Principles Calculations Research Articles

Cu-bearing HHCCI interface combined with first principles calculation and corrosive wear resistance

The corrosive wear resistance of copper-bearing HHCCI was tested through experiments and HRTEM, combined with first-principles calculations to study the atomic structure, interface fracture work, thermodynamic stability, electronic structure and bonding structure of six Fe3Cr4C3(01 1‾ 0)/γ-Fe(101) interface models with different termination methods. The HRTEM results show that the interface formed by (01 1‾ 0) of M7C3-type carbide and (101) of the austenite matrix is a coherent interface. The first principles calculation results show that the interface formed by the Fe3Cr4C3(01 1‾ 0)-Cr termination model and the γ-Fe (101)-2 termination model has the highest interface bonding strength. The Fe-end/7Fe-2 interface model is the most stable. Both the interfacial chemical energy and the interfacial elastic energy will affect the overall thermodynamic stability of the Fe3Cr4C3(01 1‾ 0)/γ-Fe (101) interface. The fracture work of Fe3Cr4C3(01 1‾ 0) and γ-Fe (101) is greater than the corresponding interface adhesion work. Moreover, the fracture work on each terminal end of M7C3 along the (01 1‾ 0) surface is higher than that on each terminal end of γ-Fe along the (101) surface. The failure of HHCCI during corrosive wear may mainly occur in the interface area or the side close to the γ-Fe matrix. The chemical bonds in the Fe-end/7Fe interface are mainly Fe-Fe metal bonds, Fe-Cr metal bonds and some Fe-C polar covalent bonds. The chemical bonds in the Cr-end/7Fe interface are mainly Cr-Fe metal bonds and some Cr-C polar covalent bonds. The chemical bonds in the C-end/7Fe interface are mainly composed of Cr-Fe metal bonds, C-Fe polar covalent bonds and part of C-Cr polar covalent bonds, as a result, each interface exhibits different corrosive wear resistance potentials.

Cs/O, Cs/NF3, and Cs/Li/NF3 coadsorption on GaAs0.5P0.5 (001) β2(2×4) reconstruction surface: A first principle study

The activation process of p-type doped GaAs0.5P0.5 (001) β2(2 × 4) reconstruction surface is investigated based on first principle calculations. For Cs-only adsorption, the adsorption energy rises as the Cs coverage increases, and when the Cs coverage is at 0.75 ML, the work function hits its minimum. Cs/O and Cs/NF3 concurrent adsorption are conducive to increasing steadiness and lowering the work function of the reconstruction surface during the phase of high Cs coverage. The work function reaches its minimum when Cs coverage is 0.75 ML in Cs/O coadsorbed models while that occurs when Cs coverage is 1.00 ML in Cs/NF3 coadsorbed models. Results indicate that the Li atoms intercalation with low coverage in Cs/NF3 coadsorbed models is conducive to establishing dipole moments, strengthening stability, and lowering the work function. The optimal adsorption recipe is 6Cs/2Li/NF3, with which the surface owns the lowest work function and shows the most stable property.

Green massive mechanical synthesis of highly efficient Zn-Doped Cs3Cu2I5 for LED and X-ray imaging applications

As a new generation of environmentally friendly halide materials, Cs3Cu2I5 shows up the promising application in scintillation, LED, and photoelectric detection fields. However, liquid phase methods cannot produce Cs3Cu2I5 on a large scale, and the preparation process is accompanied by serious pollution. Although the ball milling method can solve the problems caused by the liquid phase synthesis method, the Cs3Cu2I5 obtained has many surface defects and low luminescence efficiency. In this work, a strategy of doping-assisted ball-milling synthesis using ZnI2 is proposed. Similar to the chemical preparation of Cs3Cu2I5, the iodine-rich environment during the modified ball milling process can also play a role in repairing surface defects. And it has been demonstrated through experiments and first principles calculations that the distortion of [Cu2I5]3- under ground/excited state can be adjusted through doping Zn into the Cu1 site, the formation barrier of self-trapped excitons is reduced, and the luminous efficiency of Cs3Cu2I5 is significantly improved. Subsequently, using the simple hot-pressing method, the representative Cs3Cu2I5:1.0Zn sample was combined with commercially available polypropylene to fabricate Cs3Cu2I5:1.0Zn@PP film. Finally, the potential applications of the newly-developed film in portable and customizable WLED and scintillation fields were explored.

