Chemisorption-induced gap states at organic–metal interfaces: benzenethiol and benzeneselenol on metal surfaces

Abstract

Valence electronic states of benzenethiol (C₆H₅SH) and benzeneselenol (C₆H₅SeH) in the gas, condensed, and chemisorbed phases were examined by ultraviolet photoemission spectroscopy, metastable atom electron spectroscopy, and first-principles calculations using density functional theory. C₆H₅SH is chemisorbed on Pt(111) and Au(111) substrates to form a thiolate (C₆H₅S), and C₆H₅SeH is bound on Pt(111) substrate to form a selenolate (C₆H₅Se). In all cases, chemisorption-induced gap states (CIGSs) appear just below the Fermi level (E_F) of the substrate, yielding a metallic character around the anchor S and Se atoms. However, the local density at E_F decreases considerably from the anchor atom to the benzene ring, because strong coupling between benzene π(1e_1g) and S 3p(or Se 4p) in free molecules is apparently lifted upon chemisorption. In other words, thiolates and selenolates (especially C₆H₅S on Au(111)) act as poor mediators of the metal wave functions at E_F, which is closely related to electric conductance in the relevant metal–organic–metal junctions at zero bias.