Collection of Computational Materials(202312)
In a high vacuum environment, some organic molecular pollutants such as hydrocarbon and siloxane are released by spacecraft materials and deposited on the surface of the sensitive parts of spacecraft devices, which has become an important adverse factor restricting the development of long-life and high-performance spacecraft. Zeolite molecular adsorber coating can effectively collect spatial contaminations in real time, but the adsorption mechanism is not clear. To deeply analyze the adsorption mechanism of zeolite on the spatial contaminations, the adsorption behaviors of NaY zeolite including adsorption isotherms, adsorption heat curves and density distributions on three typical contaminations, toluene(C7H8), dimethyl phthalate (C10H10O4), octamethyl cyclotetrasiloxane (C8H24O4Si4), were calculated by the Grand Canonical Monte Carlo method in this work. The NaY zeolites and pollutant models were successfully constructed, and the rationality of the models was verified by comparing simulated data with experimental ones. These results indicated that all three classic molecules can be adsorbed by NaY zeolite in the ultra-high vacuum condition. The saturated adsorption capacity decreases in the order of C7H8>C10H10O4>C8H24O4Si4, which is significantly related to the molecule sizes and structures of contaminations. The saturated adsorption amount of C8H24O4Si4 is relatively low (8 per cell) when that of C7H8 is 36 per cell. In addition, the density distributions indicates that different contaminations are preferentially adsorbed inside the super-cage of NaY zeolite. Overall, this work analyzes the adsorption mechanism of NaY zeolite on typical contaminations, and can provide basic insights for the development of zeolite molecular adsorber coating with high adsorption capacity.
Due to the characteristics of small samples, high dimensions, and much noise, materials data often produce inconsistent results with those obtained from domain experts when used for machine learning modeling. For the whole process of machine learning, developing machine learning models embedding materials domain knowledge is a solution to this problem. The accuracy of materials data directly affects the reliability of data-driven materials performance prediction. Here, a data accuracy detection method incorporating materials domain knowledge is proposed by focusing on the data preprocessing stage in the machine learning application process. Firstly, a materials domain knowledge database is constructed based on the knowledge from materials experts. Secondly, it is coordinated with the data-driven data accuracy detection method to perform single-dimensional data accuracy detection based on the rule for value of descriptors, multi-dimensional data correlation detection based on the rule for correlation of descriptors, and full-dimensional data reliable detection based on multi-dimensional similar sample identification strategy from both data and domain knowledge perspectives. For the anomalous data identified at each stage, they are corrected by incorporating the materials domain knowledge. Furthermore, domain knowledge is incorporated into the whole process of the data accuracy detection method to ensure high accuracy of the dataset from the initial stage. Finally, experiments on the NASICON-type solid electrolyte activation energy prediction dataset demonstrate that this method can effectively identify anomalous data and make reasonable corrections. Compared with the original dataset, the prediction accuracy of all six machine learning models based on the revised dataset is improved to different degrees, among which R2 achieves a 33% improvement on the optimal model.
Density functional theory calculations play an important role in the study of defects in halide perovskites. Although the traditional semi-local functionals (such as PBE) can obtain the band gaps close to the experiments, they fail to accurately describe the positions of the band edges. Utilizing more accurate hybrid functionals combined with the spin-orbit coupling (SOC) effect with full structure relaxation is considered to be necessary for the prediction of defect properties. There are two types of hybrid functionals in the literature, namely the screened HSE and the unscreened PBE0. In this study, taking the orthorhombic phase CsPbI3 as an example, these methods were compared for the calculation of defect properties. The results show that there is no obvious difference between two methods for bulk properties, but qualitative differences appear for the defect properties. Most of the shallow-level defects predicted in the HSE calculations become deep-level defects in the PBE0 calculations. Meanwhile, there are qualitative differences between the defect transition levels and the Kohn-Sham levels. The origin of above differences lies in the fact that the Hartree-Fock exchange potential has long-range interaction. Therefore, in unscreened hybrid functionals, such as PBE0, it is more difficult to obtain convergent results with a manageable supercell size. In contrast, HSE exhibits a screening effect on the Hartree-Fock exchange potential and can obtain accurate defect energy levels using relatively small supercell sizes. Therefore, all results here demonstrate that the HSE hybrid functional owns a significant advantage in dealing with this problem even though a large Hartree-Fock mixing parameter (about 0.43) is needed.