Composition and microstructure engineering of Fe–Si–Co soft magnetic alloys with enhanced performance

The growing demand for high-efficiency and low-loss energy conversion and transportation techniques urges the development of advanced Fe–Si based soft magnet alloys. Simultaneous achievement of low coercivity (Hc) and large saturation magnetization (Ms) however, remains challenging. In this study, soft magnetic alloys with the composition Fe82–xSi18Cox (x = 0 at.%, 4 at.%, 8 at.%, 12 at.%, 16 at.%, and 20 at.%) have been designed followed by microstructural tuning. The Co incorporation results in initially decreased Hc followed by increment due to reduced magnetocrystalline anisotropy and increased saturation magnetostriction from negative to positive values of the alloys. Meanwhile, the Ms raises with subsequent reduction, which origins from competitive mechanisms of increased average moment of Fe atoms and decreased average moment of Co atoms according to first principles calculations. Microstructural evolution during annealing of the Fe70Si18Co12 with synergistically optimized Hc and Ms has been revealed that after elevated-temperature annealing, the DO3 phase is predominately transformed from the B2 phase accompanied by an increase in the degree of ordering. The growth of the DO3 phase deteriorates the Hc due to the aggravating pinning effect on the domain wall movement, which arises from the inhomogeneous magnetization distribution caused by increasing antiphase boundaries.

Prediction of comprehensive properties and their hydrogen performance of Mg2XH6(X=Mn, Fe, Co, Ni) perovskite hydrides based on first principles

Hydrogen storage materials are very critical for hydrogen energy utilization. Cubic Mg2XH6(X = Mn, Fe, Co, Ni) hydrogen storage materials are investigated by first principles calculation. The structural stability are confirmed by complex consideration of phonon spectrum, formation energies and elastic constants. They are brittle and hard materials with high temperature applicable potential. Mg2MnH6 and Mg2CoH6 are magnetic compounds while Mg2FeH6 and Mg2NiH6 are non-magnetic materials. Mg2MnH6 and Mg2FeH6 are semiconductors with bandgap of 1.867 eV and 3.318 eV, respectively, indicating they are applicable in optoelectronic field, while Mg2CoH6 and Mg2NiH6 are metallic in energy band structure. They are all ionic compounds which are confirmed by Cauchy pressure, electronic density difference and electronic localization function. Their hydrogen storage capacity are all over 5.2 wt%, which are favorable in practical use. And the moderate desorption temperature of 328.9 K of Mg2MnH6 suggests it is a promising candidate for hydrogen storage.

The first principles study of the dual-atom catalyst based on g-C3N5 for efficient nitrogen fixation

Electrocatalytic nitrogen reduction reaction (NRR) for ammonia production has broad prospects because of its mild green reaction conditions. However, the available catalysts cannot meet the needs of industrial ammonia production due to low catalytic activity and poor selectivity. In our work, the dual-atom catalysts (DAC) supported by g-C3N5 with transition metals dual-atom were designed based on the first principles calculation. Based on the criteria of stability, catalytic activity, and the competitive landscape between NRR and hydrogen evolution reaction (HER), the Ni2/g-C3N5 stands out as the most promising candidate among 30 designed diatomic catalysts for NRR, exhibiting remarkable potential for this reaction. In fact, the Ni2/g-C3N5 possesses low limiting potential (−0.27 V) and its selectivity of NRR is nearly 100 %. Importantly, by comprehensively considering the pivotal parameters of charge order, electronegativity, and d-band center, a dedicated descriptive descriptor has been formulated. This result not only profoundly uncovers the intrinsic origins of catalytic activity in the NRR but also significantly expedites the screening process for highly efficient catalysts. Our work not only accelerates the search for an NRR catalyst with high activity but also provides theoretical guidance for subsequent experiments on DACs.

