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Individual adatoms on a close-packed (111) metal surface can reside
in two different binding sites, the so-called fcc (green sphere
in the sphere model) and hcp (blue sphere) sites. Since the energy
difference between these two sites and the related diffusion barriers
can be very small, we expect that the site occupation preference
will depend critically on the size of the metal structures, i.e.
is different for monomers, dimers and trimers. Even new effects
such as a localized diffusion behavior of single dimers can be expected,
as has been predicted recently in a theory work by Bogicevic and
coworkers (see sphere model). Although in homoepitaxy Ir/Ir(111)
has been the only experimental example for a hcp site preference
so far, several similar systems may well exist.
Using scanning tunneling microscopy, we determined the adsorption
site of copper metal adatoms to be the fcc site. For this we employed
a unique diffusion method in combination with atomic manipulation.
The individual adatoms were then used as markers to identify the
adsorption sites within dimers, trimers, and close-packed copper
chains. For a wide temperature range of 7-21 K a localized diffusion
behavior of Cu dimers was observed for the first time. This behavior
exhibited the properties predicted in a theoretical work by Bogicevic
and coworkers.
It is possible to follow the individual diffusion steps by taking
a series of STM images at a temperature of 5 K. The stable fcc-fcc
and the metastable fcc-hcp configuration can be observed. The relative
probability of observation at 5 K yields an energy difference between
the two configurations of only 1.3 meV. The hopping behavior reveals
that thermally assisted tunneling dominates the adatom diffusion
at low temperatures, although copper is a rather heavy atom.
Trimers were found to be of fcc-fcc-fcc type and stable in the
temperature range of 5-21 K.
References
S. C. Wang and G. Ehrlich, Surf. Sci. 239, 301 (1990).
A. Bogicevic, P. Hyldgaard, G. Wahnström, and B. I. Lundqvist,
Phys. Rev. Lett. 81, 172 (1998).
J. Repp, G. Meyer, K. H. Rieder, and P. Hyldgaard, Phys. Rev.
Lett. 91, 206102 (2003).
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