The diffusion of Au(CH3S)2 on Au(111) observed with the Scanning Tunneling Microscope

The diffusion of a three-legged molecule, CH3S-Au-SCH3, on Au(111) has been investigated using scanning tunnelling microscopy. Each of the two S atoms forms a bond with a Au atom in the Au(111) substrate. The Au atom in the molecule provides the third anchoring point via its interaction with Au(111). CH3S-Au-SCH3 hops as a single unit without breaking any of the S-Au bonds within the molecule at temperatures below 170 K, with an activation energy of 66 meV. The CH3S-Au-SCH3 molecules have a tendency to aggregate into rows driven by an attractive potential between neighbouring molecules, with the minimum row consisting of just two molecules in the form of a dimer. The dimer is much less mobile than a single molecule due to the attractive potential between the two molecules. The dimer is observed apparently to hop as a single unit with an activation energy of 210 meV. Detachment of a CH3S-Au-SCH3 molecule from the end of a row consisting three or more molecules takes place with an activation energy of 320 meV.