First-principles Results Research Articles

First-principles mobility prediction for amorphous semiconductors

Carrier mobility in amorphous semiconductors remained unpredictable due to random electronic states in the absence of the long-range order in a lattice structure, although amorphous semiconductors have been investigated over several decades and widely used in diverse electronic devices. In this work, we develop a method to predict mobility of disordered systems by virtue of the first-principles calculation without using any empirical parameters. Quantum transport modeling based on the nonequilibrium Green's function formalism enables us to establish a formula to connect first-principles results with amorphous-phase mobility. Finally, the developed approach is quantitatively validated by comparing the theoretical predictions with previously measured mobilities of amorphous metal oxides $({\mathrm{SnO}}_{2},{\mathrm{In}}_{2}{\mathrm{O}}_{3}$, and ZnO) and amorphous silicon. Localization analysis provides further physical insight into a distinct feature between the amorphous metal oxides and amorphous silicon.

Combined first-principles and machine learning study of the initial growth of carbon nanomaterials on metal surfaces

To understand the effects of H of hydrocarbon precursors and alloying elements of catalysts on the initial growth of carbon nanomaterials, this work combined first-principles (FP) calculation and machine learning (ML) methods to study the adsorption of complete hydrocarbon molecules on various alloy surfaces. Specifically, we calculated the energetics and geometries of the adsorption of acetylene (C2H2) molecules on the pristine Ni, Co, and Fe metal and a benzene molecule on Ni-X [X = thirty 3d, 4d, and 5d transition metal (TM) elements] alloy surfaces. The results suggest that benzene has larger adsorption energies on Ni-X (X = Y, Cd, Sc, Lu, Ag, Ti, Hg, Pd, Zn, V, Cu, and Au) than that on the pristine Ni surface. Furthermore, the ML models were constructed using the surface “Center-Environment” (SCE) features to predict the FP results. The ML models suggest that the features critical to adsorption energies include the size, electronegativity, and valence of the doping element. We proposed an “add and dehydrogenate” mechanism underlying the initial growth of nanocarbon with hydrocarbon precursors. These results provide a theoretical basis in the design of alloy catalysts including in-plane single atom catalysts.

Enhancement of thermoelectric properties and first-principles calculations of Ag / Ti co-substituted Ca3Co4O9

Thermoelectric materials of Ca3Co4O9 with excellent comprehensive performance have stagnated the research of thermoelectric conversion devices due to their low thermoelectric conversion efficiency. Ca3-xAgxCo4-yTiyO9 was synthesized by double substitution at Ca and Co sites via a traditional solid-state method to change the microstructure and improve the thermoelectric properties in this paper. Ag and Ti co-substituted increase the porosity, reduce the concentration of Co4+ ions, and improve the Seebeck coefficient and power factor. The optimum substitute amount is x = 0.2, y = 0.2, and the Seebeck coefficient and power factor are 238.4 μV/K and 1481.9 μW/K2m. The first-principles calculation results show that the addition of Ti increases the covalent bond strength between Co–O atoms, and the addition of Ag narrows the gap, which is beneficial to the improvement of thermoelectric properties.

Edge stabilities, properties and growth kinetics of graphene-like two dimensional monolayers composed with Group 15 elements.

Graphene-like two dimensional (2D) monolayers composed of β-structured Group 15 (β-G15) elements have attracted great attention due to their intrinsic bandgaps, thermodynamic stabilities and high mobilities. Quite different from graphene, a buckle with amplitude ranging from 1.24 Å to 1.65 Å exists along the z direction in β-G15 films. To learn the growth behaviours and the relevant influence of such buckles, here, we performed a systematic study on the edge stabilities of monolayer films constructed with β-phase P, As, Sb and Bi, respectively. Our theoretical results show that, for free-standing films, the zigzag edge with dangling atoms is the most stable one for bare P, As and Sb and the pristine AC edge is the most stable one for Bi, while the pristine zigzag edge becomes the most stable one for all films if the edge is terminated with hydrogen atoms, both resulting in hexagonal flakes under equilibrium growth conditions. Buckles show no apparent influence on the edge stabilities in free-standing films while play a significant role in cases considering underlying metal substrates. Such an influence can be attributed to the charge transfer difference between the lower/upper β-G15 atoms and underlying substrates, which may eventually determine the growth mechanism and morphologies of 2D β-G15 films. Detailed growth kinetics and properties were also discussed based on the first-principles results. The understanding of these fundamental principles should provide useful information for guiding the synthesis of β-G15 films and other 2D materials.

