J. G. Wan

Nanjing University, Nan-ching, Jiangsu Sheng, China

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Publications (53)94.06 Total impact

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    ABSTRACT: The dielectric and ferroelectric behaviors of a ferroelectric are substantially determined by its domain structure and domain wall dynamics at mesoscopic level. A relationship between the domain walls and high frequency mesoscopic dielectric response is highly appreciated for high frequency applications of ferroelectrics. In this work we investigate the low electric field driven motion of 90°-domain walls and the frequency-domain spectrum of dielectric permittivity in normally strained ferroelectric lattice using the phase-field simulations. It is revealed that, the high-frequency dielectric permittivity is spatially inhomogeneous and reaches the highest value on the 90°-domain walls. A tensile strain favors the parallel domains but suppresses the kinetics of the 90° domain wall motion driven by electric field, while the compressive strain results in the opposite behaviors. The physics underlying the wall motions and thus the dielectric response is associated with the long-range elastic energy. The major contribution to the dielectric response is from the polarization fluctuations on the 90°-domain walls, which are more mobile than those inside the domains. The relevance of the simulated results wth recent experiments is discussed.
    Scientific reports. 01/2014; 4:5007.
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    ABSTRACT: The relationship between the bipolar resistive switching and the polarization reversal is investigated at various temperatures in the Au/BiFeO3/SrRuO3 structure. It is found that the polarization-induced barrier variation in the Au/BiFeO3 and BiFeO3/SrRuO3 junctions decreases with decreasing temperature. This explains why the resistance-switching ratio decreases with decreasing temperature below 323 K and gives evidence that the polarization modulates the resistance state of the Au/BiFeO3/SrRuO3 structure. Besides, the oxygen vacancies migration and/or the carrier trapping/detrapping mechanisms are also suggested to play a very important role in the resistive switching behavior in this structure as the temperature goes above 323 K.
    Applied Physics Letters 01/2014; 104(14):143503-143503-5. · 3.79 Impact Factor
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    ABSTRACT: The structure and properties of medium-sized (GaAs)n clusters (n = 18–36) were investigated using density functional theory with the generalized gradient approximation. In general, stuffed topologies are the lowest energy states. Tubular and other cage-like structures are less energetically preferable. Structural transition from cages to stuffed topologies occurred at an approximate size of n = 20. In addition, the binding energy per unit, the highest occupied molecular orbital and the lowest unoccupied molecular orbital gaps, the density of states, and infrared spectra were investigated.
    The Journal of Physical Chemistry C. 06/2013; 117(24):12835–12840.
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    ABSTRACT: The possibility of a new endohedral fullerene with a trapped aluminum carbide cluster, Al(4)C @C(80)-I ( h ), was theoretical investigated. The geometries and electronic properties of it were investigated using density functional theory methods. The Al(4)C unit formally transfers six electrons to the C(80) cage which induces stabilization of Al(4)C@C(80). A favorable binding energy, relatively large HOMO-LUMO gap, electron affinities and ionization potentials suggested the Al(4)C@C(80) is rather stable. The analysis of vertical ionization potential and vertical electron affinity indicate Al(4)C@C(80) is a good electron acceptor.
    Journal of Molecular Modeling 11/2012; · 1.98 Impact Factor
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    ABSTRACT: Employing the graphene sheets (GSs), the electron scattering constants are measured in the high-angle annular dark-field (HAADF) imaging by the scanning transmission electron microscopy. Single scattering is found to be dominant until the layer number of 200, complying with a simple relation of I = Io(1 - e(-tau/lambda)). The discrete layer counting of the GSs enables precise determination of incident depths. This work results values of lambda = 48.2, 61.4, 97.9 and 115.6 nm for 80, 120, 160 and 200 keV electrons, respectively. The uncertainties with the mean free paths and the cross sections are confined to 10 percent. The dependences on the electron beam energy and the collection angle are discussed based on a multislice simulation.
    Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6494-8. · 1.15 Impact Factor
