In the July 3, 2008 issue of Science magazine, a team from the Montan University in Leoben and the TU Graz explained the growth of ordered thin films of parahexaphenyl. When molecules of parahexaphenyl are evaporated on clean a clean mica surface, the molecules form crystalline needles. Within the needles the molecules are ordered in straight rows with the molecules lying flat on the mica surface.

An atomic force microscope image showing parahexaphenyl needles on mica.

The chemical structure of parahexaphenyl.

When the surface of the mica is bombarded with ions before the parahexaphenyl is deposited, the molecules stand up on the surface. Islands of the molecules have a stepped structure where the steps are one molecule high.

A scanning tunneling microscope image of parahexaphenyl islands.

The form of the island is partially determined by the difficulty that the molecules have in diffusing over the edges of the steps. The energy barrier that the molecules must overcome to go over a step is known as the Ehrlich-Schwoebel barrier. Numerical simulations show that the molecules bend when they go over a step edge. The energy it takes to bend the molecules is an important contribution to the Ehrlich-Schwoebel barrier.

An animation of a molecule diffusing over a step edge.

Gregor Hlawacek, Peter Puschnig, Paul Frank, Adolf Winkler, Claudia Ambrosch-Draxl, Christian Teichert: 'Characterization of Step-Edge Barriers in Organic Thin-Film Growth', Science 321 p. 108 (2008).

chemie.de

Solid state and materials research news: phys. stat. sol. (RRL) 4/2008