First-principles Calculation Method Research Articles

Theoretical prediction of 2D biphenylene as a potential anchoring material for lithium-sulfur batteries.

Searching for good anchoring materials that can suppress the shuttle effect is critical to large-scale commercialization of lithium-sulfur (Li-S) batteries. In this work, the adsorption behavior of lithium polysulfides (LiPSs, such as S8 and Li2Sn, n = 1, 2, 4, 6, and 8), the sulfur reduction reaction (SRR), the decomposition processes of Li2S and the diffusion behavior of Li atoms on intrinsic and doped 2D biphenylene (BIP) are systematically investigated by employing the first-principles calculation method. Calculations show that the adsorption energies of LiPSs on the electrolyte (DOL and DME) are smaller than those on the intrinsic/B doped BIP. The moderate anchoring strength (0.8-2.0 eV) between LiPSs and the BIP can effectively suppress the shuttle effect. Moreover, the Gibbs free energy barrier for SRR is 0.72/0.64 eV on intrinsic/B doped BIP. The dissociation energy barrier of Li2S on intrinsic/B doped BIP is 1.35 eV, while the diffusion energy barrier of Li atoms on intrinsic/B doped BIP is 0.18 eV/0.30 eV. Lower energy barriers are conducive to enhancing the discharging and charging efficiency. Therefore, intrinsic and B doped BIP are predicted as good anchoring materials for Li-S batteries.

Effect of stacking mode on electronic structure and optical properties of bilayer germanene

The electronic structure and optical properties of germanene and three type of bilayer germanene have been studied by first-principles calculation method. The results show that the stable germanene exhibits warped structure. Monolayer germanene exhibits massless Dirac Fermion near K-point, while bilayer germanene exhibits anisotropy Dirac bands. Meanwhile, in order to control the energy band structure, the band gap of bilayer germanene is adjusted by changing the interlayer distance and stacking mode. At the optimal layer spacing, the band gap of bilayer germanene appears at the K-point with 0.417eV, and the phase transition from semiconductor to metal can be realized by further changing the stacking mode. In terms of optical properties, bilayer germanene has better optical response. The results show that the band gap of bilayer germanene can be modulated successfully using stacking mode. These findings are of great significance to the modulation of electronic structure of germanene and the application of germanene in the field of Dirac band materials.

Alloying effect on the mechanical properties of Laves phase NbCr2: A first-principles study

The effect of doping elements X (Al, Si, Ti, V, Mn, Ni, Y, Zr, Mo, Hf, Ta and W) on the mechanical properties of Laves phase NbCr2 was investigated using the first-principles calculation method. The results showed that after alloying designated ternary compounds, the tensile properties improved by about 20 %, shear properties by about 23 % and hardness values by about 20 % when Si (Hf) atoms replaced Cr (Nb) atoms, at which point the system achieved the best toughness, plasticity and hardness. The larger the diameter of the doped atoms, or, the higher the number of outer electrons to be, the more pronounced the anisotropy of NbCr2 tends to be. Furthermore, the enhanced metallic bonding and weakened covalent bonding are the main reasons for the enhanced toughness, plasticity and hardness. Thus our finding opens an important avenue to systematically understand the mechanism in alloying effect on the mechanical properties of laves phase NbCr2.

First-Principles Computational Study of the Modification Mechanism of Graphene/Graphene Oxide on Hydroxyapatite

