Ju Li

Xi'an Jiaotong University, Ch’ang-an, Shaanxi, China

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Publications (201)1079.19 Total impact

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    ABSTRACT: Stress-driven grain boundary (GB) migration has been evident as a dominant mechanism accounting for plastic deformation in crystalline solids. Using molecular dynamics (MD) simulations on a Ti bicrystal model, we show that a uniaxial stress-driven coupling is associated with the recently observed 90° GB reorientation in shock simulations and nanopillar compression measurements. This is not consistent with the theory of shear-induced coupled GB migration. In situ atomic configuration analysis reveals that this GB motion is accompanied by the glide of two sets of parallel dislocation arrays, and the uniaxial stress-driven coupling is explained through a composite action of symmetrically distributed dislocations and deformation twins. In addition, the coupling factor is calculated from MD simulations over a wide range of temperatures. We find that the coupled motion can be thermally damped (i.e., not thermally activated), probably due to the absence of the collective action of interface dislocations. This uniaxial coupled mechanism is believed to apply to other hexagonal close-packed metals.
    Acta Materialia. 01/2015; 82:295–303.
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    ABSTRACT: We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in vanadium (V)-doped topological insulator Sb$_2$Te$_3$ has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L$_3$ and L$_2$) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T=10K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from onset of FM order. Crystal field analysis indicates that in V-doped Sb$_2$Te$_3$, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.
    12/2014;
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    ABSTRACT: In this paper we discuss the transformation of a sheet of material into a wide range of desired shapes and patterns by introducing a set of simple cuts in a multilevel hierarchy with different motifs. Each choice of hierarchical cut motif and cut level allows the material to expand into a unique structure with a unique set of properties. We can reverse-engineer the desired expanded geometries to find the requisite cut pattern to produce it without changing the physical properties of the initial material. The concept was experimentally realized and applied to create an electrode that expands to >800% the original area with only very minor stretching of the underlying material. The generality of our approach greatly expands the design space for materials so that they can be tuned for diverse applications.
    Proceedings of the National Academy of Sciences of the United States of America. 11/2014;
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    ABSTRACT: We develop a new envelope-function formalism to describe electrons in slowly-varying inhomogeneously strained semiconductor crystals. A coordinate transformation is used to map a deformed crystal back to geometrically undeformed structure with deformed crystal potential. The single-particle Schr\"{o}dinger equation is solved in the undeformed coordinates using envelope function expansion, wherein electronic wavefunctions are written in terms of strain-parametrized Bloch functions modulated by slowly varying envelope functions. Adopting local approximation of electronic structure, the unknown crystal potential in Schr\"{o}dinger equation can be replaced by the strain-parametrized Bloch functions and the associated strain-parametrized energy eigenvalues, which can be constructed from unit-cell level ab initio or semi-empirical calculations of homogeneously deformed crystals at a chosen crystal momentum. The Schr\"{o}dinger equation is then transformed into a coupled differential equation for the envelope functions and solved as a generalized matrix eigenvector problem. As the envelope functions are slowly varying, coarse spatial or Fourier grid can be used to represent the envelope functions, enabling the method to treat relatively large systems. We demonstrate the effectiveness of this method using a one-dimensional model, where we show that the method can achieve high accuracy in the calculation of energy eigenstates with relatively low cost compared to direct diagonalization of Hamiltonian. We further derive envelope function equations that allow the method to be used empirically, in which case certain parameters in the envelope function equations will be fitted to experimental data.
    Journal of physics. Condensed matter : an Institute of Physics journal. 10/2014; 26(45).
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    ABSTRACT: In nanotechnology, small-volume metals with large surface area are used as electrodes, catalysts, interconnects and antennae. Their shape stability at room temperature has, however, been questioned. Using in situ high-resolution transmission electron microscopy, we find that Ag nanoparticles can be deformed like a liquid droplet but remain highly crystalline in the interior, with no sign of dislocation activity during deformation. Surface-diffusion-mediated pseudoelastic deformation is evident at room temperature, which can be driven by either an external force or capillary-energy minimization. Atomistic simulations confirm that such highly unusual Coble pseudoelasticity can indeed happen for sub-10-nm Ag particles at room temperature and at timescales from seconds to months.
    Nature materials. 10/2014;
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    ABSTRACT: Ammonia (NH3) nitridation on an Fe surface was studied by combining density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. A DFT calculation was performed to obtain the energy barriers (Eb) of the relevant elementary processes. The full mechanism of the exact reaction path was divided into five steps (adsorption, dissociation, surface migration, penetration, and diffusion) on an Fe (100) surface pre-covered with nitrogen. The energy barrier (Eb) depended on the N surface coverage. The DFT results were subsequently employed as a database for the kMC simulations. We then evaluated the NH3 nitridation rate on the N pre-covered Fe surface. To determine the conditions necessary for a rapid NH3 nitridation rate, the eight reaction events were considered in the kMC simulations: adsorption, desorption, dissociation, reverse dissociation, surface migration, penetration, reverse penetration, and diffusion. This study provides a real-time-scale simulation of NH3 nitridation influenced by nitrogen surface coverage that allowed us to theoretically determine a nitrogen coverage (0.56 ML) suitable for rapid NH3 nitridation. In this way, we were able to reveal the coverage dependence of the nitridation reaction using the combined DFT and kMC simulations.
