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Theoretical calculations were performed to investigate the effect of isovalent substitution of Fe with Ga on the magnetic and ferroelectric properties in BiFeO3 doped with 16.6% and 8.3% Ga. Such substitution will break the G-type anti-ferromagnetic order and enhance the magnetism of BiFeO3 by forming a stable ferrimagnetic order. The doped system maintains robust spontaneous polarization that is sufficiently large for practical applications at room temperature. The findings explain experimentally observed magnetism. The coexistence of magnetism and ferroelectricity in the doped BiFeO3 render it a candidate material for application to information storage and spintronics devices.

We performed a systematic study of the electronic structures and magnetic properties of SnS 2 monolayer doped with non-magnetic elements in groups IA, IIA and IIIA based on the first-principles methods. The doped systems exhibit half-metallic and metallic natures depending on the doping elements. The formation of magnetic moment is attributable to the cooperative effect of the Hund's rule coupling and hole concentration. The spin polarization can be stabilized and enhanced through confining the delocalized impurity states by biaxial tensile strain in hole-doped SnS 2 monolayer. Both the double-exchange and p-p exchange mechanisms are simultaneously responsible for the ferromagnetic ground state in those hole-doped materials. Our results demonstrate that spin polarization can be induced and controlled in SnS2 monolayers by non-magnetic doping and tensile strain.

Auxetic materials, which possess a negative Poisson's ratio (NPR), have been a hot topic in materials science research. Through atomistic simulations, we theoretically rediscover a few novel two-dimensional (2D) VA-nitride (VA-N) binary compounds with δ-phosphorene-like structures. The structures in the δ-phase (except for δ-PN) exhibit better stability in terms of energy, thermodynamics, and mechanics with respect to their counterparts in the α- and β-phases. The structures in the δ-phase show semiconducting behaviors with direct band gaps falling in the visible light region. Interestingly, most structures in the α- and δ-phases (except for δ-BiN) exhibit large in-plane NPRs and excellent mechanical properties. The maximum NPR occurs along the zigzag (x) direction for the δ-phases and along the diagonal direction for the α-phases. Particularly, for α- and δ-SbN, the NPRs are −0.628 and −0.296, respectively. δ-SbN can sustain tensile strains of up to 22% and 35% with maximum stresses of 12.1 and 9.8 GPa in the zigzag and armchair directions, respectively. In addition, the transverse response can reach up to 6.6% at a strain of ∼18% along the armchair (y) direction for δ-SbN, which is considerably higher than those of other 2D auxetic materials. Our results reveal that 2D VA-N binary compounds have potential applications in designing 2D electromechanical and optoelectronic devices.

First-principles calculations are performed to comparatively study the structural, electronic structures and magnetic properties of Mo doped AlN, GaN and InN monolayers (MLs). After Mo atom doping, the semiconducting GaN and InN MLs transform to metal, while the AlN ML keeps semiconducting with a reduced gap. Total magnetic moments of 1.0 and 0.54 µ B , which mainly arising from the localized Mo 4d states, are induced by doping in AlN and InN MLs, respectively, while the doped GaN ML is still nonmagnetic. Nevertheless, the excessive localization and strongly ionic character of the Mo-4d states in AlN ML directly impedes the magnetic coupling, leading to a paramagnetic ground states. A similar case is observed in Mo atoms doped InN ML. The firm N-Mo interaction prevent the impurity states permeating out the range of N-Mo pair, resulting in a quick vanishing of ferromagnetic coupling as the distance between two Mo atoms increasing. All configurations of Mo atoms doped GaN ML in this paper are room temperature ferromagnetic. Spin polarized itinerant electrons mediate the magnetic interaction between two Mo atoms. Increasing the Mo concentration may stabilize the FM state and produce a higher Curie temperature. Our calculations show that GaN nanosheets with Mo atoms doped may be a nice candidate for future spintronic devices. And we conclude that a appropriate magnitude of localization (or delocalization) is what the key point to produce room temperature ferromagnetism from this comparative study.

