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

Song Xu, Sylvain J N Cruchon-Dupeyrat, Jayne C. Garno, Gang-yu Liu, G. Kane Jennings, Tseh Hwan Yong, Paul E. Laibinis

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168 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)5002-5012
Number of pages11
JournalJournal of Chemical Physics
Volume108
Issue number12
StatePublished - Mar 22 1998
Externally publishedYes

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics

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    Xu, S., Cruchon-Dupeyrat, S. J. N., Garno, J. C., Liu, G., Jennings, G. K., Yong, T. H., & Laibinis, P. E. (1998). In situ studies of thiol self-assembly on gold from solution using atomic force microscopy. Journal of Chemical Physics, 108(12), 5002-5012.