Stable Switching Research Articles

Bipolar Resistive Switching Behaviour of Polycrystalline BiFeO3 Thin Films Synthesized via Sol-gel Assisted Spin Coating Technique

Polycrystalline BiFeO3 thin films have been grown on glass substrates using a simple but efficient method commonly known as the spin coating technique. When used in a Cu / BiFeO3 / Cu configuration, the annealed BiFeO3 film (at 350 °C) exhibits bipolar resistive switching behaviour. The device shows stable resistive switching behaviour, where a stable hysteresis in the current–voltage curve was well developed by applying +/- 10 V at room temperature. The ratio of resistance in the high resistance state to the low resistance state of the device is ~ 104 with a good retention time of more than 106 min. The Poole–Frenkel emission at the Cu / BiFeO3 interface is proposed, and a redistribution of oxygen vacancies along the grain boundaries is found to play a key role in the resistance switching in the polycrystalline pure BiFeO3 films.

High-temperature operation of gallium oxide memristors up to 600 K

Memristors have attracted much attention for application in neuromorphic devices and brain-inspired computing hardware. Their performance at high temperatures is required to be sufficiently reliable in neuromorphic computing, potential application to power electronics, and the aerospace industry. This work focuses on reduced gallium oxide (GaOx) as a wide bandgap memristive material that is reported to exhibit highly reliable resistive switching operation. We prepared amorphous GaOx films to fabricate Pt/GaOx/indium tin oxide memristors using pulsed laser deposition. Stable resistive switching phenomena were observed in current–voltage properties measured between 300 and 600 K. The conduction mechanism analysis revealed that the resistive switching is caused by the transition between ohmic and space charge limiting current conductions. We elucidated the importance of appropriate control of the density of oxygen vacancies to obtain a high on/off resistance ratio and distinct resistive switching at high temperatures. These results indicate that GaOx is a promising memristor material that can be stably operated even at the record-high temperature of 600 K.

Self-powered photodetector with fast response based on Sb2Se3/Cu2S/Si heterojunction

Antimony selenide (Sb2Se3) thin film, as a potential semiconductor material, has good photoelectric response in the visible and near-infrared regions and excellent application prospects in wide-spectrum detectors. In the present work, copper sulfide (Cu2S) film was inserted to improve the separation and migration of photogenerated carriers in Sb2Se3 film. Sb2Se3 and Sb2Se3/Cu2S thin films were deposited on n-type silicon wafers using thermal evaporation. Excellent optical response and stable switching ratios at 638 nm and 980 nm can be obtained for the photodetectors based on Sb2Se3/Si and Sb2Se3/Cu2S/Si heterojunctions. The Cu2S film as mid-layer has increased on/off ratio from 119 to 155 (638 nm) and 265 to 510 (980 nm), respectively. The devices have good self-powered characteristics. Compared with Sb2Se3/Si heterojunction, the response time of Sb2Se3/Cu2S/Si is significantly improved from 207 μs to 160 μs at 450 nm, from 117 μs to 53 μs at 638 nm, and it can reach up to 37 μs at 980 nm. The sandwiched p-type Cu2S can effectively enhance the separation and migration of the photogenerated carriers by the large driving force of built-in field at the heterointerface. The Sb2Se3/Cu2S/Si photodetector has a potential application in wide-spectrum-detection field.

An Efficient Design of TaOx‐Based Memristor by Inserting an Ultrathin Al2O3 Layer with High Stability for Neuromorphic Computing and Logic Operation

AbstractNew computing‐in‐memory architecture based on memristors can achieve in situ storage and computing of data, which greatly improves the computing efficiency of the hardware system. Here, a reliable bilayer structured TaOx/Al2O3 memristor with a 2 nm Al2O3 insertion layer is demonstrated. This device exhibits stable and gradual switching behavior with a low set/reset voltage (0.61 V/−0.49 V) and multilevel conductance characteristics. It is further indicated that the device has a larger ON/Off ratio (≈148×) and better nonlinearity of conductance modulation by inserting an Al2O3 layer. Various forms of synaptic plasticity are mimicked, such as long‐term potentiation/depression (LTP/LTD), paired‐pulse facilitation (PPF), and spike‐timing‐dependent plasticity (STDP). Based on the quasi‐linear conductance modulation characteristics, excellent classification accuracy (90.4%) is achieved for the applications of handwritten digit recognition. Moreover, the logic operations (intersection, union, and complement) are implemented on a 3 × 5 memristor array, which shows an efficient way to design versatile and reliable devices and provides a novel idea for neuromorphic computing and in‐memory logic operation.

