Interfacial Electronic Properties of Thiophene and Sexithiophene Adsorbed on a Fluorinated Alkanethiol Monolayer

Abstract

X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) have been used to study the interfacial electronic properties of thiophene and α-sexithiophene (6T) adsorbed at 130 K in ultrahigh vacuum on top of 1H,1H,2H,2H-perfluorodecanethiol (PFDT) self-assembled monolayers (SAMs) on gold. The binding energy of the F 1s XPS peak decreases following thiophene and 6T adsorption, indicating charge transfer to the fluorinated SAMs. UPS measurements substantiate charge transfer, with the valence features of thiophene and 6T in contact with the monolayer appearing at higher ionization energies compared to thicker layers. The energies of the UPS-measured vacuum levels of 6T deposited on PFDT/Au illustrate the absence of a common vacuum level between the organic layers at the PFDT-6T interface and the presence of a −0.9 eV interface dipole. Similar measurements performed for 6T deposition on self-assembled octadecanethiol (ODT) give a weaker interface dipole of opposite sign (Δ = +0.4 eV). The relatively large value and sign of the 6T/PFDT/Au interface dipole suggest that charge transfer to the PFDT-covered surface results in the formation of dipoles with their negative ends toward the Au surface, in contrast to the 6T-ODT interface. The effects of a SAM layer on X-ray-induced oligomerization, which is known to occur for condensed thiophene, were also investigated. Comparison of the thickness of oligomeric thiophene formed by Mg Kα X-ray irradiation on clean and PFDT-covered gold surfaces demonstrates that a thicker oligomer layer forms on the SAM-covered surface, suggesting that the spacing provided by the SAM reduces quenching of electronic excitations that lead to X-ray-induced oligomerization.