Penghao Xiao

A generalized solid-state nudged elastic band method

A generalized solid-state nudged elastic band (G-SSNEB) method is presented for determining reaction pathways of solid-solid transformations involving both atomic and unit-cell degrees of freedom. We combine atomic and cell degrees of freedom into a unified description of the crystal structure so that calculated reaction paths are insensitive to the choice of periodic cell. For the rock-salt to wurtzite transition in CdSe, we demonstrate that the method is robust for mechanisms dominated either by atomic motion or by unit-cell deformation; notably, the lowest-energy transition mechanism found by our G-SSNEB changes with cell size from a concerted transformation of the cell coordinates in small cells to a nucleation event in large cells. The method is efficient and can be applied to systems in which the force and stress tensor are calculated using density functional theory.

Removal of Interstitial H2O in Hexacyanometallates for a Superior Cathode of a Sodium-Ion Battery

Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for large-scale storage of electrical energy, provided a host cathode for Na can be found that provides the necessary capacity, voltage, and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease of synthesis and rigid open framework that enables fast Na(+) insertion and extraction. Here we report an intriguing effect of interstitial H2O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal composition Na2MnFe(CN)6·zH2O (Na2-δMnHFC). The newly discovered dehydrated Na2-δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of 150 mAh g(-1) in a sodium half cell and 140 mAh g(-1) in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C, the half-cell capacity is 120 mAh g(-1), and at 0.7 C, the cell exhibits 75% capacity retention after 500 cycles.

Interfacial adhesion between graphene and silicon dioxide by density functional theory with van der Waals corrections

Interfacial adhesion between graphene and a SiO2 substrate is studied by density functional theory (DFT) with dispersion corrections. The results demonstrate the van der Waals (vdW) interaction as the predominant mechanism at the graphene/SiO2 interface. It is found that the interaction strength is strongly influenced by changes of the SiO2 surface structures due to surface reactions with water. The adhesion energy is reduced when the reconstructed SiO2 surface is hydroxylated, and further reduced when covered by a monolayer of adsorbed water molecules. Moreover, it is noted that vdW forces are required to accurately model the graphene/SiO2 interface with DFT and that the adhesion energy is underestimated by empirical force fields commonly used in atomistic simulations.

Simple Synthesis of Nanocrystalline Tin Sulfide/N-Doped Reduced Graphene Oxide Composites as Lithium Ion Battery Anodes

Composites of nitrogen-doped reduced graphene oxide (NRGO) and nanocrystalline tin sulfides were synthesized, and their performance as lithium ion battery anodes was evaluated. Following the first cycle the composite consisted of Li2S/LixSn/NRGO. The conductive NRGO cushions the stress associated with the expansion of lithiation of Sn, and the noncycling Li2S increases the residual Coulombic capacity of the cycled anode because (a) Sn domains in the composite formed of unsupported SnS2 expand only by 63% while those in the composite formed of unsupported SnS expand by 91% and (b) Li percolates rapidly at the boundary between the Li2S and LixSn nanodomains. The best cycling SnS2/NRGO-derived composite retained a specific capacity of 562 mAh g-1 at the 200th cycle at 0.2 A g-1 rate.

Unification of algorithms for minimum mode optimization

Minimum mode following algorithms are widely used for saddle point searching in chemical and material systems. Common to these algorithms is a component to find the minimum curvature mode of the second derivative, or Hessian matrix. Several methods, including Lanczos, dimer, Rayleigh-Ritz minimization, shifted power iteration, and locally optimal block preconditioned conjugate gradient, have been proposed for this purpose. Each of these methods finds the lowest curvature mode iteratively without calculating the Hessian matrix, since the full matrix calculation is prohibitively expensive in the high dimensional spaces of interest. Here we unify these iterative methods in the same theoretical framework using the concept of the Krylov subspace. The Lanczos method finds the lowest eigenvalue in a Krylov subspace of increasing size, while the other methods search in a smaller subspace spanned by the set of previous search directions. We show that these smaller subspaces are contained within the Krylov space for which the Lanczos method explicitly finds the lowest curvature mode, and hence the theoretical efficiency of the minimum mode finding methods are bounded by the Lanczos method. Numerical tests demonstrate that the dimer method combined with second-order optimizers approaches but does not exceed the efficiency of the Lanczos method for minimum mode optimization.

