Credit: © 2006 AAAS

The assembly of a large number of molecules into an ordered arrangement is not an easy thing to control or predict. The nuances of molecule–molecule interactions must be considered as well as other external factors. In solutions, the role of solvent molecules may be important; on surfaces, substrate–adsorbate interactions will play their part.

Ludwig Bartels and co-workers1 from the University of California, Riverside, have investigated how anthraquinone — a small organic molecule — assembles on a copper surface. Images taken with a scanning tunnelling microscope reveal the formation of a regular honeycomb lattice. The sides of each hexagon are formed from three parallel molecules, resulting in an open porous network. However, this particular pattern was only observed when a certain fraction of the surface (at least 15 molecules for every 1,000 copper atoms) was covered with anthraquinone. With a more sparse coverage, well-spaced individual rows were seen, whereas at higher densities, islands of closely packed molecules began to emerge. The assembly of a hexagonal array is thought to arise from the delicate balance of short-range molecule–molecule attractions and long-range surface–molecule repulsive interactions.

A better understanding of molecular assembly on surfaces will allow for the creation of designer films with properties that can be tailored for specific applications such as catalysis and templated synthesis.