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    ABSTRACT: Magnetoelectric (ME) laminate composite Terfenol-D/LiNbO3((zxtw)-129°/30°) was studied. It shows a much higher ME voltage coefficient (αE) compared with the laminate using normal cut type of LiNbO3 single crystal. The αE has a wide frequency peak (bandwidth of 40 kHz) and more than one optimized bias magnetic field under high frequencies. The mechanism was discussed in this article.
    Integrated Ferroelectrics 10/2011; 87(2007):50-56. · 0.38 Impact Factor
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    ABSTRACT: The structures of B n N20 − n (n = 6−18), the clusters of boron nitride, are investigated by the density functional theory calculations. The structures of the obtained low-lying isomers can be described by the following six prototypes: single ring, double ring, three-ring, graphitic-like sheet, fullerene and others. B10N10 is demonstrated to be the most stable cluster against the nonstoichiometric ones. Nonzero magnetic moments, 1.999, 1.998, 2.000, 3.999 and 1.999μ B respectively, are found in five B n N20−n (n = 6, 7, 11, 12, 13) clusters. Further analysis indicates that the magnetic moment of the B6N14 cluster is mainly originated from the N atoms, while those of others are from the B atoms. The magnetic moment are finally attributed to the interesting issues of the 2p electrons due to the breaking of local symmetries, the change of coordination number, charge distribution and orbital hybridization.
    The European Physical Journal D 01/2011; 63(2):201-207. · 1.51 Impact Factor
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    ABSTRACT: The interaction of O(2) with the doped icosahedral X@Al(12) (X = Al(-), P(+), C, Si) clusters with 40 valence electrons were investigated using density functional theory methods. A different behavior exhibited between Al(13)(-) and X@Al(12) (X = P(+), C, Si) when they interact with O(2). The dissociation of O(2) on Al(13)(-) is strongly dependent on spin state of oxygen molecule. But X@Al(12) (X = P(+), C, and Si) is not the case. The transform of spin moment from O(2) to Al(13)(-) is much faster. Small molecularly binding energy and relatively high energy barrier show that these clusters are all reluctant reacts with the ground state O(2).
    Journal of Computational Chemistry 11/2010; 31(15):2804-9. · 3.84 Impact Factor
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    ABSTRACT: We investigate the effect of Al-doping on the multiferroicity of DyMn1−xAlxO3. It is indicated that a slight doping (0.005 ≤ x ≤ 0.01) can significantly enhance the electric polarization which will be suppressed as x>0.03. The possible mechanism for the polarization enhancement is the rearrangement of the Mn spiral spin order upon the Al-doping. Our work demonstrates that the ferroelectricity of DyMnO3 is sensitive to the B-site nonmagnetic doping, allowing an unusual enhancement of the electric polarization.
    Applied Physics Letters 06/2010; 96(25):252902-252902-3. · 3.79 Impact Factor
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    ABSTRACT: A series of polycrystalline pyrochlore rare-earth titanate Ho2−xCrxTi2O7 are synthesized in order to enhance the ferroelectricity of pyrochlore Ho2Ti2O7. A giant enhancement of polarization P from 0.54 μC/m2 at x = 0 up to ∼ 660 μC/m2 at x = 0.4 is obtained, accompanied with an increment of ferroelectric transition point Tc from Tc ∼ 60 K up to Tc ∼ 140 K. A magnetic anomaly at Tc ∼ 140 K together with the polarization response to magnetic field is identified, implying the multiferroic effect in Ho2−xCrxTi2O7.
    Applied Physics Letters 06/2010; 96(24):242904-242904-3. · 3.79 Impact Factor
  • Q L Lu, J G Wan
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    ABSTRACT: Hydrogen molecules adsorption and storage in Sc coated Si@Al(12) cluster were investigated using density functional theory methods. Scandium atoms can bind strongly to the surfaces of Si@Al(12) due to the charge transfer between Sc and Si@Al(12), and do not suffer from clustering on the substrate. Si@Al(12) cluster coated with three and four Sc atoms can adsorb 16 and 18 H(2) molecules with a binding energy of 0.28-0.63 eV/H(2), corresponding to hydrogen storage capacity of 6.0 and 6.3 wt %, respectively. The stable Si@Al(12) can be applied as one of candidates for hydrogen storage materials at ambient conditions.
    The Journal of Chemical Physics 06/2010; 132(22):224308. · 3.12 Impact Factor
