How Does Moisture Affect the Physical Property of Memristance for Anionic–Electronic Resistive Switching Memories?

Memristors based on anionic–electronic resistive switches represent a promising alternative to transistor‐based memories because of their scalability and low power consumption. To date, studies on resistive switching have focused on oxygen anionic or electronic defects leaving protonic charge‐carrier contributions out of the picture despite the fact that many resistive switching oxides are well‐established materials in resistive humidity sensors. Here, the way memristance is affected by moisture for the model material strontium titanate is studied. First, characterize own‐processed Pt|SrTiO_3‐δ|Pt bits via cyclic voltammetry under ambient conditions are thoroughly characterized. Based on the high stability of a non‐volatile device structures the impact of relative humidity to the current–voltage profiles is then investigated. It is found that Pt|SrTiO_3‐δ|Pt strongly modifies the resistance states by up to 4 orders of magnitude as well as the device's current–voltage profile shape, number of crossings, and switching capability with the level of moisture exposure. Furthermore, a reversible transition from classic memristive behavior at ambient humidity to a capacitively dominated one in dry atmosphere for which the resistive switching completely vanishes is demonstrated for the first time. The results are discussed in relation to the changed Schottky barrier by adsorbed surface water molecules and its interplay with the charge transfer in the oxide.     

Nanoscale Resistive Switching in Amorphous Perovskite Oxide (a‐SrTiO3) Memristors

Memristive devices are the precursors to high density nanoscale memories and the building blocks for neuromorphic computing. In this work, a unique room temperature synthesized perovskite oxide (amorphous SrTiO₃: a‐STO) thin film platform with engineered oxygen deficiencies is shown to realize high performance and scalable metal‐oxide‐metal (MIM) memristive arrays demonstrating excellent uniformity of the key resistive switching parameters. a‐STO memristors exhibit nonvolatile bipolar resistive switching with significantly high (10³–10⁴) switching ratios, good endurance (>10⁶I–V sweep cycles), and retention with less than 1% change in resistance over repeated 10⁵ s long READ cycles. Nano‐contact studies utilizing in situ electrical nanoindentation technique reveal nanoionics driven switching processes that rely on isolatedly controllable nano‐switches uniformly distributed over the device area. Furthermore, in situ electrical nanoindentation studies on ultrathin a‐STO/metal stacks highlight the impact of mechanical stress on the modulation of non‐linear ionic transport mechanisms in perovskite oxides while confirming the ultimate scalability of these devices. These results highlight the promise of amorphous perovskite memristors for high performance CMOS/CMOL compatible memristive systems.     

Origin of the Ultra‐nonlinear Switching Kinetics in Oxide‐Based Resistive Switches

Experimental pulse length–pulse voltage studies of SrTiO₃ memristive cells are reported, which reveal nonlinearities in the switching kinetics of more than nine orders of magnitude. The results are interpreted using an electrothermal 2D finite element model. The nonlinearity arises from a temperature increase in a few‐nanometer‐thick disc‐shaped region at the Ti electrode and a corresponding exponential increase in oxygen‐vacancy mobility. The model fully reproduces the experimental data and it provides essential design rules for optimizing the cell concept of nanoionic resistive memories. The model is generic in nature: it is applicable to all those oxides which become n‐conducting upon chemical reduction and which show significant ion conductivity at elevated temperatures.     

Nociceptive Memristor

The biomimetic characteristics of the memristor as an electronic synapse and neuron have inspired the advent of new information technology in the neuromorphic computing. The application of the memristors can be extended to the artificial nerves on condition of the presence of electronic receptors which can transfer the external stimuli to the internal nerve system. In this work, nociceptor behaviors are demonstrated from the Pt/HfO₂/TiN memristor for the electronic receptors. The device shows four specific nociceptive behaviors; threshold, relaxation, allodynia, and hyperalgesia, according to the strength, duration, and repetition rate of the external stimuli. Such nociceptive behaviors are attributed to the electron trapping/detrapping to/from the traps in the HfO₂ layer, where the depth of trap energy level is ≈0.7 eV. Also, the built‐in potential by the work function mismatch between the Pt and TiN electrodes induces time‐dependent relaxation of trapped electrons, providing the appropriate relaxation behavior. The relaxation time can take from several milliseconds to tens of seconds, which corresponds to the time span of the decay of biosignal. The material‐wise evaluation of the electronic nociceptor in comparison with other material, which did not show the desired functionality, Pt/Ti/HfO₂/TiN, reveals the importance of careful material design and fabrication.     