The plasticity of synaptic memristor based on 2D-MoS2 thin film prepared in large-scale by a PLD-assisted CVD method

Two-dimensional MoS2 film was directly grown via a two-step chemical vapour deposition (CVD) method in a large-scale on a phosphorus doped silicon substrate, where a precursor of Mo was Pre-deposited by pulsed laser deposition (PLD) to ensure the homogeneity of Mo source. The film was confirmed to be dense, uniform and continuous nanocrystalline of MoS2 by the characterization of XRD, Raman, SEM, TEM and XPS. Then MoS2 film was employed as the channel materials to fabricate the memristor with lateral structure of Cu/MoS2/Cu. Consequently, the devices show stable nonvolatile resistance switching characteristics, such as low operating voltages, repeated cyclic sweeps (600 times) without decrease and long retention time (>104 s). Such mechanism of resistive switching can be attributable to the space-charge limited conduction dominated by defects. Importantly, a variety of biological synaptic functions, such as synaptic potentiation, depression and repetitive learning processes, are effectively simulated. Furthermore, the relationships between the resistance state and the parameters of pulse sequence were explored based on MoS2 thin film memristors. This research provides a feasible method for manufacturing MoS2 based on memristor, which is used for information and computing applications in neural morphological devices.

Reversible switching between two common protein folds in a designed system using only temperature

Naturally occurring metamorphic proteins have the ability to interconvert from one folded state to another through either a limited set of mutations or by way of a change in the local environment. Here, we show in a designed system that it is possible to switch reversibly between two of the most common monomeric folds employing only temperature changes. We demonstrate that a latent 3α state can be unmasked from an α/β-plait topology with a single V90T amino acid substitution, populating both forms simultaneously. The equilibrium between these two states exhibits temperature dependence, such that the 3α state is predominant (>90%) at 5°C, while the α/β-plait fold is the major species (>90%) at 30°C. We describe the structure and dynamics of these topologies, how mutational changes affect the temperature dependence, and the energetics and kinetics of interconversion. Additionally, we demonstrate how ligand-binding function can be tightly regulated by large amplitude changes in protein structure over a relatively narrow temperature range that is relevant to biology. The 3α/αβ switch thus represents a potentially useful approach for designing proteins that alter their fold topologies in response to environmental triggers. It may also serve as a model for computational studies of temperature-dependent protein stability and fold switching.

Basins of attraction and paired Hopf bifurcations for delay differential equations with bistable nonlinearity and delay-dependent coefficient

We consider a class of delay differential equations with bistable nonlinearity, in which the trivial equilibrium may coexist with two positive equilibria. Despite the difficulty caused by delay and bistable nonlinearity, we give a rather complete description on the dynamics including global stability, semi-stability, bistability and Hopf bifurcation. For the case where the stable trivial equilibrium coexists with a stable positive equilibrium, we obtain two delay-dependent intervals as subsets of basins of attraction of two stable equilibria. These subsets are sharp in some sense. Using delay as the bifurcation parameter, we analytically show that the number of local Hopf bifurcation values is finite and these local Hopf bifurcation values are neatly paired. A Nicholson's blowflies equation with Allee effect is used to illustrate our general results. Through this example, we show that delay can induce stability switches, symmetric transitions among multitype bistability and robust phase-transitions for long transients.

