In this work we suggest that certain structures present in real commercial catalysts maybe related to low dimensional structures and present a theoretical study of two MoS 2 clusters (one made of 61 atoms and the second one made of 68 atoms). The 61 atoms cluster has 7 sulfur atoms decorating at the top of sulfur (S) layer in order to analyze the effects produced on the electronic properties. The first cluster yielded semiconductor behavior with a forbidden energy gap E g of the order of 0.11 eV, while the second cluster decorated with seven S atoms yielded metallic behavior. A careful analysis to the partial density of states (PDOS) indicate that the contributions to the total DOS in the vicinity of the Fermi level, from the seven S atoms located at the surface is negligible, they serve only as promoters to the metallic behavior to the system.

We have investigated the scenario of graphene when irradiated with high energetic protons and subsequently decorated with Pd atoms on one of the layers. Theoretical analyses were performed on graphene 2L (two layers) with vacancies (carbon 3 and 13) (sample A), graphene 2L with vacancies and the two carbon atoms intercalated in between the two carbon layers (sample B), graphene 2L with the vacancies intercalated and subsequently with two Pd atoms on one of the layers, the top layer (called surface) (sample C), and, last but not least, graphene 2L with vacancies intercalated and decorated with six Pd atoms on the surface (sample D). For the four cases enunciated, energy bands were performed which provided information about the semimetallic behavior, showing more semi-metallic character for the first case, while less metallic behavior occurs for the second and third one. Moreover, sample D showed a mini gap (between the conduction and valence bands) of the order of 0.02 eV and manifest semiconductor behavior. Total and projected density of states were performed in order to provide information about the contributions from each selected atom to the total DOS in the vicinity of the Fermi level in order to analyze the effect on the electronic behavior. Pd d orbitals contribute with ∼6% to the total DOS, while graphene (carbon atoms) p orbitals contribute with ∼5%. Furthermore, a strong hybridization is manifest between these two multiple degenerate orbitals.In addition, Crystal Orbital Overlap Population (COOP) between metal (M-M) contributions were calculated in order to inquire the existence (or absence) of magnetic instabilities. Pd atoms showed an itinerant ferromagnetic behavior which induces it to the graphene 2L samples.

No abstract

Full substitution of Ni by the transition metals Co and Pt in PrM 2 B 2 C ͑M =Ni,Co,Pt͒ is analyzed using tight binding within the extended Huckel method. Electronic structure calculation, total and projected density of states ͑PDOS͒, Mulliken population, and crystal orbital overlap population analysis were examined in order to elucidate the absent or presence of superconductivity. Band-structure calculation shows small differences in Pr/ Ni and Pr/ Co compounds. The total and PDOS crossing the Fermi energy ͑E F ͒ is located in a valley and is dominated by −d states, which are principally responsible for the metallic character in both compounds. For Pr/ Pt compound, the band structure as well as the total and PDOS are fully different, since two sets of bands, the −f and −d bands, are highly localized at the E F , contributing both bands to the electronic conduction. Besides, weak hybridization with important contribution of the C −p state is observed in Pr/ Pt with respect to the Pr/ Ni and Pr/ Co compound, where in these last compounds, the C −p contribution is null. Furthermore, Mulliken population analysis and average net charge indicated unfilled −d orbital with charge transference inside the M 2 B 2 layer for Pr/ Ni͑Co͒. The situation is totally different for Pr/ Pt superconductor compounds, since an almost filled −d orbital and homogeneous charge distribution were observed. These theoretical evidences suggest that the absence of the superconductivity in Pr/ Ni and Pr/ Co are connected by the remarkable instabilities between −d, −p, and −f states, with respect to that observed in the superconductor PrPt 2 B 2 C compounds.

Calculations of electronic structure, total and projected density of states (DOS), crystal orbital overlap population (COOP), and average net charge, and also Mulliken population analysis, were performed to study electronic properties of Co and Ni silicides. Analysis of the energy bands depicts metallic behavior for both silicides.The projected DOS yields an indication that hybridization occurs for Co and Ni silicides. The hybridized band in CoSi2 is composed of Co d and p orbitals and Si p and s orbitals, while in NiSi2 the hybridized band is formed by Ni d and p orbitals with Si p orbitals. The fact that the Fermi energy crosses a small part of the DOS, as is the case of CoSi2, yields an indication of the different electronic properties of CoSi2 when compared to NiSi2. The hybridization is stronger in CoSi2 than in NiSi2. Mulliken population analysis provides an indication that a smaller charge distribution exists in NiSi2 when compared to CoSi2. This difference in charge distribution accounts for the different electronic behavior, in agreement with the DOS analysis. Moreover, COOP analysis provides an indication of the existence of covalent bonding between M and Si (M = Ni, Co), this being stronger in Co than in Ni silicides. Furthermore, the average net charge in both compounds yields an indication that there is a charge transfer from M towards Si. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)