Controlling gold atom penetration through alkanethiolate self-assembled monolayers on Au{111} by adjusting terminal group intermolecular interactions

The penetration behavior of thermally evaporated Au on S(CH ₂)₁₅CH₃, S(CH₂)₁₅CO ₂CH₃, S(CH₂)₁₅-CO₂H, K-modified S(CH₂)₁₅CO₂CH₃, and K-modified S(CH₂)₁₅CO₂H self-assembled monolayers (SAM) on Au substrates is investigated. Gold is a particularly interesting metal since vapor-deposited Au atoms are known to pass through alkanethiolate SAMs on Au{111} substrates at room temperature. Here we show that it is possible to control Au penetration by adjusting the interactions between terminal groups. It is found that Au atoms evenly penetrate into the CH ₃ and CO₂CH₃ films, forming smooth buried layers below the organic thin films. For the CO₂H film, although Au atoms can still penetrate through it, filaments and mushroomlike clusters form due to H-bonding between film molecules. In the case of the K-modified CO ₂-CH₃ or CO₂H films, however, most Au atoms form islands at the vacuum interface. These results suggest that van der Waals forces and H-bonds are not strong enough to block Au from going through but that ionic interactions are able to block Au penetration. The measurements were performed primarily using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM). The combination of these highly complementary probes provides a very useful strategy for the study of metal atom behavior on SAMs.