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Projects

Exchange force microscopy

Force microscopy can obtain atomic resolution and can be made sensitive to magnetic force by using a magnetic tip. In this project magnetic sensitivity with atomic resolution shall be combined to detect the very short-range exchange forces, which are the origin of magnetism. To proove sensitivity towards exchange forces on the atomic scale, an antiferromgnet is a suitable sample. Being directly sensitive to exchange interaction between atomc spins means nothing less than ultimate achievable magnetic resolution. In contrasts to spin polarized scanning tunneling microscopy this mechod can also be used on the large class of magnetic insulators.

Site specific force curves

It has been prooven that force microscopy in the dynamic mode is capable of true atomic resolution and can be used to record force versus distance curves (so called spectroscopy) with high sensitivity. In this project both method shall be combined to measure site specific forces with atomic resolution. Two principal techniques are possible. (i) After taking an atomically resolved image the tip is moved to a specift atomic site, where a frequency versus distanvce curve is recorded. (ii) A frequency versus distance curve is recorded on every point af an atomicolly resolve image. Thereby, individual curves can be assigned to a specific atomic site. For both methods high stability is a prerequisite, which is met by our home-built low temperature force microscope.

Chemical identification of DNA bases

The successful recording of site specific force curves with atomic resolution indicate, that force microscopy in the dynamic mode can be used to obtain a chemical contrast. In principle, this technique can be used to identify different chemical species. The four different base pairs used to code genetic information in nature are of particular interest for such an application. In this project the possibility to identify the base pairs with force microscopy in the dynamic mode shall be evaluated.

Damping

In dynamic mode force microscopy usually the frequency shift is the measured quantity. Either the cantilever excitation amplitude (constant excitation mode) or the cantilever oscilation amplitude itself (constant amplitude mode) is kept constant. It is of particular interest, if it is possible to draw conclusions about damping mechanisms on the atomic scale by measuring the excitation amplitude or oscillation amplitude, respectively, and to evaluate their role and physical meaning by experimental techniques and theoretical analysis.

Atomc resolution on van der Waals surfaces

Graphite was the first van der Waals surface that has been sucessfully imaged with atomic resolution in the dynamic non-contact mode. However, carbon atoms within the graphite sheets are held together by strong chemical bonds. Therefore, it remained an open question, whether a true van der Waals crytal can be imaged non-destructively. Moreover, van der Waals crystal are ideal test systems, since the tip sample interaction can be decribed relatively easy by analytical methods.

Noble gas van der Waals crystals like xenon exist only at crygenic temperatures. Therefore, a low temperature force microscope capable of atomic resolution is a prerequisite to perform these experiments.

The transition from non-contact to contact

Contact forces and elastic interactions have been studied extensively in contact using static and dynamic modes of operation with relatively soft cantilevers. Due to the prevalent jump-to-contact problem the tip-sample distance cannot be adjusted contineously, i.e., the transition regime was not accessible. This problem is absent in the dynamic mode used under vacuum conditions. Moreover, contamination effects are negligible on well defined surfaces prepared in situ under ultra-high vacuum conditions. This investigations might also help to verify, if the foremost tip atom is or is not in contact during atomic scale imaging.

Atomic scale contrast on graphite: contact vs. non-contact

The (0001) surface of graphite has been the first surface, on which atomc scale contrats has been achieved with atomic force microscopy in the static contact mode and serves as a standard sample. However, the observed atomic scale contrast does not reflect the the honeycomb structure, but a trigonal symmetry. Furthermore, it is known, that in the static contact mode only the translational symmetry is imaged on the atomic sclae and that the origin of the contrast is dominated by frictional forces, which result in a stick-slip motion of the tip.

On the other hand, non-contact atomic force microscopy is abel to achieve true atomic resolution, i.e., single point defects can be detected. The term non-contact also indicate, that frictional forced cannot play any role during the imaging process. Therefore, graphite is an ideal sample to clarify the contrast mechanism in force microscopy. This investigation is also important with respect to scanning tunneling experiments on graphite, which also reaveal a triganol symmetry on the atomic scale (note, that during tunneling tip and sample are not in contact, but forces are still present).

Atomic scale contrast on heterogen III-V semiconductors

Central point is the development of a high resolution atomic force microscope, to study well -defined surface under ultra-higfh vacuum conditions. To achieve the highest possible stability, it should be operated at liquid helium temperatures.

To investigate the contrast mechanism on the atomic scale, measurements with force microscopy should be compared to results obtained with other techniques like scanning tunneling microscopy and surface x-ray diffraction). Of particular interest is the so called chemical sensitivity, i.e., whether differnet atoms can be destinguished by force microscopy. Therefore, the (110) surfaces of III-V-semsconductor like InAs(110) are of interest, since the exhibit both types of two atoms at the surface.

The project is part the "Microcharacterization of Materials and Devices" program of the Volkswagen Stiftung and is been done in cooperation with Prof. R. L. Johnson.

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