Self-diffusion on low-index metallic surfaces: Ag and Au (100) and (111)

Asymmetric Interfacet Adatom Migration as a Mode of Anisotropic Nanocrystal Growth

Crystals are known to grow nonclassically or via four classical modes (the layer-by-layer, dislocation-driven, dendritic, and normal modes, which generally involve minimal interfacet surface diffusion). The field of nanoscience considers this framework to interpret how nanocrystals grow; yet, the growth of many anisotropic nanocrystals remains enigmatic, suggesting that the framework may be incomplete. Here, we study the solution-phase growth of pentatwinned Au nanorods without Br, Ag, or surfactants. Lower supersaturation conditions favored anisotropic growth, which appeared at variance with the known modes. Temporal electron microscopy revealed kinetically limited adatom funneling, as adatoms diffused asymmetrically along the vicinal facets (situated inbetween the {100} side-facets and {111} end-facets) of our nanorods. These vicinal facets were perpetuated throughout the synthesis and, especially at lower supersaturation, facilitated {100}-to-vicinal-to-{111} adatom diffusion. We derived a growth model from classical theory in view of our findings, which showed that our experimental growth kinetics were consistent with nanorods growing via two modes simultaneously: radial growth occurred via the layer-by-layer mode on {100} side-facets, whereas the asymmetric interfacet diffusion of adatoms to {111} end-facets mediated longitudinal growth. Thus, shape anisotropy was not driven by modulating the relative rates of monomer deposition on different facets, as conventionally thought, but rather by modulating the relative rates of monomer integration via interfacet diffusion. This work shows how controlling supersaturation, a thermodynamic parameter, can uncover distinct kinetic phenomena on nanocrystals, such as asymmetric interfacet surface diffusion and a fundamental growth mode for which monomer deposition and integration occur on different facets.

Influence of electric fields on metal self-diffusion barriers and its consequences on dendrite growth in batteries

Based on the results of periodic density functional theory calculations, we have recently proposed that the height of self-diffusion barriers can serve as a descriptor for dendrite growth in batteries [M. Jäckle et al., Energy Environ. Sci. 11, 3400 (2018)]. However, in the determination of the self-diffusion barriers, the electrochemical environment has not been taken into account. Still, due to the presence of electrical double layers at electrode/electrolyte interfaces, strong electric fields can be present close to the interfacial region. In a first step toward including the electrochemical environment, we have calculated barriers for terrace-diffusion on lithium, magnesium, and silver surfaces and across-step self-diffusion on lithium in the presence of electric fields. Whereas the electric field effect is more pronounced on a stepped surface than on flat terraces, overall we find a negligible influence of electric fields on self-diffusion barriers which we explain by the good screening properties of metals.

The Lattice Kinetic Monte Carlo Simulation of Atomic Diffusion and Structural Transition for Gold

For the kinetic simulation of metal nanoparticles, we developed a self-consistent coordination-averaged energies for Au atoms based on energy properties of gold bulk phases. The energy barrier of the atom pairing change is proposed and holds for the microscopic reversibility principle. By applying the lattice kinetic Monte Carlo simulation on gold films, we found that the atomic diffusion of Au on the Au(111) surface undergoes a late transition state with an energy barrier of about 0.2 eV and a prefactor between 40~50 Ų/ps. This study also investigates the structural transition from spherical to faceted gold nanoparticles upon heating. The temperatures of structural transition are in agreement with the experimental melting temperatures of gold nanoparticles with diameters ranging from 2 nm to 8 nm.

Silicon Reactivity at the Ag(111) Surface

The modelization of silicene on Ag(111) is generally based on the assumption of a complete immiscibility between silicon and silver. However, there are recent reports that growth occurs inside the first layer of the Ag(111) terraces rather than on top of them. Here, we report on a combined density functional theory and scanning tunneling microscopy study unveiling the basic exchange mechanism between Si and the topmost layer Ag atoms and modeling the nucleation process. Our findings demonstrate that a strong Si-Ag interaction must be considered to properly describe the Si/Ag(111) interface.

Ageing of out-of-equilibrium nanoalloys by a kinetic mean-field approach

Ageing of nanoalloys: the nanoparticle passes through a metastable onion-like configuration on its way to its equilibrium core–shell structure.

Effects of long jumps, reversible aggregation, and Meyer-Neldel rule on submonolayer epitaxial growth

We demonstrate, using kinetic Monte Carlo simulations of submonolayer epitaxial growth, that long jumps and reversible aggregation have a major impact on the evolution of island morphologies. Long jumps are responsible for a supra-Arrhenius behavior of the effective diffusion coefficient as the attachment and detachment kinetics give rise to a bimodal island size distribution that depends on temperature and long jump extent limits. As the islands density increases with temperature, the average size of stable islands reaches a maximum before decreasing. We have also observed that the diffusion coefficient cannot be used alone to predict the evolution of island sizes and morphologies, the relative rate of each process having a major importance. Our theoretical developments are of direct relevance for materials systems such as Au, Pd, Ag, Cu, Ni, H/Si , H/W(110), Co/Ru , and Co/Ru(S), that are known for exhibiting a compensation effect that cannot be contained within experimental uncertainties.