S. J. Stranick, A. N. Parikh, Y.-T. Tao, D. L. Allara,* and P. S. Weiss*
Journal of Physical Chemistry 98, 7636 (1994).
Scanning tunneling microscopy has been used to demonstrate phase segregation in varied composition, two-component self-assembled monolayers on gold. These monolayer films were assembled using CH3(CH2)15SH and CH3O2C(CH2)15SH, two similar alkanethiol molecules which are non-hydrogen bonding, and have identical alkyl chain lengths. X-ray photoelectron spectroscopy, infrared spectroscopy, and ellipsometry have been used to characterize the average chemical compositions and average molecular structures of these films. Scanning tunneling microscopy of single composition, self-assembled monolayers of each of these molecules shows a preponderance of defects which can be attributed to single missing chains. In mixed composition films, we observe nanometer scale molecular domains with time-dependent shapes. These observations have important implications for both the fundamental understanding of solubilities and phase segregation in quasi-two dimensional mixtures and for applications of self-assembly in which spatial patterns of adsorbate mixtures are important.