Thiols and disulfides self-assembled on gold, studied by x-ray photoelectron spectoscopy

Abstract: A number of different thiols and disulfides self-assembled on polycrystalline gold surfaceshave been studied using mainly AlKα-excited X-ray Photoelectron Spectroscopy (XPS), but also Fourier Transform Infrared Spectroscopy (FTIR), Ellipsometry, Contact Angles (CA), Voltammetry and Temperature Programmed Desorption (TPD).Evidence is presented in this thesis that the presence of disulfides in long chain thiolatefilms reported earlier is due to radiation damage from the X-rays used in the X-rayspectroscopies. The disulfide formation arises as a consequence of interchain crosslinkingnear the thiolate head group. The process is described as a one-step event where an alkane chain C-W bond is ruptured and the remaining carbon reacts with a neighboring chain to form the cross link These results are taken into account in the subsequent research.The occurrence of a low-binding energy shifted sulfur peak at 161.3 eV in films of cystamine and cysteamine is discussed, along with a thorough characterization of these short chain assemblies. The low binding energy component occurs as a result of an increase of the coordination number of sulfur to the gold atoms in the substrate. The film is shown to be polar, with an affinity for oxygen at the film/contamination interface.Long chain thiols self-assembled on gold have been shown to be ideal for biosensor applications, and a novel technique for producing gradients of ω-substituted thiol films is evaluated from an XPS point of view. The gradients are 5-6 mm wide, and the coverage is at most 3% lower in the gradient than in the pure monolayers.Mixed monolayers can be used as models for biological membranes. One such system is presented here, using a mixture of 11-mercaptoundecanoic acid (MUA) and thiocholesterol (cholest-5-ene-3 β-thiol, TC). TC is found to dominate for short adsorption times, but for long immersion times, the MUA overtakes the TC.Finally, two systems with a potential application in nanoelectronics are presented, α-functionalized terthiophenes (one long-chain thiol analogue and one disulfide) and thio-ethylpyrroles (l-(2-thioethyl)pyrrole, 1-TEP and 3-(2-thioethyl)pyrrole, 3-TEP) on gold. The thiophenes are shown to form dense and well-ordered films. The 1-TEP adsorb as intact molecules, whereas the 3-TEP lose the pyrrole ring upon adsorption.