Xinxin Chen

Mechanism for resistive switching in an oxide-based electrochemical metallization memory

A comparison of the asymmetric OFF-state current-voltage characteristics between Cu/ZnO/Pt and Cu/ZnO/Al-doped ZnO (AZO) electrochemical metallization memory (ECM) cells demonstrates that the Cu filament rupture and rejuvenation occur at the ZnO/Pt (or AZO) interface, i.e., the cathodic interface. Therefore, the filament is most likely to have a conical shape, with wider and narrower diameters formed at the anodic and cathodic interfaces, respectively. It is inferred that the filament growth starts at the anode surface and stops at the cathode surface. Our results indicate that oxide-based ECM cells strongly differ from sulfide- and selenide-based ones in the resistive switching mechanism. VC 2012 American Institute of Physics.

A Multilevel Memory Based on Proton-Doped Polyazomethine with an Excellent Uniformity in Resistive Switching

The uniformity of operating parameters in organic nonvolatile memory devices is very important to avoid false programming and error readout problems. In the present work, we fabricated an organic resistive-switching memory based on protonic-acid-doped polyazomethine (PA-TsOH), which demonstrates an excellent operative uniformity and multilevel storage capability. The deliberate tuning of the resistance states can be attributed to the electric-field-controlled molecular doping of the imine-containing polymers.

Nonvolatile bistable resistive switching in a new polyimide bearing 9-phenyl-9H-carbazole pendant

A new polyimide bearing the functional pendant 9-phenyl-9H-carbazole moieties, poly[2,2-(4,4′-di(N-benzyloxycarbazole)-3,3′-biphenylene)propane-hexafluoroisopropylidenediphthalimide] (6F-BAHP-PC PI), has been designed as a functional material for resistance memory devices. The ITO/6F-BAHP-PC PI/Ag memory device exhibits nonvolatile resistive switching (RS) with a high ON/OFF ratio (>106), long retention time (>6 × 104 s), good endurance, and low power consumption (∼100 μW). In situconductive atomic force microscopy measurements show that the RS of 6F-BAHP-PC PI originates from the formation/rupture of nanoscale conducting filaments.

Direct observation of lithium-ion transport under an electrical field in LixCoO2 nanograins.

The past decades have witnessed the development of many technologies based on nanoionics, especially lithium-ion batteries (LIBs). Now there is an urgent need for developing LIBs with good high-rate capability and high power. LIBs with nanostructured electrodes show great potentials for achieving such goals. However, the nature of Li-ion transport behaviors within the nanostructured electrodes is not well clarified yet. Here, Li-ion transport behaviors in LixCoO2 nanograins are investigated by employing conductive atomic force microscopy (C-AFM) technique to study the local Li-ion diffusion induced conductance change behaviors with a spatial resolution of ~10 nm. It is found that grain boundary has a low Li-ion diffusion energy barrier and provides a fast Li-ion diffusion pathway, which is also confirmed by our first principles calculation. This information provides important guidelines for designing high performance LIBs from a point view of optimizing the electrode material microstructures and the development of nanoionics.

Nanoscale Magnetization Reversal Caused by Electric Field-Induced Ion Migration and Redistribution in Cobalt Ferrite Thin Films

Reversible nanoscale magnetization reversal controlled merely by electric fields is still challenging at the moment. In this report, first-principles calculation indicates that electric field-induced magnetization reversal can be achieved by the appearance of unidirectional magnetic anisotropy along the (110) direction in Fe-deficient cobalt ferrite (CoFe(2-x)O4, CFO), as a result of the migration and local redistribution of the Co(2+) ions adjacent to the B-site Fe vacancies. In good agreement with the theoretical model, we experimentally observed that in the CFO thin films the nanoscale magnetization can be reversibly and nonvolatilely reversed at room temperature via an electrical ion-manipulation approach, wherein the application of electric fields with appropriate polarity and amplitude can modulate the size of magnetic domains with different magnetizations up to 70%. With the low power consumption (subpicojoule) characteristics and the elimination of external magnetic field, the observed electric field-induced magnetization reversal can be used for the construction of energy-efficient spintronic devices, e.g., low-power electric-write and magnetic-read memories.

Electrically controlled electron transfer and resistance switching in reduced graphene oxide noncovalently functionalized with thionine

We demonstrate the electrically controlled electron transfer of thionine-functionalized reduced graphene oxide (rGO–th) in the form of a homogeneous solution and films. The electron transfer can be realized in a bidirectional way, which provides a method to control the electronic properties of graphene through π–π noncovalent functionalization. Based on the aforementioned controllable electron transfer between graphene sheets and thionine, resistance random access memories with a configuration of Pt/rGO–th/Pt were fabricated and show nonvolatile resistive switching with a large ON/OFF ratio of more than 104, fast switching speed of <5 ns, long retention time of over 105 s and excellent endurance. Furthermore, the reverse electron transfer between thionine and rGO as well as the resistive switching mechanism of the Pt/rGO–th/Pt devices were confirmed by density functional theory (DFT) calculation.

Role of oxadiazole moiety in different D–A polyazothines and related resistive switching properties

Two donor–acceptor (D–A) polyazothines (PAs), incorporating the oxadiazole entity either acting as an electron acceptor (A) to form D–A structured PA-1 with the triphenylamine donor (D), or acting as a donor to form D–A structured PA-2 with the 3,3′-dinitro-diphenylsulfone acceptor, have been successfully synthesized via a polycondensation reaction. The variation in the role of the oxadiazole moiety in the D–A polymers, together with the use of different top electrode metals, leads to interesting electronic transport properties and various resistive switching behaviors of the present polyazothines. Pt-electrode devices based on a PA-1 active layer show a rewritable memory effect with poor endurance (less than 20 cycles), whereas the PA-2 based Pt devices exhibit write-once read-many-times (WORM) memory behavior. For the Al-electrode devices, both PAs demonstrate a much improved resistive switching effect, and the endurance of the PA-2 devices is better than that of the PA-1 devices. The difference in the electronic transport and memory properties of the four devices may originate from the different charge injection/extraction and electron transfer processes of the sandwich systems, and will provide guidelines for selecting both the proper D and A moieties in D–A polymers and electrode metals for high-performance resistance random access memories (RRAMs).

Nucleation dynamics of nanostructural TiO2 films with controllable phases on (001) LaAlO3

Microstructure evolution and nucleation dynamics of TiO2 nanostructural thin films on (001) LaAlO3 substrates grown by the polymer-assisted deposition technique have been systematically studied with the increase of annealing temperature. Epitaxial anatase TiO2 phase with nanometer-scaled periodic surface strip patterns can be achieved when the sample is annealed at 900 ° C. It is also found that the morphology of the surface pattern is related to the ramping rate of the temperature during annealing. The formation of the surface strip pattern can be considered to be associated with the diffusion limit growth dynamics. The surface pattern structure was found to strongly affect the hydrophilic properties of the thin films.