The atomic slide puzzle

This work was started in search of an explanation for the beneficial "surfactant" mechanism, which indium has on the growth of copper on the Cu(001) surface.

Raoul van Gastel found with his STM-measurements that, at room temperature, the indium atoms quickly embed themselves into the outermost atomic layer of copper - each one replacing a single copper atom in that layer. Directly after the deposition of In on Cu(001), indium atoms are found to be embedded in the direct vicinity of steps on the surface as is shown the image below.

The embedded indium atoms exhibit a peculiar mobility; they make long jumps separated by long time intervals. To make things even more peculiar, the jumps of nearby indium atoms appear to be synchronized in time! the embedded indium atoms slowly diffuse away from the steps at which they first entered the surface. An example of this diffusion of the embedded indium atoms is shown in the movie below.

(Click on image to see movie 1.8 Mb)

Each yellow protrusion in the movie is the STM-image of a single, embedded indium atom. The vertical scale is grossly exaggerated; the indiums "stick out" by only 40 pm. The fine corrugation on the red terrace is the atomic structure of the Cu(001) substrate. This movie was recorded at a low speed of 20 s/image. However, even in our fastest movies (up to 18 images/s) the long jumps of the embedded indium atoms appeared completely instantaneous to the STM. Typical jump lengths are a few lattice spacings.

From accurate measurements of the statistics of the observed jump lengths and of the waiting times between successive jumps, Raoul van Gastel deduced that the mobility of the indium atoms is caused by the rapid, two-dimensional diffusion of a very low density of monatomic vacancies (missing copper atoms), through the first copper layer. Due to their ultrahigh diffusion rate, these vacancies remain "invisible" for the STM at room temperature. The whole incorporation and diffusion process is illustrated in the ball model cartoon movie shown below.

(Click on image to see movie 9.6 Mb)

The situation where a minute density of vacancies causes the diffusion of surface atoms closely resembles the toy known as the "slide puzzle", where a single missing tile can be used to rearrange all the tiles of an image. At room temperature the Cu(001)-surface the atomic-scale realization of the slide puzzle.

It is important to realize that the slide-puzzle diffusion mechanism is also active when there is no embedded indium in the copper surface. The indium merely serves as a low density of "tracer" particles, which enable us to follow the rearrangements continually taking place in the surface. What we learn from these observations is that not only adatoms, but also the atoms in a close-packed terrace of a metal surface are mobile at relatively low temperatures, e.g. room temperature.

We expect that the slide-puzzle diffusion effect is not restricted to copper and also not to (001) surfaces of fcc metals. It is probably a quite general effect, providing a significant contribution to material transport on (or rather in) a large number of metal surfaces.

Popular descriptions of this work appeared in ScienceNews, in Physics News and in various other places:
(if copies are not available from the web, write to Ellie van Rijsewijk if you'd like to have a hardcopy)

Interface Physics Group Joost Frenken: E-mail Last modification: 11.08.2004