Tuning the effective work function of gold and silver using ω-functionalized alkanethiols: Varying surface composition through dilution and choice of terminal groups

Thiol-based self-assembled monolayers (SAMs) have been used to tune the effective work function of gold over a range of ca. 1.8 eV via two strategies: (i) the use of ω-functionalized alkanethiols where the tail groups have widely varying electronegativity or (ii) by the creation of two-component SAMs from selected mixtures of methyl-terminated alkanethiols (C16) and alkanethiols fluorinated at the two terminal carbon atoms (C16F2). UV-photoelectron spectroscopy (UPS) was used to monitor changes in effective work function, using shifts in the low kinetic energy edge of these photoemission spectra to quantify the shift in local vacuum level resulting from the interface dipole effect created by the surface modifier. Tail groups on alkanethiol chains varied from -CH₃, to -phenyl, -Cl, -Br, and -CF₃ or -CF ₂CF₃, which provided a shift in local vacuum level that varied linearly with the calculated molecular dipole moment of the individual modifiers, as observed previously for a more limited range of alkanethiols (J. Phys. Chem. B 2003, 107, 11690). The studies presented here confirm that the intrinsic dipole in the gold-thiolate bond is small (less than 100 meV), whereas the silver-thiolate bond appears to have a strongly polar character, in the direction Ag⁺-S^- (ca. 900 meV). The use of a simple point dipole model to rationalize these apparent shifts in vacuum level was further explored using SAMs derived from various mixtures of C16 and C16F2. The low kinetic energy edge in the UV-photoemission spectra and the effective work function are observed to increase monotonically in energy with increasing C16F2 coverage, confirming that little surface segregation occurs in these selfassembled monolayers over a wide concentration range.