Thiophene Derivatives on Gold and Molecular Dissociation Processes

We report a systematic study of thiophene derivatives on gold surfaces. These molecules are of interest in molecular electronics, and the characracteristics of the thiophene-electrode interface in devices needs to be understood as it affects electron transport characteristics. Some experiments indicated S-C bond scission in contact with metals resulting in disruption of the π-electron system that affects charge transport, which would also be affected by presence of split-off chemisorbed sulfur. We explored this dissociation aspect by photoemission for the case of monocrystalline Au(111) surfaces and Au films grown on mica for a series of polythiophenes molecules (nT, n = 1-4, 6) as well as for α,ω-diquaterthiophene (DH4T) and dihexylsexithiophene (DH6T). The S 2p X-ray photoelectron spectroscopy peaks are found to have complex line shapes corresponding to S atoms with different core level binding energies (CLBE). Density functional theory calculations of adsorption energies and CLBEs were performed for various adsorption configurations of thiophene on a perfect Au(111) plane and for comparison, calculations were also performed for bithiophene, terthiophene, alkenethiol, alkenethiol chain, and a broken thiophene related metallocycle, incorporating an Au adatom and an S atom. On the basis of these results we relate the different contributions to the S 2p peak to intact molecules on different adsorption sites and broken molecules. Calculations in particular show that the CLBEs for intact thiophene (1T) can be the same as for the alkene and alkanethiol cases as opposed to usual assumptions in the literature. The existence of intact thiophenes is confirmed by the presence of clear π resonance peaks in the near edge X-ray fine structure (NEXAFS) spectra. Spontaneous dissociation appears to a variable extent in different samples, and we tentatively relate this to the presence of a more or less large number of steps and defects sites. X-ray induced beam damage was investigated for 1T and 3T using an intense synchrotron beam of 260 eV photons, and showed changes in the S 2p spectra related to S-C bond scission.