Atom Movement and Binding on Surface Clusters: Pt on Pt(111) Clusters

Growth mechanisms from tetrahedral seeds to multiply twinned Au nanoparticles revealed by atomistic simulations

The growth pathways from tetrahedral to multiply twinned gold nanoparticles in the gas phase are studied by molecular dynamics simulations supported by density functional theory calculations. Our results show that the growth from a tetrahedron to a multiple twin can take place by different pathways: directly from a tetrahedron to a decahedron (Th → Dh pathway), directly from a tetrahedron to an icosahedral fragment (Th → Ih), and from a tetrahedron to an icosahedron passing through an intermediate decahedron (Th → Dh → Ih). The simulations allow to determine the key atomic-level growth mechanism at the origin of twinning in metal nanoparticles. This mechanism is common to all these pathways and starts from the preferential nucleation of faulted atomic islands in the vicinity of facet edges, leading to the formation and stabilization of twin planes and of fivefold symmetry axes.

Shape control of size-selected naked platinum nanocrystals

Controlled growth of far-from-equilibrium-shaped nanoparticles with size selection is essential for the exploration of their unique physical and chemical properties. Shape control by wet-chemistry preparation methods produces surfactant-covered surfaces with limited understanding due to the complexity of the processes involved. Here, we report the controlled production and transformation of octahedra to tetrahedra of size-selected platinum nanocrystals with clean surfaces in an inert gas environment. Molecular dynamics simulations of the growth reveal the key symmetry-breaking atomic mechanism for this autocatalytic shape transformation, confirming the experimental conditions required. In-situ heating experiments demonstrate the relative stability of both octahedral and tetrahedral Pt nanocrystals at least up to 700 °C and that the extended surface diffusion at higher temperature transforms the nanocrystals into equilibrium shape.

Site determination and thermally assisted tunneling in homogenous nucleation

A combined low-temperature scanning tunneling microscopy and density functional theory study on the binding and diffusion of copper monomers, dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers are found in fcc sites only, monomers as well as atoms in dimers can occupy the fcc as well as the metastable hcp site. In fact the dimer fcc-hcp configuration is only 1.3 meV less favorable with respect to the fcc-fcc configuration. This enables a confined intracell dimer motion, which at temperatures below 5 K is dominated by thermally assisted tunneling.

Mesoscopic relaxation in homoepitaxial metal growth

We demonstrate that a size-dependent mesoscopic mismatch exists in homoepitaxy, which has a strong impact on the morphology of the islands and the substrate. Atomic scale calculations for double layer Cu islands on Cu(111) reveal that mesoscopic strain relaxations in both islands and the substrate strongly influence the shape of islands and can effect the details of atomic motion near the island.

Theoretical studies of atomic-scale processes relevant to crystal growth

The study of adsorption, diffusion, island formation, and interlayer transport of atoms on a growing surface has been an active field in the past decade, because of both experimental and theoretical advances. Experiments can give detailed images of patterns formed on growing surfaces. An important challenge to the theoretical studies is the identification of dynamical processes controlling the pattern formation and overall surface morphology. This can be achieved by accurate modeling of the atomic interactions, a thorough search for active atomic-scale processes, and simulation of the growth on the experimental timescale to allow for detailed comparison with the experimental measurements. An overview of some of the theoretical methodology used in these studies and results obtained for one of the most extensively studied systems, Pt(111), is given here. A remarkable richness of phenomena has emerged from these studies, where apparently small effects can shift the balance between competing molecular processes and thereby change the morphology of a growing surface. The article concludes with a discussion of possible future directions in this research area.

Substrate mediated long-range oscillatory interaction between adatoms: Cu /Cu(111)

A quantitative study of the long-range interaction between single copper adatoms on Cu(111) mediated by the electrons in the two-dimensional surface-state band is presented. The interaction potential was determined by evaluating the distance distribution of two adatoms from a series of scanning tunneling microscopy images taken at temperatures of 9-21 K. The long-range interaction is oscillatory with a period of half the Fermi wavelength and decays for larger distances d as 1/d(2). Five potential minima were identified for separations of up to 70 A. The interaction significantly changes the growth of Cu/Cu(111) at low temperatures.