Publications (64)185.99 Total impact
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ABSTRACT: We examine two factors that could limit the doping of ntype IIIV semiconductor nanowires, namely, the solubility of the dopants and the possible formation of DXlike defect centers. We find that it is preferable to dope zincblende nanowires via anion substitution as opposed to cation substitution. The comparison with previous work on ntype IIIV semiconductor nanocrystals also allows us to determine the role of dimensionality and quantum confinement on doping characteristics of materials. Our work is based on quantum calculations of InP nanowires using realspace pseudopotentials constructed within density functional theory. 
Article: Investigation of the liquid Pb/Si(001) interface from ab initio moleculardynamics calculations
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ABSTRACT: The structure of liquid Pb on an ideal Si(001) surface was studied experimentally a decade ago by means of xray diffraction and the results were interpreted in terms of the presence of fivefold symmetry Pb structures captured transiently by the potential created by the unreconstructed Si(001) surface. We critically analyze this interpretation in the light of the results obtained in an extensive ab initio molecular dynamics study of a system comprising 314 Pb atoms and 175 Si atoms setup in 7 (001) ideal layers (a total number of 1956 valence electrons) in a slab geometry. The structure found for the first Pb layer is very different from that of bulk Pb, mostly consisting in onedimensional lines. However, we do observe the possibility of forming transient structures, in particular icosahedral caps. 
Article: Electronic properties of pure and ptype doped hexagonal sheets and zigzag nanoribbons of InP
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ABSTRACT: Unlike graphene, a hexagonal InP sheet (HInPS) cannot be obtained by mechanical exfoliation from the native bulk InP, which crystallizes in the zinc blende structure under ambient conditions. However, by ab initio density functional theory calculations we found that a slightly buckled HInPS is stable both in pristine form and when doped with Zn atoms; the same occurred for hydrogenpassivated zigzag InP nanoribbons (ZInPNRs), quasionedimensional versions of the quasitwodimensional material. We investigated the electronic properties of both nanostructures, in the latter case also in the presence of an external transverse electric field, and the results are compared with those of hypothetical planar HInPS and ZInPNRs. The band gaps of planar ZInPNRs were found to be tunable by the choice of strength of this field, and to show an asymmetric behavior under weak electric fields, by which the gap can either be increased or decreased depending on their direction; however, this effect is absent from slightly buckled ZInPNRs. The binding energies of the acceptor impurity states of Zndoped HInPS and ZInPNRs were found to be similar and much larger than that of Zndoped bulk InP. These latter findings show that the reduction of the dimensionality of these materials limits the presence of free carriers.  [Show abstract] [Hide abstract]
ABSTRACT: The synthesis of fused quantum dots offers new possibilities for the design of nanoscale devices. Here we consider semiconductor quantum dots, fused along zincblende crystal directions, to construct nanomaterials systematically in one, two, and three dimensions. We solve and explain the electronic structure of such nanomaterials. Based on this, we also discuss the potential use of these nanostructures for electronic applications such as current splitters, fieldeffect transistors, and logic gates. Our results for fusing of InP quantum dots were obtained by using a realspace implementation of pseudopotentials constructed within densityfunctional theory.  [Show abstract] [Hide abstract]
ABSTRACT: One of the most challenging issues in materials physics is to predict the properties of defects in matter. Such defects play an important role in functionalizing materials for use in electronic and optical devices. As the length scale for such devices approaches the nanoregime, the interplay of dimensionality, quantum confinement and defects can be complex. In particular, the usual rules for describing defects in bulk may be inoperative, i.e. a shallow defect level in bulk may become a deep level at the nanoscale. The development of computational methods to describe the properties of nanoscale defects is a formidable challenge. Nanoscale systems may contain numerous electronic and nuclear degrees of freedom, and often possess little symmetry. In this review, we focus on new computational methods, which allow one to predict the role of quantum confinement on the electronic, magnetic and structural properties of functionalized nanostructures. We illustrate how these methods can be applied to nanoscale systems, and present calculations for the electronic, magnetic and structural properties of dopants in semiconductor nanocrystals and nanowires.  [Show abstract] [Hide abstract]
ABSTRACT: We perform an ab initio molecular dynamics study of the static, dynamic and electronic properties of the liquid BixPb1−x alloy at three concentrations, including the eutectic one, x = 0.55. This alloy is of particular technological interest for its possible use as coolant in fast reactors. Our predictions are in good agreement with the available experimental data. In particular, the computed total static structure factors reproduce accurately the neutron diffraction results, and the predicted adiabatic sound velocity and shear viscosity compare well with the experimental values. The partial dynamic structure factors exhibit clear side peaks indicative of propagating density fluctuations, and the longitudinal and transverse dispersion relations show several branches, which we analyze in detail. The electronic density of states show that the liquid Bi–Pb alloy is a good metal, but with strong deviations from the freeelectron parabolic curve.  [Show abstract] [Hide abstract]
