Preferential nucleation and high mobility of linear Cu trimers on Ag(111)

Using scanning tunneling microscopy, we studied the formation of dimers, trimers, and other clusters of Cu on Ag(111) at low temperature. We show that trimers, once formed, have significantly higher mobilities than either atoms or dimers. While transient, this mobility makes linear trimers a major contributor to mass transport. Using the scanning tunneling microscopy tip, we constructed linear and compact trimers at 5 K and investigated their stabilities and diffusion parameters as a function of temperature. Analysis shows that the diffusion barrier for linear trimers is very low, 13.6 meV, compared to 65 meV for atoms, while the compact trimer is stable and immobile. Thus, significant diffusion of this nonequilibrium cluster can occur before the equilibrium compact trimer is reached. The details of trimer diffusion and rotation provide insights into the intermediate diffusion steps and indicate that the large lattice mismatch plays an important role. The properties of Cu trimers on Ag(111) contrast with those reported for homoepitaxial trimers on fcc(111) surfaces. Because the diffusion phenomena for Cu trimers and other clusters on Ag(111) are largely a result of lattice mismatch, similar phenomena may exist in the early stages of nucleation and growth of other heteroepitaxial systems.