Showing papers by "Zhiguo Wang published in 2013"

Electronic Origin for the Phase Transition from Amorphous LixSi to Crystalline Li15Si4

Abstract: Silicon has been widely explored as an anode material for lithium ion battery. Upon lithiation, silicon transforms to amorphous LixSi (a-LixSi) via electrochemical-driven solid-state amorphization. With increasing lithium concentration, a-LixSi transforms to crystalline Li15Si4 (c-Li15Si4). The mechanism of this crystallization process is not known. In this paper, we report the fundamental characteristics of the phase transition of a-LixSi to c-Li15Si4 using in situ scanning transmission electron microscopy, electron energy loss spectroscopy, and density function theory (DFT) calculation. We find that when the lithium concentration in a-LixSi reaches a critical value of x = 3.75, the a-Li3.75Si spontaneously and congruently transforms to c-Li15Si4 by a process that is solely controlled by the lithium concentration in the a-LixSi, involving neither large-scale atomic migration nor phase separation. DFT calculations indicate that c-Li15Si4 formation is favored over other possible crystalline phases due to t...

Controlling magnetism of MoS2 sheets by embedding transition-metal atoms and applying strain.

Abstract: Prompted by recent experimental achievement of transition metal (TM) atoms substituted in MoS2 nanostructures during growth or saturating existing vacancies (Sun et al., ACS Nano, 2013, 7, 3506; Deepak et al., J. Am. Chem. Soc., 2007, 129, 12549), we explored, via density functional theory, the magnetic properties of a series of 3d TM atoms substituted in a MoS2 sheet, and found that Mn, Fe, Co, Ni, Cu and Zn substitutions can induce magnetism in the MoS2 sheet. The localizing unpaired 3d electrons of TM atoms respond to the introduction of a magnetic moment. Depending on the species of TM atoms, the substituted MoS2 sheet can be a metal, semiconductor or half-metal. Remarkably, the applied elastic strain can be used to control the strength of the spin-splitting of TM-3d orbitals, leading to an effective manipulation of the magnetism of the TM-substituted MoS2 sheet. We found that the magnetic moment of the Mn- and Fe-substituted MoS2 sheets can monotonously increase with the increase of tensile strain, while the magnetic moment of Co-, Ni-, Cu- and Zn-substituted MoS2 sheets initially increases and then decreases with the increase of tensile strain. An instructive mechanism was proposed to qualitatively explain the variation of magnetism with elastic strain. The finding of the magnetoelastic effect here is technologically important for the fabrication of strain-driven spin devices on MoS2 nanostructures, which allows us to go beyond the current scope limited to the spin devices within graphene and BN-based nanostructures.

Single-layered V2O5 a promising cathode material for rechargeable Li and Mg ion batteries: an ab initio study

Abstract: Using first principles calculations based on density functional theory, the adsorption and diffusion properties of Li and Mg atoms on single-layered and bulk V2O5 are investigated. The simulation results show that the diffusion barrier of Li on the single-layered V2O5 is decreased compared with that of the bulk V2O5, which indicates that the Li mobility can be significantly enhanced on the single-layered V2O5. The increased binding energies of Li to single-layered V2O5 make them more attractive for promising cathode materials. Although the diffusion barrier of Mg on the single-layered V2O5 does not decrease, the binding energies of Mg to single-layered V2O5 is increased compared with that of bulk V2O5, thus the single-layered V2O5 is an attractive cathode material for rechargeable ion batteries.

Electron-Rich Driven Electrochemical Solid-State Amorphization in Li–Si Alloys

Abstract: The physical and chemical behaviors of materials used in energy storage devices, such as lithium-ion batteries (LIBs), are mainly controlled by an electrochemical process, which normally involves insertion/extraction of ions into/from a host lattice with a concurrent flow of electrons to compensate charge balance. The fundamental physics and chemistry governing the behavior of materials in response to the ions insertion/extraction is not known. Herein, a combination of in situ lithiation experiments and large-scale ab initio molecular dynamics simulations are performed to explore the mechanisms of the electrochemically driven solid-state amorphization in Li–Si systems. We find that local electron-rich condition governs the electrochemically driven solid-state amorphization of Li–Si alloys. This discovery provides the fundamental explanation of why lithium insertion in semiconductor and insulators leads to amorphization, whereas in metals, it leads to a crystalline alloy. The present work correlates electr...

