Ab Initio Density Functional Theory Research Articles

Universal Principle for Large-Scale Production of a High-Quality Two-Dimensional Monolayer via Positive Charge-Driven Exfoliation

On the basis of the intrinsic characteristics of the layered materials, here we report a universal principle for the production of intact monolayers via layer-by-layer exfoliation from their bulk via positive charge doping. At experimental accessible densities (nc) of ∼1014 cm-2, various multilayer crystals, including graphite, hexagonal boron nitride, transition metal dichalcogenides, MXenes, and black phosphorus, can be exfoliated into the corresponding monolayers through ab initio density functional theory stimulations. The carrier critical thresholds for exfoliating are found to be nearly independent of thickness but dependent on surface size. The universality of positive charge-driven exfoliation originates from the common intrinsic characteristics of electronic structures for layered materials. The positively doped charges that preferentially accumulate near the surface induce interlayer repulsion, leading to layer-by-layer exfoliation when repulsion surpasses interlayer van der Waals force. This strategy may open the possibility of producing diverse high-quality two-dimensional monolayers with a small number of defects toward large-scale manufacturing.

Nuclear moments of indium isotopes reveal abrupt change at magic number 82.

In spite of the high-density and strongly correlated nature of the atomic nucleus, experimental and theoretical evidence suggests that around particular 'magic' numbers of nucleons, nuclear properties are governed by a single unpaired nucleon1,2. A microscopic understanding of the extent of this behaviour and its evolution in neutron-rich nuclei remains an open question in nuclear physics3-5. The indium isotopes are considered a textbook example of this phenomenon6, in which the constancy of their electromagnetic properties indicated that a single unpaired proton hole can provide the identity of a complex many-nucleon system6,7. Here we present precision laser spectroscopy measurements performed to investigate the validity of this simple single-particle picture. Observation of an abrupt change in the dipole moment at N = 82 indicates that, whereas the single-particle picture indeed dominates at neutron magic number N = 82 (refs. 2,8), it does not for previously studied isotopes. To investigate the microscopic origin of these observations, our work provides a combined effort with developments in two complementary nuclear many-body methods: ab initio valence-space in-medium similarity renormalization group and density functional theory (DFT). We find that the inclusion of time-symmetry-breaking mean fields is essential for a correct description of nuclear magnetic properties, which were previously poorly constrained. These experimental and theoretical findings are key to understanding how seemingly simple single-particle phenomena naturally emerge from complex interactions among protons and neutrons.

Characterizing particle-like charge-migration dynamics with high-order harmonic sideband spectroscopy

We introduce high-harmonic sideband spectroscopy (HHSS) and show that it can be a robust probe of attosecond charge migration (CM) in a halogenated carbon-chain molecule. We simulate both the CM and harmonic-generation (HHG) dynamics using ab initio time-dependent density-functional theory. We find that CM dynamics initiated along the molecular backbone induces sidebands in the HHG spectrum driven by a delayed laser pulse that is polarized perpendicular to the molecular axis. Monitoring the spectrum as either the HHG laser frequency or the relative delay is scanned allows for the extraction of detailed information about the time-domain characteristics of the CM process.

Hydrogen-induced enhancement of thermal stability in VZr(H) metallic glasses

Prediction of crystallization temperatures in metallic glasses is still an open question. Investigations of multi component alloys are common in the literature, however, binary and ternary alloys are more suitable for fundamental studies due to their simplicity. Here, we show that a low thermodynamic driving force for crystallization can be associated with a high crystallization temperature. The driving force is determined by calculating – for the first time in metallic glasses – the temperature dependent Gibbs free energies of the alloys using ab initio density functional theory, in combination with the stochastic quenching method. The crystallization temperatures of VxZr100–x and VxZr67–xH33 have been determined using simultaneous in-situ x-ray scattering techniques and resistivity measurements. The onset of crystallization is found to exhibit a parabolic dependence throughout the composition range, whereas alloying with hydrogen increases the thermal stability up to 150 K close to the amorphous-crystalline boundaries. These findings suggest that hydrogen acts as an alloying element with the ability to dynamically tune the intrinsic properties of the material. Lastly, temperature-dependent descriptions of the Gibbs free energy and kinetic considerations of a metallic glass are necessary for a complete characterization of the crystallization process.

