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 surfaces has been investigated by classical electrochemical methods like cyclic voltammetry and capacitance-potential measurements. Additionally in situ scanning tunneling microscopy (STM) and ex situ photoelectron spectroscopy (XPS) measurements have been performed for the adsorption of thymine on the (111) surface. The capacitance measurements as well as cyclic voltammetry investigations show the three adsorption states of thymine on all Au electrodes, The first adsorption state refers to a random adsorption of thymine molecules at negative surface charges. The second state can be characterized as a condensed but weakly adsorbed adlayer on the (100) and (111) crystals, whereas a noncondensed state has been found on the (210) surface. The condensed thymine film 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. The images indicate flat adsorbing thymine molecules in this state. The third adsorption state is characterized by charge transfer from deprotonated thymine molecules to the gold surface, XPS data show one chemically modifed nitrogen atom for the chemisorbed thymine film. This adsorption state shows a commensurate 2 root 3 x 2 root 3 overstructure in the STM image. The STM images are interpreted by stacks of adsorbed thymine molecules with the molecular plane perpendicular to the surface. The stacks are connected by coadsorbed water molecules. The molecules are bound by a deprotonated nitrogen to the surface.