First-principles Generalized Gradient Approximation Research Articles

Ferromagnetism with above-room-temperature Curie temperature in Fe-doped β-Ga2O3 studied by first-principles calculations

Induced by the spin-polarized double-exchange interaction between the 3d orbitals of Fe atoms and O-2p orbitals near the O/Ga vacancy complexes, the origin of magnetism in β-Ga2O3 is investigated by using geometry optimization and energy calculation based on the first-principles generalized gradient approximation Hubbard + U method of density-functional theory. The results show that the formation energy of Fe2+ in the β-Ga2O3 matrix is lower than that of Fe3+, and the system is more stable when the Ga or O vacancies (VGa or VO) are adjacent to the Fe lattice position. It is found that the β-Ga31Fe2+/3+O47 and β-Ga30Fe2+/3+O48 systems all exhibit ferromagnetism with above-room-temperature Curie temperature. β-Ga30Fe2+/3+O48 (i.e., with Fe2+/3+ and VGa) has more spin-polarized O atoms in the same direction. The magnetism originates from the double-exchange interaction of the coupling and hybridization of the Fe-3d state of Fe3+ and the O-2p state, which increases the magnetic moment of the β-Ga30Fe3+O48 system. This study provides a theoretical reference for designing and preparing new β-Ga2O3-based dilute magnetic semiconductors and related spintronic and optoelectronic devices.

First-principles study on the effects of Zr doping and interstitial H and O vacancy on the optoelectronic properties of β-Ga2O3

In this work, the electronic structure and optical properties of Zr-doped [Formula: see text]-Ga2O3 with interstitial H and O vacancies were studied by using first-principles generalized gradient approximation combined with the Hubbard U method based on density functional theory. The energy band structure, density of states, absorption spectrum, effective mass, mobility and conductivity of the doped system were calculated and analyzed. Results showed that the doped system was more stable under the Ga-rich condition than under the O-rich condition. With the increase in Zr doping concentration, the bandgap of the [Formula: see text]-Ga2O3 system gradually narrowed and the absorption spectrum of the system blue-shifted in the wavelength range of 162–275[Formula: see text]nm. System conductivity was enhanced by Zr doping, decreased by O vacancies and greatly improved by interstitial H. Therefore, doping Zr into the [Formula: see text]-Ga2O3 system is important for improving material properties and preparing electronic and optical devices.

First-principles study on the effects of different valence VZn and Ag doping with Hi co-existence on the magnetism and photocatalysis of ZnO

In vacuum environment, metal organic chemical vapor deposition (MOCVD) method was used in experiments, with Ag doping and cation vacancy (VZn) doping in ZnO, where unintentional Hi cannot be avoided. However, studies on the effect of H interstitial physical properties are few. In this study, first-principles generalized gradient approximation plane-wave ultra-soft pseudopotential GGA + U approach within the frame of DFT is used to study the influences of VZn and Ag doping in different valencies with Hi on the magnetism and photocatalysis of ZnO.Under O-rich conditions, Zn34Ag2+HiO36(VZn0) has the best thermodynamic stability. Researches showed that Zn34Ag1+O36(VZn0) has a 2 μB magnetic moment, and magnetism is mainly generated by the roving electrons (O−) using the VZn as medium. Photocatalysis increases with the increase of Ag valence state. Especially, Zn34Ag2+HiO36(VZn0) has a relatively long electronic lifetime of 5.35 × 10−12 s, the strongest visible light absorption ability, relatively high carrier activity and relatively strong reduction ability.

