C. Z. Wang

Iowa State University, Ames, Iowa, United States

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Publications (248)699.69 Total impact

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    ABSTRACT: We present an efficient genetic algorithm, integrated with experimental diffraction data, to solve a nanoscale metastable Al 20Sm4 phase that evolves during crystallization of an amorphous magnetron sputtered Al 90Sm10 alloy. The excellent match between calculated and experimental X-ray diffraction patterns confirms an accurate description of this metastable phase. Molecular dynamic simulations of crystal growth from the liquid phase predict the formation of disordered defects in the devitrified crystal.
    Applied Physics Letters 03/2015; 106(10):101903. DOI:10.1063/1.4914399 · 3.52 Impact Factor
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    ABSTRACT: The short-range order (SRO) in Pd78Cu6Si16 liquid was studied by high energy x-ray diffraction and ab initio molecular dynamics (MD) simulations. The calculated pair correlation functions at different temperatures agree well with the experimental results. The partial pair correlation functions from ab intio MD simulations indicate that Si atoms prefer to be uniformly distributed while Cu atoms tend to aggregate. By performing structure analysis using Honeycutt-Andersen index, Voronoi tessellation, and atomic cluster alignment method, we show that the icosahedron and face-centered cubic SRO increase upon cooling. The dominant SRO is the Pd-centered Pd9Si2 motif, namely the structure of which motif is similar to the structure of Pd-centered clusters in the Pd9Si2 crystal. The study further confirms the existence of trigonal prism capped with three half-octahedra that is reported as a structural unit in Pd-based amorphous alloys. The majority of Cu-centered clusters are icosahedra, suggesting that the presence of Cu is benefit to promote the glass forming ability.
    Scientific Reports 02/2015; 5:8277. DOI:10.1038/srep08277 · 5.08 Impact Factor
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    ABSTRACT: Nanostructure arrays such as nanowire, nanopillar, and nanocone arrays have been proposed to be promising antireflection structures for photovoltaic applications due to their great light trapping ability. In this paper, the optical properties of Si nanopillar and nanocone arrays in visible and infrared region were studied by both theoretical calculations and experiments. The results show that the Mie resonance can be continuously tuned across a wide range of wavelength by varying the diameter of the nanopillars. However, Si nanopillar array with uniform diameter exhibits only discrete resonance mode, thus can't achieve a high broadband absorption. On the other hand, the Mie resonance wavelength in a Si nanocone array can vary continuously as the diameters of the cross sections increase from the apex to the base. Therefore Si nanocone arrays can strongly interact with the incident light in the broadband spectrum and the absorbance by Si nanocone arrays is higher than 95% over the wavelength from 300 to 2000 nm. In addition to the Mie resonance, the broadband optical absorption of Si nanocone arrays is also affected by Wood-Rayleigh anomaly effect and metal impurities introduced in the fabrication process.
    Scientific Reports 01/2015; 5:7810. DOI:10.1038/srep07810 · 5.08 Impact Factor
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    ABSTRACT: Classically, the onset of nucleation is defined in terms of a critical cluster of the condensed phase, which forms from the gradual aggregation of randomly diffusing adatoms. Experiments in Pb/Si(111) at low temperature have discovered a dramatically different type of nucleation, with perfect crystalline islands emerging "explosively" out of the compressed wetting layer after a critical coverage Θ_{c}=1.22 ML is reached. The unexpectedly high island growth rates, the directional correlations in the growth of neighboring islands and the persistence in time of where mass is added in individual islands, suggest that nucleation is a result of the highly coherent motion of the wetting layer, over mesoscopic distances.
    Physical Review Letters 12/2014; 113(23):236101. DOI:10.1103/PhysRevLett.113.236101 · 7.73 Impact Factor
  • Acta Materialia 11/2014; 80:498-504. DOI:10.1016/j.actamat.2014.06.049 · 3.94 Impact Factor
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    ABSTRACT: The structure of the orthorhombic Zr2Co11 phase was revisited in the present work. Selected-area electron diffraction (SAED) and high-resolution electron microscopy (HREM) techniques were used to investigate the structure. They show the orthorhombic Zr2Co11 phase has a 1-D incommensurate modulated structure. The structure can be approximately described as a B-centered orthorhombic lattice. The lattice parameters of the orthorhombic Zr2Co11 phase have been determined by a tilt series of SAED patterns. A hexagonal network with a modulation wave has been observed in the HREM image and the hexagonal motif is considered as the basic structural unit.
