Mixing of Nonsubstituted and Partly Fluorinated Alkanethiols in a Binary Self-Assembled Monolayer

An applicability of irradiation-promoted exchange reaction (IPER) to the fabrication of binary self-assembled monolayers comprised of nonsubstituted and partly fluorinated alkanethiols (NSAT and PFAT, respectively) is demonstrated. This particular combination is interesting because of the intrinsic differences between these two species in terms of molecular volume and dipole moment. Within the IPER framework, the composition of the binary NSAT-PFAT SAMs could be precisely controlled by the irradiation dose up to 40% PFAT portion. This threshold of the intermixing is presumably related to a limited ability of the primary NSAT matrix to accumulate the bulky PFAT moieties and to the onset of extensive cross-linking in this matrix, which sets the upper limit of the applied promoting dose (at ca. 1-2 mC/cm(2)). The binary SAMs fabricated by IPER were compared with the analogous films prepared by coadsorption. A detailed analysis of the experimental data suggests that the differently prepared SAMs differ to some extent. The binary NSAT-PFAT films were found to exhibit interesting effects in photoemission related to the superposition of contra-directed dipole moments of the NSAT and PFAT species. The effects could only be explained within an electrostatic framework, which shows that the standard chemical shift description is not always sufficient or even applicable to describe photoemission from a SAM-based system.