A Computational Study: Structural and Electronic Properties of Some Transition Metal Doped Bilayer Graphene Systems

Graphene, which is accepted as the main material of nanomaterials, attracts great attention thanks to its applicability in almost every field and its superior properties. The zero-band graphene gap is a problem that scientists must overcome in designing new electronics. Tailoring the electronic properties of graphene systems by making a change in the band gap allow us great advantages. In this study, Intercalation of transition metal (TM) atom to graphene systems as sandwich-like graphene|TM|graphene structures were investigated by ab initio first principle Density Functional Theory (DFT) computations. GGA with BPW91 basis set were used for DFT calculations. DFT calculations were performed on W, Re, and Os transition metal atoms intercalted between bilayer graphene (BLG). After geometry optimization of graphene|TM|graphene structures, graphene layers doped with nitrogen atoms by substitutional doping for investigation of change in electronic behavior. The electronic behaviour of metal intercalated BLG structures can be modified by type of transition metal and dopant as a result of charge transfer. Substitutional doping with nitrogen atoms to graphene structures showed a change in local density due to the charge transfer as a result of its extra one electron. Placing a transition metal atom between BLG layers leads to constriction in the band gap with a boost in conductive character.

Dearomatization of Benzenoid Arenes Triggered by Triplet Excited State Intramolecular Proton Transfer.

The detailed mechanism of photoinduced dearomatization of benzenoid arenes is investigated using both the high-level ab initio method and density functional theory. The results suggest that the optically allowed singlet excited state (S2) can quickly decay to the lowest triplet excited state (T1) through a barrierless internal conversion and intersystem crossing. Importantly, we find a triplet excited state intramolecular proton transfer (T-ESIPT) pathway to produce a diradical triplet intermediate (3MO-H), which can trigger the subsequent [4 + 2] dearomatization reaction. Furthermore, the diastereoselectivity of the reaction was illustrated by the rotation of the O-H group of 3MO-H, which could be effectively modulated by the solvent effect (arising from the strength of the intermolecular hydrogen bond) and the substituted effect (arising from the strength of the electron-donation group). This photochemical mechanism can explain well the experimental observations, and the novel T-ESIPT process can open a new door in studying the photoinduced proton transfer reactions.

Electronic and crystal structures of α- and β- gold selenides

Recent discoveries of two-dimensional (2D) materials are of significant relevance for basic research and the creation of new electronic devices. Crystal and electronic structures have been studied extensively on unexplored 2D gold selenide (AuSe) using ab-initio density functional theory. AuSe exists primarily in two phases: α- and β-AuSe. Our ground-state calculations show that the β-AuSe phase should be more stable than the α-AuSe phase. The small energy difference (∼16 meV) between these two phases allows them to interchange from α- to β-AuSe or vice-versa under different external thermodynamic conditions. We found that α-AuSe is metallic, whereas β-AuSe is semiconductors with an indirect bandgap of ∼193 meV. Fermi surface calculations of metallic α-AuSe show that hole-like inter-penetrating cylindrical Fermi surfaces around Γ point, indicating the one-dimensional character along the b* direction. The calculated electron localization function demonstrates the nature of bonding and various shared-electron interactions in AuSe. It has been found that ionic bonding is significant among gold and selenium nuclei. Our results indicate the significance of 2D AuSe and thus encourage more experimental research.

