Spin-polarized Density Functional Theory Research Articles

Defect induced magnetism on SiC monolayer

Using spin-polarized density functional theory, magnetic induction by means of vacancy defect was studied on SiC monolayer. While pristine SiC monolayer is found to be a semiconductor with a direct band gap of 2.64eV, vacancy of both Si and C defect transforms it into metallic ferromagnetic material. Calculated magnetization density shows that Si vacancy results in a magnetic moment of while C vacancy results to a magnetic moment of . Observations reveals that magnetism mostly arises from the unpaired electrons that arise as a result of the removed Si/C atom and spin asymmetry arises mainly from the p-orbitals of Si/C as a result of vacancy. Our result shows that magnetism can be introduced in SiC monolayer effectively using vacancy defect and can further be altered depending on the type of defect

Spin-Topological Electronic Valve in Ni/hBN–Graphene–hBN/Ni Magnetic Junction

A spin-topological electronic valve was discovered in a Ni/hBN–graphene–hBN/Ni magnetic junction to control the in-plane conductance of graphene. By manipulating the mass-gapped Dirac cone (MGDC) of graphene’s topology using the magnetic proximity effect, the spin-topological electronic valve was made possible. The first-principles investigation was conducted to show how the mechanism of graphene’s MGDC is controlled. Twelve stacking configurations for the anti-parallel configuration (APC) and parallel configuration (PC) of the magnetic alignment of Ni slabs were calculated using spin-polarized density functional theory. Three groups can be made based on the relative total energy of the 12 stacking configurations, which corresponds to a van der Waals interaction between hBN and graphene. Each group exhibits distinctive features of graphene’s MGDC. The configuration of the Ni(111) surface state’s interaction with graphene as an evanescent wave significantly impacts how the MGDC behaves. By utilizing the special properties of graphene’s MGDC, which depend on the stacking configuration, a controllable MGDC using mechanical motion was proposed by suggesting a device that can translate the top and bottom Ni(111)/hBN slabs. By changing the stacking configuration from Group I to II and II to III, three different in-plane conductances of graphene were observed, corresponding to three non-volatile memory states. This device provides insight into MJs having three or more non-volatile memory states that cannot be found in conventional MJs.

Magnetic Moment Is an Effective Descriptor for Electrocatalytic Nitrogen Reduction Reaction on Two-Dimensional Organometallic Nanosheets

Electrocatalytic reduction of nitrogen to ammonia (eNRR) under ambient condition is a potential sustainable and promising alternative to the traditional Haber-Bosch process. However, this electrochemical transformation is limited by the high overpotential, poor selectivity, low efficiency, and low yield. Herein, a new class of two-dimensional (2D) organometallic nanosheets c-TM-TCNE (c = cross motif, TM = 3d/4d/5d transition metals, TCNE = tetracyanoethylene) were comprehensively investigated as potential electrocatalysts for eNRR through high-throughput screening combined with spin-polarized density functional theory computations. After a multistep screening and follow-up systematic evaluation, c-Mo-TCNE and c-Nb-TCNE were selected as eligible catalysts, and c-Mo-TCNE showed the lowest limiting potential of -0.35 V via a distal pathway, displaying high catalytic performance. In addition, the desorption of NH3 from the surface of c-Mo-TCNE catalyst is also easy, with the free energy being 0.34 eV. Furthermore, the stability, metallicity, and eNRR selectivity are preeminent, making c-Mo-TCNE a promising catalyst. Unexpectedly, the magnetic moment of the transition metal shows a strong correlation with the catalytic activity (limiting potential), i.e., the larger the magnetic moment of the transition metal, the smaller the limiting potential of the electrocatalyst. The Mo atom has the largest magnetic moment and the c-Mo-TCNE catalyst features the smallest magnitude of limiting potential. Thus, the magnetic moment can be used as an effective descriptor for eNRR on c-TM-TCNE catalysts. The present study opens a way toward the rational design of highly efficient electrocatalysts for eNRR with novel two-dimensional functional materials. This work will promote further experimental efforts in this field.

