Ni Qin

CoNi2S4 Nanosheet Arrays Supported on Nickel Foams with Ultrahigh Capacitance for Aqueous Asymmetric Supercapacitor Applications

We report that CoNi2S4 nanosheet arrays exhibit ultrahigh specific capacitance of 2906 F g(-1) and areal capacitance of 6.39 F cm(-2) at a current density of 5 mA cm(-2), as well as good rate capability and cycling stability, and superior electrochemical performances with an energy density of 33.9 Wh kg(-1) at a power density of 409 W kg(-1) have been achieved in an assembled aqueous asymmetric supercapacitor. The CoNi2S4 nanosheet arrays were in situ grown on nickel foams by a facile two-step hydrothermal method. The formation mechanism of the CoNi2S4 nanosheet arrays was based on an anion-exchange reaction involving the pseudo Kirkendall effect. The two aqueous asymmetric supercapacitors in series using the CoNi2S4 nanosheet arrays as the positive electrodes can power four 3-mm-diameter red-light-emitting diodes. The outstanding supercapacitive performance of CoNi2S4 nanosheet arrays can be attributed to ravine-like nanosheet architectures with good mechanical and electrical contact, low crystallinity and good wettability without an annealing process, rich redox reactions, as well as high conductivity and transport rate for both electrolyte ions and electrons. Our results demonstrate that CoNi2S4 nanosheet arrays are promising electrode materials for supercapacitor applications.

Opportunity of Spinel Ferrite Materials in Nonvolatile Memory Device Applications Based on Their Resistive Switching Performances

The opportunity of spinel ferrites in nonvolatile memory device applications has been demonstrated by the resistive switching performance characteristics of a Pt/NiFe(2)O(4)/Pt structure, such as low operating voltage, high device yield, long retention time (up to 10(5) s), and good endurance (up to 2.2 × 10(4) cycles). The dominant conduction mechanisms are Ohmic conduction in the low-resistance state and in the lower-voltage region of the high-resistance state and Schottky emission in the higher-voltage region of the high-resistance state. On the basis of measurements of the temperature dependence of the resistances and magnetic properties in different resistance states, we explain the physical mechanism of resistive switching of Pt/NiFe(2)O(4)/Pt devices using the model of formation and rupture of conducting filaments by considering the thermal effect of oxygen vacancies and changes in the valences of cations due to the redox effect.

Bipolar and tri-state unipolar resistive switching behaviors in Ag/ZnFe2O4/Pt memory devices

We report on the co-existence of bipolar and unipolar resistive switching behaviors in Ag/ZnFe2O4/Pt structures of which the ZnFe2O4 layer was fabricated by a chemical solution deposition method. The memory devices show reproducible and stable bipolar resistive switching, tri-state unipolar resistive switching under only applied negative bias voltage, and unipolar resistive switching transited from bipolar resistive switching with different electroforming conditions. Excellent switching cycling in both unipolar resistive switching and bipolar resistive switching is demonstrated. Based on the conducting filament model, electrochemical metallization effect has been proposed to explain the bipolar resistive switching behavior, whereas the unipolar resistive switching behavior is attributed to electrochemical metallization effect and thermochemical effect.

Highly uniform resistive switching properties of amorphous InGaZnO thin films prepared by a low temperature photochemical solution deposition method.

We report on highly uniform resistive switching properties of amorphous InGaZnO (a-IGZO) thin films. The thin films were fabricated by a low temperature photochemical solution deposition method, a simple process combining chemical solution deposition and ultraviolet (UV) irradiation treatment. The a-IGZO based resistive switching devices exhibit long retention, good endurance, uniform switching voltages, and stable distribution of low and high resistance states. Electrical conduction mechanisms were also discussed on the basis of the current-voltage characteristics and their temperature dependence. The excellent resistive switching properties can be attributed to the reduction of organic- and hydrogen-based elements and the formation of enhanced metal-oxide bonding and metal-hydroxide bonding networks by hydrogen bonding due to UV irradiation, based on Fourier-transform-infrared spectroscopy, X-ray photoelectron spectroscopy, and Field emission scanning electron microscopy analysis of the thin films. This study suggests that a-IGZO thin films have potential applications in resistive random access memory and the low temperature photochemical solution deposition method can find the opportunity for further achieving system on panel applications if the a-IGZO resistive switching cells were integrated with a-IGZO thin film transistors.

Significantly enhanced red photoluminescence properties of nanocomposite films composed of a ferroelectric Bi3.6Eu0.4Ti3O12 matrix and highly c-axis-oriented ZnO nanorods on Si substrates prepared by a hybrid chemical solution method.

We have developed a hybrid chemical solution method for preparing nanocomposite thin films composed of a ferroelectric Bi(3.6)Eu(0.4)Ti(3)O(12) (BEuT) matrix and highly c-axis-oriented ZnO nanorods on Si substrates. First, a seed-layer solution growth approach was used to prepare the highly c-axis-oriented ZnO nanorods, and then a chemical solution deposition method was employed to fabricate the BEuT matrix by coating the ZnO nanorods using a spin-coating technique. The nanocomposite thin films obtained exhibited significantly enhanced red photoluminescence (PL) properties. The PL enhancement can be attributed to very efficient radiation energy transfer from the ZnO nanorods to Eu(3+) ions in the BEuT matrix due to two spectral overlaps between the emission spectra of the ZnO nanorods and the excitation bands of Eu(3+) ions in the BEuT matrix: the spectral overlap between the sharp UV emission of ZnO centered at 380 nm and the excitation spectrum of the (7)F(0) --> (5)L(6) transition of Eu(3+) ions at 395 nm and that between the defect-related deep-level green emission band of ZnO centered at 525 nm and the excitation spectrum of the (7)F(0) --> (5)D(2) transition of Eu(3+) ions at 465 nm. Our study opens possibility of realizing highly efficient photoluminescent ferroelectric multifunctional integrated thin-film devices. In addition, the hybrid chemical solution method also provides a useful route for the synthesis of some new nanocomposite thin films consisting of other inorganic matrix and c-axis-oriented ZnO nanorods.

