Sanwu Wang

University of Tulsa, Tulsa, Oklahoma, United States

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Publications (35)101.19 Total impact

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    ABSTRACT: The enhancement of thermal stability in nanocrystalline-amorphous (NC–a) materials was investigated by a modified Monte Carlo Potts method. The thermal stability, including the grain size stability and amorphous distribution stability, was found to be controlled by the amorphous fraction f and the energy ratio (Jgb/Jint) of the NC–NC grain boundary energy to the NC–a interfacial energy. The best thermal stability for temperatures up to 1550 K was observed with f = 0.279 and Jgb/Jint = 12, where finer grains are surrounded by thin and coherent NC–a boundaries.
    Scripta Materialia 09/2014; 87:33–36. · 2.82 Impact Factor
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    ABSTRACT: With extensive first-principles calculations, we investigated the geometric structure, phase transition, and electronic properties of orthorhombic, monoclinic, and tetragonal K1−xNaxNbO3 (KNN) as functions of the Na content. We found that KNN undergoes an orthorhombic-to-monoclinic-to-orthorhombic phase transition when the Na content is gradually increased. We also found that the polarization vector of the monoclinic phase can be rotated more easily than those of the orthorhombic and tetragonal phases, giving rise to an enhanced piezoelectric response of the monoclinic KNN. Furthermore, our calculations provide an interpretation for the experimentally observed unusual broad peak of the KNN piezoelectric parameters.
    Journal of the American Ceramic Society 08/2014; · 2.43 Impact Factor
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    ABSTRACT: We report quantum-mechanical investigations that predict the formation of white graphene and nano-sized white graphite from the first-order phase transformations of nano-sized boron nitride thin-films. The phase transformations from the nano-sized diamond-like structure, when the thickness d > 1.4 nm, to the energetically more stable nano-sized white graphite involve low activation energies of less than 1.0 eV. On the other hand, the diamond-like structure transforms spontaneously to white graphite when d ≤ 1.4 nm. In particular, the two-dimensional structure with single-layer boron nitride, the so-called white graphene, could be formed as a result of such transformation.
    Applied Physics Letters 01/2014; 104(9):093104-093104-4. · 3.52 Impact Factor
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    ABSTRACT: Two-dimensional graphenelike BC_{3} honeycomb structures, which have been grown on the NbB_{2}(0001) surface, possess intriguing properties that offer new opportunities for applications in nanoelectronics, mechanical materials, and superconductivity. The bonding configuration of BC_{3} on the substrate has not yet been determined, however. We report ab initio thermodynamics calculations and analysis that lead to a prediction about the most stable bonding configuration of a monolayer BC_{3} on NbB_{2}, namely the C-top configuration in which carbon is bonded directly on the top of the niobium atoms. We also find that the BC_{3} monolayer on NbB_{2} is thermodynamically stable, accounting for the experimental observation that the BC_{3} sheet can be grown on NbB_{2} by carbon substitution and surface segregation of carbon impurities. We further provide detailed information about the atomic structures and electronic properties of the BC_{3}-bonded NbB_{2} surface.
    Physical Review B 09/2013; 88(11). · 3.66 Impact Factor
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    ABSTRACT: Catalytic biomass conversion sometimes occurs at the liquid-solid interfaces. We report ab initio molecular dynamics simulations at finite temperatures for the catalytic reactions involving furfural at the water-Pd and water-Cu interfaces. We found that, during the dynamic process, the furan ring of furfural prefers to be parallel to the Pd surface and the aldehyde group tends to be away from the Pd surface. On the other hand, at the water-Cu(111) interface, furfural prefers to be tilted to the Cu surface while the aldehyde group is bonded to the surface. In both cases, interaction of liquid water and furfural is identified. The difference of dynamic process of furfural at the two interfaces suggests different catalytic reaction mechanisms for the conversion of furfural, consistent with the experimental investigations.
    03/2013;
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    ABSTRACT: Furfural conversion over metal catalysts plays an important role in the studies of biomass-derived feedstocks. We report ab initio molecular dynamics simulations for the decarboxylation process of furfural on the palladium surface at finite temperatures. We observed and analyzed the atomic-scale dynamics of furfural on the Pd(111) surface and the fluctuations of the bondlengths between the atoms in furfural. We found that the dominant bonding structure is the parallel structure in which the furfural plane, while slightly distorted, is parallel to the Pd surface. Analysis of the bondlength fluctuations indicates that the C-H bond is the aldehyde group of a furfural molecule is likely to be broken first, while the C=O bond has a tendency to be isolated as CO. Our results show that the reaction of decarbonylation dominates, consistent with the experimental measurements.
    03/2013;
