Coalescence dynamics of 3D islands on weakly-interacting substrates

Abstract

We use kinetic Monte carlo simulations and analytical modelling to study coalescence of threedimensional (3D) nanoscale faceted silver island pairs on weakly-interacting fcc(111) substrates, with and without concurrent supply of mobile adatoms from the vapor phase. Our simulations show that for vapor flux arrival rates F < 1 monolayer/second (ML/s) coalescence manifests itself by one of the islands absorbing the other via sidewall facet migration. This process is mediated by nucleation and growth of two-dimensional (2D) layers on the island facets, while the supply of mobile atoms increases the nucleation probability and shortens the time required for coalescence completion. When F is increased above 1 ML/s, coalescence is predominantly governed by deposition from the vapor phase and the island pair reaches a compact shape via agglomeration. The crucial role of facets for the coalescence dynamics is further supported by a mean-field thermodynamic description of the nucleation energetics and kinetics. Our findings explain experimental results which show that two-dimensional film growth morphology on weakly-interacting substrates is promoted when the rate of island coalescence is suppressed. The present study also highlights that deviations of experimentally reported film morphological evolutions in weakly-interacting film/substrate systems from predictions based on the sintering and particle growth theories may be understood in light of the effect of deposition flux atoms on the energetics and kinetics of facet-layer nucleation during coalescence. Coalescence-the process of merging two or more atomic islands or grains into a single cluster-is an ubiquitous phenomenon in material synthesis technologies, and largely determines the morphology and microstructure of metallurgical alloys 1 and vapor-deposited thin films 2,3 , as well as the size and shape of free-standing and supported nanoparticles 4,5. The most established description of island coalescence relies on the sintering theory, according to which shape equilibration of coalescing clusters proceeds via isotropic atomic surface diffusion 6. This scenario is relevant for temperatures above the roughening transition limit T R for which generation of ample amounts of adatom-kink pairs facilitates continuous and uniform mass transport from the convex to the concave areas of the island cluster 7. However, many material synthesis processes, including vapor-based growth of thin films and supported nanoparticles on weakly-interacting substrates, occur below T R. At these conditions, generation of adatom kink-pairs is infrequent, leading to formation of three-dimensional (3D) islands bounded by flat sidewall facets 8. Combe et. al. 9 studied shape equilibration of single free-standing faceted 3D nanoparticles upon annealing at temperatures smaller than T R showing that mass transport is mediated by nucleation and growth of two-dimensional (2D) layers on the facet surfaces. The relevance of this mechanism for coalescence of 3D clusters supported on a substrate has not b...

Manipulation of thin silver film growth on weakly interacting silicon dioxide substrates using oxygen as a surfactant

“…From the latter it follows that in an oxygen-free system material will be, primarily, redistributed between islands during growth, while the presence of atomic oxygen will promote material redistribution post-deposition, i.e., coarsening during growth will be hindered. Furthermore, exposure of vicinal Ag surfaces to atomic oxygen has been shown to promote sidewall facet formation 76 , which is known to decrease the rate of material transport between the coalescing islands 29,77 . Moreover, atomic oxygen adsorption on the surface of Ag islands residing on SiO2 surfaces has been suggested to lower the island surface and the island/substrate interface energies; which yields a smaller driving force for cluster reshaping 78 .…”

“…The kinetic pathways leading to 3D morphologies in homo-and hetero-epitaxial film/substrate systems are well established in the literature 11 . This understanding has enabled the development of growth manipulation strategies in which surfactants (i.e., minority metal or gaseous species) are deployed and temporary adsorbed at the film growth front to allow navigation between 2D and 3D morphologies 12-26 . The atomistic mechanisms that govern morphological evolution of metal films on weakly-interacting substrates are different from those in strongly-interacting epitaxial systems 7,[27][28][29] . As such, established knowledge for surfactant-based growth manipulation is not directly applicable to the case of noble-metal film deposition on 2D materials and oxides.…”

Silver nanoparticle array on weakly interacting epitaxial graphene substrate as catalyst for hydrogen evolution reaction under neutral conditions

“…Another aspect is that facets disappear as the increase in temperature leads to larger kink density on the surface of the islands. 31 Next, we find that the morphology of the Ag films is very sensitive to the modification of the deposited Ag amount. As can be seen from Fig.…”

In Situ and Real-Time Nanoscale Monitoring of Ultra-Thin Metal Film Growth Using Optical and Electrical Diagnostic Tools

“…Atomic mobility, in a first approximation, scales with [ 21 ], i.e., Ag exhibits the largest mobility. This allows Ag to: (i) diffuse longer distances on the substrate and self-assemble into larger and fewer nuclei; (ii) exhibit more pronounced upward diffusion from the base to the top of atomic islands [ 106 , 107 ]; and (iii) diffuse faster on Ag islands so the coalescence completion is promoted [ 108 ]. All the aforementioned effects favor 3D growth morphology, and thereby, yield the largest and values.…”