Magnetic proximity induced valley-contrasting quantum anomalous Hall effect in a graphene- CrBr3 van der Waals heterostructure

The magnetic proximity effect is an imperative tool to comprehend the valley contrasting quantum anomalous Hall (QAH) effect in van der Waals (vdW) heterostructure. The introduction of magnetic exchange and spin orbit interaction together enables to realize topological phases, with a particular emphasis on an interface can be envisioned towards dissipationless electronics. Herein, the proximity coupled valley contrasting QAH effect is predicted in vdW heterostructure consisting of graphene and a ferromagnetic (FM) semiconductor ${\mathrm{CrBr}}_{3}$ with the implication of relativistic effect, from the ab initio density functional theory (DFT) simulation. The valley contrasting QAH effect is observed with a nonzero Chern number at a high-symmetry point stemming from Berry curvature and Wannier charge center (WCC), leading to a topologically nontrivial state. The occurrence of strong magnetic proximity coupling between graphene and ${\mathrm{CrBr}}_{3}$ monolayer is realized intrinsically, from shifting of Hall coefficient value near Fermi level. The opening of the global band gap (178 meV) is observed with the inclusion of spin orbit coupling (SOC). The anomalous Hall conductivity (AHC) demonstrates the presence of two maxima peaked at valley ${K}^{\ensuremath{'}}$ and $K$. The observation of AHC is mainly dominated by nonzero surface charge and localized potential at the heterointerface due to proximity interaction. The Fermi level is found to be located exactly inside the nontrivial global band gap, which can be tuned effectively by applying the external electric field or by introducing a staggered sublattice potential. This robustness makes experimental fabrication highly favorable for developing a valley contrasting QAH device prototype.

Spin splitting in monoperiodic systems described by magnetic line groups

In this paper we report the classification of all the 81 magnetic line group families into seven spin splitting prototypes, in analogy to the similar classification previously reported for the 1651 magnetic space groups, 528 magnetic layer groups, and 394 magnetic rod groups. According to this classification, electrically induced (Pekar–Rashba) spin splitting is possible in the antiferromagnetic structures described by magnetic line groups of type I (no anti-unitary operations) and III, both in the presence and in the absence of the space inversion operation. As a specific example, a group theoretical analysis of spin splitting in CoO (8, 8) nanotube is carried out and its predictions are confirmed by ab initio density functional theory calculations.

Surface-Preferred Crystal Plane Growth Enabled by Underpotential Deposited Monolayer toward Dendrite-Free Zinc Anode.

Aqueous Zn batteries with ideal energy density and absolute safety are deemed the most promising candidates for next-generation energy storage systems. Nevertheless, stubborn dendrite formation and notorious parasitic reactions on the Zn metal anode have significantly compromised the Coulombic efficiency (CE) and cycling stability, severely impeding the Zn metal batteries from being deployed in the proposed applications. Herein, instead of random growth of Zn dendrites, a guided preferential growth of planar Zn layers is accomplished via atomic-scale matching of the surface lattice between the hexagonal close-packed (hcp) Zn(002) and face-centered cubic (fcc) Cu(100) crystal planes, as well as underpotential deposition (UPD)-enabled zincophilicity. The underlying mechanism of uniform Zn plating/stripping on the Cu(100) surface is demonstrated by ab initio molecular dynamics simulations and density functional theory calculations. The results show that each Zn atom layer is driven to grow along the exposed closest packed plane (002) in hcp Zn metal with a low lattice mismatch with Cu(100), leading to compact and planar Zn deposition. In situ optical visualization inspection is adopted to monitor the dynamic morphology evolution of such planar Zn layers. With this surface texture, the Zn anode exhibits exceptional reversibility with an ultrahigh Coulombic efficiency (CE) of 99.9%. The MnO2//Zn@Cu(100) full battery delivers long cycling stability over 548 cycles and outstanding specific energy and power density (112.5 Wh kg-1 even at 9897.1 W kg-1). This work is expected to address the issues associated with Zn metal anodes and promote the development of high-energy rechargeable Zn metal batteries.

