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√3 × 2√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.

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We have examined tetramethylthiourea (TMTU) adsorption on Au(111) with cyclic voltammetry, capacitance measurements, and in situ scanning tunneling microscopy (STM). Capacitance measurements indicate that TMTU is adsorbed over a wide potential range. A capacitance peak at higher potentials (ca. 650 mV vs SCE) is interpreted as arising from the displacement of TMTU by bisulfate/sulfate ions. High-resolution STM images show an ordered structure of TMTU on the gold surface. Three distinctive spots for each molecule are seen in the STM images. From these molecular resolution images a (3 × 3) structure for the TMTU adsorbate is deduced. Large changes are observed in the step and terrace morphology of the Au(111) substrate at potentials (+400 mV) slightly before the capacitance maximum assigned to TMTU desorption. The STM images indicate a slow etching of the gold surface at potentials slightly before the onset of TMTU desorption. This deduction has been further supported by inductively coupled plasma optical emission spectroscopy (ICP-OES) and atomic absorption spectroscopy (AAS), with gold being detected in electrolyte solution which had been in contact with the gold electrode polarized at +400 mV. The etching is seen in our images as a slow retreat of step edges, which have the same height as monoatomic gold steps. Upon subsequently lowering the potential to below 0.3 V, a slow advance of step edges is then seen and this is assigned to the deposition of complexed gold from solution. At more positive potentials, where TMTU desorption occurs, the formation of pits is seen, which expand and coalesce. These pits have a height of 0.12 ± 0.02 nm, which is significantly less than that of a monoatomic step on a Au(111) surface. This observation has been assigned to the dissolution of the TMTU film.