Organic Stable Radical Oligomers as Spin Filters

Single-molecule magnetic junctions provide versatile platforms for use in manipulating spin-polarized currents via fine-tuning the electronic properties of the sandwiched molecule. We employed the nonequilibrium Green’s function and spin-polarized density functional theory simulations to investigate the performances of high-spin organic oligoradicals as spin filters sandwiched between two gold electrodes. The spin-polarized current could be obtained using the junction bearing a high-spin molecule with almost unity spin filtering efficiency. Moreover, the spin filtering efficiency could be tuned by varying the number of magnetic units, e.g., di-, tri-, and tetraradicals. Transmission eigenchannel, spin orbital, and density of states analyses revealed how the magnetic exchange interaction in a molecule modulates the dominant electron conducting channels and spin filtering efficiency. The use of a tunable organic magnetic molecular junction system may be a promising approach in manipulating the spin-polarized currents in spintronic devices.

Binuclear spin-crossover [Fe(bt)(NCS)2]2(bpm) complex: A study using first principles calculations.

The spin-crossover [Fe(bt)(NCS)2]2(bpm) complex is studied using spin-polarized density functional theory within the generalized gradient approximation, the Hubbard U and the weak van der Waals interactions in conjunction with the projector augmented wave method in its molecular and periodic arrangements. It is shown that the considered complex has three magnetic configurations [high spin state (HS)-HS, HS-low spin state (LS), and LS-LS] corresponding to those observed experimentally after two transition temperatures Tc (1) of 163K and Tc (2) of 197K. For the HS-HS magnetic state, we found that the two Fe centers are antiferromagnetically coupled for both molecular and periodic structures in good agreement with the experimental observations. Our results show that the computed total energy difference between the magnetic state configurations of the considered Fe2 complex is significantly smaller compared to those reported in the literature for other mono- or binuclear compounds.

Small bimetallic clusters Agn-1M (M = Au, Co, Cu, Ni, Pd, Pt; n = 3, 9, 15): Density functional theory and genetic algorithm

We investigated the effect of size and composition on the properties of bimetallic nanoclusters. The geometric structures, stabilities, and electronic properties of size-selected Agn-1M (M = Au, Co, Cu, Ni, Pd, Pt; n = 3, 9, 15) bimetallic nanoclusters are systematically analyzed using spin-polarized density functional theory (DFT) within the generalized gradient approximation (GGA). We determine the most stable geometries for these clusters using a genetic algorithm (GA) in combination with DFT. Our results show that doping pure silver clusters with an M atom (transition metal), referred to as a “guest atom”, increases the stability as compared to pure Agn (n = 3, 9, 15) clusters. The results for various properties including formation energy per atom, electronic structure, magnetic moments, and vibrational density of states (VDOS) are evaluated as a function of both size and composition of the system. The adsorption of selected bimetallic clusters on hydroxylated alumina substrate shows weak binding and minor changes in geometric properties except for Ag8Pt.

Accelerating self-consistent field iterations in Kohn-Sham density functional theory using a low-rank approximation of the dielectric matrix

We present an efficient preconditioning technique for accelerating the fixed point iteration in real-space Kohn-Sham density functional theory (DFT) calculations. The preconditioner uses a low rank approximation of the dielectric matrix (LRDM) based on G\^ateaux derivatives of the residual of fixed point iteration along appropriately chosen direction functions. We develop a computationally efficient method to evaluate these G\^ateaux derivatives in conjunction with the Chebyshev filtered subspace iteration procedure, an approach widely used in large-scale Kohn-Sham DFT calculations. Further, we propose a variant of LRDM preconditioner based on adaptive accumulation of low-rank approximations from previous SCF iterations, and also extend the LRDM preconditioner to spin-polarized Kohn-Sham DFT calculations. We demonstrate the robustness and efficiency of the LRDM preconditioner against other widely used preconditioners on a range of benchmark systems with sizes ranging from $\sim$ 100-1100 atoms ($\sim$ 500--20,000 electrons). The benchmark systems include various combinations of metal-insulating-semiconducting heterogeneous material systems, nanoparticles with localized $d$ orbitals near the Fermi energy, nanofilm with metal dopants, and magnetic systems. In all benchmark systems, the LRDM preconditioner converges robustly within 20--30 iterations. In contrast, other widely used preconditioners show slow convergence in many cases, as well as divergence of the fixed point iteration in some cases. Finally, we demonstrate the computational efficiency afforded by the LRDM method, with up to 3.4$\times$ reduction in computational cost for the total ground-state calculation compared to other preconditioners.

