XPS studies of octadecylphosphonic acid (OPA) monolayer interactions with some metal and mineral surfaces

摘要

Partial and complete self-assembled monolayers (SAMs) of octadecylphosphonic acid (OPA) have been deposited onto air-exposed surfaces of the metals copper, silver, gold, iron, silicon and aluminium, as well as onto freshly cleaved, air-exposed surfaces of the minerals muscovite and biotite. The line width of the C(1s) signal in the XPS spectra of the surface narrowed, as the extent of coverage increased to 100%, to a half-width of 0.9 eV. Moreover, the line widths associated with the insulating muscovite substrate also became substantially narrower as OPA coverage increased. Binding energy differences on this charge-shifted surface were found to be more consistent when OPA was used as a charge reference, compared to using adventitious carbon as a reference. OPA coverage of the air-exposed metals copper, silver, gold and iron also produced narrow C(1s) spectra whose binding energies were consistently close to 284.9 eV. The C(1s) binding energy positions on Al and Si samples were charge-shifted by the insulating nature of the thin oxide formed on air exposure, or by the insulating nature of the substrate in the case of the minerals. Correction of the observed C(1s) energy position to 284.9 eV gave sets of elemental binding energies for the substrate materials that were reproducible. Thus, OPA coverage could be a possible alternative candidate for use in charge correction of binding energies of insulating materials. The OPA coverage cases were modelled using the software QUASES (TM) Analyse. For the substrates copper, silver, gold, iron and aluminium, analyses of the metal core line spectra gave OPA overlayer thicknesses close to those measured by AFM (1.6 nm). However, QUASES (TM) analyses of the C(1s) extrinsic backgrounds for the same surfaces required the use of an attenuation length of only 0.4 mn to derive a comparable thickness - much lower than literature values for carbon. This discrepancy is ascribed to the structured nature of the SAM. Copyright (c) 2005 John Wiley & Sons, Ltd.