First-principles study of excitons in optical spectra of silver chloride

Silver chloride is a material that has been investigated and used for many decades. Of particular interest are its optical properties, but only few fundamental theoretical studies exist. We present first-principles results for the optical properties of AgCl, obtained using time-dependent density functional theory and many-body perturbation theory. We show that optical properties exhibit strong excitonic effects, which are correctly captured only by solving the Bethe-Salpeter equation starting from quasiparticle self-consistent GW results. Numerical simulations are made feasible by using a model screening for the electron-hole interaction in a way that avoids the calculation of the static dielectric constant. A thorough analysis permits us to discuss localization in bright and dark excitons of silver chloride.

A Differentiable Neural-Network Force Field for Ionic Liquids.

We present NeuralIL, a model for the potential energy of an ionic liquid that accurately reproduces first-principles results with orders-of-magnitude savings in computational cost. Built on the basis of a multilayer perceptron and spherical Bessel descriptors of the atomic environments, NeuralIL is implemented in such a way as to be fully automatically differentiable. It can thus be trained on ab initio forces instead of just energies, to make the most out of the available data, and can efficiently predict arbitrary derivatives of the potential energy. Using ethylammonium nitrate as the test system, we obtain out-of-sample accuracies better than 2 meV atom–1 (<0.05 kcal mol–1) in the energies and 70 meV Å–1 in the forces. We show that encoding the element-specific density in the spherical Bessel descriptors is key to achieving this. Harnessing the information provided by the forces drastically reduces the amount of atomic configurations required to train a neural network force field based on atom-centered descriptors. We choose the Swish-1 activation function and discuss the role of this choice in keeping the neural network differentiable. Furthermore, the possibility of training on small data sets allows for an ensemble-learning approach to the detection of extrapolation. Finally, we find that a separate treatment of long-range interactions is not required to achieve a high-quality representation of the potential energy surface of these dense ionic systems.

Cr-Doped Fe1-xCrxF3·0.33H2O Nanomaterials as Cathode Materials for Sodium-Ion Batteries.

Due to the high theoretical specific capacity and low cost, FeF3·0.33H2O has become one of the potential choices of cathode materials for sodium-ion batteries. However, the poor intrinsic conductivity limits its practical applications. Herein, the atomic substitution is used to improve its intrinsic conductivity. The first-principles calculation results show that Cr3+ doping can reduce the band gap of FeF3·0.33H2O to improve its intrinsic conductivity. The discharge specific capacity of Fe0.95Cr0.05F3·0.33H2O with a narrowest band gap is 194.02 mA h/g at 0.1 C within the range of 1.4-4.0 V, which is higher than that of FeF3·0.33H2O (136.47 mA h/g). Using the electrochemical impedance spectroscopy and galvanostatic intermittent titration technique tests, it is found that Rct of Fe0.95Cr0.05F3·0.33H2O is reduced and DNa+ is almost unchanged, as compared to FeF3·0.33H2O.

Quantum Spin-Valley Hall Kink States: From Concept to Materials Design.

We propose a general and tunable platform to realize high-density arrays of quantum spin-valley Hall kink (QSVHK) states with spin-valley-momentum locking based on a two-dimensional hexagonal topological insulator. Through the analysis of Berry curvature and topological charge, the QSVHK states are found to be topologically protected by the valley-inversion and time-reversal symmetries. Remarkably, the conductance of QSVHK states remains quantized against both nonmagnetic short- and long-range and magnetic long-range disorder, verified by the Green-function calculations. Based on first-principles results and our fabricated samples, we show that QSVHK states, protected with a gap up to 287meV, can be realized in bismuthene by alloy engineering, surface functionalization, or electric field, supporting nonvolatile applications of spin-valley filters, valves, and waveguides even at room temperature.