First principles study on p-type conductivity and new magnetic mechanism of ZnO:Sm with point defects in different strains

Iterant electrons exist in the f and d states of ZnO:Sm with point defects (VZn, Hi), which are often overlooked. Moreover, p-type ZnO is difficult to realize. To solve such problems, this study adopts the first-principles generalized gradient approximation (GGA + U) plane wave supersoft pseudopotential method to construct a ZnO:Sm system with point defects (VZn, Hi). The stability and conductivity of the doped system and the new magnetic mechanism are studied under a biaxial strain. Result shows that the Zn34SmHiO36 system has the lowest formation energy, simplest doping, lowest binding energy, and highest stability and is a relatively p-typed system when the Sm–H spacing is closer compared with the Zn34SmHiO36 systems with different Sm–H spacings. The hole conductivity of the Zn34SmHiO36-doped system in the z-axis direction is high when no external strain is applied. The result demonstrates that the Zn34SmHiO36 system is magnetic. The charge transfer from s state of single ion Sm to f and d states of Sm causes magnetism in the doped system, which differs from the traditional theory of orbital hybrid coupling and double exchange magnetic source between distinct ions. The Sm-4f and Sm-5d states in the Zn34SmHiO36 system have itinerant and local electron properties, which affect the magnetic source and p-type of the system. This work has certain reference value for the design and preparation of new ZnO magnetoelectric-integrated functional materials.

Non- d0 ferroelectricity from semicovalent superexchange in bismuth ferrite

Empirical ${d}^{0}$ rule, i.e., magnetic unpairing $d$ occupation tends to be chemically incompatible for ferroelectrics, has for long inhibited the availability of room-temperature single-phase multiferroics. Here, we readdress the ferroelectricity of type-I multiferroic $\mathrm{BiFe}{\mathrm{O}}_{3}$, and conclusively clarify hybrid origins of lone-pair electrons' instability. Surprisingly, the sole lone-pair mechanism cannot account for overall polarization. Thus, beyond the popular belief, we propose that, with half-filling ${t}_{2g}^{3}{e}_{g}^{2}$ configuration and stereochemical activity of Bi's lone pair preparticipating, renormalized nonempty Fe $3d$ orbitals render $\mathrm{BiFe}{\mathrm{O}}_{3}$ a strong p-d charge-transfer insulator, and further induce non-${d}^{0}$ ferroelectricity through antiferromagnetic semicovalent superexchange. The magnetoelectric coupling scenario is numerically elaborated by mean-field p-d model, and first-principles generalized gradient approximation $\mathrm{GGA}+U$ calculations. The magnetically induced ferroelectricity also reconciles the competitivity of octahedral rotations and ferroelectricity, which turns into cooperative behavior as octahedral rotations straighten the $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Fe}$ bonds and strengthen superexchange interactions. Such mechanism is in principle ubiquitous in magnets with superexchange interaction and manipulatable to hybridization engineering.

First-Principles Studies for Electronic Structure and Optical Properties of p-Type Calcium Doped α-Ga2O3.

Gallium oxide (Ga2O3) is a promising wide-band-gap semiconductor material for UV optical detectors and high-power transistor applications. The fabrication of p-type Ga2O3 is a key problem that hinders its potential for realistic power applications. In this paper, pure α-Ga2O3 and Ca-doped α-Ga2O3 band structure, the density of states, charge density distribution, and optical properties were determined by a first-principles generalized gradient approximation plane-wave pseudopotential method based on density functional theory. It was found that calcium (Ca) doping decreases the bandgap by introducing deep acceptor energy levels as the intermediate band above the valence band maximum. This intermediate valence band mainly consists of Ca 3p and O 2p orbitals and is adequately high in energy to provide an opportunity for p-type conductivity. Moreover, Ca doping enhances the absorptivity and reflectivity become low in the visible region. Aside, transparency decreases compared to the pure material. The optical properties were studied and clarified by electrons-photons interband transitions along with the complex dielectric function’s imaginary function.

