Gaoxue Wang

Atomically Thin Group V Elemental Films: Theoretical Investigations of Antimonene Allotropes

Group V elemental monolayers including phosphorene are emerging as promising 2D materials with semiconducting electronic properties. Here, we present the results of first-principles calculations on stability, mechanical and electronic properties of 2D antimony (Sb), antimonene. Our calculations show that free-standing α and β allotropes of antimonene are stable and semiconducting. The α-Sb has a puckered structure with two atomic sublayers and β-Sb has a buckled hexagonal lattice. The calculated Raman spectra and STM images have distinct features thus facilitating characterization of both allotropes. The β-Sb has nearly isotropic mechanical properties, whereas α-Sb shows strongly anisotropic characteristics. An indirect-direct band gap transition is expected with moderate tensile strains applied to the monolayers, which opens up the possibility of their applications in optoelectronics.

Degradation of phosphorene in air: understanding at atomic level

Phosphorene is a promising two dimensional (2D) material with a direct band gap, high carrier mobility, and anisotropic electronic properties. Phosphorene-based electronic devices, however, are found to degrade upon exposure to air. In this paper, we provide an atomic level understanding of stability of phosphorene in terms of its interaction with O2 and H2O. The results based on density functional theory together with first principles molecular dynamics calculations show that O2 could spontaneously dissociate on phosphorene at room temperature. H2O will not strongly interact with pristine phosphorene, however, an exothermic reaction could occur if phosphorene is first oxidized. The pathway of oxidation first followed by exothermic reaction with water is the most likely route for the chemical degradation of the phosphorene-based devices in air.

Effects of extrinsic point defects in phosphorene: B, C, N, O, and F adatoms

Phosphorene is emerging as a promising 2D semiconducting material with a direct band gap and high carrier mobility. In this paper, we examine the role of the extrinsic point defects including surface adatoms in modifying the electronic properties of phosphorene using density functional theory. The surface adatoms considered are B, C, N, O and F with a [He] core electronic configuration. Our calculations show that B and C, with electronegativity close to P, prefer to break the sp3 bonds of phosphorene, and reside at the interstitial sites in the 2D lattice by forming sp2 bonds with the native atoms. On the other hand, N, O and F, which are more electronegative than P, prefer the surface sites by attracting the lone pairs of phosphorene. B, N and F adsorption will also introduce local magnetic moment to the lattice. Moreover, B, C, N and F adatoms will modify the band gap of phosphorene yielding metallic transverse tunneling characters. Oxygen does not modify the band gap of phosphorene, and a diode like tunneling behavior is observed. Our results therefore offer a possible route to tailor the electronic and magnetic properties of phosphorene by the adatom functionalization, and provide the physical insights of the environmental sensitivity of phosphorene, which will be helpful to experimentalists in evaluating the performance and aging effects of phosphorene-based electronic devices.

Out-of-plane structural flexibility of phosphorene.

Phosphorene has been rediscovered recently, establishing itself as one of the most promising two-dimensional group-V elemental monolayers with direct band gap, high carrier mobility, and anisotropic electronic properties. In this paper, surface buckling and its effect on its electronic properties are investigated by using molecular dynamics simulations together with density functional theory calculations. We find that phosphorene shows superior structural flexibility along the armchair direction allowing it to have large curvatures. The semiconducting and direct band gap nature are retained with buckling along the armchair direction; the band gap decreases and transforms to an indirect band gap with buckling along the zigzag direction. The structural flexibility and electronic robustness along the armchair direction facilitate the fabrication of devices with complex shapes, such as folded phosphorene and phosphorene nano-scrolls, thereby offering new possibilities for the application of phosphorene in flexible electronics and optoelectronics.

Observation of rotatable stripe domain in permalloy films with oblique sputtering

Stripe domain (SD) in obliquely sputtered permalloy films were investigated by comparing with normally sputtered ones. The critical thickness for SD formation of obliquely sputtered films was about 100 nm thinner than that of normally sputtered films. The hysteresis loops of obliquely sputtered films showed a peculiar shape. A rotation of SD towards easy axis was observed in the obliquely sputtered films, which was confirmed by permeability spectra under a bias field. The origin of the rotation could result from in-plane uniaxial anisotropy, which is induced by the shape effect of the oblique columnar growth of permalloy grains.

