Showing papers by "Zhiguo Wang published in 2012"
TL;DR: Examination of a series of transition metal dichalcogenides shows that the biaxial tensile strained NbS (2) and NbSe(2) structures can be magnetized with a ferromagnetic character due to the competitive effects of through-bond interaction and through-space interaction.
Abstract: Developing approaches to effectively induce and control the magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we have d...
320 citations
TL;DR: The crystallization of Li(15)Si(4) from amorphous Li(x)Si is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process.
Abstract: It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous LixSi phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li15Si4 or Li22Si5. Presently, the detailed atomistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory molecular dynamic calculation. We found the crystallization of Li15Si4 from amorphous LixSi is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is o...
249 citations
TL;DR: The unexpected discovery of a thermodynamically driven, yet kinetically controlled, surface modification in the widely explored lithium nickel manganese oxide cathode material, which may inhibit the battery charge/discharge rate is reported.
Abstract: A variety of approaches are being made to enhance the performance of lithium ion batteries. Incorporating multivalence transition-metal ions into metal oxide cathodes has been identified as an essential approach to achieve the necessary high voltage and high capacity. However, the fundamental mechanism that limits their power rate and cycling stability remains unclear. The power rate strongly depends on the lithium ion drift speed in the cathode. Crystallographically, these transition-metal-based cathodes frequently have a layered structure. In the classic wisdom, it is accepted that lithium ion travels swiftly within the layers moving out/in of the cathode during the charge/discharge. Here, we report the unexpected discovery of a thermodynamically driven, yet kinetically controlled, surface modification in the widely explored lithium nickel manganese oxide cathode material, which may inhibit the battery charge/discharge rate. We found that during cathode synthesis and processing before electrochemical cy...
223 citations
TL;DR: In this article, the creation and evolution of monovacancy, divacancy and grain boundaries under irradiation in graphene are investigated using density functional theory and time-dependent molecular dynamics simulations.
Abstract: Defects play an important role on the unique properties of the sp2-bonded materials, such as graphene. The creation and evolution of monovacancy, divacancy, Stone-Wales (SW), and grain boundaries (GBs) under irradiation in graphene are investigated using density functional theory and time-dependent density functional theory molecular dynamics simulations. It is of great interest that the patterns of these defects can be controlled through electron irradiation. The SW defects can be created by electron irradiation with energy above the displacement threshold energy (Td, ∼19 eV) and can be healed with an energy (14–18 eV) lower than Td. The transformation between four types of divacancies—V2(5–8–5), V2(555–777), V2(5555–6–7777), and V2(55–77)—can be realized through bond rotation induced by electron irradiation. The migrations of divancancies, SW defects, and GBs can also be controlled by electron irradiation. Thus, electron irradiation can serve as an important tool to modify morphology in a controllable m...
65 citations
TL;DR: Taken together, theoretical results show that the recombinative H(2)O desorption is preferred over both H bulk diffusion and H( 2) desOrption processes, which is in agreement with experimental data.
Abstract: Combined scanning tunneling microscopy, temperature programmed desorption, photo stimulated desorption, and density functional theory studies have probed the formation and reactivity of highly-hydroxylated rutile TiO2(110) surfaces, which were prepared via a novel, photochemical route using trimethyl acetic acid (TMAA) dissociative adsorption and subsequent photolysis at 300 K. Deprotonation of TMAA molecules upon adsorption produces both surface bridging hydroxyls (OHb) and bidentate trimethyl acetate (TMA) species with a saturation coverage of nearly 0.5 monolayers (ML). Ultra-violet light irradiation selectively removes TMA species, producing a highly-hydroxylated surface with up to ~0.5 ML OHb coverage. At high coverages, the OHb species typically occupy second-nearest neighbor sites along the bridging oxygen row locally forming linear (2 × 1) structures of different lengths, although the surface is less ordered on a long scale. The annealing of the highly-hydroxylated surface leads to hydroxyl recombination and H2O desorption with ~100% yield, thus ruling out the diffusion of H into the bulk that has been suggested in the literature. In agreement with experimental data, theoretical results show that the recombinative H2O desorption is preferred over both H bulk diffusion and H2 desorption processes.