Due to its unique crystal structure and nano-properties, hydroxyapatite (HA) has become an important inorganic material with broad development prospects in electrical materials, for fire resistance and insulation, and in bone repair. However, its application is limited to some extent because of its low strength, brittleness and other shortcomings. Graphene (G) and its derivative graphene oxide (GO) are well known for their excellent mechanical properties, and are widely used to modify HA by domestic and foreign scholars, who expect to achieve better reinforcement and toughening effects. However, the enhancement mechanism has not been made clear. Accordingly, in this study, G and GO were selected to modify HA using the first-principles calculation method to explore the theory of interfacial bonding of composites and explain the microscopic mechanism of interfacial bonding. First-principles calculation is a powerful tool used to solve experimental and theoretical problems and predict the structure and properties of new materials with precise control at the atomic level. Therefore, the bonding behaviors of hydroxyapatite (100), (110) and (111) crystal planes with G or GO were comprehensively and systematically studied using first-principles calculation; this included analyses of the density of states and differential charge density, and calculations of interfacial adhesion work and elastic moduli. Compared to HA (100) and (111) crystal planes, HA (110) had the best bonding performance with G and with GO, as revealed by the calculation results. The composite material systems of HA (110)/G and HA (110)/GO had the smallest density of states at the Fermi level, the largest charge transfers of Ca atoms, the largest interfacial adhesion work and the most outstanding elastic moduli. These results provide a theoretical basis for the modification of HA to a certain extent, and are beneficial to the expansion of the scope of its application.

A first-principles study of magnetic switching and hysteresis effect in the ferromagnetic Fe metal

The first-principles calculation method used in this study has an extended Hamiltonian, which contains both spin states, so that the spin magnetic moment per unit cell, m , and magnetization vector, M , can be obtained via the spinors of atomic orbitals. Both spin- and orbital- B -field couplings are considered. The magnetic field, B , induces not only components of m and M along their directions, which vanish when the field is cut off, but also bi-stable spontaneous z components, m z and M z . This study reveals that the spin- B -field coupling is energetically too weak to reverse m z / M z directly. The B field is found to reverse the spin polarization of itinerant s p electrons first when B reaches a threshold, which then triggers the reversal of m z / M z via nonlocal exchange interactions with localized d electrons. The hysteresis effect is found to be due to the existence of bi-stable m z / M z and that without a reversed B the spin-polarization of itinerant s p electrons won’t be reversed to trigger magnetic switching. • The first use of spinors to obtain the 3-D magnetization vector in Fe. • The conventional magnetic field is energetically too weak to cause magnetic switching directly. • The magnetic field reverses spin polarization of sp electrons first, which then triggers reversal of magnetization through nonlocal exchange couplings. • The relationship between the calculated magnetization and the magnetic field is hysteresis-loop-like. • The magnetic switching can be assisted by an accompanying strong transverse magnetic field.

A first-principles study: single-layer TiS2 modified by non-metal doping.

In this paper, the effect of substitutional doping of boron (B), carbon (C), nitrogen (N), oxygen (O), and phosphorus (P) at the S-site on the electronic structure of the monolayer TiS2 system is investigated using a first-principles calculation method. The effect of nonmetal doping on the band gap of TiS2 and its mechanism were studied by analyzing the stability, electronic structure, and charge transfer of the system. By calculating the electron cloud overlap population and formation energy, we confirmed that the system is stable, and the Ti-X bond length of the doped system undergoes different degrees of distortion. The properties of the B, C, and N doped system show different properties with different doping concentrations; the band gap of the O-atom system gradually expands with the rise of the doping rate. The hybridization positions of the impurity and the intrinsic atoms are found by comparing the DOS; combined with the charge density difference, we confirmed the interaction of impurity atoms with the system and revealed the bonding process of different systems.

Photoelectric properties of cubic mixed-cation lead halide perovskites (Cs MA1-PbI3) from first-principles

