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.

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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.

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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Classical molecular dynamics simulation is used for structural thermodynamic and dynamic analysis of Au-Pd bimetallic clusters. It is observed that the Pd-core/Au-shell structure is the most stable, and can be formed through annealing of other structures such as Au-core/Pd-shell, eutecticlike, or solid solution. Depending on the starting temperature and initial composition, three-layer icosahedral nanorod, face-centered cubic (fcc) nanorod, and fcc cluster can be obtained on slow cooling. The three-layer icosahedral nanorod structure is not as stable as the Pd-core/Au-shell decahedron; however it is more stable than the solid-solution decahedron structure up to 400 K. Our findings provide valuable insight into catalysis using Au-Pd and other similar bimetallic clusters.

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