Yuanxi Wang

Non-oxidative intercalation and exfoliation of graphite by Brønsted acids

Graphite intercalation compounds are formed by inserting guest molecules or ions between sp(2)-bonded carbon layers. These compounds are interesting as synthetic metals and as precursors to graphene. For many decades it has been thought that graphite intercalation must involve host-guest charge transfer, resulting in partial oxidation, reduction or covalent modification of the graphene sheets. Here, we revisit this concept and show that graphite can be reversibly intercalated by non-oxidizing Brønsted acids (phosphoric, sulfuric, dichloroacetic and alkylsulfonic acids). The products are mixtures of graphite and first-stage intercalation compounds. X-ray photoelectron and vibrational spectra indicate that the graphene layers are not oxidized or reduced in the intercalation process. These observations are supported by density functional theory calculations, which indicate a dipolar interaction between the guest molecules and the polarizable graphene sheets. The intercalated graphites readily exfoliate in dimethylformamide to give suspensions of crystalline single- and few-layer graphene sheets.

Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers

We investigate Second Harmonic Generation (SHG) in monolayer WS2 both deposited on a SiO2/Si substrate or suspended using transmission electron microscopy grids. We find unusually large second order nonlinear susceptibility, with an estimated value of deff ~ 4.5 nm/V nearly three orders of magnitude larger than other common nonlinear crystals. In order to quantitatively characterize the nonlinear susceptibility of two-dimensional (2D) materials, we have developed a formalism to model SHG based on the Greens function with a 2D nonlinear sheet source. In addition, polarized SHG is demonstrated as a useful method to probe the structural symmetry and crystal orientation of 2D materials. To understand the large second order nonlinear susceptibility of monolayer WS2, density functional theory based calculation is performed. Our analysis suggests the origin of the large nonlinear susceptibility in resonance enhancement and a large joint density of states, and yields an estimate of the nonlinear susceptibility value deff = 0.77 nm/V for monolayer WS2, which shows good order-of-magnitude agreement with the experimental result.

Lattice dynamics, thermodynamics, and bonding strength of lithium-ion battery materials LiMPO4 (M = Mn, Fe, Co, and Ni): a comparative first-principles study

Gaps in our knowledge of phonon and thermodynamics still remain despite significant research efforts on cathode materials LiMPO4 (M = Mn, Fe, Co, and Ni) for rechargeable Li-ion batteries. Here, we employ a mixed-space approach of first-principles phonon calculations to probe the lattice dynamics including LO–TO splitting (longitudinal and transverse optical phonon splitting), quantitative bonding strength between atoms, and finite-temperature thermodynamic properties of LiMPO4. In order to take into account the strong on-site Coulomb interaction (U) presented in transition metals, the GGA + U calculations are used for LiMPO4. It is found that the oxygen–phosphorus (O–P) bond with the minimal bond length is extremely strong, which is roughly five times larger than the second strongest O–O bond. The atom P-containing bonds are apparently stronger than the corresponding atom O-containing bonds, indicating the stability of LiMPO4 is mainly due to atom P. It is observed that the equilibrium volume of LiMPO4 decreases from Mn, Fe, Co, to Ni, and the bulk modulus, zero-point vibrational energy, and Debye temperature increase. Phonon results indicate that the largest vibrational contribution to Gibbs energy is for LiMnPO4, followed by LiFePO4, LiCoPO4, and then LiNiPO4, due to the decreasing trend of phonon densities of state at the low frequency region of LiMPO4. Computed phonon and thermodynamic properties of LiMPO4 are in close accord with available experiments, and provide knowledge to be validated experimentally.

Temperature-dependent ideal strength and stacking fault energy of fcc Ni: a first-principles study of shear deformation

Variations of energy, stress, and magnetic moment of fcc Ni as a response to shear deformation and the associated ideal shear strength (τ(IS)), intrinsic (γ(SF)) and unstable (γ(US)) stacking fault energies have been studied in terms of first-principles calculations under both the alias and affine shear regimes within the {111} slip plane along the <112> and <110> directions. It is found that (i) the intrinsic stacking fault energy γ(SF) is nearly independent of the shear deformation regimes used, albeit a slightly smaller value is predicted by pure shear (with relaxation) compared to the one from simple shear (without relaxation); (ii) the minimum ideal shear strength τ(IS) is obtained by pure alias shear of {111}<112>; and (iii) the dissociation of the 1/2[110] dislocation into two partial Shockley dislocations (1/6[211] + 1/6[121]) is observed under pure alias shear of {111}<110>. Based on the quasiharmonic approach from first-principles phonon calculations, the predicted γ(SF) has been extended to finite temperatures. In particular, using a proposed quasistatic approach on the basis of the predicted volume versus temperature relation, the temperature dependence of τ(IS) is also obtained. Both the γ(SF) and the τ(IS) of fcc Ni decrease with increasing temperature. The computed ideal shear strengths as well as the intrinsic and unstable stacking fault energies are in favorable accord with experiments and other predictions in the literature.

