Liuwan Zhang

Surface-layer effect in CaCu3Ti4O12

Low-frequency (∼100Hz) dielectric properties of CaCu3Ti4O12 were investigated in detail near room temperature. A dielectric relaxation peak was observed at about 250 K with an activation energy of 0.54 eV. This peak can be tuned by annealing treatment in nitrogen and oxygen. The results can be well explained by considering the surface-layer effect. The inhomogeneous distribution of oxygen vacancies leads to the formation of surface layer, resulting in the observed Maxwell–Wagner-type dielectric relaxation.

Dielectric properties of TbMnO3 ceramics

The complex dielectric properties for ceramic samples of TbMnO3 were investigated as functions of temperature (100K⩽T⩽360K) and frequency (100Hz⩽f⩽100kHz). Two thermally activated dielectric relaxations were found with the activation energies of 0.30 and 0.22eV for the high- and low-temperature relaxations, respectively. By means of complex impedance analysis the high-temperature relaxation was identified to originate from the internal barrier-layer capacitor effects related to the grain boundaries, and the low-temperature relaxation was ascribed to the dipolar effects induced by charge-carrier-hopping motions inside the grains.

Polaron relaxation related to localized charge carriers in CaCu3Ti4O12

The dielectric properties of CaCu3Ti4O12 were studied in the frequency range from 100Hzto100kHz at temperatures ranging from 23K to room temperature. The properties can be perfectly described by the so-called universal dielectric response, indicating that they are intimately related to the hopping conductivity caused by localized carriers. A deviation from the Arrhenius behavior in the lower frequency (temperature) region has been attributed to Anderson localization. These results strongly indicate that the relaxation in CaCu3Ti4O12 might be a dipolar-type relaxation associated with the hopping localized carriers.

Properties and mechanism study of Ag doped SnO2 thin films as H2S sensors

Abstract Ag doped tin oxide thin films were prepared by evaporating the mixture of SnO2 and Ag powder. The granular film with a high Ag incorporation (16 at.%) was found to possess excellent sensitivity, selectivity and rapid response to the presence of H2S gas at low temperature. X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis were carried out to investigate the sensing mechanism. The film was composed of tin oxide with oxygen deficiency (Sn3O4) and Ag2O crystals. Direct evidence of H2S adsorption on the Ag2O grain surface was observed.

Fabrication of large area hexagonal boron nitride thin films for bendable capacitors

AbstractHighly reliable and bendable dielectrics are desired in flexible or bendable electronic devices for future applications. Hexagonal boron nitride (h-BN) can be used as bendable dielectric due to its wide band gap. Here, we fabricate high quality h-BN films with controllable thickness by a low pressure chemical vapor deposition method. We demonstrate a parallel-plate capacitor using h-BN film as the dielectric. The h-BN capacitors are reliable with a high breakdown field strength of ∼9.0 MV/cm. Tunneling current across the h-BN film is inversely exponential to the thickness of dielectric, which makes the capacitance drop significantly. The h-BN capacitor shows a best specific capacitance of 6.8 μF/cm2, which is one order of magnitude higher than the calculated value.

Strain-assisted tunneling current through TbMnO3∕Nb-1 wt %-doped SrTiO3 p–n junctions

Simple oxide heterostructures have been fabricated by growing multiferroic TbMnO3 thin film on Nb-1 wt %-doped SrTiO3 substrate. In addition to the beneficial rectifying characteristics in a temperature range from 350 to 30 K, the intriguing observation is that at constant reverse bias the current increases with decreasing temperature below 275 K. By analyzing the band diagram, the anomalous increasing current with decreasing temperature was ascribed to the strain-assisted tunneling current through TbMnO3∕Nb-doped p–n junctions.

Investigation of the resistance switching in Au/SrTiO3:Nb heterojunctions

The electric properties of as-prepared and annealed Au/SrTiO3:Nb heterojunctions are comparatively investigated. The former junction has hysteretic current-voltage and apparent capacitance-voltage characteristics, showing significant resistance switching effects. The latter behaves like a conventional semiconductor Schottky contact with negligible resistance switching phenomena. Based on the equivalent circuit analysis, we demonstrated that the hysteresis of the measured apparent capacitance-voltage relation of the as-prepared junction is caused by the leakage resistance. Its real junction capacitance is not hysteretic and independent on the resistance states, following the ideal Schottky model. Our results suggest that during the resistance switching, the Schottky barrier profile is kept unchanged, and the conductive filaments play a vital role.

Dielectric behaviour of cobalt titanium oxide

The complex dielectric properties of CoTiO3 ceramics were investigated as functions of temperature (130 K ≤ T ≤ 430 K) and frequency (100 Hz ≤ f ≤ 100 kHz). Two thermally activated dielectric relaxations were observed. TiO2 addition and annealing treatments were used to modify the behaviour of these relaxations. It was found that the low-temperature relaxation can be greatly enhanced by the addition of TiO2. Annealing in N2 at 800 °C for 2 h eliminates the low-temperature relaxation but largely strengthens the high-temperature relaxation, whereas annealing in O2 at 800 °C for 2 h causes the opposite effects. It was suggested that the low-temperature relaxation can be related to the dipolar effect induced by charge-carrier hopping motions while the high-temperature relaxation might be ascribed to the defect dipolar polarization.

Anomalous thermal hysteresis in dielectric permittivity of CaCu3Ti4O12

We herein report an anomalous thermal hysteresis in dielectric permittivity in CaCu3Ti4O12. The anomalous behavior was well explained in terms of the low-temperature Maxwell–Wagner relaxation induced by frozen carriers. A multirelaxation mechanism, i.e., the coupling of the dipole relaxation to the frozen carrier-induced and blocked carrier-induced Maxwell–Wagner relaxations in the low-temperature and high-temperature regions, respectively, is proposed to be the origin of the colossal dielectric constant.

Fabrication and transport properties of ZnO∕Nb-1wt%-doped SrTiO3 epitaxial heterojunctions

(110) ZnO/(001) Nb-1 wt %-doped SrTiO3 n-n type heteroepitaxial junctions were fabricated using the pulse laser deposition method. A diodelike current behavior was observed. Different from conventional p-n junctions or Schottky diodes, the diffusion voltage was found to increase with temperature. At all temperatures, the forward current was perfectly fitted on the thermionic emission model. The band bending at the interface can qualitatively explain our results, and the extracted high ideality factor at low temperatures, as well as large saturation currents, is ascribed to the deep-level-assisted tunneling current through the junction