Adsorption states of thiophene-thiols in self-assembled monolayers on Au(111) studied by X-ray photoelectron spectroscopy and thermal desorption spectroscopy

Abstract

Self assembled monolayers (SAMs) of S-containing organic molecules on Au electrode have been extensively investigated because of ease of preparation of a well-ordered organic monolayer film. They have attracted wide attention for application to organic electronic devices and surface modification of metal electrodes. In order to fabricate organic nano-devices, growth mechanism of the SAM should be understood as well as the surface structures and adsorption states. Recently, the adsorption energy of the thiol-derivatives was theoretically discussed (S. Higai et al., J. Chem. Phys. 121 (2004) 970). It is expected that the chemical environment of the -carbon atom significantly affects the adsorption energy or adsorption state on the Au(111) surface. Their results showed the weakest adsorption energy in the SAM of 2-thiophene-thiol (TT) on Au(111). However, no experimental results of these molecules have been carried out yet. In order to compare their theoretical prediction with the experiments, the adsorption states of TT SAMs were investigated by X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). For comparison, we also measured SAMs of propanethiol (PT), benzenethiol (BT), and 2-thenylmercaptan (TM). The chemical bonds of S atoms on the Au surface were elucidated by S2p XPS. PT and BT SAMs are chemisorbed through the Au-S chemical bond. On the other hand, the TT and TM SAMs have several chemical configurations of S atoms, although most S atoms of the thiol group are chemisorbed on the Au surface. Peak positions of the bound S were independent of the molecules, indicating no significant difference of the electronic structure of the bound S among these SAMs. TDS results showed that the desorption process was dependent on the molecular structure, and that the desorption temperature cannot be explained by the adsorption energy predicted theoretically. TT and TM showed decomposed desorption on the Au surface, while the molecules were desorbed without decomposition in the PT and BT SAMs. It is suggested that this difference of desorption process results from the difference of the intermolecular interaction or the reactivity of the molecules with the Au surface. From our experimental results, the adsorption energies of the thiol-derivatives depend on not only the strength of the Au-S bond but also intermolecular interaction and thermal stability of the molecules. In the presentation, we will show the results of angle-resolved XPS of these SAMs in order to examine the change on the electronic structure of the Au surface layer by adsorbing the molecules.