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 latti ce gas into the stability region of the (root 3x root 3)R30 degrees phase, this structure is formed via fast homogeneous nucleation and subsequent lat eral growth, resulting in a domain wall network which coarsens on a time sc ale of several minutes. The subsequent transition into a (1x1) phase upon f urther decreasing the potential occurs via instantaneous nucleation at the prevailing domain walls of the (root 3x root 3)R30 degrees and subsequent o ne-dimensional growth. Hence, this phase transition depends crucially on th e 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 th e reverse transition from the (1x1) to the (root 3x root 3)R30 degrees phas e is determined by the magnitude of the potential step. Small potential ste ps lead to heterogeneous nucleation at step edges, whereas for large potent ial steps homogeneous nucleation and growth predominate.