Publications (32)80.33 Total impact

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ABSTRACT: A simple method to obtain a gapcorrected band structure of cadmium telluride within density functional theory is presented. Onsite Coulomb selfinteractionlike correction potential has been applied to the 5pshell of Te and the 4dshell of Cd. The predicted physical properties are similar to or better than those obtained with hybrid functionals and at largely reduced computational cost. In addition to the corrected electronic structure, the lattice parameters and the bulk modulus are improved. The relative stabilities of the different phases (zincblende, wurtzite, rocksalt and cinnabar) are preserved. The formation energy of the cadmium vacancy remains close to the values obtained from hybrid functional calculations.Physica B Condensed Matter 11/2014; 452:119–123. DOI:10.1016/j.physb.2014.07.015 · 1.28 Impact Factor 
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ABSTRACT: The electronic structure and properties of the orthorhombic phase of the CH3NH3PbI3 perovskite are computed with density functional theory. The structure, optimized using a van der Waals functional, reproduces closely the unit cell volume. The experimental band gap is reproduced accurately by combining spinorbit effects and a hybrid functional in which the fraction of exact exchange is tuned selfconsistently to the optical dielectric constant. Including spinorbit coupling strongly reduces the anisotropy of the effective mass tensor, predicting a low electron effective mass in all crystal directions. The computed binding energy of the unrelaxed exciton agrees with experimental data, and the values found imply a fast exciton dissociation at ambient temperature. Also polaron masses for the separated carriers are estimated. The values of all these parameters agree with recent indications that fast dynamics and large carrier diffusion lengths are key in the high photovoltaic efficiencies shown by these materials.Physical Review B 07/2014; 90(4). DOI:10.1103/PhysRevB.90.045207 · 3.66 Impact Factor 
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ABSTRACT: The UV absorption spectrum of C60 in nhexane solvent has been revised by means of numerical analysis and timedependent density functional theory (TDDFT). The absorption spectrum in the range 3–7 eV has been fitted by a spectral function that includes fourteen transitions with Gaussian lineshape, providing reference transition energies and intensities. The agreement between the experimental and theoretical UV absorption intensities has been considerably improved with respect to previous calculations, by including the solvent dielectric response via the polarizable continuum model (PCM).Chemical Physics Letters 02/2014; 593:72–76. DOI:10.1016/j.cplett.2013.12.067 · 1.99 Impact Factor 
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ABSTRACT: Point defects in Ga and Aldoped ZnO thin films are studied by means of first principles electronic structure calculations. Candidate defects are identified to explain recently observed differences in electrical and spectroscopical behavior of both systems. Substitutional doping in GaZnO explain the metallic behavior of the electrical properties. Complexes of interstitial oxygen with substitutional Ga can behave as acceptor and cause partial compensation, as well as gap states below the conduction band minimum as observed in photoemission experiments. Zn vacancies can also act as compensating acceptors. On the other hand, the semiconducting behavior of AlZnO and the small variation in the optical gap compared with pure ZnO, can be explained by almost complete compensation between acceptor Zn vacancies and substitutional Al donors. Interstitial Al can also be donor levels and can be the origin of the small band observed in photoemission experiments below the Fermi level. Combinations of substitutional Al with interstitial oxygen can act simultaneously as compensating acceptor and generator o the mentioned photoemission band. The theoretical calculations have been done using density functional theory (DFT) within the generalized gradient approximation with onsite Coulomb interaction. In selected cases, DFT calculations with semilocalexact exchange hybrid functionals have been performed. Results are compared with photoelectron spectra for GaZnO and AlZnO at the corresponding doping levels. 
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ABSTRACT: A metastable carbon nanotube with single, double, and triple bonds has been predicted from ab initio simulation. It results from the relaxation of an ideal carbon nanotube with chirality (2,1), without any potential barrier between the ideal nanotube and the new structure. Tenmembered carbon rings are formed by breaking carbon bonds between adjacent hexagons; eightmembered rings, already present in the ideal structure, become the smallest rings. This structure is stable in molecular dynamics simulations at temperatures up to 1000 K. Raman, infrared, and optical absorption spectra are simulated to allow its identification in the laboratory. The structure can be described as a double helical chain with alternating single, double, and triple bonds, where the chains are bridged by single bonds.Physical Review Letters 09/2012; 109(1010):105501. DOI:10.1103/PhysRevLett.109.105501 · 7.73 Impact Factor 