Recently, hybrid cationic perovskite solar cells (PSC) have attracted much attention because of their higher stability and better photovoltage performance than pure ABX 3 perovskite solar cells. Methylammonium lead iodide (MAPbI 3 ) is one of the most widely studied perovskite solar cell materials. However, the instability of organic cations has become the main obstacle to its commercialization. Here, we address the issues of forming a photostable MA-based perovskite by partially substituting the methylammonium with cesium ions (Cs x MA 1- x PbI 3 ). Its structure and photoelectric properties are studied by first-principles calculation method. The results show that the phase stability of MAPbI 3 can be improved for appropriate concentration of cesium ion. In addition, the band gap decreases with the increase of the proportion of doped cesium ions. Cs x MA 1- x PbI 3 maintains the basic characteristics of MAPbI 3 and has better thermodynamic stability, carrier mobility and light absorption characteristics in the case of small amount of Cs doping, which make it a candidate material for high-efficiency perovskite solar cells. • Cs-doped MAPbI 3 increases the thermal stability. • Cs-doped MAPbI 3 has a better band gap value, which is conducive to the visible light absorption in solar cells. • The doped material has higher carrier mobility and visible light absorption coefficient

Initial oxidation on 12 grain boundaries in pure TiN coating and on (Al,Si,Cr) doped Σ3 (111) grain boundaries: A first-principles calculations

In this work, using first-principles calculation method, the effect of twelve <110> symmetric tilt grain boundaries (GBs) on the initial oxidation behavior of TiN was studied. Firstly, the calculated GB energies show that for the TiN (1–10) surface with GBs from Σ3 to Σ27, Σ3 (111) GB possesses an extremely small GB energy (0.404 J/m2), while the other GBs possess a high energy (2.681∼4.834 J/m2). The electronic structure results show that the intrinsic characteristic of GB with high energy is well associated with the extra DOS peak existing in high energy region. Secondly, the oxygen adsorption results show that for TiN surfaces without GB, the order of adsorption energy of O is (100) > (110) > (111). When TiN (1–10) surface with different GBs, the sites at GBs are chemically more favorable sites for oxygen. The degree of weakening of oxidation resistance varies depending on the type of GBs, due to the apparent difference in oxygen adsorbability. Moreover, an interesting discovery is that Σ3 (111) GB provides a possibility to enhance the oxidation resistance of TiN, attributed to the easy introduction of Σ3 (111) GBs and suppressed inward progression of oxidation. Finally, the doping investigation in Σ3 (111) GB shows that among the three doping elements (Al, Si, and Cr), Si doping can further improve the oxidation resistance of (1–10) surface with Σ3 (111) GB in TiN, owing to the easy introduction of Si and formation of diffusion barrier layer of silicon oxides.

Large in-plane and out-of-plane piezoelectricity in 2D γ-LiMX2 (M=Al, Ga and In; X=S, Se and Te) monolayers

Atomically thin two-dimensional (2D) materials have attracted tremendous attention owing to their unique properties compared to bulk counterparts and great application potential in the field of piezoelectric nanodevices. In present work, the non-centrosymmetric γ-phase 2D Li-based ternary chalcogenide monolayer structures (LiMX2; M = Al, Ga and In; X = S, Se and Te) have been studied through the first-principles calculation method. According to the results, γ-LiAlS2, γ-LiAlSe2, γ-LiAlTe2, γ-LiGaSe2, γ-LiInSe2 and γ-LiInTe2 monolayers were stable structures, being direct bandgap semiconductors with a bandgap greater than 0.1 eV. The charges of six stable monolayers were distributed in a mirror asymmetry manner, meanwhile, they possessed electrostatic potential gradients. Consequently, the γ-LiMX2 monolayers showed the better in-plane piezoelectric coefficients (d11 = 14.48 p.m./V for LiGaSe2) than most of other 2D piezoelectric materials, as well as the plurality of bulk piezoelectric systems. In the meanwhile, some candidates (γ-LiAlS2 and γ-LiAlSe2) also had superior out-of-plane piezoelectric coefficients (d31), which were up to twice those of the corresponding β-phase monolayers. These outstanding properties imply the γ-LiMX2 structures are very promising transducer materials for lightweight and high-performance piezoelectric-nanodevices.

Interlayer Coupling Induced Sharp Increase of the Curie Temperature in a Two-Dimensional MnSn Multilayer.