First principles prediction of nitrogen-doped carbon nanotubes as a high-performance cathode for Li–S batteries

Abstract: The insulating nature of sulfur and the solubility of polysulfide in an organic electrolyte are two main factors that limit the application of lithium sulfur (Li–S) battery systems. Enhancement of Li conductivity, identification of a strong adsorption agent for polysulfides and the improvement of the whole sulfur-based electrode are of great technological importance. The diffusion of Li atoms in the outer-wall, inner-wall and inter-wall spaces in nitrogen-doped double-walled carbon nanotubes (CNTs) and penetrations of Li and S atoms through the walls are studied using density functional theory. We find that N-doping does not alter the diffusion behavior of Li atoms throughout the CNTs, but the energy barrier for Li atoms to penetrate the wall is greatly decreased by N-doping (from ∼9.0 eV to ∼1.0 eV). On the other hand, the energy barrier for S atoms to penetrate the wall remains very high, which is caused by the formation of chemical bonds between S and nearby N atoms. The results indicate that Li atoms are able to diffuse freely, whereas S atoms can be encapsulated inside the N-doped CNTs, suggesting that the N-doped CNTs can be potentially used in high performance Li–S batteries.

Kinetic Monte Carlo simulations of excitation density dependent scintillation in CsI and CsI(Tl)

Abstract: Nonlinear quenching of electron–hole pairs in the denser regions of ionization tracks created by γ-ray and high-energy electrons is a likely cause of the light yield non-proportionality of many inorganic scintillators. Therefore, kinetic Monte Carlo (KMC) simulations were carried out to investigate the scintillation properties of pure and thallium-doped CsI as a function of electron–hole pair density. The availability of recent experimental data on the excitation density dependence of the light yield of CsI following ultraviolet excitation allowed for an improved parameterization of the interactions between self-trapped excitons (STE) in the KMC model via dipole–dipole Forster transfer. The KMC simulations reveal that nonlinear quenching occurs very rapidly (within a few picoseconds) in the early stages of the scintillation process. In addition, the simulations predict that the concentration of thallium activators can affect the extent of nonlinear quenching as it has a direct influence on the STE density through STE dissociation and electron scavenging. This improved model will enable more realistic simulations of the non-proportional γ-ray and electron response of inorganic scintillators.

Regulating energy transfer of excited carriers and the case for excitation-induced hydrogen dissociation on hydrogenated graphene

Abstract: Understanding and controlling of excited carrier dynamics is of fundamental and practical importance, particularly in photochemistry and solar energy applications. However, theory of energy relaxation of excited carriers is still in its early stage. Here, using ab initio molecular dynamics (MD) coupled with time-dependent density functional theory, we show a coverage-dependent energy transfer of photoexcited carriers in hydrogenated graphene, giving rise to distinctively different ion dynamics. Graphene with sparsely populated H is difficult to dissociate due to inefficient transfer of the excitation energy into kinetic energy of the H. In contrast, H can easily desorb from fully hydrogenated graphane. The key is to bring down the H antibonding state to the conduction band minimum as the band gap increases. These results can be contrasted to those of standard ground-state MD that predict H in the sparse case should be much less stable than that in fully hydrogenated graphane. Our findings thus signify the importance of carrying out explicit electronic dynamics in excited-state simulations.

