In a major breakthrough in nanoelectronics, a team led by Michael Crommie has used a scanning tunneling microscope (STM) to attach individual potassium atoms to isolated C60 molecules (“buckyballs”) to precisely control their electronic properties. Up to seven potassium atoms were added and removed in a controlled manner, demonstrating that the electronic properties of a molecular structure can be reversibly tuned with atomic precision.

Atomic control here was achieved using a scanning tunneling microscope capable of moving single atoms with great precision. First, widely separated C60 molecules and potassium atoms were deposited on the surface of a silver crystal polished to virtually perfect flatness and held at 7 K. Under these conditions, when the tip of the STM is brought close enough to attract individual atoms or molecules, they can be moved at will. The team discovered that when the STM tip was used to move a C60 molecule over a potassium (K) atom, the buckyball “picked up” the potassium. Marked changes in the electrical characteristics of the C60-K complex confirmed that the atom was attached in the complex. The same technique was used to add more K atoms, up to a total seven. In a process that has no real analogy in standard IC manufacturing, dopants were also removed. By moving the buckyball over an impurity in the silver surface (thought now to be an oxygen atom), the potassium atoms were "pulled off" one at a time.
The effect of the K atoms is well understood. They add charge to the C60 and cause its molecular orbital states to fill with electrons in a manner analogous to the way electrons are reordered in a semiconductor when it is “n-doped” (on a much larger scale) using conventional techniques. Work is continuing to create the opposite electrical effect through “p-type” doping in C60 by incorporating atoms which would pull electrons out of the C60. Then a molecular p-type/n-type junction (the most essential solid-state junction) for electronic devices could be made.