The MnSn monolayer synthesized recently is a novel two-dimensional ferromagnetic material with a hexagonal lattice, in which three Mn atoms come together to form a trimer, making it remarkably different from other magnetic two-dimensional materials. Most impressively, there occurs a sharp increase of the Curie temperature from 54 to 225 K when the number of layers increases from 1 to 3. However, no quantitative explanation has been reported in previous studies. Herein, by means of the first-principles calculation method and the Monte Carlo method, we demonstrate that strong interlayer ferromagnetic coupling plays an essential role in enhancing its critical temperature, which acts as a magnetic field to stabilize the ferromagnetism in MnSn multilayers. Our work not only explains the sharp increase of the Curie temperature of the MnSn film in experiments but also reveals that the interlayer coupling is a new routine to achieve high-temperature ferromagnetism in two-dimensional materials.

Phase Stability and Mechanical Properties Analysis of AlCoxCrFeNi HEAs Based on First Principles

With the in-depth research on high-entropy alloys (HEAs), most of the current research uses experimental methods to verify the effects of the main elements of HEAs on the mechanical properties of the alloys. However, this is limited by the long experimental period and the influence of many external factors. The computer simulation method can not only effectively save costs and shorten the test cycle, but also help to discover new materials and broaden the field of materials. Therefore, in this paper, the physical properties (such as lattice constant, density and elastic constant) of AlCoxCrFeNi (x = 0, 0.25, 0.5, 0.75, 1) HEAs were calculated based on the first-principles calculation method and virtual crystal approximate modeling method. It is found that AlCoxCrFeNi HEAs have the best hardness and toughness properties, with a Co content of 0.5~0.7. The research results can provide theoretical guidance for the preparation of HEAs with optimal mechanical properties.

Special roles of two-dimensional octahedral frameworks in photodynamics of Cs3Bi2Br9 nanoplatelets: Electron and lattice-wave localization

The low-dimensional cesium bismuth halides are intriguing wide-bandgap semiconductors with fruitful photophysics. However, their photodynamics is rather intricate and remains debated. We study the optical properties of the Cs3Bi2Br9 nanoplatelets (NPLs) by using the combined experimental and first-principles calculation methods. The results indicate that the exhibited dominant blue emission band and weak green band arise from two kinds of shallow color centers. The Cs3Bi2Br9 NPLs exhibit Raman active and inactive vibrational modes that are separately ascribed to the localized lattice waves propagating along the edges and interiors of the quantum well-like bromide–bismuth octahedral frameworks in Cs3Bi2Br9. These findings improve our understanding of the unique photodynamics of these multiple quantum well-like semiconductor nanocrystals.

Novel Li rich perovskites Li4NBI3 (B = Ge, Sn, or Pb) with high mobility based on super alkali cation Li4N

It is an important research goal to find new environment-friendly and stable materials for sustainable and green energy applications. This paper reports a new type of super alkali perovskite Li4NBI3 (B = Ge, Sn, or Pb) with high mobility that was designed and studied by using the first-principle calculation method based on density functional theory. The geometrical structures, thermodynamic stability, electronic properties, transport properties, mechanical properties, and optical absorption properties of Li4NBI3 perovskites were calculated. The effects of the super alkali cluster Li4N on the structure and electronic properties of these perovskites were investigated. The results show that ionic bonds are formed between Li4N and I, and that the local energy state provided by Li4N increases the density of states at the edge of the energy band, which affects the energy band structure and pseudo-bandgap, and further affects the optical absorption coefficient and carrier mobility. Mechanical parameters showed that these three perovskites have high ductility, so they are suitable for making thin films. With high conductivity, strong light absorption, these perovskites are likely to become potential candidates for new energy materials.