Fabrication and characterization of high efficiency and stable Ag/AgFeO2/Ag3PO4 ternary heterostructures nanocatalyst

An Ag/AgFeO2/Ag3PO4 ternary was synthesized by hydrothermal method with polyvinylpyrrolidone (PVP). The composite materials were characterized by XRD, SEM, TEM, DRS and XPS. XRD, SEM and TEM results are used to characterize the structure and morphology of Ag/AgFeO2/Ag3PO4 samples, DRS results are mainly used to characterize the light absorption capacity of Ag/AgFeO2/Ag3PO4 samples. Photocatalytic results showed that the photocatalytic performance of Ag/AgFeO2/Ag3PO4 photocatalyst was significantly improved, which was 10.6 times than that of pure AgFeO2. The optimal photocatalyst can degrade MO (Methyl Orange) up to 98% in 1 h. Simultaneously, the cyclic experiments showed that it had good stability, from 80% for Ag/AgFeO2/AgPO4 to 58% for pure Ag3PO4 after five cycles. To obtain further insight into the high photooxidative activity of AgFeO2, ab initio density functional theory (DFT) calculations have also been carried out. The mechanism study shows that the synergistic effect of heterojunction and strong SPR of silver nanorods make the catalyst have higher photocatalytic performance and better stability.

Local atomic ordering strategy for high strength Mg alloy design by first-principle calculations

In this study, solute X (X = Li, Al, Mn, Zn, Y, Zr, Nd, and Gd) solution strengthening in Mg alloys have been screened by ab initio density functional theory calculations to quantify not only the substitutional stacking-fault configurations but also the solute ordering sequence as a function of local segregation. Interestingly, it has been found that the strengthening effects of single atom addition to a supercell made of 64 atoms can be mostly attributed to lattice distortions (Mn>Nd>Gd>Y>Zn>Al>Zr>Li), while the local ordering arrangements of Mg-X complex actually contribute most to the strengthening when the solute concentration rises. For example, Nd can induce a large local atomic ordering and significantly increase the basal critical-resolved shear stress (CRSS). A linear relationship between solute concentrations and ideal strength, and the quasi-quadratic relationship between solute atomic radii and ideal strength have been observed. Simultaneously, the higher the solute concentration, the higher degree of the solid solution strengthening, resulting in a smaller quasi-quadratic function curve opening. Based on the screening of the chemical (including stacking fault energy and atomic bonds) and strain (lattice distortion energy) energy calculations, we have discovered that the solute strengthening follows the ordering sequence of Nd> Mn> Gd> Y> Zn> Al> Zr> Li. • Local ordering of Mg-X alloys was found by thermodynamic evaluations using DFT. • Both chemical and strain energy contribute to the solute strengthening ordering. • Local ordering configuration in solid solution strengthening guide the alloy by design.

High-entropy transition metal nitride thin films alloyed with Al: Microstructure, phase composition and mechanical properties

Deviation from equimolar composition in high-entropy multielement ceramics offers a possibility of fine-tuning the materials’ properties for targeted application. Here, we present a systematic experimental and theoretical study on the effects of alloying equimolar pentanary (TiHfNbVZr)N and hexanary (TiHfNbVZrTa)N high-entropy nitrides with Al. Although being predicted to be metastable by ab initio density-functional theory calculations, single-phase fcc NaCl-structured solid solution thin films with Al solubility limits as high as x ∼ 0.51–0.61 in (TiHfNbVZr)1-xAlxN and x ∼ 0.45–0.64 in (TiHfNbVZrTa)1-xAlxN are synthesised utilizing a hybrid deposition technique that offers dynamic mixing of film atoms from Al+ subplantation and non-equilibrium growth conditions leading to quenching of the desired film structure. In experimental studies supplemented with density-functional theory calculations, it is demonstrated that Al concentration in alloys with the multielement compositions of high-entropy nitride thin films determine hardness, yield strength, toughness, and ability to deform plastically up to fracture due to different deformation mechanisms arising from the electronic structure and phase compositions.

