Packing density of HS(CH2)nCOOH self-assembled monolayers

Abstract

Self-assembled monolayers (SAMs) of HS(CH₂)_nCOOH, n = 5, 10, 15 deposited from ethanol solution onto gold are prepared by five approaches, and their packing densities are evaluated by X-ray photoelectron spectroscopy (XPS) measurements. The five approaches are: (1) direct deposition; (2) acetic-acid-assisted deposition; (3) butyl-amine-assisted deposition; (4) displacement of a preformed HS(CH₂)_nCH₃ (n = 5, 10, 15) SAMs; and (5) co-deposition with HS(CH₂)_nCH₃ (n = 5, 10, 15). Packing density metrics are calculated from measurements of SAM and substrate photoemission intensities and their attenuations by two methods. In one case the attenuated photoemissions are expressed as a ratio relative to comparable measurements on an experimental HS(CH₂)_nCH₃ model system. In the other case a new method is introduced where a calculated attenuation based on theoretical random coil and extended chain models is used as the reference to determine a packing density fraction. Packing densities are also correlated with the S2p(Au-bonded):Au4f peak area ratios and with shifts in the C1s binding energies. SAMs prepared by the direct deposition are a partial multilayer where a second molecular layer is physisorbed onto the SAM and not removable by solvent washing. The addition of acetic acid to the deposition solution disrupts dimer associations of HS(CH₂)_nCOOH in solution and at the surface of the monolayer and yields the most ordered monolayer with the highest density of –COOH groups. The addition of butyl amine results in a labile ammonium carbonate ion pair formation but results in a lower packing density in the SAM. The displacement of the preformed HS(CH₂)_nCH₃ SAM and the co-deposition of HS(CH₂)_nCH₃ with HS(CH₂)_nCOOH result in SAMs with little incorporation of the –COOH component.