The effects of hydrogen on the bonding properties of SiC/Cu interface via first-principles calculations

Ceramic/metal interfaces own great importance of technology, but the weak interface bond limits their applications. In this work, the effects of doping H element on the stability and bonding properties of SiC/Cu interface are studied by first principles calculations. In order to study the stability of interface, the potential surfaces were firstly studied for the C and Si terminated SiC/Cu interfaces. The C terminated interface is more stable than the Si terminated interface, which contributes the largest work of adhesion of 3.998 J/m2. The effects of hydrogen on the interface bonding properties were further studied. The occupation properties of hydrogen atoms were then studied, the hydrogen tends to occupy the octahedral position with negative formation energy. To reveal the bonding strength of SiC/Cu interface, the tensile simulations were carried out. The hydrogen owns obvious effects on the tensile strength depends on its occupation site. The movement and diffusion properties of hydrogen atoms in the interface zone were studied combining molecular dynamics and climbing image nudged elastic band method. Finally, the bonding mechanisms were analyzed by combining charge density, state density, differential charge and bader charge. Based on the above calculation results, it is found that for SiC(001)/Cu(111) interface, the H element generally has a negative impact on the bonding properties of SiC/Cu interface.

Theoretical investigations on exploring Si-based heterostructures with cubic and ground-state perovskites

First principles calculations in the framework of density functional theory (DFT) are performed to exploit replaceable perovskite oxides against the instability of traditional cubic SrTiO3 grown on silicon substrate. In this work, we consider more thermostable CaTiO3 and CaZrO3 with cubic and ground-state phase as the candidates to perform the gate dielectric. After the calibration of total energy and lattice transformation of ground-state perovskites to match with Si substrate, three categories of 2 × 2 heterostructures have been constructed and investigated through the geometrical change, atom displacement around interfacial layer, total energy and electronic structure change, under the variations on perovskite species, the concentration of interfacial layer and the thickness of perovskite film. And the results demonstrate that the category with half a monolayer of Ca/Sr atoms in interfacial layer and two unit-cell perovskite thicknesses show the type-I band alignment through their band alignment. Among this category, the heterostructure with Pnma CaZrO3 shows semiconductivity and excellent electrical property with larger band offset for the candidate of gate dielectric. This work provides a creative perspective to expand the perovskite diversity for the booming growth of integrated circuit technology.

Promoting homogeneous tungsten doping in LiNiO2 through a grain boundary phase induced by excessive lithium

LiNiO2 (LNO) is one of the most promising cathode materials for lithium-ion batteries. Tungsten element in enhancing the stability of LNO has been researched extensively. However, the understanding of the specific doping process and existing form of W is still not perfect. This study proposes a lithium-induced grain boundary phase W doping mechanism. The results demonstrate that the introduced W atoms first react with the lithium source to generate a Li-W-O phase at the grain boundary of primary particles. With the increase of lithium ratio, W atoms gradually diffuse from the grain boundary phase to the interior layered structure to achieve W doping. The feasibility of grain boundary phase doping is verified by first principles calculation. Furthermore, it is found that the Li2WO4 grain boundary phase is an excellent lithium ion conductor, which can protect the cathode surface and improve the rate performance. The doped W can alleviate the harmful H2↔H3 phase transition, thereby inhibiting the generation of microcracks, and improving the electrochemical performance. Consequently, the 0.3wt% W-doped sample provides a significant improved capacity retention of 88.5% compared with the pristine LNO (80.7%) after 100 cycles at 2.8-4.3 V under 1 C.

