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We demonstrate a flexible nonvolatile multilevel memory and artificial synaptic devices based on the Cu/P(VDF-TrFE)/Ni memtranstor which exhibits pronounced nonlinear magnetoelectric effects at room temperature. The states of the magnetoelectric voltage coefficient α

Spiking neural network, inspired by the human brain, consisting of spiking neurons and plastic synapses, is a promising solution for highly efficient data processing in neuromorphic computing. Recently, memristor-based neurons and synapses are becoming intriguing candidates to build spiking neural networks in hardware, owing to the close resemblance between their device dynamics and the biological counterparts. However, the functionalities of memristor-based neurons are currently very limited, and a hardware demonstration of fully memristor-based spiking neural networks supporting in-situ learning is very challenging. Here, a hybrid spiking neuron combining a memristor with simple digital circuits is designed and implemented in hardware to enhance neuron functions. The hybrid neuron with memristive dynamics not only realizes the basic leaky integrate-and-fire neuron function but also enables the in-situ tuning of the connected synaptic weights. Finally, a fully hardware spiking neural network with the hybrid neurons and memristive synapses is experimentally demonstrated for the first time, and in-situ Hebbian learning is achieved with this network. This work opens up a way towards the implementation of spiking neurons, supporting in-situ learning for future neuromorphic computing systems.

Surface electronic structures of the photoelectrodes determine the activity and efficiency of the photoelectrochemical water splitting, but the controls of their surface structures and interfacial chemical reactions remain challenging. Here, we use ferroelectric BiFeO3 as a model system to demonstrate an efficient and controllable water splitting reaction by large-area constructing the hydroxyls-bonded surface. The up-shift of band edge positions at this surface enables and enhances the interfacial holes and electrons transfer through the hydroxyl-active-sites, leading to simultaneously enhanced oxygen and hydrogen evolutions. Furthermore, printing of ferroelectric super-domains with microscale checkboard up/down electric fields separates the distribution of reduction/oxidation catalytic sites, enhancing the charge separation and giving rise to an order of magnitude increase of the photocurrent. This large-area printable ferroelectric surface and super-domains offer an alternative platform for controllable and high-efficient photocatalysis.

Two-dimensional (2D) materials and van der Waals heterostructures have attracted tremendous attention because of their appealing electronic, mechanical, and optoelectronic properties, which offer the possibility to extend the range of functionalities for diverse potential applications. Here, we fabricate a novel multiterminal device with dual-gate based on 2D material van der Waals heterostructures. Such a multiterminal device exhibited excellent nonvolatile multilevel resistance switching performance controlled by the source-drain voltage and back-gate voltage. Based on these features, heterosynaptic plasticity, in which the synaptic weight can be tuned by another modulatory interneuron, has been mimicked. A tunable analogue weight update (both on/off ratio and update nonlinearity) of synapse with high speed (50 ns) and low energy (∼7.3 fJ) programming has been achieved. These results demonstrate the great potential of the artificial synapse based on van der Waals heterostructures for neuromorphic computing.

Memristive switching in materials attracts intensive attention due to its potential application for nonvolatile memories. The environmental effect on the switching stability is of crucial importance to the fabrication and performance of a real memory devices. In this work, a solid state electrochemical cell with Cu/Si/Pt sandwich structure has been investigated. The cell shows a forming-free and gradual memristive switching behavior. The environmental atmosphere has significant effect on the switching behavior. We suggest that Cu electrode is oxidized by the atmosphere, forming a CuO x layer at the Cu/Si interface. The memristive switching can be attributed to the redox reaction between the CuO x and Si layers with an equilibrium of oxygen exchange between the cell and the environment. By pre-fabricating a CuO x layer during the cell preparation, the oxygen exchange with the environmental atmosphere is avoided and the switching degradation in vacuum condition is improved. These results provide a fundamental insight into improvement of memristive devices close to a real service condition.

An important potential application of solid state electrochemical reactions is in redox-based resistive switching memory devices. Based on the fundamental switching mechanisms, the memory has been classified into two modes, electrochemical metallization memory (ECM) and valence change memory (VCM). In this work, we have investigated a solid state electrochemical cell with a simple Ag/MoO3-x/fluorine-doped tin oxide (FTO) sandwich structure, which shows a normal ECM switching mode after an electroforming process. While in the lower voltage sweep range, the switching behavior changes to VCM-like mode with the opposite switching polarity to the ECM mode. By current-voltage measurements under different ambient atmospheres and X-ray photoemission spectroscopy analysis, electrochemical anodic passivation of the Ag electrode and valence change of molybdenum ions during resistance switching have been demonstrated. The crucial role of moisture adsorption in the switching mode transition has been clarified based on the Pourbaix diagram for the Ag-H2O system for the first time. These results provide a fundamental insight into the resistance switching mechanism model in solid state electrochemical cells.