    Journal of Alloys and Compounds 10/2014; 611:167–170. DOI:10.1016/j.jallcom.2014.05.072 · 2.73 Impact Factor
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    ABSTRACT: The role of silicon oxide shell in oxide-assisted SiNWs growth is studied by performing ab initio molecular dynamics simulations on the structural and dynamical properties of the interface between crystalline Si(111) surface and disorder SiO thin film. Si atoms in the SiO film tends to aggregate into the vicinity of the Si(111)/SiO interface. In addition, the diffusion of Si atoms at the interface is anisotropic - the diffusion along the interface is several times faster than that perpendicular to the interface. The segregation and anisotropic diffusion of Si atoms at the Si(111)/SiO interface shed interesting light into the mechanism of oxide-assisted silicon nanowire growth.
    Materials Chemistry and Physics 10/2014; 148(3):1145-1148. DOI:10.1016/j.matchemphys.2014.09.036 · 2.13 Impact Factor
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    ABSTRACT: We present an efficient \textit{ab initio} method for calculating the electronic structure and total energy of strongly correlated electron systems. The method extends the traditional Gutzwiller approximation for one-particle operators to the evaluation of the expectation values of two particle operators in a full many-electron Hamiltonian. The method is free of adjustable Coulomb parameters, and has no double counting issues in the calculation of total energy, and has the correct atomic limit. We demonstrate that the method describes well the bonding and dissociation behaviors of the hydrogen and nitrogen clusters. We also show that the method can satisfactorily tackle great challenging problems faced by the density functional theory recently discussed in the literature. The computational workload of our method is similar to the Hartree-Fock approach while the results are comparable to high-level quantum chemistry calculations.
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    ABSTRACT: The local atomic structure evolution in Al2Au alloy during solidification from 2000 K to 400 K was studied by ab initio molecular dynamics simulations and analyzed using the structure factor, pair correlation functions, bond angle distributions, the Honeycutt–Anderson (HA) index and Voronoi tessellation methods. It was found that the icosahedral-like clusters are negligible in the Al2Au stable liquid and supercooled liquid states, and the most abundant clusters are those having HA indices of 131 and 120 or Voronoi indices of 〈0, 4, 4, 0〉, 〈0, 3, 6, 0〉 and 〈0, 4, 4, 2〉 with coordination numbers of 8, 9 and 10, respectively. These clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, revealing the existence of structure heredity between liquid and crystalline phase in Al2Au alloy.
    Acta Materialia 04/2014; 68(15):1–8. DOI:10.1016/j.actamat.2014.01.004 · 3.94 Impact Factor
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    ABSTRACT: The temperature dependent optical properties of tin film from solid to liquid were studied by spectroscopic ellipsometry and ab initio molecular dynamics simulations. The dielectric function of liquid Sn was different from solid, and an interband transition near 1.5 eV was easily observed in solid while it apparently disappeared upon melting. From the evolution of optical properties with temperature, an optical measurement to acquire the melting point by ellipsometry was presented. From first principles calculation, we show that the local structure difference in solid and liquid is responsible for this difference in the optical properties observed in experiment.
    Applied Physics Letters 03/2014; 104(12):121907-121907-5. DOI:10.1063/1.4869722 · 3.52 Impact Factor
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    ABSTRACT: Solving the crystal structures of novel phases with nanoscale dimensions resulting from rapid quenching is difficult due to disorder and competing polymorphic phases. Advances in computer speed and algorithm sophistication have now made it feasible to predict the crystal structure of an unknown phase without any assumptions on the Bravais lattice type, atom basis, or unit cell dimensions, providing a novel approach to aid experiments in exploring complex materials with nanoscale grains. This approach is demonstrated by solving a long-standing puzzle in the complex crystal structures of the orthorhombic, rhombohedral, and hexagonal polymorphs close to the Zr2Co11 intermetallic compound. From our calculations, we identified the hard magnetic phase and the origin of high coercivity in this compound, thus guiding further development of these materials for use as high performance permanent magnets without rare-earth elements.
