Juanjuan Jia

On sulfur core level binding energies in thiol self-assembly and alternative adsorption sites: An experimental and theoretical study

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.

Adsorption and desorption kinetics of NTCDA molecules on Ag(111) and Au(111) surfaces studied by ion scattering

ABSTRACT A study of adsorption of NTCDA (1,4,5,8-naphthalene tetracarboxylic dianhydride) on Ag(111) and Au(111) surfaces is presented. The adsorption kinetics is followed at room temperature from the submonolayer range upto thin film formation using time-of-flight scattering and recoiling spectroscopy in which atomic fragments and incident scattered particles are detected. The desorption kinetics is similarly followed from room temperature to upto a few hundred degrees. On Ag adsorption it first proceeds rapidly and then a slower uptake is observed prior to multilayer formation. This is ascribed to the formation of first a relaxed (as indicated by others) and then compressed monolayer phases. We attribute compressed phase desorption to temperatures below 400 K and desorption of the relaxed phase is ascribed to the 400–450 K region confirming conclusions of Braatz et al. In case of the Au surface the kinetics of adsorption at low deposition flux is very slow and accelerates after some molecules are initially adsorbed. We associate this to the very low binding energy of single molecules as predicted by theory. Subsequent higher sticking probability can be associated with intermolecular Van der Waals forces which contribute strongly to the adsorption energy.