Even the smoothest-looking coatings are very rough on the atomic scale, with islands of atoms peppered abundantly across the microscopic landscape. Roughness is not always a bad thing. In fact, it's often beneficial in adhesives, aerodynamic materials, and surfaces that "catalyze" or speed up chemical reactions. Understanding its root causes will help researchers to make surfaces smoother or rougher.
Depositing copper atoms on a Cu surface, researchers at the University of Twente in the Netherlands identified a largely ignored mechanism which contributes to introducing roughness when a beam of atoms is deposited onto a surface. Known as steering, it arises when surface atoms, including already deposited ones, exert chemical forces on incoming atoms and cause them to veer towards the surface. Steering causes incoming atoms to arrive preferentially on the top of protruding islands of atoms on the surface.
Because of the properties of their electrons, metal surfaces exert the greatest steering forces, while semiconductors and insulators are expected to exert weaker forces. The steering effect is most markedly pronounced when the beam of incoming atoms is at grazing angles with respect to a metal surface. This effect is less pronounced for beams of atoms aimed perpendicularly at a surface.
Besides providing insights into the causes of roughness, understanding this effect may help researchers to prepare arrays of surface ridges, which could serve as templates for making magnetic nanowires and other customized materials.
This research is reported by Sebastiaan van Dijken, Louis C. Jorritsma, and Bene Poelsema in the 17 May 1999 issue of Physical Review Letters.