In situ STM imaging of individual molecules in two-component self-assembled monolayers of 3-mercaptopropionic acid and 1-decanethiol on Au(111)

Two-component self-assembled monolayers composed of 3-mercaptopropionic aci d (MPA) and 1-decanethiol (CH3(HC2)(9)SH:C10SH) on Au(111) were investigate d with in situ scanning tunneling microscopy (STM) and cyclic voltammetry, where the monolayers I and II were prepared in 0.1 mM ethanol + water solut ions with the ratios of MPA:C10SK = 95:5 and 90:10, respectively. In situ S TM images revealed that both monolayer I and II consisted of phase-separate d domains with molecularly ordered structures, each of which was predominan tly formed by one of the constituent molecules. Based on the STM images, th e surface fractions of constituent molecules were evaluated as MPA:C10SH=0. 61:0.28 for monolayer I, and MPA:C10SH = 0.34:0.57 for monoiayer II. Cyclic voltammograms of the reductive desorption of the thiols provided the surfa ce fractions of MPA:C10SH = 0.69:0.31 for monolayer I, and MPA:C10SH = 0.41 and 0.59 for monolayer II, which were in good agreement with the results o btained from STM images. Molecular resolution imaging allowed us to visuali ze the individual MPA and C10SH molecules and to determine the interfacial structures in the molecularly ordered domains of the phase-separated monoIa yer. The C10SH domains in the monoIayer exhibited ordered phases with dense ly packed (root3 x root3)R30 degrees and p(3 x 2 root 3R-30 degrees) struct ures, which are well-characterized structures for alkanethiol monolayers. O n the other hand, a completely different molecular arrangement of MPA defin ed as a (3 x 3) structure was consistently observed in the MPA domains, whe re the molecular arrangement is almost the same as that of (root3 x root3)R 30 degrees but intermolecular hydrogen bending is thought to exist in the t hree neighboring MPA molecules located around the corner of the (3 x 3) uni t cell. It was demonstrated that individual molecules of the monolayer cons tituents were successfully imaged in the two-component, phase-separated mon olayer in solution. (C) 2001 Published by Elsevier Science B.V.