Hans Joachim Güntherodt

Characterization of molecular overlayers on metal surface in dynamic equilibrium by scanning tunneling microscope

The overlayer structure of [tetra-(3,5-di-tertiary-butylphenyl)porphyrinato] copper (Cu-TBPP) molecules on Ag (001) and chloro(subphthalocyaninato)boron (III) (SubPc) molecules on Ag (111) were analyzed by a UHV scanning tunneling microscope. In the case of approximately 20% coverage, both molecules showed high mobility on the substrates, which caused them to form an overlayer island on terraces. Cu-TBPP molecules formed an oblique lattice on the Ag (001) surface and SubPc molecules formed a hexagonal lattice on Ag (111). From a high-resolution STM image of the overlayer, characteristic protrusion corresponding to the molecular shapes, such as four-lobes of Cu-TBPP and three-lobes of SubPc, were clearly identified.

Feasibility Studies on a Nanometric Oscillator Fabricated by Surface Diffusion for Use as a Force Detector in Scanning Force Microscopy.

A metal ball supported by a nanometric filiform neck, made by surface diffusion in vacuum, has the potential to be used as an oscillating force detector in scanning force microscopy. Although in most cases, the oscillator is extremely fragile and does not survive the transport from one vacuum chamber to the other, there still remains the possibility that it can be used if fabricated and utilized in situ. With the aim of characterizing the oscillator without breaking the vacuum, we have made a scanning tunneling/force microscope (STM, SFM) with a heating filament for fabrication of the oscillator in a scanning electron microscope (SEM). The formation of the oscillator was observed with the SEM, and then, the tip of the STM/SFM was used for the application of force to verify its feasibility as an oscillator.

SPM for Functional Identification of Individual Biomolecules

The identification of specific binding molecules is of increasing interest in the context of drug development based on combinatorial libraries. Scanning Probe Microscopy (SPM) is the method of choice to image and probe individual biomolecules on a surface. Functional identification of biomolecules is a first step towards screening on a single molecule level. As a model system we use recombinant single- chain Fv fragment (scFv) antibody molecules directed against the antigen fluorescein. The scFvs are covalently immobilized on a flat gold surface via the C-terminal cysteine, resulting in a high accessibility of the binding site. The antigen is immobilized covalently via a long hydrophilic spacer to the silicon nitride SPM-tip. This arrangement allows a direct measurement of binding forces. Thus, closely related antibody molecules differing in only one amino acid at their binding site could be distinguished. A novel SPM-software has been developed which combines imaging, force spectroscopic modes, and online analysis. This is a major prerequisite for future screening methods.

Instrumental developments and recent experiments in near-field optical microscopy

Recent advances in the understanding of light propagation in small dimensions as well as in instrumentation make scanning near-field optical microscopy (SNOM) a very promising tool for studying optical phenomena on a nanometer scale. In this talk, we will demonstrate experiments carried out with the recently developed tunneling near-field optical microscope. We found superior image contrast, as compared with images taken with conventional aperture SNOM, along with the high resolution commonly achieved with fiber probes. This work was motivated by the theoretical investigations presented in Dr. Pohls talk. We will further describe two recently built instruments. The first is a scanning tunneling optical microscope combined with a scanning force microscope. The second instrument is an aperture-type SNOM mounted on the sample stage of a conventional inverted optical microscope. Of particular interest to us is imaging with polarization contrast. One of the goals is to study liquid-crystal films which have been micropatterned with the help of a force microscope tip. These samples are promising as waveguides and potential electro-optical devices. Additionally, they represent very convenient test samples for polarization SNOM.