Christoph Deneke

Self-Propelled Nanotools

We describe nanoscale tools in the form of autonomous and remotely guided catalytically self-propelled InGaAs/GaAs/(Cr)Pt tubes. These rolled-up tubes with diameters in the range of 280-600 nm move in hydrogen peroxide solutions with speeds as high as 180 μm s(-1). The effective transfer of chemical energy to translational motion has allowed these tubes to perform useful tasks such as transport of cargo. Furthermore, we observed that, while cylindrically rolled-up tubes move in a straight line, asymmetrically rolled-up tubes move in a corkscrew-like trajectory, allowing these tubes to drill and embed themselves into biomaterials. Our observations suggest that shape and asymmetry can be utilized to direct the motion of catalytic nanotubes and enable mechanized functions at the nanoscale.

Rolled-up nanomembranes as compact 3D architectures for field effect transistors and fluidic sensing applications.

We fabricate inorganic thin film transistors with bending radii of less than 5 μm maintaining their high electronic performance with on-off ratios of more than 10(5) and subthreshold swings of 160 mV/dec. The fabrication technology relies on the roll-up of highly strained semiconducting nanomembranes, which compacts planar transistors into three-dimensional tubular architectures opening intriguing potential for microfluidic applications. Our technique probes the ultimate limit for the bending radius of high performance thin film transistors.

Process integration of microtubes for fluidic applications

Three-dimensional InGaAs∕GaAs microtubes are integrated by photolithography into a microfluidic device. The integration process, made possible due to advances in fabricating long, homogeneous rolled-up microtubes, is described in detail. Liquid filling and emptying of individual microtubes, and the final microfluidic device are investigated by video microscopy. The authors find an agreement for their channels with the Washburn equation [Phys. Rev. 17, 273 (1921)] for filling using a modified capillary pressure fit to experimental conditions. Emptying of a vacuum pumped microfluidic device also qualitatively agrees with theory. The results suggest rolled-up micro- and nanotubes as possible systems to provide fully integrative fluid analysis on a chip.

Semiconductor tubes, rods and rings of nanometer and micrometer dimension

Strainedsemicond uctor bilayers are releasedfrom their substrate andformedinto free-stand ing tubes, rod s and rings on semiconductor surfaces. We present nanotubes based on the InGaAs=GaAs, SiGe=Si, andInGaP material system. The inner diameters of our tubes vary from 15 to ? 500 nm. We present rolled-up nanotubes, which have performed up to 30 revolutions. The wall thickness of the nanotubes depends on the number of rotations. As a consequence, the inner to outer diameter ratio can be tuned over a wide range, i.e. from 1 to 0.125. In the latter case the nanotubes become nanorods. Strained layers are bent in two directions, vertical and horizontal. The result are ultra-thin and ring-like vertical free-standing membranes. The wide range of possible materials and structural properties make these tubes and rings interesting candidates for fundamental investigations as well as for applications. ? 2002 Elsevier Science B.V. All rights reserved.

Hybrid organic/inorganic molecular heterojunctions based on strained nanomembranes.

In this work, we combine self-assembly and top-down methods to create hybrid junctions consisting of single organic molecular monolayers sandwiched between metal and/or single-crystalline semiconductor nanomembrane based electrodes. The fabrication process is fully integrative and produces a yield loss of less than 5% on-chip. The nanomembrane-based electrodes guarantee a soft yet robust contact to the molecules where the presence of pinholes and other defects becomes almost irrelevant. We also pioneer the fabrication and characterization of semiconductor/molecule/semiconductor tunneling heterojunctions which exhibit a double transition from direct tunneling to field emission and back to direct tunneling, a phenomenon which has not been reported previously.

Wrinkled-up Nanochannel Networks: Long-Range Ordering, Scalability, and X-ray Investigation

Highly ordered two-dimensional self-organized nanochannel networks as well as free-standing nanomembranes are produced on rigid substrates by means of III-V semiconductor compressively strained layers grown on top of an etchant-sensitive material. The releasing process is controlled by regularly spaced pits obtained from photolithography and a subsequent wet chemical etching. By tuning basic film parameters such as strain and thickness, one obtains periodic arrays of two-dimensional nanochannel networks with symmetries defined by the shape and periodicity of the photolithographic starting pits. Such nanochannel networks with a submicroscale lateral feature size exhibit a surprising flexibility with respect to the crystal lattice symmetry, retaining the original film crystalline quality as confirmed by X-ray grazing-incidence diffraction (GID) measurements. Finite element modeling helps in understanding the particular process of the cross-nanochannel formation.

The smallest man-made jet engine

The design of catalytic engines powered by chemical fuels is an exciting and emerging field in multidisciplinary scientific communities. Recent progress in nanotechnology has enabled scientists to shrink the size of macroengines down to microscopic, but yet powerful, engines. Since a couple of years ago, we have reported our progress towards the control and application of catalytic microtubular engines powered by the breakdown of hydrogen peroxide fuel which produces a thrust of oxygen bubbles. Efforts were undertaken in our group to prove whether the fabrication of nanoscale jets is possible. Indeed, the smallest jet engine (600 nm in diameter and 1 picogram of weight) was synthesized based on heteroepitaxially grown layers. These nanojets are able to self-propel in hydrogen peroxide solutions and are promising for the realisation of multiple tasks.

Nanomembrane-Based Mesoscopic Superconducting Hybrid Junctions

A new method for combining top-down and bottom-up approaches to create superconductor-normal metal-superconductor niobium-based Josephson junctions is presented. Using a rolled-up semiconductor nanomembrane as scaffolding, we are able to create mesoscopic gold filament proximity junctions. These are created by electromigration of gold filaments after inducing an electric field mediated breakdown in the semiconductor nanomembrane, which can generate nanometer sized structures merely using conventional optical lithography techniques. We find that the created point contact junctions exhibit large critical currents of a few milliamps at 4.2 K and an I(c)R(n) product placing their characteristic frequency in the terahertz region. These nanometer-sized filament devices can be further optimized and integrated on a chip for their use in superconductor hybrid electronics circuits.

Interfaces in semiconductor/metal radial superlattices

Semiconductor/metal radial superlattices are produced by the roll-up of inherently strained InGaAs∕Ti∕Au as well as InAlGaAs∕GaAs∕Cr films. Cross sections of the obtained structures are prepared and investigated in detail by diverse transmission electron microscopy as well as microanalysis techniques. Special attention is paid to the interfaces of the semiconductor/metal hybrid superlattice. The study reveals amorphous, noncrystalline layers for the semiconductor/metal as well as for the metal/semiconductor interface. The chemical analysis suggests that the observed interlayers are oxides giving rise to a semiconductor/oxide/metal/oxide superlattice rather than a pure semiconductor/metal superlattice.

Tuning magnetic properties by roll-up of Au/Co/Au films into microtubes

Au/Co/Au trilayers are fabricated by tilted deposition on prestructured polymer sacrificial layers. The metal trilayers are released by selectively dissolving the sacrificial layer and roll-up into microtubes. Magnetization properties are strongly affected by the roll-up process. In addition to a modified shape anisotropy, the magnetostrictive anisotropy due to the anisotropic stress release is reversed. Low temperature measurements support the presence of significant exchange bias in these rolled-up structures.