First-principles Simulations Research Articles

New Insights on the Tensile Strength and Fracture Mechanism of c-ZrO2/α-Al2O3 Interfaces

The tensile strength and fracture properties of the c-ZrO2(001)/α-Al2O3(11¯02) interfaces were investigated by first-principle tensile simulations. Models with different stacking sequences of c-ZrO2(001) were examined. The theoretical tensile strength and work of adhesion were present. It was found that the adhesive strength of the interface was strongly influenced by the termination of c-ZrO2(001), and the c-ZrO2(001)/α-Al2O3(11¯02) interfaces adhered weakly. Then, variations of the atomic bonds were observed to clarify the fracture characteristics of the interfaces. Our study indicates that the fracture modes of the O- and Zr-model tend to be ductile fractures, while the fracture mode of the 2O-model is a brittle fracture. Furthermore, all three models were completely separated along the intermediate layer between the initial ZrO2 and Al2O3 slabs. Finally, we compared our results with those available in the published literature, and the potential application of the first-principle results will be further discussed.

First-principles simulations of liquid iron-heavy element alloys at high pressure

Iron-rich metallic liquid in the Earth's outer core is thought to contain many elements. While the incorporation of light elements in liquid iron has been widely studied, liquid iron-heavy element mixtures, particularly considering Co, Mo, and W which are relevant in constraining core-forming conditions need more studies. Here we investigate the thermodynamic and structural behavior of these siderophile elements and Ni dissolved in liquid iron in the amount of 2.67 atom% using first-principles molecular dynamics over the pressure range of the entire Earth's interior at temperatures 4000 to 7000 K. The calculated pressure-volume-temperature results of these iron-rich alloys are accurately described by adding the appropriative terms to the equation of state of pure iron liquid to account for the effects of temperature and impurity. The calculated mean iron coordination number of Mo and W is somewhat larger than that of Ni and Co and host iron atoms thus implying substitutional incorporation mechanism. The lack of clustering activity among the impurity atoms means that each of these impurity elements is soluble in liquid iron. Our analysis also shows that Mo and W increase the liquid density much more than Ni and Co mainly for mass reason while the bulk modulus remains essentially unaffected in all cases. The presence of heavy elements widens the apparent density gap between iron-rich liquid and the outer core so additional amounts of lighter impurity elements should be considered when constraining the core composition.

Accuracy of ghost rotationally invariant slave-boson and dynamical mean field theory as a function of the impurity-model bath size

We compare the accuracy of the ghost rotationally invariant slave-boson (g-RISB) theory and dynamical mean field theory (DMFT) on the single-band Hubbard model, as a function of the number of bath sites in the embedding impurity Hamiltonian. Our benchmark calculations confirm that the accuracy of g-RISB can be systematically improved by increasing the number of bath sites, similar to DMFT. With a few bath sites, we observe that g-RISB is systematically more accurate than DMFT for the ground-state observables. On the other hand, the relative accuracy of these methods is generally comparable for the quasiparticle weight and the spectral function. As expected, we observe that g-RISB satisfies the variational principle in infinite dimensions, as the total energy decreases monotonically towards the exact value as a function of the number of bath sites, suggesting that the g-RISB wave function may approach the exact ground state in infinite dimensions. Our results suggest that the g-RISB is a promising method for first-principles simulations of strongly correlated matter, which can capture the behavior of both static and dynamical observables, at a relatively low computational cost.

Time-dependent multiconfiguration self-consistent-field and time-dependent optimized coupled-cluster methods for intense laser-driven multielectron dynamics

We reviewed the time-dependent multiconfiguration self-consistent-field (TD-MCSCF) method and the time-dependent optimized coupled-cluster (TD-OCC) method for first-principles simulations of high-field phenomena such as tunneling ionization and high-order harmonic generation in atoms and molecules irradiated by a strong laser field. These methods provide a flexible and systematically improvable description of the multielectron dynamics by expressing the all-electron wavefunction by configuration interaction expansion or coupled-cluster expansion, using time-dependent one-electron orbital functions. The time-dependent variational principle plays a key role in deriving these methods while satisfying gauge invariance and Ehrenfest theorem. The real-time/real-space implementation with an absorbing boundary condition enables the simulation of high-field processes involving multiple excitation and ionization. We present a detailed, comprehensive discussion of such features of the TD-MCSCF and TD-OCC methods.

