In situ studies of thiol self-assembly on gold from solution using atomic force microscopy

The kinetics and mechanism for the solution-phase adsorption of n-alkanethiols onto gold to form self-assembled monolayers (SAMs) have been monitored in situ using atomic force microscopy (AFM). Time-dependent AFM images reveal detailed structural information about the adsorbed layer during its growth. In 2-butanol, CH₃(CH₂)₁₇SH molecules initially adsorb on gold with the molecular axis of their hydrocarbon chains oriented parallel to the surface. As the surface coverage increases to near saturation, a two-dimensional phase transition occurs and produces islands composed of molecules with their hydrocarbon axis oriented ∼30° from the surface normal. Continued exposure to the thiol solution results in a greater number of these islands and the growth of these nuclei until a SAM is formed with a commensurate (√3 × √3)R30° structure. The growth of the lying-down phase follows a first-order Langmuir adsorption isotherm, while the phase transition is best described by a second-order reaction. The kinetics of the self-assembly process also depends on the chain length of the alkanethiol and the cleanness of the gold surface. Longer-chained thiols, such as CH₃(CH₂)₁₇O(CH₂)₁₉SH, formed complete SAMs more rapidly than did shorter-chained thiols, such as CH₃(CH₂)₁₇SH. The physisorbed, lying-down phase for CH₃(CH₂)₁₇O(CH₂)₁₉SH was less homogeneous and its two-dimensional phase transition was more complicated than for CH₃(CH₂)₁₇SH and CH₃(CH₂)₂₁SH, as the CH₃(CH₂)₁₇O(CH₂)₁₉SH molecules adopt multiple conformations. Of these, the two dominant ones are an all-trans, and another where the hydrocarbon chain adopts an all-trans conformation except for a gauche bond on both sides of the ether unit. These conformers coexist on the surface during the initial adsorption and its transition to the standing-up phase, but change to the all-trans structure in the complete SAM.