Abstract
Self-assembled monolayers (SAMs) of anthracene-2-thiol (AnT) on Au(111) have been investigated using scanning tunneling microscopy (STM). A preparation of AnT-SAMs from ethanolic solutions results in a deposition of multilayer films. As a result, the general features that have been frequently observed for different systems of thiol-modified gold surfaces are hidden in AnT-SAMs. The thin overlayers on top of the chemisorbed anthracenethiolate monolayer are removed by the STM-tip after a repetitive scanning over the same part of the SAM at nondestructive imaging conditions. After 2 h of consecutive and continuous STM scanning, smooth AnT-SAM surfaces were formed. The polished surfaces contain vacancy depressions rather than the elevated gold islands which are typically formed after the adsorption of purely aromatic thiols such as AnT on Au(111). The STM data showed the coexistence of two distinct stable commensurate phases, namely, alpha and beta. High-resolution STM images revealed a (root 3 x 8) structure for the alpha phase and a (root 7 x 4) R11 degrees structure for the beta phase whose unit cells contain, respectively, four and two molecules. The beta phase was found to be 50% less densely packed than the alpha phase. The lower molecular density of the beta phase should be correlated with a significantly larger tilt angle of the AnT molecular backbone with respect to the surface normal. (C) 2016 Elsevier B.V. All rights reserved.