MAX/MAB phases are a series of non-van der Waals ternary layered ceramic materials with a hexagonal structure, rich in elemental composition and crystal structure, and embody physical properties of both ceramics and metals. They exhibit great potential for applications in extreme environments such as high temperature, strong corrosion, and irradiation. In recent years, two-dimensional (2D) materials derived from the MAX/MAB phase (MXene and MBene) have attracted enormous interest in the fields of materials physics and materials chemistry and become a new 2D van der Waals material after graphene and transition metal dichalcogenides. Therefore, structural modulation of MAX/MAB phase materials is essential for understanding the intrinsic properties of this broad class of layered ceramics and for investigating the functional properties of their derived structures. In this paper, we summarize new developments in MAX/MAB phases in recent years in terms of structural modulation, theoretical calculation, and fundamental application research and provide an outlook on the key challenges and prospects for the future development of these layered materials.
Ultraviolet (UV) nonlinear optical (NLO) crystals play an irreplaceable role as the key materials to realize the frequency conversion for all solid state lasers. To date, it is still difficult to design UV NLO crystals with large second harmonic generation (SHG) coefficients, moderate birefringences and wide band gaps. Benefiting from the large band gap, sulfate has become an important research direction of UV NLO crystals. However, since SO4 is isotropic tetrahedral building units with nearly nonpolar Td symmetry, it exhibits small microscopic second order polarizability and polarizability anisotropy, which tends to result in a weak SHG effect and small birefringence. In this work, we introduced Hg2+ ions that are easy to form distorted polyhedrons into the sulfates, resulting in a new NLO material, Rb3Hg2(SO4)3Cl. It crystallizes in a monoclinic space group (P21) with the lattice parameters a=0.78653(2) nm, b=0.97901(2) nm, c=1.00104(3) nm, and β=110.95(3) (Z=2). Structure of Rb3Hg2(SO4)3Cl consists of [SO4] tetrahedra, [HgO5] and [HgO4Cl] polyhedral, which connected by a common corner to form a spatial 3D network. All the Rb atoms reside in the cavity of 3D network. The powder SHG measurement proposed by Kurtz and Perry indicates that Rb3Hg2(SO4)3Cl is a phase-matchable material in the visible region and exhibits a moderate SHG response about 1.5 times that of KH2PO4 (KDP). In addition, the UV-Vis-NIR diffuse reflectance spectral measurement indicates that Rb3Hg2(SO4)3Cl has a short UV cut-off edge of 251 nm, corresponding to the band gap of 4.94 eV. Its polarizing microscope measurement reveals that Rb3Hg2(SO4)3Cl has a moderate birefringence (The birefringence of Rb3Hg2(SO4)3Cl crystal at 546.1 nm is 0.04). Moreover, first-principles calculations uncover that the distorted [HgO5], [HgO4Cl] and [SO4] polyhedral are responsible for its SHG effect. Our study shows that Rb3Hg2(SO4)3Cl may have potential applications as a UV NLO crystal.
Wide band gap γ-CuI is a p-type transparent semiconductor with excellent optoelectronic and thermoelectric property, which has recently attracted worldwide attention. However, as an emerging material, its luminescence mechanism that is impacted by defects is rarely reported and remains obscure, limiting its further applications. In this work, Cl-doped CuI film was prepared by gas-phase reaction method. Using cathodoluminescence spectroscopy, effects of Cl doping on the surface morphology and cathodoluminescence property of CuI films were investigated in detail, and main defects of Cl presence in CuI films were explored by combining first-principle calculations, revealing relationship between structure and luminescent property of Cl-doped CuI films. These data showed Cl-doped region had a smoother surface than that of the undoped region with granular morphology, which clearly demonstrated that Cl dopant altered surface structure of the undoped region. Compared with the undoped region, the Cl dopant induced doubled fluorescence signal of band-edge emission at 410 nm, but reduced the defect peak at 720 nm, indicating that a small amount of Cl dopant brought a great luminescent improvement to CuI. The formation energy calculations of various crystal defects suggest that Cl can inhibit the formation of deep-level defects such as I vacancy in CuI and reduce the probability of non-radiative transition of excitons, which is consistent with the cathodoluminescence results. The full width at half maximum of the band-edge luminescence peak of Cl-doped CuI film is as small as 7 nm, showing extremely high luminescence monochromaticity. Therefore, the present findings deepen our understanding on how halogen doping boosts the luminescence performance of CuI-based materials.
