The kinetics of phase transitions in underpotentially deposited Cu adlayers on Au(111)

The formation and phase transitions of underpotentially deposited (root 3x root 3)R30 degrees and (1x1) Cu adlayers on Au(111) were studied by in-situ STM. Upon a negative potential sweep from the region of a disordered lattice gas into the stability region of the (root 3x root 3)R30 degrees phase, this structure is formed via fast homogeneous nucleation and subsequent lateral growth, resulting in a domain wall network which coarsens on a time scale of several minutes. The subsequent transition into a (1x1) phase upon further decreasing the potential occurs via instantaneous nucleation at the prevailing domain walls of the (root 3x root 3)R30 degrees and subsequent one-dimensional growth. Hence, this phase transition depends crucially on the history of the sample, i.e., the density of morphological defects in the (root 3x root 3)R30 degrees adlayer. On the other hand, the mechanism of the reverse transition from the (1x1) to the (root 3x root 3)R30 degrees phase is determined by the magnitude of the potential step. Small potential steps lead to heterogeneous nucleation at step edges, whereas for large potential steps homogeneous nucleation and growth predominate.