Claudia Schäfle

Morphological wetting transitions at ring-shaped surface domains

The wetting behavior of ring-shaped (or annular) surface domains is studied both experimentally and theoretically. The ring-shaped domains are lyophilic and embedded in a lyophobic substrate. Liquid droplets deposited on these domains can attain a variety of morphologies depending on the liquid volume and on the dimensions of the ringlike surface domains. In the experiments, the liquid volume is changed in a controlled manner by varying the temperature of the sample. Such a volume change leads to a characteristic sequence of droplet shapes and to morphological wetting transitions between these shapes. The experimental observations are in good agreement with analytical and numerical calculations based on the minimization of the interfacial free energy. Small droplets form ringlike liquid channels (or filaments) that are confined to the ring-shaped domains and do not spread onto the lyophobic disks enclosed by these rings. As one increases the volume of the droplets, one finds two different morphologies depending on the width of the ring-shaped domains. For narrow rings, the droplets form nonaxisymmetric liquid channels with a pronounced bulge. For broad rings, the droplets form axisymmetric caps that cover both the lyophilic rings and the lyophobic disks.

Submicron metal oxide structures by a sol-gel process on patterned substrates

Abstract We report about a combination of micro-contact-printing and the sol-gel technique which results in structures in the micron- and submicron range. This technique which is here demonstrated on electrochromic tungsten oxide does not rely on vacuum methods and may therefore be easily upscaled to large areas.

From Mesoscopic to Nanoscopic Surface Structures : Lithography with Colloid Monolayers

A current trend in research is the fabrication and characterization of smaller and smaller surface structures approaching dimensions of a few nanometers. They are expected to exhibit a number of new physical and chemical properties when compared to micrometer-sized structures of the same material. These can include, for example, magnetic, optical, or catalytic properties. As conventional lithographic techniques are limited either by the wavelength of light or because they are serial in nature and costly (e.g., e-beam lithography), a series of new and unconventional approaches to nanofabrication have been put forward. Among these, a technique in which submicroscopic colloidal particles are used as a mask for, for example, etching or vacuum deposition, has proved more and more successful, especially in applications where a periodic arrangement of the structures is required. This technique, which has already been dubbed anatural lithographyo or ananosphere lithographyo, works in principle as follows: by some means colloidal particles of equal size and shape (normally spherical) are brought onto the surface to be structured. The particles usually arrange themselves randomly. However, under favorable conditions, they form hexagonally arranged, close-packed arrays in a self-assembly process. The surface is subsequently exposed, for example, to an ion beam or light. Vacuum deposition is also possible (see Fig. 1). Afterwards, the particles are removed by a lift-off process. Different kinds of structures can then be observed: ion beam etching usually leaves isolated posts of the substrate material, whereas vacuum deposition leads to holey thin films (for random arrangement) or, in the case of a regular arrangement, to the formation of triangular-shaped, elevated structures arranged in a honeycomb pattern. The formation of ring-like patterns has also been reported recently. As suspensions of colloidal particles are commercially available with particle sizes covering three orders of magnitude (10 nm to 10 mm), surface structures of almost any desired size and periodicity can, in principle, be fabricated. Besides the aforementioned possibilities of fundamental research on na-

Submicron contacts for electrical characterization of semiconducting WS2 thin films

We report a new method to characterize the local electronic properties of polycrystalline semiconducting thin films. A lattice of triangular gold electrodes, with a typical area of 0.2 μm2, is evaporated on a p-type WS2 film. With the help of a conductive atomic force microscope, the current–voltage characteristics of the contacts established between the gold electrodes and the WS2 film are measured. A linear dependence of the current versus voltage is obtained on gold triangles in contact with grain edges. This indicates a high level of doping or degeneracy of the semiconductor at the grain edges. The electrodes deposited on flat WS2 crystallites form rectifying diodes with the underlying grains. Barrier heights of 0.56–0.74 eV and diode ideality factors between 1.15 and 2 are determined. Under illumination, open-circuit voltages up to 500 mV can be measured on some contacts. A short response time of the photocurrent is observed (<0.1 ms) when the diodes are reversed biased, which is related to intrinsic...