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March 16, 2009

Atomic Force Microscopes can Swap Atoms and Are Getting Faster

A recent Science paper describes interchanging atoms between an Atomic Force Microscope (AFM) tip and the surface. Some tips deposit Si [silicon], others deposit Sn [Tin], some alternate. [H/T Eric Drexler who describes the process and its significance]

The ‘Si’ structure took 1.5 hours to make. Most of this time was spent imaging, and some of this time was spent waiting for a new Si atom to appear on the tip. I say “appear”, because the tips recharge spontaneously, not by picking up atoms from the surface.


Separate UK Work Speeding Up Atomic Force Microscopes

One limiting factor of conventional AFM operation is the speed at which images can be acquired. Over the past five years, researchers at the Nanophysics and Soft Matter Group at the University of Bristol have been developing a high-speed AFM capable of video-rate image capture. An AFM with this ability enables nanoscale processes to be observed in real-time, rather than capturing only snap-shots in time.

An obvious application of this instrument is to modify the sample surface while observing changes in the surface topography. Successful demonstration of this would indicate the potential for a new generation of fabrication tools. James Vicary and Mervyn Miles, scientists at the above-mentioned Nanophysics and Soft Matter Group,
have now done exactly that.

In the March 4, 2009 online edition of Nanotechnology, they describe the application of the high-speed AFM developed by their group for nanofabrication (In Situ Real-time nanofabrication with high-speed atomic force microscopy).

While the two scientists did not observe any damage to the nanostructures, despite the tip having passed over the features in excess of 250 times, they found that combined high-speed imaging and nanostructuring does, however, lead to degradation of the AFM tip over time.

An immediate solution to this problem would be to lower the electric field strength and accept longer fabrication times. Ultimately, however, an alternative tip composition or tip coating would be favorable.

An immediate solution to this problem would be to lower the electric field strength and accept longer fabrication times. Ultimately, however, an alternative tip composition or tip coating would be favorable.


Oxide features were fabricated during imaging, with relative tip–sample velocities of up to 10 cm/second, and with a data capture rate of 15 fps.








Interchanging Atoms
Complex Patterning by Vertical Interchange Atom Manipulation Using Atomic Force Microscopy.

Researchers assembled complex atomic patterns at room temperature by the vertical interchange of atoms between the tip apex of an atomic force microscope and a semiconductor surface. At variance with previous methods, these manipulations were produced by exploring the repulsive part of the short-range chemical interaction between the closest tip-surface atoms. By using first-principles calculations, we clarified the basic mechanisms behind the vertical interchange of atoms, characterizing the key atomistic processes involved and estimating the magnitude of the energy barriers between the relevant atomic configurations that leads to these manipulations.



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