Zhang Wei-feng

A Win-Win Effect for Both the Ferromagnetism and the Dopability of p-Type Doping in ZnO:(Cu+N)

We investigate the electronic structures and magnetic properties of ZnO doped with N, Cu, and (Cu+N) by using a first principles method and considering the strong correlation effect. It is interesting to compare these three systems. ZnO:N has weak p-type doping and unstable ferromagnetism, while ZnO:Cu becomes insulating due to the Jahn—Teller effect. Cu and N codoping can not only greatly improve the dopability of p-type doping accompanying the Jahn—Teller fading, but also enhance the ferromagnetism at the same time. The calculation results indicate that there is a win-win effect between N and Cu dopants in the ZnO:(Cu+N) system, which could possibly find applications in spintronics besides optoelectronics.

Optical study of Ba(MnxTi(1−x)O3) thin films by spectroscopic ellipsometry

Stoichiometric Ba(MnxTi(1−x)O3) (BMT) thin films with various values of x were deposited on Si(111) substrates by the sol-gel technique. The influence of Mn content on the optical properties was studied by spectroscopic ellipsometry (SE) in the UV—Vis—NIR region. By fitting the measured ellipsometric parameter (Ψ and Δ) with a four-phase model (air/BMT+voids /BMT/Si(111)), the key optical constants of the thin films have been obtained. It was found that the refractive index n and the extinction coefficient k increase with increasing Mn content due to the increase in the packing density. Furthermore, a strong dependence of the optical band gap Eg on Mn/Ti ratios in the deposited films was observed, and it was inferred that the energy level of conduction bands decreases with increasing Mn content.

Homogeneous interface-type resistance switching in Au/La0.67Ca0.33MnO3/SrTiO3/F:SnO2 heterojunction memories

La0.67Ca0.33MnO3 thin films are fabricated on fluorine-doped tin oxide conducting glass substrates by a pulsed laser deposition technique with SrTiO3 used as a buffer layer. The current-voltage characteristics of the heterojunctions exhibit an asymmetric and resistance switching behaviour. A homogeneous interface-type conduction mechanism is also reported using impedance spectroscopy. The spatial homogeneity of the charge carrier distribution leads to field-induced potential-barrier change at the Au-La0.67Ca0.33MnO3 interface and a concomitant resistance switching effect. The ratio of the high resistance state to the low resistance state is found to be as high as 1.3 × 104% by simulating the AC electric field. This colossal resistance switching effect will greatly improve the signal-to-noise ratio in nonvolatile memory applications.

Temperature-Dependent Raman Spectrum of Hexagonal YMnO3 Films Synthesized by Chemical Solution Method

We study the temperature-dependent Raman spectrum of hexagonal YMnO3 films prepared by a chemical solution method. There are seven Raman peaks (3A1 + E1 + 4E2) of the film identified at room temperature. From the results of temperature dependence of the Raman spectrum, it is deduced that the YMnO3 film has a magnetic phase transition temperature of about 123 K. The temperature variation phonon mode at 685 cm?1 shows an anomalous frequency variation near 123 K, suggesting either a more complex mechanism of spin-phonon coupling or strong mixing of phonon modes. The reason for the higher antiferromagnetic N?el temperature TN of the film than that of the bulk counterpart is also discussed.

Study of Cr6+ in Water by Surface Enhanced Raman Scattering

Cr6+ in K2Cr2O7, (NH4)2Cr2O7, and K2CrO4 aqueous solution showed the same new strong band of 794 cm-1 in surface enhanced Raman scattering (SERS) by using SERS silver substrates. This new feature could be assigned to a surface complex of silver and Cr6+, for instance Ag2CrO4. The feature of 794 cm-1 could be a fingerprint band for detecting pollutant Cr6+ in water from SERS by using the SERS active substrate of silver.

Bipolar resistance switching in the fully transparent BaSnO3-based memory device

The fully transparent indium—tin-oxide/BaSnO3/F-doped SnO2 devices that show a stable bipolar resistance switching effect are successfully fabricated. In addition to the transmittance being above 87% for visible light, an initial forming process is unnecessary for the production of transparent memory. Fittings to the current—voltage curves reveal the interfacial conduction in the devices. The first-principles calculation indicates that the oxygen vacancies in cubic BaSnO3 will form the defective energy level below the bottom of conduction band. The field-induced resistance change can be explained based on the change of the interfacial Schottky barrier, due to the migration of oxygen vacancies in the vicinity of the interface. This work presents a candidate material BaSnO3 for the application of resistive random access memory to transparent electronics.

Interface-related switching behaviors of amorphous Pr0.67Sr0.33MnO3-based memory cells

The resistive switching properties in amorphous Pr0.67Sr0.33MnO3 films deposited by pulsed laser deposition are investigated. Reproducible and bipolar counter-8-shape and 8-shape switching behaviours of Au/Pr0.67Sr0.33MnO3/F:SnO2 junctions are obtained at room temperature. Dramatically, the coexistence of two switching polarities could be reversibly adjusted by an applied voltage range. The results allocated those two switching types to areas of different defect densities beneath the same electrode. The migration of oxygen vacancies and the trapping effect of electrons under an applied electric field play an important role. An interface-effect-related resistance switching is proposed in an amorphous Pr0.67Sr0.33MnO3-based memory cell.