    Physical Review Letters 01/2014; 112(4):045502. DOI:10.1103/PhysRevLett.112.045502 · 7.73 Impact Factor
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    ABSTRACT: Reverse Monte Carlo simulations (RMC) have been widely used to generate three dimensional (3D) atomistic models for glass systems. To examine the reliability of the method for metallic glass, we use RMC to predict the atomic configurations of a "known" structure from molecular dynamics (MD) simulations, and then compare the structure obtained from the RMC with the target structure from MD. We show that when the structure factors and partial pair correlation functions from the MD simulations are used as inputs for RMC simulations, the 3D atomistic structure of the glass obtained from the RMC gives the short- and medium-range order in good agreement with those from the target structure by the MD simulation. These results suggest that 3D atomistic structure model of the metallic glass alloys can be reasonably well reproduced by RMC method with a proper choice of input constraints.
    Journal of Applied Physics 01/2014; 115(5). DOI:10.1063/1.4865164 · 2.19 Impact Factor
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    ABSTRACT: The structural and dynamical properties of rapidly quenched Al are studied by molecular dynamics simulations. The pair-correlation function of high temperature liquid Al agrees well with the experimental results. Different cooling rates are applied with high cooling rates leading to glass formation, while low cooling rates leading to crystallization. The local structures are characterized by Honeycutt Andersen indices and Voronoi tessellation analysis. The results show that for high cooling rates, the local structures of the liquid and glassy Al are predominated by icosahedral clusters, together with considerable amount of face-centered cubic and hexagonal close packed short-range orders. These short-range order results are further confirmed using the recently developed atomic cluster alignment method. Moreover, the atomic cluster alignment clearly shows the crystal nucleation process in supercooled liquid of Al. Finally, the mean square displacement for the liquid is also analyzed, and the corresponding diffusion coefficient as a function of temperature is calculated.
    Journal of Non-Crystalline Solids 01/2014; DOI:10.1016/j.jnoncrysol.2013.05.004 · 1.72 Impact Factor
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    ABSTRACT: We present a genetic algorithm (GA) for structural search that combines the speed of structure exploration by classical potentials with the accuracy of density functional theory (DFT) calculations in an adaptive and iterative way. This strategy increases the efficiency of the DFT-based GA by several orders of magnitude. This gain allows a considerable increase in the size and complexity of systems that can be studied by first principles. The performance of the method is illustrated by successful structure identifications of complex binary and ternary intermetallic compounds with 36 and 54 atoms per cell, respectively. The discovery of a multi-TPa Mg-silicate phase with unit cell containing up to 56 atoms is also reported. Such a phase is likely to be an essential component of terrestrial exoplanetary mantles.
    Journal of Physics Condensed Matter 12/2013; 26(3):035402. DOI:10.1088/0953-8984/26/3/035402 · 2.22 Impact Factor
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    ABSTRACT: Atomic structure transitions of liquid niobium during solidification, at different temperatures from 3200 to 1500 K, were studied by using ab initio molecular dynamics simulations. The local atomic structure variations with temperature are investigated by using the pair-correlation function, the structure factor, the bond-angle distribution function, the Honeycutt-Anderson index, Voronoi tessellation and the cluster alignment methods. Our results clearly show that, upon quenching, the icosahedral short-range order dominates in the stable liquid and supercooled liquid states before the system transforms to crystalline body-center cubic phase at a temperature of about 1830 K.
    Journal of Physics Condensed Matter 12/2013; 26(5):055004. DOI:10.1088/0953-8984/26/5/055004 · 2.22 Impact Factor
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    ABSTRACT: The effects of strontium modification on the structure and dynamics of Al88Si12 liquid are studied by means of ab initio molecular dynamics simulations. By replacing 0.5% and 4.0% of Al with Sr, we show that the addition of Sr lowers the self-diffusion of Al and Si of the liquid and reduces the nearest-neighbor correlation between Si atoms. The simulation provides an explanation for the change in morphology of the eutectic phases observed in rapidly solidified Al-Si alloys modified with Sr.
    Journal of Physics Condensed Matter 05/2013; 25(24):245102. DOI:10.1088/0953-8984/25/24/245102 · 2.22 Impact Factor
