Structure and stability of cytosine adlayers on Au(111): An in-situ STM study

The formation, structure and stability of cytosine adlayers on Au(111) in aqueous solution has been studied by current-potential, capacitance-potential, transient- and in-situ scanning tunneling microscopy (STM) measurements. In addition to adsorption of cytosine on gold at low coverages and negative potentials (state I), we found a complicated reorientational transition region (state II) and a densely packed two-dimensional condensed adlayer at rather positive potentials (state III). The stability of the various adlayers shifts towards more positive potentials with decreasing pH. The kinetics of dissolution of the "chemisorbed" adlayer III was studied with current vs. time transients employing a potential-step technique. The experimental curves were analyzed by comparison with a model based on hole nucleation and growth in combination with a parallel Langmuir-type desorption process. The structure and stability of state III was characterized employing in-situ STM. We found highly ordered domains and derived a unit cell with the following dimensions: a = 7.3 ± 0.3 Å, b = 8.7 ± 0.3 Å and γ = 50° ± 5°. The proposed packing model assumes the coordination of the N(3) ring nitrogen of cytosine with gold atoms. Finally, we followed the dissolution of the condensed cytosine adlayer using in-situ STM: two pathways of layer disintegration seem to exist.