The energetics and the electronic and magnetic properties of iridium nanoparticles in the range of 2-64 atoms were investigated using density functional theory calculations. A variety of different geometric configurations were studied, including planar, three-dimensional, nanowire, and single-walled nanotube. The binding energy per atom increases with size and dimensionality from 2.53 eV/atom for the iridium dimer to 6.09 eV/atom for the 64-atom cluster. The most stable geometry is planar until four atoms are reached and three-dimensional thereafter. The simple cubic structure is the most stable geometric building block until a strikingly large 48-atom cluster, when the most stable geometry transitions to face-centered cubic, as found in the bulk metal. The strong preference for cubic structure among small clusters demonstrates their rigidity. This result indicates that iridium nanoparticles intrinsically do not favor the coalescence process. Nanowires formed from linear atomic chains of up to 4-atom rings were studied, and the wires formed from 4-atom rings were extremely stable. Single-walled nanotubes were also studied. These nanotubes were formed by stacking 5- and 6-atom rings to form a tube. The ring stacking with each atom directly above the previous atom is more stable than if the alternate rings are rotated.

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The accuracy of the Sutton-Chen potential energy surface (PES) for describing atomic interactions in small metal clusters was investigated by comparison with density functional theory (DFT) calculation results. The binding energies calculated using the Sutton-Chen PES for the dimers, trimers, and 8- and 13-atom clusters of four transition metals, Ir, Pt, Au, and Ag, differ from those obtained with DFT calculations. As the DFT results agree well with the available experimental data, the above disagreement indicates that the original Sutton-Chen PES cannot accurately describe the interactions among atoms in the cluster for these metals. The parameters of the Sutton-Chen potential were therefore optimized to the DFT results for each of the metals. Molecular dynamics (MD) simulations were carried out on the coalescence of a dimer with a single atom for these metals. Both the original bulk and the cluster optimized Sutton-Chen PESs were tested with various incident angles and initial kinetic energies. The MD results show that the coalescence is highly dependent on the PES. This demonstrates that use of an accurate PES is critical, particularly at low-energy regime. The kinetic energy, incident angle, and choice of metal were examined for their role in the outcome of the coalescence process.

The coalescence of small iridium clusters was studied using a molecular dynamics (MD) technique with a cluster Sutton−Chen potential. MD simulations were carried out on the reaction of two clusters from 1 to 7 atoms at various incident angles and initial kinetic energies. The threshold energy of product formation was investigated for the various systems, and the effect of initial orientation between two clusters on the product was also studied. A comparison of diffusion and Ostwald-ripening coalescence mechanisms was performed for 8-atom cluster formation, and Ostwald-ripening led to cluster formation with higher initial energies. An atom-to-bond collision resulted in the formation of a larger cluster more often and at higher energies than an atom-to-atom collision.