X-RAY PHOTOELECTRON-SPECTROSCOPY SULFUR 2P STUDY OF ORGANIC THIOL ANDDISULFIDE BINDING INTERACTIONS WITH GOLD SURFACES

The presence of two sulfur species was detected in X-ray photoelectron spectroscopy (XPS) studies of thiol and disulfide molecules adsorbed onto gold surfaces. These species are assigned to bound thiolate (S-2p 3/2 binding energy of 162 eV) and unbound thiol/disulfide (S-2p3/2 bin ding energy from 163.5 to 164 eV). These assignments are consistent wi th XPS data obtained from different thiols (C12, C16, C18, and C22 alk ane thiols, a fluorinated thiol, and a cyclic polysiloxane thiol) and different adsorption conditions (solvent type, thiol concentration, te mperature, and rinsing). In particular, the use of a poor solvent for thiol adsorption solutions (e.g., ethanol for long chain alkanethiols) and the lack of a rinsing step both resulted in unbound thiol molecul es present at the surface of the bound thiolate monolayer. This has im plications for recent studies asserting the presence of multiple bindi ng sites for gold-thiolate species in organic monolayers.