Hans-Werner Fink

Electrical conduction through DNA molecules

The question of whether DNA is able to transport electrons has attracted much interest, particularly as this ability may play a role as a repair mechanism after radiation damage to the DNA helix. Experiments addressing DNA conductivity have involved a large number of DNA strands doped with intercalated donor and acceptor molecules, and the conductivity has been assessed from electron transfer rates as a function of the distance between the donor and acceptor sites,. But the experimental results remain contradictory, as do theoretical predictions. Here we report direct measurements of electrical current as a function of the potential applied across a few DNA molecules associated into single ropes at least 600 nm long, which indicate efficient conduction through the ropes. We find that the resistivity values derived from these measurements are comparable to those of conducting polymers, and indicate that DNA transports electrical current as efficiently as a good semiconductor. This property, and the fact that DNA molecules of specific composition ranging in length from just a few nucleotides to chains several tens of micrometres long can be routinely prepared, makes DNA ideally suited for the construction of mesoscopic electronic devices.

Solution to the twin image problem in holography.

While holography truly constitutes an ingenious concept, ever since its invention by Gabor it has been troubled by the so-called twin-image problem limiting the information that can be obtained from a holographic record. For symmetry reasons there are always two images appearing in the reconstruction of a hologram and the unwanted out of focus twin-image obscures the object. Here we show a universal method of reconstructing a hologram completely free of twin-image disturbances while no assumptions about absorbing or phase shifting properties of the object need to be imposed. Thus, truthful amplitude and phase distributions are retrieved.

Point source for ions and electrons

Field-ion techniques have been used to create physical point sources for ions and electrons with emission areas and angles orders of magnitude smaller than in any other available source. The monatomic pyramidal tips emit electrons or ionize noble-gas atoms originating from the single front atom. The angular divergence from the normal direction above the single W atom is less than 0.5° for ion beams produced by field ionization. The angular spread for emission from small clusters is somewhat larger for field ionization and electron emission. By employing tips as sources for free electrons in an STM-like setup, we were able to obtain high-resolution images of surfaces with an electron beam of only 15 eV primary energy. The image information is contained in the yield of the secondary electrons created at the sample surface.

Theory of the point source electron microscope

Abstract The theory of the point source electron microscope including multiple scattering events is formulated. Images are calculated and analyzed for carbon clusters, varbon fibers and large metal films. A Fourier-like transform is then shown to be appropriate for the reconstruction of the object with atomic resolution. Effects due to higher partial waves, multiple scattering and finite image size are examined in detail.

Low-energy electron and ion projection microscopy

Abstract We have built a projection microscope by positioning an ultrasharp tip, prepared by field-ion microscopy techniques, in close proximity to a partly transparent carbon foil. The short distance between the perforated carbon film and the tip, achieved with an STM-like approach mechanism, leads to electron emission from the emitter at only 30 V. This arrangement provides a bright source for low-energy electrons or noble gas ions. With a detector at a macroscopic distance opposite the emitter side, a projection image of the holes and structures in the foil of about 30 nm diameter is generated by the low-energy electron beam.

Electron holography of individual DNA molecules

High-contrast holograms of unstained DNA molecules, deposited on a dedicated sample holder fabricated by silicon micromachining techniques, have been obtained in the low-energy electron point-source microscope. Recording at television rates allows dynamic processes to be observed. Numerical reconstruction of the holograms reveals the structure of the molecules depicted as the magnitude of the electron object wave front.

Carbon nanotubes are coherent electron sources

The observation of electron interference effects provides direct evidence of the coherence of the electrons emitted from a carbon nanotube. To demonstrate this, the low-energy electron point source microscope has been used to mount an individual carbon nanotube onto a tungsten tip. In subsequent experiments, the electrons emitted from the nanotube were used to generate holograms. Comparison with a standard tungsten atomic point source emitter establishes a high degree of coherence for a nanotube emitter.

Simultaneous reconstruction of phase and amplitude contrast from a single holographic record

We present a reconstruction technique for simultaneous retrieval of absorption and phase shifting properties of an object recorded by in-line holography. The routine is experimentally tested by applying it to optical holograms of a pure phase respectively a pure amplitude object of micrometer dimensions that has been machined into a glass-plate using a focused ion beam. The method has also been applied to previously published electron holograms of single DNA molecules.

Nondestructive imaging of individual biomolecules

Radiation damage is considered to be the major problem that still prevents imaging an individual biological molecule for structural analysis. So far, all known mapping techniques using sufficient short wavelength radiation, be it x rays or high energy electrons, circumvent this problem by averaging over many molecules. Averaging, however, leaves conformational details uncovered. Even the anticipated use of ultrashort but extremely bright x-ray bursts of a free electron laser shall afford averaging over 10{6} molecules to arrive at atomic resolution. Here, we present direct experimental evidence for nondestructive imaging of individual DNA molecules. In fact, we show that DNA withstands coherent low energy electron radiation with deBroglie wavelength in the Angstrom regime despite a vast dose of 10{8} electrons/nm{2} accumulated over more than one hour.

In‐line holography using low‐energy electrons and photons: Applications for manipulation on a nanometer scale

An in‐line holography setup is used to obtain electron and photon holograms of high magnification from various samples. Electron holograms are obtained using the coherent beam of low‐energy electrons that originates from the electron point‐source tip. Light optical holograms are generated for comparison using a divergent laser beam. In both cases the numerical reconstruction of the holograms yields the wave front at the sample. The low‐energy electron point source microscope has been modified by incorporating an additional tip for manipulating the sample under observation. With this manipulating tip, individual nanometer‐sized wires have been contacted, and an electrical current has been passed through carbon fibers as well as through carbon nanotubes while the holographic electron pattern has been observed in situ.