to fabricate metal oxide nanotubes and thin films with embedded biomolecules.(1-13).

Adsorption↗
PubMed2005

Liquid crystalline nanowires in porous alumina: geometric confinement versus influence of pore walls.

Aligned liquid crystalline nanowires within ordered porous alumina templates show a pronounced texture on a macroscopic scale. We have investigated the influence of the geometric confinement and the nature of the pore walls on the mesophase formation by means of X-ray diffraction. The apparent texture is the result of a complex interplay of the pore geometry, interfacial phenomena, and the thermal history. Pores with a diameter of a few hundred nm guide the mesophase formation more efficiently than those with a diameter below 100 nm.

Aluminum Oxide↗
PubMed2005

Diameter-dependent growth direction of epitaxial silicon nanowires.

We found that silicon nanowires grown epitaxially on Si (100) via the vapor-liquid-solid growth mechanism change their growth direction from 111 to 110 at a crossover diameter of approximately 20 nm. A model is proposed for the explanation of this phenomenon. We suggest that the interplay of the liquid-solid interfacial energy with the silicon surface energy expressed in terms of an edge tension is responsible for the change of the growth direction. The value of the edge tension is estimated by the product of the interfacial thickness with the surface energy of silicon. For large diameters, the direction with the lowest interfacial energy is dominant, whereas for small diameters the surface energy of the silicon nanowire determines the preferential growth direction.

Anisotropy↗
PubMed2005

Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy.

Because of their importance in fundamental research and possible applications in nanotechnology and nanoelectronics, semiconductor nanowires have attracted much interest. In addition to the growth itself, the control of the size and location is an essential problem. Here we show the growth of ordered arrays of vertically aligned silicon nanowires by molecular beam epitaxy using prepatterned arrays of gold droplets on Si(111) substrates. The ordered arrays of gold particles were produced by nanosphere lithography.

Crystallization↗
PubMed2004

Channel activity of a viral transmembrane peptide in micro-BLMs: Vpu(1-32) from HIV-1.

We report for the first time on pore-suspending lipid bilayers, which we call micro-black lipid membranes (micro-BLMs), based on a highly ordered macroporous silicon array. Micro-BLMs were established by first functionalizing the backside porous silicon surface with gold and then chemisorbing 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol followed by spreading 1,2-diphytanoyl-sn-glycero-3-phosphocholine dissolved in n-decane. Impedance spectroscopy revealed the formation of single lipid bilayers confirmed by a mean specific capacitance of 0.6 +/- 0.2 microF/cm2. Membrane resistances were in the G omega-regime and beyond. The potential of the system for single channel recordings was demonstrated by inserting the transmembrane domain of the HIV-1 accessory peptide Vpu(1-32), which forms helix bundles with characteristic opening states. We elucidated different amilorides as potential drugs to inhibit channel activity of Vpu.

Amiloride↗