MnSb2Se4 is a narrow band semiconductor having large Seebeck coefficients and intrinsically low thermal conductivities, but modest thermoelectric zT values due to having low carrier concentrations and high electrical resistivity. Here, we report that Cu substituted Mn1−xCuxSb2Se4 (0 ≤ x ≤ 0.35) materials display resonant doping behavior, leading to significantly enhanced power factors (PFs) and overall thermoelectric zT values in the measured temperature range. For the optimized composition Mn0.75Cu0.25Sb2Se4, the PF reaches 0.26 mW m−1 K−2 at 773 K, coupled with low thermal conductivities of 0.61 W K−1 m−1 to 0.32 W K−1 m−1 over the measured temperature range. A peak zT of 0.64 at 773 K is achieved, which is a 100% increase in comparison to undoped MnSb2Se4. Such a high zT has rarely been seen in thermoelectric materials with a low symmetry of crystallization, implying that Cu-doped MnSb2Se4 could be considered as a new platform in thermoelectric research for intermediate temperature power generation.

A type of synaptic devices have been constructed by employing the nonlinear magnetoelectric effects in multiferroic materials. The state of the magnetoelectric coefficient of the device can be tuned continuously with a large number of nonvolatile levels by engineering the applied electric-field pulses. Therefore, the magnetoeletric coefficient may be regarded as the synaptic weight, and the subsequent magnetoelectric voltage V ME acts as excitatory or inhibitory postsynaptic potentials. Synaptic behaviors including the long-term potentiation, long-term depression, and spiking-time-dependent plasticity have been demonstrated in a multiferroic heterostructure made of Metglas/PMN-PT/Metglas. The consumed energy density is estimated to be ~7.7 mJ cm−3 per spike, comparable to the biological synapse in the brain. Our work presents an alternative way towards artificial synaptic devices with very low energy consumption.

The magnetoelectric effects in multiferroics have a great potential in creating next-generation memory devices. We conceive a new concept of non-volatile memories based on a type of nonlinear magnetoelectric effects showing a butterfly-shaped hysteresis loop. The principle is to utilize the states of the magnetoelectric coefficient, instead of magnetization, electric polarization or resistance, to store binary information. Our experiments in a device made of the PMN-PT/Terfenol-D multiferroic heterostructure clearly demonstrate that the sign of the magnetoelectric coefficient can be repeatedly switched between positive and negative by applying electric fields, confirming the feasibility of this principle. This kind of non-volatile memory has outstanding practical virtues such as simple structure, easy operations in writing and reading, low power, fast speed, and diverse materials available., Comment: 10 pages, 4 figures

The electric-pulse–induced resistance switching of the Ag–La0.7Ca03MnO3(LCMO)–Pt heterostructures was studied. The multilevel resistance switching (MLRS), in which several resistance states can be obtained, was observed in the switching from high to low resistance state (HRS → LRS) by applying electric pulse with various pulse voltages. The threshold pulse voltages of MLRS are related to the initial resistance values as well as the switching directions. On the other hand, the resistance switching behavior from low to high resistance states (LRS → HRS) shows unobvious MLRS. According to the resistance switching behavior in serial and parallel modes, MLRS was explained by the parallel effect of multifilament forming/rupture in the Ag–LCMO interface layer. The present results suggest a possible application of Ag–LCMO–Pt heterostructures as multilevel memory devices.

The asymmetric fatigue behaviour of electric-pulse-induced resistance switching, that is, only switching from a low to a high resistance state, exhibits obvious fatigue behaviour and has been investigated in Ag–La0.7Ca0.3MnO3–Pt heterostructures. This asymmetric fatigue is strongly dependent on the applied electric-pulse voltage and can be refreshed by applying a voltage sweep in vacuum condition (

Ag/Ln1−x CaxMnO3/Pt (Ln = Pr, La, X = 0.3) sandwich structures showing electric-pulse induced reversible resistance switching properties provide a route for a new resistance random access memory (RRAM), because the electric-pulse induced high-resistance state and low-resistance state can be assimilated to logic 1 and 0 in RRAM systems, respectively. In this work the fatigue and the retention behavior of the electric-pulse induced resistance states have been studied. It is found that the retention properties of the EPIR materials can be modified by post-annealing of the materials and by the inner adjusting of the pulse applied mode. Moreover, the EPIR devices can be optimized by initializing using voltage sweep or pulse sweep circles before application to possess the appropriate original resistance and the stable resistance switching behavior.

