Gold-sulfur bonding in 2D and 3D self-assembled monolayers: XPS characterization

Although self-assembled monolayers (SAMs) of alkylthiols on planar gold (2D SAMs) and on gold nanoparticles (3D SAMs) have been intensely studied, the actual nature of the Au-S bonding remains poorly characterized. Comparison of the X-ray photoelectron spectroscopy (XPS) spectra of 2D and 3D SAMs and "reference" Au(I) complexes, sometimes referred to as Au(I) thiolate polymers, provides detailed insight into this problem. We report high-resolution XPS spectra and Au 4f(7/2) and S 2p(3/2) binding energies (BE) in 2D SAMs, 3D SAMs and the Au(I) thiolate complexes for two short-chain thiols (n-C4SH and n-C5SH). Sulfur 2p(3/2) BE shifts are used to compare the different states of bonding in the SAM systems and the Au(I) complexes and establish that the S atom in the SAM systems bears a charge of about 0.2e. The 2D and 3D SAMs exhibit similar XPS characteristics and are both distinguishable from the Au(I) complexes. The origins of the observed BE values are discussed in the context of the nature of the gold substrate and the oxidation state of the chemisorbed sulfur atom. Comparison of C-13 NMR chemical shift data and XPS BE data further clarifies the nature of the surface interactions as well as the use of the Au(I) complexes as reference materials.