Photoelectric Multilevel Memory Device based on Covalent Organic Polymer Film with Keto–Enol Tautomerism for Harsh Environments Applications

Covalent organic polymers (COPs) memristors with multilevel memory behavior in harsh environments and photoelectric regulation are crucial for high-density storage and high-efficiency neuromorphic computing. Here, a donor–acceptor (D–A)-type COP film (Py-COP-3), which is initiated by keto–enol tautomerism, is proposed for high-performance memristors. Satisfactorily, the indium tin oxide (ITO)/Py-COP-3/Ag device demonstrates multilevel memory performance, even in high temperatures, acid-base corrosion, and various organic solvents. Moreover, the performance can be modulated by the photoelectric effect to maintain a great switching behavior. By contrast, Py-COP-0, with similar structure and chemical composition to Py-COP-3 but without keto–enol tautomerism, exhibits binary storage performance. Further studies unravel that both the formation of conductive filaments and charge transfer within D-A Py-COP-3 film contribute to the resistive switching behavior of memory devices.     

Self‐Limited Switching in Ta2O5/TaOx Memristors Exhibiting Uniform Multilevel Changes in Resistance

To facilitate the development of memristive devices, it is essential to resolve the problem of non‐uniformity in switching, which is caused by the random nature of the filamentary switching mechanism in many resistance switching memories based on transition metal oxide. In addition, device parameters such as low‐ and high‐state resistance should be regulated as desired. These issues can be overcome if memristive devices have switching limits for both the low‐ and high‐resistance states and if their resistance values are highly controllable. In this study, a method termed self‐limited switching for uniformly regulating the values of both the low‐ and high‐resistance states is suggested, and the circuit configuration required for the self‐limited switching is established in a Ta₂O₅/TaO_x memristive structure. A method of improving the uniformity of multi‐level resistance states in this memristive system is also proposed.     

Environmentally Robust Memristor Enabled by Lead‐Free Double Perovskite for High‐Performance Information Storage

Memristors are emerging as a rising star of new computing and information storage techniques. However, the practical applications are severely challenged by their instability toward harsh conditions, including high moisture, high temperatures, fire, ionizing irradiation, and mechanical bending. In this work, for the first time, lead‐free double perovskite Cs₂AgBiBr₆ is utilized for environmentally robust memristors, enabling highly efficient information storage. The memory performance of the typical indium‐tin‐oxide/Cs₂AgBiBr₆/Au sandwich‐like memristors is retained after 1000 switching cycles, 10⁵ s of reading, and 10⁴ times of mechanical bending, comparable to other halide perovskite memristors. Most importantly, the memristive behavior remains robust in harsh environments, including humidity up to 80%, temperatures as high as 453 K, an alcohol burner flame for 10 s, and ⁶⁰Co γ‐ray irradiation for a dosage of 5 × 10⁵ rad (SI), which is not achieved by any other memristors and commercial flash memory techniques. The realization of an environmentally robust memristor from Cs₂AgBiBr₆ with a high memory performance will inspire further development of robust electronics using lead‐free double perovskites.     

Memristors Based on 2D Materials as an Artificial Synapse for Neuromorphic Electronics

The memristor, a composite word of memory and resistor, has become one of the most important electronic components for brain-inspired neuromorphic computing in recent years. This device has the ability to control resistance with multiple states by memorizing the history of previous electrical inputs, enabling it to mimic a biological synapse in the neural network of the human brain. Among many candidates for memristive materials, including metal oxides, organic materials, and low-dimensional nanomaterials, 2D layered materials have been widely investigated owing to their outstanding physical properties and electrical tunability, low-power-switching capability, and hetero-integration compatibility. Hence, a large number of experimental demonstrations on 2D material-based memristors have been reported showing their unique memristive characteristics and novel synaptic functionalities, distinct from traditional bulk-material-based systems. Herein, an overview of the latest advances in the structures, mechanisms, and memristive characteristics of 2D material-based memristors is presented. Additionally, novel strategies to modulate and enhance the synaptic functionalities of 2D-memristor-based artificial synapses are summarized. Finally, as a foreseeing perspective, the potentials and challenges of these emerging materials for future neuromorphic electronics are also discussed.