Spin-polarized Calculations Research Articles

Electronic Properties of SiB Nanoribbons in Density Functional Theory

The present study investigates the electronic and magnetic properties of hydrogenated armchair/zigzag SiB nanoribbons with different widths. The calculations are carried out within the framework of the density functional theory using the full potential linearized augmented plane waves and the generalized gradient approximation for the exchange-correlation functional. Based on the results, it has been found that the nanoribbons have a metallic behavior, meaning that the density of states around the Fermi level increases as the width of the nanoribbon increases. Also, spin polarization calculations showed that the ribbons have magnetic ordering properties. Overall, in this work, a method has been introduced to investigate the electronic properties of SiB nanoribbons. The method has the capability to be extended to other nanoribbons.

Theoretical investigation of electronic and superconducting properties of Beryllium-doped iron-based superconductor: A first-principles study

We have performed a comprehensive set of calculations to study the influence of Be-alloying on the electronic and superconducting properties of Iron-Selenium (FeSe). We have considered FeSe1-xBex (where x = 0.0–1.0) and performed density functional theory (DFT) calculations implemented in the Vienna ab initio simulation package (VASP) and SPRKKR electronic structure code. In our study, both spin-unpolarized and spin-polarized calculations using LDA functional have been executed to assess the role of substitutional alloying in this material. We found that these systems are non-magnetic under Be alloying into FeSe. In addition, we calculated Bloch spectral function and Fermi surface of FeSe1-xBex alloys and found that alloying through Be, makes the system disordered and shows its robust nature against alloying. The effect of disorder on the total density of states (DOS) has been investigated and as a result, it shifts the DOS towards higher values at Fermi level. Coherent potential approximation implemented in SPRKKR and supercell method using VASP have been used to handle the substitutional disorder for our studies. Based on the DFT calculations at zero temperature, we found that electrical conductivity decreases by increasing the concentration of Be into FeSe at Se sites. Furthermore, we have also estimated superconducting transition temperature (Tc) for FeSe1-xBex alloys. However, we have experimental evidence for FeSe and FeSe0.94Be0.06 alloys, we have reported the Tc for other concentrations hypothetically and that might be helpful for the synthesis of new superconducting materials, experimentally.

Adsorption of propylene carbonate on the LiMn2O4 (100) surface investigated by DFT + U calculations* *Project supported by the National Natural Science Foundation of China (Grant No. 51962010).

Understanding the mechanism of the interfacial reaction between the cathode material and the electrolyte is a significant work because the interfacial reaction is an important factor affecting the stability, capacity, and cycling performance of Li-ion batteries. In this work, spin-polarized density functional theory calculations with on-site Coulomb energy have been employed to study the adsorption of electrolyte components propylene carbonate (PC) on the LiMn2O4 (100) surface. The findings show that the PC molecule prefers to interact with the Mn atom on the LiMn2O4 (100) surface via the carbonyl oxygen (Oc), with the adsorption energy of −1.16 eV, which is an exothermic reaction. As the adsorption of organic molecule PC increases the Mn atoms coordination with O atoms on the (100) surface, the Mn3 + ions on the surface lose charge and the reactivity is substantially decreased, which improves the stability of the surface and benefits the cycling performance.

On-Surface Synthesis and Characterization of [7]Triangulene Quantum Ring.

The ability to engineer geometrically well-defined antidots in large triangulene homologues allows for creating an entire family of triangulene quantum rings (TQRs) with tunable high-spin ground state, crucial for next-generation molecular spintronic devices. Herein, we report the synthesis of an open-shell [7]triangulene quantum ring ([7]TQR) molecule on Au(111) through the surface-assisted cyclodehydrogenation of a rationally designed kekulene derivative. Bond-resolved scanning tunneling microscopy (BR-STM) unambiguously imaged the molecular backbone of a single [7]TQR with a triangular zigzag edge topology, which can be viewed as [7]triangulene decorated with a coronene-like antidot in the center. Additionally, dI/dV mapping reveals that both inner and outer zigzag edges contribute to the edge-localized and spin-polarized electronic states of [7]TQR. Both experimental results and spin-polarized density functional theory calculations indicate that [7]TQR retains its open-shell septuple ground state (S = 3) on Au(111). This work demonstrates a new route for the design of high-spin graphene quantum rings for future quantum devices.

