Misfit-dislocation-mediated heteroepitaxial island diffusion

Abstract

Scanning tunneling microscopy combined with molecular dynamics simulations reveal a dislocation-mediated island diffusion mechanism for Cu on Ag(111), a highly mismatched system. Cluster motion is tracked with atomic precision at multiple temperatures and diffusion barriers and prefactors are determined from direct measurements of hop rates. The nonmonotonic size dependence of the diffusion barrier is in good agreement with simulations and can lead to enhanced mass transport upon coarsening, in surprising contrast to the traditional island diffusion models where diffusivity reduces with cluster size.

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

“…Using low-temperature scanning tunneling microscopy (STM), we have previously shown that larger clusters have surprisingly low diffusion barriers and a novel dislocation mechanism that is related to the ~12% lattice mismatch for Cu on Ag(111) [25,26]. Here, we show that Cu trimers diffuse large distances following their formation.…”

“…The extremely low diffusion barrier for the linear trimer, combined with the preference for this structure during nucleation, make the trimer an extremely important contributor to mass transport for Cu on Ag(111). The effects of lattice mismatch are manifest in the observed directions of hopping and rotation that require intermediate steps involving trimers of mixed fcc-hcp character that shorten Cu-Cu bonds to relieve strain, in a one-dimensional analog of the misfit-dislocation mechanism responsible for the low diffusion barriers of larger Cu clusters on Ag(111) [25,26]. Because the novel diffusion phenomena for both small and large clusters of Cu on Ag(111) result from lattice mismatch, it is likely that similar behavior will occur in other heteroepitaxial systems.…”

“…1 for individual atoms, though it is possibly much lower than that. This low barrier is surprising in light of molecular dynamics simulations that predict a barrier of ~290 meV-prohibitively large to allow diffusion at 24 K [25,26,37]. In these simulations, the ground state of a Cu trimer on Ag(111) was a compact triangle with all atoms in fcc sites, and diffusion occurred in a concerted fashion.…”

“…Because the majority of cluster diffusion studies have focused on homoepitaxial systems, even less is known about the role of lattice mismatch in determining the properties of heteroepitaxial clusters despite predictions of interesting phenomena [19,20]. While the mobility of larger islands generally decreases with size, several cases of non-monotonic size dependencies [21][22][23][24][25][26][27][28][29][30][31][32] and, remarkably, cases where clusters have even higher mobility than individual atoms, have been reported [27][28][29][30][31][32].…”

Atomic mechanisms and diffusion anisotropy of Cu tetramers on Cu(111)

“…For even larger sizes, other mechanisms may come into play, such as vacancy diffusion inside the islands [46]. Finally, in heteroepitaxial systems where lattice mismatch also participates in the total energy, the formation of misfit dislocations within the islands represents another significant contribution to consider [47,48].…”