Room‐Temperature‐Processable Highly Reliable Resistive Switching Memory with Reconfigurability for Neuromorphic Computing and Ultrasonic Tissue Classification

AbstractReversible metal‐filamentary mechanism has been widely investigated to design an analog resistive switching memory (RSM) for neuromorphic hardware‐implementation. However, uncontrollable filament‐formation, inducing its reliability issues, has been a fundamental challenge. Here, an analog RSM with 3D ion transport channels that can provide unprecedentedly high reliability and robustness is demonstrated. This architecture is realized by a laser‐assisted photo‐thermochemical process, compatible with the back‐end‐of‐line process and even applicable to a flexible format. These superior characteristics also lead to the proposal of a practical adaptive learning rule for hardware neural networks that can significantly simplify the voltage pulse application methodology even with high computing accuracy. A neural network, which can perform the biological tissue classification task using the ultrasound signals, is designed, and the simulation results confirm that this practical adaptive learning rule is efficient enough to classify these weak and complicated signals with high accuracy (97%). Furthermore, the proposed RSM can work as a diffusive‐memristor at the opposite voltage polarity, exhibiting extremely stable threshold switching characteristics. In this mode, several crucial operations in biological nervous systems, such as Ca2+ dynamics and nonlinear integrate‐and‐fire functions of neurons, are successfully emulated. This reconfigurability is also exceedingly beneficial for decreasing the complexity of systems—requiring both drift‐ and diffusive‐memristors.

Modeling of ultrafast polarization switching in PbTiO[formula omitted

Ultrafast switching of electric polarization in ferroelectric materials is an open scientific problem. Switching has not been achieved experimentally. In this paper, using an improved model describing switching in a two-dimensional space of generalized coordinates, we show that for PTO, one pulse with linear polarization cannot lead to stable switching. Only depolarization of the sample can be observed if the threshold pulse intensity is reached. It is also shown that friction plays an insignificant role in dynamics of switching, and the optimal frequency of the perturbing pulse is calculated at which the intensity is the lowest.

Real world use of dolutegravir two drug regimens.

Since 2015, we prescribed dolutegravir (DTG)-based two drug regimens (DTG-2DR) for 620 people [total cohort 3133 (19.8%)]. Clinic database search 1 January 15 to 31 October 21. Demographic, tolerability and HIV related data analysed. In total, 620 people identified; 561 had complete data. 446 male (79.5%); median age 54 years (interquartile range 46, 59). 343 (61.1%) MSM. Nine people who initiated naïvely achieved viral suppression (100%). 546/552 (99.0%) switched or continued and were suppressed at data censor. 460/552 (83.3%) received DTG-lamivudine (DTG/3TC), 74/552 (13.4%) received DTG-rilpivirine (DTG/RPV) and 18/552 (3.3%) received DTG-emtricitabine (DTG/FTC). 70 (12.5%) switched off DTG-2DR (55 DTG/3TC, 13 DTG/RPV, two DTG/FTC) due to side-effects. 41 episodes of blip (1 off >50 copies/ml) occurred in 30 people (5.3%). 11/41 on DTG-RPV [ n = 7 multi-tablet regimen (MTR), n = 4 single tablet regimen (STR)]. 27/41 DTG-3TC, 3/41 DTG/FTC ( n = 26 MTR, n = 4 STR). Six people (1.1%) failed (confirmed viral load >200 copies/ml or persistent low level viraemia) ( n = 4 DTG-3TC STR, n = 1 DTG-3TC MTR, n = 1 DTG-RPV MTR). Four failures due to low level viraemia, one due to non-adherence and one due to high viral load. Resistance tests performed for 5/6 - mutations detected only in latter person with high viral load failure (on DTG-3TC MTR) who developed triple class resistance. Majority of experience is in DTG/3TC stable switch. Minority of patients developed side-effects. Low number of virological failures, one developed integrase inhibitor resistance. Viral failure associated with MTR, commensurate with trial data showing no failure with resistance if DTG/3TC STR used. Overall DTG-2DR demonstrates high efficacy in real-world setting.

