Rui-Ning Wang

Impurity induced spin filtering in graphene nanoribbons

Zigzag-edged graphene nanoribbons (ZGNRs) are used to build devices in which transport with high spin polarization is realized by first-principles simulations. At first, we dope the semiconducting ZGNRs with B atoms on both sides to obtain a metallic ribbon. Then, in one segment of this ribbon which acts as the scattering region, the B atoms at one edge are substituted by C atoms or by N atoms. It is found that such devices show very good spin filtering effect which originates from the interaction between the impurity atoms and the edge states.

Electronic structures and transverse electrical field effects in folded zigzag-edged graphene nanoribbons

Folded graphene nanoribbons were observed and their unusual transport properties were reported recently. In this work, the electronic structures and the effects of external transverse electrical field in the folded zigzag-edged graphene nanoribbons (ZGNRs) are investigated by first-principles calculations. It is found that, the folding does not break the semiconductivity, the antiferromagnetically coupled edge states and edge magnetism characteristic of ZGNRs. Interestingly, when external electrical field is applied, the ribbons can be turned to metal or half-metal, depending on the direction of the field. The robustness of the half-metallicity and the edge magnetism to the electrical field is also discussed.

First-principles study on electron transport of carbon dumbbells C60-Cn-C60

Electronic transport properties of carbon dumbbells, a new type of carbon hybrid nanostructures formed by connecting carbon atomic chains to two fullerenes C(60), are investigated by using nonequilibrium Greens functions in combination with the density-functional theory. Specifically, the transport properties as a function of the carbon chain length n are examined. An obvious metal-insulator-like oscillation has been achieved with the increase of the carbon chain length. When n is even, the device behaves as a metal. In contrast, when n is odd, it behaves as an insulator. It is quite different to the carbon chains directly sandwiched between metallic leads where the low conductance states are observed for even n while the high conductance states are observed for odd n. Such a difference arises from the screening effect of C(60)s.

Structural, magnetic and transport properties of carbon chains sandwiched between zigzag graphene nanoribbons

We investigate the structural, magnetic properties and spin polarized transport of monatomic carbon chains covalently interconnected between zigzag graphene nanoribbon (ZGNR) leads with first-principles and non-equilibrium Greens function methods. It is revealed that an even numbered carbon chain is nonmagnetic when connected at the center of the armchair side of ZGNRs, but it becomes spin polarized and a net magnetic moment (about 0.1 μB) is developed when it moves to the edge. In contrast, an odd-numbered carbon chain is magnetic when it lies at the center of the armchair side and the magnetic moment decreases when the carbon chain moves to the edge. The total magnetic moment of the ZGNR/Cn/ZGNR junction depends on the magnetic alignment between two zigzag edges. Compared with that of ZGNR/C9/ZGNR with C9 chain anchoring at the center, the magnetic moment of the junction increases by 2.2 μB in the antiferromagnetic configuration and decreases by 1.5 μB in the ferromagnetic alignment when the carbon chains migrate from the center to one edge. Thus, the ZGNR/Cn/ZGNR junctions show rich magnetic varieties by modulating the carbon chain anchoring sites and the zigzag edge magnetic coupling. Further, spin-dependent transport calculations indicate that the ZGNR/C9/ZGNR junction behaves as a perfect spin-filter in spintronic circuits.

Semiconductor-metal transition of titanium sesquioxide nanopowder

Titanium sesquioxide (Ti2O3) nanopowders have been successfully synthesized using TiCl4 assisted hydrogen reduction method from P25 (TiO2) powders at 970 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy reveal its high purity. The temperature dependent XRD, resistance, and infrared absorption investigations exhibit Z-shaped curves (cell parameter a, resistance, and infrared transmittance) indicating the semiconductor-metal transition (SMT) in the range of 135–220 °C. With prolonging the annealing time, the starting temperature of SMT is found to move toward the low-temperature side, and this was also discussed. The c/a ratio alteration is considered to result in the modulation of property. Besides, the results are verified comparing with the theoretical calculation of band structure.