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    ABSTRACT: We report extensive first-principles electronic structure modeling and calculations for the SiC–SiO 2 interface, a solid–solid interface formed during oxidation of silicon carbide (SiC). The interface modeling provides atomic-scale understanding about the nature of the interface defects as well as passivation effects due to the modification of the interface bonding. In particular, simulation results show that incorporation of hydrogen and fluorine decreases the defect density, thus enhancing the performance of SiC-based electronic devices.
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    ABSTRACT: Oxidation behavior of the two-phase Nb∕Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb∕Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.
    The Journal of Chemical Physics 01/2013; 138(1):014708. · 3.12 Impact Factor
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    ABSTRACT: Ab initio density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the α(2)-Ti(3)Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the α(2)-Ti(3)Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the α(2)-Ti(3)Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e. the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O-α(2)-Ti(3)Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized.
    Physical Chemistry Chemical Physics 07/2012; 14(31):11160-6. · 4.20 Impact Factor
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    ABSTRACT: First-principles density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the α2-Ti3Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the α2-Ti3Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the α2-Ti3Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e., the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O/α2-Ti3Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized.
    02/2012;
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    ABSTRACT: Biomass pyrolysis and other relevant catalytic reactions often occur at the liquid-solid interface. It is therefore of great importance to investigate the interfacial structure and other properties in order to achieve a deep understanding about the catalytic reactions for biomass conversion. We used ab initio molecular dynamics simulations to study the interfaces formed by liquid water and the palladium surfaces. Such interfaces are involved in many catalytic reactions for biomass conversion. We report results about the structural properties of the water/Pd(100) and water/Pd(111) interfaces, the interaction between liquid water and the metal surfaces, and how the interaction affects the structure. We found that while the interaction between water and the metal surface is weak, it could still cause considerable effects. In particular, the interaction promotes the formation of close-packed local clusters of liquid water.
    02/2012;
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    ABSTRACT: With extensive first-principles density-functional-theory calculations, we investigate the stability and the atomic and electronic structures of the CrB(2)(0001) and MoB(2)(0001) surfaces, each with two different terminations. It is found that the boron-terminated surface is energetically more favorable over the wide range of thermodynamically allowed chemical potentials than the metal-terminated surface for both CrB(2)(0001) and MoB(2)(0001), suggesting a stable layer of graphene-like boron on the surfaces. Our results also show the similarities and the differences in relaxation and in bonding characteristics between the boron-terminated and metal-terminated surfaces.
    Journal of Physics Condensed Matter 06/2011; 23(22):225501. · 2.22 Impact Factor
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    ABSTRACT: High concentrations of defects at the SiC–SiO2 interface significantly reduce the efficiency of the SiC-based microelectronics. Investigations of the defect passivation are thus of great importance. We report first-principles density-functional-theory calculations for the effects of fluorine and hydrogen in passivating the defects at the SiC–SiO2 interface. The calculations show that the isolated point defects involving excessive carbon atoms can be passivated by atomic fluorine and hydrogen, separately or combined. The results further suggest that molecular fluorine may be more effective for the passivation of the interface defects than molecular hydrogen and hydrogen fluoride.
    Applied Physics Letters 12/2010; 97(24):242111-242111-3. · 3.52 Impact Factor
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    ABSTRACT: The channel mobilities in SiC-based metal-oxide-semiconductor field-effect transistors are significantly reduced by the interface defects that produce states in the band gap of the SiC-SiO2 interface. Therefore, it is of great importance to investigate the nature of the interface defects and the ways for passivating such defects. We used first-principles quantum-mechanical calculations to study the interface defects due to excessive carbon atoms. We report the results about the atomic configurations of the defects and the associated electronic structures, as well as the effects of hydrogen and fluorine in passivating such interface defects.
    03/2010;