59 citations
TL;DR: In this paper, the GNF-BN structure was explored and a specific configuration to obtain a large spin was proposed, together with the high stability of spin alignment, the proposed configuration can be exploited for spintronic devices.
Abstract: Novel electronic and magnetic properties of various-sized graphene nanoflakes (GNFs) embedded in a boron nitride (BN) layer were studied by ab initio methods. The feasibility of synthesizing hybrid GNF-BN structure, a desirable quantum dot structure, was explored. In this structure, photoexcited electrons and holes occupy the same spatial region—the GNF region—which offers an effective way to generate a GNF-based light-emitting device and adjust its emitted optical properties by controlling the size and array of GNF in the BN layer. On the basis of the important magnetism properties of embedded GNF, we propose a specific configuration to obtain a large spin. Together with the high stability of spin alignment, the proposed configuration can be exploited for spintronic devices.
40 citations
TL;DR: Wang et al. as discussed by the authors extended the Monte Carlo model of electron thermalization in inorganic scintillators to include electron scattering with both longitudinal optical (LO) and acoustic phonons as well as the effects of internal electric fields.
Abstract: A Monte Carlo model of electron thermalization in inorganic scintillators, which was developed and applied to CsI in a previous publication [Wang et al., J. Appl. Phys. 110, 064903 (2011)], is extended to another material of the alkali halide class, NaI, and to two materials from the alkaline-earth halide class, CaF2 and BaF2. This model includes electron scattering with both longitudinal optical (LO) and acoustic phonons as well as the effects of internal electric fields. For the four pure materials, a significant fraction of the electrons recombine with self-trapped holes and the thermalization distance distributions of the electrons that do not recombine peak between approximately 25 and 50 nm and extend up to a few hundreds of nanometers. The thermalization time distributions of CaF2, BaF2, NaI, and CsI extend to approximately 0.5, 1, 2, and 7 ps, respectively. The simulations show that the LO phonon energy is a key factor that affects the electron thermalization process. Indeed, the higher the LO pho...
38 citations
TL;DR: In this paper, the electronic and optical properties of two-dimensional covalent organic films with a macrocyclic network were systematically investigated using density-functional theory, and the results shed considerable light on the nature of spatial carrier confinement with band offset.
Abstract: Two-dimensional covalent organic films with a macrocyclic network were recently synthesized experimentally under simple solvothermal conditions [J. W. Colson et al., Science, 2011, 332, 228], which offers immense potential for optoelectronic applications as in the case of graphene. Here we systematically investigate the electronic and optical properties of such novel covalent organic frameworks (COF-5, TP-COF and NiPc-PBBA COF) as free-standing sheets using density-functional theory. The results shed considerable light on the nature of spatial carrier confinement with band offset. COF-5 exhibits a type-II heterojunction alignment with significant valence and conduction band offsets, suggesting an effective spatial carrier separation of electrons and holes. In TP-COF, the valence offset is close to zero, related to the dispersed distribution of photoexcited holes over the entire structure, while the conduction band offset is still remarkable, indicating the effective confinement of photoexcited electrons. NiPc-PBBA COF presents a type-I heterojunction alignment where the band-edged wave functions are localized in the same region, achieving effective spatial carrier congregation. The calculated absorption peaks of the optical absorption of TP-COF and NiPc-PBBA COF frameworks are in agreement with experimental measurements, thus providing theoretical insights into experimental observed transmission spectra of these frameworks.
38 citations
TL;DR: In this article, the authors investigated the H+ ion diffusion properties in LATP glass and their surface interactions using both experimental and modeling approaches, and they found that the apparent H+ related current observed in the initial cyclic voltammetry scan should be attributed to the adsorption of H+ ions on the LatP glass rather than the bulk diffusion of H+) ions in the glass, and the density functional theory calculations indicate that the H+) ion diffusion energy barrier (3.21 eV) is much higher than that of Li+ ion (0.79 eV
29 citations
TL;DR: In this article, the electronic and magnetic properties of graphene nanoflakes (GNFs) can be tuned by patterned adsorption of hydrogen, which can control the H coverage from bare GNFs to half hydrogenated and then to ful...
