Understanding Temperature Dependence in the Supramolecular Structure of Coronene and Alkane Acid Monolayers on Au [111]

Current issues in solution-solid interfaces have focused greatly on the room temperature structure of self-assembled monolayers and nanostructures.  Much less understood is how the various interactions (e.g.; solute-solute, solute-solvent, solute-surface, and solvent‑surface interactions) drive assembly at the solution-substrate interface.  A key component for understanding these processes is measuring and interpreting their temperature dependence.  The present work is an exploratory study in the 0 ºC to 80 ºC temperature range for a particular model system – the Au(111), coronene, heptanoic acid system.  Epitaxial Au (111) provides a well defined surface, coronene acts as solute, and heptanoic acid is the solvent.

Scanning tunneling microscopy (STM) was used to obtain sub-nanometer characterization of these monolayers at different temperatures.  Two distinct morphologies exist for this system; a densely packed coronene structure that does not incorporate the alkane acids into the monolayer and a more lightly packed monolayer that does include acids.  The spacing for these layers was found to be 1.2 nm and 1.5 nm respectively.  The distinct coronene spacing in these films is concentration dependent. The system was also studied by STM at a fixed concentration of 2.0 x 10-5 M while the temperature was held at 0 ºC, 40 ºC and 60 ºC.  It was found that a highly ordered monolayer persisted at these various temperatures as the coadsorbed coronene-acid structure.  The monolayer eventually broke down after several hours of imaging.