Molecules terminated at each end with a chemical group that is capable of binding to a metal surface can be used to form a junction between two electrodes. The conductance characteristics of such molecules can be greatly influenced by factors such as their chemical structure, length and shape. However, little is known about the effect that different molecular end groups can have on the electronic properties of these systems.

Now, Latha Venkataraman and colleagues1 at Columbia University in the US have compared the conductance of a series of alkane molecules terminated by three different chemical groups — amines, sulphides and phosphines. Junctions were created by moving the gold tip of a scanning tunnelling microscope in and out of contact with the surface of a gold electrode immersed in solutions of the different molecules. As the tip is moved away from the electrode, the conductance decreases until the contact is reduced to a single chain of Au atoms. Pulling the tip further away breaks the contact, but in the presence of the alkanes, conductance is observed at lengths beyond this point. This suggests that the molecules are bound across the gap between the two electrodes.

When the different end groups were tested, it was observed that the alkanes terminated by phosphines gave the highest conductance of the three types. Although it is thought that this result stems from the fact that Au–P bonds are stronger than either Au–S or Au–N bonds, the size and shape of the electron orbitals that interact with the gold surface also have a role to play.