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    ABSTRACT: A ferromagnetic shape memory alloy was utilized to construct a Ni <sub>43</sub> Mn <sub>41</sub> Co <sub>5</sub> Sn <sub>11</sub>/ Pb ( Zr , Ti ) O <sub>3</sub> laminate. Large direct and converse magnetoelectric (ME) effects were observed at resonance frequencies around the martensitic transition temperature of Ni <sub>43</sub> Mn <sub>41</sub> Co <sub>5</sub> Sn <sub>11</sub> alloy, and the temperature affects the ME properties remarkably. The ME coefficients respond almost linearly to the applied ac electric or magnetic field. The mechanism of both direct and converse ME effects in the laminate is discussed.
    Journal of Applied Physics 06/2010; · 2.21 Impact Factor
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    ABSTRACT: Free-standing graphene sheets (GS) with several to over a hundred layers are prepared by splitting the expandable graphite. The raw material is firstly transferred to a heating chamber for the thermal flashing at 1500 οC in a hydrogen chamber. The expanded powder (1 mg) is then dispersed in 10 ml of the PmPV/DCE suspension for high-power sonication and further centrifugation. After drop-cast onto a holey Formvar film, the GSs are suspended on the grid. The electron scattering is carried out in a Tecnai F20 TEM/STEM with a field emission gun. A high-angle annular dark field (HAADF) detector is fitted to collect the scattered electrons with a tunable collecting angle. The quantitative STEM image simulation is carried out using amultislice algorithm [4].
    01/2010;
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    ABSTRACT: Plasmon coupling of dimers of Ag nanoparticles is studied by the EELS as shown in Fig 1a [1]. The spectroscopic images in the above Fig. 1a are extracted from 2.8eV, 3.4eV and 3.6eV respectively, where one can see the preferable exciting points of the features around the Ag nanoparticles. With the aid of DDA calculation(Fig. 1b), the three features are identified as the in-phase plasmon coupling, non-coupling and anti-phase coupling modes. The in-phase mode is able to conduct the light transportation due to its dipolar nature, which redshifts from the 3.4eV at large spacings to 2.7eV near touching (Fig. 1c). Such behaviour is also observed in the chain of 3 and 4 nanoparticles. The experiment provides clear evidence on the plasmon coupling of Ag nanoparticles. Extracting the dopant states in the Co-doped ZnO nanoparticles [2]. In order to assign the dopant-introduced states in the low-loss EELS, we employ both the core-loss EELS and the valence EELS on the same Co-doped ZnO nanoparticles by spatially mapping the dopant concentration distribution via the core-loss EELS (Fig 2a 2b 2c) and simultaneously by identifying the dopant features in the accumulated valence EELS via the measured dopant richness (Fig 2d). Three Coincidentally-growing Co dopant states are successfully determined in Fig. 2e. The experimental inputs feed the first-principles calculation, which generates full electronic structure of the doped ZnO nanoparticles. The experiment supports the carrier mediated room-temperature magnetism, which is a main confusion with current dilute magnetic semiconductors.
    01/2010;
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    ABSTRACT: We studied the structure and electronic properties of Al7OH clusters using density functional theory with generalized gradient approximation. OH prefers to the ontop site of Al atoms, and the hollow form is not stable. The Al7 moieties can keep their structures in ontop form Al7OH cluster, while it dramatically changes for bridge form ones. The binding strength between Al7 and OH are much larger than that of Al7I and Al7H. The bonding characteristic of Al7-OH is mainly ionic according to the population analysis. Large binding energy, HOMO-LUMO gap, and high ionization potential imply that Al7OH cluster is physically high and chemically stable.
    International Journal of Modern Physics B 01/2010; 24(10):1253-1259. · 0.46 Impact Factor
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    ABSTRACT: The lowest-energy geometries and electronic-structure properties have been obtained for AlnHn (n=1-10) clusters within the density-functional theory using the generalized gradient approximation for the exchange correlation potential. The resulting geometries show that the hydrogen atoms tend to occupy outside positions and no hollow positions are found. The subunit Aln of AlnHn (n=1-5) have little distortion, in comparison with corresponding pure Aln cluster, whereas the subunit Aln have large distortion from n=6. The stability has been investigated by analyzing the binding energy per atom and the second difference in energy, indicating that Al8H8 exhibit higher stability than others. The bonding property has been analyzed by calculating the Mulliken charges and Al–H distances. The calculated energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO), the vertical ionization potential, and the vertical electron affinity also confirm that Al8H8 is a stable cluster. The density of states (DOS) shows that AlnHn exhibit changes from molecular-like (Al1H1) to band-like structure (Al10H10) as n increases.
