Scanning tunneling microscopy tip-assisted modification of Ti(IV) dithiothreitol self-assembled monolayers on Au(111): restructuring of the gold surface

Au(111) surfaces modified by exposure to a dilute solution of a Ti(IV)/dithiothreitol ligand to metal charge transfer complex (Ti(DTT)2) show a remarkable response to continuous scanning by scanning tunneling microscopy (STM). Vacancy islands on the gold surface, which are formed during the self-assembly of the molecular overlayer, coarsen and even merge during STM scanning at negative sample bias. In certain instances, the vacancy islands even line up to form parallel nanostructures. We believe the effect is due to mechanical interactions between tip and sample, which is enhanced by electrostatic effects. The Ti(DTT)2 complex is anchored to the gold surface via Au−S bonds, but due to the fact that there are multiple thiol groups, there may be “uncoordinated” thiols left “dangling”. The tip-induced modification involves the interaction of the tip with these “dangling” sulfurs, which in turn causes movement of single complex molecules with the attached sulfur-bonded gold. Under negative sample bias, the electric field weakens the binding between the sulfur-bonded gold atoms and the surrounding gold atoms in the surface allowing for the observed tip-induced dynamics. In contrast with the Ti(DTT)2 complex, a similar Ti(IV)/3-mercapto-1,2-propanediol complex (Ti(MPD)2) does not exhibit any tip-induced effects. In this case, there are no dangling sulfurs to interact with the STM tip. In addition, similarly prepared dithiothreitol, dithiothreitol titanium isopropoxide, and 3-mercapto-1,2-propanediol self-assembled monolayers (without dangling sulfurs) do not exhibit the tip-induced effect.