Design of molecular M[sbnd]N[sbnd]C dual-atom catalysts for nitrogen reduction starting from surface state analysis

Under electrocatalytic conditions, the state of a catalyst surface (e.g., adsorbate coverage) can be very different from a pristine form due to the existing conversion equilibrium between water and H- and O-containing adsorbates. Dismissing the analysis of the catalyst surface state under operating conditionsmay lead to misleading guidelines for experiments. Given that confirming the actual active site of the catalyst under operating conditions is indispensable to providing practical guidance for experiments, herein, we analyzed the relations between the Gibbs free energy and the potential of a new type of molecular metal-nitrogen-carbon (MNC) dual-atom catalysts (DACs) with a unique 5 N-coordination environment, by spin-polarized density functional theory (DFT) and surface Pourbaix diagram calculations. Analyzing the derived surface Pourbaix diagrams, we screened out three catalysts, N3-Ni-Ni-N2, N3-Co-Ni-N2, and N3-Ni-Co-N2, to further study the activity of nitrogen reduction reaction (NRR). The results display that N3-Co-Ni-N2 is a promising NRR catalyst with a relatively low ΔG of 0.49 eV and slow kinetics of the competing hydrogen evolution. This work proposes a new strategy to guide DAC experiments more precisely: the analysis of the surface occupancy state of the catalysts under electrochemical conditions should be performed before activity analysis.

Improvement in the resistance to KCl and PbCl2 synergistic poisoning of the commercial SCR catalyst by Ce(SO4)2 modification: A combined experimental and spin-polarized DFT study

The commercial selective catalytic reduction (SCR) catalysts are vulnerable to the synergistic poisoning of K and Pb species in flue gas, two common and typical components that can trigger severe damage on the physical and chemical properties of the catalysts. In this study, a Ce(SO4)2-V2O5-MoO3/TiO2 catalyst with 15 wt% Ce(SO4)2 loading was developed which maintained high activity under the synergistic poisoning of 2 wt% KCl and PbCl2. The resistance mechanisms were explored by a series of characterization techniques and density functional theory (DFT) calculations. The results indicated that the enhancement on the resistance to the synergistic poisoning of K and Pb species by the introduction of Ce(SO4)2 could be elucidated in three aspects. Firstly, the interactions between Ce(SO4)2 and K/Pb species protected the V and Mo species from the synergistic poisoning. Secondly, the Ce4+/Ce3+ redox couples and large amounts of chemisorbed oxygen improved the redox ability of the catalyst. Finally, the Ce(SO4)2 modified catalyst maintained a high surface acidity despite the synergistic poisoning of K and Pb species, which was beneficial for the adsorption of NH3 species and the following SCR process.

Structure and size-dependent vibrational and thermal properties of Ni clusters: A systematic ab initio approach