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    ABSTRACT: Triangular graphene nanoflakes (GNFs) embedded in fluorinated graphene are systematically investigated using first-principles density functional theory (DFT) calculations. The electronic and magnetic properties of such nanoflakes depend sensitively on the topology (armchair or zigzag) and the length of the edge of nanofakes. All embedded armchair triangular GNFs are semiconducting with size-dependent band gaps. The embedded zigzag triangular GNFs are magnetic with a ferrimagnetic ground state due to topological frustration of the π bonds. The magnetic moment scales linear with the size of the triangular GNF.
    Chemical Physics Letters 05/2013; 572:48–52. DOI:10.1016/j.cplett.2013.03.084 · 1.99 Impact Factor
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    ABSTRACT: Electronic structures of (4, n) and (m, 4) (the NW has m layers parallel to the {1 1 1} facet and n layers parallel to {1 1 0}) Si/Ge core/shell nanowires (NWs) along the [1 1 2] direction with cross-sectional aspect ratio (m/n) from 0.36 to 2.25 are studied by first-principles calculations. An indirect to direct band gap transition is observed as m/n decreases, and the critical values of m/n and diameter for the transition are also estimated. The size of the band gap also depends on the aspect ratio. These results suggest that m/n plays an important role in modulating the electronic properties of the NWs.
    Journal of Physics D Applied Physics 04/2013; 46(13):135302. DOI:10.1088/0022-3727/46/13/135302 · 2.52 Impact Factor
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    ABSTRACT: Structural and electronic properties, including deformation, magnetic moment, Mulliken population, bond order, as well as electronic transport properties, of zigzag graphene nanoribbon (ZGNR) with Co adatoms on hollow sites are investigated by quasi-atomic minimal basis orbits (QUAMBOs), a first-principles tight binding (TB) scheme based on density functional theory (DFT), combined with a non-equilibrium Green's function. For electronic transport, below the Fermi level the transmission is strongly suppressed and spin dependent as a result of magnetism by Co adatom adsorption, while above the Fermi level the transmission is slightly distorted and spin independent. Due to the local environment dependence of QUAMBOs-TB parameters, we construct QUAMBOs-TB parameters of ZGNR leads and ZGNR with Co adatoms on hollow center sites by a divide-and-conquer approach, and accurately reproduce the electronic transmission behavior. Our QUAMBO-NEGF method is a new and promising way of examining electronic transport in large-scale systems.
    Journal of Physics Condensed Matter 03/2013; 25(10):105302. DOI:10.1088/0953-8984/25/10/105302 · 2.22 Impact Factor
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    ABSTRACT: The recently introduced correlation matrix renormalization approximation (CMRA) was further developed by adopting a completely factorizable form for the renormalization z-factors, which assumes the validity of the Wick's theorem with respect to Gutzwiller wave function. This approximation (CMR-II) shows better dissociation behavior than the original one (CMR-I) based on the straightforward generalization of the Gutzwiller approximation to two-body interactions. We further improved the performance of CMRA by redefining the z-factors as a function of f(z) in CMR-II, which we call CMR-III. We obtained an analytical expression of f(z) by enforcing the equality in energy functional between CMR-III and full configuration interaction for the benchmark minimal basis H2. We show that CMR-III yields quite good binding energies and dissociation behaviors for various hydrogen clusters with converged basis set. Finally, we apply CMR-III to hydrogen crystal phases and compare the results with quantum Monte Carlo.
    Physical Review B 03/2013; 89(4):24003-. DOI:10.1103/PhysRevB.89.045131 · 3.66 Impact Factor

Publication Stats

5k Citations
699.69 Total Impact Points

Institutions

  • 1991–2015
    • Iowa State University
      • • Ames Laboratory
      • • Department of Physics and Astronomy
      • • Department of Chemistry
      Ames, Iowa, United States
  • 2014
    • Zhejiang University
      • State Key Lab of Silicon Materials
      Hang-hsien, Zhejiang Sheng, China
  • 2013
    • Fudan University
      • Department of Optical Science and Engineering
      Shanghai, Shanghai Shi, China
  • 2011
    • University of Science and Technology of China
      • Department of Physics
      Luchow, Anhui Sheng, China
  • 2010–2011
    • U.S. Department of Energy
      • Ames Lab
      Washington, Washington, D.C., United States
    • Qingdao University
      • College of Physics
      Tsingtao, Shandong Sheng, China
  • 2007–2008
    • Jilin University
      • State Key Lab of Theoretical and Computational Chemistry
      Jilin, Jilin Sheng, China
  • 2006
    • Northeast Normal University
      Hsin-ching, Jilin Sheng, China