Showing the power of scanning tunneling microscopy (STM), scientists from the Free University of Berlin have used an STM tip to tease single molecules through a complicated chemical dance, called an Ullmann reaction. Normally, making C12H10 molecules from iodobenzene (C6H5I) requires thermal activation on a copper catalyst. But Saw-Wai Hla and colleagues accomplished the same feat, taking the reaction one step at a time. Hla and company suggest that this STM technique, which they describe in the September 25 issue of Physical Review Letters, might also be used to engineer entirely man-made molecules.
The team began with several C6H5I molecules resting on a terraced copper substrate at 20 degrees Kelvin. They then pried them apart into iodine and phenyl (C6H5) by injecting electrons from the STM tip (a). Next they used the tip to pull the iodine away (b and c) and draw the phenyl molecules closer together (d). Bonding the two was as simple as adding in another shot of electrons (e). To prove that the phenyls were chemically joined, they pulled one, and the other followed (f).