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ABSTRACT: Excitation properties of the isolated C(60) and (C(60))(N) model clusters (N = 2, 3, 4, 6 and 13) are studied using an a priori parameterized and selfconsistent Hamiltonian, the Complete Neglect of Differential Overlap considering the l azimuthal quantum number method. This method properly describes electron excitations of the isolated C(60) after the configuration interaction of singles (CIS) procedure, when those are compared with experimental data in nhexane solution and in a molecular beam. Geometry models of (C(60))(N) clusters to model the effect of aggregation were obtained from the fullerene fcc crystal. Some peaks in the low energy edge of the absorption spectrum appear corresponding to clustering effects, as well as small increases of bandwidths in the strong bands at the UV region. An analysis of the theoretical absorption spectrum for dimer models has been carried out, taking into account the influence of the distance between fullerene centers. The density of states of CIS for fullerene clusters in the range from 2.0 to 6.5 eV shows the possibility of electron transitions as functions of the size of the clusters.Physical Chemistry Chemical Physics 08/2012; 14(37):1305866. DOI:10.1039/c2cp41979c · 4.20 Impact Factor 
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ABSTRACT: Hydroxyapatite (HAP) and fluorapatite (FAP) are essential components of dental enamel and bone. In this paper, we report a computational study of the elastic properties of HAP and FAP using ab initio and force field techniques. We have obtained the HAP and FAP elastic stiffness constants in hexagonal symmetry by fitting the Hooke law for both the energystrain and stressstrain relations. Our ab initio HAP stiffness constants differ from the results of previous calculations, but follow similar trends. The HAP and FAP stiffness constants calculated with the ab initio method are very similar, although FAP is slightly stiffer than HAP in the hexagonal plane, and more compliant along the hexagonal axis. The pseudosinglecrystal HAP experimental stiffness constants in current use are critically reviewed. Combining the data from the ab initio simulations with the experimental FAP stiffness constants, several alternative sets of HAP stiffness constants are proposed. The mismatch in properties between HAP and FAP is evidently too small to assume it to be directly responsible for dental enamel mechanical degradation with fluorosis disease.10/2011; 4(7):101120. DOI:10.1016/j.jmbbm.2011.03.001 
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ABSTRACT: The interaction between implanted nitrogen atoms, adsorbed gold atoms, and oxygen vacancies at the anatase TiO(2)(101) surface is investigated by means of periodic density functional theory calculations. Substitutional and interstitial configurations for the Ndoping have been considered, as well as several adsorption sites for Au adatoms and different types of vacancies. Our total energy calculations suggest that a synergetic effect takes place between the nitrogen doping on one hand and the adsorption of gold and vacancy formation on the other hand. Thus, while preimplanted nitrogen increases the adsorption energy for gold and decreases the energy required for the formation of an oxygen vacancy, preadsorbed gold or the presence of oxygen vacancies favors the nitrogen doping of anatase. The analysis of the electronic structure and electron densities shows that a charge transfer takes place between implantedN, adsorbed Au and oxygen vacancies. Moreover, it is predicted that the creation of vacancies on the anatase surface modified with both implanted nitrogen and supported gold atoms produces migration of substitutional N impurities from bulk to surface sites. In any case, the most stable configurations are those where N, Au and vacancies are close to each other.Physical Chemistry Chemical Physics 06/2011; 13(23):1134050. DOI:10.1039/c0cp02470h · 4.20 Impact Factor 
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ABSTRACT: The photoluminescence of annealed CdxZn1−xSe quantum dots (QDs) under the influence of an external magnetic field has been studied in this paper. Postgrowth annealing was performed for different annealing times. Above a critical annealing time, the QD luminescence shows a pronounced redshift of the Zeeman split magnetic subcomponents. This observation is in contrast to the blueshift caused by the diamagnetic behavior that is usually observed in nonmagnetic QDs. We attribute our finding to the paramagnetism caused by the mixing of heavy and light hole states. Hence, postgrowth thermal annealing treatment might be employed to render undoped epitaxial QDs intrinsically magnetic in a controlled manner. Two theoretical models were developed: a fewparticle model to account for excitonic complex effects and a multiband calculation that describes the valence band hybridization. Contrasting the two models allowed us to unambiguously elucidate the nature of such an effect.New Journal of Physics 04/2011; 14(4):043038. DOI:10.1088/13672630/14/4/043038 · 3.67 Impact Factor 