Resistive switching properties and physical mechanism of cobalt ferrite thin films

We report reproducible resistive switching performance and relevant physical mechanism of sandwiched Pt/CoFe2O4/Pt structures in which the CoFe2O4 thin films were fabricated by a chemical solution deposition method. Uniform switching voltages, good endurance, and long retention have been demonstrated in the Pt/CoFe2O4/Pt memory cells. On the basis of the analysis of current-voltage characteristic and its temperature dependence, we suggest that the carriers transport through the conducting filaments in low resistance state with Ohmic conduction behavior, and the Schottky emission and Poole-Frenkel emission dominate the conduction mechanism in high resistance state. From resistance-temperature dependence of resistance states, we believe that the physical origin of the resistive switching refers to the formation and rupture of the oxygen vacancies related filaments. The nanostructured CoFe2O4 thin films can find applications in resistive random access memory.

Strong red emission in lead-free ferroelectric Pr3+-doped Na0.5Bi0.5TiO3 thin films without the need of charge compensation

We report on the strong red emission properties of lead-free perovskite ferroelectric Pr3+-doped Na0.5Bi0.5TiO3 (NBT: Pr3+) thin films without the need of charge compensation. The thin films were prepared by a chemical solution deposition method combined with a rapid thermal annealing process at 700 °C. The strong red emission assigned to prominent 1D2→3H4 transitions of the Pr3+ ions at 611 nm was observed together with a very weak blue-green emission, indicating the commercial use potential of the lead-free perovskite NBT: Pr3+ ferroelectric thin film materials. The energy transfer from the host lattice to the Pr3+ ions should be responsible for the strong red emission, while the weak blue-green emission might be due to the quenching of 3P0 induced by a low-lying Pr3+/Ti4+↔Pr4+/Ti3+ intervalence charge transfer state. The lead-free NBT: Pr3+ thin films also exhibit good ferroelectric and dielectric properties.

Bright up-conversion photoluminescence of Bi4−xErxTi3O12 ferroelectric thin films

The up-conversion (UC) photoluminescence and ferroelectric properties of Bi4−xErxTi3O12 (BErT) thin films were studied in terms of annealing temperature and Er3+ doping concentration. The thin films were prepared by chemical solution deposition method. There are two green emission bands centered at 527 and 548 nm, and a red emission band centered at 663 nm in UC luminescence spectra measured under a 980 nm laser excitation at room temperature, which correspond to the radiative transitions from 2H11/2, 4S3/2, and 4F9/2 to 4I15/2, respectively. The quenching concentration of Er3+ ions for green emission was as high as 20 mol % for Bi3.2Er0.8Ti3O12 thin films. The large Er3+ quenching concentration and efficient energy transfer between two neighboring Er3+ ions result in the improved UC emission. The dependence of UC emission intensity on pumping power indicated a two-photon UC emission process in the thin films. The combination of UC emission and ferroelectricity was realized in the capacitors of Pt/Bi3.25E...

Combination of Strong Blue Up-Conversion Photoluminescence and Greatly Enhanced Ferroelectric Polarization in Tm3+-Yb3+-W6+- Doped Bi4Ti3O12 Thin Films

A strong blue up-conversion photoluminescence and a greatly enhanced ferroelectric polarization were observed in Tm 3+ -Yb 3+ -W 6+ -triply-doped bismuth titanate thin films. The thin films were prepared on fused silica and Pt/Ti/SiO 2 /Si (111) substrates by chemical solution deposition and annealed at 700°C. The strong blue up-conversion emission centered at 478 nm, corresponding to 1 G 4 → 3H6 transitions of Tm 3+ ions, was confirmed to be a three-photon energy-transfer process by pumping laser power dependence of emission intensity. The enhanced ferroelectricity with a high remnant polarization (Pr) value of about 32 μC/cm 2 is attributed to the weakened domain pinning due to the reduced oxygen vacancies by W 6+ substitution for Ti 4+ ions. The thin films combining the strong blue up-conversion photoluminescence with the good ferroelectric properties would provide possibility of realizing novel multifunctional optoelectronic integration device applications.

W-doping induced antiferroelectric to ferroelectric phase transition in PbZrO3 thin films prepared by chemical solution deposition

We reported on W-doping induced antiferroelectric to ferroelectric phase transition in PbZrO3 (PZO) thin films prepared on Pt/Ti/SiO2/Si substrates by a chemical solution deposition method. The phase transition has been studied through polarization-electric field hysteresis loop, capacitance-voltage characteristic, and Raman scattering measurements. Suitable amount W-doping increased the saturated polarization of antiferroelectric W-doped PZO thin films, whereas the ferroelectric W-doped PZO thin films exhibited higher dielectric constant with a high dielectric-bias voltage tunability of about 70%. With increasing W-doping content, the orientation of the thin films changed from preferred (111)Cubic to complete (100)Cubic, due to W-doping-induced lattice distortion, meanwhile the Curie temperature dropped, and dielectric maximum broadened. Our study demonstrates that W-doping is an effective way to tailor the electrical properties of PZO thin films through the induced antiferroelectric-ferroelectric phase ...