Monte Carlo simulation of gamma-ray response of BaF2 and CaF2

Abstract: We have employed a Monte Carlo (MC) method to study intrinsic properties of two alkaline-earth halides, namely, BaF2 and CaF2, relevant to their use as radiation detector materials. The MC method follows the fate of individual electron-hole (e-h) pairs and thus allows for a detailed description of the microscopic structure of ionization tracks created by incident γ-ray radiation. The properties of interest include the mean energy required to create an e-h pair, W, Fano factor, F, the maximum theoretical light yield, and the spatial distribution of e-h pairs resulting from γ-ray excitation. Although W and F vary with incident photon energy at low energies, they tend to constant values at energies higher than 1 keV. W is determined to be 18.9 and 19.8 eV for BaF2 and CaF2, respectively, in agreement with published data. The e-h pair spatial distributions exhibit a linear distribution along the fast electron tracks with high e-h pair densities at the end of the tracks. Most e-h pairs are created by interband...

Transition Metal Adsorption Promotes Patterning and Doping of Graphene by Electron Irradiation

Abstract: We present an ab initio molecular dynamics study of the effect of transition metal (TM) adatoms (Sc–Zn) on the threshold displacement energy (TDE) of graphene. Our calculations predict that it is s...

Antisite defects in La0.7Sr0.3MnO3 and La0.7Sr0.3FeO3

Abstract: Complex oxide thin films and superlattices with the perovskite ABO3 structure have been found to possess multifunctional properties. Here, we present our discovery of antisite defects, La ions in Fe(Mn) sites (denoted as LaB), in a La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 superlattice. The antisite defect was directly characterized by atomic resolution Z-contrast imaging and the composition and electronic structure were analyzed by electron energy loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Density functional theory was used to calculate the formation energy, showing that the formation of the detected antisite defects is a consequence of the slightly reducing conditions during sample growth.

Passively Q-switched erbium-doped fiber laser using a graphene saturable absorber

Abstract: We demonstrate a graphene-based passively Q-switched erbium-doped fiber laser. The laser produces stable pulses with a typical pulse width of ∼ 9.0 µs at a pulse repetition rate ranging from 8.3 to 12.5 kHz.

A compact high-gain patch antenna using stacked rings in LTCC technology

Abstract: A compact high-gain patch antenna using stacked rings based on low-temperature co-fired ceramic (LTCC) multilayer technology is proposed in this paper. Three metal stacked rings are employed to surround the patch antenna, acting as directors to guide the electromagnetic wave radiating toward broadside and, hence, improve the antenna's gain. Also, three ground planes are connected by metallic vias to form a cavity-backed structure, which suppresses the outward propagating diffraction wave and contributes to the improvement of the antenna’s gain either. The fabricated antenna achieves a high gain of 8.1 dBi at 36.7 GHz including the loss of the feeding parts and radio-frequency (RF) probe, and the gain is relatively stable within the bandwidth. Furthermore, the antenna only occupies a compact size of mm3 which is much smaller than most of the other high-gain patch antennas on LTCC.

Science-Driven Candidate Search for New Scintillator Materials FY 2013 Annual Report

Abstract: ........................................................................................................................................... 1.1 Acronyms and Abbreviations .......................................................................................................... 1.2 1.0 Objective and Summary .......................................................................................................... 1.6 2.0 Task 1: Ab initio calculations of electronic properties, electronic response functions and secondary particle spectra ....................................................................................................... A.1 2.1 Summary of progress ..................................................................................................... A.1 2.2 Publications/Presentations .............................................................................................. A.1 2.3 Progress during FY13..................................................................................................... A.1 3.0 Task 2: Intrinsic response properties, theoretical light yield, and microscopic description of ionization tracks ...................................................................................................................... A.3 3.1 Summary of progress ..................................................................................................... A.3 3.2 Publications/Presentations .............................................................................................. A.3 3.3 Progress during FY13..................................................................................................... A.3 4.0 Task 3: Kinetics and efficiency of scintillation: nonlinearity, intrinsic energy resolution, and pulse shape discrimination ...................................................................................................... A.6 4.1 Summary of progress ..................................................................................................... A.6 4.2 Publications/Presentations .............................................................................................. A.6 4.3 Progress during FY13..................................................................................................... A.6 Appendix A Publications and Invited Presentations ....................................................................... A.8