Li, Ju

Email Address
Research Projects
Organizational Units
Research Interests

Search Results

Now showing 1 - 10 of 20
  • Publication
    Computing the Viscosity of Supercooled Liquids
    (2009-06-11) Kushima, Akihiro; Lin, Xi; Li, Ju; Eapen, Jacob; Mauro, John C.; Qian, Xiafeng; Diep, Phong; Yip, Sidney
    We describe an atomistic method for computing the viscosity of highly viscous liquids based on activated state kinetics. A basin-filling algorithm allowing the system to climb out of deep energy minima through a series of activation and relaxation is proposed and first benchmarked on the problem of adatom diffusion on a metal surface. It is then used to generate transition state pathway trajectories in the potential energy landscape of a binary Lennard-Jones system. Analysis of a sampled trajectory shows the system moves from one deep minimum to another by a process that involves high activation energy and the crossing of many local minima and saddle points. To use the trajectory data to compute the viscosity we derive a Markov Network model within the Green–Kubo formalism and show that it is capable of producing the temperature dependence in the low-viscosity regime described by molecular dynamics simulation, and in the high-viscosity regime (102–1012 Pa s) shown by experiments on fragile glass-forming liquids. We also derive a mean-field-like description involving a coarse-grained temperature-dependent activation barrier, and show it can account qualitatively for the fragile behavior. From the standpoint of molecular studies of transport phenomena this work provides access to long relaxation time processes beyond the reach of current molecular dynamics capabilities. In a companion paper we report a similar study of silica, a representative strong liquid. A comparison of the two systems gives insight into the fundamental difference between strong and fragile temperature variations.
  • Publication
    One-particle-thick, Solvent-free, Course-grained Model for Biological and Biomimetic Fluid Membranes
    (2010-07-12) Yuan, Hongyan; Huang, Changjin; Li, Ju; Lykotrafitis, George; Zhang, Sulin
    Biological membranes are involved in numerous intriguing biophysical and biological cellular phenomena of different length scales, ranging from nanoscale raft formation, vesiculation, to microscale shape transformations. With extended length and time scales as compared to atomistic simulations, solvent-free coarse-grained membrane models have been exploited in mesoscopic membrane simulations. In this study, we present a one-particle-thick fluid membrane model, where each particle represents a cluster of lipid molecules. The model features an anisotropic interparticle pair potential with the interaction strength weighed by the relative particle orientations. With the anisotropic pair potential, particles can robustly self-assemble into fluid membranes with experimentally relevant bending rigidity. Despite its simple mathematical form, the model is highly tunable. Three potential parameters separately and effectively control diffusivity, bending rigidity, and spontaneous curvature of the model membrane. As demonstrated by selected examples, our model can naturally simulate dynamics of phase separation in multicomponent membranes and the topological change of fluid vesicles.
  • Publication
    Lithium Fiber Growth on the Anode in a Nanowire Lithium Ion Battery During Charging
    (2011-05-04) Liu, Xiau Hua; Zhong, Li; Kushima, Akihiro; Zhang, Li Qiang; Li, Ju; Mao, Scott X.; Ye, Zhi Zhen; Sullivan, John P.; Huang, Jian Yu
    Lithium (Li) dendrite formation has been recognized as one of the major safety concerns for Li metal batteries but not for conventional Li ion batteries (LIBs) where Li metal is not used. With the advanced in situ transmission electron microscopy enabling direct observation of battery operation, we found that Li fibers with length up to 35 µm grew on nanowire tip after charging. The Li fibers growth were highly directional, i.e., nucleating from the nanowire tip, and extending along the nanowire axis, which was attributed to the strong electric field enhancement effect induced by the sharp nanowire tip. This study reveals a potential safety concern of short-circuit failure for LIBs using nanowire anodes.
  • Publication
    Temperature and Strain-Rate Dependence of Surface Dislocation Nucleation
    (2008-05-05) Li, Ju; Zhu, Ting; Samanta, Amit; Leach, Austin; Gall, Ken
    Dislocation nucleation is essential to the plastic deformation of small-volume crystalline solids. The free surface may act as an effective source of dislocations to initiate and sustain plastic flow, in conjunction with bulk sources. Here, we develop an atomistic modeling framework to address the probabilistic nature of surface dislocation nucleation. We show the activation volume associated with surface dislocation nucleation is characteristically in the range of 1–10b3, where b is the Burgers vector. Such small activation volume leads to sensitive temperature and strain-rate dependence of the nucleation stress, providing an upper bound to the size-strength relation in nanopillar compression experiments.
  • Publication
    Diffusive Molecular Dynamics and its Application to Nanoindentation and Sintering
    (2011-08-04) Li, Ju; Sarkar, Sanket; Cox, William; Lenosky, Thomas J; Bitzek, Erik; Wang, Yunzhi
    The interplay between diffusional and displacive atomic movements is a key to understanding deformation mechanisms and microstructure evolution in solids. The ability to handle the diffusional time scale and the structural complexity in these problems poses a general challenge to atomistic modeling. We present here an approach called diffusive molecular dynamics (DMD), which can capture the diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations and evolving a smooth site-probability representation. The model is applied to nanoindentation and sintering, where intimate coupling between diffusional creep, displacive dislocation nucleation, and grain rotation are observed.