High temperature and pressure dependent structural and thermophysical properties of Co2VN (N = Sn, Sb) ferromagnetic materials

Ab-initio framework investigation of structural stability and thermophysical behavior of two Co-based Heusler alloys is carried out using spin-polarized calculations at high temperature and pressure. The structural characterization in nonmagnetic and ferromagnetic states reveals the ordered ferromagnetic Cu2MnAl-prototype structure as stable phase. The optimized lattice constant is found to be consistent with the available experimental value. The continuity in the P–V plot indicates the absence of any structural phase transition from highly symmetric cubic structure to other structural phase. The band structure profile shows perfect half-metallic character with integral 3.00 μ B magnetic moment for Co2VSn and 4.00 μ B for Co2VSb according to the Slater-Pauling rule. Elastic constants convey that these alloys are mechanically stable with a high Debye and melting temperatures. With the incorporation of modified version of Beck-Johnson potential these alloys displays a perfect half-metallic character having an indirect band gap of 1.12 eV and 1.34 eV in spin down orientation of Co2VN (N = Sn, Sb) Heuslers respectively. The density of states along with their corresponding band structure delivers the semiconducting nature of alloys in spin down channel for the present set of alloys. Semi-classical Boltzmann theory for heat transport is used to check the applicability of the material for thermoelectric technology. Insight into variation of lattice thermal conductivity shows an exponential decreasing trend for both the compounds intriguing their experimental exploration. Also, a detailed description of thermodynamic behavior of the vital quantities like entropy, thermal expansion, Grüneisen parameter and specific heat were examined using quasi harmonic Debye approximation.

First-principles study of structural, electronic and magnetic properties of A-site-ordered quadruple perovskite LaMn3Rh4O12

The introduction of magnetic ions at A-site is a typical modification in traditional ABO3 perovskite materials to achieve the target physical properties. Here, the impact of Mn3+ incorporation on the magnetic coupling and electronic structures of A-site-ordered quadruple perovskite LaMn3Rh4O12 was investigated using spin-polarized first-principles calculations. Our calculations demonstrate that LaMn3Rh4O12 is antiferromagnetic with G-type antiferromagnetic coupling within Mn sublattice and nonmagnetic Rh, which is quite different from the diamagnetic host perovskite LaRhO3. The electronic structure analyses reveal that the formal oxidation state of B-site Rh is +3 with 4d6 electronic configuration and it splits into t2g and eg orbitals (t2g6eg0) in low-spin state, indicating that Rh has no contribution to the magnetic properties and the introduced Mn3+ at the A′-site drives the ordered antiparallel spins coupling. Moreover, LaMn3Rh4O12 exhibits semiconducting behavior and the chemical valence is La3+Mn3+3Rh3+4O2-12. Our study provides an alternative method for beneficial modification of traditional ABO3 perovskites by introducing A-site magnetic ions.

O2 Activation over Ag-Decorated CeO2(111) and TiO2(110) Surfaces: A Theoretical Comparative Investigation

Periodic spin-polarized hybrid density functional theory calculations have been performed to investigate the reactivity of pristine, O-defective, and Ag-decorated CeO2(111) and TiO2(110) surfaces w...

