Valence Change Research Articles

Dual‐Mode Operation of Epitaxial Hf0.5Zr0.5O2: Ferroelectric and Filamentary‐Type Resistive Switching

Since the discovery of its ferroelectricity, hafnium oxide is widely used for applications in ferroelectric field‐effect transistors and ferroelectric tunnel junctions. is especially favored for its robust ferroelectricity and high remanent polarization at low thicknesses. In addition, is well established as amorphous or crystalline oxide layer in resistive switching devices. Herein, ferroelectric switching is found coexisting with high on/off ratio resistive switching in sub‐10 nm epitaxial . The resistive switching shows typical characteristics of a filamentary‐type valence change memory (VCM), clearly contradicting the polarization charges as the origin of different resistance states. In contrast to previous observations, no electroforming step is required to initiate VCM switching. The bottom electrode enables a RESET to the virgin state, allowing subsequent ferroelectric hysteresis measurements. It is possible to change between both switching schemes repeatedly without impacting the ferroelectric performance. This indicates that ferroelectric switching and oxygen vacancy movement do not interfere with each other, and both switching phenomena can exist independently. This finding opens up ways to unite the different strengths of both switching mechanisms in the same stack. It becomes possible to assign the two operating principles to artificial neural network training and inference according to their respective advantages.

Redox-controlled differential weathering of andesitic pumice: Possible catalytic effects of ferrous/ferric iron on rapid halloysite accumulation in a Holocene tephra layer

Differential weathering process of Holocene andesitic tephra in Atsuma, Hokkaido, northern Japan, were investigated with a specific focus on the flame-like occurrence of halloysite bands and their formation mechanism. The parent material was coarse-grained pumice (Ta-d2) deposited 9 ka to form a highly permeable metre-thick layer atop fine-grained impervious periglacial loam, which is covered by humic soil–tephra sequences. The shallower part of the Ta-d2 layer was weathered to a reddish or yellowish brown colour, which is characterized by intensive leaching of Si and Al, with abundant precipitation of iron (hydr)oxide and amorphous secondary minerals in an oxidizing environment. A reductive grey colour was observed only at the Ta-d2 bottom below the halloysite band, where pumice grains survived significant elemental leaching and contained ferrous iron in the solid phase with abundant dissolved Fe in the liquid phase. Between these oxidized and preserved zones, an irregularly shaped, fine-grained, white iron-rich halloysite layer with a decimetre-scale thickness developed. In this clay band, the molar ratio of Si to (Al + Fe) was nearly 1:1, with a small amount of remnant Ca. Mössbauer analysis revealed that Fe2+ was initially incorporated into the aluminosilicate sheet via Al3+ substitution and transformed into Fe3+ within the structure. The halloysite band was formed at depths in the tephra layer corresponding to the redox front. The abundance of Si and Al in pore water in any zone of the tephra layer meets the stability requirements of halloysite, indicating that the redox condition controls the rapid local accumulation of halloysite through the isomorphous substitution followed by valence change of iron. We propose a new hypothesis stating that iron catalyses halloysite nucleation via a stepwise function, including layer charging to absorb hydrated cations and subsequent charge loss resulting in cation desorption and passive intercalation of water molecules within the 1:1 layer.

Freestanding NiMoO4/carbon cloth electrodes for Li–O2 batteries with high current densities and preferable limited capacities

The development of power batteries with high specific energy and long cycle life has broad application prospects in the field of electrocatalysis. Li–O2 batteries are known as the most promising power batteries, in which the performance of the critical materials (cathode catalytic materials) directly determines various performance indices of Li–O2 batteries. The wolframite-type ABO4 catalysts can provide enough reactive active sites to improve catalytic activity for Li–O2 batteries because there are abundant valence changes of A and B elements. Here, NiMoO4 nanoflakes with a wolframite structure were used as the electrode materials in Li–O2 batteries, which were directly prepared on carbon cloth using the hydrothermal method. The first discharge capacities of the NiMoO4/carbon cloth electrodes reached 23600 mA h g-1 at a current density of 600 mA g-1 and 10726 mA h g-1 at a current density of 2000 mA g-1. The NiMoO4/carbon cloth electrodes exhibit stable performance for 100 cycles under a fixed limited capacity of 1000 mA h g-1 at current densities of 1000, 3000 and 5000 mA g-1. The above results illustrate that the synthesized NiMoO4 has good stable performance with extremely high current densities and preferable limited capacities in Li–O2 batteries.

