Chunli Liu

Occurrence of both unipolar memory and threshold resistance switching in a NiO film.

We observed two types of reversible resistance switching (RS) effects in a NiO film: memory RS at low temperature and threshold RS at high temperature. We were able to control the type of RS effects by thermal cycling. These phenomena were explained using a new dynamic percolation model that can describe the rupture and formation of conducting filaments. We showed that the RS effects are governed by the thermal stability of the filaments, which arise from competition between Joule heating and thermal dissipation. This work provides us understandings on basic mechanism of the RS effects and their interrelation.

Effects of heat dissipation on unipolar resistance switching in Pt/NiO/Pt capacitors

We fabricated Pt∕NiO∕Pt capacitor structures with various bottom electrode thicknesses tBE and investigated their resistance switching behaviors. The capacitors with tBE⩾50nm exhibited typical unipolar resistance memory switching, while those with tBE⩽30nm showed threshold switching. This interesting phenomenon can be explained in terms of the temperature-dependent stability of conducting filaments. In particular, the thinner tBE makes dissipation of Joule heat less efficient, so the filaments will be at a higher temperature and become less stable. This study demonstrates the importance of heat dissipation in resistance random access memory.

Hydrothermally grown ZnO nanostructures on few-layer graphene sheets

This study describes the hydrothermal growth of ZnO nanostructures on few-layer graphene sheets and their optical and structural properties. The ZnO nanostructures were grown on graphene sheets of a few layers thick (few-layer graphene) without a seed layer. By changing the hydrothermal growth parameters, including temperature, reagent concentration and pH value of the solution, we readily controlled the dimensions, density and morphology of the ZnO nanostructures. More importantly, single-crystalline ZnO nanostructures grew directly on graphene, as determined by transmission electron microscopy. In addition, from the photoluminescence and cathodoluminescence spectra, strong near-band-edge emission was observed without any deep-level emission, indicating that the ZnO nanostructures grown on few-layer graphene were of high optical quality.

The evolution of conducting filaments in forming-free resistive switching Pt/TaOx/Pt structures

The forming-free (FF) and forming-required (FR) resistive switching behaviors of Pt/TaOx/Pt structures were analyzed. Changes in the length and shape of the conducting filaments (CFs) suggested different evolution processes in the FF and FR cells when the CFs grew from weaker filaments to stronger ones. In the FR cells, the filament changed from a conical shape to a cylindrical shape with no change in length. In the FF cells, the filament maintained a cylindrical shape while increasing its diameter and decreasing its length.

Tunable blue-green-emitting wurtzite ZnS:Mg nanosheet-assembled hierarchical spheres for near-UV white LEDs

Mg-doped ZnS hierarchical spheres have been synthesized via hydrothermal method using mixed solvents of ethylenediamine and DI water without any surface-active agent. The surface morphology and microstructure studies revealed that the hierarchical spheres were consisted of many well-aligned nanosheets with width 10 nm and length about 50 ~ 100 nm. X-Ray diffraction results show that the ZnS:Mg hierarchical spheres have wurtzite structure with high crystallinity. The absorption edge in the diffuse reflection spectra shifts towards lower wavelength with increasing Mg concentration, indicating an expansion in the bandgap energy that is estimated to be in the range of 3.28 to 3.47 eV. Blue-green photoluminescence with tunable intensity and peak position was observed depending on the Mg content. The Mg2+-activated ZnS phosphor can be good candidates for blue-green components in near-UV white light-emitting diodes.

Initial growth behavior and resulting microstructural properties of heteroepitaxial ZnO thin films on sapphire (0001) substrates

The initial growth behavior and resulting microstructural properties of heteroepitaxial ZnO thin films prepared by pulsed laser deposition on sapphire (0001) substrates were investigated. High-resolution x-ray diffraction and transmission electron microscopy studies revealed that the initial growth behavior and the microstructure of the films were significantly dependent on the growth parameters employed. ZnO films grown at 700°C with an O2 partial pressure of 20mTorr initiated in a columnar growth mode and contained two types of domains. These domains were in plane orientated either ZnO[112¯0]‖Al2O3[101¯0] or ZnO[101¯0]‖Al2O3[101¯0] and were surrounded by highly defective domain boundaries with threading dislocations. ZnO films grown at 800°C with 1mTorr O2 showed a two-dimensional layered growth with only one in-plane epitaxial relationship, ZnO[112¯0]‖Al2O3[101¯0]. Most of the defects in the layered grown films were basal plane stacking faults near the interface between the ZnO and the substrate. The m...

