STRUCTURE AND STABILITY OF CYTOSINE ADLAYERS ON AU(111) - AN IN-SITU STM STUDY

The formation, structure and stability of cytosine adlayers on Au(lll) in aqueous solution has been studied by current-potential, capacitanc e-potential, transient- and in-situ scanning tunneling microscopy (STM ) measurements. In addition to adsorption of cytosine on gold at low c overages and negative potentials (state I), we found a complicated reo rientational transition region (state II) and a densely packed two-dim ensional condensed adlayer at rather positive potentials (state III). The stability of the various adlayers shifts towards more positive pot entials with decreasing pH. The kinetics of dissolution of the ''chemi sorbed'' adlayer III was studied with current vs. time transients empl oying a potential-step technique. The experimental curves were analyze d by comparison with a model based on hole nucleation and growth in co mbination with a parallel Langmuir-type desorption process. The struct ure and stability of state III was characterized employing in-situ STM . We found highly ordered domains and derived a unit cell with the fol lowing dimensions: a = 7.3 +/- 0.3 Angstrom, b = 8.7 +/- 0.3 Angstrom and gamma = 50 degrees +/- 5 degrees. The proposed packing model assum es the coordination of the N(3) ring nitrogen of cytosine with gold at oms. Finally, we followed the dissolution of the condensed cytosine ad layer using in-situ STM: two pathways of layer disintegration seem to exist.