Surface Phase Behavior of <i>n</i>-Alkanethiol Self-Assembled Monolayers Adsorbed on Au(111):  An Atomic Force Microscope Study

Abstract

n -Alkanethiol (C n H 2 n +1 SH) self-assembled monolayers (SAMs) adsorbed on Au(111) were studied with an atomic force microscope (AFM) to confirm the influence of the lateral interaction between adsorbed thiols on the film morphology. Two experiments were performed: firstly, a study of the domain formation at the initial stage of SAM growth (single component) and, secondly, investigations of the coadsorption phenomenon in mixed SAMs composed of two alkanethiols having different chain lengths. For the kinetics study, Au(111) was immersed into the 10 -2 mM ethanol solutions with the single component alkanethiol (C 4 H 9 SH, C 12 H 25 SH, or C 18 H 37 SH), for varying times (1 s to 10 min). In all cases, the film coverage increased as the immersion time became longer, and finally the surface was totally covered with thiols after an immersion time of 3 min or more. Clear island formations were observed in the partially covered C 12 H 25 SH and C 18 H 37 SH SAMs, while C 4 H 9 SH formed meshlike domains. The mixed SAMs were prepared by immersing Au(111) into 1 mM ethanol solutions with mixed alkanethiols (C 4 H 9 SH/C 18 H 37 SH) of various compositions, R soln = [C 4 H 9 SH]/[C 18 H 37 SH] = 1/1 to 100/1, for a time of 1 h. Clear phase separation was observed at R soln = 20/1 and 40/1. Above or below these compositions, the film surface appeared very flat, covered with a nearly single component, C 4 H 9 SH or C 18 H 37 SH, respectively. This is the first systematic study of the surface phase behavior of alkanethiol SAMs by AFM imaging. It reveals more direct information about the film morphology than previous studies with conventional surface analytical techniques such as X-ray photoelectron spectroscopy, ellipsometry, contact angles, etc .