Credit: © 2008 APS

Scanning probe microscopes are routinely used to pattern surfaces by pushing and pulling single atoms or molecules across the surface with the tip of the microscope. Now, Michiaki Ohara, Yousoo Kim and Maki Kawai at the RIKEN Surface Chemistry Laboratory in Saitama and the University of Tokyo have moved molecules on a surface with a scanning tunnelling microscope (STM), without actually touching them (Phys. Rev. B 78, 201405; 2008). By using a local electric field produced by the tip, rather than the tip itself, there is less risk of damaging the microscope or breaking up the molecule.

Kim and co-workers placed a negatively charged STM tip just above the centre of a methylthiolate molecule on a copper surface. Electrons tunnelling from the STM tip to the copper surface excite vibrations in the methylthiolate molecule, causing it to jump randomly in one of three directions defined by the copper lattice.

To achieve some level of control over the jumping direction, the researchers positioned the STM tip just off the molecule centre. In this case, the molecules always jumped directly away from the STM tip — a process the team called 'repulsive-hopping'. The molecules could also be made to jump towards the tip, by reversing the electric field.

The Japanese team demonstrated the potential of their approach by spelling out the letters 'S', 'T' and 'M' with methylthiolate molecules on the copper surface. Each of the images shown above is 6 nm across.