First-principles spin-polarized calculations on the adsorption of ethanethiol molecule upon the nonpolar (100) ZnO surface

The study of the molecular adsorption of ethanethiol upon the nonpolar () ZnO surface has been performed throughout the density functional theory, including the effect of spin polarization. To obtain a more representative model, we have considered the presence of an oxygen vacancy within the ZnO surface, which triggered the appearance of an F-center within the band gap of the material, located 0.32 eV above the valence band. The geometric structure, adsorption energy, charge transfer, and electronic properties of the adsorption of ethanethiol over the surface have been calculated and analyzed. This analysis led to the identification of three different adsorption patterns, where dissociative and non-dissociative chemisorption were observed with adsorption energies ranging from −0.96 eV to −1.75 eV, as well as physisorption with lower energies.

Magneto-electronic property in zigzag phosphorene nanoribbons doped with transition metal atom

The magneto-electronic properties of zigzag phosphorene nanoribbons (ZPNRs) doped, respectively, with iron (Fe), cobalt (Co) and nickel (Ni) atoms are investigated by the first-principles method based on density functional theory. The calculated results show that the structures of doped and undoped ZPNR are stable because their binding energy and Gibbs free energy are negative, and the Forcite annealing dynamics simulation shows that the thermal stabilities of all doped ZPNRs are extremely high. The ground states of pristine ZPNRs and ZPNRs doped with Co atoms are nonmagnetic states, while the ground states of ZPNRs doped with Fe or Ni atoms are ferromagnetic states. When they are in the nonmagnetic states, the pristine ZPNRs and ZPNRs doped with Co atoms turn into semiconductors, while the ZPNRs doped with Fe or Ni atoms become metals. The undoped ZPNRs are direct band gap semiconductors, while the ZPNRs doped with Co atoms are indirect band gap semiconductors, and the band gaps of the latter are smaller than those of the former. The changes of the properties of the ZPNRs are due to the introduction of impurity energy band into the energy band structures. The spin-polarized calculation displays that the pristine ZPNRs and ZPNRs doped with Co atoms are non-magnetic, and the ZPNRs doped with Fe or Ni atoms are magnetic but only in the ferromagnetic state. In the ferromagnetic state, the ZPNRs doped with Fe atoms are spin semiconductors, while the ZPNR doped with Ni atoms are spin half-metals. This means that the half-metal feature can be realized by doping Ni atom into ZPNR. The magnetism of ZPNRs doped with Fe or Ni atoms is mainly contributed by impurity atoms, and the occurrence of magnetism is due to the existence of unpaired electrons in ZPNR. The doping position has a certain influence on the electromagnetic properties of ZPNR. In the ferromagnetic state, the ZPNRs are half-metals when the Ni atoms are doped near the edge of the nanoribbons, while the ZPNRs are spin semiconductors as the Ni atoms are doped near the symmetric center of the nanoribbons. These results might be of significance for developing the phosphorene based electronic nanodevices

Opto-electronic properties of HfO2: A first principle-based spin-polarized calculations

Wide band gap and highly dielectric, HfO 2 compound has been investigated for two different phases viz. cubic and monoclinic using density functional theory (DFT) calculations to study structural and optoelectronic properties. For the calculations, PBE-GGA and Tb-mBJ exchange correlations have been used. Volume optimization curve confirmed that the monoclinic phase is most stable as compared to the cubic one, which is consistent with its experimental outcome. Both density of states (DOS) and band structure profile represent the wide band gap for the HfO 2 compound, having the gap in the range of 5.4-5.8 eV for the cubic and monoclinic phase of HfO 2 compound under the TB-mBJ potential. The observed bandgap matches greatly with its experimental and other theoretical investigations on HfO 2 compound. In addition,we investigated the different optical parameters such as dielectric function, refractive index, extinction coefficient, absorption coefficient, optical conductivity, reflectivity and electron energy loss in detail for the HfO 2 compound. The maximum value of the dielectric constant found to be ∼ 2.89 at zero frequency. Estimated refractive index found to be ∼ 1.7, which validates the standard Penn Model. Our calculations show a high static dielectric constant for the HfO 2 compound. Absorption found to be maximum in UV region of em radiation, which is suitable for its application in UV based devices. It is noted that the estimated optical parameters, along with electronic properties, are in good agreement with experimental and theoretical studies.