Ionic memristive effects on the nanometre scale in metal oxides: Understanding the process of valence change

Relevance. The relevance of the study is due to the great potential of memristive effects, which are manifested in the change of material resistance under the influence of an external electric field and ionic fluxes. Aim. The aim is to analyse and study the mechanisms of ionic memristive effects, with a detailed consideration of the process of changing the valence of metal cations. Methodology. The work was based on the study of nanometre-sized metal oxides TiO2 and ZrO2. The materials were obtained by synthesis by chemical deposition using high-purity precursors. Results. The obtained results open up wide opportunities for the practical use of ionic membrane effects. The study of ionic memristive effects in TiO2 and ZrO2-based films has shown that the change in resistance occurs due to various mechanisms, including ionic migration, electrochemical reactions, and defect reorganization. Under the influence of an external electric field, a change in the resistance of both materials is observed. In TiO2, the resistance decreases with increasing voltage, while in ZrO2, an increase in resistance is observed. During additional experiments in the temperature range of 25-200 ℃, it was found that temperature significantly affects the ionic membrane effects. With its increase, a noticeable increase in the intensity of these effects in both materials is observed. Conclusions. The use of X-ray diffractometry and infrared spectroscopy revealed that changes in the valence of metal cations in both films occur under the influence of an electric field. The analysis of changes in the X-ray and infrared spectra showed the presence of modifications in the crystal and molecular structure in response to the electric field. In particular, the change in the positions and intensity of the peaks indicates a restructuring of the bonds in the crystal lattice. The paper proposes new studies to expand the understanding of these effects and to consider possible ways to improve membrane devices. The study of ionic memristive effects in TiO2 and ZrO2 is of great practical importance for the development of electronics and the creation of new generations of memristors and neuromorphic systems.

ZnS-embedded porous carbon for peroxydisulfate activation: Enhanced electron transfer for bisphenol A degradation

Transition metal sulfides have garnered increasing attention for their role in persulfate activation, a crucial process in environmental remediation. However, the function of metal sulfides without reversible valence changes, such as ZnS, remains largely unexplored in this context. Here we report ZnS-embedded porous carbon (ZnS-C), synthesized through the pyrolysis of Zn-MOF-74 and dibenzyl disulfide. ZnS-C demonstrates remarkable activity in activating peroxydisulfate (PDS) across a wide pH range, enabling the efficient mineralization removal of bisphenol A (BPA). Through electrochemical investigation and theoretical simulations of charge density distributions, we unveil that the electron transfer from BPA to PDS mediated by the ZnS-C catalyst governs the reaction. This study, both in theory and experiment, demonstrates metal sulfide as electron pump that enhances electron transfer efficiency in PDS activation. These findings redefine the role of metal sulfide catalysts, shedding new light on their potential for regulating reaction pathways in PDS activation processes.

Reliability Aspects of 28 nm BEOL‐Integrated Resistive Switching Random Access Memory

Approaching the application of redox‐based resistive switching random access memory (ReRAM), the research focus shifts more and more toward different aspects of reliability. Herein, it is vital to account for the statistics in large memory blocks, as certain failure mechanisms are observed to only affect a few bits per million. In a cooperation between RWTH Aachen and Infineon Technologies, the variability, retention, and endurance of filamentary valence change memory, integrated into 28 nm CMOS on Mbit scale are comprehensively studied. This article reviews the main findings of this project. It is found that the programmed states follow characteristic normal or log‐normal statistics based on dynamic equilibrium in the random walk of oxygen vacancies in the switching layer, experimentally observed as read noise. On long timescales, these statistics are remarkably stable, providing high data retention. However, the existing long‐term degradation can be characterized by shifting and broadening of the programmed high resistive state distributions. A high endurance of more than 500 k cycles is demonstrated on the Mbit scale. Only a tail of a few devices appears to fail to reset. Via kinetic Monte Carlo modeling, the voltage divider of ReRAM cell and periphery has been identified as the origin of this failure.

Enhanced resistivity and piezoelectric response in BiFeO3-BaTiO3 via two-step sintering

Due to the defects from Bi volatilization and Fe ion valence change during normal sintering (NS), the BiFeO3-BaTiO3 (BF-BT) ceramic is always accompanied by serious leakage current and deteriorated electrical performance, which impeded its further application. In this work, the two-step sintering (TSS) method was applied to the BF-BT ceramics preparation, and numerous microstructures of samples were observed and verified that it can decrease the sintering temperature from 1000 ℃ to 900 ℃ and enhance the relative density to above 96%. This reduced the defects and solved the issue of large leakage current, and the resistivity of TSS samples is an order of magnitude higher than NS samples and reaches the maximum among the already reported. Moreover, the experimental and theoretical results conclude that the TSS samples obtained a stronger intrinsic piezoelectric effect, which increased the strain from 0.13% to 0.19%. The TSS method displayed enormous potential for BF-BT preparation.

Effect of electrokinetic process on in situ stabilization and detoxification of arsenic-contaminated soil with high content of calcium

