Probing electrochemically induced resistive switching of TiO2 using SPM techniques.

Resistive switching on the nanoscale is an emerging research field and Scanning Probe Microscopy (SPM) is a powerful tool for studies in this area. Under the SPM tip, the electrical field is very high due to the small tip radius on the order of tens of nanometers, and this can enable a range of ionic/electrochemical phenomena during the resistive switching of the materials under the SPM tip. Although the ionic/electrochemical phenomena have long been considered vital for the resistive switching of materials, a few pieces of experimental evidence, as well as the decoupling of the effects of the electrochemical processes at different stages, are still needed. In this work, we applied SPM based techniques to study resistive switching as well as the electrochemical phenomena during the resistive switching of the TiO2 thin films prepared using Pulse Laser Deposition (PLD). It was found that the reversible or irreversible electrochemical processes initiated at different voltages can promote or degrade the resistive switching behavior of TiO2. Combined with an electrical cell with environmental control, these electrochemical processes have been shown to require the involvement of moisture; the accumulation of oxygen vacancies, protons, and hydroxyls at the tip/TiO2 junction may contribute to the promoting effect of the reversible electrochemical process on resistive switching, while the oxygen vacancy ordering and the injection of protons and hydroxyls into the lattice may lead to the irreversible electrochemical process. This work provides a detailed insight into the characteristics, origins, and the effects of the electrochemical phenomena on resistive switching performance, and will provide a further understanding of the electrochemical phenomena in various functional materials.

Origin of High Thermoelectric Performance in Earth-Abundant Phosphide-Tetrahedrite.

Phosphide-based thermoelectrics are a relatively less studied class of compounds, primarily due to the presence of light elements, which result in high thermal conductivity and inherent stability problems. In this work, we present a stable phosphide-tetrahedrite, Ag6Ge10P12, which possesses the highest zT (∼0.7) among all known phosphides at intermediate temperatures (750 K). We examine the intrinsic electronic and thermal transport properties of this compound by expressing the transport properties in terms of weighted mobility (µW), transport coefficient (σE0), and material quality factor (B), from which we are able to elucidate that the origin of its high zT can be attributed to the platelike Fermi surface and high level of band multiplicity related to its complex band structure. Finally, we discuss the origin of the low lattice thermal conductivity observed in this compound using experimental sound velocity, elastic properties, and Debye-Callaway model, thus laying the foundation for similar stable phosphides as potentially earth-abundant and nontoxic intermediate-temperature thermoelectric materials.

MoS2 /Polymer Heterostructures Enabling Stable Resistive Switching and Multistate Randomness.

Resistive random-access memories (ReRAMs) based on transition metal dichalcogenide layers are promising physical sources for random number generation (RNG). However, most ReRAM devices undergo performance degradation from cycle to cycle, which makes preserving a normal probability distribution during operation a challenging task. Here, ReRAM devices with excellent stability are reported by using a MoS2 /polymer heterostructure as active layer. The stability enhancement manifests in outstanding cumulative probabilities for both high- and low-resistivity states of the memory cells. Moreover, the intrinsic values of the high-resistivity state are found to be an excellent source of randomness as suggested by a Chi-square test. It is demonstrated that one of these cells alone can generate ten distinct random states, in contrast to the four conventional binary cells that would be required for an equivalent number of states. This work unravels a scalable interface engineering process for the production of high-performance ReRAM devices, and sheds light on their promising application as reliable RNGs for enhanced cybersecurity in the big data era.

Probing the Ionic and Electrochemical Phenomena during Resistive Switching of NiO Thin Films.

Ionic transport and electrochemical reactions underpin the functionality of the memory devices. NiO, as a promising transition metal oxide for developing resistive switching random access memory, has been extensively explored in the terms of the resistive switching. However, there is limited experimental evidence to visualize the ionic processes of the NiO under the external electrical field. In addition, the correlation between the ionic processes and the resistive switching has not been established. To close this gap and also to determine the role of the ionic processes in resistive switching of the NiO, in this study, a series of scanning probe microscopy techniques, including electrochemical strain microscopy (ESM), conductive atomic force microscopy, Kelvin probe force microscopy, and a newly developed first-order reversal curve-IV, are employed to measure the ESM response, the resistive switching performance, the work function, and the ionic dynamics of NiO, respectively. The results in this work have clearly visualized the ionic transport and electrochemical reactions of NiO when subjected to the electrical field. It has been found that the ionic processes and the resistive switching accompanied each other. Furthermore, it is found that the electrochemical reactions play a determinative role in the resistive switching of the NiO, and this electrochemically induced resistive switching performance can be explained by an integrated mechanism that has combined the filamentary and the interfacial effects underlying resistive switching. In addition to providing a better understanding of the resistive switching of NiO, this work also provides effective methods to probe the ionic processes and to correlate these ionic processes to the performance of functional materials.