High energy particle bombardment of silicon carbide can lead to the accumulation of defects and lattice disorder, which can negatively affect physical property and reduce lifetime of SiC devices. Thus, it is essential to systematically study the damage of SiC in different radiation environment. Herein, 6H-SiC was irradiated by neutrons at the fluence of 5.74×1018, 1.74×1019, 2.58×1020 and 1.27×1021 n/cm2, and then annealed. Changes in lattice parameters from post-irradiation isochronal annealing for 30 min in the range of 500-1650 ℃ were measured using X-ray single crystal diffraction. The results showed that the lattice swelling and recovery behavior were isotropic. Based on the swelling data, it was concluded that the neutron irradiation-induced defects in 6H-SiC were primarity point defects. Both intrinsic and irradiation defects can introduce defect energy levels, which were mainly caused by vacancies and led to the absorption band edge redshift and band gap narrowing of SiC. The defect energy levels of these vacancies and vacancy-associated defects were determined by absorption spectra, luminescence spectra and Raman spectra. Experiments and first principles calculation showed that the silicon vacancies introduced defect levels above the valence band, while the carbon vacancies introduced levels below the conduction band. The infrared absorption at 1382 nm and 1685 nm and the emission at 550 nm of unirradiated 6H-SiC were mainly due to the intrinsic carbon vacancies. The luminescence of post-irradiated SiC at 415, 440 and 470 nm was mainly due to the silicon vacancy produced by irradiation and its related defect configuration. All above data revealed the luminescence mechanism of SiC based on the charge state and the defect energy level distribution.
Large-sized crystalline materials are the basic raw materials in semiconductors, lasers, and communications. Preparation of large-scale, high-quality crystalline materials has become a bottleneck restricting the development of related industries. Breaking through the preparation theory and technology of large-sized crystal materials is the key to obtaining high-quality large-sized crystals. Preparation process of crystal materials often undergoes nucleation and growth stages, including multiple processes at spatiotemporal scale: from atom/molecules, through clusters and nuclei, to bulk crystals. To further explore and accurately understand the crystal growth mechanism, we need intensively study the multiscale process,multi-scale in situ characterization techniques, and computational simulation methods. Among them, the latest in situ characterization methods for crystal growth includes optical microscopy, electron microscopy, vibration spectra, synchrotron radiation, neutron technology, and especially, machine learning method. Thus, the multi-scale computational simulation techniques for crystallization is introduced, for example, first principles calculation at atom/molecular scale, molecular dynamics simulation, Monte Carlo simulation, phase field simulation at mesoscopic scale, and finite element simulation at macroscopic scale. A single in situ characterization or simulation technique can only explore crystallization information over a specific time and space scale. To accurately and fully reflect the crystallization process, a combination of multi-scale methods is introduced. It can be speculated that the establishment of in situ characterization technology and computational simulation methods for the actual large-sized crystal growth environment will be the future development trend, which provides an important experimental and theoretical basis for developing crystallization theory and controlling crystal quality. Furthermore, it can be deduced that the combination of in situ characterization technology with machine learning and big data technology will be the trend for large-sized crystal growth.
In this study, the first-principles method was used to predict the vacancy ordered structures of ternary Hf-Ta-C system and the effect of vacancy on its mechanical properties. Crystal structure of (Hf, Ta)C1-x under ambient pressure were predicted by first-principles evolutionary using USPEX software. This calculation found 5 stable and 3 metastable vacancy ordered structures which all share the rock-salt structure. Then, mechanical properties of (Hf, Ta)C1-x vacancy ordered structures were calculated by the first-principles method, and change of mechanical properties with the concentration of vacancy was analyzed. They all showed high bulk modulus, shear modulus, elastic modulus, and Vickers hardness. Their moduli and hardness decreased with the increase of the concentration of vacancy at the same Hf/Ta ratio. Finally, their electronic density of states are calculated, revealing that their chemical bonding is a mixture of strong covalence and weak metallic. Data from this study are promising for understanding vacancy ordered structures, mechanical properties and applications of Hf-Ta-C system.
