Guang-Sheng Fu

Influence of structure characteristics on room temperature ferromagnetism of Ni-doped ZnO thin films

The influence of structure characteristics on the ferromagnetism of Ni-doped ZnO (ZnO:Ni) thin films prepared by pulsed laser deposition has been investigated by magnetization and resistivity measurements. Structural and optical absorption analyses revealed that Ni has been incorporated into ZnO lattice as Ni2+ substituting for Zn2+ ions and the microstructure of the films becomes more inhomogeneous with increasing Ni content. The room temperature ferromagnetic (FM) behavior only occurs when Ni content is not high and the saturation magnetic moment per Ni atom decreases with increasing Ni content. No FM signal was detected for the film with 7at.% Ni content. Interestingly, the carrier density decreases with the increase of Ni doping from 1to5at.%, whereas an increase of carrier density is observed for 7at.% doped film, which indicates that carrier density alone cannot account for the FM property in ZnO:Ni films. All the results demonstrated that the microstructure of ZnO:Ni films gets more deteriorated wi...

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 exceedsu2009u2009-0.194, there is an insulator-metal transition and the system becomes metallic with sizable pocket Fermi surfaces.

Enhanced polarization and dielectricity in BaTiO3:NiO nanocomposite films modulated by the microstructure

Parallel and vertical interfaces in vertically and parallelly aligned nanocomposite thin films have been shown to be an effective method to manipulate functionalities. However, tuning the physical properties by modulating the microstructure of the self-assembled nanocomposite films and understanding the physical properties underlying the manipulation is still a challenge. In this work, BaTiO3:NiO (BTO:NiO) nanocomposite films with nanomultilayer, nanocolumnar and nanogranular structures have been prepared on Nb:SrTiO3 (Nb:STO) substrates by a pulsed laser deposition (PLD) method. These films have been used as a model system to investigate the relationship between the microstructure and the ferroelectric properties. The polarization, dielectricity and leakage can be separately modulated by tuning the microstructures. The experimental results show that the remanent polarization of the nanocomposite films is much higher than that of the pure BTO films. By precisely modulating the microstructure, a significantly enhanced polarization (>70% higher than pure BTO for the nanomultilayer structure) and dielectricity (>60% and >240% higher than pure BTO for the nanomultilayer and nanocolumnar structure, respectively) are realized in these films. These results demonstrate that tunable ferroelectric properties can be realized by controlling the microstructures in the epitaxial BTO:NiO nanocomposite thin films, which will be expected to be applied in the devices such as supercapacitors, solar cells and non-volatile memory applications.

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 ...

Charge transfer effect at the La 0.7 Ca 0.3 MnO 3 /NiO heterojunction and the novel interface ferromagnetism

We report the formation of a new ferromagnetic (FM) states in the antiferromagnet (AFM) NiO at the interface with ferromagnet (FM) La_0.7Ca_0.3MnO₃ (LCMO). The new magnetization temperature at 80 K is observed and can be ascribed to Ni³⁺-O-Mn³⁺ superexchange interactions. Mn 3s and Ni 3p core-level spectra have been measured by x-ray photoelectron spectroscopy (XPS) which show a direct evidence of charge transfer effects of the type Mn⁴⁺-Ni²⁺ → Mn³⁺-Ni³⁺ at the interface region. The valence band offset (VBO) at the LCMO/NiO interface can be determined to be ΔE_VBO ~ 0.77 eV. The valence band of NiO is shifted to higher binding energy compared to LCMO. Thus, charge transfer occurred owing to the valence band edge shifting at the heterostructure interfaces.

Control of the ferromagnetic to paramagnetic transition in SrRuO 3 epitaxial films by electric field

The external electric field controlled the phase transition behavior for ferromagnetic thin films is important to realize its practical applications as spintronic devices. In this work, we report the electric field controlled ferromagnetic (FM) to paramagnetic (PM) transition in a SrRuO3/Pb(Mg1/3Nb2/3)O3-PbTiO3 (SRO/PMN-PT) heterostructures by adding a bias voltage. The FM to PM transition and the magnetoresistance (MR) have been dynamic controlled by the electric field. The Curie temperature (Tc) is changed from 154 K to 162 K as a result of external electric field applied to the PMN-PT substrates. It is demonstrated that the SRO film is closely associated with the strain states, which can be directly controlled by the bias voltage. Our finding offer novel opportunities for modulating phase transitions in a reversible non-volatile way.