Electric fieldwork

The electric field from a scanning tunnelling microscope tip can be used to move individual molecules without touching them

Credit: © 2008 APS

Many useful nanoscale assemblies have been made by pushing and pulling molecules with the tip of a scanning tunnelling microscope (STM). Now, Yousoo Kim and co-workers at the RIKEN Surface Chemistry Laboratory in Saitama and the University of Tokyo¹ have moved molecules without touching them, by using the electric field from an STM tip. This way there is less risk of damaging the STM tip or breaking up the molecule.

The researchers placed a negatively charged STM tip just above the centre of a methylthiolate (CH₃S) molecule on a copper surface. Electrons tunnelling from the STM tip to the copper surface excite vibrations in the CH₃S molecule, causing the molecule to jump randomly in one of three directions defined by the copper lattice.

To achieve control over the jumping, the researchers tried positioning the STM tip just off the molecule centre. In this case, the molecules always jumped directly away from the STM tip. The molecules could also be made to jump towards the tip, by reversing the electric field.

The new method is so precise, the researchers managed to arrange several CH₃S molecules to spell out the letters 'S', 'T' and 'M'.