We investigate the electronic structures and magnetic properties of SnS2 monolayers substitutionally doped with 4-d transition-metal through systematic first principles calculations. The doped complexes exhibit interesting electronic and magnetic behaviors, depending on the interplay between crystal field splitting, Hund’s rule, and 4d levels. The system doped with Y is nonmagnetic metal. Both the Zr- and Pd-doped systems remain nonmagnetic semiconductors. Doping results in half-metallic states for Nb-, Ru-, Rh-, Ag, and Cd doped cases, and magnetic semiconductors for systems with Mo and Tc dopants. In particular, the Nb- and Mo-doped systems display long-ranged ferromagnetic ordering with Curie temperature above room temperature, which are primarily attributable to the double-exchange mechanism, and the p-d/p-p hybridizations, respectively. Moreover, The Mo-doped system has excellent energetic stability and flexible mechanical stability, and also possesses remarkable dynamic and thermal (500 K) stability. Our studies demonstrate that Nb- and Mo-doped SnS2 monolayers are promising candidates for preparing 2D diluted magnetic semiconductors, and hence will be a helpful clue for experimentalists.

We theoretically studied the structural, electronic, and magnetic properties in the two-dimensional (2D) AlN nanosheets (AlNNSs) doped with nonmagnetic (NM) atoms X(=Mg, Ca, Zn, and Sr), based on first-principles calculations. The structure relaxations show Mg and Zn atoms locate in the 2D AlNNS plane, while the Ca and Sr atoms lie out of it. The results based on GGA-PBE scheme show that all the doped AlN monolayers (ML) are half-metallic. Further, results within HSE06 scheme show that Mg-, Zn- and Sr-doped AlN ML remain half-metallic while Ca-doped case changes into magnetic semiconductor. Each dopant induces a total magnetic moment of 1.0 μB per supercell which mainly stemmed from the spin-polarized holes resided on the three nearest-neighboring N atoms. Calculations illustrate that ferromagnetic (FM) states are energetically stable when two X atoms separate far away from each other, while anti-ferromagnetic (AFM) states are energetically favorable when two X atoms locate at adjacent lattice sites. The long-range FM coupling and AFM one are attributed to the strong p-d/p-p interaction and the virtual hopping mechanism, respectively. Remarkably, because of the participation of the delocalized Zn-d orbitals in Zn-doped case, strong p-d/p-p interactions gain supremacy in the competition between virtual hopping mechanism and p-d/p-p interaction, resulting in FM coupling even two Zn atoms locate at close crystal lattice. Calculations show the two-X-doped AlNNSs have FM states with Curie temperatures (Tc) higher than 600 K, indicating that X-doped AlN systems are promising candidates for spintronic devices in the future.

Based on first-principles spin-polarized density functional theory calculations, the electronic structures, and magnetic properties of Cu and Zn-doped BiFeO 3 are investigated. The calculated formation energies show that Cu prefers to occupy Fe site, while the Zn prefer to occupy Bi site. All the doped BiFeO 3 systems turn out to be favorable for G-type antiferromagnetic arrangement. The substitution of Cu and Zn for Fe produces a magnetic moment of 3.0 and 4.0 μ B per dopant, respectively. The net magnetic moments are from the broken symmetry of the AFM spin ordering network. For the substitution of Cu and Zn for Bi, the net magnetic moment originates from Cu/Zn itself and hole introduced by Cu/Zn. Two-Cu/Zn-doped cases show various magnetic behaves. If O vacancy is far away from dopants, the O vacancies don't affect the net magnetic moment of the substitution of Cu and Zn for Fe, but have notable effect for Bi site doping. The O vacancies result in metallicity in all doped cases. Our study demonstrates that the nonmagnetic Cu and Zn doping will lead to the diversity and complexity of magnetic properties depending on doping sites, distance between dopants, intrinsic defect, and so on, which could be responsible for the observed various magnetic behaviors in Cu/Zn-doped BiFeO 3 samples.

