M. M. G. Alemany

University of Santiago de Compostela, Santiago de Compostela, Galicia, Spain

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Publications (65)170.51 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We examine two factors that could limit the doping of n-type III-V semiconductor nanowires, namely, the solubility of the dopants and the possible formation of DX-like defect centers. We find that it is preferable to dope zinc-blende nanowires via anion substitution as opposed to cation substitution. The comparison with previous work on n-type III-V 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 real-space pseudopotentials constructed within density functional theory.
    Physical Review B 09/2013; 88(11). · 3.66 Impact Factor
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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 x-ray 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 one-dimensional lines. However, we do observe the possibility of forming transient structures, in particular icosahedral caps.
    03/2013;
  • Source
    R C Longo, J Carrete, M M G Alemany, L J Gallego
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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 hydrogen-passivated zigzag InP nanoribbons (ZInPNRs), quasi-one-dimensional versions of the quasi-two-dimensional 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 Zn-doped HInPS and ZInPNRs were found to be similar and much larger than that of Zn-doped bulk InP. These latter findings show that the reduction of the dimensionality of these materials limits the presence of free carriers.
    Journal of Physics Condensed Matter 01/2013; 25(8):085506. · 2.22 Impact Factor
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    ABSTRACT: The synthesis of fused quantum dots offers new possibilities for the design of nanoscale devices. Here we consider semiconductor quantum dots, fused along zinc-blende 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, field-effect transistors, and logic gates. Our results for fusing of InP quantum dots were obtained by using a real-space implementation of pseudopotentials constructed within density-functional theory.
    Physical review. B, Condensed matter 11/2011; 84(20). · 3.77 Impact Factor
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    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 free-electron parabolic curve.
    Modelling and Simulation in Materials Science and Engineering 04/2011; 411:163-170. · 1.93 Impact Factor
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    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 nano-regime, 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.
    Reports on Progress in Physics 01/2011; 74(4). · 13.23 Impact Factor
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    ABSTRACT: Semiconductors are intentionally doped n-type by replacing one host atom by an impurity atom that has one more electron. In the case of III-V semiconductors this can be done at both anion and cation sites. Here we show that III-V semiconductor nanocrystals with zinc-blende 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 first-principles calculations based on a real-space implementation of density-functional theory and pseudopotentials by using the PARSEC code.
    03/2010;
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    ABSTRACT: We characterize the impurity state responsible for current flow in Zn-doped InP nanocrystals through first-principles calculations based on a real-space implementation of density-functional 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 p-type InP materials strongly inhibits the capability of the materials to generate free carriers.
    03/2010;
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    ABSTRACT: We perform a comprehensive study of the static, dynamic and electronic properties of liquid Bi near melting by means of 124-atom ab initio molecular dynamics simulations based on PARSEC, a real-space implementation of pseudopotentials constructed within the density-functional 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 non-simple 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.
    03/2010;
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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 x-ray 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 one-dimensional 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.
    Physical review. B, Condensed matter 01/2010; 82(13). · 3.77 Impact Factor
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    ABSTRACT: We demonstrate that it is preferable to dope III-V semiconductor nanocrystals by n-type anion substitution as opposed to cation substitution. Specifically, we show the dopability of zinc-blende nanocrystals is more efficient when the dopants are placed at the anion site as quantified by formation energies and the stabilization of DX-like defect centers. Our results are based on first-principles calculations of InP quantum dots by using a real-space implementation of density-functional theory and pseudopotentials.
    Physical review. B, Condensed matter 01/2010; 81(12). · 3.77 Impact Factor
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    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 124-atom ab initio molecular dynamics simulations based on PARSEC, a real-space implementation of pseudopotentials constructed within the density-functional 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 low-q 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 second-order 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.
