Hans Werner Fink
Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
The Low Energy Electron Point Source (LEEPS) Microscope is based on an atomic sized electron source that provides a bright beam of electrons with kinetic energies ranging from 20 to 300 eV. Owing to the atomic sized emission of this source a coherent ensemble of electrons is provided directly without the need for lenses. This makes it possible to carry out interference experiments in a conceptually simple fashion just as with a laser in light optics. An object, placed in the path of the divergent electron beam, scatters part of the wave elastically. At a distant detector the interference between this object-wave and the unscattered part of the coherent primary wave is observed. Since the invention of the concept of holography by Denis Gabor it is known that this hologram contains information about both, the amplititude and phase of the object wave and the wave front at the object can be retrieved from the hologram by solving the Kirchhoff-Fresnel integral. The low energy of the electrons enshures a high phase contrast when scattering at light elements. This, in combination with the lack of radiation damage, makes this imaging technique particular appealing for the study of unstained biological molecules. High contrast holograms of unstained DNA molecules, deposited on a dedicated sample holder fabricated by micromachining techniques, have been obtained and the numerical reconstruction reveals the structure of the molecules depicted as the magnitude of the electron object wave front. Experiments to address the electrical conductivity of DNA molecules have also been carried out. For this, a manipulation tip has been employed to achieve mechanical contact with individual DNA fibers in order to measure their I-V charachteristics. From the linear I-V curve a restistance value of 1.4 MW can be deduced for a 600nm long DNA fiber. Since the contact restances at both ends at the DNA fiber are not known a priori, the above quoted electrical resitivity value has to be taken as an upper limit. However, these preliminary experiments clearly indicate that DNA molecules are actually small conducting wires. This findings will not only have consequences in molecular biology where the conductivity of DNA molecules has been debated for a long time in the context of possible repair mechanism, but will also open up possibilities to employ DNA molecules, whose chemistry happens to be extremely well understood, as tailored molecular conductors in combination with micromachined supports on which they are to be arranged.