Tunable topologically nontrivial states in newly discovered graphyne allotropes: from Dirac nodal grid to Dirac nodal loop

By means of quotient-graph associated crystal prediction method, a new graphyne allotrope with unique Dirac nodal grid state is reported in this work. It is named as 191-E24Y24-1 according to its hexagonal lattice (with P6/mmm symmetry, No. 191) containing 24 sp2-hybridized carbon atoms and 24 sp-hybridized ones. The first-principles results show that the total energy of 191-E24Y24-1 is more favorable than that of recent synthesized -graphdiyne and carbon ene-yne. It is also demonstrated to be dynamically, thermally, and mechanically stable. Interestingly, the 191-E24Y24-1 harbors intrinsic semimetal features showing intriguing hexagonal Dirac nodal grid state in the reciprocal space. Such unique electronic state is stable against small external tensile strains, and it is tunable under compression strains which will transform to new triangle Dirac nodal grid state. Moreover, a new metastable graphyne allotrope named 191-E12Y36-4 with Dirac nodal loop state is also observed in the process of stretching 191-E24Y24-1 with large tensile strains. The results presented in this work reveal two novel graphyne allotropes with exotic electronic properties. These discoveries are not only physical interesting, but also provide potential material candidates for carbon-based high performance electronic nanodevices.

Microstructures and mechanical properties of (MoNbTa)6TixVy (x + y = 4) multi-component alloys

In order to reveal the effect of homogenization treatment on the phase composition and chemical distribution of MoNbTa-based refractory multi-component alloys, the microstructures and mechanical properties of as-cast and homogenized (MoNbTa)6TixVy (x + y = 4) alloys were investigated by using X-ray diffraction, scanning electron microscopy and mechanics performance tests. Besides, the theoretical elastic properties and bonding behavior were analyzed by the first principle calculations. The results show that except for (MoNbTa)6Ti2.5V1.5 alloy, there are no obvious differences of crystal structures between other as-cast and homogenizing alloys. The lattice constants of (MoNbTa)6TixVy alloys increase with the increase of Ti content, while the lattice constants of alloys decrease after homogenization treatment. Typically (Mo, Ta)-rich dendritic and (Ti, V)-rich interdendritic morphologies are observed, and the atoms would diffuse during the homogenization process resulting in the uniform composition. The as-cast (MoNbTa)6Ti2V2 alloy exhibit the largest values of compressive strength, plasticity, and hardness in these alloys. Following homogenization, the mechanical properties of (MoNbTa)6TixVy alloys decrease significantly. The results of first principles reveal that the covalent bonding interactions between Ti and V atoms are stronger than that between Mo and Ta atoms in these MoNbTa-based refractory multi-component alloys.

Ultrathin Fe2P nanosheet co-catalyst CdS nanorod: The promising photocatalyst with ultrahigh photocatalytic H2 production activity

Developing 2D ultra-thin materials as co-catalysts is an effective way to boost the photocatalytic activity of semiconductors, but it remains challenging to unveil the design principle. In this work, we prepared the ultrathin 2D Fe2P nanosheets with an average thickness of ca. 1.41 nm via the facile solvothermal method. Such 2D Fe2P nanosheets were used as co-catalyst to fabricate a 2D-1D type of Fe2P-CdS photocatalysts for visible-light-driven photocatalytic H2 evolution. Experimental and first-principles results show that the Fe2P nanosheets co-catalyst with metallic characteristics modified CdS nanorods to form a 2D-1D heterointerface with strong ionic interaction, which induces the electron density redistribution in Fe2P-CdS composite, resulting in a lower recombination rate of carriers. Moreover, Fe2P nanosheets also reduce the overpotential for hydrogen evolution, provide abundant active sites and enhance H2O adsorption, significantly promoting the release of H2. As a result, the 11 wt% Fe2P-CdS exhibited an extremely high photocatalytic performance (207.82 mmol·h−1·g−1), which is more than 78 times higher than that of the pristine CdS. This work demonstrates the potential of 2D Fe2P as a promising co-catalyst in photocatalysis and provides a useful guide on phosphide-based material designing.

Microstructure and electronic structure of Cr2C and Fe2Y in the Cr-coating prepared by pack-cementation on the surface of ODS steel

In this study, a combination of experimental characterization and first-principles calculation was employed to systematically study the structure and electronic properties of Cr-coating prepared on the surface of oxide dispersion strengthened (ODS) steel by pack-cementation chromizing. X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) results show that the Cr-coating is mainly composed of Cr2C and Fe2Y phases. The first-principles calculation results show that the Cr atoms and C atoms in the Cr2C crystal are bonded by ionic bonds, while the Fe atoms and Y atoms in the Fe2Y crystal are mainly boned by polar covalent bonds. Compared with Fe2Y, Cr2C crystal has greater elastic anisotropy. The elastic constant of Cr2C crystal increases significantly with the increase of pressure. When the pressure reaches 2.4 GPa, Cr2C undergoes a transition from ductile to brittle. The elastic modulus of Fe2Y increases slowly with increasing pressure, and always appears as a ductile phase under the pressure value of 0–6 GPa, which is considered to have a good effect on the impact properties of the Cr-coating. The average work function of Cr2C crystal electrons is greater than that of Fe2Y crystal electrons, indicating that when the coating is in a corrosive medium, the Fe2Y phase will serve as the anode in the electrode first, and will be corroded before the Cr2C phase.