    The Journal of chemical physics. 10/2014; 141(13):134108.
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    ABSTRACT: Although lithium-sulfur batteries exhibit a high initial capacity, production cost and lack of cyclability are major limitations. Here we report a liquid-based, low-cost and reliable synthesis method of lithium-sulfur composite cathode with improved cyclability. An open network of Conductive Carbon Black nanoparticles (Cnet) is infused with sulfur (Snet) to form sponge-like networks (Cnet + Snet). Initially, Snet is open to the outside, allowing liquid electrolyte to infiltrate and impart Snet Li+ conductivity. During lithiation, Cnet could accommodate the volume expansion of Snet without largely losing electrical contact. During delithiation, the carbon nanoparticles would preferably flocculate on outer surface due to polysulfide dissolution an depletion of sulfur, to form a passivation layer that still allows Li+ exchange, but preventing more polysulfides from getting out, thus slowing the leaching of polysulfides into bulk electrolyte liquid. The plausibility of carbonaceous passivation layer was checked using an extra carbon deposition layer to achieve an improved performance of ~400 mAh/g after 250 cycles under a high rate 2.0 C. A 763 mAh/g discharge specific capacity of this sulfur nanosponge cathode (abbreviated as “SULFUN”) was obtained after 100 cycles under a rate of 0.2 C. 520 mAh/g and 290 mAh/g discharge capacities were attained after 300 and 500 cycles, respectively, making this cathode material attractive for powering portable electronics.
    J. Mater. Chem. A. 10/2014;
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    ABSTRACT: A spatially varying bandgap drives exciton motion, and can be used to funnel energy within a solid. This bandgap modulation can be created by composition variation (traditional heterojunction), elastic strain, or in the work shown next, by a small twist between two identical semiconducting atomic sheets, creating an internal stacking translation u(r) which varies gently with position r that controls the local bandgap Eg(u(r)). Recently synthesized carbon/boron nitride (CBN) (Nat. Nanotech. 8, 119, 2013) and phosphorene (Nat Nanotech. 9, 372, 2014) may be used to construct this twisted semiconductor bilayer that may be regarded as an in-plane crystal but an out-of-plane molecule, which could be useful in solar energy harvesting and electroluminescence. Here by first-principles methods we compute the bandgap map and delineate its material and geometric sensitivities. Eg(u(r)) is predicted to have multiple local minima ("funnel centers") due to secondary or even tertiary periodic structures in-plane, which leads to a hitherto unreported pattern of multiple "exciton flow basins". A compressive strain or electric field will further enhance Eg-contrast in different regions of the pseudoheterostructure3 so as to absorb or emit even broader spectrum of light.
    Nano Letters 08/2014; 14(9):5350-5357. · 13.03 Impact Factor
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    ABSTRACT: A microscopic phase field (MPF) model is formulated to describe quantitatively the core structure and energy of dislocations using ab initio data as input. Based on phase field microelasticity theory implemented in the slip plane using Green’s function to describe the long-range elastic interaction, the MPF model is a three-dimensional generalization of the Peierls model. Using the same generalized stacking fault energy as input, the core structure and energy predicted for straight dislocations by the MPF model show complete agreement with those predicted by the Peierls model. The ability of the MPF model to treat dislocations of arbitrary configurations is demonstrated by calculating the structure and energy of a twist grain boundary in aluminum. After discrete lattice sampling a la Nabarro, the grain boundary energy manifests Read–Shockley behavior for low-angle boundaries as well as deep cusps for high-angle special boundaries, indicating a “Peierls torque friction” effect for grain boundaries that has the same physical origin as the Peierls lattice friction for dislocation cores.
    Acta Materialia 08/2014; 74:125–131. · 3.94 Impact Factor
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    ABSTRACT: We model the mechanical response of alkanethiol-passivated gold nanoparticle superlattice (supercrystal) at ambient and elevated pressures using large-scale molecular dynamics simulation. Due to the important roles of soft organic ligands in mechanical response, the supercrystals exhibit entropic viscoelasticity during compression at ambient pressure. Applying a hydrostatic pressure of several hundred megapascals on the superlattice, combined with a critical deviatoric stress of the same order along the [110] direction of the face-centered-cubic supercrystal, can drive the room-temperature sintering ("fusion") of gold nanoparticles into ordered gold nanowire arrays. We discuss the molecular-level mechanism of such phenomena, and map out a non-equilibrium stress-driven processing diagram, which reveals a region in stress space where fusion of nanoparticles can occur, instead of other competing plasticity or phase transformation processes in the supercrystal. We further demonstrate that, for silver-gold (Ag-Au) binary nanoparticle superlattices in sodium-chloride type superstructure, stress-driven fusion along the [100] direction leads to the ordered formation of Ag-Au multi-junction nanowire arrays.
    Nano letters. 07/2014;
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    ABSTRACT: Initial condition dependence is the key to understanding the difference between ideal strength and actual strength of both crystalline and amorphous materials. Besides intrinsic structural heterogeneities in metallic glasses (MGs), a class of “extended defects” based on the “connected atomistic free volume” (CAFV) is proposed to define the microstructure (initial condition), which is crucial to understanding the strength. To explore these concepts and theories, deformation of finite-sized MG samples with different populations of pre-existing extended defects (damages) are simulated using a nanometer-scale shear transformation zone (STZ) model based on microelasticity and the kinetic Monte Carlo method. A “smaller is stronger” effect on the peak stress of simulated true stress–strain curves is seen in samples with pre-existing damage introduced as post-activated STZ clusters. Samples with “chemically contaminated” surface STZs also exhibit a size effect on the peak stress, and depending on whether the surface STZs are softer or harder than the bulk STZs, smaller can be either weaker or stronger.
    Acta Materialia 07/2014; 73:149–166. · 3.94 Impact Factor