First-principle study on the magnetic and optical properties of SnO2 doped with Fe2+/3+ and oxygen vacancies at different ratios

The effects of different ratios of Fe2+/3+ and oxygen vacancy doping on the magneto-optical properties and mechanism of SnO2 have not been fully investigated. Therefore, the electronic structure and magneto-optical properties of SnO2 doped with different concentrations of Fe and oxygen vacancies were studied through geometry optimization and energy calculation in accordance with the first-principle generalized gradient approximation + U(GGA + U) method of density functional theory. The effects of changes in Fe2+/3+ and oxygen vacancy concentration on the electronic structure, energy, magnetic exchange mode, and optical absorption properties of SnO2 materials were explored. Calculation results showed that doping with different concentrations of Fe2+/3+ and oxygen vacancies made the stability and magnetic moment of the doping system vary. The comparison of several doping systems with the same high magnetic moment showed that the Sn22[Fe3+]2O46 system demonstrates the best stability and magnetic value. Doping with different concentrations of Fe2+/3+ and oxygen vacancies exerted varying effects on the energy band structure and optical properties of the doped system. The static dielectric constant of all doping systems increased, and the absorption band edge was red-shifted, which helped improve the photocatalytic activity of SnO2.

Effects of triaxial strain on carrier activity, carrier lifetime, and absorption spectrum of (S, Se, Te) and 2 N Co-doped ZnO

The acceptor (N)-isovalent (S, Se, Te) elements that separately co-dope ZnO is a novel doping technology. However, the effects of triaxial strain on the photocatalytic performance and mechanism of doped systems are often neglected in experimental studies. Moreover, the influence of interstitial H on the doped ZnO system is not considered. This paper adopted first-principles and selected the appropriate strain range and step length. Assuming the equivalent effect of several strain points of tensile strain or compressive strain, we used simulation calculation to fit the complex strain problem in the actual process (such as doping, the strain problem caused by the difference of the lattice constant and thermal expansion coefficient of the substrate and the doping system), and then studied physical properties of the doped system. The planar wave ultrasoft pseudopotential + U method based on the first-principles generalized gradient approximation under the framework of density functional theory was used to calculate (S, Se, Te) and 2 N co-doped ZnO. The interstitial H-doped Zn36SO33N2 was studied as a representative ZnO system. Results showed that the binding energy of all doping systems under unstrained conditions was negative with a relatively high stability. The strain increased under tensile strain and compressive strain. As the formation energy of the doping systems increased, system stability decreased. Although the band gap of all compressive strain systems widened, impurities formed in the band gap, and valence band electrons absorbed photons of low energy to transition to the energy level of the impurities and then to the energy level of conduction band. The photoexcited electron-graded transitions induced the absorption spectrum to red-shift. The band gap of the system widened, whereas the band gaps of other tensile strain systems narrowed, except for when the tensile strain was 1%. However, regardless of whether the band gap of the tensile strain system became narrow or wide, the photoexcited electrons were still able to transition in stages. Hence, the absorption spectrum of the tensile strain system still red-shifted. The stability, activity, carrier separation, and lifetime of Zn36SHiO33N2 were relatively better than those of Zn36SO33N2 at 5% tensile strain. These properties make Zn36SHiO33N2 an ideal photocatalyst. This study provides a theoretical reference for the experimental design and preparation of novel ZnO photocatalysts.

First-principle study on the magnetic and optical properties of TiO2 doped with different valence Fe

The effect of Fe doping with different valence states on the magnetic and the optical properties of rutile TiO2 has not been recognized. This paper adopts the First-principles generalized gradient approximation + U (GGA + U) method. The band gaps of Ti14Fe23+O32 and Ti14Fe22+O32 systems are narrowed compared with those of pure rutile TiO2, and the absorption spectra red-shifted. Holes or electron capture positions are introduced to reduce the probability of the hole–electron pair recombination. The Ti14Fe23+O32 system has narrower band gap, more remarkable red- shift in the absorption spectrum, slower electron and hole recombination, faster separation, and better photocatalytic performance than the Ti14Fe22+O32 system. The semimetallization is performed. P-type dilute magnetic semiconductor with semimetallization is advantageous as a hole injection source. Moreover, the ferromagnetic mechanism can be achieved at room temperature. The Ti14Fe23+O32 system has certain reference value for designing and preparing novel photocatalysts or dilute magnetic semiconductors.

