Force microscope achieves resolution record

Atomic details in two hundred million magnification

Force microscope University of Augsburg
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A research team from the University of Augsburg has succeeded in significantly improving the spatial resolution of microscopy, reports the science journal Science. The scientists modeled a single tungsten atom with an atomic force microscope and found four regions of increased electron density within the atom, which appear in the images as electron clouds. With a width of the image of five centimeters, the magnification corresponds to two hundred million times.

The resolution of the image is 77 pm, an unprecedented value. The electron structure shown has its origin in the quantum mechanical properties of crystalline tungsten. The crystal structure of tungsten is body centered cubic, so each tungsten atom is surrounded by eight nearest neighbors and forms these bonds with locally increased electron densities. From these eight electron clouds, four clouds can be observed at the crystal surface.

Roll of probe and sample turned over

In atomic force microscopy, the samples to be examined are mechanically scanned with a very fine tip. The microscopic image is obtained from the spatial variation of the forces between sample and tip. In order to get the best possible resolution, it was important for the researchers to use a very small, light atom as an atomic probe. Carbon atoms in graphite crystals are excellent candidates for this. Because graphite crystals are flat, the scientists simply reversed the role of probe and sample: the last atom protruding from a sharp tungsten tip is imaged by a light carbon atom of graphite. This progress has been made possible by several innovations.

Evaluation of the harmonics

So far, the force acting between the tip and the sample has been measured either by the static deflection of a cantilevered cantilever or by the frequency change of a vibrating cantilever (in Figure 2, top left). Actually, one does not care for the entire force acting between tip and sample, but only for the proportion between the atom which protrudes furthest from the tip (front atom) and the next sample atom. A central problem of force microscopy is the detachment of the front-atom contribution. Instead of a static deflection or a change in frequency, this experiment evaluates cantilever harmonics resulting from the tip-sample interaction. These harmonics respond to the short-range intra-atomic forces much more sensitive than the static deflection of the beam or its frequency change.

Five degrees above absolute zero

The experiment was performed in a novel microscope cooled to a temperature of only five degrees above the absolute temperature zero. In addition, the instrument operates in ultrahigh vacuum with a pressure of about 1 × 10 -13 of one atmosphere. The microscope is built on a 30 t foundation and isolated from external disturbances such as sound and electromagnetic interference by a metallic soundproofing chamber. The construction of this microscope at the Institute for Physics of the University of Augsburg was supported by a long-term joint research project (EKM) of the Free State of Bavaria and the Federal Ministry of Education and Research with a project support by the VDI. display

Resolution tripled

Already in the year 2000 the research group found structures within single atoms. The results at that time were achieved on silicon, a material that shows pronounced covalent bonds, with a large gap between the two charge lobes of about 230 pm. In the new experiment, the spatial resolution is tripled; moreover, the covalent bonding character was first imaged in a metal. Improvements in microscopy have in many cases been the basis for major advances in science. It is expected that this further development of atomic force microscopy will also be of great benefit to nanotechnology.

(University of Augsburg, 11.06.2004 - NPO)