Strain and electric field co-modulation of electronic properties of bilayer boronitrene

The electronic properties of bilayer strained boronitrenes are investigated under an external electric field using density functional methods. Our result is just the same as the previous conclusion: ie, that the electric field will reduce their band gaps. Except for the decrease of their band gaps, the degeneracy of π valence bands at K points will be lifted and the degenerate gap will increase with the electric field increasing. Moreover, the widths of π valence bands are nearly robust and increase a little. In addition, a simple tight-binding model, where different electrostatic potentials are applied to boronitrene layers, can be sufficient to describe the variations of their band gaps. It is found that the interlayer hopping interaction increases while the intralayer hopping parameter changes little with increasing the electric field. Furthermore, a band gap phase diagram is determined within the in-plane strain [-0.2, 0.2] and the interlayer bias [0, 10] V nm(-1). The strain could make the bottom of conduction bands shift from K to M, then to Γ in the Brillouin zone, while the top of valence bands shifts from K to Γ. Thus, a direct-gap semiconductor at K points is changed into an indirect-gap semiconductor, and then a semiconductor with the direct band gap at Γ points. When bilayer boronitrene is a semiconductor with a direct gap at K points, the electric field and strain are inverse proportional relationships. Particularly, when the compressive strain exceeds  -0.194, there is an insulator-metal transition and the system becomes metallic with sizable pocket Fermi surfaces.

Impact of contact couplings on thermoelectric properties of anti, Fano, and Breit-Wigner resonant junctions

Quantum interference is a well-known phenomenon which results in unique features of the transmission spectra of molecular junctions at the nanoscale. We investigate and compare the thermoelectric properties of three types of junctions like the anti, Breit-Wigner, and Fano resonances. Due to its asymmetric line-shaped transmission function, Fano resonances lead to a larger thermoelectric figure of merit (ZT) than the symmetric anti and Breit-Wigner resonances. The occurrence of quantum interference in molecular and other nanoscale junctions is independent of contact couplings between the sandwiched molecules and left/right electrodes. However, it is found that the contact couplings determine the electric and thermoelectric performances of quantum interference junctions. In anti-resonant junctions, the Seebeck coefficient is enhanced by strong contact couplings. By contrast, for Breit-Wigner resonant junctions, this same property will increase in the weak contact coupling regime. Contrary to what is observe...

Intra- and inter-layer charge redistribution in biased bilayer graphene

We investigate the spatial redistribution of the electron density in bilayer graphene in the presence of an interlayer bias within density functional theory. It is found that the interlayer charge redistribution is inhomogeneous between the upper and bottom layers and the transferred charge from the upper layer to the bottom layer linearly increases with the external voltage which further makes the gap at K point linearly increase. However, the band gap will saturate to 0.29 eV in the strong-field regime, but it displays a linear field dependence at the weak-field limit. Due to the AB-stacked way, two carbon atoms per unit cell in the same layer are different and there is also a charge transfer between them, making the widths of π valence bands reduced. In the bottom layer, the charge transfers from the direct atoms which directly face another carbon atom to the indirect atoms facing the center of the hexagon on the opposite layer, while the charge transfers from the indirect atoms to the direct atoms in ...

Topological insulator in tellurium-based perovskites

Perovskites have a cubic structure with general formula of ABO3. The meritorious features such as colossal magnetoresistance, ferroelectricity, superconductivity and charge ordering are observed in this class of materials and make perovskites a hot-spot which receive considerable research interest in recent years. Here, we predict a new topological insulator in tellurium-based perovskites and calculate its band structures, electronic density of states (DOS) and Z2 quantum number. By orbital-projected band structure analysis we find that the orbitals involved in the topological band-inversion process are s- and p-orbitals. The topological surface state is also given. This new material can provide dissipationless signal current for devices and is expected to be applied in spintronics devices.

Spin-flip effect on transport properties of a Mn3 molecule

Electron transport through a single-molecule magnet [NEt4]3[Mn3Zn2(salox)3O(N3)6Cl2] is investigated by spin-polarized density functional theory combined with the Keldysh nonequilibrium Green’s function technique. Our study demonstrates that spin-filtering effect and negative differential resistance exist in the ground state of this molecule. When the magnetic state of the molecule is changed from its ground state to the spin-flip state, substantial changes are induced not only in energy levels of the molecule, but also in the coupling of molecular states with eigenstates of Ag(100) nano-electrodes, which lead to the disappearance of spin-filtering effect and negative differential resistance.