Effect of Cu/Al doping on electronic structure and optical properties of ZnO

The stability, electronic structure, conductivity and optical properties of Cu/Al doped ZnO in the condition of different concentrations and different positions were studied by using the first-principles generalized gradient approximation plane-wave pseudopotential method combined with Hubbard U correction based on density functional theory. Geometric optimization and energy calculations for the original structure of pure ZnO, Zn0.96875R0.03125O (R replaces Cu or Al) and Zn1−x−yCuxAlyO (x, y = 0.03125, 0.03125; 0.0625, 0.03125 and 0.03125, 0.0625). The results show that the co-doped systems are more stable than single-doped, and each of the systems is easier to form under O-rich conditions. When doping at the same concentration, the volume of the system with different impurity atoms on the same horizontal plane is small. In this paper, the co-doping can greatly reduce the band-gap of ZnO, so that the absorption edge of each system is red-shifted, the absorption of Zn0.90625Cu0.0625Al0.03125O in visible light is the strongest. The carrier mobility and conductivity of each system were calculated by the relative quantity of carriers and the effective mass. It is found that co-doping is more beneficial to the conduction than single-doping, and the influence of the concentration of Al on the conductivity of the system is larger than that of Cu.

Influence of valence states and vacancy defects on the magnetic properties of Mn doped InN

The effects of valence states and vacancy defects on the properties of Mn doped InN were systematically investigated by the first-principle generalized gradient approximation + U (GGA + U) calculations. Calculations show the systems exhibits lower formation energy and better stability after Mn doping. The energies calculation results show that the double Mn2+ doped InN exhibits only antiferromagnetism, but mixed Mn2+/3+ doping in the nearest neighboring cation sites and mixed Mn2+/3+ doping with vacancy defects can obtain a better ferromagnetism and high Curie temperature. The electronic structures indicate that the p-d exchange interaction and double exchange interaction play important roles in the magnetic formation of the doping system. The vacancies in mixed Mn2+/3+ doped InN enhance the dominance of the p-d exchange interaction, and favor the doping system to obtain a high Curie temperature. It can be inferred that the mixed 2+/3 + valence of Mn ions and introduction of vacancy defects in InN is beneficial to stabilize the ferromagnetism.

Physical properties of B4N4-I and B4N4-II: First-principles study**Project supported by the National Natural Science Foundation of China (Grant No. 61601468), the Fundamental Research Funds for the Central Universities, China (Grant No. 3122014C024), and the Fund for Scholars of Civil Aviation of the University of China (Grant No. 2013QD06X).

The structural, mechanical, electronic, mechanical anisotropy, and thermal properties of boron nitride (BN) polymorphs, such as B4N4-I and B4N4-II, are investigated under ambient pressure utilizing first-principles generalized gradient approximation calculations using an ultrasoft pseudopotential scheme. The phonon spectra and elastic constants reveal that B4N4-I is dynamically and mechanically stable at the pressure of 0 GPa and temperature of 0 K. Anisotropic calculations indicate that both B4N4-I and B4N4-II exhibit higher anisotropy of Young’s modulus than cubic BN (c-BN). B4N4-II and B4N4-I present indirect and wide band gaps of 5.32 eV and 4.86 eV, respectively. In addition, B4N4-I is more brittle than B4N4-II. Moreover, the minimum thermal conductivity, κmin, of B4N4-II at 300 K is 1.92 W/(cm·K), which is slightly higher than those of B4N4-I and c-BN (1.84 W/(cm·K) and 1.83 W/(cm·K), respectively. However, κmin of B4N4-I is slightly higher than that of c-BN.

Cu-Doped ZnO Electronic Structure and Optical Properties Studied by First-Principles Calculations and Experiments.

The band structure, the density of states and optical absorption properties of Cu-doped ZnO were studied by the first-principles generalized gradient approximation plane-wave pseudopotential method based on density functional theory. For the Zn1-xCuxO (x = 0, x = 0.0278, x = 0.0417) original structure, geometric optimization and energy calculations were performed and compared with experimental results. With increasing Cu concentration, the band gap of the Zn1-xCuxO decreased due to the shift of the conduction band. Since the impurity level was introduced after Cu doping, the conduction band was moved downwards. Additionally, it was shown that the insertion of a Cu atom leads to a red shift of the optical absorption edge, which was consistent with the experimental results.