Magnetic core-shell ZnFe2O4/ZnS nanocomposites for photocatalytic application under visible light.

Magnetic core-shell ZnFe2O4/ZnS composites were synthesized through a two-step chemical process including the hydrothermal and the co-precipitation methods. The structural characterization revealed that the composites consisted of a layer of ZnS clusters on the surface of ZnFe2O4 nanoparticles. The band gap energy of the composite was estimated to be 2.2eV through the Kubelka-Munk plot, implying the possible application as a photocatalyst under the visible light radiation. The improved photocatalytic efficiency of the ZnFe2O4/ZnS composites was confirmed through the photocatalytic degradation of Methyl Orange. The increased absorption of the visible light and the enhanced separation of the electron-hole pairs due to the relative energy band positions in ZnFe2O4 and ZnS are considered as the main advantages. Additionally, the moderate magnetization of the ZnFe2O4 core insured the easy magnetic collection of the composite materials without affecting the photocatalytic performance. Our results showed that ZnFe2O4-based nanocomposites could be used as an effective and magnetic retrievable photocatalyst.

Abnormal resistance switching behaviours of NiO thin films: possible occurrence of both formation and rupturing of conducting channels

We report a detailed study on the abnormal resistance switching behaviours observed in NiO thin films which show unipolar resistance switching phenomena. During the RESET process, in which the NiO film changed from a low resistance state to a high resistance state, we sometimes observed that the resistance became smaller than the initial value. We simulated the resistance switching by using a random circuit breaker network model. We found that local conducting channels could be formed as well as ruptured during the RESET process, which result in the occurrence of such abnormal switching behaviours.

Epitaxial Brownmillerite Oxide Thin Films for Reliable Switching Memory.

Resistive switching memory, which is mostly based on polycrystalline thin films, suffers from wide distributions in switching parameters-including set voltage, reset voltage, and resistance-in their low- and high-resistance states. One of the most commonly used methods to overcome this limitation is to introduce inhomogeneity. By contrast, in this paper, we obtained uniform resistive switching parameters and sufficiently low forming voltage by maximizing the uniformity of an epitaxial thin film. To achieve this result, we deposited an SrFeOx/SrRuO3 heteroepitaxial structure onto an SrTiO3 (001) substrate by pulsed laser deposition, and then we deposited an Au top electrode by electron-beam evaporation. This device exhibited excellent bipolar resistance switching characteristics, including a high on/off ratio, narrow distribution of key switching parameters, and long data retention time. We interpret these phenomena in terms of a local, reversible phase transformation in the SrFeOx film between brownmillerite and perovskite structures. Using the brownmillerite structure and atomically uniform thickness of the heteroepitaxial SrFeOx thin film, we overcame two major hurdles in the development of resistive random-access memory devices: high forming voltage and broad distributions of switching parameters.

One-pot solvothermal synthesis of magnetic SnFe2O4 nanoparticles and their performance in the photocatalytic degradation of chlortetracycline with visible light radiation

Highly crystalline SnFe2O4 nanoparticles with high saturation magnetization and superior chlortetracycline (CTC) degradation efficiency was developed using a one-pot solvothermal method. The SnFe2O4 nanoparticles about 50 nm were prepared with a simple and cost-effective process performed at 200 °C. Superparamagnetism was observed from the SnFe2O4 nanoparticles with a high saturation magnetization of 74.3 emu g−1. The excellent photocatalytic activity of the SnFe2O4 nanoparticles was demonstrated through the high degradation efficiency of CTC under a visible light/SnFe2O4/9 mM H2O2 process. The effective degradation of CTC with the SnFe2O4 nanoparticles was attributed to the effective absorption of the visible light and the good separation ability of the electron–hole pairs. The synergy effect between SnFe2O4 and H2O2 was analysed, and the optimum initial concentration of H2O2 was determined to be 9 mM to achieve the best photocatalytic result on CTC. The SnFe2O4 nanoparticles also exhibited a fast and easy magnetic retrieval and a stable performance with continuous recycled usage.