ABSTRACT: The structure of liquid Pb on an ideal Si(001) surface was studied experimentally a decade ago by means of xray diffraction and the results were interpreted in terms of the presence of fivefold symmetry Pb structures captured transiently by the potential created by the unreconstructed Si(001) surface. We critically analyze this interpretation in the light of the results obtained in an extensive ab initio molecular dynamics study of a system comprising 314 Pb atoms and 175 Si atoms setup in 7 (001) ideal layers (a total number of 1956 valence electrons) in a slab geometry. The structure found for the first Pb layer is very different from that of bulk Pb, mostly consisting in onedimensional lines. However, we do observe the possibility of forming transient structures, in particular icosahedral caps. We discuss if this result can explain the modulation found experimentally in the diffracted intensity.  [Show abstract] [Hide abstract]
ABSTRACT: We perform a comprehensive study of the static, dynamic, and electronic properties of liquid Bi at T=600 K, ρ=0.02876 Å−3 by means of 124atom ab initio molecular dynamics simulations based on PARSEC, a realspace implementation of pseudopotentials constructed within the densityfunctional theory. The predicted results are in good agreement with available experimental data, thus confirming the adequacy of this technique to achieve a reliable description of a nonsimple liquid metal such as liquid Bi, whose static structure has reminiscences of the rhombohedral structure of the crystal. The calculated intermediate scattering function F(q,t) shows at lowq values a strong diffusive component which imposes a slow decay of this function. The dynamic structure factor S(q,ω) exhibits side peaks, indicative of collective density excitations, over a range of wave numbers up to q≈1.4 Å−1. Moreover, our simulations suggest an important “positive dispersion effect” for the density fluctuations of around 20%. We have also investigated the relaxation mechanisms for the density fluctuations by analyzing the different contributions to the secondorder memory function of F(q,t). Our results suggest that the thermal relaxation is the slow decaying channel, whereas the viscoelastic relaxation is the fast decaying channel. This behavior is just the opposite of that found in some liquid metals and may be attributed to the semimetal character of Bi.  [Show abstract] [Hide abstract]
ABSTRACT: We perform a comprehensive study of the static, dynamic and electronic properties of liquid Bi near melting by means of 124atom ab initio molecular dynamics simulations based on PARSEC, a realspace implementation of pseudopotentials constructed within the densityfunctional theory. The predicted results are in good agreement with available experimental data, thus confirming the adequacy of this technique to achieve a reliable description of a nonsimple liquid metal such as liquid Bi, whose static structure has reminiscences of the rhombohedral structure of the crystal. Our results for the intermediate scattering function, density of states and electrical conductivity also show markedly differences to those of simple liquid metals.  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate that it is preferable to dope IIIV semiconductor nanocrystals by ntype anion substitution as opposed to cation substitution. Specifically, we show the dopability of zincblende nanocrystals is more efficient when the dopants are placed at the anion site as quantified by formation energies and the stabilization of DXlike defect centers. Our results are based on firstprinciples calculations of InP quantum dots by using a realspace implementation of densityfunctional theory and pseudopotentials.  [Show abstract] [Hide abstract]
ABSTRACT: Semiconductors are intentionally doped ntype by replacing one host atom by an impurity atom that has one more electron. In the case of IIIV semiconductors this can be done at both anion and cation sites. Here we show that IIIV semiconductor nanocrystals with zincblende structure should not be doped by cation substitution, but rather by anion substitution. We found that, as result of quantum confinement, the formation of defects that affect the main characteristics of the dopants is favored when the nanocrystals are doped at the cation site. Our study is performed through firstprinciples calculations based on a realspace implementation of densityfunctional theory and pseudopotentials by using the PARSEC code.  [Show abstract] [Hide abstract]
ABSTRACT: We characterize the impurity state responsible for current flow in Zndoped InP nanocrystals through firstprinciples calculations based on a realspace implementation of densityfunctional theory and pseudopotentials. We found the activation energy of the acceptor state to range from 0.03 eV in the bulk to 2.5 eV in smaller nanocrystals as a result of quantum confinement. The maximum value for nanocrystals is an order of magnitude larger than the maximum value found for InP nanowires (0.2 eV). Our results show that reducing the dimensionality in ptype InP materials strongly inhibits the capability of the materials to generate free carriers. 
Article: Role of dimensionality and quantum confinement in ptype semiconductor indium phosphide quantum dots
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ABSTRACT: We characterize the impurity state responsible for current flow in zincdoped indium phosphide nanocrystals through firstprinciples calculations based on a realspace implementation of densityfunctional theory and pseudopotentials. We found the activation energy of the acceptor state to range from the value of the acceptor state in the bulk (0.03 eV) to up to values of ∼2.5 eV in the smaller nanocrystals as a result of the threedimensional quantum confinement. This maximum value for the nanocrystals is an order of magnitude bigger than the maximum value found for onedimensional nanomaterials (nanowires) within the same theoretical approach (∼0.2 eV). Our results show that the progressive reduced dimensionality in ptype indium phosphide materials strongly reduces the capability of the materials to generate free carriers.  [Show abstract] [Hide abstract]
ABSTRACT: We propose to tailor the magnetic structure of atomic clusters by suitable doping, which produces the nanometric equivalent to alloying. As a proof of principle, we perform a theoretical analysis of Fe(6x)Mn(x) clusters (x = 05), which shows a modulation of the magnetic moment of the clusters as a function of Mn doping and, more importantly, a collinear to noncollinear transition at x = 4. 