Two-dimensional (2D) monolayer MoSi2N4 has attracted wide attention due to its excellent carrier transport capacity and chemical stability. However, the relationship between its photoelectric properties and applied plane strain has not been thoroughly explored. The effect of plane strain on band structures and photoelectric properties of 2D monolayer MoSi2N4 is revealed by the plane-wave ultrasoft pseudopotentials. The results show that the monolayer MoSi2N4 is an indirect band gap semiconductor. Its top of valance band is dominated by Mo4d orbitals and partly contributed by N2p orbitals, while its bottom of conduction band is mainly contributed by Mo4d orbitals. Under tensile strain, band gap of monolayer MoSi2N4 narrows gradually and effective mass of photogenerated carriers decreases continuously. Under compressive strain, the band gap widens gradually and the effective mass increases slowly. It is worth noting that a compressive strain (ε=-2.8%) results in transition form indirect to direct band gap. Optical absorption of monolayer MoSi2N4 exhibits obvious anisotropy, which edge shifts in different degree under the plane strain, effectively expanding the spectral response range of the system and beneficial to the photoelectric properties. These results provide a theoretical guidance for further research on the application of 2D monolayer MoSi2N4 in the field of new tunable nano optoelectronic devices.
Oxygen evolution reaction (OER) plays an important role in solving energy shortage and environmental problems, but it requires a huge overpotential to overcome the slow kinetic barriers, so the development of high- efficiency electrocatalysts has become an indispensable step. In this work, the performance of α-MnO2(001) and Mo doped α-MnO2(001) electrocatalytic oxygen evolution reaction were studied by using density functional theory. Gibbs free energy, density of states and differential charge density were calculated according to the reaction path. The research results show that Mo doping can effectively modulate the electronic structure of α-MnO2(001) surface, improve desorption and adsorption capacity between intermediates and the catalyst, and provide more electrons for OER. Gibbs free energy calculation results indicate that the formation of O2 from *OOH is the rate-determining step for OER in the Mo doped α-MnO2(001) system. Mo doping reduces the overpotential to 1.01 V, which presents a good catalytic performance for oxygen evolution.
An “aluminum/graphene oxide/aluminum (Al/GO/Al)” interface model with different carbon/oxygen ratio or with different defects was established. Effects of oxygen-containing functional groups and different defects on the interface of reduced graphene oxide/aluminum composites was studied using first principle method based on density functional theory (DFT). The results show that the epoxy group is better than carbon atom to produce obvious charge interaction with aluminum atom in the interface model of Al/GO/Al. The net charge of oxygen atom is -0.98 e while aluminum atom is 0.46 e, which is conducive to the interfacial bonding between reduced graphene oxide (RGO) and aluminum matrix in composites. When the defects exists, the net charge of carbon atoms at the defects in the Al/GO/Al interface model is in the range of -0.05 e to -0.38 e. Interaction between epoxy group and carbon atoms is weak, while interaction between epoxy group and aluminum atoms is significantly intensified. The existence of epoxy group can inhibit reaction between carbon atom and aluminum atom in the vacancy defects, and protect integrity of carbon structure in RGO with vacancy defects. Therefore, this research may provide theoretical guidance for development of high-performance Al/GO/Al matrix composites.
MXenes have been widely studied for their excellent specific surface area, high conductivity and composition tunability, which have been used as a highly efficient electrode material for lithium-ion batteries (LIBs). However, limited storage capacity and severe lattice expansion caused by Li-ions diffusion restrict the application of MXenes as electrode materials. Here, Ti3C2 MXenes with surface halogenation (fluorination, chlorination and bromination) as representative MXene materials were designed. Effects of surface functionalization on the atomic structures, electronic properties, mechanical properties, and electrochemical performance of Ti3C2T2 (T = F, Cl and Br) anode in LIBs were investigated using first-principles calculations based on density functional theory with van der Waals correction. The results reveal that Ti3C2T2 MXenes exhibit metallic conductivity with improved structural stability and mechanical strength. Compared with Ti3C2F2 and Ti3C2Br2, Ti3C2Cl2 exhibits the large elastic modulus (321.70 and 329.43 N/m along x and y directions, respectively), low diffusion barrier (0.275 eV), high open circuit voltage (0.54 eV), and storage capacity (674.21 mA·h/g) with stoichiometric ratio of Ti3C2Cl2Li6, which renders the enhanced rate performance and endures the repeated lattice expansion and contraction during the charge/ discharge process. Moreover, surface chlorination yields expanded interlayer spacing, which can improve Li-ion accessibility and fast charge-discharge rate in Ti3C2Cl2. The research demonstrates that Cl- terminated Ti3C2 is a promising anode material, and provides effective and reversible routes to engineering other MXenes as anode materials for LIBs.