  • Publication
    Calculating Phase-Coherent Quantum Transport in Nanoelectronics with ab initio Quasiatomic Orbital Basis Set
    (2010-11-23) Qian, Xiaofeng; Li, Ju; Yip, Sidney
    We present an efficient and accurate computational approach to study phase-coherent quantum transport in molecular and nanoscale electronics. We formulate a Green’s-function method in the recently developed ab initio nonorthogonal quasiatomic orbital basis set within the Landauer-Büttiker formalism. These quasiatomic orbitals are efficiently and robustly transformed from Kohn-Sham eigenwave functions subject to the maximal atomic-orbital similarity measure. With this minimal basis set, we can easily calculate electrical conductance using Green’s-function method while keeping accuracy at the level of plane-wave density-functional theory. Our approach is validated in three studies of two-terminal electronic devices, in which projected density of states and conductance eigenchannel are employed to help understand microscopic mechanism of quantum transport. We first apply our approach to a seven-carbon atomic chain sandwiched between two finite crosssectioned Al(001) surfaces. The emergence of gaps in the conductance curve originates from the selection rule with vanishing overlap between symmetry-incompatible conductance eigenchannels in leads and conductor. In the second application, a (4,4) single-wall carbon nanotube with a substitutional silicon impurity is investigated. The complete suppression of transmission at 0.6 eV in one of the two conductance eigenchannels is attributed to the Fano antiresonance when the localized silicon impurity state couples with the continuum states of carbon nanotube. Finally, a benzene-1,4-dithiolate molecule attached to two Au(111) surfaces is considered. Combining fragment molecular orbital analysis and conductance eigenchannel analysis, we demonstrate that conductance peaks near the Fermi level result from resonant tunneling through molecular orbitals of benzene- 1,4-dithiolate molecule. In general, our conductance curves agree very well with previous results obtained using localized basis sets while slight difference is observed near the Fermi level and conductance edges.
  • Publication
    Quasiatomic orbitals for ab initio tight-binding analysis
    (2008-12-16) Li, Ju; Qian, Xiaofeng; Qi, Liang; Wang, Cai-Zhuang; Chan, Tzu-Liang; Yao, Yong-Xin; Ho, Kai-Ming; Yip, Sidney
    Wave functions obtained from plane-wave density-functional theory (DFT) calculations using norm-conserving pseudopotential, ultrasoft pseudopotential, or projector augmented-wave method are efficiently and robustly transformed into a set of spatially localized nonorthogonal quasiatomic orbitals (QOs) with pseudoangular momentum quantum numbers. We demonstrate that these minimal-basis orbitals can exactly reproduce all the electronic structure information below an energy threshold represented in the form of environment-dependent tight-binding Hamiltonian and overlap matrices. Band structure, density of states, and the Fermi surface are calculated from this real-space tight-binding representation for various extended systems (Si, SiC, Fe, and Mo) and compared with plane-wave DFT results. The Mulliken charge and bond order analyses are performed under QO basis set, which satisfy sum rules. The present work validates the general applicability of Slater and Koster's scheme of linear combinations of atomic orbitals and points to future ab initio tight-binding parametrizations and linear-scaling DFT development.
  • Publication
    Near Neutrality of an Oxygen Molecule Adsorbed on a Pt(111) Surface
    (2008-10-03) Qi, Liang; Qiang, Xiaofeng; Li, Ju
    The charge state of paramagnetic or nonmagnetic O2 adsorbed on a Pt(111) surface is analyzed using density functional theory. We find no significant charge transfer between Pt and the two adsorbed molecular precursors, suggesting these oxygen reduction reaction (ORR) intermediates are nearly neutral, and changes in magnetic moment come from self adjustment of O2 spin-orbital occupations. Our findings support a greatly simplified model of electrocatalyzed ORR, and also point to more subtle pictures of adsorbates or impurities interacting with crystal than literal integer charge transfers.
  • Publication
    Dynamical Behavior of Heat Conduction in Solid Argon
    (2011-04-01) Kaburaki, Hideo; Li, Ju; Yip, Sidney; Kimizuka, Hajime
    Equilibrium molecular dynamics is performed to obtain the thermal conductivity of crystalline argon using the Green-Kubo formalism, which permits the study of dynamical details of the transport process. A large system run to longer times is used to derive the heat flux autocorrelation functions from the low temperature solid to the liquid state. The power spectrum of an autocorrelation function reveals the change in the nature of the underlying atomic motions across the temperature range.
  • Publication
    Pressure-temperature phase diagram for shapes of vesicles: A coarse-grained molecular dynamics study
    (2009-10-06) Liu, Ping; Li, Ju; Zhang, Yong-wei
    Coarse-grained molecular dynamics simulations are performed to obtain the phase diagram for shapes of a vesicle with a variation in temperature and pressure difference across the membrane. Various interesting vesicle shapes are found, in particular, a series of shape transformations are observed for a vesicle with an initial spherical shape, which changes to a prolate shape, then an oblate shape, and then a stomatocyte shape, with either increasing temperature or decreasing pressure difference across the membrane.