How much force is needed to move an atom or molecule on a crystal surface?
Scientists have long been able to manipulate single atoms and molecules with atomic force microscopes (AFM) but they have not tried to measure the force needed to do so. Now Markus Ternes and co-workers1 at the IBM Almaden Research Center and the University of Regensburg have measured these forces, thus providing new insights into atomic-scale friction and assembly mechanisms.
When an atom or molecule is placed on a crystal surface it sits in preferential adsorption sites between crystal atoms. Ternes and co-workers moved their AFM tip back and forth above an atom sitting on a surface, progressively lowering the height of the tip and thus applying larger and larger forces on the atom until it hopped to a neighbouring adsorption site.
The driving forces in this game of very miniature golf depend on the details of the atom or molecule, the surface, and the chemical interactions between them. For instance, a cobalt atom can be moved along a copper surface with a tenth of the force that is needed to move a carbon monoxide molecule. However, moving a cobalt atom on a platinum surface takes over 12 times the force needed to move a cobalt atom on a copper surface, even though both surfaces have the same crystal structure.
Ternes, M. Lutz, C. P. Hirjibehedin, C. F. Giessibl, F. J. & Heinrich, A. J. The force needed to move an atom on a surface. Science 319, 1066–1069 10.1126/science.1150288 (2008).
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Reid, T. Feeling the force. Nature Nanotech (2008). https://doi.org/10.1038/nnano.2008.64