Abstract: The electronic and magnetic properties of graphene nanoflakes (GNFs) can be tuned by patterned adsorption of hydrogen. Controlling the H coverage from bare GNFs to half hydrogenated and then to ful...
24 citations
TL;DR: In this article, the structure, stability, and electronic properties of BN sheets with grain boundaries were investigated using first-principles calculations, and two types of GBs, i.e., zigzag-and armchair-oriented GBs were considered.
Abstract: Using first-principles calculations, the structure, stability, and electronic properties of BN sheets with grain boundaries (GBs) are investigated. Two types of GBs, i.e., zigzag- and armchair-oriented GBs, are considered. Simulation results reveal that the zigzag-oriented GBs are more stable than the armchair-oriented ones. The GBs induce defect levels located within the band gap, which must be taken into account when building nanoelectronic devices.
TL;DR: In this article, the structure and electronic properties of carbon nanotubes (CNTs) with grain boundaries (GBs) were investigated using density-functional calculations, where the GBs parallel and perpendicular to the tube axis were considered.
Abstract: Structure and electronic properties of carbon nanotubes (CNTs) with grain boundaries (GBs) are investigated using density-functional calculations, where the GBs parallel and perpendicular to the tube axis are considered. Simulation results show that the GBs have a great effect on the electronic properties of the CNTs. For the GBs along the tube axis, the CNTs are narrow or zero band gap (<0.16 eV) materials, independent of the misoritentaion angle and diameter. For the GBs perpendicular to the tube axis, localized electronic states appear within the GBs regions, leading to a larger band gap of up to 0.6 eV. It is convenient to transport and localize the electrons and holes by engineering the GBs. These findings are of great significance for developing carbon-based nanomaterials and electronic devices.
TL;DR: In this paper, the authors employed the three-dimensional finite-difference time-domain method to quantify the electric field distribution within the subsurface in the presence of such a defect under normal incidence irradiation.
Abstract: Theoretical studies show that a Hertzian-conical crack can be considered to be composed of double cone faces for simplicity. In the present study, the three-dimensional finite-difference time-domain method is employed to quantify the electric-field distribution within the subsurface in the presence of such a defect under normal incidence irradiation. Both impurities (inside the crack) and the chemical etching have been investigated. The results show that the maximum electric field amplitude |E|max is 9.57374 V/m when the relative dielectric constant of transparent impurity equals 8.5. And the near-field modulation will be improved if the crack is filled with the remainder polishing powders or water vapor/drops. Meanwhile, the laser-induced initial damage moves to the glass—air surface. In the etched section, the magnitude of intensification is strongly dependent on the inclination angle θ. There will be a highest modulation when θ is around π/6, and the maximum value of |E|max is 18.57314 V/m. When θ ranges from π/8 to π/4, the light intensity enhancement factor can easily be larger than 100, and the modulation follows a decreasing trend. On the other hand, the modulation curves become smooth when θ > π/4 or θ < π/8.
07 Nov 2012
TL;DR: In this paper, a graphene-based passively Q-switched erbium-doped fiber laser is demonstrated, which 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.
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.
TL;DR: Using density functional theory, a series of calculations of structural and electronic properties of hydrogen vacancies in a fully hydrogenated boron nitride (fH-BN) layer were conducted.
Abstract: Using density functional theory, a series of calculations of structural and electronic properties of hydrogen vacancies in a fully hydrogenated boron nitride (fH-BN) layer were conducted. By dehydrogenating the fH-BN structure, B-terminated vacancies can be created which induce complete spin polarization around the Fermi level, irrespective of the vacancy size. On the contrary, the fH-BN structure with N-terminated vacancies can be a small-gap semiconductor, a typical spin gapless semiconductor, or a metal depending on the vacancy size. Utilizing such vacancy-induced band gap and magnetism changes, possible applications in spintronics are proposed, and a special fH-BN based quantum dot device is designed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)