In Primo Levi's novel The Monkey's Wrench, a chemist explains to a construction engineer how chemists sometimes fantasize about building molecules:

"We don't have those tweezers we often dream of at night, the way a thirsty man dreams of springs, that would allow us to pick up a segment, hold it firm and straight, and paste it in the right direction on the segment that has already been assembled. If we had those tweezers (and it's possible that, one day, we will), we would have managed to create some lovely things that so far only the Almighty has made."

A team of physicists in Germany now claim in Physical Review Letters1 to have found those tweezers, and to have used them to paste together a molecule from its fragments. They too imagine that wonders may come of this technology: "novel man-designed molecules or [molecular-scale] devices".

These delicate tweezers have in fact been around for many years now: the scanning tunnelling microscope (STM). Invented by Swiss scientists in the early 1980s, the STM was originally meant to be a device for taking snapshots of objects with atomic-scale resolution. But it became apparent that it was also a tool for manipulating molecules.

The STM is basically an extremely fine metal needle, which can be moved across a flat surface with great precision. When a voltage is applied between the tip and surface, electrons 'tunnel' from one to the other, creating a flow of electrical current. The size of the current depends on what is on the surface, and how big the gap is between it and the tip.

If the needle is scanned across a metal or silicon surface, for example, the current rises and falls periodically as the tip moves from the 'peaks' of atoms to the 'valleys' of spaces between them.

The STM will also trace out the shapes of molecules lying flat on a surface, and in some cases each atom in the molecule can be distinguished. More often, molecules appear as more vaguely defined blobs.

These blobs can be pulled around by the electrical forces of attraction between the molecules and the tip. Applying a voltage pulse to the tip can push an atom from the tip to the surface, or blast away an atomic-scale crater in the surface.

Saw-Wai Hla and colleagues at the Free University of Berlin have done something more refined. They say they have chopped two molecules in half and joined together fragments from different molecules, thereby carrying out a chemical transformation on a single molecule that chemists normally conduct many trillions of molecules at a time.

The researchers used the STM to dissect molecules of iodobenzene, lying on the surface of copper. These consist of a benzene molecule -- basically a ring of six carbon atoms -- modified by the appendage of an iodine atom.

Hla and colleagues positioned the STM tip above a molecule and applied a pulse, squirting electrons into the molecule. This transformed it from a single blob to two separate blobs, which they interpreted as the carbon-ring ('phenyl') fragment and the split-off iodine atom.

They then dragged two such phenyl groups together, using the STM as a prod rather than a scalpel. Once these were side by side, the researchers changed the tip voltage again to inject electrons into the two fragments. These electrons welded them together into a biphenyl molecule.

Iodobenzene can be converted into biphenyl using a copper catalyst at a temperature of about minus 50 °C -- a piece of textbook chemistry. Hla and colleagues achieved this with STM surgery at minus 253 °C.

The blurry molecular portraits that the STM usually generates are notoriously difficult to interpret, and some researchers may prefer to reserve judgement until the products of the process come into sharper focus. But if the claim holds up, it will introduce a whole new dimension to the chemist's art of making molecules.