Lan-Ting Shi

Discovering inch-diameter metallic glasses in three-dimensional composition space

We have discovered low-cost bulk metallic glasses (BMGs) with diameters as large as 16 to 25 mm based on the ordinary engineering metal Mg. We also use this example to present a new strategy, to illustrate how to navigate in three-dimensional composition space for locating the best glass-forming compositions. The extraordinary glass-forming ability demonstrated by the inch-sized BMGs has implications for future development and applications of amorphous metals

Improvement of reproducible resistance switching in polycrystalline tungsten oxide films by in situ oxygen annealing

The electric-field-induced resistance switching in polycrystalline tungsten oxide films was investigated. Compared with the as-deposited film, the film annealed in an oxygen atmosphere shows a more stable switching behavior, a higher low-to-high resistance ratio, and a better endurance and retention. Based on the x-ray photoemission spectroscopy analysis, the resistance switching was attributed to the change in the interfacial barrier potential, due to the electron trapping/detrapping in the surface states, and the switching improvement was attributed to the decrease in the surface density of states. The present work suggests a possible approach controlling the resistance switching property by surface modification.

Photoresponse of the Schottky junction Au/SrTiO3:Nb in different resistive states

A systematic study on photovoltaic effects has been performed for the Schottky junction Au/SrTiO3:0.05 wt %Nb, the resistance of which can be tuned, by applied electric pulses, between ∼1 and ∼200 MΩ. It is found that, despite the great change in junction resistance, the photocurrent across the junction is constant when the power and wavelength of incident light are fixed. The corresponding Schottky barrier, deduced from the photoresponse data is ∼1.5 eV, independent of junction resistance. This result suggests the invariance of the interfacial barrier during resistance switching and the occurrence of filamentary conduction channels.

Electronic transport and colossal electroresistance in SrTiO3:Nb-based Schottky junctions

Current-voltage characteristics and colossal electroresistance (CER) have been experimentally investigated in the temperature range from 293 to 454 K for the Schottky junctions Au/SrTiO3:0.5 wt % Nb and Au/SrTiO3:0.05 wt % Nb. Both junctions show electron tunneling-dominated transport behavior. Postannealing of SrTiO3:0.05 wt % Nb in oxygen atmosphere causes a transition of the transport behavior from electron tunneling to thermionic emission. The CER effect appears in the junctions with the transport behavior dominated by electron tunneling and greatly weakens when thermionic emission prevails after postannealing. This result reveals the presence of a close relation between CER and electron tunneling.

Roles of silver oxide in the bipolar resistance switching devices with silver electrode

Three devices, Ag/WO3−x/Pt, Ag/AgOx/Pt, and Ag/AgOx/WO3−x/Pt, were investigated to elucidate the influence of the silver oxide on the bipolar resistive switching behavior. The silver oxide films were obtained by depositing silver at oxygen atmosphere. We find that the resistive switching behavior was determined by the silver oxide layer. Bulk and interface resistive switching were observed in the Ag/AgOx/Pt and Ag/AgOx/WO3−x/Pt devices, respectively. By the micro-x-ray photoemission spectroscopy analysis, it was demonstrated that the electrochemical redox reaction occurred in the AgOx layer is responsible for the resistive switching behavior at silver/oxide interface.

Improved resistance switching in ZnO-based devices decorated with Ag nanoparticles

ZnO is especially attractive among the materials showing resistance switching because of its excellent properties such as light emitting and transparency for visible light. Unfortunately, the resistance switching in a ZnO-based device is usually unstable. By dispersing Ag particles of size ~20 nm at the electrode–ZnO interface, we significantly improved the resistance uniformity, set/reset repeatability of Ag–ZnO–Pt devices. Conducting atomic force microscope analysis revealed the appearance of micro-regions where resistance switching, with an improved stability, is more easily triggered. It is suggested that Ag particles act as seeds for conducting filaments, leading to depressed randomness and reduced diameter of the conducting paths.

Resistance dependence of photovoltaic effect in Au/SrTiO3:Nb(0.5 wt %) Schottky junctions

Photoresponse in the Au/SrTiO3:0.5 wt % Nb Schottky junction with an electric field–tunable resistance between ∼70 kΩ and ∼900 MΩ has been experimentally studied. The most remarkable observation is the strong dependence of the open-circuit photovoltage on junction resistance and the invariance of the short-circuit photocurrent during resistance switching. These results, combined with a theoretical calculation based on the equivalent circuit model consisting of a diode in parallel with a resistor, suggest the occurrence of filamentary conductive channels across the interface of the junction under the impact of electric pulses, whereas the remaining Schottky barrier keeps completely unchanged.

Threshold optimization of polymeric opal photonic crystal cavity as organic solid-state dye-doped laser

The authors have demonstrated the optimization of the active layer thickness on the threshold of organic solid-state dye-doped laser (SSDL), which was fabricated by three-dimensional polymeric opal photonic crystal and tert-butyl Rhodamine B (t-Bu-RhB) doped polymer film. Gain media were produced by incorporating t-Bu-RhB into polymer film at 3.0 wt %. The sandwiched laser resonator cavities with different active layer thickness displayed single-mode lasing oscillations in the reflection band gap of the resonator structure. The lasing threshold as low as 1.13 μJ/pulse was achieved. The optimization of active layer would provide an opportunity to accelerate the development of low threshold polymeric SSDLs.

Visualization of the conductive channel in a planar resistance switching device based on electrochromic materials

In this work, bipolar resistance switching behavior was realized in an Au/tungsten oxide/Au planar device, and the evolution of the conductive channel during resistance switching was successfully visualized by the in situ optical image technique based on the color-conductivity dependence of tungsten oxide. We found that there are two types of conductive channel, named parabolic channel and bar-like channel, exist in the planar device. The parabolic channel formed firstly near the cathode and then extended to but could not touch the anode. By applying opposite electric-field, the bar-like channel formed from the cathode (i.e., foregoing anode) and extended to the parabolic channel. With alternating the external electric-field polarity, the bar-like channel showed an indirect connection and nonmonotonic disconnection with the parabolic channel at the region near the foregoing anode, corresponding to the high-to-low and low-to-high resistance switching processes of the planar device, respectively. The instable RS behavior was caused by the change of bar-like channel occurring position under the high external field condition. The conductive channel formation was ascribed to the sodium ion immersion from the soda-lime glass substrate into the tungsten oxide film and then migration driven by the electric field to form sodium tungsten bronze. These results will give some insight into the resistance switching property improvement and mechanism elucidation as well as a possibility to develop electric/optical-coupled switch and data storage devices

Electric field-induced resistance switching in (Bi2O3)0.7(Y2O3)0.3 films

Single-phase (Bi2O3)0.7(Y2O3)0.3 (BYO) films are obtained by pulse laser deposition under the temperatures from 300 to 500 °C and the oxygen pressure of 90 Pa. Electric field-induced resistance switching is observed in the Ag/BYO/Pt structure. Postannealing in 750 °C improves the repeatability of the resistance switching and narrows the distribution of the set and reset voltages triggering the resistance switching. A linear variation of reset current with resistance is obtained for the Ag/BYO/Pt systems, either as-prepared or postannealed, despite the great dispersion of these two quantities. A reduction in the activation energy, from ∼0.63 to ∼0.33 eV, for the migration of oxygen vacancies is observed after the forming process. Possible mechanisms are discussed.