IN-SITU SCANNING-TUNNELING-MICROSCOPY STUDY OF URACIL ON AU(100)

The adsorption of uracil on Au(100) has been studied by in-situ scanni ng tunneling microscopy (STM). Defending on the potential applied to t he gold electrode? uracil forms three highly ordered monolayers, which were imaged with molecular resolution. ''Steady state'' and time-depe ndent experiments were performed to follow structural changes of the r espective substrate surface in the presence of these organic layers. A physisorbed film of planar oriented and via hydrogen bonds connected molecules exists at negative electrode charges. Coupled with changes o f the electronic state of adsorbed uracil this monolayer undergoes a f irst order phase transition when altering the electrode potential towa rds more positive values. A third (chemisorbed) adlayer is finally for med simultaneously with the substrate surface structural transition (h ex) --> (1 x 1). Uracil changes its orientation from planar to perpend icular, and a substrate specific surface coordination complex is creat ed. The ''equilibrium structure'' is determined by the competing actio n of substrate-adsorbate coordination and lateral stacking interaction s. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.