Mixed Aliphatic Self-Assembled Monolayers with Embedded Polar Group

We studied structure, molecular orientation, morphology, and electrostatic properties of mixed self-assembled monolayers (SAMs) comprised of alkanethiolates (ATs) which are modified by a dipolar ester group embedded into the alkyl backbone at two different orientations, namely, with the dipole directed upward and downward from the substrate. The packing density and molecular orientation in these SAMs were found to be similar to those of the “parent” single-component monolayers. Applying X-ray photoelectron spectroscopy (XPS) as a morphology tool, we could estimate that the mixed SAMs represent homogeneous intermolecular mixtures of both components, down to the molecular level, excluding existence of “hot spots” for charge injection. The analysis of the C 1s XPS spectra and the work function data suggests that the composition of the mixed SAMs fully mimicked the mixing ratio of both components in the solutions from which these SAMs were prepared, which suggests a minor role of the dipole–dipole interaction in the overall balance of the structure-building forces. Varying this composition, work function of the gold substrate could be tuned linearly, in controlled fashion within a ∼1.1 eV range, between the ultimate values for the single-component monolayers with a fixed orientation of the embedded ester group, namely, 3.83 and 4.92 eV. This adjustment could be performed keeping the chemical composition at the SAM-ambient interface unchanged, which, along with tunability and homogeneity of these films, is a great advantage of the midchain substituted monolayers, qualifying them for energy-level alignment in model systems and organic electronics devices.