NANOMETER-SCALE PATTERNING OF SURFACES USING SELF-ASSEMBLY CHEMISTRY .2. PREPARATION, CHARACTERIZATION, AND ELECTROCHEMICAL-BEHAVIOR OF 2-COMPONENT ORGANOTHIOL MONOLAYERS ON GOLD SURFACES

We report a study of the physical and electrochemical properties of tw o-component self-assembled monolayers (SAMs) composed of both electroa ctive (4-aminothiophenol, 4-ATP) and electroinactive (n-octadecanethio l, ODT) species. In all of the experiments reported here, relatively s hort (3 h) assembly times were used to prepare the mixed SAMs. We have characterized the macroscopic composition and the microscopic structu re of these SAMs using Auger electron spectroscopy (AES), coulometry, grazing angle Fourier transform infrared spectroscopy, and lateral for ce microscopy (LFM). The adsorption isotherms determined by AES and co ulometry show significant deviation from Langmuirian behavior and are suggestive of phase separation. LFM images obtained at three points ne ar the critical region of the isotherm ([4-ATP]/[ODT] similar to 4) in dicate that these two-component SAMs display complex phase behavior: A t relatively low 4-ATP coverages, the surface consists of small island s of 4-ATP imbedded in an ordered film of ODT. At higher coverages of 4-ATP, however, we find evidence of both separation into distinct phas es and mixing of the two components. In a second series of experiments , we demonstrate that phase domains of 4-ATP are electroactive and can be used to carry out localized electrochemistry. That is, the islands of 4-ATP, which are randomly distributed in the ODT matrix, behave as an array of ultramicroelectrodes. Surface-confined 4-ATP molecules ca n be used to nucleate the growth of polyaniline selectively from the p hase separated domains of 4-ATP. We find that if a 4-ATP/ODT mixed mon olayer is oxidized in the presence of aniline, nanometer scale polyani line features are formed. The size and distribution of these features have been characterized by AFM and can be controlled through a combina tion of monolayer composition and the concentration of aniline in solu tion.