Strain-mediated phase transition of MoTe2 monolayer

MoTe2 has emerged as a promising candidate in the field of integrated circuits, memristive devices, and catalysts, owing to its polymorphic nature across different phases. Experimentally, strain engineering has been demonstrated as an effective approach for manipulating the phase transition of MoTe2, but the mechanism remains unclear. The strain-dependent phase transition and its micro-mechanisms have been investigated based on first principle calculations. As demonstrated, critical strain and phase transition path from H → T′ phases are strongly governed by the applied strain's orientation, magnitude, and triaxiality. At the atomic level, nonzero movements of Te atoms within the phase transition domain with mechanical unloading have been clarified, together with an advanced understanding on the impact of strain on Te-vacancies migration. These insights advanced the knowledge of MoTe2 phase transition behavior and demonstrated the large space to explore potential applications through strain, defect, and phase engineering.

Performance for CO gas sensing of janus ReSSe monolayer doped with Fe, Ru and Os from first principles calculation

Abstract Transition metal dichalcogenides (TMDs) have many excellent properties as promising class of two-dimensional materials. In this study, we conducted rigorous calculations utilizing density functional theory to evaluate the potential of Janus ReSSe monolayers, doped with transition metals such as Fe, Ru, and Os, in gas-sensitive applications specifically targeting CO detection. Three stable structures of X-Re15S16Se16 Janus doped with X elements (X = Fe, Os, Ru) were designed. Our findings indicate that the C atom of the CO molecule exhibits a higher affinity for adsorbing onto the X (X = Fe, Os, Ru) transition metal atoms, forming robust X–C bonds, rather than the O atom. Among these bonds, the Os-C bond exhibits the strongest bonding states, followed by the Ru-C bond, while the Fe-C bond behaves the weakest. Notably, the d-orbital peaks of the X (X = Fe, Os, Ru) transition metals display distinct bonding strengths with the C atom. This research may provide a theoretical foundation for the development of new gas sensors based on two-dimensional materials.

Modulation of photoluminescence properties of transition metal dichalcogenides through temperature and Au/Ag nanoparticles

Abstract Transition metal dichalcogenides (TMDCs) attract significant attention because of their remarkable optical properties. Nowadays, the light emission efficiency of TMDCs is still inferior. Temperature and plasma resonance effects can be important approaches in the modulation of their luminescence performance. Here, multilayer WSe2 and monolayer MoSe2 were synthesized. Under the temperature control system, two photoluminescence (PL) peaks are observed from multilayer WSe2. The higher-energy PL peak results from K-Γ indirect band gap transition, which is then demonstrated by the first principle calculation. Otherwise, PL enhancement is realized on monolayer MoSe2 decorated with Au nanoparticles. The PL of WS2 is inhibited by hybridization with Ag nanoparticles.

Structure regulation and performance optimization mechanism of Sr0.7Bi0.2TiO3-based energy storage ceramics based on charged defect design engineering

The linear-like relaxor ferroelectric Sr0.7Bi0.2TiO3 with regulable microstructure offers a new platform to reveal the essential mechanism of energy storage properties improvement and develop advanced pulse capacitors. Herein, Li with relatively weak volatility accompanied by Bi was introduced in Sr0.7Bi0.2TiO3 to form a charged defect and increase the maximum polarization (Pmax) while avoiding the deterioration of dielectric breakdown strength (BDS). The occupation of Li and the formation of Li-Bi defect dipole polar were analyzed using first principle calculations. A unique local superlattice induced by the lattice distortion was first observed in related systems. The Pmax was enhanced due to the increased size and number of polar structures. The breakdown behaviour and local electric field distribution were analyzed using numerical simulation. The BDS was improved due to the decreased oxygen vacancies, high thermal conductivity and enhanced electrical insulation. The excellent and stable energy storage performance was achieved in different temperatures, frequencies and cycles. Meanwhile, the optimized sample possesses excellent and stable charge–discharge properties, especially fatigue resistance (105 cycles under 25 °C and 150 °C). After the process optimization, the sample shows a higher energy storage density of 3.44 J/cm3 with a high efficiency of 93.74 % at 345 kV/cm due to the further enhanced BDS. In this work, the performance improvement mechanism was studied in-depth, providing a new strategy for optimizing linear-like lead-free relaxor ferroelectric energy storage properties.