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    ABSTRACT: We report a new class of large-gap quantum spin Hall insulators in two-dimensional transition metal dichalcogenides, namely, MX$_2$ with M=(Mo, W) and X=(S, Se, and Te), whose topological electronic properties are highly tunable by external electric field. We propose a novel topological field effect transistor made of these atomic layer materials and their van der Waals heterostructures. Our device exhibits parametrically enhanced charge-spin conductance through topologically protected transport channels, and can be rapidly switched off by electric field through topological phase transition instead of carrier depletion. Our work provides a practical material platform and device architecture for topological quantum electronics.
    arXiv:1406.2749. 06/2014;
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    ABSTRACT: To improve the contact between platinum catalyst and titanium substrate, a layer of TiO2 nanotube arrays has been synthesized before depositing Pt nanoflowers by pulse electrodeposition. Dramatic improvements in electrocatalytic activity (3×) and stability (60×) for methanol oxidation were found, suggesting promising applications in direct methanol fuel cells. The 3× and 60× improvements persist for Pt/Pd catalysts used to overcome the CO poisoning problem.
    Nano Research 06/2014; · 7.39 Impact Factor
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    ABSTRACT: Nanostructured LiFePO4 (LFP) electrodes have attracted great interest in the Li-ion battery field. Recently there have been debates on the presence and role of metastable phases during lithiation/delithiation, originating from the apparent high rate capability of LFP batteries despite poor electronic/ionic conductivities of bulk LFP and FePO4 (FP) phases. Here we report a potentiostatic in situ transmission electron microscopy (TEM) study of LFP electrode kinetics during delithiation. Using in situ high-resolution TEM, a Li-sublattice disordered solid solution zone (SSZ) is observed to form quickly and reach 10-25 nm × 20-40 nm in size, different from the sharp LFP|FP interface observed under other conditions. This 20 nm scale SSZ is quite stable and persists for hundreds of seconds at room temperature during our experiments. In contrast to the nanoscopically sharp LFP|FP interface, the wider SSZ seen here contains no dislocations, so reduced fatigue and enhanced cycle life can be expected along with enhanced rate capability. Our findings suggest that the disordered SSZ could dominate phase transformation behavior at non-equilibrium condition when high current/voltage is applied; for larger particles, the SSZ could still be important as it provides out-of-equilibrium but atomically wide avenues for Li+/e- transport.
    Nano Letters 05/2014; 14(7):4005-4010. · 13.03 Impact Factor
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    ABSTRACT: The ability to fine-tune band gap and band inversion in topological materials is highly desirable for the development of novel functional devices. Here we propose that the electronic properties of a free-standing nanomembrane of topological crystalline insulator (TCI) SnTe and Pb$_{1-x}$Sn$_x$(Se,Te) are highly tunable by engineering elastic strain and controlling membrane thickness, resulting in tunable band gap and giant piezoconductivity. Membrane thickness governs the hybridization of topological electronic states on opposite surfaces, while elastic strain can further modulate the hybridization strength by controlling the penetration length of surface states. We propose a frequency-resolved infrared photodetector using force-concentration induced inhomogeneous elastic strain in TCI nanomembrane with spatially varying width. The predicted tunable band gap accompanied by strong spin-textured electronic states will open up new avenues for fabricating piezoresistive devices, thermoelectrics, infrared detectors and energy-efficient electronic and optoelectronic devices based on TCI nanomembrane.
    arXiv:1403.3952. 03/2014;
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    ABSTRACT: Structural defects and their dynamics play an important role in controlling the behavior of phase-change materials (PCM) used in low-power nonvolatile memory devices. However, not much is known about the influence of disorder on the electronic properties of crystalline PCM prior to a structural phase-change. Here we show that the application of voltage pulses to single-crystalline GeTe nanowire memory devices introduces structural disorder in the form of dislocations and anti-phase boundaries (APB). The dynamic evolution and pile-up of APBs increases disorder at a local region of the nanowire, which electronically transforms it from a metal to a dirty metal to an insulator, while still retaining single-crystalline long-range order. We also observe that close to this metalinsulator transition, precise control over the applied voltage is required to create an insulating state; else the system ends up in a more disordered amorphous phase suggesting the role of electronic instabilities during the structural phase-change.
    Nano Letters 03/2014; 14(4):2201-2209. · 13.03 Impact Factor
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    ABSTRACT: Twinning on the plane is a common mode of plastic deformation for hexagonal-close-packed metals. Here we report, by monitoring the deformation of submicron-sized single-crystal magnesium compressed normal to its prismatic plane with transmission electron microscopy, the reorientation of the parent lattice to a 'twin' lattice, producing an orientational relationship akin to that of the conventional twinning, but without a crystallographic mirror plane, and giving plastic strain that is not simple shear. Aberration-corrected transmission electron microscopy observations reveal that the boundary between the parent lattice and the 'twin' lattice is composed predominantly of semicoherent basal/prismatic interfaces instead of the twinning plane. The migration of this boundary is dominated by the movement of these interfaces undergoing basal/prismatic transformation via local rearrangements of atoms. This newly discovered deformation mode by boundary motion mimics conventional deformation twinning but is distinct from the latter and, as such, broadens the known mechanisms of plasticity.
    Nature Communications 02/2014; 5:3297. · 10.74 Impact Factor
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    ABSTRACT: In purely bent ZnO microwires, the excitons can be effectively driven and concentrated by the elastic strain-gradient towards the tensile outer side of the purely bent wire. Experimental and theoretical approaches are combined to investigate the dynamics of excitons in inhomogeneous strain field with uniform elastic strain-gradient. Cathodoluminescence spectroscopy analysis on purely bent ZnO microwires verifies that excitons can be effectively driven and concentrated along the elastic strain-gradient.
    Advanced Materials 01/2014; · 14.83 Impact Factor