Materials showing reversible resistance switching at room temperature are attractive for today’s semiconductor technology with its wide interest in nonvolatile random access memories. In this letter, the switching between the high resistance state (OFF-state) and the low resistance state (ON-state) was demonstrated, which depends on both polarity and magnitude of the applied electric pulse. Such a phenomenon could be useful for various magnetic and electric device applications. A “conductance channel” model is proposed to explain the observed effect. This mechanism is supported by the experimental data obtained from the multi-resistance states measured synchronously.

Resistance memory devices based on a Gi/Mg doped ZnO/indium in oxide structure on a PET (polytheylene-re-rephitalate) flexible substrate were fabricated. The devices shows table bipolor resistance switching property and good flexibility. The high to low resistance ration was larger than 30 times, the endurane was more than 10(2)-cycle and there resistance retention was longer than 10(4)s. The resistance values of both high and low resistance states were not significantly changed by bendgin a radius (

We report a memristive switching effect in Pt/CuOx/Si/Pt devices prepared by the rf sputtering technique at room temperature. Differently from other Cu-based metal filament switching systems, a gradual electroforming process, marked by a gradual increase of the device resistance and a gradual decrease of the device capacitance, was observed in the current-voltage and capacitance characteristics. After the gradual electroforming, the devices show a uniform memristive switching behavior. By Auger electron spectroscopy analysis, a model based on the thickness change of the SiOx layer at the CuOx/Si interface and Cu ion migration is proposed for the gradual electroforming and uniform memristive switching, respectively. This work should be meaningful for the preparation of forming-free and homogeneous memristive devices.

Electric-field-induced resistance switching (RS) phenomena have been studied for over 60 years in metal/dielectrics/metal structures. In these experiments a wide range of dielectrics have been studied including binary transition metal oxides, perovskite oxides, chalcogenides, carbon- and silicon-based materials, as well as organic materials. RS phenomena can be used to store information and offer an attractive performance, which encompasses fast switching speeds, high scalability, and the desirable compatibility with Si-based complementary-metal-oxide-semiconductor fabrication. This is promising for nonvolatile memory technology, i.e. resistance random access memory (RRAM). However, a comprehensive understanding of the underlying mechanism is still lacking. This impedes a faster product development as well as an accurate assessment of the device performance potential. Generally speaking, RS occurs not in the entire dielectric but only a small, confined region, which results from the local variation of conductivity in dielectrics. In this review, we focus on the RS in oxides with such an inhomogeneous conductivity. According to the origin of the conductivity inhomogeneity, the RS phenomena and their working mechanism are reviewed by dividing them into two aspects: interface RS, based on the change of contact resistance at metal/oxide interface due to the change of Schottky barrier and interface chemical layer, and bulk RS, realized by the formation, connection, and disconnection of conductive channels in the oxides. Finally the current challenges of RS investigation and the potential improvement of the RS performance for the nonvolatile memories are discussed., 36 pages, 20 figures

Transtance change random access memory (TCRAM) is a type of nonvolatile memory based on the nonlinear magnetoelectric coupling effects of multiferroics. In this work, ferroelectric P(VDF-TrFE) thin films were prepared on Metglas foil substrates by the sol-gel technique to form multiferroic heterostructures. The magnetoelectric voltage coefficient of the heterostructure can be switched reproducibly to different levels between positive and negative values by applying selective electric-field pulses. Compared with bulk multiferroic heterostructures, the polarization switching voltage was reduced to 7 V. Our facile technological approach enables this organic magnetoelectric heterostructure as a promising candidate for the applications in multilevel TCRAM devices.

The memtranstor has been proposed to be the fourth fundamental circuit memelement in addition to the memristor, memcapacitor, and meminductor. Here, we demonstrate the memtranstor behavior at room temperature in a device made of the magnetoelectric hexaferrite (Ba0.5Sr1.5Co2Fe11AlO22) where the electric polarization is tunable by external magnetic field. This device shows a nonlinear q– relationship with a butterfly-shaped hysteresis loop, in agreement with the anticipated memtranstor behavior. The memtranstor, like other memelements, has a great potential in developing more advanced circuit functionalities.

We studied the resistive switching (RS) effect in LaAlO3/Nb:SrTiO3 heterostructures at different temperatures with AC impedance technique in addition to the conventional I–V measurements. It was demonstrated that the bipolar RS effect originates from LaAlO3/Nb:SrTiO3 interface and the resistance and capacitance states are controlled by the filling status of traps. A model based on the variation of trap state was proposed to explain the RS effect and the thermal history dependent electronic transport behavior. This work demonstrates the key role of trap state in the RS effect and electronic transport.

