The doping of materials with atoms that accept or donate electrons, and therefore modify the electronic behaviour of the material, plays a crucial role in semiconductor electronics. Crommie and colleagues have now applied this idea to the fullerenes -- molecules that consist of 60 carbon atoms arranged in a spherical shell (figure 1).

The Berkeley team used a scanning tunnelling microscope to drag a carbon-60 molecule over a silver surface containing potassium atoms. They found that they could attach an arbitrary number of potassium atoms to a single molecule. Each potassium atom donates a well-defined number of electrons to the molecule and so allows the electronic structure of the resulting potassium-fullerene complex to be controlled (figure 2). The process can be reversed by simply moving the structures back over the surface, where impurities – such as oxygen – can remove the potassium atoms one by one.

“Previously only extended monolayers and bulk crystals of carbon-60 have been modified through alkali metal adsorption,” Crommie told PhysicsWeb. “Our work opens a completely new regime by showing that it is possible to controllably dope a single, isolated molecule. This puts us in the unique position of knowing and controlling precisely how many dopant atoms are attached to a specific molecule.”

The team now hopes to extend its technique to more complex molecules and other dopant atoms. “We expect that our paper will inspire a whole new class of experiments on new and exciting nanostructured systems,” added Crommie.