Glancing angle deposition of large-scale helical Si@Cu3Si nanorod arrays for high-performance anodes in rechargeable Li-ion batteries.

Silicon (Si) anode materials have attracted substantial interest due to their high theoretical capacity. Here, the growth of helical Si@Cu3Si nanorod arrays via glancing angle deposition (GLAD) followed by an annealing process is reported. Pre-deposited Cu atoms were driven into Si-nanorods and successfully reacted with Si to form a Si-Cu alloy at a high temperature. By varying the rotation rate and annealing temperature, the resultant Si@Cu3Si nanorod arrays showed a reasonably accessible surface area with precise control spacing behavior in favor of accommodating Si volume expansion. Meanwhile, the Si@Cu3Si anode materials showed higher electrical conductivity, facilitating Li+ ion diffusion and electron transfer. The Si@Cu3Si nanorod arrays in half cells exhibited a volumetric capacity as high as 3350.1 mA h cm-3 at a rate of 0.25 C and could maintain 1706.7 mA h cm-3 after 100 cycles, which are superior to those of pristine Si materials. This facile and innovative technology provided new insights into the development of Si-based electrode materials.

Self-Selecting Resistive Switching Scheme Using TiO2 Nanorod Arrays.

In this study, the resistive switching scheme using TiO2 nanorod arrays synthesized by a large-scale and low-cost hydrothermal process was reported. Especially, the nonlinear I-V characteristics of TiO2 nanorod arrays with a nonlinearity of up to ~10, which suppress the leakage current less than 10-4 Acm-2, were demonstrated, exhibiting a self-selecting resistive switching behavior. It provides a simple pathway for integration of RRAM crossbar arrays without additional stacking of active devices. The mechanisms of the nonlinear resistive switching behaviors were discussed in detail. In addition, the maximum array numbers of 79 for self-selecting RRAM cells were estimated. The results demonstrate an opportunity of using the concept of self-selecting resistive switching characteristics in a single material, which offers a new strategy to tackle the sneak path issue of RRAM in the crossbar arrays structure.

Resistive switching of Sn-doped In2O3/HfO2 core-shell nanowire: geometry architecture engineering for nonvolatile memory.

Core-shell NWs offer an innovative approach to achieve nanoscale metal-insulator-metal (MIM) heterostructures along the wire radial direction, realizing three-dimensional geometry architecture rather than planar type thin film devices. This work demonstrated the tunable resistive switching characteristics of ITO/HfO2 core-shell nanowires with controllable shell thicknesses by the atomic layer deposition (ALD) process for the first time. Compared to planar HfO2 thin film device configuration, ITO/HfO2 core-shell nanowire shows a prominent resistive memory behavior, including lower power consumption with a smaller SET voltage of ∼0.6 V and better switching voltage uniformity with variations (standard deviation(σ)/mean value (µ)) of VSET and VRESET from 0.38 to 0.14 and from 0.33 to 0.05 for ITO/HfO2 core-shell nanowire and planar HfO2 thin film, respectively. In addition, endurance over 103 cycles resulting from the local electric field enhancement can be achieved, which is attributed to geometry architecture engineering. The concept of geometry architecture engineering provides a promising strategy to modify the electric-field distribution for solving the non-uniformity issue of future RRAM.

Tunable multilevel storage of complementary resistive switching on single-step formation of ZnO/ZnWO(x) bilayer structure via interfacial engineering.

Tunable multilevel storage of complementary resistive switching (CRS) on single-step formation of ZnO/ZnWOx bilayer structure via interfacial engineering was demonstrated for the first time. In addition, the performance of the ZnO/ZnWOx-based CRS device with the voltage- and current-sweep modes was demonstrated and investigated in detail. The resistance switching behaviors of the ZnO/ZnWOx bilayer ReRAM with adjustable RESET-stop voltages was explained using an electrochemical redox reaction model whose electron-hopping activation energies of 28, 40, and 133 meV can be obtained from Arrhenius equation at RESET-stop voltages of 1.0, 1.3, and 1.5 V, respectively. In the case of the voltage-sweep operation on the ZnO-based CRS device, the maximum array numbers (N) of 9, 15, and 31 at RESET-stop voltages of 1.4, 1.5, and 1.6 V were estimated, while the maximum array numbers increase into 47, 63, and 105 at RESET-stop voltages of 2.0, 2.2, and 2.4 V, operated by the current-sweep mode, respectively. In addition, the endurance tests show a very stable multilevel operation at each RESET-stop voltage under the current-sweep mode.