First principle understanding of antiferroelectric ordering in La-doped silver niobate

The antiferroelectric Pbcm phase of silver niobate (AgNbO3) has received increasing attention owing to its environmental safety (as it is lead-free), compatibility, and superior energy-storage density compared with its ferroelectric counterparts. We comprehensively investigated the effects of La doping at Ag sites (i.e., Ag0.88La0.12NbO3) on the structural, electronic, and ferroelectric properties of AgNbO3 by using ab initio density functional theory calculations and the Berry phase method for polarization calculations, respectively. The polarization in the [001] direction of pristine AgNbO3 is estimated to be 1.9 μC/cm2, which is significantly enhanced (to 14.86 μC/cm2) for Ag0.88La0.12NbO3. The enhancement in the spontaneous polarization of AgNbO3 after the incorporation of La is attributed to suppressed distortion in the Nb–O–Nb antiparallel ordering and tilting of the NbO6 octahedra. Furthermore, the electron density maps of AgNbO3 and its doped counterpart were calculated to investigate the lattice-distortion and bond-change observations.

First Principle Calculation of Accurate Electronic and Related Properties of Zinc Blende Indium Arsenide (zb-InAs).

We carried out a density functional theory (DFT) study of the electronic and related properties of zinc blende indium arsenide (zb-InAs). These related properties include the total and partial densities of states and electron and hole effective masses. We utilized the local density approximation (LDA) potential of Ceperley and Alder. Instead of the conventional practice of performing self-consistent calculations with a single basis set, albeit judiciously selected, we do several self-consistent calculations with successively augmented basis sets to search for and reach the ground state of the material. As such, our calculations strictly adhere to the conditions of validity of DFT and the results are fully supported by the theory, which explains the agreement between our findings and corresponding, experimental results. Indeed, unlike some 21 previous ab initio DFT calculations that reported zb-InAs band gaps that are negative or zero, we found the room temperature measured value of 0.360 eV. It is a clear achievement to reproduce not only the locations of the peaks in the valence band density of states, but also the measured values of the electron and hole effective masses. This agreement with experimental results underscores not only the correct description of the band gap, but also of the overall structure of the bands, including their curvatures in the vicinities of the conduction band minimum (CBM) and of the valence band maximum (VBM).

Structural, electronic and magnetic properties of inverse spinel NiFe2O4: DFT + U investigation

We have used ab initio density functional theory (DFT) to explore the structural, mechanical, electronic and magnetic properties of inverse spinel NiFe2O4. The ultra-soft pseudopotential method parametrized under generalized gradient approximation (GGA) for the exchange-correlation functional with Hubbard correction (GGA + U) was applied. The equilibrium lattice parameter was estimated to be 8.4620 Å and the 3 independent elastic constants C11, C12 and C44 were found to be 304.12 GPa, 144.45 GPa and 57.46 GPa, respectively. GGA + U formalism could successfully explain the semiconducting and ferrimagnetic behaviour of NiFe2O4. The spin-polarized electronic density of states reveals that the p−d hybridization between O and Fe atoms is responsible for the semiconducting nature of NiFe2O4. The ferrimagnetic behaviour of inverse spinel NiFe2O4 can be explained through the anti-parallel spin configuration of tetrahedrally and octahedrally occupied Fe atoms. The resultant magnetic moment is thus solely contributed by the octahedrally occupied Ni atoms. Spin-polarized band structure calculations reveal indirect band gap of 1.6 eV for spin-up channel and direct band gap of 2.2 eV for spin-down channel.