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ABSTRACT: We report a systematic study of magnetooptical properties in CdZnSe/ZnSe semiconductor quantum dots (QDs) subjected to postgrowth thermal annealing. The theoretical and experimental characterizations are combined in order to understand the evolution of Zeeman splitting and the blueshift as well as redshift of the magnetic subcomponents with magnetic field strength of excitons confined in annealed QDs as a function of different annealing times. A combination of a multiband model as well as parameter interpolation supported by ab initio calculations is presented and points toward an inversion of the lightholeheavyhole states due to annealing. The band mixing of exciton states explains besides the variation in the Zeeman splitting for differently annealed QD excitons, the evolution of the diamagnetic blueshift into a paramagnetic redshift of the magnetic subcomponents. A discussion of the effects associated to the Coulomb interaction on the polarized excitonic recombinations modulated by magnetic field is given.Physical Review B 11/2010; 82(20):205318. DOI:10.1103/PhysRevB.82.205318 · 3.66 Impact Factor 
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ABSTRACT: A study of the vibrational density of states (DOS) of γAl2O3 is presented. Four structural models from the recent literature are considered: vacant spinel model and three nonspinel models. The vacant spinel and one of the nonspinel models have unit cells with 40 atoms, while the other two models have 160 atoms. The interatomic interactions are computed using classical force fields that include Coulomb and van der Waals attractive interactions, short range repulsive interactions, as well as threebody terms. The oxygen polarizability is included via a coreshell potential. The DOS is compared with ab initio calculations recently published for the vacant spinel model. The classical and ab initio DOS show some differences for frequencies higher than 200cm−1, the ab initio having more peaks and having a frequency cutoff 100cm−1 lower than the classical DOS. The DOS of all models present some small differences. While the 160atoms nonspinel models present a rather structureless DOS, 40atoms models present peaks and dips relative to the 160atoms models. The elastic constants of polycrystalline γAl2O3 are also estimated using several force fields. In general, the classical force field predict higher elastic moduli than the ab initio method. The infrared spectra of the four models are calculated.Journal of Materials Science 09/2010; 45(18):50945100. DOI:10.1007/s1085301044775 · 2.31 Impact Factor 
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ABSTRACT: Molecular dynamic (MD) simulations, both classical and ab initio, of amorphous GeO2 (germania), Al2O3 (alumina), and CdTeO compounds are presented. We focus our attention on the structural and vibrational properties, giving an atomic description of the short and intermediaterange order. Amorphous germanium oxide under pressure was studied by means of classical MD simulations. At normal density, the analysis of the interatomic distances reveals that in the amorphous state there is a shortrange order dominated by a slightly distorted (GeO4) tetrahedron. Beyond that, there is an intermediaterange order composed by vertexsharing tetrahedra. As density increases, there is a structural transformation, from a shortrange order defined by the basic tetrahedron to a basic octahedron. Consistent with this picture, the vibrational density of states also presents big changes, where the low frequency band shrinks, and the high frequency becomes wider and flatter. In the case of alumina, both classical and first principles MD calculations of amorphous Al2O3 are reported, comparing both methodologies. Interatomic correlations allow us to conclude that the shortrange order is mainly composed by AlO4 tetrahedra, but in contrast to classical MD results, also an important number of AlO5 unit is present. The vibrational density of states presents two main bands, a low frequency one related to the intertetrahedron vibration and a high frequency band related to the intratetrahedron vibration. Finally, we present an ab initio MD calculation for the complex ternary material CdTeO3. According our calculations, the shortrange order of this compound consists of a number of basic building blocks, greater than in the case of its crystalline counterpart. The compound is characterized using pair and angular distribution functions, coordination numbers, and a description of the molecular units of the compound. For example, Cd is coordinated by either six or five atoms. In the case of Te, the chemical unit is TeO3. The most frequent clusters are CdO6, CdO5,TeO3, and TeO4.Journal of Materials Science 09/2010; 45(18):51245134. DOI:10.1007/s1085301045790 · 2.31 Impact Factor 