Fast‐Switching Speed and Ultralong Lifespan Au/Prussian Blue Electrochromic Film for Iris Devices Applications

AbstractPrussian Blue (PB) is an excellent electrochromic (EC) material, possessing many praiseworthy advantages such as proper redox potential, large transmittance, environmental benignity, neutral colored state, and low cost. However, the co‐influence of weak interaction force and low electrical conductivity results in poor cyclic stability and slow switching speed. In this work, the novel Pt/Au and Au/PB films are prepared by electrodeposition to improve the EC properties of PB. The introduction of Au and Pt nanoparticles not only enhances the conductivity and inhibits polarization, but also increases the contact area and alleviates the stress and strain caused by volume expansion during ion insertion/extraction. Furthermore, the Pt/PB and Au/PB heterojunction with a strong inner electronic field promote the separation of electrons and holes and quicken charge transfer. In consideration of the better electrical conductivity, higher transmittance and strong local plasma resonance effect of Au nanoparticles, the Au/PB film shows ultrastable EC cyclic stability (96.4% after 2000), fast coloration/bleaching switching time (1.36/2.32 s), small charge transfer resistance (38.2 Ω), and high coloration efficiency (131.3 cm2 C−1). In addition, the EC iris devices are designed and assembled using Au/PB film as an EC layer to confirm the feasibility of the material in digital cameras and mobile phones.

Diverse synaptic weight adjustment of bio-inspired ZrOx-based memristors for neuromorphic system

In this article, we demonstrate the bio-inspired synaptic features of the TiN/ZrOx/Pt capacitor structure for neuromorphic engineering. The chemical and material compositions and the thicknesses of each of the layers are verified by using transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) maps. Stable resistive switching with a low set voltage (∼1 V) was determined by scanning the DC I–V curves of many cells. The DC endurance of ∼104 cycles and retention (10,000 s) in five states was achieved. Multi-level cells (MLC) characteristics were achieved based on the compliance current and reset stop voltage in DC sweep and pulses. Finally, we emulated paired-pulse facilitation (PPF), paired-pulse depression (PPD), electric excitatory postsynaptic current (EPSC), and spike-timing-dependent plasticity (STDP) of the artificial synapse by using the RRAM device.

Hopf Bifurcation in a reaction–diffusion–advection model with two nonlocal delayed density-dependent feedback terms

We consider a reaction–diffusion–advection logistic model with two nonlocal delayed density-dependent terms and zero-Dirichlet boundary conditions. The existence of positive non-homogeneous steady states and the associated Hopf bifurcation are obtained. By taking the time delay τ as the bifurcation parameter, we find that the system may admit no stability switches, a single stability switch and multiple stability switches. Moreover, we investigate the influence of advection on the Hopf bifurcation and stability switches, and find that the critical values of τ at which a Hopf bifurcation occurs increase (decrease) as the advection rate α increases in a positive (negative) range, which implies that the advective effect has decelerated (accelerated) the generation of Hopf bifurcation to some extent if the advection rate α lies in a certain positive (negative) range.

Stability switch and Hopf bifurcations for a diffusive plankton system with nonlocal competition and toxic effect

<abstract><p>Since the distribution of plankton is always uneven, the nonlocal phytoplankton competition term indicates the spatial weighted mean of phytoplankton density, which is introduced into the plankton model with toxic substances effect to study the corresponding dynamic behavior. The stability of the positive equilibrium point and the existence of Hopf bifurcations are discussed by analysing the distribution of eigenvalues. The direction and stability of bifurcation periodic solution are researched based on an extended central manifold method and normal theory. Finally, spatially inhomogeneous oscillations are observed in the vicinity of the Hopf bifurcations. Through numerical simulation, we can observe that the system without nonlocal competition term only generates homogeneous periodic solution, and inhomogeneous periodic solution will produce only when both diffusion and nonlocal competition exist simultaneously. We can also see that when the toxin-producing rate of each phytoplankton is in an appropriate range, the system with nonlocal competition generates a stability switch with inhomogeneous periodic solution, when the value of time delay is in a certain interval, then Hopf bifurcations will appear, and with the increase of time delay, the quantity of plankton will eventually become stable.</p></abstract>

Stability switches, periodic oscillations and global stability in an infectious disease model with multiple time delays.