Polyetheretherketone/Zinc-aluminum 합금 복합체의 열전도율 및 접합본딩 특성에 관한 연구

In order to improve the thermal conductivity of polyetheretherketone (PEEK), the ZA8 zinc aluminum alloy powder was used as a thermal conductive filler. PEEK / ZA8 thermal composites were prepared by the molding process. The effects of surface modification and ZA8 contents on the thermal conductivity and mechanical properties of PEEK / ZA8 composites were investigated. Meanwhile, the first-principle calculation method based on density functional theory was used to study the surface adsorption characteristics and interface bonding mechanism of the PEEK / ZA8. Results show that the thermal conductivity of the composites is improved after ZA8 surface modification, but the mechanical properties of the composites decreased with the increase of ZA8 content. When the ZA8 content is 40%, the thermal conductivity of the composites is 0.298 Wm<SUP>-1</SUP>K<SUP>-1</SUP>, which is 72.25% higher than that of pure PEEK. On this basis, 5 wt% graphite is added, the thermal conductivity is 0.418 Wm<SUP>-1</SUP>K<SUP>-1</SUP>, which is 40.27% higher than that of PEEK / ZA8 without graphite, and 141.62% higher than that of pure PEEK. The first-principle calculation results show that the existence of hydroxyl group can change the adsorption properties of aluminate on Zn (001) surface. From physical adsorption to chemical adsorption, the bond strength between aluminate and Zn surface is greatly improved.

Uniform Magnesium Electrodeposition via Synergistic Coupling of Current Homogenization, Geometric Confinement, and Chemisorption Effect.

Unevenly distributed magnesium (Mg) electrodeposits have emerged as a major obstacle for Mg-metal batteries. A comprehensive design matrix is reported for 3D magnesiophilic hosts, which regulate the uniform Mg electrodeposition through a synergistic coupling of homogenizing current distribution, geometric confinement, and chemisorptive interaction. Vertically aligned nitrogen- and oxygen-doped carbon nanofiber arrays on carbon cloth (denoted as "VNCA@C") are developed as a proof of concept. The evenly arranged short nanoarray architecture helps to homogenize the surface current density and the microchannels built in this 3D host allow the preferential nucleation of Mg due to their geometrical confinement effect. Besides, the nitrogen-/oxygen-doped carbon species exhibit strong chemisorptive interaction toward Mg atoms, providing preferential nucleation sites as demonstrated by first-principle calculation results. Electrochemical analysis reveals a peculiar yet highly reversible microchannel-filling growthbehavior of Mg metals, which empowers the delicately designed VNCA@C host with the ability to deliver a reduced nucleation overpotential of 429mV at 10.0mA cm-2 and an elongated Mg plating/stripping cycle life (110 cycles) under high current density of 10.0 mA cm-2 . The proposed design matrix can be extended to other metal anodes (such as lithium and zinc) for high-energy-density batteries.

Dimensional crossover and band topology evolution in ultrathin semimetallic NiTe2 films

Nickel ditelluride (NiTe2), a recently discovered Type-II Dirac semimetal with topological Dirac fermions near the Fermi energy, is expected to exhibit strong thickness-mediated electronic tunability and intrinsic two-gap superconductivity in the single-layer limit. Realizing such intriguing phenomena requires the fabrication of ultrathin NiTe2 films and an understanding of the underlying physics that is still under debate. By conducting experimental band mappings of ultrathin films prepared with molecular beam epitaxy, we reveal spectroscopic evidence for the dimensionality crossover of single-crystalline ultrathin NiTe2 films as a function of film thickness. As the film thickness increases from one to five layers, the gap in the conical topological surface states closes. Comparisons of experimental to first-principles results also highlight difficulties in fabricating atomically smooth single-layer NiTe2 films. Our results not only provide further impetus for studying emergent phenomena in NiTe2 but also underscore the limitations of fabricating NiTe2 films for device applications.