Local lattice structures and electronic properties of β-Li3PS4/CuS interface

In all-solid-state lithium–sulfur batteries (ASSLBs), unfavorable solid–solid contact at the interfaces with sulfide electrolyte (SE) is one of the important reasons affecting the properties of ASSLBs. CuS is a promising cathode material, and [Formula: see text]-Li3PS4 (LPS) is a newly found SE used for ASSLBs. In order to deeply explore the electronic properties at the interface between these two materials, we used the first-principles calculation method to investigate the local lattice structures, electronic properties, work-function, and charge distribution of LPS/CuS interface. The interface binding energy of the LPS/CuS interface is about −0.568 J/m2. Hence, the LPS/CuS interface structure is thermodynamically reasonable and stable with newly formed chemical bonds at the interface. The interfacial electron density of state shows the metallic properties, which are mainly contributed by Cu-[Formula: see text], [Formula: see text] orbitals and S-[Formula: see text], [Formula: see text] orbitals in CuS and S-[Formula: see text] orbital electrons in LPS. Furthermore, the work function of the interface means the formation of a space charge layer, which has a certain influence on the charge and discharging process.

First-principles calculations on brazed diamond with FeCoCrNi high entropy alloys doped with strong carbide-forming elements

FeCoCrNi alloy is one of the most widely studied high-entropy alloys due to its excellent properties such as extreme hardness, excellent wear resistance, and good chemical stability. However, the feasibility of using FeCoCrNi alloy as the filler for brazing diamond and the related interfacial bonding mechanisms are unclear. In this work, the interfacial bonding behaviors between FeCoCrNi filler and diamond, and the influences of strong carbide-forming elements X (X = Mo, Mn, Ti, V, W, Zr, Nb, Hf, Ta) doping into FeCoCrNi filler on the brazed diamond were systematically investigated through first-principles calculations method. The calculated interfacial segregation work and interfacial energies show FeCoCrNi/diamond interface has a higher interfacial bonding strength and stability than Ni–Cr/diamond interface. The calculated embedding energies show that these strong carbide-forming elements X are more likely to segregate in the interfacial region close to the diamond. The doping of X, especially Hf, Zr, Ta, and Nb, significantly enhances the interfacial bonding strength and stability of the FeCoCrNi/diamond interface. Combining with further analysis of electronic structures, it is suggested that the FeCoCrNiNb is an ideal high entropy alloy filler for brazing diamond due to its dual effects on interface strengthening of FeCoCrNi/diamond and graphitization inhibiting of diamond.

First-principle calculations of the structural, vibrational, mechanical, electronic, and optical properties of ε-O8 under pressure.

The vibrational, mechanical, electronic, and optical properties of the ε-O8 phase in the pressure range of 11.4-70 GPa were studied by the first-principle calculation method. The phonon dispersion curves have a tiny virtual frequency at 60 GPa, which indicates that ε-O8 is dynamically unstable at 60 GPa. However, the 3-BM EOS demonstrates that the unit cell is stable up to 70 GPa. It has been shown that ε-O8 remains ductile within the whole applied pressure range. Concurrently, we calculated the variation of the band gap of ε-O8 in the pressure range of 11.4-70 GPa. The results show that the band gap of ε-O8 decreases with increasing pressure. Notably, the band gap disappears within the range of 50-60 GPa, which reveals that the metallic phase transition occurs within this pressure range.

An Improved Artificial Neural Network Elastic Constant Prediction Method Based on Particle Swarm Optimization

As machine learning methods keep developing in materials science, using machine learning to mine novel materials has become a research hotspot. For microscopic crystals, the first-principles calculation of elastic constants has always been a conventional research method. However, the first-principles calculation method is time-consuming and laborious, which limits the development of materials. In order to achieve efficient discovery of new materials, this paper proposed an improved artificial neural network (ANN) method to predict the elastic modulus of crystals improved by particle swarm optimization (PSO), which build the PSO-ANN model in use of structures and elastic constants of crystals. Experiments show that the root mean square error (RMSE) can reach within 1GPa. To verify the accuracy of the method for other crystal predictions, the MgZn2 crystal is modelled and calculated by using the first-principles calculation method. Comparing with the prediction method proposed, the average error of the predicted elastic constants is within the range 5.6%.

