Hanbin Wang

High resolution transmission electron microscopy and Raman scattering studies of room temperature ferromagnetic Ni-doped ZnO nanocrystals

Ni-doped ZnO nanocrystals have been synthesized by a wet chemical reaction. The nanocrystals have been investigated carefully by high resolution transmission electron microscopy and all the particles are found to be the known wurtzite ZnO. X-ray photoelectron spectroscopy and Raman spectra results provide the evidence that Ni2+ is incorporated into the ZnO lattice at Zn2+ site. Magnetic property measurements reveal that the as-grown Zn1−xNixO nanocrystals exhibit room temperature ferromagnetic behaviors with saturation magnetization of 0.01emu∕g and Curie temperature above 340K for Ni concentration of ∼1% in atomic ratio.

Bendable ITO-free Organic Solar Cells with Highly Conductive and Flexible PEDOT:PSS Electrodes on Plastic Substrates

UNLABELLED Flexible and transparent electrodes have great potential for photon transmission and charge-carrier collection for next generation electronics compared to rigid electronics with indium tin oxide (ITO)-coated glass substrates. This study describes a comprehensive study of the electrical, morphological, optical, structural, and mechanical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT PSS) films treated by methanol and methanesulfonic acid (MSA), which are coated on hydrophobic flexible plastic substrates. Such a film coated on hydrophobic plastic substrates exhibits a high conductivity up to 3560 S cm(-1) and a good mechanical flexibility. Moreover, the use of the films to fabricate bendable ITO-free organic solar cells (OSCs) integrated on plastic substrates was presented. The bendable devices based on P3HT:PCBM not only exhibit a high power conversion efficiency (PCE) up to 3.92%, which is comparable to 4.30% of the rigid devices with ITO-coated glass substrates, but also keep about 80% in PCE of the initial value after 100 time bending with a bending radius of 14 mm in the ambient atmosphere. This work provides a novel route to dramatically improve the conductivity of PEDOT PSS electrodes, as well as the mechanical flexibility of highly efficient organic electronics with the flexible electrodes.

Double-shelled ZnO/CdSe/CdTe nanocable arrays for photovoltaic applications: microstructure evolution and interfacial energy alignment

A series of high-density double- and single-shelled ZnO/CdSe/CdTe, ZnO/CdTe/CdSe, ZnO/CdTe and ZnO/CdSe nanocable arrays were synthesized as photoanodes by an electrodeposition method using ZnO nanorod arrays as cores. For ZnO/CdSe/CdTe nanocable arrays, the uniform CdSe and CdTe nanoshells were composed of zinc-blende phase nanocrystals with a respective average size range of 10–20 nm and 7–15 nm, and formed a compact and continuous interface in between. Based on the band offset of the bulk material before contact and the interfacial Fermi level shift after contact, the energy level alignments at the CdSe/CdTe and CdTe/CdSe interface were deduced for the double-shelled nanocable arrays. The CdTe/CdSe interface of the ZnO/CdTe/CdSe nanocables has a negative band offset of −0.16 eV whilst for ZnO/CdSe/CdTe nanocables, the band edge of CdTe lies above CdSe with a conduction band offset of 0.16 eV at the CdSe/CdTe interface. Such a stepwise band alignment, together with the compact interface, fewer grain boundaries along the radial direction, and the fast transfer rate along the axial direction of the nanocables, makes the ZnO/CdSe/CdTe nanocable arrays photoanode have a saturated photocurrent of ∼14.3 mA cm−2. This is under the irradiation of AM1.5G simulated sunlight at 45 mW cm−2, which is greatly higher than ZnO/CdTe/CdSe, ZnO/CdSe or ZnO/CdTe nanocable arrays.

Investigation on the electrical conductivity of transformer oil-based AlN nanofluid

Aluminum-nitride-(AlN)-transformer oil-based nanofluid was prepared by dispersing AlN nanoparticles in transformer oil. The composition-dependent electrical conductivity of AlN-transformer oil nanofluid was investigated at different ambient temperatures. The results indicate the nonlinear dependences of the electrical conductivity on volumetric fraction and temperature. In comparison to the pure transformer oil, the electrical conductivity of nanofluid containing 0.5% AlN nanoparticles has increased by 1057 times at 60°C. By considering the electrophoresis of the AlN nanoparticles, a straightforward electrical conductivity model is established to modulate and understand the experiment results.

The trap state relaxation related polarization in CaCu3Ti4O12

Electrical conditioning properties and dielectric behavior of CaCu3Ti4O12 (CCTO) were investigated. A reversible current-voltage characteristic relaxation effect together with a corresponding dielectric relaxation was found in this material. A trap state model which combines a monopole and dipole relaxation was proposed to be the key element to explain the reported huge permittivity in CCTO. The work also supports the extrinsic origin of the giant dielectric constant.

