Comparison of a Fluorinated Aryl Thiol Self-Assembled Monolayer with Its Hydrogenated Counterpart on Polycrystalline Ag Substrates

The effects of perfluorination of aryl thiols on surface coverage, surface electronic properties, and molecular orientation of self-assembled monolayers of thiophenol (TP) and pentafluorothiophenol (F5TP) on polycrystalline At; were evaluated using linear sweep voltammetry, ultraviolet photoelectron spectroscopy (UPS), and surface Raman spectroscopy, respectively. Electrochemical reductive desorption by linear sweep voltammetry indicates a. surface coverage for the TP monolayer of (5.07 +/- 1.29) x 10(-10) mol/cm(2), equating to a molecular area of 32.8 +/- 8.3 angstrom(2), and a surface coverage for the F5TP monolayer of (1.95 +/- 0.59) x 10(-10) mol/cm(2), equating to an area of 85.2 +/- 25.8 angstrom(2)/molecule. TP-modified Ag exhibits a change in work function (Delta Phi) of -0.64 eV relative to bare Ag, whereas F5TP-modified Ag exhibits a Delta Phi of +0.54 eV relative to bare Ag. Quantitative analysis of the UPS and reductive desorption results yields molecular pictures of the proposed interfaces with TP molecules tilted <20 degrees from the surface normal in a herringbone pattern spaced 6.4 angstrom apart and F5TP molecules in a more disordered arrangement tilted 67 degrees from the surface normal with an intermolecular distance of 10.4 angstrom. Qualitative surface Raman spectroscopic analysis of in-plane and out-of-plane modes for these systems confirms that TP molecules are oriented more vertical than F5TP molecules in these monolayers.