Alvaro Posada-Amarillas

Global Minimum Pt13M20 (M = Ag, Au, Cu, Pd) Dodecahedral Core–Shell Clusters

In this work, we report finding dodecahedral core-shell structures as the putative global minima of Pt13M20 (M = Ag, Au, Cu, Pd) clusters by using the basin hopping method and the many-body Gupta model potential to model interatomic interactions. These nanoparticles consist of an icosahedral 13-atom platinum core encapsulated by a 20 metal-atom shell exhibiting a dodecahedral geometry (and Ih symmetry). The interaction between the icosahedral platinum core and the dodecahedral shell is analyzed in terms of the increase in volume of the icosahedral core, and the strength and stickiness of M-Pt and M-M interactions. Low-lying metastable isomers are also obtained. Local relaxations at the DFT level are performed to verify the energetic ordering and stability of the structures predicted by the Gupta potential finding that dodecahedral core-shell structures are indeed the putative global minima for Pt13Ag20 and Pt13Pd20, whereas decahedral structures are obtained as the minimum energy configurations for Pt13Au20 and Pt13Cu20 clusters.

Tetrahelix conformations and transformation pathways in Pt1Pd12 clusters.

The threshold method is used to explore the potential energy surface of the Pt(1)Pd(12) bimetallic cluster, defined by the Gupta semiempirical potential. A set of helical structures, which follow a Bernal tetrahelix pattern, correspond to local minima for the Pt(1)Pd(12) cluster, characterizing the region of the energy landscape where these structures are present. Both right-handed and left-handed chiral forms were discovered in our searches. Energetic and structural details of each of the tetrahelices are reported as well as the corresponding transition probabilities between these structures and with respect to the icosahedron-shaped global minimum structure via a disconnectivity graph analysis.

Experimental and theoretical DOS of Co and Ni silicides

Abstract A set of samples of Co–Ni silicide thin-films were deposited on Si wafers by PLD and were submitted to thermal annealing to promote silicidation. Samples were characterized by XPS, including in-depth profiles. Experimental results are complemented with theoretical density of states (DOS). Calculations were performed by means of extended Huckel theory approximation. Tendency of DOS behavior of Co and Ni silicides at valence level about similarities/differences between theoretical calculations and experimental results is discussed alongside this work.

STRUCTURAL AND DYNAMICAL PROPERTIES IN LIQUID Ni AND Ag BY COMPUTER SIMULATION

We studied the temperature dependence of the structural and dynamical properties for liquid nickel and silver by modeling the interatomic interaction through a tight-binding n-body potential in the second-moment approximation (TB-SMA). A common-neighbor analysis was performed to explore the microstructural changes with temperature in Ni and Ag in the liquid state in terms of their inherent structures (IS). A tendency to the formation of more symmetric IS characteristic of the BCC crystalline and icosahedral order is observed as the temperature is raised. At the same time, the FCC and HCP crystalline order is reduced while the BCC-type order increases. The simulated diffusion coefficients are in reasonable agreement with the available experimental information and with other theoretical results. The simple liquid behavior is shown by an Arrhenius plot for both nickel and silver.

Computational study of Au_4 cluster on a carbon nanotube with and without defects using QM/MM methodology

We use ONIOM (QM/MM) methodology to carry out geometry calculations in a 4-atom nanocluster supported by an (8, 8) armchair carbon nanotube with and without defects employing LSDA/SDD for the QM system and UFF for MM. In two particular cases, defects were added in the carbon nanotube wall. These defects are a double oxygenated vacancy (Vac2O2) and a double vacancy but without oxygen which creates two pentagons and an octagon. Our results show how geometries using QM/MM and energies calculations carried out with QM, change on both the gold nanocluster and the carbon nanotube. In addition, an application of ONIOM methodology in a comparative study to predict behavior of structures as hybrid materials based in carbon nanotubes combined with gold nanoclusters is shown. In this work we examine geometry changes on both the gold nanocluster and the carbon nanotube. A comparison is made with the binding energy resulting values as well as with the orbital energies such as the frontier orbitals HOMO and LUMO.

DFT study of composites formed by M2 metallic clusters (M = Ni, Cu, Fe and Au) embedded in faujasite

The present work is a theoretical study of the different frameworks and composites of faujasite (FAU) zeolite. Most of the computations for composites were performed by embedding M2 metal clusters (M = Ni, Cu, Fe and Au) within the Al2Si40O96 FAU zeolite framework. Results showed that, in this framework, the difference between the α and β spin induced an unfolding band structure for Cu2 and Au2 clusters. Thus, their respective band gap energy decreased. In addition, low energy band gap values are associated with composites in which the cluster presents practically no hybridized orbitals. In particular, the charge trace of a zeolitic framework was identified as a fingerprint that is different for other types of zeolites and composites. In this charge trace, the framework atoms on equivalent sites with lower charge values were identified as being responsible for clusters adsorption.