Experimental evidence of icosahedral and decahedral packing in one-dimensional nanostructures.

The packing of spheres is a subject that has drawn the attention of mathematicians and philosophers for centuries and that currently attracts the interest of the scientific community in several fields. At the nanoscale, the packing of atoms affects the chemical and structural properties of the material and, hence, its potential applications. This report describes the experimental formation of 5-fold nanostructures by the packing of interpenetrated icosahedral and decahedral units. These nanowires, formed by the reaction of a mixture of metal salts (Au and Ag) in the presence of oleylamine, are obtained when the chemical composition is specifically Ag/Au = 3:1. The experimental images of the icosahedral nanowires have a high likelihood with simulated electron micrographs of structures formed by two or three Boerdijk-Coxeter-Bernal helices roped on a single structure, whereas for the decahedral wires, simulations using a model of adjacent decahedra match the experimental structures. To our knowledge, this is the first report of the synthesis of nanowires formed by the packing of structures with 5-fold symmetry. These icosahedral nanowire structures are similar to those of quasicrystals, which can only be formed if at least two atomic species are present and in which icosahedral and decahedral packing has been found for bulk crystals.

Nanoparticle stability from the nano to the meso interval.

Nanoparticles are the cornerstone of nanotechnology. Their crystal structure and relation to shape are still open problems despite a lot of advances in the field. The classical theory of nanoparticle stability predicts that for sizes <1.5-2 nm the icosahedral structure should be the most stable, then between around 2-5 nm, the decahedral shape should be the most stable. Beyond that, face-centered-cubic (FCC) structures will be the predominant phase. However, in the experimental side, icosahedral (I(h)) and decahedral (D(h)) particles can be observed much beyond the 5 nm limit. In fact, it is possible to find I(h) and D(h) particles even in the mesoscopic range. Conversely, it is possible to find FCC particles with a size <1.5 nm. In this paper we review a number of the mechanisms proposed in the literature that allow the stabilization of nanoparticles. Some of the mechanisms are very interrelated and it becomes difficult to distinguish between them.

Exploring the surface reactivity of 3d metal endofullerenes: a density-functional theory study.

Changes in the preferential sites of electrophilic, nucleophilic, and radical attacks on the pristine C60 surface with endohedral doping using 3d transition metal atoms were studied via two useful reactivity indices, namely the Fukui functions and the molecular electrostatic potential. Both of these were calculated at the density functional BPW91 level of theory with the DNP basis set. Our results clearly show changes in the preferential reactivity sites on the fullerene surface when it is doped with Mn, Fe, Co, or Ni atoms, whereas there are no significant changes in the preferential reactivity sites on the C60 surface upon endohedral doping with Cu and Zn atoms. Electron affinities (EA), ionization potentials (IP), and HOMO-LUMO gaps (Eg) were also calculated to complete the study of the endofullerene's surface reactivity. These findings provide insight into endofullerene functionalization, an important issue in their application.