Effects of Ag Doping and Point Defect on the Magnetism of ZnO

The magnetism sources and magnetic mechanism of Ag doping and point defect, which coexist in the presence of ZnO, are frequently controversial. To solve these problems, the effects of Ag doping and Zn or O vacancy on the magnetism of ZnO were investigated using geometry optimization and energy calculation according to the first-principles generalized gradient approximation + U (GGA+U) method of density functional theory. Results revealed that the system which Ag doping and Zn vacancy coexists in ZnO could achieve room-temperature ferromagnetism. This system had an extremely high spin polarization, which was advantageous for the spin electron injection sources applied in dilute magnetic semiconductors. Result also revealed that the magnetism of Zn14AgO16 was caused by the electron hybrid coupling effects among the O-2p, Ag-4d, Ag-5s, and Zn-4s orbits. And these effects were produced by the hole carriers after complexes were formed by the Ag doping and Zn vacancy. With regard to the most stable structure of the ground state, all doping systems which Ag replacing Zn and O vacancy coexisted in ZnO and those which Ag replacing Zn, interstitial Ag, and O vacancy coexisted in ZnO were all non-magnetic. Thus, these doping systems were unsuitable for dilute magnetic semiconductors. By contrast, the doping systems of interstitial Ag and Zn vacancy were magnetic, although the magnetism was relatively weak, thus rendering such systems inapplicable as well.

Effects of La Doping and Zn or O Vacancy on the Magnetic Property of ZnO

Research on the magnetic origin and mechanism of La-doped ZnO system has been controversial. To solve this problem, we studied the effects of La doping and Zn or O vacancy on the magnetic property of ZnO using geometry optimization and energy calculation on the basis of first-principle generalized gradient approximation method of density functional theory. The electronic structures and magnetic properties of undoped ZnO, La-doped ZnO system, and systems with coexisting La doping and Zn or O vacancies were calculated. Results showed that the systems of La-doped ZnO and the coexistence of La doping and O vacancy are nonmagnetic. In addition, the system of La replacing Zn and one Zn vacancy exhibits long-range orderly ferromagnetism, and the Curie temperature of the doping system can achieve room temperature. The magnetism source of the systems with La doping and Zn vacancy coexisting in ZnO demonstrates strong hybrid coupling electron exchange effects existing among Zn-4s, Zn-3p, O-2p, and La-5s orbits that are nearest to the Zn vacancy. The next nearest distance between doping and Zn vacancy leads to the lowest formation energy and highest stability under the same doping condition.

Electronic structures and optical properties of HfO2–TiO2 alloys studied by first-principles GGA + U approach**Project supported by the National Natural Science Foundation of China (Grant Nos. 11672087, 11502058, and 11402252).

The phase diagram of HfO2–TiO2 system shows that when Ti content is less than 33.0 mol%, HfO2–TiO2 system is monoclinic; when Ti content increases from 33.0 mol% to 52.0 mol%, it is orthorhombic; when Ti content reaches more than 52.0 mol%, it presents rutile phase. So, we choose the three phases of HfO2–TiO2 alloys with different Ti content values. The electronic structures and optical properties of monoclinic, orthorhombic and rutile phases of HfO2–TiO2 alloys are obtained by the first-principles generalized gradient approximation (GGA)+U approach, and the effects of Ti content and crystal structure on the electronic structures and optical properties of HfO2–TiO2 alloys are investigated. By introducing the Coulomb interactions of 5d orbitals on Hf atom (), those of 3d orbitals on Ti atom (), and those of 2p orbitals on O atom (Up) simultaneously, we can improve the calculation values of the band gaps, where , , and Up values are 8.0 eV, 7.0 eV, and 6.0 eV for both the monoclinic phase and orthorhombic phase, and 8.0 eV, 7.0 eV, and 3.5 eV for the rutile phase. The electronic structures and optical properties of the HfO2–TiO2 alloys calculated by (Up = 6.0 eV or 3.5 eV) are compared with available experimental results.

