PHASE-SEPARATION OF MIXED-COMPOSITION SELF-ASSEMBLED MONOLAYERS INTO NANOMETER-SCALE MOLECULAR DOMAINS

Scanning tunneling microscopy has been used to demonstrate phase segre gation in varied composition, two-component self-assembled monolayers on gold. These monolayer films were assembled using CH3(CH2)(15)SH and CH3O2C(CH2)(15)SH, two similar alkanethiol molecules which are non-hy drogen-bonding and have identical alkyl chain lengths. X-ray photoelec tron spectroscopy, infrared spectroscopy, and ellipsometry have been u sed to characterize the average chemical compositions and average mole cular structures of these films. Scanning tunneling microscopy of sing le composition, self-assembled monolayers of each of these molecules s hows a preponderance of defects which can be attributed to single miss ing chains. In mixed composition films, we observe nanometer scale mol ecular domains with time-dependent shapes. These observations have imp ortant implications both for the fundamental understanding of solubili ties and phase segregation in quasi-two-dimensional mixtures and for a pplications of self-assembly in which spatial patterns of adsorbate mi xtures are important.