Force microscopy resolves atomic details less than 100 picometers apart

Science, the Washington-based scientific news journal reports in its June 10th online edition of Science Express about a breakthrough in the resolving power of microscopy achieved by a team of physicists at Augsburg University (S. Hembacher, F.J. Giessibl, J. Mannhart, "Force microscopy with light atom probes", www.scienceexpress.org, 10 June 2004). The scientists have imaged an individual tungsten atom by atomic force microscopy and found four distinct peaks that are attributed to highly located electron clouds (Fig. 1). A printed image width of 5 cm corresponds to a magnification of two hundred million. A world-record resolution of 77 pm is demonstrated. The electron structure originates from the quantum-mechanical nature of tungsten bonding. Tungsten develops a body centered cubic crystal structure such that every tungsten atom is surrounded by eight nearest neighbor atoms, causing "arms" of increased charge density which point to the next neighbors. Four of these highly localized electron clouds are visible on surface atoms.


Fig 1	Fig 2, Fig 3

Reversing the role of tip and sample

Atomic force microscopy (AFM) works by mechanically profiling samples with extremely sharp tips. An image is created by recording the spatial variations of tip-sample forces. For optimal resolution, the imaging tip atom should be small. Carbon atoms in graphite are excellent candidates for probing charge structures within atoms. Because graphite has planar surfaces, the role of tip and sample is switched in the experiment: the front atom in a sharp tungsten tip is imaged by a light carbon atom of a graphite surface. This progress is possible because of several innovations:

Detecting higher harmonic oscillations

So far, the force between tip and sample was detected by the deflection of a cantilever beam that holds a sharp tip or by the frequency change of the oscillating cantilever (see Fig. 2). Ideally, AFM would not map the total force that acts between tip and sample, but only the contribution of the tip's front atom. The isolation of the front atom contribution has been a central problem in AFM. Instead of measuring static deflections or frequency changes, higher harmonics triggered by tip-sample forces are analyzed in the improved technique. These higher harmonics are much more sensitive to short-range interactions than the previously used signals.

Taking data at temperatures 5 degrees above absolute zero temperature

The experiment was conducted with a new microscope that is cooled to a temperature of 5 degrees Kelvin and it operates at a pressure of 10^-13 of an atmosphere. The microscope sits on a 30 t foundation and is isolated from sound and electromagnetic stray fields by a metal chamber. The setup of the microscope at the Physics Institute of Augsburg University was funded by a joint research project (EKM) of the state of Bavaria and the federal Bundesministeriums für Bildung und Forschung, managed by VDI.

Already in 2000, the group found structures within single atoms (Giessibl, Hembacher, Bielefeldt, Mannhart, "Subatomic Features on the Silicon (111)-(7x7) Surface Observed by Atomic Force Microscopy" Science 289, 422, 2000). These results have been observed on Silicon, a material that displays pronounced covalent bonding with large distances of 230 pm between lobes. In the new experiment, the resolution is increased threefold, and the covalent character of metal bonding has been imaged for the first time.

In many cases, improvements in microscopy have been a foundation for significant further progress in the natural sciences. It is expected that this improvement will also be of great value to nanotechnology.

10-jun-2004