Dislocation mediated alignment during metal nanoparticle coalescence

Abstract

Dislocation mediated alignment processes during gold nanoparticle coalescence were studied at low and high temperatures using molecular dynamics simulations and transmission electron microscopy. Particles underwent rigid body rotations immediately following attachment in both low temperature (500 K) simulated coalescence events and low temperature (~315 K) transmission electron microscopy beam heating experiments. In many low temperature simulations, some degree of misorientation between particles remained after rigid body rotations, which was accommodated by grain boundary dislocation nodes. These dislocations were either sessile and remained at the interface for the duration of the simulation or dissociated and crossslipped through the adjacent particles, leading to improved co-alignment. Minimal rigid body rotations were observed during or after attachment in high temperature (1100 K) simulations, which is attributed to enhanced interfacial diffusion. However, particle rotation was eventually induced by {111} slip on planes parallel to the neck groove. These deformation modes led to the formation of single and multi-fold twins whose structures depended on the initial orientation of the particles. The driving force for {111} slip is attributed to high surface stresses near the intersection of low energy {111} facets in the neck region. The details of this twinning process were examined in detail using simulated trajectories, and the results reveal possible mechanisms for the nucleation and propagation of Shockley partials on consecutive planes. Deformation twinning was also observed in-situ using transmission electron microscopy, which resulted in the co-alignment of a set of the particles' {111} planes across their grain boundary and an increase in their dihedral angle. This constitutes the first detailed experimental observation of deformation twinning during nanoparticle coalescence, validating simulation results presented here and elsewhere.

Computational Modeling of Nanoparticle Coalescence

“…According to our scheme, Figure 4. [75] Copyright 2016, Elsevier. The strain is accommodated by sheared interfacial layers, shown along their common [100] axis.…”

“…However, we should emphasize that there exists no clearcut temporal distinction or succession of these stages, some of which may occur simultaneously; our scheme is just an attempt to identify and evaluate the importance of each elementary component of the overall coalescence mechanism: 1) Coagulation. [75] Eventually, the particles rotate as rigid bodies, driven by mutual torques, in order to maximize their contact area. It can be due to either in-flight collisions during gas-phase condensation, or cluster diffusion on a support, after deposition; a phenomenon often referred to as Smoluchowski ripening.…”

“…[74] In Section 2.2.2 we have referred to how MD addresses the issues of initial kinetic energies and the effect of local environment on the approach of the clusters. [42,75,76] 4) Heat Release Due to Free-Surface Annihilation. Once the clusters have approached enough to feel the presence of each other, an interface is formed between them, as shown in the previously discussed hybrid KMC&RBD study by Theissmann et al [24,72] Due to randomness in the coagulation geometry, this initial interface is usually misoriented, which leads to the shearing of a number of interface layers, as shown in Figure 4 depicting the initial twist misorientation after neck formation in coalescing Au nanoparticles.…”

“…Once the clusters have approached enough to feel the presence of each other, an interface is formed between them, as shown in the previously discussed hybrid KMC&RBD study by Theissmann et al [24,72] Due to randomness in the coagulation geometry, this initial interface is usually misoriented, which leads to the shearing of a number of interface layers, as shown in Figure 4 depicting the initial twist misorientation after neck formation in coalescing Au nanoparticles. [75] Copyright 2016, Elsevier. This results in the formation of a more coherent, larger-area interface between the two interacting clusters.…”

“…It results in the formation of a dumbbell configuration, either with perfect alignment of the clusters, if their geometry allows it, or with the formation of a twin boundary, when the rotational motion is blocked by an excessive number of newly formed bonds; the latter case should be far more usual than the former, considering the expected randomness in the original relative orientation of the clusters. Simultaneously, twin boundary formation is commonly accompanied by the appearance of misfit dislocations, as shown in Figure Heat Release Due to Free‐Surface Annihilation .…”