Bonding of Gold Nanoclusters on Graphene with and without Point Defects.

Hybrid nanostructures of size-selected nanoparticles (NPs) and 2D materials exhibit striking physical and chemical properties and are attractive for many technology applications. A major issue for the performance of these applications is device stability. In this work, we investigate the bonding of cuboctahedral, decahedral and icosahedral Au NPs comprising 561 atoms on graphene sheets via 103-atom scale ab initio spin-polarized calculations. Two distinct cases we considered: (i) the Au NPs sit with their (111) facets on graphene and (ii) the NPs are oriented with a vertex on graphene. In both cases, we compare the binding energies with and without a graphene vacancy under the NP. We find that in all cases, the presence of the graphene vacancy enhances the bonding of the NPs. Significantly, in the vertex-on-graphene case, the binding energy is considerably increased by several eVs and becomes similar to the (111) facet-on-graphene case. The strain in the NPs is found to be minimal and the displacement of the carbon atoms in the immediate neighborhood of the vacancy is on the 0.1 Å scale. The work suggests the creation of stable NP-graphene systems for a variety of electronic, chemical and photonic applications.

Electronic properties of substitutional impurities in graphenelike C2N , tg−C3N4 , and hg−C3N4

We study the electronic and structural properties of substitutional impurities of graphenelike nanoporous materials C$_2$N, $tg$-, and $hg$-C$_3$N$_4$ by means of density functional theory calculations. We consider four types of impurities; boron substitution on carbon sites (B(C)), carbon substitution on nitrogen sites (C(N)), nitrogen substitution on carbon sites (N(C)), and sulfur substitution on nitrogen sites (S(N)). From cohesive energy calculations, we find that the C(N) and B(C) substitutions are the most energetically favorable and induce small bond modifications in the vicinity of the impurity, while the S(N) induces strong lattice distortions. Though all of the studied impurities induce defect levels inside the band gap of these materials, their electronic properties are poles apart depending on the behavior of the impurity as an acceptor or a donor. It is also observed that acceptor (donor) wavefunctions are composed only of $\sigma$ ($\pi$) orbitals from the impurity itself and/or neighboring sites. Consequently, acceptor wavefunctions are directed towards the pores and donor wavefunctions are more extended throughout the neighboring atoms, a property that could further be explored to modify the interaction between these materials and adsorbates. Moreover, impurity properties display a strong site sensitivity and ground state binding energies ranging from $0.03$ to $1.13$ eV, thus offering an interesting route for tuning the optical properties of these materials. Finally, spin-polarized calculations reveal that all impurity configurations have a magnetic ground state that rises from the spin splitting of the impurity levels. In a few configurations, more than one impurity level can be found inside the gap and two of them could potentially be explored as two-level systems for single-photon emission, following similar proposals recently made on defect complexes on TMDCs.

Half-metallic ferromagnetism in molybdenum doped methylammonium lead halides (MAPbX3, X = Cl, Br, I) system: First-principles study

Magnetic properties of molybdenum doped methylammonium lead halides (MAPbX3, X = Cl, Br, I) system has been studied using density functional theory for the very first time. Spin-polarized magnetic calculations indicate that the doping of Mo-atoms at Pb-site generates a significant magnetic moment in all the three MAPbX3 systems, where the d-orbital electrons of the dopant primarily generate the induced magnetism in the system. The Spin density distribution and Bader charge analysis also support that the magnetic moment is concentrated around the dopant. The unpaired spins of the Mo-dopants prefer parallel alignment, which makes a stable ferromagnetic spin ordered system. The defect formation energy of the Mo-doping at Pb-site is also small, which makes it a potential candidate for spintronics application.