The structural, electronic, and magnetic properties of the two-dimensional (2D) AlN nanosheet doped with nonmagnetic (NM) atoms X(=Li, Na, and K) are investigated by first principle calculations. We find that the X atoms lie out of the 2D AlN nanosheet, while the structures are metastable when the dopants are situated in the plane of the nanosheets. The total magnetic moments induced by doping are 2.0μB per supercell which mainly originated from the spin-polarized holes localized on the three N atoms surrounding the dopant for all the doped AlN nanosheets. The substitution results in deep p-type acceptor levels which consist of the unoccupied N-2p orbitals. Magnetic coupling calculations demonstrate that FM states are energetically favorable when two X atoms are far away from each other while anti-ferromagnetic states are energetically favorable when two X atoms adjoin in the crystal lattice. Remarkably, calculations show that K-doped AlN nanosheet has room temperature ferromagnetism within fairly low concentration (of 5.56% doping).

Searching for low-dimensional materials with high Curie temperature and full spin–polarization ratio is an urgent demand for the rapidly evolving spintronic industry. Here, the stability, electronic structures, and magnetic properties of two-dimensional hexagonal CrOX (X = F, Cl, Br) monolayers were investigated with density functional theory. The CrOF and CrOCl monolayers exhibit good stability and are ferromagnetic semiconductors with a Curie temperature of ~ 215 and 64 K, respectively, as determined from Monte Carlo simulations. High-temperature ferromagnetism and controllable half-metallicity can be realized by hole doping from a gate voltage. At a doping concentration of 0.1|e|per unit cell, the Curie temperatures of CrOF and CrOCl are increased to 385 and 576 K, respectively, and the doped systems change from semiconductors into half-metals. These findings suggest a route for controlling and tuning the electronic and magnetic properties of CrOX monolayers, which are promising materials for spintronic devices.

Auxetic materials, which possess a negative Poisson's ratio (NPR), have been a hot topic in materials science research. Through atomistic simulations, we theoretically rediscover a few novel two-dimensional (2D) VA-nitride (VA-N) binary compounds with δ-phosphorene-like structures. The structures in the δ-phase (except for δ-PN) exhibit better stability in terms of energy, thermodynamics, and mechanics with respect to their counterparts in the α- and β-phases. The structures in the δ-phase show semiconducting behaviors with direct band gaps falling in the visible light region. Interestingly, most structures in the α- and δ-phases (except for δ-BiN) exhibit large in-plane NPRs and excellent mechanical properties. The maximum NPR occurs along the zigzag (x) direction for the δ-phases and along the diagonal direction for the α-phases. Particularly, for α- and δ-SbN, the NPRs are -0.628 and -0.296, respectively. δ-SbN can sustain tensile strains of up to 22% and 35% with maximum stresses of 12.1 and 9.8 GPa in the zigzag and armchair directions, respectively. In addition, the transverse response can reach up to 6.6% at a strain of ∼18% along the armchair (y) direction for δ-SbN, which is considerably higher than those of other 2D auxetic materials. Our results reveal that 2D VA-N binary compounds have potential applications in designing 2D electromechanical and optoelectronic devices.

Based on first-principles spin-polarized density functional theory calculations, the relative stability, electronic structures, and magnetic properties of B-, C-, N-, and F-doped BiFeO3 are investigated. The substitution of B, C, N, and F for O produces a magnetic moment of 3.0, 2.0, 1.0, and 1.0 μB per dopant, respectively. The net magnetic moments are from the broken of the symmetry of the AFM spin ordering network. We find that the BiFeO3 with one O atom substituted by a C atom leads to a ferrimagnetic half-metallic property with a C-type spin alignment. The B- and N-doped BiFeO3 are ferrimagnetic semiconductors, and ordered an A-type and a G-type spin alignment, respectively. As for F-doped case, system becomes metallic in its G-type spin alignment. Our study demonstrates that the nonmagnetic elements doping is an efficient route to tune magnetic and electronic properties in BiFeO3.