Vertical stacking approach for tailoring electronic and optical properties of ultrathin two-dimensional vdW heterostructures of P-MAB (M=Ti, Zr, Hf; A=S, B=Se)

The vertical staking in the form of hetersotructures, is an effective way to enhance the performance of corresponding isolated monolayers. Strong interactions between Janus monolayers (MAB) and Phosphorus (P) monolayers result in significant improvements in both physical and chemical properties compared to single layer configurations. Using first- principle (hybrid) calculations, we explore electronic structure and optical properties of P-MAB (M = Ti, Zr, Hf; A = S, B=Se) van der Waals heterostructures, for potential applications in electronic and optoelectronic devices. Ab initio molecular dynamic (AIMD) simulations confirm the thermal stability, while phonon spectra confirm the dynamical stabilities of these systems. Our analysis reveals that all configurations maintain semiconducting nature with an indirect bandgap. The type-II band alignment in the P-MAB heterostructures is demonstrated through calculations of the partial density of states (PDOS). Bader charge analysis and planar-averaged charge density differences indicate an electron transfer from the P monolayers to the MAB monolayer. Furthermore, our investigation of the dielectric function ϵ2(ω) shows that the lowest energy shift are primarily influenced by excitonic effects.

Studying the optical and electronic properties of Eu-doped CdSe phosphors using first principles calculations incorporating the spin orbit coupling method of DFT

We studied the electronic and optical properties of pure and Eu-doped CdSe phosphor materials using the first-principles calculation method, which is a feature of Density Functional Theory. The FP-LAPW method was employed for the study as incorporated in the Wien2k code. The GGA + U + SOC method of DFT was used for the first time in our study to investigate the electronic as well as optical characteristics of Eu-doped CdSe materials. Eu doping was done in two different ratios to study the effects of altered dopant concentration. The doping increased the band gap of the pure CdSe parental material. The calculated band gaps in our study were 0.42 eV for pure CdSe, 0.67 eV for a single Eu atom doped in CdSe, and 1.64 eV for two Eu atoms doped in CdSe. The measured electronic and optical characteristics of undoped i.e, pure CdSe and Eu-doped CdSe showed that the doping significantly changed the optical behavior of CdSe, which may be useful for applications in phosphor-converted LEDs.

Beyond the random phase approximation (RPA): First principles calculation of the valence EELS spectrum for KBr including local field, quasiparticle, excitonic and spin orbit coupling effects

The low energy region (< 50 eV) of the electron energy loss spectrum (EELS) can contain a great deal of spectral detail associated with excitations of the valence electrons. Calculation of the spectra from first principles can assist with interpretation and the most widely used method is the random phase approximation (RPA), usually neglecting local field effects (LFE). For KBr this approach is insufficient due to the importance of quasiparticle and excitonic effects. Calculations including these multi-electron effects are shown to give much improved agreement with the experimental spectra, and the inclusion of spin-orbit coupling (SOC) reproduces the excitonic doublet just above band-edge onset. A review of the complex theory behind these methods is given along with practical guidance on performing these calculations.

Effect of V-Ti multi-microalloying on enhancing hydrogen embrittlement resistance in hot-stamping steel

Hydrogen embrittlement (HE) has become a significant concern that limits the usage of ultra-high strength hot-stamping steel. In this work, the utilization of a novel approach employing V-Ti multi-microalloying has yielded substantial improvements in the resistance of hot-stamping steel to HE. The experimental findings demonstrate that V-Ti multi-microalloying effectively restrained the coarsening of precipitates, leading to the generation of high-density nanosized (V, Ti) C complex precipitates. The first principle calculation results indicate that V-Ti multi-microalloying improved the hydrogen trapping capacity of (V, Ti) C compared with TiC. Additionally, this approach refined the prior austenite grain size, fortifying the material's resistance to hydrogen-induced cracking.