There is scarce information on the vibrational and thermal properties of small Ni clusters. Here, the outcomes of ab initio spin-polarized density functional theory calculations on the size and geometry effects upon the vibrational and thermal properties of Nin (n = 13 and 55) clusters, are discussed. For theses clusters a comparison is presented between the closed shell symmetric octahedral (Oh) and the icosahedral (Ih) geometries. The results indicate that the Ih isomers are lower in energy. Besides, ab initio molecular dynamics runs at T = 300K show that Ni13 and Ni55 clusters transform from their initial Oh geometries towards the corresponding Ih ones. For Ni13, we also consider the lowest energy less symmetric layered 1-3-6-3 structure, and the cuboid, recently observed experimentally for Pt13, which is competitive in energy but is unstable, as phonon analysis reveals. We calculate their vibrational density of states (νDOS) and heat capacity, and compare with the Ni FCC bulk counterpart. The characteristic features of the νDOS curves of these clusters are interpreted in terms of the clusters’ sizes, the interatomic distance contractions, the bond order values as well as the internal pressure and strains of the clusters. We find that the softest possible frequency of the clusters is size and structure-dependent, being the smallest for the Oh ones. We identify mostly shear, tangential type displacements involving mainly surface atoms for the lowest frequency of the spectra of both Ih and Oh isomers. For the maximum frequencies of these clusters the central atom shows anti-phase movements against groups of nearest neighbor atoms.An excess of heat capacity at low temperatures with respect to the bulk is found, while at high temperatures a constant limiting value, close but lower to the Dulong and Petit value, is determined.

Effect of Magnetic Coupling on the Optical Properties of Oxide Co Nanowires on Vicinal Pt Surfaces

Nowadays, modern scientific research has sparked a renewed interest to study the interaction of electromagnetic field (EM) with magnetic nanostructures and in particular in nanophotonics and spintronics. The current work is devoted to an ab initio study of the magneto-optical properties of step-decorated oxide Co nanowires (1D oxides) on vicinal Pt surfaces. Theoretical calculations of the magnetic moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The first-principles calculations revealed the effect of magnetic coupling between cobalt spins on refractivity and extinction spectra of these 1D oxides governed by atomic structure and cobalt-oxygen interaction within a nanowire at the step edge. The emergence of a sharp pronounced peak in the spectral difference of the refractive indexes has been observed between ferromagnetic and antiferromagnetic configurations of the nanowire. Anisotropy of an extinction coefficient in the terahertz (THz) range of the spectra was established for oxide Co nanowires in an antiferromagnetic state in contrast with a ferromagnetic one.

Heterogeneous metal trimer catalysts on Mo2TiC2O2 MXene for highly active N2 conversion to NH3

Activating the inert basal plane of functional-group-terminated MXenes is highly desirable for non-noble metal MXenes applied for the N2 reduction reaction (NRR) to value-added ammonia (NH3), but remains a significant challenge. Herein, we report the coupled interactions and catalytic mechanism of the supported trimeric metal M3 single-cluster catalysts (SCCs) on oxygen-vacancy-rich Mo2TiC2O2 (VO−Mo2TiC2O2) MXene materials for catalyzing this difficult reaction via spin-polarized density functional theory (DFT) calculations. Energy and electronic property analysis showed that the robust structural stability of the M3/VO−Mo2TiC2O2 catalysts stemmed from the vacancy defects, electronegativity difference, and significant charge redistribution. Based on the unique bidirectional electron transfer activation mechanism, the N2 adsorption strength can be well represented by a simple descriptor of the valence electrons in the d orbitals of M3 trimers. Remarkably, these confined Rh3, Ru3, Ni3 and Co3 SCCs on VO−Mo2TiC2O2 were capable of providing superior electrocatalytic activity and selectivity toward the NRR with ultra-low limiting potentials of −0.23, −0.32, −0.42 and −0.44 V, respectively. Finally, a mechanistic investigation illustrated that the valence electrons, charge transfer, and d-band center of the M3 active sites could collectively act as good descriptors to correlate the structure and NRR activity.

Vacancy defects in nitrogen doped diamond

The introduction of nitrogen-vacancy (N–V) defects in diamond is important as they have impact on the optical and electronic properties of diamond-based devices. Here, spin polarised density functional theory simulations are used to find the lowest energy structures of V, VV, NN, NV, NV2, N2V and N2V2 and calculate the energies associated with the formation of clusters from point defects. The results show that there is a significant charge transfer from nearest neighbour C to nitrogen. The energy penalties to introduce N–V defects in diamond are very high, much higher than in Si or Ge. Strong exoergic binding energies are calculated to form clusters from isolated defects though formation energies of both the vacancy and N substitutional defects are high, indicating that there is a very strong driving force to form stable defect clusters. Calculated binding energies in Si and Ge are much lower than that calculated in diamond.