    The European Physical Journal D 01/2010; 57(2):197-205. · 1.51 Impact Factor
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    ABSTRACT: Upon the development of the quantitative scanning transmission electron microscopy (QSTEM), individually counting the gold atom has been achieved. The correlation of the signals to the atom quantity (N), i.e. calibration, is its crucial work. One has to seek the ultra-fine mass standards, where the size-selected gold clusters rank the best recently. However, practically this is difficult for light elements, since STEM intensity is proportional to Zα, where Z is atomic number of the elements and α is in the range of 1.5-1.9 depending on the detector collection angle, sample thickness, and the Debye-Waller factor of the atomic species. As a result, the light elements would have a much weaker image contrast than that of heavy elements. This could be even more difficult for the carbon particles supported by the Formvar films. Graphene layers are featured by large scale uniformity and discrete layer steps, which shed light on precision calibration of the carbon atomic scales. To establish direct correlation between the STEM intensity with the number of layers in graphene sheets, we apply an independent layer counting method by utilizing dark lines in bright field TEM images at the edge of graphene sheets [2]. We search over a large area of the sample for the freely suspending graphene sheets (for example, the area indicated by the arrows in Fig. 1a)[3]. Fig. 1b displays a typical example of such an edge, where the layer number can be counted in the high-resolution bright field image and the STEM intensity is also obtained. The calibration in the few-layer range gives Fig. 1c, where a linear relationship is apparent. The calibration performed over an extended thickness presents a precise exponential growth. The behaviour can be described by the incoherent scattering picture of single scattering. This leads to accurate measurement of the electron mean free path (The error is reduced to 5 percent). The calibration relation works from individual up- - to millions of carbon atoms.
    Vacuum Electron Sources Conference and Nanocarbon (IVESC), 2010 8th International; 01/2010
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    ABSTRACT: Rare-earth titanate cubic pyrochlore Ho <sub>2</sub> Ti <sub>2</sub> O <sub>7</sub> ceramics are prepared by the standard solid state reaction. The magnetic, dielectric, and ferroelectric behaviors are measured in order to investigate the multiferroic behaviors of Ho <sub>2</sub> Ti <sub>2</sub> O <sub>7</sub> . Upon the gradual spin ordering sequence with decreasing temperature, a ferroelectric transition with a small polarization is identified. We discuss the origin of ferroelectricity and its possible correlation with the spin order at low temperature.
    Journal of Applied Physics 12/2009; · 2.21 Impact Factor
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    ABSTRACT: Bilayer PVDF/Metglas laminates with remarkably anisotropic ME coupling effect have been fabricated. By a careful evaluation of these anisotropic behaviors both experimentally and theoretically, we demonstrate that an ultra-sensitive detection of weak dc magnetic field and its spatial orientation is practically applicable using the bilayer PVDF/Metglas laminate. This magnetic field probe has quite high sensitivity. The reason for such a high magnitude and orientation-dependent magnetoelectric coupling is explained.
    Sensors and Actuators A Physical 01/2009; 153(1):64-68. · 1.84 Impact Factor
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    ABSTRACT: The effect of the orientation of 0.52 Pb(Fe1/2Nb1/2)O3-0.48 PbTiO3 single crystal (PFN-PT) on the magnetoelectric (ME) properties in tri-layer laminate composites consisting of TbxDy1-xFe2 alloy and PFN-PT has been studied. The different alphaE as a function of applied magnetic field was observed in the laminate composites with different PFN-PT orientations. The origin of alphaE dependent on the orientation of PFN-PT is discussed based on an analysis of crystal physics. It was found that the shear strains also play an important role in the ME effect besides the normal strains.
    Journal of Physics D Applied Physics 01/2009; 42(1). · 2.53 Impact Factor

Publication Stats

455 Citations
94.06 Total Impact Points

Institutions

  • 2003–2014
    • Nanjing University
      • Department of Physics
      Nan-ching, Jiangsu Sheng, China
  • 2008–2012
    • Anhui University
      Luchow, Anhui Sheng, China
  • 2004
    • Tsinghua University
      • School of Materials Science and Engineering
      Beijing, Beijing Shi, China