First-principles study of size effects on electrical properties of AlN/GaN heterostructured nanofilms

GaN-based heterostructures and their electrical properties play a crucial role in micro- and nanoelectronic device applications. In this work, the size-dependent electrical properties of wurtzite AlN/GaN heterostructured nanofilms are investigated by using the first-principles simulations. The electronic structures of 1.4 nm/1.4 nm, 1.8 nm/1.8 nm, 2.2 nm/2.2 nm, 2.6 nm/2.6 nm AlN/GaN heterostructured nanofilms are systemically analyzed. The simulations demonstrated that the surface and interface enable to affect the electron density of states of AlN/GaN nanofilms, and the potential well at the valence band of the AlN intermediate layer increases with increasing nanofilm thickness. The electrical conductivity of AlN/GaN nanofilms increases significantly with increasing the thickness from 2.8 nm to 4.4 nm, and the conductivity values stabilize from 4.4 nm to 5.2 nm. Therefore, it can be concluded that the size dependence of the conductivity becomes weaker with increasing the nanofilm thickness. The results of this work could be useful for realizing the tuning of electrical properties of AlN/GaN heterostructured nanofilms through the size effects and surface/interface engineering.

Water Oxidation-Induced Surface Reconstruction and Dissolution at the RuO2(110) Surface Revealed by First-Principles Simulation

The lack of atomic-scale understanding of the dissolution of RuO2 electrocatalysts for water splitting has hindered the development of durable RuO2-based electrocatalysts. In this study, using the first-principles electrochemical nudged elastic band method, we have discovered a facile water oxidation-induced Ru reconstruction and dissolution mechanism at the RuO2(110) surface. The potential-dependent activation energies of the dissolution process are well below 1.0 eV at electrode potentials higher than 1.5 V vs SHE. Furthermore, the Ru atoms in the close neighborhood of a Ru vacancy are found to be less prone to dissolution due to their higher oxidation state that could reduce the driving force for water oxidation-induced Ru reconstruction. These findings not only deepen the atomic-scale understandings of the Ru dissolution process but also provide routes for improving the stability of RuO2-based water splitting electrocatalysts.

Superior Effect of Si Doping on Interfacial Reaction of LiPON Solid Electrolyte for the Silicon-Rich Oxide (SiO1/2) Anode in All-Solid-State Batteries: A First-Principles Prediction

Silicon oxide is considered an alternative silicon anode for lithium-ion batteries, possessing high lithium capacity and excellent cycling performance. However, the application of silicon oxide in a solid-state lithium battery is still not reported; exploring the atomic-scale mechanism at its interface with the solid-state electrolyte is critical for further study. In this work, atomic-scale electrochemistry of Si-, B-, and C-doped LiPON solid electrolytes for the silicon-rich oxide (SiO1/2) electrode is explored by first-principles simulations. Our calculations reveal that the interfacial stability and conductivity are significantly enhanced upon doping of Si in LiPON. The SiO1/2/LiSiPON interface presents the highest adhesion energy and the lowest interface formation energy, suggesting a superior stability of the interface. An obvious shift of the DOS curve and large charge overlap area can be observed for LiSiPON with incorporation of the SiO1/2 layer, which shows a much smaller band gap compared with primitive LiSiPON. Moreover, lithium tends to diffuse along the LiSiPON/SiO1/2 interface, and the doped elements provide a channel for lithium transport due to mutual electrostatic interaction. Our work provides theoretical guidance for designing electrode/SSE interfaces for silicon oxide-based ASSBs.

On the collisional damping of plasma velocity space instabilities

For plasma velocity space instabilities driven by particle distributions significantly deviated from a Maxwellian, weak collisions can damp the instabilities by an amount that is significantly beyond the collisional rate itself. This is attributed to the dual role of collisions that tend to relax the plasma distribution toward a Maxwellian and to suppress the linearly perturbed distribution function. The former effect can dominate in cases where the unstable non-Maxwellian distribution is driven by collisionless transport on a timescale much shorter than that of collisions, and the growth rate of the ideal instability has a sensitive dependence on the distribution function. The whistler instability driven by electrostatically trapped electrons is used as an example to elucidate such a strong collisional damping effect of plasma velocity space instabilities, which is confirmed by first-principles kinetic simulations.