A delay differential equation model of an infectious disease is considered and analyzed. In this model, the impact of information due to the presence of infection is considered explicitly. As information propagation is dependent on the prevalence of the disease, the delay in reporting the prevalence is an important factor. Further, the time lag in waning immunity related to protective measures (such as vaccination, self-protection, responsive behaviour etc.) is also accounted. Qualitative analysis of the equilibrium points of the model is executed and it is observed that when the basic reproduction number is less unity, the local stability of the disease free equilibrium (DFE) depends on the rate of immunity loss as well as on the time delay for the waning of immunity. If the delay in immunity loss is less than a threshold quantity, the DFE is stable, whereas, it loses its stability when the delay parameter crosses the threshold value. When, the basic reproduction number is greater than unity, the unique endemic equilibrium point is found locally stable irrespective of the delay effect under certain parametric conditions. Further, we have analyzed the model system for different scenarios of both delays (i.e., no delay, only one delay, and both delay present). Due to these delays, oscillatory nature of the population is obtained with the help of Hopf bifurcation analysis in each scenario. Moreover, at two different time delays (delay in information's propagation), the emergence of multiple stability switches is investigated for the model system which is termed as Hopf-Hopf (double) bifurcation. Also, the global stability of the endemic equilibrium point is established under some parametric conditions by constructing a suitable Lyapunov function irrespective of time lags. In order to support and explore qualitative results, exhaustive numerical experimentations are carried out which lead to important biological insights and also, these results are compared with existing results.

Stability and bifurcation of a delayed prey-predator eco-epidemiological model with the impact of media

<abstract><p>In this paper, a delayed prey-predator eco-epidemiological model with the nonlinear media is considered. First, the positivity and boundedness of solutions are given. Then, the basic reproductive number is showed, and the local stability of the trivial equilibrium and the disease-free equilibrium are discussed. Next, by taking the infection delay as a parameter, the conditions of the stability switches are given due to stability switching criteria, which concludes that the delay can generate instability and oscillation of the population through Hopf bifurcation. Further, by using normal form theory and center manifold theory, some explicit expressions determining direction of Hopf bifurcation and stability of periodic solutions are obtained. What's more, the correctness of the theoretical analysis is verified by numerical simulation, and the biological explanations are also given. Last, the main conclusions are included in the end.</p></abstract>

Resistive switching characteristics of HfO<sub><i>x</i></sub>-based resistance random access memory under photoelectric synergistic regulation

Cu/HfO<sub><i>x</i></sub>/Pt and Cu/HfO<sub><i>x</i></sub>-ZnO/Pt resistance random access memory (RRAM) devices are prepared by magnetron sputtering. The results show that the Cu/HfO<sub><i>x</i></sub>/Pt device has the stable bipolar resistive switching characteristics, good retention (as long as 10<sup>4</sup> s), and a switching ratio greater than 10<sup>3</sup>. The current conduction mechanism of HfO<sub><i>x</i></sub> device is ohmic conduction at low resistance, while space charge limited current (SCLC) mechanism dominates at high resistance, and the conductive filament is composed of oxygen vacancies. Owing to the low content and random distribution of oxygen defects in the HfO<sub><i>x</i></sub> film, the endurance and uniformity of the device are poor. Compared with HfO<sub><i>x</i></sub> device, HfO<sub><i>x</i></sub>-ZnO device exhibits lower operating voltage and better uniformity and stability. The main reason is that ZnO material has smaller formation energy of oxygen vacancy, which can produce more oxygen defects under electric field to participate in the resistive switching behavior of the device, thereby reducing the operating voltage and improving the uniformity of the device. In addition, owing to the existence of the interface between HfO<sub><i>x</i></sub> and ZnO film, the random distribution of oxygen defects is inhibited, that is, the random fracture and formation of conductive filament are inhibited, which is beneficial to improving the uniformity of the device. In addition, the resistive switching behaviors of Cu/HfO<sub><i>x</i></sub>/Pt and Cu/HfO<sub><i>x</i></sub>-ZnO/Pt RRAM devices under different intensities of 255 nm ultraviolet illumination are studied. For Cu/HfO<sub><i>x</i></sub>/Pt device, the light of 255 nm wavelength shows little effect on its resistive switching characteristics. For the Cu/HfO<sub><i>x</i></sub>-ZnO/Pt RRAM device, the operating voltage and stability of the device can be improved by increasing the light intensity. Although the switching ratio of the device decreases with the increase of light intensity, the device can exhibit multiple resistance states by adjusting different light intensities to achieve multi-level storage. Finally, the analysis of the <i>I</i>-<i>V</i> curves of the devices indicates that the two types of devices show similar resistive switching mechanisms under the illumination of light or no light, which can be explained by the resistive switching mechanism of oxygen vacancy conductive filament. Therefore, a physical model based on the oxygen vacancy conductive filament is established to explain the resistive switching behavior of the device in this paper.