Publication Stats

3k Citations
1,079.19 Total Impact Points

Institutions

  • 2010–2014
    • Xi'an Jiaotong University
      • State Key Laboratory for Mechanical Behavior of Materials
      Ch’ang-an, Shaanxi, China
    • Osaka University
      • Department of Mechanical Science and Bioengineering
      Ōsaka-shi, Osaka-fu, Japan
  • 1997–2014
    • Massachusetts Institute of Technology
      • • Department of Nuclear Science and Engineering
      • • Department of Mechanical Engineering
      Cambridge, Massachusetts, United States
  • 2013
    • University of Maryland, College Park
      • Department of Chemical and Biomolecular Engineering
      College Park, MD, United States
    • China University of Petroleum
      Ch’ang-p’ing-ch’ü, Beijing, China
    • Korea University
      • Department of Materials Science and Engineering
      Sŏul, Seoul, South Korea
  • 2009–2013
    • Sandia National Laboratories
      • Advanced Materials Laboratory
      Albuquerque, New Mexico, United States
  • 2008–2013
    • University of Pennsylvania
      • • Department of Chemistry
      • • Department of Materials Science and Engineering
      Philadelphia, Pennsylvania, United States
    • Los Alamos National Laboratory
      • Theoretical Division
      Los Alamos, NM, United States
  • 2012
    • Peking University
      • International Center for Quantum Materials
      Beijing, Beijing Shi, China
    • Jiangsu Normal University
      Hsü-chuang, Shaanxi, China
  • 2009–2012
    • Pennsylvania State University
      • • Department of Materials Science and Engineering
      • • Department of Engineering Science and Mechanics
      University Park, MD, United States
  • 2003–2010
    • The Ohio State University
      • Department of Materials Science and Engineering
      Columbus, OH, United States
    • Princeton University
      • Department of Chemical and Biological Engineering
      Princeton, NJ, United States
  • 2007
    • Georgia Institute of Technology
      Atlanta, Georgia, United States