Extreme Anisotropic Dispersion and One-Dimensional Confined Electrons in 2-D SiP₂ FETs With High Transmission Coefficients

2-D semiconductors are promising Si alternatives for channels in next-generation electronic devices. Considering the 2-D SiP2 with unique dispersion at the band edge and intrinsic P–P chains with 1-D confined electrons, we provide a comprehensive investigation on the electronic and ballistic transport properties of SiP2. Based on first-principle and ballistic quantum transport simulations, the 2-D SiP2 exhibits a strong anisotropy in both geometric structures and electronic properties, playing a critical role in the anisotropy of transmission path and device performance for SiP2 field-effect transistors (FETs). Considering the ultrahigh saturation current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{\text {sat}}$ </tex-math></inline-formula> ) of SiP2 FETs in along-chain direction, the transfer characteristic with different gate lengths (3–12 nm) is calculated to thoroughly evaluate the performance. As a result, 5–12 nm SiP2 n-FETs with underlap (UL) configuration can fulfill the International Technology Roadmap for Semiconductors (ITRS) and International Roadmap for Devices and Systems (IRDS) goals, showing an ultrahigh ON-state current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3420 ~{\mu } \text{A}/ {\mu } \text{m}$ </tex-math></inline-formula> with 10 nm gate length. In addition, the figures of merits, such as delay time and power dissipation, are comprehensively assessed, proving the fast-switching and low-energy consumption ability of SiP2 FETs. Hence, our study demonstrates the great potential of 2-D SiP2 for future competitive electronic devices.

Origin of superconductivity in hole doped SrBiO3 bismuth oxide perovskite from parameter-free first-principles simulations

The recent discovery of nickel oxide superconductors have highlighted the importance of first-principles simulations for understanding the formation of the bound electrons at the core of superconductivity. Nevertheless, superconductivity in oxides is often ascribed to strong electronic correlation effects that density functional theory (DFT) cannot properly take into account, thereby disqualifying this technique. Being isostructural to nickel oxides, Sr1-xKxBiO3 superconductors form an ideal testbed for unveiling the lowest theory level needed to model complex superconductors and the underlying pairing mechanism yielding superconductivity. Here I show that parameter-free DFT simulations capture all the experimental features and related quantities of Sr1-xKxBiO3 superconductors, encompassing the prediction of an insulating to metal phase transition upon increasing the K doping content and of an electron-phonon coupling constant of 1.22 in sharp agreement with the experimental value of 1.3 ± 0.2. The proximity of a disproportionated phase is further demonstrated to be a prerequisite for superconductivity in bismuthates.

Ti-6Al-4V to over 1.2 TPa: Shock Hugoniot experiments, ab initio calculations, and a broad-range multiphase equation of state

Titanium alloys are used in a large array of applications. In this work we focus our attention on the most used alloy, Ti-6Al-4V (Ti64), which has excellent mechanical and biocompatibility properties with applications in aerospace, defense, biomedical, and other fields. Here we present high-fidelity experimental shock compression data measured on Sandia's Z machine. We extend the principal shock Hugoniot for Ti64 to more than threefold compression, up to over 1.2 TPa. We use the data to validate our ab initio molecular dynamics simulations and to develop a highly reliable, multiphase equation of state (EOS) for Ti64, spanning a broad range of temperature and pressures. The first-principles simulations show very good agreement with Z data and with previous three-stage gas gun data from Sandia's STAR facility. The resulting principal Hugoniot and the broad-range EOS and phase diagram up to 10 TPa and ${10}^{5}$ K are suitable for use in shock experiments and in hydrodynamic simulations. The high-precision experimental results and high-fidelity simulations demonstrate that the Hugoniot of the Ti64 alloy is stiffer than that of pure Ti and reveal that Ti64 melts on the Hugoniot at a significantly lower pressure and temperature than previously modeled.

Performance evaluation and assessment of the corrosion inhibition mechanism of carbon steel in HCl medium by a new hydrazone compound: Insights from experimental, DFT and first-principles DFT simulations

