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Abstract The defect formation energies of transition metals (Cr, Fe, and Ni) doped in the pseudo-H passivated ZnO nanowires and bulk are systematically investigated using first-principles methods. The general chemical trends of the nanowires are similar to those of the bulk. We also show that the formation energy increases as the diameter of the nanowire decreases, indicating that the doping of magnetic ions in the ZnO nanowire becomes more difficult with decreasing diameter. We also systematically calculate the ferromagnetic properties of transition metals doped in the ZnO nanowire and bulk, and find that Cr ions of the nanowire favor ferromagnetic state, which is consistent with the experimental results. We also find that the ferromagnetic coupling state of Cr is more stable in the nanowire than in the bulk, which may lead to a higherT cuseful for the nano-materials design of spintronics.

Abstract Tetragonal ferroelectric/ferroelectric superlattice under uniaxial tensile stress along the c axis is investigated from first principles. We show that the calculated ideal tensile strength is 6.85 GPa and that the superlattice under the loading of uniaxial tensile stress becomes soft along the nonpolar axes. We also find that the appropriately applied uniaxial tensile stress can significantly enhance the piezoelectricity for the superlattice, with piezoelectric coefficient d 33 increasing from the ground state value by a factor of about 8, reaching 678.42 pC/N. The underlying mechanism for the enhancement of piezoelectricity is discussed.

Abstract Glomerate GaN nanowires were synthesized on Si(111) substrates by annealing sputtered Ga2O3/Co films under flowing ammonia at temperature of 950 °C. X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and Fourier transformed infrared spectra were used to characterize the morphology, crystallinity and microstructure of the as-synthesized samples. Our results show that the samples are of hexagonal wurtzite structure. For the majority of GaN nanowires, the length is up to tens of microns and the diameter is in the range of 50–200 nm. The growth process of the GaN nanowires is dominated by Co–Ga–N alloy mechanism.

Structural phase transition, spontaneous polarization, piezoelectric response, and band structure of aluminum nitride under hydrostatic pressure are systematically studied via first-principles calculations. The band structures are obtained from the HSE06 range-separated hybrid functional. Our calculated results exhibit interesting behaviors: (i) Just like the cases of uniaxial and in-plane strains, the material undergoes a structural transition from the equilibrium wurtzite phase to a pseudographitic h-MgO phase at large pressure. (ii) Although the new phase is nonpolar, the spontaneous polarization of wurtzite phase is greatly enhanced by pressure and reaches the maximum value at the phase transition. (iii) The appropriately applied pressure remarkably enhances the piezoelectric response for wurtzite phase, with the strongest behavior appearing at the phase transition. This is consistent, in that the wurtzite structure becomes markedly soft along the polar axis as pressure increases and similar to the structural transition of perovskites from ferroelectric to paraelectric phases. (iv) The wurtzite phase under pressure undergoes a direct-indirect bandgap transition, with the conduction band minimum (CBM) no longer at zone center Γ, but at the zone-edge K point and the valence band maximum (VBM) at Γ. In addition, the polar-nonpolar structural transformation simultaneously gives rise to another bandgap transition from indirect to indirect with the CBM shifting from K to M point, but the VBM still at Γ. This is remarkably different from the results of uniaxial and in-plane strains. The anomalous behaviors of the band structure originate from that the top valence and bottom conduction states at different reciprocal lattice points show the different dependence on hydrostatic pressure. [ABSTRACT FROM AUTHOR]

Tetragonal PbTiO3 under in-plane biaxial stress is investigated from first principles. We show that before the biaxial tensile stress applied in the plane perpendicular to the c axis reaches its ideal tensile strength of 28.95 GPa, PbTiO3 tend to show the paraelectric tetragonal symmetry. We also find that the biaxial tensile stress can enhance the piezoelectric stress coefficients, whereas the biaxial compressive stress can enhance the piezoelectric strain coefficients. The underlying mechanisms for the predicted piezoelectric responses are also discussed. [ABSTRACT FROM AUTHOR]

Tetragonal PbTiO3 under in-plane biaxial stress is investigated from first principles. We show that before the biaxial tensile stress applied in the plane perpendicular to the c axis reaches its ideal tensile strength of 28.95 GPa, PbTiO3 tend to show the paraelectric tetragonal symmetry. We also find that the biaxial tensile stress can enhance the piezoelectric stress coefficients, whereas the biaxial compressive stress can enhance the piezoelectric strain coefficients. The underlying mechanisms for the predicted piezoelectric responses are also discussed. [ABSTRACT FROM AUTHOR]