Reversible intercalation of hexagonal boron nitride with Brønsted acids.

Hexagonal boron nitride (h-BN) is an insulating compound that is structurally similar to graphite. Like graphene, single sheets of BN are atomically flat, and they are of current interest in few-layer hybrid devices, such as transistors and capacitors, that contain insulating components. While graphite and other layered compounds can be intercalated by redox reactions and then converted chemically to suspensions of single sheets, insulating BN is not susceptible to oxidative intercalation except by extremely strong oxidizing agents. We report that stage-1 intercalation compounds can be formed by simple thermal drying of h-BN in Brønsted acids H2SO4, H3PO4, and HClO4. X-ray photoelectron and vibrational spectra, as well as electronic structure and molecular dynamics calculations, demonstrate that noncovalent interactions of these oxyacids with the basic N atoms of the sheets drive the intercalation process.

Effects of alloying element and temperature on the stacking fault energies of dilute Ni-base superalloys

A systematic study of stacking fault energy (γ(SF)) resulting from induced alias shear deformation has been performed by means of first-principles calculations for dilute Ni-base superalloys (Ni(23)X and Ni(71)X) for various alloying elements (X) as a function of temperature. Twenty-six alloying elements are considered, i.e., Al, Co, Cr, Cu, Fe, Hf, Ir, Mn, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Sc, Si, Ta, Tc, Ti, V, W, Y, Zn, and Zr. The temperature dependence of γ(SF) is computed using the proposed quasistatic approach based on a predicted γ(SF)-volume-temperature relationship. Besides γ(SF), equilibrium volume and the normalized stacking fault energy (Γ(SF) = γ(SF)/Gb, with G the shear modulus and b the Burgers vector) are also studied as a function of temperature for the 26 alloying elements. The following conclusions are obtained: all alloying elements X studied herein decrease the γ(SF) of fcc Ni, approximately the further the alloying element X is from Ni on the periodic table, the larger the decrease of γ(SF) for the dilute Ni-X alloy, and roughly the γ(SF) of Ni-X decreases with increasing equilibrium volume. In addition, the values of γ(SF) for all Ni-X systems decrease with increasing temperature (except for Ni-Cr at higher Cr content), and the largest decrease is observed for pure Ni. Similar to the case of the shear modulus, the variation of γ(SF) for Ni-X systems due to various alloying elements is traceable from the distribution of (magnetization) charge density: the spherical distribution of charge density around a Ni atom, especially a smaller sphere, results in a lower value of γ(SF) due to the facility of redistribution of charges. Computed stacking fault energies and the related properties are in favorable accord with available experimental and theoretical data.

Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy

Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS2.

Icosahedral ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass

This paper presents a computational evidence of icosahedral short and medium range ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass using ab initio molecular dynamics simulation. It is found that 1551, 1541, and 1431 types of bond pairs are pronounced in both the liquid and glass states, resulting in icosahedral coordinate polyhedra at low temperatures. By linking the individual icosahedra through vertex-, edge-, face-, and intercross-shared atoms, icosahedral medium range ordering is formed. The predicted homogenized structure factor and pair correlation function of the glass structure have been confirmed to be in agreement with the experimental results.

Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide

Bound exciton is a signature of sulfur vacancies, and thus, it can be used to investigate defects in atomically thin materials. Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence (PL) spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved PL spectroscopy at low temperatures revealed bound excitons that were present only on the edges of monolayer tungsten disulfide and not in the interior. Optical pumping of the bound excitons was sublinear, confirming their bound nature. Atomic-resolution images reveal that the areal density of monosulfur vacancies is much larger near the edges (0.92 ± 0.45 nm−2) than in the interior (0.33 ± 0.11 nm−2). Temperature-dependent PL measurements found a thermal activation energy of ~36 meV; surprisingly, this is much smaller than the bound-exciton binding energy of ~300 meV. We show that this apparent inconsistency is related to a thermal dissociation of the bound exciton that liberates the neutral excitons from negatively charged point defects. First-principles calculations confirm that sulfur monovacancies introduce midgap states that host optical transitions with finite matrix elements, with emission energies ranging from 200 to 400 meV below the neutral-exciton emission line. These results demonstrate that bound-exciton emission induced by monosulfur vacancies is concentrated near the edges of as-grown monolayer tungsten disulfide.

First-principles calculation of structural, mechanical, magnetic and thermodynamic properties for γ-M23C6 (M = Fe, Cr) compounds

We report the results of our first-principles calculations of structural stability, mechanical, magnetic, and thermodynamic properties for γ-M(23)C(6) (M = Fe, Cr) compounds with each of the four metal Wyckoff sites being occupied in turn by Fe. The thermodynamic properties and the temperature dependence of the mechanical behavior of γ-M(23)C(6) compounds are investigated based on the quasi-harmonic Debye model. The results show that the thermodynamic properties of γ-M(23)C(6) (M = Fe, Cr) compounds are more dependent on the position of Fe atoms than the amount of Fe.