    Physical review. B, Condensed matter 01/2010; 81(13). · 3.77 Impact Factor
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    ABSTRACT: We characterize the impurity state responsible for current flow in zinc-doped indium phosphide nanocrystals through first-principles calculations based on a real-space implementation of density-functional 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 three-dimensional quantum confinement. This maximum value for the nanocrystals is an order of magnitude bigger than the maximum value found for one-dimensional nanomaterials (nanowires) within the same theoretical approach (∼0.2 eV). Our results show that the progressive reduced dimensionality in p-type indium phosphide materials strongly reduces the capability of the materials to generate free carriers.
    Physical review. B, Condensed matter 12/2008; 78(23). · 3.77 Impact Factor
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    R C Longo, M M G Alemany, A Vega, J Ferrer, L J Gallego
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    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(6-x)Mn(x) clusters (x = 0-5), 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.
    Nanotechnology 06/2008; 19(24):245701. · 3.84 Impact Factor
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    ABSTRACT: The impurity state responsible for current flow in zinc-doped indium phosphide is characterized through first-principles calculations based on a real-space implementation of density-functional 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 self-consistent computation of the electronic properties of crystals containing thousands of atoms carried out within the context of modern first-principles 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.
    Solid State Communications 05/2008; 146(5):245-248. · 1.53 Impact Factor
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    R C Longo, M M G Alemany, J Ferrer, A Vega, L J Gallego
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    ABSTRACT: We investigated the possibility of noncollinear magnetism in small Mn(n) clusters (n=2-6) using the density-functional method SIESTA with the generalized gradient approximation (GGA) to exchange and correlation. The lowest-energy 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.
    The Journal of Chemical Physics 04/2008; 128(11):114315. · 3.12 Impact Factor
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    ABSTRACT: p-type indium phosphide nanowires are known to function as working devices when assembled with n-type 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 zinc-doped indium phosphide nanowires through first-principles calculations based on a real-space implementation of density-functional 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 two-dimensional quantum confinement. Our results show that, in thin nanowires, quantum confinement can move the defect level deep into the energy gap.
    03/2008;
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    ABSTRACT: We investigated the possibility of noncollinear magnetism in small Mnn clusters (n = 2-6) using the density-functional method SIESTA with the generalized gradient approximation (GGA) to exchange and correlation. The lowest-energy 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.
    03/2008;
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    ABSTRACT: Using the fully unconstrained version of the density-functional method SIESTA with the generalized gradient approximation for exchange and correlation, we compute the structures and magnetic configurations of the lowest-energy isomers of the free-standing 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 finite-size effects on magnetism.
    Physical review. B, Condensed matter 01/2008; 77(21). · 3.77 Impact Factor
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    ABSTRACT: The impurity state responsible for current flow in zinc-doped indium phosphide nanowires is characterized through first-principles calculations based on a real-space implementation of density functional theory and pseudopotentials. The binding energy of the acceptor state is predicted to range from the value of the acceptor state in the bulk up to values of 0.2 eV in the thinner nanowires as a result of the two-dimensional quantum confinement. The location of the impurity atom within the nanomaterial is not found to play a prominent role in determining the characteristic properties of the state. Our results show that, in thin nanowires, quantum confinement can move the defect level deep into the energy gap.
    Nano Letters - NANO LETT. 06/2007;

Publication Stats

558 Citations
170.51 Total Impact Points

Institutions

  • 1996–2013
    • University of Santiago de Compostela
      • • Facultad de Física
      • • Departamento de Física de la Materia Condensada
      Santiago de Compostela, Galicia, Spain
  • 2006–2011
    • University of Texas at Austin
      • Institute for Computational Engineering and Sciences
      Austin, Texas, United States
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, NJ, United States
  • 2004–2005
    • University of Minnesota Twin Cities
      • Department of Chemical Engineering and Materials Science
      Minneapolis, MN, United States
    • Institute for Systems and Computer Engineering of Porto (INESC Porto)
      Oporto, Porto, Portugal
  • 2003–2005
    • University of Minnesota Duluth
      • Department of Computer Science
      Duluth, Minnesota, United States