Adsorption of 3-Mercaptopropyltrimethoxysilane on Silicon Oxide Surfaces and Adsorbate Interaction with Thermally Deposited Gold

Abstract

Adsorption of 3-mercaptopropyltrimethoxysilane (MPS) on hydroxylated silicon oxide substrates by immersion in MPS solution or exposure to MPS vapor has been compared using X-ray photoelectron spectroscopy (XPS). To aid the interpretation, MPS has also been cryogenically condensed in ultrahigh vacuum (UHV) onto gold surfaces. Condensation of MPS vapor on gold in the absence of water does not result in MPS polymerization, as evidenced by multilayer desorption upon warming to room temperature. The C 1s XPS spectrum has been used to infer the relative abundance of methoxy groups. Vapor-deposition on hydroxylated silicon oxide leads to an unpolymerized MPS monolayer consisting of molecules with two methoxy groups. Solution deposition yields a 2−3 layer thick film. The layer in contact with the surface is cross-linked and methoxy-depleted, but the layers on top of it contain some methoxy groups. Step-wise thermal deposition of gold in UHV onto solution-deposited MPS has also been investigated. Gold-thiolate bond formation is evidenced by an S 2p XPS peak at a binding energy of 162.6 eV, and C 1s and Au 4f XPS intensities indicate that gold remains on top of the MPS layer. Ultraviolet photoelectron spectroscopy (UPS) has been used to measure the energies of the valence states and the work function. A work function decrease of 0.8 eV is observed by a gold dose of 9.7 × 1014 atoms/cm2, and the work function eventually reaches 4.7 eV for several layers worth of gold. Interaction of gold with MPS layers has also been studied with atomic force microscopy. Force−distance measurements using gold-coated colloidal probes demonstrate ca. four times higher normalized adhesive forces on MPS compared to octadecyltrichlorosilane layers.