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    ABSTRACT: High state densities in the band gap of the SiC-SiO2 interface significantly reduce the channel mobilities in SiC-based high-temperature/high-power microelectronics. Investigations of the nature of the interface defects are thus of great importance. While several possible defects including very small carbon clusters with up to four carbon atoms have been identified by first-principles theory, larger carbon clusters as possible defects have attracted less attention. Here, we report first-principles quantum-mechanical calculations for two larger carbon clusters, the C10 ring and the C20 fullerence, in the SiC-SiO2 interface. We find that both carbon clusters introduce significant states in the band gap. The states extend over the entire band gap with higher densities in the upper half of the gap, thus accounting for some of the interface trap densities observed experimentally.
    Journal of Computational and Theoretical Nanoscience 05/2009; 6(6):1305-1310. · 1.03 Impact Factor
  • Hongli Dang, Y. G. Shen, Sanwu Wang
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    ABSTRACT: We report first-principles quantum-mechanical calculations that predict a novel phase transition of nano-sized boron nitride (BN) thin-films. When the thickness of the BN thin-film is below 1.4 nm, a spontaneous phase transition from the diamond-like structure to a graphite phase is predicted. The process would involve no energy barriers. When the thickness of BN increases, on the other hand, energy barriers for the phase transition would appear and gradually increase with the thickness. Calculations show that while the graphite structure has a lower total energy than the corresponding diamond-like structure for the BN thin-film with any thickness, the spontaneous phase transition would occur only when the size is small. We attribute this phenomenon to the quantum size effect.
    01/2009;
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    ABSTRACT: Nitrogen incorporation at the SiO <sub>2</sub>/ SiC interface via high temperature nitric oxide annealing leads to the passivation of electrically active interface defects, yielding improved inversion mobility in the semiconductor. However, we find that such nitrided oxides can possess a larger density of hole traps than as-grown oxides, which is detrimental to the reliability of devices (e.g., can lead to large threshold voltage instabilities and to accelerated failure). Three different charge injection techniques are used to characterize this phenomenon in metal–oxide–semiconductor structures: x-ray irradiation, internal photoemission and Fowler–Nordheim tunneling. Some nitrogen-based atomic configurations that could act as hole traps in nitrided SiO <sub>2</sub> are discussed based on first-principles density functional calculations.
    Journal of Applied Physics 07/2008; · 2.21 Impact Factor
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    ABSTRACT: Nanostructured^ superhard materials have been successfully synthesized in recent years. The hardness of nanosuperlattices^ and nanocomposites significantly exceeds that of the component materials. While it is believed that the nanodimensions are needed to impede dislocation activity and grain-boundary sliding, relevant calculations are rare. We report first-principles density-functional calculations for the core structures and energetics of various dislocations including the [110]110, [110]111, [110]100, [100]100 and [100]110 edge dislocations in bulk TiN. We found that the formation energies of the core dislocations were continuously increased when their sizes decreased. We also found that the most common types of dislocations in TiN are the [110]110 and [110]111 edge dislocations. The obtained results are helpful for elucidating the atomic-scale mechanism for the superhardness of nanocomposites.
    03/2008;
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    ABSTRACT: The authors report the first-principles density-functional calculations for the structure and the elastic properties of superlattices containing nanoscale crystalline titanium nitride (TiN) and thin layer of silicon nitride. The authors found that the elastic properties are strongly dependent on the size of the components. Superlattices with TiN thickness smaller than 2.5 nm have far smaller values of bulk and shear moduli than bulk crystalline TiN, while ∼ 3 nm TiN can make the superlattice have the elastic properties close to those of crystalline TiN.
    Applied Physics Letters 08/2007; 91(8):081916-081916-3. · 3.52 Impact Factor
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    ABSTRACT: We report scanning-tunneling-microscopy observations and first-principles calculations for the formation and evolution of self-organized Ge nanostructures on Si(111)-(7×7) surfaces for Ge coverages up to 0.5 ML. We show that individual Ge atoms initially form a triangular lattice. At higher coverages, Ge nanoparticles 1nm in diameter gradually form in both the faulted and unfaulted half unit cells with an initial preference in the faulted halves, ultimately driving ordered hexagonal arrays. The underlying 7×7 surface periodicity, the triangular single-Ge lattice, and the nanoparticle hexagonal superstructures coexist. Charge transfer from Si adatoms to Ge nanoparticles is shown to play a key role in the self-organization.
    Physical Review B 04/2007; · 3.66 Impact Factor

Publication Stats

234 Citations
101.19 Total Impact Points

Institutions

  • 2007–2014
    • University of Tulsa
      • Department of Physics and Engineering Physics
      Tulsa, Oklahoma, United States
  • 2011–2013
    • Tianjin Normal University
      T’ien-ching-shih, Tianjin Shi, China
  • 2004–2007
    • Vanderbilt University
      • Department of Physics and Astronomy
      Nashville, Michigan, United States
  • 2005
    • Northeast Institute of Geography and Agroecology
      • Institute of Physics
      Beijing, Beijing Shi, China