Single CuO(x) nanowire memristor: forming-free resistive switching behavior.

CuOx nanowires were synthesized by a low-cost and large-scale electrochemical process with AAO membranes at room temperature and its resistive switching has been demonstrated. The switching characteristic exhibits forming-free and low electric-field switching operation due to coexistence of significant amount of defects and Cu nanocrystals in the partially oxidized nanowires. The detailed resistive switching characteristics of CuOx nanowire systems have been investigated and possible switching mechanisms are systematically proposed based on the microstructural and chemical analysis via transmission electron microscopy.

Single-step formation of ZnO/ZnWO(x) bilayer structure via interfacial engineering for high performance and low energy consumption resistive memory with controllable high resistance states.

A spontaneously formed ZnO/ZnWOx bilayer resistive memory via an interfacial engineering by one-step sputtering process with controllable high resistance states was demonstrated. The detailed formation mechanism and microstructure of the ZnWOx layer was explored by X-ray photoemission spectroscopy (XPS) and transmission electron microscope in detail. The reduced trapping depths from 0.46 to 0.29 eV were found after formation of ZnWOx layer, resulting in an asymmetric I-V behavior. In particular, the reduction of compliance current significantly reduces the switching current to reach the stable operation of device, enabling less energy consumption. Furthermore, we demonstrated an excellent performance of the complementary resistive switching (CRS) based on the ZnO/ZnWOx bilayer structure with DC endurance >200 cycles for a possible application in three-dimensional multilayer stacking.

Manipulated transformation of filamentary and homogeneous resistive switching on ZnO thin film memristor with controllable multistate.

A bias polarity-manipulated transformation from filamentary to homogeneous resistive switching was demonstrated on a Pt/ZnO thin film/Pt device. Two types of switching behaviors, exhibiting different resistive switching characteristics and memory performances were investigated in detail. The detailed transformation mechanisms are systematically proposed. By controlling different compliance currents and RESET-stop voltages, controllable multistate resistances in low resistance states and a high resistance states in the ZnO thin film metal-insulator-metal structure under the homogeneous resistive switching were demonstrated. We believe that findings would open up opportunities to explore the resistive switching mechanisms and performance memristor with multistate storage.

Improved performance of ZnO-based resistive memory by internal diffusion of Ag atoms.

An Ag/ZnO/Pt memory device, which has much better resistive switching behaviour than Pt/ZnO/Pt device was demonstrated. The detailed resistive mechanisms for the Pt/ZnO/Pt and the Ag/ZnO/Pt systems are proposed and investigated. Microstructures are observed by transmission electron microscope (TEM), indicating that the formation of conducting path for both systems is different. For the Pt/ZnO/Pt device, the conductive filament path is constructed by the oxygen vacancies from top to bottom electrodes under a larger enough bias at a forming process. For the Ag/ZnO/Pt device, the filament path was grown by oxygen vacancies combined with an internal diffusion of Ag atoms under a large bias and can provide the lowest energy barrier for electrons transported between two electrodes during set and reset processes, which reduces formation of other conducting paths after each switching. Accordingly, the stable switching performance of the Ag/ZnO/Pt device can be achieved over 100 cycles even the thickness of ZnO film <25 nm.

ZnO1-x nanorod arrays/ZnO thin film bilayer structure: from homojunction diode and high-performance memristor to complementary 1D1R application.

We present a ZnO(1-x) nanorod array (NR)/ZnO thin film (TF) bilayer structure synthesized at a low temperature, exhibiting a uniquely rectifying characteristic as a homojunction diode and a resistive switching behavior as memory at different biases. The homojunction diode is due to asymmetric Schottky barriers at interfaces of the Pt/ZnO NRs and the ZnO TF/Pt, respectively. The ZnO(1-x) NRs/ZnO TF bilayer structure also shows an excellent resistive switching behavior, including a reduced operation power and enhanced performances resulting from supplements of confined oxygen vacancies by the ZnO(1-x) NRs for rupture and recovery of conducting filaments inside the ZnO TF layer. A hydrophobic behavior with a contact angle of ~125° can be found on the ZnO(1-x) NRs/ZnO TF bilayer structure, demonstrating a self-cleaning effect. Finally, a successful demonstration of complementary 1D1R configurations can be achieved by simply connecting two identical devices back to back in series, realizing the possibility of a low-temperature all-ZnO-based memory system.