Dynamic friction in controlled molecular manipulation

抄録

Friction is a ubiquitous phenomenon caused by the relative motion of two bodies and involves the dissipation of energy. Although this phenomenon is well known and has been studied for many years, it is not easy to understand because of the multiple contributions at different scales. To understand the phenomenon, it is ideal to study systems defined at the atomic scale, a typical example of which is the manipulation of a molecule on surfaces using atomic force microscopy (AFM) or scanning tunneling microscopy (STM). Such molecular manipulation has the advantage of producing fascinating nanostructures on surfaces, and its elementary processes have been studied. One of the most impressive manipulation studies is the measurement of the force required to move a molecule with AFM, i.e. the observation of static friction [1]. On the other hand, the dynamics and energy dissipation during molecular manipulation, i.e. kinetic friction, has not been extensively studied due to the hurdle of measurement and the difficulty of interpretation based on ab initio calculations.

Here we have studied the manipulation dynamics of a single CO molecule on a Cu(110) surface with a metallic tip by combining AFM, inelastic electron tunneling spectroscopy, and density functional theory calculations [2,3]. We found the following: (1) When the tip is far away from the surface, a CO molecule is adsorbed on a top site, but when the tip approaches the molecule just above the top site, the adsorption site switches to the bridge site. (2) When the oscillating tip is brought close to the molecule, the switching between the top and bridge sites occurs in correlation with the oscillation of the tip, and an energy dissipation signal is observed. (3) When the tip is placed close to the neighboring top site, molecular manipulation occurs from the top site to the neighboring top site. (4) In this case, even when the oscillating tip is brought close to the molecule, the switching occurs only once, so the energy dissipation is not measured in the time-averaged measurements. (5) Such a bridge site plays an important role as an intermediate state in molecular manipulations where the tip is scanned laterally. (6) By considering this intermediate state, the dynamics of molecular manipulations, including the difference between static and dynamic friction, can be understood. (7) Such an intermediate state has also been observed for CO manipulation on a Cu(111) surface, suggesting the generality of our results.

References

[1] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl, A. J. Heinrich. Science 319, 1066-1069 (2008).

[2] N. Okabayashi, T. Frederiksen, A. Liebig, F. J. Giessibl, accepted in Phys. Rev. Lett.

[3] N. Okabayashi, T. Frederiksen, A. Liebig, F. J. Giessibl, accepted in Phys. Rev. B