Nanoparticles are a state of matter that has properties different from either molecules or bulk solids. In the present work, we review the shape and structure of nanometer-sized particles; several shapes are discussed, such as the octahedron and truncated octahedron, the icosahedron, the Marks decahedron, the truncated “star-like” decahedron, the rounded decahedron and the regular decahedron. Experimental high-resolution transmission electron microscopy (TEM) images of each type of particle are presented together with the Fast Fourier Transform and a model of the particle. We consider only gold particles grown by vapor deposition or by colloidal methods. High-resolution TEM images of the particles in different orientations are shown. We discuss two basic types of particles uncapped and capped. Data for other metals and semiconductors are reviewed. We have also performed very extensive simulations obtaining the total energy and pair correlation functions for each cluster under study. Furthermore, distributions of single atom energy for every cluster are displayed in order to reveal the effect of surface on the stability of different types and sizes of clusters. We discuss the structure of the particles from ∼1 to ∼100 nm. The mechanisms for stress release as the particles grow larger are reviewed and a mechanism is suggested. Finally, we discuss the parameters that define the shape of a nanoparticle and the possible implications in technological applications.

Starting from an amorphous C film, single-walled carbon nanotubes were obtained in situ in a high-resolution electron microscope by the combined effect of irradiation and axial strain. Ductile nanotubes developed either a junction or a linear chain of C atoms before failure. These facts have been put in direct evidence for the first time. Tight-binding calculations indicate that the bonding in the linear chain is of a cumulene type.

Classical molecular dynamics simulations were carried out to study the thermodynamic stability and melting behavior of Au-Pt nanoclusters of most common structural variants like decahedra, icosahedra, and cuboctahedra. It has been shown that the Pt-core/Au-shell structures are most stable, while the eutectic-like structures are more stable than solid solution ones, and the Au-core/Pt-shell are least stable, on thermal heating. On the other hand, the large difference between the melting points of the constituent elements can be a dominating factor on the melting mechanism of the bimetallic nanoparticles. The bimetallic clusters transform to the most stable Pt-core/Au-shell structure from whatever initial structures on heating above certain temperatures.

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Colloidal bimetallic nanoclusters of Au-Pd were synthesized by simultaneous reduction of the metal ions from their corresponding chloride salts with polymer ͑PVP͒ stabilizer. Structural characterization of the samples with different Au/ Pd ratios was made using high-resolution electron microscopy. Classical molecular dynamics simulation is used for structural thermodynamics and dynamic analysis of the bimetallic clusters. Structural incoherency and structure reversal mechanism in such bimetallic clusters were studied systematically for different atomic configurations, which explain the anomalies on the reported experimental results on such nanoclusters mainly by electron microscopy. Our simulation and experimental results revealed that stable ordered structures of the bimetallic cluster are Pd core/Au shell, random solid solutions and eutecticlike configurations. Though the Au-core/Pd-shell structure is stable at low temperature, the structure changes to Pd core/Au shell on heating at about 500 K.

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We present a detailed structural analysis for small Tin (n = 2-15) clusters based on ab initio quantum mechanical calculations of their binding energies, frontier orbital gaps, and second energy derivatives. Local density approximation calculations revealed that while the smaller clusters (n < or = 8) prefer hexagonal atomic arrays with bulklike crystal symmetry, the bigger clusters prefer pentagonal atomic arrays. From the stability criteria of the magic number clusters we could identify three magic number clusters Ti7, Ti13, and Ti15. While the most stable configuration of Ti7 is a decahedral bipyramid induced by tetrahedral atomic arrays, the most stable configuration of Ti13 is an icosahedron. The other stable cluster Ti15 takes a closed-shell icosahedron-like configuration with both pentagonal and hexagonal symmetries. The stability of the Tin clusters strongly depends on their geometries and charge states. The HOMO-LUMO gap of the Tin clusters approaches its bulk value for n > 8. While there is not much difference between the HOMO and LUMO isosurface charge distributions for the Ti7 and Ti13 clusters in their most stable configurations, they are very different in the case of Ti15. Such a distinct charge distribution in Ti15 indicates its singular chemical selectivity over the other two magic number clusters.