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 an d "reference" Au(I) complexes, sometimes referred to as Au(I) thiolate poly mers, provides detailed insight into this problem. We report high-resolutio n 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-C4 SH 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 f rom the Au(I) complexes. The origins of the observed BE values are discusse d in the context of the nature of the gold substrate and the oxidation stat e of the chemisorbed sulfur atom. Comparison of C-13 NMR chemical shift dat a 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.