S. Frey

The effect of sulfur–metal bonding on the structure of self-assembled monolayers

The equilibrium structure of alkanethiol monolayers self-assembled on metal substrates is determined by a delicate interplay of the intermolecular chain–chain and chemisorptive substrate–head group inter actions. To understand the role of the individual constituents of this interplay, we studied the structure of biphenyl and perfluoroalkyl terminated alkanethiol monolayers self-assembled on Au and Ag. The structural characteristics of the monolayers derived from NEXAFS, FTIRRAS and XPS measurements point to a decisive role of the directional substrate–head group interactions.

Modification of alkanethiolate monolayers on Au-substrate by low energy electron irradiation: Alkyl chains and the S/Au interface

Low-energy electron irradiation damage in alkanethiol (AT) self-assembled monolayers (SAM) has been studied by using hexadecanethiolate [HDT: CH3–(CH2)15–S-] film on Au-substrate as a model system. The induced changes were monitored by insitu photoelectron spectroscopy and angle resolved near edge X-ray absorption fine structure spectroscopy. AT SAMs are found to be very sensitive to low-energy electron irradiation. Both the alkyl chains and the S/Au interface are affected simultaneously through the electron-induced dissociation of C–H, C–C, C–S, and Au–thiolate bonds. The most noticeable processes are the loss of the orientational and conformational order, partial dehydrogenation and desorption of the film, and the appearance of new sulfur species. The latter process can be related to the formation of disulfide at the S/Au interface or an incorporation of the thiolate (or the corresponding radical) into the alkyl matrix via bonding to irradiation-induced carbon radicals in the adjacent aliphatic chains. The most essential damage in the AT films occurs in the early stages of irradiation. Irradiation with a dose of 1000 µC cm-2 (about 13 electrons per HDT chain) at the primary electron energy of 50 eV results in almost complete breakdown of the orientational order in the initially well-ordered HDT film, a decrease of its thickness by about 25%, and a destruction of ≈40% of the original Au–thiolate bonds. The film becomes a disordered structure comprising both saturated and unsaturated hydrocarbons. Further irradiation of the residual film is accompanied by a continuous C–C bond cleavage and the desorption of the remaining hydrogen, which merely leads to increasing cross-linking and the transformation of saturated hydrocarbons into unsaturated ones through C2C double bond formation.

An extension of the mean free path approach to X-ray absorption spectroscopy

Abstract In the partial electron yield (PEY) acquisition mode commonly used in X-ray absorption spectroscopy (XAS) both elastically and inelastically scattered electrons are acquired, the latter contribution dominating the detector signal. Hence, the majority of the inelastic scattering events will not result in signal attenuation as happens in the case of X-ray photoelectron spectroscopy (XPS). To determine the respective changes in the effective mean free paths (MFP) we have performed XPS and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements for a series of self-assembled monolayers of alkanethiols on gold substrates. The length of the alkyl chain and, therefore, the film thickness was varied. In agreement with expectations, the obtained MFPs for the Au 4f photoelectrons and C KLL Auger electrons in the PEY acquisition mode (with the respective inelastic contributions) exceed the corresponding values for the Au 4f and C KLL electrons of the same kinetic energies in the constant final state acquisition mode. Furthermore, the effective PEY-MFP for the C KLL Auger electrons increased with decreasing retarding voltage of the PEY detector, which correlates with the enhanced contribution of the inelastically scattered electrons in the acquired signal. The results obtained are of importance for the analysis of XAS spectra of thin organic films and polymers.

Modification of semifluorinated alkanethiolate monolayers by low energy electron irradiation

The low energy electron induced damage in self-assembled monolayers (SAM) formed from semifluorinated alkanethiolates (SFAT) of CF3(CF2)9(CH2)nSH (F10HnSH) with different hydrocarbon chain length (n=2, 11 and 17) on polycrystalline gold has been monitored in-situ by X-ray photoelectron spectroscopy and angle resolved near edge X-ray absorption fine structure spectroscopy. All investigated SFAT SAMs exhibit qualitatively similar behavior with respect to low energy electron irradiation. Both the fluorocarbon and hydrocarbon parts and the S/Au interface are affected simultaneously. Progressive disordering of initially well-ordered, densely packed SAMs, desorption of film constituents, and chemical changes within the residual film are observed. Desorption of sulfur-containing fragments, which probably include the complete SFAT chains, was only found for F10H2S/Au. The desorbed carbon-containing fragments originate almost exclusively from the fluorocarbon part of the SFAT SAMs. Fluorine desorbs not only as a constituent of the carbon-containing fragments, but through irradiation-induced scission of C–F bonds. The accumulated chemical changes within the residual SFAT films include the complete disappearance of CF3 tail group, partial transformation of CF2 moieties into CF entities, appearance of CC double bonds in the fluorocarbon and hydrocarbon (predominantly) parts, and transformation of the thiolate head groups into new irradiation-induced sulfur species. Some general tendencies in the reaction of SAMs toward electron-irradiation are noticed in full agreement with previous findings for conventional AT SAMs.