Theoretical investigation of the AlN (0 0 0 1)-(2 × 2) surface doped with nickel: Structural, electronic and magnetic properties

Structural, electronic and magnetic properties of the nickel doped AlN (0 0 0 1)-(2 × 2) surfaces are investigated by spin-polarized first-principle total-energy calculations. The exchange–correlation potential energies are treated within the generalized gradient approximation (GGA) with the Perdew, Burke and Enzernof (PBE) functional. Electron-ion interactions are dealt with the pseudopotential approach. According to results the H3 site is the most favorable structure in the adsorption of Ni at different coverage from ¼ to 1 monolayer (ML). When coverage is in the range from ½ to 1 ML, we found ferromagnetic (FM) behavior as a consequence of the Ni presence with magnetic moment (>0.7 μB/atom). Also, we found the formation of dimers, trimers and atomic chains in the ½, ¾ and 1ML coverage, respectively. The minimum energy pathways in the Ni adsorption and incorporation show activation energies of 1.53 and 1.26 eV, respectively. The deposit of Ni into substitutional sites suggests the possibility of growing NiN epitaxially. We employed the surface formation energy formalism to investigate the different structures stability. Results indicate that the Ni doping is favorable under Ni-rich conditions in all the range of chemical potential ΔμAlN, finding four AlN surfaces with different magnetic properties: the 1Ni ads and 4Ni subs models are non-magnetic, however the 1Ni subs model is FM and the 2Ni subs model is antiferromagnetic (AFM) with magnetic moments of the order of 0.8 μB/atom. Therefore, we can conclude that high concentrations of Ni induce a non-magnetic behavior, and low concentrations favor the magnetic properties, provided that the Ni atoms occupy substitutional sites. The density of states shows that only the 1st ML of surfaces is metallic, in contrast layers beneath the first layer are semiconductor, where the valence and conductions bands are mainly formed by the N-p and Al-p orbitals, respectively.

Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-Doped Tungsten Diselenide Monolayers at Room Temperature.

The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long-range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step toward modern TMD-based spintronics, but also enables exploration of new and exciting dimensionality-driven magnetic phenomena. To this end, tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V-doped WSe2 are shown. As vanadium concentration increases, the saturation magnetization increases, which is optimal at ≈4 at% vanadium; the highest doping level ever achieved for V-doped WSe2 monolayers. The TISF occurs at ≈175 K and becomes more pronounced upon increasing the temperature toward room temperature. The TISF can be manipulated by changing the vanadium concentration. The TISF is attributed to the magnetic-field- and temperature-dependent flipping of the nearest W-site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin-polarized density functional theory study. The findings pave the way for the development of novel spintronic and valleytronic nanodevices and stimulate further research.

Structural parameters and optical spectra of Zn1-xCoxO ternary alloys with zinc-blende, rocksalt and wurtzite phases

We present spin-polarized density-functional theory calculations of the structural properties and optical spectra of Zn1-xCoxO ternary alloys in the hypothetical zinc-blende, rocksalt and wurtzite structures. Our results are very accordant with those existing in the literature. Most of our findings are reported for the first time. Generally, the rocksalt structure is found to have the highest stiffness as compared to those of zinc-blende and wurtzite structures. The lattice parameters of alloys of interest vary in a non-linear way and hence do not obey to the Vegard's law. Analysis of the optical spectra shows that all optical features of interest are affected by both alloying and variation of structure giving thus novel optical properties. Moreover, it shows a good optical absorption in a wide photon energy range which seems to be more important for the rocksalt structure. The static refractive index values are reported for the different structures and for various compositions x.

Density functional study on electronic properties of transition metal-based vacancy-ordered halide perovskites

Replacing toxic Pb of halide perovskites is a serious challenge in the current investigation of perovskite solar cells. Vacancy-ordered double halide perovskites have attracted considerable research interest due to their excellent stability and optoelectronic properties. A series of vacancy-ordered double perovskites based on transition metal cations are comprehensively investigated in this work through first-principles calculations. Various electronic configurations of tetravalent transition metals are considered in spin-polarized calculation. Our calculations reveal electronic properties of vacancy-ordered double halide perovskites and provide a theoretical basis for the exploration of novel lead-free perovskites.

