Self-Assembled Monolayer of Short Carboxyl-Terminated Molecules Investigated with ex Situ Scanning Tunneling Microscopy

Self-assembled monolayers (SAMs) of thiolated molecules have numerous potential applications and are ideal models for complex supramolecular systems. The structure of these two-dimensional crystals is not understood fully yet and deserves in-depth investigations. Here we show that a short carboxylic acid omega-terminated molecule (3-mercaptopropionic acid, MPA) forms highly ordered SAMs on Au(111) surfaces. Molecular resolution scanning tunneling microscopy (STM) images were taken at room temperature in air (ex situ). They showed a root 3 x root 3 unit cell that, when imaged at high set current, appears reconstructed as a c(4 x 2) superlattice. This structure is quantitatively the same as that of intermediate-length alkane thiols, but is different from the structure found previously for SAMs of the same molecule investigated with liquid-STM during the formation of the monolayer (in situ). This work highlights the possibility for a short omega-terminated molecule to form ordered monolayers over large areas with few defects.