Dynamic processes of resistance switching have been systemically investigated for the Ti/HfO2/Pt bipolar devices. Different transient characteristics were observed in the set and reset processes. The set process consisted of a waiting step and a following abrupt transition, whereas the reset process demonstrated a gradual resistance change. Nonlinear dependence of set time on pulse voltage was observed and explained by the thermally accelerated dielectric breakdown of local switching regions. The accumulation and dissipation effects observed for different pulse treatments strongly supported the proposed model, which suggests a possible approach to overcome the voltage-time dilemma.

Electric field-induced resistive switching (RS) and related effects are studied for the ZnO-based device Ag/AgO x /Mg0.2Zn0.8O/Pt. The system exhibits a bipolar resistive switching (BRS) for the current (I)–voltage (V) cycles, with the set/reset voltage distributing in a narrow region around 0.15 V/0.16 V. The high to low resistance ratio is ∼10, and the resistive state is well retainable. However, the RS becomes unipolar (unipolar resistive switching—URS) when electric pulses are applied, with a fairly wide distribution of the set/reset voltages, though the resistive state is still well retainable. It was further found that a backward transition from the URS to the BRS state can be occasionally triggered by simply performing I–V cycling in the negative branch, which shows the strong competition of the BRS and URS states. Both the BRS and URS states were stable and reproducible over 90 cycles. Possible mechanisms for the BRS and URS state and their mutual transition were discussed.

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.

Rectifying and photoelectronic properties of the La0.67Ba0.33MnO3/SrTiO3:Nb junctions with the film thickness from d=0.5 to 30 nm have been systematically studied. It is found that the electronic transport of the junction is dominated by quantum tunneling or thermoionic emission when film thickness is below or above 1 nm. The rectifying ratio and ideality factor, correspondingly, experience a sudden change as film thickness grows from 0.5 to 1 nm and a smooth variation with film thickness above 1 nm. The threshold film thickness for the establishment of a mature depletion layer is therefore 1 nm. The photoemission properties of the junctions also exhibit a strong dependence on film thickness. As experimentally shown, the photocurrent vanishes in the zero thickness limit, and grows rapidly with the increase in film thickness until d=6 nm, where a maximal photocurrent of ∼770 nA/mm2 under the irradiance of the laser of 5 mW and 532 nm is obtained. After this maximum, an increase-to-decrease turning appears ...

The resistance switching characteristics of the La0.7Ca0.3MnO3-based devices with the top electrodes of Ag, Ag–Al alloys with the atomic ratios of Ag:Al=2:1 (2AgAl) and Ag:Al=1:2 (Ag2Al), and Al have been investigated. The device with 2AgAl top electrode shows excellent endurance, where more than 1000 cycles of reproducible current-voltage hysteresis with stable high and low resistance states have been observed. Based on Auger electron spectroscopy measurement and the detailed investigation of current-voltage curves of these devices, it is suggested that the oxygen affinity of the metal electrode, which is determined by the chemical component of Ag and Al, has an important influence on the interface structure and the resistance switching endurance. The present work provides a possible way for the improvement of the resistance switching endurance by modulating oxygen affinity of the electrode.

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.

Transparent indium–tin-oxide/ZnO:Mg/F-doped SnO2 devices that show bipolar resistance switching have been successfully fabricated. In addition to the transmittance above 80% for visible light, the devices show a high-to-low resistance ratio greater than 2.5, an endurance more than 105 cycles, and a resistance retention longer than 5000 s even at the temperature of 110 °C. The field-induced resistance change can be explained based on the formation/rupture of conduction filaments, due to the migration of structural defects in electric field. The present work shows the potential application of resistive random access memory to invisible electronics.

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.

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.

Resistance random access memory (RRAM) has attracted intense attention in recent years for the potential application as nonvolatile memory. One of the tempting properties of RRAM is the multi-level memory, in which several resistance states can be obtained and each of them can be used to save information. In this paper, the electric-pulse-induced multi-level resistance switching of the Ag-La0.7Ca03MnO3-Pt heterostructures was studied. The multi-level resistance switching (MLRS) was observed in the switching from high to low resistance state (HRS→LRS) by applying electric pulse with various pulse voltages. The threshold pulse voltages of MLRS are related to the initial resistance values as well as the switching directions. On the other hand, the resistance switching behavior from low to high resistance states (LRS→HRS) shows unobvious MLRS. MLRS was explained by the parallel effect of multi-filament forming/rupture in the Ag/La0.7Ca0.3MnO3 interface layer. The present results suggest a possible application of Ag-La0.7Ca03MnO3-Pt heterostructures as multi-level memory devices.