ADSORPTION OF THYMINE ON GOLD SINGLE-CRYSTAL ELECTRODES

We have examined the adsorption of thymine on (111), (100), and (210) gold single-crystal surfaces, The adsorption behavior on these three s urfaces has been investigated by classical electrochemical methods lik e cyclic voltammetry and capacitance-potential measurements. Additiona lly in situ scanning tunneling microscopy (STM) and ex situ photoelect ron spectroscopy (XPS) measurements have been performed for the adsorp tion of thymine on the (111) surface. The capacitance measurements as well as cyclic voltammetry investigations show the three adsorption st ates of thymine on all Au electrodes, The first adsorption state refer s to a random adsorption of thymine molecules at negative surface char ges. The second state can be characterized as a condensed but weakly a dsorbed adlayer on the (100) and (111) crystals, whereas a noncondense d state has been found on the (210) surface. The condensed thymine fil m is stabilized mainly by hydrogen bonding. High-resolution STM images for this film on the (111) electrode point to an ordered adlayer with a unit cell which is incommensurate with the underlying Au surface. T he images indicate flat adsorbing thymine molecules in this state. The third adsorption state is characterized by charge transfer from depro tonated thymine molecules to the gold surface, XPS data show one chemi cally modifed nitrogen atom for the chemisorbed thymine film. This ads orption state shows a commensurate 2 root 3 x 2 root 3 overstructure i n the STM image. The STM images are interpreted by stacks of adsorbed thymine molecules with the molecular plane perpendicular to the surfac e. The stacks are connected by coadsorbed water molecules. The molecul es are bound by a deprotonated nitrogen to the surface.