S and N coordinated single-atom catalysts for electrochemical CO2 reduction with superior activity and selectivity

As the energy crisis and global warming become ever more serious, the electrochemical reduction of carbon dioxide (CO2RR) using single-atom catalysts (SAC) is a potential strategy to solve these problems by converting notorious CO2 into high value-added products. The electronic structures and the catalytic activity of SAC can be adjusted through changing the coordination environment. In this paper, 28 TM@SxNy SACs (TM = Fe, Co, Ni, Cu; x + y = 4) are constructed and the CO2RR performances toward C1 products are exploited. The calculations based on spin-polarized density functional theory (DFT) show that Cu@S3N1 has the best catalytic performance toward CO, CH3OH and CH4, with the favorable limiting potentials of −0.20 V, −0.36 V, and −0.36 V, respectively. Fe@S1N3 exhibits the best catalytic activity and selectivity toward HCOOH, with the limiting potential of −0.25 V. Electronic structure analysis reveals the catalytic origin of excellent Cu@S3N1 SAC toward CO product. SxNy coordination can partially weaken the scaling relationship between *COOH and *CO, especially for S3N1. Furthermore, the solvation effect and the selectivity of Cu@S3N1 and Fe@S1N3 under the applied potential are illustrated. The results provide a theoretical reference to regulate the CO2RR activity using coordinated SAC.

Synthesis of Graphene Quantum Dots Enhanced Nano Ca(OH)2 from Ammoniated CaCl2

Ca(OH)2 nanoparticles are effective materials for cultural heritage restoration, hazardous substance absorption and photocatalyst. However, many methods are complex, and the particle sizes are usually above 80-100 nm, involving mediocre efficacy for application in the stone restoration field. In this work, Nano Ca(OH)2 with diameters less than 70 nm and composited with Graphene Quantum Dots (GQDs) were successfully synthesized in aqueous media. The morphology and structure of the nanoparticles were investigated with TEM, HRTEM, XRD, Raman and FTIR. The particle size distribution and relative kinetic stability of the Ca(OH)2 in ethanol were performed using a laser particle size analyzer and spectrophotometer. Firstprinciple calculations based on the spin-polarized density functional theory (DFT) were carried out to study the reaction process and combination model. The nanoparticles, as prepared, are composed of primary hexagonal crystals and high ammoniated precursors, which have a positive effect on reducing the grain size, and interacted with the GQDs hybrid process. According to the First-principle calculations results, the energy variation of the whole reaction process and the bonding mode between Ca(OH)2 and GQDs can be understood better.

Pyridinic Dominance N-Doped Graphene: A Potential Material for SO2 Gas Detection.

The sensors based on graphene have shown great promise in the detection of toxic air pollutants that are detrimental to nature and create risks to human health. Many recent experimental and computational efforts have been dedicated to sensor concepts incorporating pure graphene, graphene oxide, and doped graphene. Herein, a combination of spin-polarized density functional theory (DFT) with van der Waals correction and ab initio molecular dynamics (AIMD) approaches are utilized to assess the gas sensing potential of pyridinic dominance N-doped graphene (PNG) toward SO2 detection. The potential of PNG systems as SO2 sensing can be explored through an in-depth analysis of adsorption energies, electronic parameters, charge transfer, selectivity, and thermal stability. It is further demonstrated that external strains and the modulation of external electric fields are two effective ways to modify the adsorption strength. In light of these findings, our studies suggest that PNG monolayers have the potential to be an essential substrate for the detection of SO2.

