Griselda García

Alternative search strategy for minimal energy nanocluster structures: The case of rhodium, palladium, and silver

An alternative strategy to find the minimal energy structure of nanoclusters is presented and implemented. We use it to determine the structure of metallic clusters. It consists in an unbiased search, with a global minimum algorithm: conformational space annealing. First, we find the minima of a many-body phenomenological potential to create a data bank of putative minima. This procedure assures us the generation of a set of cluster configurations of large diversity. Next, the clusters in this data bank are relaxed by ab initio techniques to obtain their energies and geometrical structures. The scheme is successfully applied to magic number 13 atom clusters of rhodium, palladium, and silver. We obtained minimal energy cluster structures not previously reported, which are different from the phenomenological minima. Moreover, they are not always highly symmetric, thus casting some doubt on the customary biased search scheme, which consists in relaxing with density functional theory global minima chosen among high symmetry structures obtained by means of phenomenological potentials.

The structure and properties of small Pd clusters

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.

Physical and chemical characterization of Pt12−nCun clusters via ab initio calculations

The physical, structural, and chemical properties of bimetallic Pt(12-n)Cu(n) clusters, where n goes from 0 to 12, have been investigated within density functional theory. We find that the electronic and magnetic properties depend a lot on the atomic fraction of Cu atoms, mainly as the number of Cu atoms changes from even to odd. The chemical potential increases monotonically as a function of the Cu concentration, whereas other chemical properties such as electrophilicity depend on local changes and decreases monotonically, as well as the ionization potential. The hardness has an oscillatory behavior, which depends on the total number of electrons. The reactivity has been spatially analyzed by studying the highest occupied molecular orbital and lowest unoccupied molecular orbital. Charge delocalization is largely increased by the number of copper atoms, whereas for largely Pt concentrations, the charge is more atomiclike. That charge dependence gives another cluster outside view, which shows a rich spatial reactivity. The magnetic dependence of the cluster on the Cu atom concentration opens the door to potential chemistry applications on bimetallic magnetic nanostructures in the field of spintronics.

Metallic behavior of Pd atomic clusters

We report a study of the nonmetal?metal transition of free-standing PdN?clusters (2?N?21) carried out through two different theoretical approaches that are extensively employed in electronic structure calculations: a semi-empirical tight-binding (TB) model and an ab?initio DFT pseudopotential model. The calculated critical size for the metallic transition decreases rapidly with the temperature and an oscillatory dependence with the cluster size is obtained, particularly in the DFT approach. The TB model describes the metallic behavior for cluster sizes beyond N?12 well. Our obtained critical size at room temperature is of the order of the experimental estimation.

Mechanical properties of iron filled carbon nanotubes: Numerical simulations

The deformation process of Fe encapsulated in a carbon nanotube (CNT) is investigated by means of classical molecular dynamics. The [100], [110], and [111] Fe crystal orientations parallel to the CNT symmetry axis, as well as the temperature dependence, are studied. The system encompasses approximately 80 000 atoms. While crystal orientation and temperature determine the systems response, the results are almost independent of the strain rate that is applied. This behavior is only slightly modified by the Fe encapsulation in the CNT. The principal energy release mechanism is the generation of dislocations and twin boundaries, at low and intermediate temperatures (T ≤ 600 K). The dislocations and twin boundaries interact, but do not interlock. For large temperatures (T ∼ 1000 K), a different reaction to deformation sets in, and no elastic response of the Fe–CNT system is observed.