The density of components in silicon microchips has grown exponentially for more than four decades, but this progress is likely to slow down as devices approach the nanometre scale. Heinrich and colleagues have shown in principle how to overcome this problem by using a pair of low-temperature scanning tunnelling microscopes to arrange pairs of carbon monoxide molecules on a copper surface.
They moved a single carbon monoxide molecule alongside one of these pairs, so that the three molecules formed a chevron -- the shape of an arrow head. However, this formation was unstable because it raised the energy of the system. The molecule at the tip of the chevron therefore hopped to the next pair of molecules, creating a new chevron, which in turn decayed. This process cascaded throughout the pairs of molecule, in a similar way to the motion of falling dominoes.
The IBM researchers used this principle to make an AND gate. They placed three rows of molecule pairs in a Y shape, with a single molecule at the point where they met. Two rows acted as the inputs and the third acted as the output. Only if there was a cascade (i.e. data) in both of the input rows would a molecule hop from the end of each row to form a chevron with the single molecule at its tip. This chevron would then decay, generating a cascade (i.e. a signal) at the output.
The researchers used a similar arrangement to make an OR gate, by placing three molecules in a stable chevron formation at the head of the output row. Only when a fourth molecule from either of the two inputs was added did the chevron decay and create a signal at the output.
Heinrich and co-workers were then able to join several AND and OR gates together to make more complicated logic devices. One such device, a three-input sorter, would have an area of about 50 um2 if made using current technology, but it measured just 200 nm2 when constructed from molecular cascades.
Unfortunately the molecular cascade devices made by the IBM researchers were very slow and could only be used to perform a single operation. To re-use the devices the researchers had to place the molecules back into their original position using one of the scanning tunnelling microscopes. To be useful, molecular cascade computers would need an automatic mechanism that would reset some of the molecules and leave the others intact to act as data registers.