Metal overlayers on organic functional groups of self-assembled monolayers: VIII. X-ray photoelectron spectroscopy of the Ni/COOH interface

The interaction of vacuum deposited Ni with the COOH organic functional groups of mercaptoundecanoic acid $[HS(CH2)10COOH]$ self-assembled monolayers formed on an Au substrate has been characterized with in situ x-ray photoelectron spectroscopy. Steady loss and complete disappearance of the hydroxyl component of $O 1s$ peak at 532.8 eV provides evidence for the formation of a Ni–O bond at the Ni/COOH interface. Further evidence for interfacial compound formation is provided by a binding energy shift in the high binding energy component of the $Ni 2p3/2$ peak from 854.8 to 854.4 eV for Ni coverages below ∼0.2 nm. These results are consistent with the donation of electrons from the deposited Ni to the COOH oxygens at low Ni coverages. The absence of the characteristic satellite feature in the $Ni 2p$ peak excludes the possibility that a full electron charge is donated to each COOH organic functional group as a bidentate complex. However, the data are consistent with a two step reaction mechanism in which Ni initially reacts weakly with the oxygens in COOH below one monolayer coverage and then forms a complex above this coverage.