Chemisorption-Induced Gap States At Organic-Metal Interfaces: Benzenethiol And Benzeneselenol On Metal Surfaces

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 pi(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.