Improved thermal stability, interface, and electrical properties of HfO2 films prepared by pulsed laser deposition using in situ ionized nitrogen

Nitrogen is incorporated into thin HfO2 films by pulsed laser deposition using in situ ionized nitrogen. The improved thermal stability and interfacial microstructure are further confirmed by high-resolution transmission electron microscopy. The composition of the thin film is investigated by x-ray photoelectron spectroscopy and electron energy-loss spectroscopy. Electrical studies show a property permittivity of 27.7 and low leakage current density were achieved by incorporation of a small amount (about 1 at. %) of nitrogen. The dominant conduction mechanisms of the Pt/HfO2/p-Si structure are trap-assisted tunneling and Schottky emission at low electric field for the gate and substrate injection, respectively.

Mn-doped ZnO nanotubes: from facile solution synthesis to room temperature ferromagnetism

Mn-doped ZnO nanotubes have been synthesized via a simple, soft solution route at 70 °C without any surfactants and templates using zinc acetate, manganese acetate and sodium hydroxide as raw materials. The nanotubes are of 1.0–2.5 μm in length, 300–500 nm in outer diameter, and 50–80 nm in wall thickness. The microstructure related to Mn doping in the ZnO matrix was characterized by X-ray diffraction, Raman scattering and X-ray photoelectron spectroscopy. The formation mechanism of the tubes, as well as the effect of the reaction temperature and Mn dopants on the structures and magnetic properties, was analyzed in detail. Room temperature ferromagnetic behavior was observed in Mn-doped ZnO nanotubes with a Mn concentration of 3% in atomic ratio; however, ferromagnetism is gradually suppressed by paramagnetic effect with increasing Mn concentration up to 6%. The origin and reduction of the ferromagnetism may be closely associated with the portion of Mn compounds containing Mn atoms in different oxidation states.

Selective Growth of Vertical-aligned ZnO Nanorod Arrays on Si Substrate by Catalyst-free Thermal Evaporation

By thermal evaporation of pure ZnO powders, high-density vertical-aligned ZnO nanorod arrays with diameter ranged in 80–250 nm were successfully synthesized on Si substrates covered with ZnO seed layers. It was revealed that the morphology, orientation, crystal, and optical quality of the ZnO nanorod arrays highly depend on the crystal quality of ZnO seed layers, which was confirmed by the characterizations of field-emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and photoluminescence measurements. For ZnO seed layer with wurtzite structure, the ZnO nanorods grew exactly normal to the substrate with perfect wurtzite structure, strong near-band-edge emission, and neglectable deep-level emission. The nanorods synthesized on the polycrystalline ZnO seed layer presented random orientation, wide diameter, and weak deep-level emission. This article provides a C-free and Au-free method for large-scale synthesis of vertical-aligned ZnO nanorod arrays by controlling the crystal quality of the seed layer.

Interfacial Reaction and Electrical Properties of HfO2 Film Gate Dielectric Prepared by Pulsed Laser Deposition in Nitrogen: Role of Rapid Thermal Annealing and Gate Electrode

The high-k dielectric HfO(2) thin films were deposited by pulsed laser deposition in nitrogen atmosphere. Rapid thermal annealing effect on film surface roughness, structure and electrical properties of HfO(2) film was investigated. The mechanism of interfacial reaction and the annealing atmosphere effect on the interfacial layer thickness were discussed. The sample annealed in nitrogen shows an amorphous dominated structure and the lowest leakage current density. Capacitors with high-k HfO(2) film as gate dielectric were fabricated, using Pt, Au, and Ti as the top gate electrode whereas Pt constitutes the bottom side electrode. At the gate injection case, the Pt- and Au-gated metal oxide semiconductor devices present a lower leakage current than that of the Ti-gated device, as well as similar leakage current conduction mechanism and interfacial properties at the metal/HfO(2) interface, because of their close work function and chemical properties.

Spin-dependent varistor-like behavior of Nd0.7Sr0.3MnO3−δ (0⩽δ⩽0.20)

Spin-dependent varistor-like electrical transport is found in Nd0.7Sr0.3MnO3−δ polycrystalline ceramics with slight oxygen deficient δ⩾0.05. The current-voltage I(V) curve shows nonlinear Ohmic characteristics below a critical temperature. Electrical-field-induced changes of the spin array orientations inside of and between the magnetic domains over grain or phase boundaries have to be concluded for the strong electroresistive effect in Nd0.7Sr0.3MnO3−δ series.