J. Oviedo

Energetics and structure of stoichiometric SnO2 surfaces studied by first-principles calculations

Abstract First-principles calculations based on density-functional theory, ultrasoft pseudopotentials and plane-wave basis sets are used to investigate the energetics and the relaxed ionic positions of several low-index stoichiometric SnO 2 surfaces. We find that, in order of increasing energy, the surfaces form the sequence (110) 2 are presented.

Amorphous structures of Cu, Ag, and Au nanoclusters from first principles calculations

We have carried out first-principles density functional calculations for clusters of the coinage metals containing thirteen atoms (M13, where M=Cu, Ag, or Au). We find that for this geometric “magic number” the low energy isomers are actually disordered, forming almost a continuous distribution as a function of energy.

First-principles study of the interaction of oxygen with the SnO2(1 1 0) surface

First-principles calculations based on density functional theory in the generalised gradient approximation, together with pseudopotentials and plane-wave basis sets, have been used to investigate the energetics of oxygen adsorption on stoichiometric and weakly and strongly reduced SnO2(1 1 0) surfaces. It is shown that, if the surface species formed by oxygen adsorption are restricted to be charge neutral, then oxygen cannot be exothermically adsorbed from the gas phase on the stoichiometric surface. A variety of molecular and dissociative modes of adsorption are examined on the reduced surface produced by removing all bridging oxygens and on the weakly reduced surface that results from removal of only a fraction of these oxygens, with the adsorbed species being in both the singlet and the triplet states, and we identify a number of modes not discussed before in the literature. We use the calculated adsorption energies to propose a tentative assignment of these adsorption modes to the peaks observed in temperature programmed desorption experiments on the SnO2 and TiO2(1 1 0) surfaces

Oxygen vacancies on TiO2 (110) from first principles calculations

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.

Molecular modification of coumarin dyes for more efficient dye sensitized solar cells

In this work, new coumarin based dyes for dye sensitized solar cells (DSSC) have been designed by introducing several substituent groups in different positions of the NKX-2311 structure. Two types of substitutions have been considered: the introduction of three electron-donating groups (-OH, -NH(2), and -OCH(3)) and two different substituents with steric effect: -CH(2)-CH(2)-CH(2)- and -CH(2)-HC=CH-. The electronic absorption spectra (position and width of the first band and absorption threshold) and the position of the LUMO level related to the conduction band have been used as theoretical criteria to evaluate the efficiency of the new dyes. The introduction of a -NH(2) group produces a redshift of the absorption maximum position and the absorption threshold, which could improve the cell efficiency. In contrast, the introduction of -CH(2)-CH(2)-CH(2)- does not modify significantly the electronic structure of NKX-2311, but it might prevent aggregation. Finally, -CH(2)-HC=CH- produces important changes both in the electronic spectrum and in the electronic structure of the dye, and it would be expected as an improvement of cell efficiency for these dyes.

A first principles study of sub-monolayer Ge on Si(001)

Experimental observations of heteroepitaxial growth of Ge on Si(001) show a (2xn) reconstruction for sub-monolayer coverages, with dimer rows crossed by missing-dimer trenches. We present first-principles density-functional calculations designed to elucidate the energetics and relaxed geometries associated with this reconstruction. We also address the problem of how the formation energies of reconstructions having different stoichiometries should be compared. The calculations reveal a strong dependence of the formation energy of the missing-dimer trenches on spacing n, and demonstrate that this dependence stems almost entirely from elastic relaxation. The results provide a natural explanation for the experimentally observed spacings in the region of n \~ 8.

Adsorption of Acetone onto MgO: Experimental and Theoretical Evidence for the Presence of a Surface Enolate

A new band at 1640 cm-1 is revealed by diffuse reflectance FT-IR spectroscopy of acetone adsorbed on a MgO surface (shown schematically). On the grounds of ab initio quantum-mechanical calculations, this band is assigned to an adsorbed enolate species. This evidence proves the catalytic role of the metal oxide surface in the condensation reaction mechanism.

PARALLEL COMPUTATION OF SECOND DERIVATIVES OF RHF ENERGY ON DISTRIBUTED MEMORY COMPUTERS

A parallel implementation of the computation of RHF energy second derivatives with respect to the nuclear coordinates is described. The algorithm and organization of the code are described in detail on the most computationally demanding steps with special emphasis on the integral transformation code. Key features of the proposed algorithm are its large degree of concurrency, limited interprocessor communication, and critical memory needs distributed over the processors. The cpu times and computer and network resources used are reported and discussed for a few examples.

MOLECULAR DYNAMICS SIMULATIONS OF THE MGO(001) SURFACE HYDROXYLATION

A first principles study of the MgO(001) surface hydroxylation is reported. Using ab initio Hartree–Fock embedded cluster calculations, pair potentials for the species H+-Osurf, H+-Mgsurf, HO−-Osurf, and HO−-Mgsurf pairs have been obtained. These potentials are used to simulate, by means of molecular dynamics, the hydroxylation of the MgO(001) surface. The hydroxyl groups are found to remain at the surface, bound to Mg cations, inducing a noticeable relaxation. The protons are found to leave the surface, stabilizing beyond the third layer. This proton penetration agrees with recent elastic recoil detection analysis in which the protonation is observed to at least 5000 A. The structure of the surface, as well as the coordination environment of inner protons, is discussed in light of radial distribution functions and spectra of density of states.

A theoretical study of the NaTiO2 (001) rutile interaction

A theoretical study of the reduction of the TiO2 (001) rutile surface by atomic Na is reported. The study is based on ab initio embedded cluster calculations carried out for two different sites of this surface. The first one (A) is represented by a TiO4−4 cluster and the second (B), an oxygen ending site, by a cluster of formula TiO8−6. The calculations show that adsorption of Na is possible on both sites although site A is energetically favored. The electron transfer process has been examined from state averaged complete active self consistent field (CASSCF) calculations. The analysis of the these wave functions shows that the Na 3s electron is transferred to a Ti 3d orbital of the surface only on site A. For site B such a transfer would involve an excited state of the cluster. The dissociation curves for several electronic states are reported.