Potential-Induced Strain Relaxation in Au Mono- and Bilayer Films on Pt(111) Electrode Surfaces

In situ scanning tunneling microscopy observations of 1--2 monolayer thick Au films on Pt(111) electrodes are presented, which show a complex, potential-dependent structural phase behavior of the Au film. Starting from a pseudomorphic structure at <0.35 VAg/AgCl, a sequence of transitions into dislocation network structures occurs with increasing potential: (1×1)→ ``striped phase'' → ``hex phase'' in the Au bilayer, (1×1)→ striped phase in the Au monolayer. This is explained by a reduction of the in-plane stress in the Au surface layer(s) due to anion adsorption and a strain energy minimization as described by the Frenkel-Kontorova model.