Magnetic Moments, Electronic Structure, and Optical Spectroscopy of Cobalt-Based Intermetallic Compounds YCo3, Y2Co7, and LaCo5

We report on the results of analysis of magnetic properties and electronic structure, which were obtained by self-consistent ab initio spin-polarized calculations, as well as experimentally determined optical properties of cobalt-based intermetallic compounds YCo3, Y2Co7, and LaCo5. Optical characteristics were measured using ellipsometry in a wide spectral interval. The mechanism of quantum absorption of light in these materials is considered based on comparative analysis of experimental and theoretical spectra of interband optical conduction. Analysis of the crystalline structure and the resultant magnetic, electronic, and optical characteristics revealed peculiarities in the variation of physical properties. which appeared upon an increase in the cobalt content in the intermetallides.

Effect of an interfacial oxygen monolayer on the spin-polarization at the Fe/MgO interface: First-principles calculations

Density functional theory is applied to calculate the structural, electronic, and magnetic properties of the Fe/MgO/Fe (001) magnetic tunnel junctions (MTJs) as well as the influence of an interfacial oxygen monolayer by the addition of oxygen atoms at the Fe/MgO interface. The formation of Fe–O bonded atoms, as well as the changes in the spin polarization around the Fermi level of the Fe/MgO/Fe (001) with an interfacial FeO layer, is observed. The calculated spin-polarized density of states of the Fe/MgO/Fe with a Fe–O interface layer showed that the peaks of the O 2s, O 2p, and Fe 3d states are shifted due to the coupling between the O atoms and Fe layer. The Fe/MgO/Fe structure exhibits metallic behavior due to the position of the density of states (DOS) peak at the Fermi level with both majority and minority spins. The calculated spin polarization (P) is only 7% for the metallic Fe/MgO/Fe structure, whereas the metallic Fe/MgO/Fe structure with FeO interfacial layer has P of 24%. Thus, the polarization is found to be extremely sensitive to the nature of the interface layer. Likewise, the nature of spin-charge transfer in both investigated MTJ structures was found to be different at the Fe/MgO interface due to the hybridization interaction between Fe and O. We have also calculated the structural, magnetic, and electronic properties of Fe/MgO/Fe with FeO layer with different sizes of the junction by varying the number of MgO layers, with Fe thickness fixed at 5 ML.

Band-Edge Engineering To Eliminate Radiation-Induced Defect States in Perovskite Scintillators.

Under radiative environments such as extended hard X- or γ-rays, degradation of scintillation performance is often due to irradiation-induced defects. To overcome the effect of deleterious defects, novel design mitigation strategies are needed to identify and design more resilient materials. The potential for band-edge engineering to eliminate the effect of radiation-induced defect states in rare-earth-doped perovskite scintillators is explored, taking Ce3+-doped LuAlO3 as a model material system, using density functional theory (DFT)-based DFT + U and hybrid Heyd-Scuseria-Ernzerhof (HSE) calculations. From spin-polarized hybrid HSE calculations, the Ce3+ activator ground-state 4f position is determined to be 2.81 eV above the valence band maximum in LuAlO3. Except for the oxygen vacancies which have a deep level inside the band gap, all other radiation-induced defects in LuAlO3 have shallow defect states or are outside the band gap, that is, relatively far away from either the 5d1 or the 4f Ce3+ levels. Finally, we examine the role of Ga doping at the Al site and found that LuGaO3 has a band gap that is more than 2 eV smaller than that of LuAlO3. Specifically, the lowered conduction band edge envelopes the defect gap states, eliminating their potential impact on scintillation performance and providing direct theoretical evidence for how band-edge engineering could be applied to rare-earth-doped perovskite scintillators.