Metal-to-semiconductor transitions in constituent-tunable layered two-dimensional Nb W1-Se2 based on first principles calculations

Using the first-principles calculation method, we have studied the electronic structure characteristics of monolayer, bilayer and trilayer NbxW1-xSe2 alloy structures with different concentrations of Nb at different supercell sizes. These alloy materials are formed by replacing one W atom with a Nb atom in supercells of layered two-dimensional (2D) WSe2 structure. Critical values of x in NbxW1-xSe2 were obtained to realize the transition from metal to semiconductor in monolayer, bilayer and trilayer alloy structures. Our results indicate that the transition metal dichalcogenides (TMDs), represented by WSe2, can retain semiconducting feature with increasing number of layers by means of concentration regulation under much lower doping of group-VB metal atoms represented by Nb. This doping approach opens up more possibilities for studying doping effects of layered materials with adjustable structure and properties.

First-principles investigation on Schottky barrier modification of graphene/CdSe heterojunction by the interlayer distance

Based on the first-principles calculation method of quantum mechanics, we systematically studied the geometric structure and electronic properties of graphene/CdSe heterojunction, and considered the influence of vertical strain on its performance. The calculated results show that the model II has the smallest binding energy of −1.09 eV when the interlayer distance is 3.4 Å. Graphene opens a small band gap of 3 meV, which indicates that it has a good application prospect in future micro-nano devices. Differential charge density found that the electrons and holes at the interface gathered on the surface of CdSe and graphene, respectively. When the interlayer distance increases from 2.6 Å to 4.0 Å, the band gap decreases from 19 meV to 1 meV. In addition, the most stable heterojunction has n- and p-type Schottky barrier heights of 0.224 eV and 1.324 eV, respectively, thus the system forms n-type Schottky contact. As the interlayer distance increases, the n- and p-type Schottky barrier heights continuously decrease and increase, respectively. When the interlayer distance increases from 2.6 Å to 2.8 Å, the heterojunction changes from p-type Schottky contact to n-type Schottky contact, and when the interlayer distance reaches 4.2 Å, the heterojunction can realize the transformation from n-type Schottky contact to n-type Ohmic contact. These findings may have potential guiding significance for the design and preparation of graphene/CdSe heterojunction nanodevices.

A multielement alloying strategy to tune magnetic and mechanical properties in NiMnTi alloys via Co and B

The phase stability, martensitic transformation, and magnetic and mechanical properties of (Ni2-xCoxMn1.5Ti0.5)1-yBy (0 ≤ x ≤ 0.625; y = 0.03 and 0.06) alloys are systematically studied through the first-principles calculations method. The Co and B atoms are inclined to be aggregated distribution in the Ni2Mn1.5Ti0.5 alloy, and the phase stability of the austenite and non-modulated (NM) martensite decreases by co-doping. The ferromagnetic activation effect in the austenite occurs when x = 0.03 and y = 0.625. The magnetism of the austenite changes from an antiferromagnetic to a ferromagnetic state, which is ascribed to the elongation of the nearest neighboring distance of Mn–Mn, the nearest Mn–Mn distance increases from 2.50–2.79 to 2.90–2.94 Å, while the NM martensite always shows antiferromagnetism. Additionally, the doped B accelerates the change from antiferromagnetic to ferromagnetic for the austenite, but B-doping decreases the stability of the whole alloy system. The Co and B co-doping increases the stiffness of the NiMnTi alloy but decreases toughness and plasticity. However, the toughness and plasticity of the NiCoMnTiB alloy are better than those of the NiMnTiB alloy, indicating that the Co doping increases the d-orbital hybridization in the NiMnTiB alloy. The above results are expected to support the performance design of the NiMnTi-based alloy.