Structural Characterization and Regulation of Organodithiol Self-Assembled Monolayers on Gold: An AFM Study
Agilent Technologies, Inc., US
Keywords: self-assembled monolayers(SAMs), organodithiol, AFM, nanografting
Abstract:a,w-organodithiols absorbed on gold are good candidates to generate thiol-terminated self-assembled monolayers (SAMs) if molecules attach to the substrate through only one end and adopt a standing-up configuration. However, the existence of a thiol group at each end of the molecule raises the debate on whether only one or both sulfur atoms would bind to gold. In addition to the formation of a desired standing-up monothiolate case, dithiol molecules may adopt a lying-down or looped orientation due to the dithiolates interactions. Using α, -aliphatic dithiols as an example, alkanedithiol SAMs prepared from the most widely used natural growth approach are well characterized by AFM. Combined high-resolution imaging and accurate layer thickness measurements offer the capability of differentiating dithiol molecular configuration. It clearly reveals that most dithiol molecules are lying-down in the naturally grown SAMs. Furthermore, a new approach to achieve high-quality thiol-terminated assemblies is developed, using an AFM-based lithography method known as nanografting performed in a dithiol solution. The spatially constrained reaction environment in nanografting sterically favors the adoption of an upright configuration for the initially adsorbed dithiol molecules. As a result, dithiol molecules adopt are standing-up and densely packed to present free SH groups on the surface. Nanografting also enables the fabrication of thiol-terminated layers on gold with nanometer-scale precision in geometry, size and location.