CHEMICAL IMPACT
Seething Surfaces
hen we picture a solid, we think of the particles as being packed closely together with relatively little motion. The particles are thought to vibrate randomly about
their positions but stay in nearly the same place. Recent research, however, indicates surface particles are a great deal
more mobile than was previously thought. Independent
teams of scientists from the University of Leiden in the
Netherlands and Sandia National Laboratory in New Mexico
have found a surprising amount of atom-swapping occurring
on the surface of a copper crystal.
The Dutch scientist Raoul van Gastel and his colleagues
used a scanning tunneling microscope (STM) to study the
surface of a copper crystal containing indium atom impurities. They noted that a given patch of surface would stay the
same for several scans and then, suddenly, the indium atoms
would appear at different places. Surprisingly, the indium
atoms seemed to make “long jumps,” moving as many as
five atom positions between scans. The most likely explanation for these movements is a “hole” created by a copper
atom escaping the surface. This hole moves around as other
atoms shift to fill it in succession (see accompanying figure). The best analogy to the movement of the hole is the
toy slide puzzle with 15 numbered pieces and one missing
W
piece in a 4 4 array. The object of the game is to slide a
piece into the hole and then repeat the process until the numbers appear in order.
The hole on the copper surface moves very fast—up to
100 million times per second—shuffling copper atoms and
allowing the indium atoms to change positions. Van Gastel
believes that all of the observed motion results from just
a few fast-moving holes. In fact, he suggests that just one
in 6 billion copper atoms is missing at a given time, analogous to one person in the entire earth’s population. Its
absence causes a given atom on the surface to move every
30 or 40 seconds. Brian Swartzentruber of Sandia National
Laboratories came to similar conclusions using an STM
to track the movement of palladium atoms on a copper
surface.
These results have important implications. For example, metal surfaces are often used to speed up particular reactions. The motions on the metal surface could significantly
influence the way that reactants interact with the surface.
Also, a lot of effort is now being expended to construct tiny
“machines” (called nanoscale devices) by assembling individual atoms on a solid surface. These devices could be literally torn apart by excess surface motions.
A section of a surface containing copper atoms
(red) and an indium atom (yellow). A hole due
to a missing copper atom is shown on the left.
The blue line on the right shows the movement
of this hole. As an atom moves to fill the hole,
the hole moves as well. In the process, the
indium atom jumps to a new position.
A toy slide puzzle.
Reprinted with permission from Physical Review Letters, Feb. 19, 2001,
Volume 86, Issue 5, “Nothing Moves A Surface: Vacancy Mediated
Surface Diffusion” by Dr. Raoul van Gastel et al, pp. 1562–1565.
Copyright © 2001 by the American Physical Society.