An Examination of the Electronic, Optical, and Thermodynamic Properties of Β-Gan

The current work involves the systematic examination of the features of Gallium Nitride (GaN) including the electronic, optical, structural, and thermodynamic features, depending on first principles calculation. This is according to approximations, (LDA), (GGA), and (m-GGA). The bandgap energies of Gallium nitride are (1.85 eV, 1.93 eV, and 2.179 eV), respectively. Furthermore, our study also reveals that GaN has a direct bandgap and is highly stable. Finally, our results indicate that the m-GGA method accurately predicts the bandgap energy of GaN. The m-GGA method outperforms both LDA and GGA methods in accuracy to predict the bandgap energy of GaN, as evidenced by its closest approximation to the experimental value. To determine the orbital nature of the Gallium and Nitrogen atoms, the state density and state partial density of Gallium nitride were simulated. The absorption coefficient of Gallium nitride is computed and analyzed in depth for the optical transitions. The absorption coefficient of Gallium nitride is affected by various factors, including the material's band structure, temperature, doping level, and the energy of the incident photons. In addition to that, the thermodynamic properties that can be calculated like enthalpy, entropy, heat capacity, free energy, and Debye temperature enable us to understand the thermal behavior of the compound better. The heat capacity of α-GaN is detected to be (39.9, 25.5, and 32.4) Jmole-1K-1, and a Debye temperature of 807 K, 1134 K, and 866 K for LDA, GGA, and m-GGA, respectively. This research will offer a detailed interpretation of β-GaN, covering all its basic properties and possible applications in electronic devices and optoelectronic devices. The results of this study are very important and the new technologies that will be developed based on the Cubic phase - GaN research will be very beneficial.

Effects of defects on the structure, electronic and elastic properties of Nb2GaC: A first principle calculations

MAX phase materials possess unique chemical and physical properties such as comparably electrical and thermal conductivity of metals, as well as the high hardness, elastic modulus, and temperature stability of ceramics. As a result, MAX phase materials can be used in extremely harsh conditions, such as high temperature, strong corrosion, and high irradiation environments. This study utilize DFT to predict the effects of Nb2GaC point defects on the crystal's structural stability, electronic, and elastic properties. And the effects of doping of O and He atoms on the structural stability and elastic properties of Nb2GaC is also predicted by using density functional theory. Through calculating vacancy formation energy and cell binding energy, the presence of point defects and oxygen, helium atoms are evidenced that does not compromise the stability of the crystal structure. Defects such as point defects and oxygen, helium atoms doping will weaken the interatomic bonding force of Nb2GaC, but the material's interatomic bonding can still remain unaffected and maintain a hybrid bond of metallic and covalent bonds. The introduction of point defects and oxygen, helium atoms provide a feasible route to facilitate the properties of Nb2GaC ceramics.

Theoretical investigation of structural, mechanical, electronic, optical, and thermal properties of ternary compounds of heusler alloy ANiSn (A= TI, TH, U) using first principles calculations

In this study, we investigated the ANiSn (A = Ti, Th, U) half-Heusler materials for various properties, including structural, electronic, mechanical, elastic anisotropic, optical, and thermal properties, using Density Functional Theory (DFT) with the Cambridge Serial Total Energy Package (CASTEP) code. The elastic constants satisfied Born's criteria, confirming the thermodynamic and mechanical stability of the ANiSn compounds. Mechanical stability was further assessed through bulk modulus, shear modulus, and Poisson's ratio. Our analysis revealed that TiNiSn and ThNiSn exhibit ductile behaviour, whereas UNiSn is brittle. The calculated elastic modulus indicated that the compounds we studied are elastically anisotropic. The electronic and optical properties confirmed the semiconducting nature of these materials, with significant absorption and conductivity observed in the ultraviolet region. ANiSn is suitable for manufacturing various optoelectronic devices, such as laser diodes (LDs), photodetectors, LEDs, and UV sensors, due to its high absorption coefficient in the IR to UV regions. Additionally, measured Debye and melting temperatures confirmed that TiNiSn is more thermally conductive and can be used in high-temperature structural substances. The low minimum thermal conductivity suggests that UNiSn may be a more efficient material for thermal barrier coatings (TBC) compared to TiNiSn and ThNiSn.