Strain engineering of Dirac cones in graphyne

6,6,12-graphyne, one of the two-dimensional carbon allotropes with the rectangular lattice structure, has two kinds of non-equivalent anisotropic Dirac cones in the first Brillouin zone. We show that Dirac cones can be tuned independently by the uniaxial compressive strain applied to graphyne, which induces n-type and p-type self-doping effect, by shifting the energy of the Dirac cones in the opposite directions. On the other hand, application of the tensile strain results into a transition from gapless to finite gap system for the monolayer. For the AB-stacked bilayer, the results predict tunability of Dirac-cones by in-plane strains as well as the strain applied perpendicular to the plane. The group velocities of the Dirac cones show enhancement in the resistance anisotropy for bilayer relative to the case of monolayer. Such tunable and direction-dependent electronic properties predicted for 6,6,12-graphyne make it to be competitive for the next-generation electronic devices at nanoscale.

Growth, structure, morphology, and magnetic properties of Ni ferrite films

The morphology, structure, and magnetic properties of nickel ferrite (NiFe2O4) films fabricated by radio frequency magnetron sputtering on Si(111) substrate have been investigated as functions of film thickness. Prepared films that have not undergone post-annealing show the better spinel crystal structure with increasing growth time. Meanwhile, the size of grain also increases, which induces the change of magnetic properties: saturation magnetization increased and coercivity increased at first and then decreased. Note that the sample of 10-nm thickness is the superparamagnetic property. Transmission electron microscopy displays that the film grew with a disorder structure at initial growth, then forms spinel crystal structure as its thickness increases, which is relative to lattice matching between substrate Si and NiFe2O4.

Physics and chemistry of oxidation of two‐dimensional nanomaterials by molecular oxygen

The discovery of graphene has inspired extensive interest in two‐dimensional (2D) materials, and has led to synthesis/growth of additional 2D materials, generally referred to as ‘Beyond Graphene’. Notable among the recently discovered exotic 2D materials are group IV elemental monolayers silicene and germanene, group V elemental monolayer phosphorene, and binary monolayers, such as hexagonal boron nitride (h‐BN), and molybdenum disulfide (MoS2 ). Environmental effect on the physical and chemical properties of these 2D materials is a fundamental issue for their practical applications in devices operating under ambient conditions, especially, exposure to air often leads to oxidation of nanomaterials with significant impact on the functional properties and performances of devices built with them. In view of its importance, we present here a review of the recent experimental and theoretical studies on the oxidation of 2D materials focusing on the relationship between the oxidation process and the energy values which can be calculated by first principles methods. The complement of experiments and theory facilitates the understanding of the underlying oxidation process in terms of cohesive energy, energy barrier to oxidation and dissociation energy of oxygen molecule for 2D materials including graphene, silicene, germanene, phosphorene, h‐BN, and MoS2. WIREs Comput Mol Sci 2017, 7:e1280. doi: 10.1002/wcms.1280

Stripe domain and enhanced resonance frequency in ferrite doped FeNi films

Stripe domain (SD) structures of FeNi films were controlled by doping different concentrations of ferrite. With increasing concentration of doped ferrite, SD width decreases, which indicated that perpendicular magnetic anisotropy (H⊥) increases. SD disappears and bubble-like features are observed with doping ferrite at 32% due to large H⊥. The origin for the enhancement of H⊥ is exchange coupling between ferrimagnet and ferromagnet. Moreover, different doping concentrations of ferrite realize an increase of resonant frequency from 1.3 to 2.3 GHz. (Some figures may appear in colour only in the online journal)

Adjustable Microwave Properties in FeCoZr/Cu Multilayers

The FeCoZr/Cu multilayers were prepared by radio frequency sputtering. The static and microwave properties of these films were investigated by measuring the hysteresis loops and microwave permeability spectra. According to the static magnetic results, the multilayers were well in-plane uniaxial anisotropic samples with high saturation magnetization 1.7 T. The anisotropic field of these multilayer thin films can be adjusted from 18 Oe to 64 Oe by changing the thickness of Cu interlayer from 8.7 nm to 1.8 nm. As a consequence, the microwave permeability and resonance frequency can also be adjusted from 1.70 GHz to 2.88 GHz. This work might facilitate search for new materials with high permeability at high frequency.