Cs2SnI6 is a stable and environmentally friendly halide perovskite material with great potential for photovoltaic and optoelectronic applications. While the surface properties are of paramount importance for device fabrications, there have been no such theoretical studies on this material. Using density functional theory calculations with the SCAN+rVV10 functional, the (001), (011) and (111) surfaces of Cs2SnI6 were studied to reveal their thermodynamic stability. We constructed seven models for these surfaces, including two along the (001) orientation (CsI2- and SnI4-terminated surfaces), two along the (011) orientation (I4- and Cs2SnI2-terminated surfaces) and three along the (111) orientation (non-stoichiometric CsI3-, Sn- and stoichiometric CsI3-terminated surfaces). Because most of the surfaces are non-stoichiometric, their relative stability depends on the experimental preparation condition, which is reflected by the chemical potentials of the constituent elements in the calculation. By determining the allowed chemical potential region, the thermodynamic stability of these Cs2SnI6 surfaces is analyzed. The results show that the surface energies of the (001) and (011) surfaces are affected by the chemical potentials, while the stoichiometric CsI3-terminated (111) surface is unaffected by the chemical potentials and is energetically the most stable surface of Cs2SnI6. Thus, the observed exposure of (111) surface of Cs2SnI6 crystals in several recent experiments is determined to be driven by thermodynamics.
PbTiO3 (PTO) is an important ferroelectric functional material, but its structure, stability, mechanical property, and thermodynamic property under pressure is still unknown, leading to restriction in applying in the field of electronic communication. Here, first-principles calculations based on density functional theory was performed to study the structure and thermal properties of pre-perovskite phase PbTiO3 (PP-PTO), ferroelectric tetragonal phase PbTiO3 (TP-PTO), and paraelectric cubic phase PbTiO3 (CP-PTO) under pressure. It is found that their compressibility in descending order is PP-PTO>TP-PTO>CP-PTO. Under considered pressure, three PTO phases have not undergone a phase transition analyzed by band structure and density of states, and their band gap gradually decreases with increasing pressure. Among them, the TP-PTO changes from an indirect to a direct band gap semiconductor at 20 GPa, while the others remain a direct band gap semiconductor. Those PTO phases are mechanically stable and anisotropy from 0 to 30 GPa. Furthermore, their comprehensive mechanical properties increase and anisotropy firstly decreases and then increases with increasing pressure. Analysis based on quasi- harmonic Debye approximation theory was performed to study the influence of temperature and pressure on Debye temperature, entropy and heat capacity. The results illuminate that Debye temperature decreases with temperature increase, nevertheless, pressure has the opposite effect, which elucidates that the order of covalent bond from strong to weak is CP-PTO>TP-PTO>PP-PTO. Entropy and heat capacity of PTO increase with rising temperature, but decrease with the increase of pressure.
Lithium-ion batteries are widely used as energy storage and dynamic power, while the capacity life of battery is one of the key factors affecting its further application. The electrochemical-mechanical multi-field coupling effect of the lithium-ion batteries during the cyclic charging and discharging process cause the damage accumulation for the electrode materials, thereby deteriorates the mechanical stability of the electrode materials, leading to multi-scale damage to the electrode materials, ultimately declining the battery life. In this study, the multi-scale failure behavior of LiNixCoyMnzO2 (NCM) cathode materials were summarized through our previous research, and the experimental and simulation analysis method for studying the damage of electrode material are introduced systematically, to provide reference for selecting damage analysis methods at different scales. In addition, the failure mechanisms of NCM cathode materials at the scale of active particles and electrode coating were studied in-depth based on combination of experimental and simulated analysis, including electrochemical experimental of lithium-ion batteries, extended finite element method (XFEM), linear matching method (LMM) framework. The research work provides important guidance for the mechanism analysis of multi-scale failure behavior and microstructure modification of electrode materials.