Benchmarks for Characterization of Minima, Transition States, and Pathways in Atomic, Molecular, and Condensed Matter Systems.

A set of benchmark systems is defined to compare different computational approaches for characterizing local minima, transition states, and pathways in atomic, molecular, and condensed matter systems. Comparisons between several commonly used methods are presented. The strengths and weaknesses are discussed, as well as implementation details that are important for achieving good performance. All of the benchmarks and methods are provided in an online database to make the implementation details available and the results reproducible. While this paper provides a snapshot of the benchmark results, the online framework is structured to be dynamic and incorporate new methods and codes as they are developed.

Engineering the Mechanical Properties of Monolayer Graphene Oxide at the Atomic Level

The mechanical properties of graphene oxide (GO) are of great importance for applications in materials engineering. Previous mechanochemical studies of GO typically focused on the influence of the degree of oxidation on the mechanical behavior. In this study, using density functional-based tight binding simulations, validated using density functional theory simulations, we reveal that the deformation and failure of GO are strongly dependent on the relative concentrations of epoxide (-O-) and hydroxyl (-OH) functional groups. Hydroxyl groups cause GO to behave as a brittle material; by contrast, epoxide groups enhance material ductility through a mechanically driven epoxide-to-ether functional group transformation. Moreover, with increasing epoxide group concentration, the strain to failure and toughness of GO significantly increases without sacrificing material strength and stiffness. These findings demonstrate that GO should be treated as a versatile, tunable material that may be engineered by controlling chemical composition, rather than as a single, archetypical material.

Suppressing the bipolar contribution to the thermoelectric properties of Mg2Si0.4Sn0.6 by Ge substitution

The optimized thermoelectric figure of merit (ZT) of Mg2Si0.4Sn0.6 peaks at about 750 K because its relatively narrow band gap results in pronounced bipolar transport at higher temperatures. To suppress the bipolar transport, we have conducted a combined experimental and theoretical investigation of Ge-substitution effects on the band structures and thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6−yGey (y = 0, 0.1, and 0.2) synthesized by solid state reaction. The measured transport properties of these compositions can be interpreted by a triple-parabolic-band model based on first-principle calculation of band structures. The results show that the bipolar conduction in the temperature range up to 800 K was effectively suppressed by Ge substitution that widens the band gap. As a side effect, Ge substitution induces the separation of two otherwise converged conduction bands in Mg2Si0.4Sn0.6, leading to reduced thermoelectric performance at low temperatures. The result of these two competing effects is th...

Solid-state dimer method for calculating solid-solid phase transitions

The dimer method is a minimum mode following algorithm for finding saddle points on a potential energy surface of atomic systems. Here, the dimer method is extended to include the cell degrees of freedom for periodic solid-state systems. Using this method, reaction pathways of solid-solid phase transitions can be determined without having to specify the final state structure or reaction mechanism. Example calculations include concerted phase transitions between CdSe polymorphs and a nucleation and growth mechanism for the A15 to BCC transition in Mo.

Communication: From graphite to diamond: Reaction pathways of the phase transition

Phase transitions between carbon allotropes are calculated using the generalized solid-state nudged elastic band method. We find a new reaction mechanism between graphite and diamond with nucleation characteristics that has a lower activation energy than the concerted mechanism. The calculated barrier from graphite to hexagonal diamond is lower than to cubic diamond, resolving a conflict between theory and experiment. Transitions are calculated to three structures of cold compressed graphite: bct C4, M, and Z-carbon, which are accessible at the experimentally relevant pressures near 17 GPa.