Memristors based on NdNiO3 nanocrystals film as sensory neurons for neuromorphic computing.

By mimicking the behavior of the human brain, artificial neural systems offer the possibility to further improve computing efficiency and solve the von Neumann bottleneck. In particular, neural systems with perceptual capability expand the application field and lay a good foundation for the construction of perceptual storage and computational systems. However, research on neurons with perceptual functions is still relatively scarce, with most works focusing on optoelectronic synapses. The neuron is important for neuromorphic computing systems because neurons output excitatory or inhibitory stimuli to regulate the weight of synapses. Therefore, the construction of sensory neurons is crucial to expand the application range of brain-like neural computing. Here, an artificial sensory neuron is proposed, which is constructed using a photosensitive bipolar threshold switching memristor based on NdNiO3 (NNO) nanocrystals. These metallic phase nanocrystals can not only enhance the local electric field, but also act as a reservoir for defects (VoS) to guide the growth of conductive filaments and stabilize the performance of the device. They present stable bipolar threshold switching behavior with a low 120 nW set power, and the operating voltages decreased in light due to photocarrier action. A leaky integrate firing (LIF) neuron has been realized, which achieved key biological neuron functions, such as all-or-nothing spiking, threshold-driven firing, refractory period, and spiking frequency modulation. The LIF neurons receiving optical inputs have the properties of an artificial sensory neuron. It could regulate the spiking output frequency at different light densities, which could be used for a ship approaching a port. This work provides a promising hardware implementation towards constructing high-performance artificial intelligence to assist ships at night in a sensory system.

Charge trapped CdS quantum dot embedded polymer matrix for a high speed and low power memristor.

The data storage requirement in the digital world is increasing day by day with the advancement of the internet of things. In this respect, nonvolatile resistive random-access memory is an option that provides high density and low power data storage capabilities. In this work, zero-dimensional colloidal CdS quantum dots and a polymer composite at an appropriate ratio were used to fabricate a memristive device. Comparison with a pristine CdS quantum dot-based device reveals that a surrounding matrix around the quantum dots is needed for observing memristive behavior. The quantum dots embedded in the polymer matrix device showed extremely stable electrical switching behavior that can be operated for more than 300 cycles and 60 000 seconds. Moreover, the device needs extremely low power to operate at a very high speed. The smooth surface morphology dictates a charge trapping mechanism for the switching phenomenon; however, an interplay between different charge transport mechanisms leads to the fast switching and high on-off ratio of the device.

Gamma radiation-induced nanodefects in diffusive memristors and artificial neurons.

Gamma photons with an average energy of 1.25 MeV are well-known to generate large amounts of defects in semiconductor electronic devices. Here we investigate the novel effect of gamma radiation on diffusive memristors based on metallic silver nanoparticles dispersed in a dielectric matrix of silica. Our experimental findings show that after exposure to radiation, the memristors and artificial neurons made of them demonstrate much better performance in terms of stable volatile resistive switching and higher spiking frequencies, respectively, compared to the pristine samples. At the same time we observe partial oxidation of silver and reduction of silicon within the switching silica layer. We propose nanoinclusions of reduced silicon distributed across the silica layer to be the backbone for metallic nanoparticles to form conductive filaments, as supported by our theoretical simulations of radiation-induced changes in the diffusion process. Our findings propose a new opportunity to engineer the required characteristics of diffusive memristors in order to emulate biological neurons and develop bio-inspired computational technology.