Atoms that Climb Uphill
Contrary to contemporary thought regarding the behavior of atoms deposited
on thin films, under some conditions atoms climb up crystal protrusions.
This image shows the veritable mountains that are formed by upward bound
aluminum atoms that are deposited on an initially smooth aluminum crystal.
Generally, atoms deposited by a method known as molecular beam epitaxy
form bumps, such as the small structures scattered across the blue field
in this picture. If the structure is heated slightly, the atoms move
about and usually diffuse downward, smoothing out the bumps. New research
shows for the first time that, within a narrow temperature range, aluminum
atoms deposited on an aluminum crystal surface instead crawl upward,
forming large crystalline mountains. The processes leading to upward
atom diffusion are likely to be important for other crystals grown via
molecular beam epitaxy, leading to much richer dynamics in the growth
of thin films than previously suspected.