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ABSTRACT: Optical properties of three kinds of zigzag (5,0), (13,0), and (9,0) singlewalled carbon nanotubes (SWCNTs) are studied using an approximate quantum mechanical method named complete neglect of differential overlap, which distinguishes basis atomic orbitals with different azimuthal l quantum numbers (CNDOL). This method models the electron energy transitions and excited state charge distributions through a configuration interaction of singly (CIS) excited determinants allowing the direct understanding of properties related with the total electronic wave function of nanoscopic systems, projecting a reliable quantum mechanical understanding to real life objects. The finite SWCNT’s structures were obtained by replicating the unit cells of periodic SWCNTs and saturating the edge dangling bonds with hydrogens. The unit cell was previously relaxed using standard density functional theory methods. The behavior of these SWCNTs were interpreted in the framework of the CNDOL scheme by increasing the lengths of the tubes above 3 nm. As the nanotubes grow in length, the position of excited states for each SWCNT evolve differently: in contrast with (9,0) SWCNT, which exhibits favorable conditions for photoexcitation, the (13,0) and (5,0) SWCNTs do not show a lowering of the lowest excited states. This behavior is discussed by taking into account electron—electron interactions as considered in the framework of the CIS procedure. Furthermore, the (13,0) and (5,0) SWCNTs present forbidden transitions for the lowest excitations and its first dipoleallowed transitions are at 0.9–1.0 and 1.4–1.6 eV, respectively. In contrast, (9,0) SWCNT allows excitations by photon at less than 0.4 eV as the length of the nanotube tends to infinite. Excitons appear more bounded, energetically and spatially, in the (13,0) than in the (9,0) and (5,0) SWCNTs. 
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ABSTRACT: We perform densityfunctionaltheory calculations of electronic core levels to obtain the tellurium xray photoelectron spectra in the amorphous solarenergy materials CdTeOx (x=0.2, 1, 2, and 3). We quantify the distribution of local tellurium environments that sum up to the total twopeak structure in the experimental spectrum. The general trend is that the more oxygen neighbors tellurium has the bigger the shift of its corelevel energy. However, due to the structural complexity, the relation between the corelevel shift and the number of oxygen neighbors does not obey simple rules. Hence, we show the importance of computer simulations when interpreting xray photoelectron spectra in this system, in particular, and amorphous oxides in general.Physical review. B, Condensed matter 01/2010; 81(1). DOI:10.1103/PhysRevB.81.014210 · 3.66 Impact Factor 
Article: Nonhydrostatic compression of bismuth to 222GPa: Some constraints on elasticity of the bccphase
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ABSTRACT: Polycrystalline bismuth sample was compressed in a diamond anvil cell and Xray diffraction patterns from the body centered cubic phase of bismuth (bccBi) recorded in the pressure range 12–222 GPa. The analysis of diffraction data indicates that the factor S=(S11−S12−S44/2) is positive, where Sij are the singlecrystal elastic compliances. The data suggest S/S11≅0.5 in the entire pressure range if stress continuity across the grain boundaries is assumed, and S/S11≅0.9 if a condition halfway between the stress and strain continuities is assumed. These results are compared with the first principle calculations of the elastic moduli carried out recently. The upper bound of the uniaxial stress component (the difference between the axial and radial stress components) increases linearly from very low value at 12 to ≈3 GPa at 222 GPa.Journal of Physics and Chemistry of Solids 09/2006; DOI:10.1016/j.jpcs.2006.06.002 · 1.59 Impact Factor 
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ABSTRACT: A method to calculate the quantum states of exciton–phonon complexes in semiconductor nanocrystals is presented. The exciton–phonon complexes are built from a basis set made of products of phonon states and electron–hole pairs, which are coupled through the electron–phonon Fröhlich interaction, and the electron–hole Coulomb and exchange interactions. In CdSe nanocrystals, the conduction band electrons are described by the effective mass equation, while the holes are represented by the spherical 4 × 4 Baldereschi–Lipari Hamiltonian. It is shown that a flexible and complete electron–hole basis, not limited to the 1s–1S3/2 octet, is essential to obtain converged eigenvalues and the correct polaron shift to the exciton energy. A study of the spectral properties is presented; in particular, the spectral region which involves the lowest exciton–phonon complex eigenstates is analysed in details. Specifically, the nonadiabatic nature of the exciton–phonon dynamics in the nanocrystals examined is clearly shown by the vibron eigenstates that were obtained.Journal of Physics Condensed Matter 07/2006; 18(31):7283. DOI:10.1088/09538984/18/31/022 · 2.22 Impact Factor 
Physical Review B 08/2005; 72(7). DOI:10.1103/PhysRevB.72.079904 · 3.66 Impact Factor

