Credit: © 2007 ACS

Tetracyanoethylene (TCNE) is a small organic molecule that readily accepts electrons from other compounds to form so-called charge-transfer complexes. Some of the materials formed in this way exhibit potentially useful properties such as superconductivity or magnetism. In particular, certain metal salts of TCNE have ferromagnetic properties that suggest they may be important for molecular magnetism-based applications.

A detailed structural characterization of these materials is often lacking, however, and this limits our understanding of their magnetic and electronic properties. Now, a team from the University of California, Berkeley led by Daniel Wegner and Michael Crommie have studied how TCNE molecules behave on different noble metal surfaces (Au, Ag and Cu) in order to gain some insight into their structural and electronic properties at the organic/inorganic interface1. Using a scanning tunnelling microscope, it was found that the TCNE molecules interact very differently with each metal surface. On a Au(111) surface, the molecules tilt to one side and cluster together loosely to form islands, whereas on a Ag(100) surface, they lie flat and remain isolated from one another. In contrast, on a Cu(100) surface the TCNE molecules lie flat, but are tightly bound into ordered islands and chains.

Scanning tunnelling spectroscopy experiments and theoretical calculations suggest that a number of factors influence the behaviour of the TCNE molecules on the metal surfaces. The strength of the charge-transfer interaction between the molecule and the surface, combined with how well the substrate lattice matches with that of the assembled molecules, determines whether TCNE–TCNE or TCNE–metal interactions dominate the growth processes. These results suggest it may be possible to tune the assembly of functional molecular building blocks in thin films to make nanoscale devices.