Scanning electron microscopy and atomic force microscopy of alkanethiol monolayers on gold

Alkanethiolates are adsorbed rapidly at gold and silver surfaces, forming monomolecular layers. These structures find applications in non-linear optics, molecular electronics, biosensors, adhesion and wetting. With the aim of studying the surface morphology of such structures at various magnifications, gold film surfaces covered with 1-octadecanethiol monolayers were inspected by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The evaporated gold films were covered using both the Langmuir-Blodgett (LB) and self-assembly methods. SEM images of films deposited by the vertical-dipping method show ridges roughly perpendicular to the dipping direction. The self-assembled (SA) monolayers have a more uniform aspect. Both LB and SA films can be distinguished from the gold surface by larger heap-shaped structures. Patchwork structures appear in films partially removed. AFM images with a magnification similar to that of the SEM images reveal more details of the surface morphology. Thioalkane monolayers appear less well defined than gold films, but the quantitative evaluation of the roughness of both surfaces brings about very similar results. Molecular resolution images of octadecanethiol monolayers show periodical structures attributed to vertically oriented alkane tails.