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ABSTRACT: The electronic structure of bulk gamma alumina is investigated using Density Functional Theory. Recent theoretical structures, including occupation of nonspinel positions, are used as starting point for energy minimization, allowing for relaxation of the cell shape and the ionic positions. A comparison of simulated diffraction patterns of the present and recent theoretical and experimental structural models is presented. The electronic structure is described in terms of band structure, density of states, charge density, electron localization function, and ionic charges. The valence band density of states of gamma alumina is similar to alpha  and kappa alumina, although a smaller bandgap is found. It is shown that gamma alumina ionicity is similar to other alumina phases, with a high localization of the electrons at oxygen atoms. The smaller bandgap of gamma alumina is attributed to the structural disorder.Physical Review B 07/2005; 72(3). DOI:10.1103/PhysRevB.72.035116 · 3.66 Impact Factor 

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ABSTRACT: We report firstprinciples calculations of the electronic structure of the simplest compounds that may be present in Cd–Te–O mixtures: CdTe, CdO, αTeO2, CdTeO3 and Cd3TeO6. The calculations are carried out in the Local Density Approximation (LDA) and predict the insulating character of these compounds, underestimating the optical bandgaps by nearly 1 eV, as usual for LDA. In the four oxides, the top valence bands originate mainly from the O 2p atomic levels. (© 2004 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)physica status solidi (c) 07/2004; 1(S1):S104  S107. DOI:10.1002/pssc.200304873
Publication Stats
377  Citations  
80.33  Total Impact Points  
Top Journals
Institutions

2012–2014

Universidad Autónoma de Madrid
 Department of Physical Applied Chemistry
Madrid, Madrid, Spain


2004–2014

University of Santiago, Chile
 Departamento de Física
CiudadSantiago, Santiago, Chile


2005

Uppsala University
Uppsala, Uppsala, Sweden


1998–2002

University of Havana
 • Departamento de Física Teórica
 • Facultad de Física
La Habana, Ciudad de La Habana, Cuba 
Center for Research and Advanced Studies of the National Polytechnic Institute
Ciudad de México, The Federal District, Mexico