Based on density functional theory using ab initio calculations, the electronic structure and magnetic properties of the B-, C-, N-, O-, F-, Cl-, Br-, and I-doped MoS 2 monolayer are investigated. The B-, C-, and N-doped systems are p -type doping, while the halogen-doped systems are n -type doping. C-doping leads fairly deep and highly localized states in the gap of MoS 2 without spin-splitting. O-doping result in a narrower band-gap for MoS 2 . At the PBE level, results show that all the doped systems, expect for C-doping and O-doping, exhibit half-metallic behaviors with a total magnetic moment of ∼1.0 μ B . At the HSE level, the Br-doped case remains half-metallic, while the other doped cases revert to magnetic semiconductor. The magnetic moment mainly comes from the unpaired Mo-4 d orbitals and the p orbital of the dopant, as well as the S-3 p states. The formation of local magnetic moment depend on the local structure around the dopant, the localization of Mo-4 d , variable valency of Mo cation, and the partially filled anion- p orbitals of the dopant itself. The long-range half-metallic ferromagnetic order is attributed to the double-exchange interactions.

The electronic, structural, and magnetic properties of the perovskite Co-doped BiFeO 3 have been investigated using density functional theory within the generalized gradient approximation plus Hubbard U correction (GGA+ U ). We discuss the changes that occur in the structural parameters, electronic structure, and magnetic properties of the Co-doped BiFeO 3 under the consideration of the impact of the 3 d electrons. The results show that a substitutional Co for Fe in BiFeO 3 produces a magnetic moment of − 1.0 μ B and a half-metallic property emerges. The ferromagnetic (FM) coupling is more stable and still presents a half-metallic property when two Co atoms substitute for Fe atoms in BiFeO 3 .

Titanium dioxide (TiO2) has promising applications in photocatalysis and energy-conversion devices due to its low cost, outstanding conductivity, and excellent electrochemical activity. However, its large band gap and insufficient-sized surface hinder its applications under visible-light radiation, so designing a highly efficient TiO2-based electrode structure is challenging. Herein, we constructed novel van der Waals (vdW) heterostructures using two-dimensional hexagonal TiO2 (H-TiO2) and 2D MoS2 (WS2) components. By density functional theory, we found that the 2D H-TiO2 has robust stability in energy and mechanics, as well as higher ductility than graphene and MoS2. The estimated indirect band gap ranged within 4.30–4.62 eV, resulting in hardly any visible-light absorbance. The vdW heterostructures of MoS2/TiO2 and WS2/TiO2 had the following characteristics: direct band gap, type-II band alignment, built-in electronic field, mobility as high as that of MoS2 (WS2), and remarkably improved visible-light absorption. These features enabled the heterostructures to have highly improved photocatalytic performance and solar-to-electric power conversion efficiency. Thus, these materials have high potential application in photocatalytic water splitting and solar energy-conversion devices.

We systematically search for the stable structures of two-dimensional (2D) V-V binary compounds with honeycomb-like structure by using the first-principles calculation. We identify 26 stable structures out of 54 2D V-V compounds based on various assessments of stabilities: total energy, thermodynamics, and mechanics. Among them, 12 2D V-V compounds are previously unrecognized structures. For each class V-V isomer, the most stable structures are found to be β-AsP, β-SbAs, α-BiAs, α-BiSb, α 2-SbP, and α 2-BiP. For all isomers of the AsP, they are always stable, and hence PAs monolayer is most likely to be prepared experimentally. All the stable structures are semiconductors with bandgaps ranging from 0.06 eV to 2.52 eV at the Heyd–Scuseria–Ernzerhof level. Therefore, they are potential materials for versatile semiconductor devices. Our findings provide a new clue to facilitate the design of 2D materials for potential applications.

The magnetic property induced by the native defects in ZnS bulk, thin film, and quantum dots are investigated comprehensively based on density functional theory within the generalized gradient approximation + Hubbard U (GGA + U) approach. We find the origin of magnetism is closely related to the introduction of hole into ZnS systems. The relative localization of S-3p orbitals is another key to resulting in unpaired p-electron, due to Hund's rule. For almost all the ZnS systems under study, the magnetic moment arises from the S-dangling bonds generated by Zn vacancies. The charge-neutral Zn vacancy, Zn vacancy in 1− charge sate, and S vacancy in the 1+ charge sate produce a local magnetic moment of 2.0, 1.0, and 1.0 μB, respectively. The Zn vacancy in the neutral and 1− charge sates are the important cause for the ferromagnetism in ZnS bulk, with a Curie temperature (TC) above room temperature. For ZnS thin film with clean (111) surfaces, the spins on each surface are ferromagnetically coupled but antiferr...