The zero-temperature minimal energy structure of small free-standing Pd clusters (14≤N≤21, where N is the number of atoms in the cluster), their characteristics and their magnetic configurations are investigated. Results obtained using five different phenomenological many-body potentials (implemented in combination with a genetic algorithm search) are refined by means of various density functional theory (DFT) techniques. The agreement and differences between the results obtained with our procedure, using these five potentials, are displayed in detail. While phenomenological potentials yield values that approach the minimal energies of larger clusters, as compared with DFT results, they fail to predict the right symmetry group for some of the clusters with N>14. We find that the minimal energy configurations are not necessarily associated with high symmetry of the atomic arrangement. Actually, several cases of previously overlooked low symmetry structures turn out to have lower energies than more symmetric ones.

Imogolite is an attractive inorganic nanotube, formed from weathered volcanic ashes, that also can be synthesized in nearly monodisperse diameters. It has found a variety of uses, among them as an effective arsenic retention agent, as a catalyst support and as a constituent of nanowires. However, long after its successful synthesis, the details of the way it is achieved are not fully understood. Here we develop a model of the synthesis, which starts with a planar aluminosilicate sheet that is allowed to evolve freely, by means of classical molecular dynamics, until it achieves its minimum energy configuration. The minimal structures that the system thus adopts are tubular, scrolled, and more complex conformations as well, depending mainly on temperature. The minimal nanotubular configurations that we obtain are monodispersed in diameter and quite similar in diameter both to those of weathered natural volcanic ashes and to the ones that are synthesized in the laboratory. A tendency toward nanotube agglomeration is also observed, in agreement with experiment.

The existence of polycrystalline shells has been widely reported in the synthesis of hollow nanoparticles; however, the exact role displayed by the grain boundaries on the stability has been scarcely studied. By including them, in this work, we study for the first time the contribution of the polycrystalline structure in the stability of this unique kind of nanostructures, addressing at the same time, a more realistic modeling of hollow nanoparticles. The role of the polycrystalline structure was studied in gold hollow nanoparticles using molecular dynamics simulations for a wide range of shell thickness and grain sizes. One of the main findings is that the shell thickness necessary for transition from a spherical to a shrunk structure is related to the grain size reduction. The results suggest that to achieve larger hollow nanoparticles, less defective shells are necessary, with single-crystal shells establishing an upper limit in the size that a structure can attain. The cavity shrinkage in a polycrystalline HNP is due to a complex combination of grain diffusion, rotations, dislocation emission, and twining, all of them activated from the grain boundary regions. Our findings suggest that the polycrystalline structure is a crucial parameter to control and improve the stability of the hollow nanoparticles.

Hollow nanoparticle structures play a major role in nanotechnology and nanoscience since their surface to volume ratio is significantly larger than that of filled ones. While porous hollow nanoparticles offer a significant improvement of the available surface area, there is a lack of theoretical understanding, and scarce experimental information, on how the porosity controls or dominates the stability. Here we use classical molecular dynamics simulations to shed light on the particular characteristics and properties of gold porous hollow nanoparticles and how they differ from the nonporous ones. Adopting gold as a prototype, we show how, as the temperature increases, the porosity introduces surface stress and minor transitions that lead to various scenarios, from partial shrinkage for small filling factors to abrupt compression and the loss of spherical shape for large filling. Our work provides new insights into the stability limits of porous hollow nanoparticles, with important implications for the design and practical use of these enhanced geometries.

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