Ca-driven stable regulatory of alkalinity within desilication products: Experimental, modeling, transformation mechanism and DFT study

The prevalent pH rebound phenomenon in the bauxite residue alkalinity regulation is primarily caused by the presence of alkaline minerals, including sodalite and cancrinite. Calcium ion is widely used to remove the free alkali for reducing the alkalinity of bauxite residue, but its underlying mechanism on alkaline minerals is still unclear. In this work, we investigated the action mechanism of calcium ion on sodalite and cancrinite by various microspectroscopic methods, and then employed spin-polarized density functional theory (DFT) calculations to reveal the reaction pathways of calcium ion substitution and migration in minerals. The calcium ion can effectively regulate the stability of alkaline minerals by inhibiting alkaline ions release, which respectively enters sodalite and cancrinite by displacing Na adsorbed inside the mineral lattice and on the mineral surface. The entered calcium ion acts as competitive protection against sodium during the neutralization process, thus inhibiting the proton-promoted dissolution of sodalite and cancrinite. Moreover, the amount of entry calcium ion controls their acid neutralization ability. DFT calculations revealed calcium ions readily replaced sodium on the internal channels of minerals rather than on the surface. These new findings contribute to the understanding of potential options to directly stabilize critical alkaline components in bauxite residue.

Spin transport property of Cr2C based nanoscale devices: A first principle study

Edge states and interfacial design are important influencing factors for spin transport property modulation of nanoscale devices. Electronic structure and spin charge transmission path of both armchair/zigzag Cr2C nanoribbon and single-porphyrin molecular junctions with Cr2C nanoribbon electrodes are studied based on spin polarized density functional theory and non-equilibrium Green's function method. The results show that both zigzag and armchair oriented Cr2C nanoribbons have intrinsic magnetic half-metallic behavior. Volt-ampere characteristics of zigzag Cr2C nanodevices are linear, and that of armchair Cr2C nanodevice shows obvious saturation effect and negative differential resistance effect (NDR). These two cases maintain both strong spin polarization current intensity and 100% spin filtering efficiency (SFE). In addition, the intercalation of porphyrin molecule significantly reduces electron accumulation near the Fermi level in case of armchair Cr2C nanodevice which retains 100% spin polarization behavior. This work expands a potential application of intrinsic magnetic two-dimensional materials in field of molecular spintronic devices.

Spin-polarized linear dispersions, lattice dynamics and transport properties of quaternary Heusler alloys (LiMgXSb)

In this study, we are predicting a new class of materials obtained by substituting a magnetic transition metal ion in quaternary Heusler alloys with the formula LiMgXSb (X = Co, Cr, Fe, Mn). We found that all the studied alloys exhibit a ferromagnetic ground state except LiMgCoSb. The phonon dispersions of the studied alloys computed within spin-polarized density functional perturbation theory (DFPT) do not exhibit any imaginary frequencies indicating that the proposed alloys are dynamically stable. The band structure calculations show that the studied alloys are metallic and exhibit the states near Fermi level for both spins. The fixed spin moment (FSM) calculations have been employed to exclude the possibility of other ferromagnetic ground states. One of the most interesting aspects of the band structure of these compounds is the existence of multiple band crossings leading to multiple Weyl points in the Brillouin zone. We have also computed the spin-polarized transport properties of these alloys. Our results find that doping charge carriers in the unit cell can significantly enhance the power factor and ZT. Electron doping is more effective for improving the power factor. Our findings show that among all the alloys LiMgFeSb has the highest ZT value with electron doping. Our studied materials constitute a new family of three-dimensional materials exhibiting band degeneracies that lead to Weyl points.

Magnetic semiconducting borophenes and their derivatives.

Two semiconducting borophenes with layer-dependent magnetism are predicted based on spin-polarized density functional theory. Both monolayer borophenes are ferromagnetic. One is composed of B3 and B15 triangular motifs, exhibiting bipolar spin polarization and a magnetic moment of 1.00 μB per primitive cell. The other consists of B15 triangular motifs, possessing a Curie temperature of about 437 K and a magnetic moment of 3.00 μB per primitive cell. B atoms located between the triangular motifs are essential for inducing ferromagnetism in monolayer borophenes. However, bilayer borophenes with high-symmetry stacking orders are nonmagnetic. Furthermore, magnetic boron nanotubes and fullerenes could be made of monolayer borophenes. Finally, we propose to fabricate these magnetic semiconducting borophenes from the buckled triangular structure of borophenes via selective electron beam ionization of B atoms by scanning transmission electron microscopy.