Chemical Imaging and Distribution Analysis of Mono-, Bi-, and Tridentate Alkanethiol Self-Assembled Monolayers on Gold by Sum Frequency Generation Imaging Microscopy

Sum frequency generation (SFG) microscopy was used to investigate the structure and orientation of multidentate alkanethiolate monolayers on gold surfaces. Adsorbate molecules in which one, two, or three sulfur atoms coordinated to the surface of gold afforded self-assembled monolayers (SAMs) with controlled spacing between the alkyl chains. SFG imaging was used to determine the orientation of the molecules on the surface and to visualize spatial variations across the surface. Using SFG imaging, the average orientation of the terminal methyl group is measured as well as the standard deviation. Further, the presence of film defects is measured by the CH2/CH3 intensity ratio and the distribution quantified through the standard deviation. The results demonstrate that the tridentate adsorbate forms a monolayer in which the alkyl chains are highly disordered on the surface when compared to SAMs derived from normal alkanethiols (i.e., monodentate adsorbates). The results are consistent with a model in which the tridentate alkanethiols generate SAMs with lower alkyl chain density. SFG microscopy provides a detailed view of the origin of the monolayer disorder by statistical spectroscopic analysis of the surface.