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Article

Steric Effects on the Adsorption of Alkylthiolate Self-Assembled Monolayers on Au (111)^†

Yanping Cao,^‡ Qingfeng Ge,^§ Daniel J. Dyer,^‡ and Lichang Wang*^‡

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901-4409, and Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904

J. Phys. Chem. B, 2003, 107 (16), pp 3803–3807

DOI: 10.1021/jp021989+

Publication Date (Web): March 29, 2003

†

This paper is dedicated to James W. Neckers on the occasion of his 100^th birthday.

‡

Southern Illinois University.

University of Virginia.

To whom correspondence should be addressed. E-mail: lwang@ chem.siu.edu. Phone: 618-453-6476. Fax: 618-453-6408.

Abstract

Steric effects on the adsorption of self-assembled monolayers (SAMs) formed by alkylthiolates on the Au(111) surface were investigated using density functional theory. Based on the (√3 × √3) R30° structure, the current results on methylthiolate (CH₃S) show that the adsorption prefers the face-centered cubic-bridge and hexagonal close-packed-bridge sites. Furthermore, the adsorption energy decreases slightly compared to the CH₃S adsorption on the p(2 × 2) structure due to lateral interactions. Comparison between the results on CH₃S and 1-propylthiolate (C₃H₇S) illustrates that the adsorption energy increases with chain length. Strong steric effects were found due to the chain length of the alkylthiolates and the hydrogen atoms in the CH₂ unit adjacent to the S atom (α hydrogens). The energetically favored tilt angle is 20° for C₃H₇S. The preferred geometry for both CH₃S and C₃H₇S adsorptions has the two α hydrogens pointing toward the bridge Au atoms of the surface. The results suggest a flat potential energy surface, which correlates well with the dynamic nature of alkylthiolate SAMs observed experimentally.