Magnetoelastic coupling and effects of uniaxial strain in α−RuCl3 from first principles

We present first-principles results on the magnetoelastic coupling in $\alpha$-RuCl$_3$ and uncover a striking dependence of the magnetic coupling constants on strain effects. Different magnetic interactions are found to respond very unequally to variations in the lattice, with the Kitaev interaction being the most sensitive. Exact diagonalization results on our magnetoelastic model reproduce recent measurements of the structural Gr\"uneisen parameter and explain the origin of the negative magnetostriction of $\alpha$-RuCl$_3$, disentangling contributions related to different anisotropic interactions and g factors. Uniaxial strain perpendicular to the honeycomb planes is predicted to reorganize the relative coupling strengths, strongly enhancing the Kitaev interaction while simultaneously weakening the other anisotropic exchanges under compression. Uniaxial strain may therefore pose a fruitful route to experimentally tune $\alpha$-RuCl$_3$ nearer to the Kitaev limit.

Progress on wide-range equation of state for hydrogen and deuterium

Hydrogen is the most abundant element in nature and an important object of astrophysics and ICF research. This paper briefly presents an overview of the research progress in wide-range equation of state and especially comments assessment of the most recent shock compression experiments on Omega laser facility and the theoretical models. Based on the previous work, the wide-range equation of state of hydrogen is constructed by using the improved chemical free energy model, the first-principle numerical simulation results and the multi-parameter equation of state model, which is applicable in the temperature range of 20−108 K and the density range of 10−7−2000 g/cm3. Compared with experimental results, such as those of shock compression experiment, static high pressure isotherm experiment and sound velocity experiment, the newly-constructed wide-range equation of state for hydrogen has high confidence and provides high precision data for astrophysics, inertial confinement fusion, international thermonuclear experimental reactor and other engineering physics designs. The construction and validation method of the hydrogen wide-range equation of state can also be applied to its isotope deuterium. In comparison with current models published abroad, the deuterium wide-range equation of state constructed by this method is in better agreement with the experimental data of principal and secondary Hugoniot published in 2019. This paper also points out the temperature-density regimesthat need attention in future research.

Finite-volume energy spectrum of the K−K−K− system

The dynamics of multi-kaon systems are of relevance for several areas of nuclear physics. However, even the simplest systems, two and three kaons, are hard to prepare and study experimentally. Here we show how to extract this information using first-principle lattice QCD results. We (1) extend the relativistic three-body quantization condition to the strangeness sector, predicting for the first time the excited level finite-volume spectrum of three kaon systems at maximal isospin, and (2) present a first lattice QCD calculation of the excited levels of this system in a finite box. We compare our predictions with the lattice results reported here and with previous ground state calculations and find very good agreement.

Diverse Properties of Carbon-Substituted Silicenes

The theoretical framework, which is built from the first-principles results, is successfully developed for investigating emergent two-dimensional materials, as it is clearly illustrated by carbon substitution in silicene. By the delicate VASP calculations and analyses, the multi-orbital hybridizations are thoroughly identified from the optimal honeycomb lattices, the atom-dominated energy spectra, the spatial charge density distributions, and the atom and orbital-decomposed van Hove singularities, being very sensitive to the concentration and arrangements of guest atoms. All the binary two-dimensional silicon-carbon compounds belong to the finite- or zero-gap semiconductors, corresponding to the thoroughly/strongly/slightly modified Dirac-cone structures near the Fermi level. Additionally, there are frequent π and σ band crossings, but less anti-crossing behaviors. Apparently, our results indicate the well-defined π and σ bondings.

Phase competition in HfO2 with applied electric field from first principles

In this work, the results of first-principles density-functional-theory calculations are used to construct the energy landscapes of HfO$_2$ and its Y and Zr substituted derivatives as a function of symmetry-adapted lattice-mode amplitudes. These complex energy landscapes possess multiple local minima, corresponding to the tetragonal, oIII ($Pca2_1$), and oIV ($Pmn2_1$) phases. We find that the energy barrier between the non-polar tetragonal phase and the ferroelectric oIII phase can be lowered by Y and Zr substitution. In Hf$_{0.5}$Zr$_{0.5}$O$_2$ with an ordered cation arrangement, Zr substitution makes the oIV phase unstable, and it become an intermediate state in the tetragonal to oIII phase transition. Using these energy landscapes, we interpret the structural transformations and hysteresis loops computed for electric-field cycles with various choices of field direction. The implications of these results for interpreting experimental observations, such as the wake-up and split-up effects, are also discussed. These results and analysis deepen our understanding of the origin of ferroelectricity and field cycling behaviors in HfO$_2$-based films, and allow us to propose strategies for improving their functional properties.