In the present work, a new hydrazone compound, namely N'-[(Z)-(4-chlorophenyl)methylidene]-2-(5-methoxy-2-methyl-1H-indol-3-yl)acetohydrazide, noted HTH, was selected to protect carbon steel against corrosion in 1.0 mol/L HCl. Different chemical, electrochemical, and surface characterization techniques such as scanning electron microscope coupled with X-ray energy dispersion (SEM/EDX) were used to investigate the corrosion inhibition performance. Electrochemical data showed that the effectiveness of the inhibitor improved with increasing concentration, reaching 98% at the optimal concentration of 10-3 mol/L. The results of potentiodynamic polarization measurements showed that hydrazone acted as a mixed-type inhibitor. The EIS results showed an increase in polarization resistance accompanied by a noticeable decrease in Ceff,dl values. In the temperature range of 303 K-333 K, hydrazone protected carbon steel by 89%, showing high resistance to temperature effect. The analysis of the steel surface by SEM/EDX confirmed that the effectiveness of the hydrazone was attributed to the formation of a protective layer on the surface of the metal. Quantum chemical calculations revealed insights into the chemical reactivity of the tested hydrazone while first-principles density functional theory (DFT) and molecular dynamics (MD) simulation supported the experimental conclusions and showed outstanding adsorption ability of HTH on the Fe(110) surface. First-principles DFT simulations showed that the HTH molecule was more stable in a parallel adsorption mode.

Understanding ZnO surface defects from first-principles simulation

The single native defects on ZnO surfaces have been experimentally found to play an essential role in different applications of ZnO nanostructures. In this work, by means of first-principles density functional theory combined with atomistic thermodynamics and the nudged-elastic band method, we investigated the electronic properties, stability under the temperatures and pressures, and migration mechanism of the six different point defects on the non-polar (101̄0) and (112̄0) ZnO surfaces. Our results elucidate the defective energy states formed within the band gap of the ZnO surfaces, their orbital properties, and the defect-induced local surface magnetic moments. The former quantity is shown to be in good agreement with the available experimental photoluminescence measurements, reflecting this work’s highly accurate theoretical method. The thermodynamic phase diagrams of the pristine surfaces against the formation of the defects demonstrate that under the thermodynamic equilibrium condition with the oxygen reservoir, the most stable surface structures are the free-defect structures, consistent with the scanning tunneling microscopy measurement, while the oxygen-vacancy defect surfaces are only stable at the lower oxygen chemical potentials. Furthermore, we simulate the energy barriers of the different defect migration mechanisms, which describe how the defects are suppressed to form pristine surfaces.

Local structural investigation of non-crystalline materials at high pressure: the case of GeO2 glass

Local structures play a crucial role in the structural polyamorphism and novel electronic properties of amorphous materials, but their accurate measurement at high pressure remains a formidable challenge. In this article, we use the local structure of network-forming GeO2 glass as an example, to present our recent approaches and advances in high-energy x-ray diffraction, high-pressure x-ray absorption fine structure, and ab initio first-principles density functional theory calculations and simulations. Although GeO2 glass is one of the best studied materials in the field of high pressure research due to its importance in glass theory and geophysical significance, there are still some long-standing puzzles, such as the existence of appreciable distinct fivefold [5]Ge coordination at low pressure and the sixfold-plus [6+]Ge coordination at ultrahigh pressure. Our work sheds light on the origin of pressure-induced polyamorphism of GeO2 glass, and the [5]Ge polyhedral units may be the dominant species in the densification mechanism of network-forming glasses from tetrahedral to octahedral amorphous structures.

Theoretical Study on Zigzag Boron Nitride Nanowires.

In this work, two kinds of BN-nanowires (BNnws): a-BNnw and d-BNnw, respectively composed of azo (N-N) and diboron (B-B) bonds, are proposed with the aid of the first-principles simulations. Their structural stabilities are carefully verified from the energetics, lattice dynamics, and thermodynamic perspectives. Similar to the other common boron nitride polymorph, the a-BNnw and d-BNnw are semiconductors with relatively wide band gaps of 3.256 and 4.631 eV at the HSE06 level, respectively. The corresponding projected DOS patterns point out that their band edges are composed of different atomic species, which can help with the separation of their excitons. The band gaps can be manipulated monotonically by axial strains within the elastic ranges. The major charge carriers are electron holes. Significantly, a-BNnw possesses very high carrier mobilities around 0.44×104 cm2 V-1 s-1 .

