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A new category of tubular nanostructures solely composed of selenium was explored by using the density functional theory. Similar to those in carbon nanotubes, the armchair and zigzag tube-like structures formed as a roll-up α-Se nanoribbon. The zigzag (n ≤ 16, 0) nanotubes exhibit decreasingly narrow direct band gap, and the armchair (n, n) possess decreasingly moderate indirect gap. The strain energy of nanotubes is inversely proportional to the second power of the nanotube diameter. With increasing tube diameter, an initial rise and consequent decrease occurred in the effective masses to the limit value of α-Se monolayer. The carrier mobility of nanotubes is comparable with that of its monolayer with orders of 103 and 102 cm2V−1s−1 for electron and hole, respectively. The optical absorption spectra of α-Se nanotubes exhibit high absorbance in energy window cover the entire visible light spectrum. Therefore, α-Se nanotubes hold great potential for future applications in high-performance optoelectronics.

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.

Based on density functional theory, the electronic structures and magnetic properties have been studied in MoS2 monolayer via substitutional doping of nonmagnetic elements (IA, IIA, and IIIA elements). The magnetic moment of those doped systems origins form the interplay between the crystal-field of MoS2 matrix and localized Mo 4d states. On the whole, these doped MoS2 monolayers exhibit a half-metal→spin gapless (or narrow gap) semiconductor→ferromagnetic semiconductor transition as the dopants change from IA to IIIA groups. Electron and hole doping by a potential gate can realize a transition from ferromagnetic semiconductor to half metal. In important, the spin-polarization direction is switchable depending on the doped carrier’s type.

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.

We explore the electronic structures and magnetic properties in Cu-doped MX 2 (=MoS 2 , MoSe 2 , MoTe 2 , and WS 2 ) based on density functional theory. A Cu dopant leads to a net moment of 5.0 or 1.0 μ B in MX 2 , which mainly depend on the size of crystal-field splitting relative to that of the spin splitting. No magnetism is observed in Cu-doped MoTe 2 . The local distortion around the Cu atom reduces the total magnetic moment in two-Cu-doped MX 2 . The magnetic coupling between the nearest neighboring Cu atoms is ferromagnetic for all the cases, but they demonstrate various magnetic ground states with the increasing distance between Cu atoms: the Cu-doped MoS 2 and WS 2 exhibit anti-ferromagnetic and nonmagnetic ground state, respectively. A long-range ferromagnetic or ferrimagnetic coupling is attributed to double-exchange interaction in Cu-doped MoSe 2 . Half-metallic ferromagnetism with Curie temperature above room temperature in Cu-doped MoSe 2 provides a useful guidance to engineer the magnetic properties of MoSe 2 in experiments.

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.

Basing on ab initio calculations, we investigate the electronic structures and magnetic properties of triazine-based graphitic carbon nitride (gt-C3N4) sheets with adsorbed by Nb and Tc. Both Nb and Tc atoms energetically prefer to be adsorbed on the cavity site. Adsorptions lead to total magnetic moments of 1.00 μB per Nb or Tc atom. The magnetic moments on Nb anti-ferromagnetically couples with those on C and N atoms, but the spins between Nb atoms strongly ferromagneticially couple to each other. The system therefore becomes ferromagnetic (FM) semiconductor with reduced band-gap, and the estimated Curie temperature (TC) is much higher than room temperature. The magnetic moments of Tc adsorbed system ferromagnetically couple with each other. The adsorption of Tc atom changes the semiconducting gt-C3N4 into half metallic material. These findings indicate adsorptions of Nb/Tc lead to various magnetic characters that has potential application in future spintronics.