Nanostructuring, Imaging and Molecular Manipulation of Dithiol Monolayers on Au(111) Surfaces by Atomic Force Microscopy

In this paper, we use atomic force microscopy (AFM) to nanostructure and image 1,10-decanedithiol (DDT) and biphenyl 4,4‘-dithiol (BPDT) layers on Au(111) surfaces comparing them to those prepared by self-assembly. First, layers of dithiols are self-assembled from solution onto gold surfaces and are imaged in situ with an AFM to examine the roughness of the layers. Second, 100 nm × 100 nm monolayer patches made of dithiol molecules are nanografted into a self-assembled monolayer (SAM) inert matrix made of 1-decanethiol (DT). Although nanografting of thiols routinely generates very compact layers with good height uniformity, nanostructuring of dithiols using this method always yields multilayers that form through intermolecular S−S bonds. We demonstrate in this paper two possible ways of tailoring, layer by layer, the structure of dithiols. First, we form multilayers by nanografting, using then the AFM tip to gradually shave away the top layers. In the second way, we add antioxidant to the solution while doing nanografting to suppress the oxidative coupling of the −SH groups. We found that nanografting in the presence of excess amounts of antioxidant can produce monolayers of dithiols. The so-produced DDT monolayer patches are lower than what can be calculated by the 30° tilt model, while the height of nanografted patches of BPDT closely corresponds to a vertical configuration. Finally, we use conductive-probe AFM to investigate the electron tunneling properties through BPDT multilayers. The molecules in these layers turn out to behave as conductive molecular wires and making these nanostructures good candidates for constructing molecular electronic devices.