Multi-task graph neural networks for simultaneous prediction of global and atomic properties in ferromagnetic systems * * This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable,

We introduce a multi-tasking graph convolutional neural network, HydraGNN, to simultaneously predict both global and atomic physical properties and demonstrate with ferromagnetic materials. We train HydraGNN on an open-source ab initio density functional theory (DFT) dataset for iron-platinum with a fixed body centered tetragonal lattice structure and fixed volume to simultaneously predict the mixing enthalpy (a global feature of the system), the atomic charge transfer, and the atomic magnetic moment across configurations that span the entire compositional range. By taking advantage of underlying physical correlations between material properties, multi-task learning (MTL) with HydraGNN provides effective training even with modest amounts of data. Moreover, this is achieved with just one architecture instead of three, as required by single-task learning (STL). The first convolutional layers of the HydraGNN architecture are shared by all learning tasks and extract features common to all material properties. The following layers discriminate the features of the different properties, the results of which are fed to the separate heads of the final layer to produce predictions. Numerical results show that HydraGNN effectively captures the relation between the configurational entropy and the material properties over the entire compositional range. Overall, the accuracy of simultaneous MTL predictions is comparable to the accuracy of the STL predictions. In addition, the computational cost of training HydraGNN for MTL is much lower than the original DFT calculations and also lower than training separate STL models for each property.

Modeling magnesium surfaces and their dissolution in an aqueous environment using an implicit solvent model.

Magnesium has attracted growing interest for its use in various applications, primarily due to its abundance, lightweight properties, and relatively low cost. However, one major drawback to its widespread use remains to be its reactivity in aqueous environments, which is poorly understood at the atomistic level. Ab initio density functional theory methods are particularly well suited to bridge this knowledge gap, but the explicit simulation of electrified water/metal interfaces is often too costly from a computational viewpoint. Here, we investigate water/Mg interfaces using the computationally efficient implicit solvent model VASPsol. We show that the Mg (0001), (101̄0), and (101̄1) surfaces each form different electrochemical double layers due to the anisotropic smoothing of the electron density at their surfaces, following Smoluchowski rules. We highlight the dependence that the position of the diffuse cavity surrounding the interface has on the potential of zero charge and the electron double layer capacitance, and how these parameters are also affected by the addition of explicit water and adsorbed OH molecules. Finally, we calculate the equilibrium potential of Mg2+/Mg0 in an aqueous environment to be -2.46V vs a standard hydrogen electrode, in excellent agreement with the experiment.

Inelastic scattering of electrons in water from first principles: cross sections and inelastic mean free path for use in Monte Carlo track-structure simulations of biological damage.

Modelling the inelastic scattering of electrons in water is fundamental, given their crucial role in biological damage. In Monte Carlo track-structure (MC-TS) codes used to assess biological damage, the energy loss function (ELF), from which cross sections are extracted, is derived from different semi-empirical optical models. Only recently have first ab initio results for the ELF and cross sections in water become available. For benchmarking purpose, in this work, we present ab initio linear-response time-dependent density functional theory calculations of the ELF of liquid water. We calculated the inelastic scattering cross sections, inelastic mean free paths, and electronic stopping power and compared our results with recent calculations and experimental data showing a good agreement. In addition, we provide an in-depth analysis of the contributions of different molecular orbitals, species and orbital angular momenta to the total ELF. Moreover, we present single-differential cross sections computed for each molecular orbital channel, which should prove useful for MC-TS simulations.

Stable Al2O3 Encapsulation of MoS2‐FETs Enabled by CVD Grown h‐BN

AbstractMolybdenum disulfide (MoS2) has great potential as a two‐dimensional semiconductor for electronic and optoelectronic application, but its high sensitivity to environmental adsorbents and charge transfer from neighboring dielectrics can lead to device variability and instability. Aluminum oxide (Al2O3) is widely used as an encapsulation layer in (opto)‐electronics, but it leads to detrimental charge transfer n‐doping to MoS2. Here, this work reports a scalable encapsulation approach for MoS2 field‐effect transistors (FETs) where hexagonal boron nitride (h‐BN) monolayers are employed as a barrier layer in‐between each of the Al2O3 and MoS2 interfaces. These devices exhibit a significant reduction of charge transfer, when compared to structures without h‐BN. This benefit of h‐BN in the gate stack is confirmed by ab initio density functional theory calculations. In addition, the devices with h‐BN layers show very low hysteresis even under ambient operating conditions.