Force required to move an atom or molecule

The images show the 2D potential landscapes of the tip–sample interaction energies during the manipulation of (a) cobalt and (b) CO on a Cu(111) surface. The energy scales of the color-coded images are shifted so that U = 0 at the preferred adsorption site for cobalt and CO. The superimposed ball-and-stick model represents the Cu(111) lattice. The size of each image is 550 × 480 pm. (Adapted with permission. Copyright 2008 American Association for the Advancement of Science.)

Exactly how much force do you need to move an atom or a molecule along a surface? Markus Ternes and colleagues at the IBM Research Division Almaden Research Center and the University of Regensburg (Germany) combined the techniques of scanning tunneling microscopy (STM) and atomic force microscopy in a single instrument to get the answer for individual cobalt atoms and CO molecules.

The investigators set up a quartz tuning fork so that one prong was fixed and the other, with a tip at its end, was hanging free. The free prong acted as a cantilever; changes in its bending were measured as the quartz deformed and produced small piezoelectric currents.

The free prong was oscillated at constant amplitude. A force between the tip and the surface changed the resonant frequency of the prong. By recording the change in frequency as a function of the tip–surface distance, Ternes and colleagues quantified the interaction forces between the tip and the atom or molecule.

The investigators discovered that moving a cobalt atom on Pt(111) needed a lateral force of 210 pN that was independent of the vertical force. For cobalt on Cu(111), it only took 17 pN to get the atom moving, suggesting that the lateral force depended on the chemical identity of the surface. For both surfaces, the cobalt atom’s force on the tip was nearly spherically symmetric. In contrast, the forces in manipulating a CO molecule deviated from spherical symmetry.

Forces for cobalt and CO on Cu(111) differed dramatically, even though they have similar tunneling conductance in STM experiments. The investigators measured a 160 pN lateral force for CO molecules that was an order of magnitude larger than that for cobalt atoms. The investigators say that their experiments can lead to a better understanding of the controlled bottom-up assembly mechanisms needed to create nanoscale devices. (Science 2008, 319, 1066–1069)