Effects of Li doping and point defect on the magnetism of ZnO

The magnetism sources and magnetic mechanism of Li doping and point defect, which coexist in the presence of hexagonal wurtzite ZnO, are controversial. To solve these problems, the effects of Li doping and point defect on the magnetism of ZnO were studied using geometry optimization and energy calculation based on the first-principle generalized gradient approximation + U (GGA + U) method of density functional theory. Results showed that the coexistence of Li doping and Zn vacancy can achieve ferromagnetic long-range order, and the Curie temperature of the doping system can achieve room temperature. In addition, results showed that magnetic moments are significantly different when the structures of the systems are also different with the same doping amount and doping method, which are advantageous for the enhancement of the magnetic properties of dilute magnetic semiconductors. The magnetism source of Zn14LiO16 is the hybrid coupling electron exchange effect between O-2p and Zn-3d orbits. With the coexistence of Li replacing Zn and Zn vacancy, the closest relative distance between doping and vacancy leads to the lowest formation energy and highest stability. In the condition of the highest stability of the ground state, all the doping systems of Li replacing Zn and O vacancy, doping system of interstitial Li and Zn vacancy, and doping system with the coexistence of Li replacing Zn, interstitial Li, and Zn vacancy are non-magnetic, which are considered worthless in the design and preparation of diluted magnetic semiconductors (DMSs).

Optical properties of anatase and rutile TiO studied by GGA + U**Project supported by the National Natural Science Foundation of China (Grant Nos. 11672087, 11272107, and 11402252).

The optical properties of thermally annealed TiO samples depend on their preparation process, and the TiO thin films usually exist in the form of anatase or rutile or a mixture of the two phases. The electronic structures and optical properties of anatase and rutile TiO are calculated by means of a first-principles generalized gradient approximation (GGA) +U approach. By introducing the Coulomb interactions on 3d orbitals of Ti atom and 2p orbitals of O atom , we can reproduce the experimental values of the band gap. The optical properties of anatase and rutile TiO are obtained by means of the GGA+U method, and the results are in good agreement with experiments and other theoretical data. Further, we present the comparison of the electronic structure, birefringence, and anisotropy between the two phases of TiO. Finally, the adaptability of the GGA+U approach has been discussed.

First-principles generalized gradient approximation (GGA) +U studies of electronic structure and optical properties in cubic ZrO2

Abstract The electronic structures and optical properties of cubic ZrO 2 are investigated by means of generalized gradient approximation (GGA) + U approach. By including the Coulomb interaction of 4 d orbitals on Zr atom ( U d ), as well as of 2 p orbitals on O atom ( U p ), the experimental value of the band gap can be reproduced when U d =8.25 eV, and U p =4.6 eV. With these U values, the electronic structure and optical properties are calculated, in comparison to experiments and other theoretical results, e.g., by scissor operations. Our results are found to be in good agreement with available experimental data, and more accurate than those obtained by a scissor operation.

First-principles generalized gradient approximation (GGA) + Ud + Up studies of electronic structures and optical properties in cubic HfO2

The electronic structures and optical properties of cubic HfO2 are calculated by means of generalized gradient approximation (GGA)+U approach. Without on-site Coulomb interactions, the band gap of cubic HfO2 is 2.92eV, much lower than the experimental value (5.7eV). Introducing the Coulomb interactions of 5d orbitals on Hf atom (Ud) and of 2p orbitals on O atom (Up), we can reproduce the experimental value of the band gap. The resulting dielectric function of cubic HfO2 by the GGA+Ud+Up approach predicts the presence of a shoulder structure below the main peak of the absorption spectrum. These indicate that the GGA+Ud+UP approach is a convenient and powerful method to calculate and predict the electronic structures and the optical properties of wide-gap optical materials.