Article: Possibility of collinear magnetic order in frustrated freestanding Fe_ {2} Cr_ {4} clusters
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ABSTRACT: Using the fully unconstrained version of the densityfunctional method SIESTA with the generalized gradient approximation for exchange and correlation, we compute the structures and magnetic configurations of the lowestenergy isomers of the freestanding cluster Fe2Cr4. The two most stable isomers of distorted octahedral geometry exhibit magnetic frustration but collinear magnetic order, in contrast to the behavior of extended Cr/Fe frustrated systems. Confinement effects and structural relaxation are shown to be the origin of this difference, which illustrates the importance of finitesize effects on magnetism.  [Show abstract] [Hide abstract]
ABSTRACT: The impurity state responsible for current flow in zincdoped indium phosphide is characterized through firstprinciples calculations based on a realspace implementation of densityfunctional theory and pseudopotentials. The identification of the acceptor state is performed via an hybridization process between states of same symmetry introduced in the host system by the impurity, and involves the selfconsistent computation of the electronic properties of crystals containing thousands of atoms carried out within the context of modern firstprinciples approaches. The character of the wave function of the acceptor state, the variation of the binding energy of the state with dopant concentration and the value of the binding energy in the bulk limit are reported in this work.  [Show abstract] [Hide abstract]
ABSTRACT: We investigated the possibility of noncollinear magnetism in small Mn(n) clusters (n=26) using the densityfunctional method SIESTA with the generalized gradient approximation (GGA) to exchange and correlation. The lowestenergy states identified were collinear, with the atomic spin magnetic moments pointing in the same direction, for Mn(2) and Mn(3), and noncollinear for Mn(4), Mn(5) and, most decidedly, Mn(6). These SIESTA/GGA results, which are compared with those of an earlier SIESTA study that used the local spin density approximation, are qualitatively in keeping with the result obtained by VASP/GGA calculations. 
Article: Ab initio calculations for the electronic properties of zincdoped indium phosphide nanowires
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ABSTRACT: ptype indium phosphide nanowires are known to function as working devices when assembled with ntype nanowires, and thus are seen as very promising building blocks for highly integrated electronic devices within the semiconductor industry. In this work, we have characterized the impurity state responsible for current flow in zincdoped indium phosphide nanowires through firstprinciples calculations based on a realspace implementation of densityfunctional theory and pseudopotentials. The binding energy of the acceptor state is predicted to range from the value of the acceptor state in the bulk to up to values of approximately 0.2 eV in the thinner nanowires as a result of the twodimensional quantum confinement. Our results show that, in thin nanowires, quantum confinement can move the defect level deep into the energy gap.  [Show abstract] [Hide abstract]
ABSTRACT: We investigated the possibility of noncollinear magnetism in small Mnn clusters (n = 26) using the densityfunctional method SIESTA with the generalized gradient approximation (GGA) to exchange and correlation. The lowestenergy states identified were ferromagnetic for Mn2 and Mn3, and magnetically noncollinear for Mn4, Mn5 and, most decidedly, Mn6. These SIESTA/GGA results, which are compared with those of an earlier SIESTA study that used the local spin density approximation, are qualitatively in keeping with the result obtained by VASP/GGA calculations.  [Show abstract] [Hide abstract]
ABSTRACT: We performed a comprehensive study of the static, dynamic and electronic properties of liquid Pb at T = 650 kelvins, density 0.0309 angstroms^{3} by means of 216particle ab initio molecular dynamics simulations based on a realspace implementation of pseudopotentials constructed within densityfunctional theory. The predicted results and available experimental data are very in good agreement, which confirms the adequacy of this technique to achieve a reliable description of the behavior of liquid metals, including their dynamic properties. Although some of the computed properties of liquid Pb are similar to those of simple liquid metals, others differ markedly. Our results show that an appropriate description of liquid Pb requires the inclusion of relativistic effects in the determination of the pseudopotentials of Pb.
Publication Stats
967  Citations  
185.99  Total Impact Points  
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Institutions

19962013

University of Santiago de Compostela
 Department of Condensed Matter Physics
Santiago, Galicia, Spain


20032005

University of Minnesota Duluth
 Department of Computer Science
Duluth, Minnesota, United States 
Institute for Systems and Computer Engineering of Porto (INESC Porto)
Oporto, Porto, Portugal