Tuning magnetism by electric field in MnPS3/Sc2CO2 van der Waals heterostructure

Combining a two-dimensional (2D) antiferromagnetic (AFM) material, MnPS3 and a 2D ferroelectric material, Sc2CO2, we propose 2D van der Waals (vdW) heterostructure multiferroics to realize strong magnetoelectric coupling, which is important for designing high-performance magnetoelectric devices. By using first-principles simulations, it is found that the transition from an AFM state to a ferromagnetic (FM) state of a MnPS3 layer could be realized by reversing the polarization direction of a Sc2CO2 layer. We further reveal that such strong magnetoelectric effects originate from the large inter-layer charge transfer due to the competitive interaction between the difference of the interface work functions between MnPS3 and Sc2CO2 and the strong electronegativity of the O atom interface in the Sc2CO2 layer. Our results suggest a feasible scheme for constructing 2D vdW heterostructure multiferroics with very strong inter-layer magnetoelectric coupling effect.

Cooperative diffusion in body-centered cubic iron in Earth and super-Earths’ inner core conditions

The physical chemistry of iron at the inner-core conditions is key to understanding the evolution and habitability of Earth and super-Earth planets. Based on full first-principles simulations, we report cooperative diffusion along the longitudinally fast directions of body-centered cubic (bcc) iron in temperature ranges of up to 2000–4000 K below melting and pressures of ∼300–4000 GPa. The diffusion is due to the low energy barrier in the corresponding direction and is accompanied by mechanical and dynamical stability, as well as strong elastic anisotropy of bcc iron. These findings provide a possible explanation for seismological signatures of the Earth’s inner core, particularly the positive correlation between P wave velocity and attenuation. The diffusion can also change the detailed mechanism of core convection by increasing the diffusivity and electrical conductivity and lowering the viscosity. The results need to be considered in future geophysical and planetary models and should motivate future studies of materials under extreme conditions.

Molecular hydrogen storage in binary H2-CH4 clathrate hydrates

Clathrate hydrate has emerged as a promising candidate for hydrogen storage, but the major challenge is the high pressure required for its formation. The addition of natural gas can significantly reduce the formation pressure, but also can improve the energy density. In this work, we estimated the hydrogen storage capacity of the H2-CH4 binary hydrate by performing the first-principles calculations and simulations. The thermodynamical and mechanical stability of the hydrate was studied as a function of the cage occupancy of both 512 and 51264 cages. For the most stable structure, the molar ratio of H2 and H2O is 0.353:1. The H2-CH4 binary hydrate can maintain its structure during dynamics under the moderate temperature and pressure. The self-preservation effect was observed at ∼270 K, which can be used for the hydrogen storage and transport. These findings provide a better understanding of the mixed hydrate as a viable hydrogen storage technology, which could enable us to achieve a sustainable hydrogen economy.

Li-phobicity of polyvinyl alcohol for the control of Li electrodeposition in Li-secondary batteries

As various types of new electrode designs for Li-secondary batteries are researched, development of technologies to control the electrodeposition of Li is required. Herein, the Li affinity of polyvinyl alcohol (PVA) is investigated to examine its applicability as a coating material for the control of Li plating position in battery anodes, via first-principle simulation and experimental studies. The simulation indicates that the oxygen ions in the PVA provide favorable adsorption sites for Li at the initial stage of Li deposition, but the sites are readily passivated because Li is combined with multiple O ions. Therefore, the binding force for the subsequent Li becomes weak and the PVA surface generally exhibits Li-phobic characteristics. The experiment of a patterned PVA coating on the Li-ion battery anode demonstrates the anti-Li-plating function of the PVA. From the investigation results, it is concluded that PVA is Li-phobic, and can be utilized for guiding the Li plating in the future design of Li-secondary battery anodes.

Energy Storage Mechanism of C12-3-3 with High-Capacity and High-Rate Performance for Li/Mg Batteries.

The low specific capacity and Mg non-affinity of graphite limit the energy density of ion rechargeable batteries. Here, we first identify that the monolayer C12-3-3 in sp2-sp3 carbon hybridization with high Li/Mg affinity is an appropriate anode material for Li-ion batteries and Mg-ion batteries via the first-principles simulations. The monolayer C12-3-3 can achieve high specific capacities of 1181 mAh/g for Li and 739 mAh/g for Mg, higher than those of most previous anodes. The Li storage reaction is an "adsorption-conversion-intercalation mechanism", while the Mg storage reaction is an "adsorption mechanism". The 2D carbon material of C12-3-3 displays fast diffusion kinetics with low diffusion barriers of 0.41 eV for Li and 0.21 eV for Mg. As a new carbon-based anode material, the monolayer C12-3-3 will promote the practical application of batteries with high-capacity and high-rate performance.