title: On sulfur core level binding energies in thiol self-assembly and alternative adsorption sites: An experimental and theoretical study.
abstract: Characteristic core level binding energies (CLBEs) are regularly used to infer the modes of molecular adsorption: orientation, organization, and dissociation processes. Here, we focus on a largely debated situation regarding CLBEs in the case of chalcogen atom bearing molecules. For a thiol, this concerns the case when the CLBE of a thiolate sulfur at an adsorption site can be interpreted alternatively as due to atomic adsorption of a S atom, resulting from dissociation. Results of an investigation of the characteristics of thiol self-assembled monolayers (SAMs) obtained by vacuum evaporative adsorption are presented along with core level binding energy calculations. Thiol ended SAMs of 1,4-benzenedimethanethiol (BDMT) obtained by evaporation on Au display an unconventional CLBE structure at about 161.25 eV, which is close to a known CLBE of a S atom on Au. Adsorption and CLBE calculations for sulfur atoms and BDMT molecules are reported and allow delineating trends as a function of chemisorption on hollow, bridge, and atop sites and including the presence of adatoms. These calculations suggest that the 161.25 eV peak is due to an alternative adsorption site, which could be associated to an atop configuration. Therefore, this may be an alternative interpretation, different from the one involving the adsorption of atomic sulfur resulting from the dissociation process of the S-C bond. Calculated differences in S(2p) CLBEs for free BDMT molecules, SH group sulfur on top of the SAM, and disulfide are also reported to clarify possible errors in assignments.
authors.affiliation: Institut des Sciences Moléculaires d'Orsay, Université-Paris Sud, 91405 Orsay, France.
authors.forename: Juanjuan
authors.initials: J
authors.lastname: Jia
authors.affiliation: Department of Physics, University of Central Florida, Orlando, Florida 32816, USA.
authors.forename: Abdelkader
authors.initials: A
authors.lastname: Kara
authors.affiliation: Dipartimento di Ingegneria "E. Ferrari," Università di Modena e Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Pasquali
authors.affiliation: Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France.
authors.forename: Azzedine
authors.initials: A
authors.lastname: Bendounan
authors.affiliation: Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France.
authors.forename: Fausto
authors.initials: F
authors.lastname: Sirotti
authors.affiliation: Institut des Sciences Moléculaires d'Orsay, Université-Paris Sud, 91405 Orsay, France.
authors.forename: Vladimir A
authors.initials: VA
authors.lastname: Esaulov