Campus Universitario da Federacao

Salvador, Brazil

Campus Universitario da Federacao

Salvador, Brazil
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De Almeida Jr. E.F.,Campus Universitario da Federacao | De Brito Mota F.,Campus Universitario da Federacao | De Castilho C.M.C.,Campus Universitario da Federacao | De Castilho C.M.C.,Federal University of Bahia | And 2 more authors.
European Physical Journal B | Year: 2012

Theoretical calculations focused on the stability of an infinite hexagonal AlN (h-A1N) sheet and its structural and electronic properties were carried out within the framework of DFT at the GGA-PBE level of theory. For the simulations, an h-A1N sheet model system consisting in 96 atoms per super-cell has been adopted. For h-A1N, we predict an Al-N bond length of 1.82 A and an indirect gap of 2.81 eV as well as a cohesive energy which is by 6% lower than that of the bulk (wurtzite) AlN which can be seen as a qualitative indication for synthesizability of individual h-A1N sheets. Besides the study of a perfect h-A1N sheet, also the most typical defects, namely, vacancies, anti-site defects and impurities were also explored. The formation energies for these defects were calculated together with the total density of states and the corresponding projected states were also evaluated. The charge density in the region of the defects was also addressed. Energetically, the anti-site defects are the most costly, while the impurity defects are the most favorable, especially so for the defects arising from Si impurities. Defects such as nitrogen vacancies and Si impurities lead to a breaking of the planar shape of the h-A1N sheet and in some cases to the formation of new bonds. The defects significantly change the band structure in the vicinity of the Fermi level in comparison to the band structure of the perfect h-A1N which can be used for deliberately tailoring the electronic properties of individual h-A1N sheets. © EDP Sciences Societa Italiana di Fisica Springer-Verlag 2012.

Medeiros P.V.C.,Campus Universitario da Federacao | Mota F.D.B.,Campus Universitario da Federacao | Mascarenhas A.J.S.,Federal University of Bahia | De Castilho C.M.C.,Campus Universitario da Federacao | De Castilho C.M.C.,Federal University of Bahia
Solid State Communications | Year: 2011

This work uses first-principles calculations to investigate the aspects of the bonding character of lithium atoms adsorbed on a graphene layer. The presented results are in contradiction to other results that have recently appeared in the specialized literature, although they confirm some previous claims. In particular, a discussion of the characteristics of the bonding between lithium and carbon atoms and whether they interact via an sp 2 or an sp 3 hybridization is intended to clarify the problem. It is also found that the carbonlithium bond is not purely covalent but instead presents a significant ionic character. The local geometry is governed by the π-acceptor character of lithium atoms which occupy reverse positions relative to the carbon atoms as compared to the positions of hydrogen in graphane. © 2010 Elsevier Ltd. All rights reserved.

Freitas R.R.Q.,Campus Universitario da Federacao | Rivelino R.,Federal University of Bahia | De Brito Mota F.,Campus Universitario da Federacao | De Castilho C.M.C.,Campus Universitario da Federacao | De Castilho C.M.C.,Federal University of Bahia
Journal of Physical Chemistry C | Year: 2012

Aggregation, molecular adsorption, and dissociation of water on the Fe(100) surface were investigated using spin-polarized density functional calculations. The preferential sites for H 2O, HO, O, and H were carefully investigated on this surface. Also, the dissociation of H 2O into H + OH species, and further OH into O + H species, was examined. The charge transfer mechanism during these dissociation processes, as well as of small water aggregates at different orientations on the Fe(100) surface, was studied within the Bader charge analysis. The coverage dependence on the adsorption properties was examined by comparing the results of a (2 × 2) with a (3 × 3) supercell. These calculations predicted that H 2O is weakly adsorbed (physisorption) on hollow, bridge, and on-top sites, with the on-top site being slightly preferred for both coverages of 0.11 and 0.25 monolayer. As expected, OH was predicted to be strongly adsorbed (chemisorption) on the Fe(100) sites, producing a large charge transfer from the surface to p-orbitals of the O atom. A dissociation barrier of about 1.0 eV, for the dissociation H 2O → OH + H, was calculated from the on-top site to the next most stable bridge and hollow sites, respectively. In contrast, a smaller barrier of ca. 0.8 eV was calculated for the dissociation of OH. Regarding the adsorption of small water aggregates on Fe(100), the present study has demonstrated that they are strongly reoriented on the surface in comparison to the isolated structures, leading to stable adsorbates. Most interestingly, the dissociation of a water molecule, after the dimer formation, leads to an energy barrier of 1.25 eV, about 25% higher than the corresponding value of an adsorbed single water molecule. © 2012 American Chemical Society.

Dos Reis D.D.,Federal University of Minas Gerais | Negreiros F.R.,Federal University of Minas Gerais | De Carvalho V.E.,Federal University of Minas Gerais | Soares E.A.,Federal University of Minas Gerais | And 2 more authors.
Surface Science | Year: 2013

In this paper we present a combined low-energy electron diffraction (LEED) and a DFT study of the Au(110)c(2 × 2)-Sb surface in order to determine its atomic structure. The DFT calculations, using both LDA and GGA approaches, have indicated that the adsorption of antimony atoms on the Au(110) surface hollow sites is energetically more favored as compared with other possible adsorption sites. The LEED analysis also showed a Sb-overlayer termination with the Sb atoms segregating to the hollow sites instead of forming an Au-Sb surface alloy. This overlayer results to be contracted of about 0.16 Å with the first gold layer presenting a small expansion (0.04 Å) with respect to the Au bulk interlayers distance. The agreement between the LEED and DFT results is very good. © 2012 Elsevier B.V. All rights reserved.

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