Until recently the well-publicized ability of the scanning tunnelling microscope to move individual atoms and molecules on surfaces has been restricted to cryogenic temperatures. At room temperature the atoms suffered from 'thermal judder', which caused them to jump erratically across the surface. Now John Pethica and colleagues from Oxford University have discovered how to move single atoms of bromine around a super-smooth copper surface. The technique resembles the 'cut' shot in snooker or billiards (Nature 404 743).
Scanning tunnelling microscopes (STMs) rely on an ultrafine ‘tip’ to push atoms over a surface. The atoms are pushed either by the tip itself, or by an electric field. The new technique relies on the current that tunnels from the tip to the atom. As the tip approaches the bromine atom, the atom moves closer to the surface. At a critical point, the atom is repelled, breaks contact with the surface, and jumps away from the tip along the easiest available direction. This is at right angles to the direction that the tip is moving in — just like the ‘cut’ shot in snooker. Pethica and colleagues solved the thermal judder problem by including a ‘dither’ – a rapid, small amplitude oscillation – perpendicular to the direction that the tip was moving in. This stops the tip from missing the atom and also prevents ‘miscues’.