Owing to the high risk of human exposure to arsenic-contaminated soil, reducing its toxicity is essential. This study used the electrokinetic (EK) process with iron-rich electrodes to synchronously achieve the accumulate, stabilize and detoxify soil arsenic. Changes in arsenic valence, leaching toxicity, and microbial communities related to toxicity were comprehensively considered. The results demonstrated that arsenic was mainly transported toward the anode and accumulated by electromigration owing to the negatively charged arsenic anions under EK conditions. The cathode approaching effectively promote arsenic movement to the anode; the largest As(T) transportation rate of 30.61% was achieved near the cathode (S4). The transportation ratio of As(III) was 1.84 times more than that of As(V). The As(III) content and leaching toxicity of soil arsenic in all treatments decreased after applying the EK process. In particular, the anode approaching effectively elevated the average ratios of soil As(III) oxidation and stabilization to 37.88% and 61.73%, respectively. Correspondingly, the total phospholipid fatty acid content increased substantially after EK treatment and showed a pollution stress elimination effect. The electrokinetic effect can essentially cause highly active and easily migrated arsenic to accumulate near the anode and middle sections. The electric field mediated iron mineralization and stabilized arsenic by oxidizing As(III) and reacting with newly formed iron-rich phases (S). Meanwhile, the electric field regulated the form of soil calcium from CaCO3 to CaSO4 and caused calcium-bound arsenic to change to a more stable form. According to these results, in situ stabilization and detoxification of arsenic-contaminated soil can be realized by the EK process, avoiding stabilizer addition and excavation.

Resistive Mechanisms and Microscopic Electrical Models of Metal Oxide Resistive Memory

Resistive random access memory (RRAM) has emerged as a competitive candidate for nonvolatile memory due to its high speed, low power consumption, simple structure, strong scalability, and CMOS compatibility. Herein, an overview of the electrodes and resistive layers is first discussed according to the material properties and structure of RRAM. Then, recent advances in the resistive mechanisms of RRAM are comprehensively discussed and evaluated based on experimental research results and fundamental physics principles. Then, the electrochemical metallization mechanism and the valence change metallization mechanism are thoroughly studied, as well as the formation, fracture, and resistance state change behavior of conductive filaments in RRAM based on the ion motion type. Then, various electron transport‐dominated resistance mechanisms are reviewed, and electron transport and resistance switching in the space–charge‐limited current mode is thoroughly examined using energy band theory. As a result, a theoretical reference is provided for the development of nanothin‐film devices.

On the Application Potential of Chemically Tailored Metal Oxide and Higher Chalcogenide Nanoparticles for Nanoscale Resistive Switching Devices

Resistive switching (RS) for nonvolatile data storage is a highly relevant field of research. Up to now, RS devices are fabricated via semiconductor processing technologies. This poses the question of whether integration of chemically tailored nanoparticles, either consisting of valence change or phase change materials, can be integrated in nanoelectrode configurations in order to explore their functionality for RS applications. This review discusses the RS properties of such nanoparticles by means of selected examples of both nanoparticle assemblies as well as on the individual particle level. Although this field of research is rather unexplored, it becomes evident that chemically tailored nanoparticles bear great potential for RS applications.

Local symmetry of manganese ions in MgAl2O4 optical nanoceramics

Optically transparent nanoceramics of the composition Mg1-xMnxAl2O4(x = 0.005) with a grain size of 15–40 nm were obtained by thermobaric synthesis in a toroid-type high-pressure chamber in the range P = 4–9 GPa at a temperature of 600 °C. EPR spectroscopy indicates the presence of divalent manganese ions with a characteristic HFS signal. In addition, the signal from F+ centers were registered in the EPR spectrum. It has been established that in the HFS of impurity Mn2+ there is an additional contribution from manganese centers split in the field of rhombic symmetry. Photoluminescence spectroscopy methods revealed the presence of optically active centers based on Mn3+ and Mn4+. With an increase in the synthesis pressure, the conversion of manganese ions localized in octa- and tetra-positions occurs, accompanied by a change in valence. It has been found that some manganese ions with a valence of 2+ can be localized in an octahedral oxygen environment, occupying anti-site positions with reduced point symmetry. The formation of F-type centers, as well as polyvalent manganese centers, was confirmed by the cathodoluminescence method. Changing the synthesis parameters makes it possible to control the ratio of valence manganese ions in the corresponding sites of the spinel crystal lattice, thereby expanding the methodological base of defect nanoengineering.

Self-Assembled Au Nanoelectrodes: Enabling Low-Threshold-Voltage HfO2-Based Artificial Neurons.

Filamentary-type resistive switching devices, such as conductive bridge random-access memory and valence change memory, have diverse applications in memory and neuromorphic computing. However, the randomness in filament formation poses challenges to device reliability and uniformity. To overcome this issue, various defect engineering methods have been explored, including doping, metal nanoparticle embedding, and extended defect utilization. In this study, we present a simple and effective approach using self-assembled uniform Au nanoelectrodes to controll filament formation in HfO2 resistive switching devices. By concentrating the electric field near the Au nanoelectrodes within the BaTiO3 matrix, we significantly enhanced the device stability and reduced the threshold voltage by up to 45% in HfO2-based artificial neurons compared to the control devices. The threshold voltage reduction is attributed to the uniformly distributed Au nanoelectrodes in the insulating matrix, as confirmed by COMSOL simulation. Our findings highlight the potential of nanostructure design for precise control of filamentary-type resistive switching devices.

Redox-induced structural changes in Keggin-type tungstophosphate investigated by high-energy-resolution fluorescence detection X-ray absorption spectroscopy

Redox-active heteropolyanions are widely applied to redox catalysts and electron acceptors and the understanding of their reduced states in solution is essential. In this study, the structural changes of Keggin-type tungstophosphate: [PW12O40]3− induced by electrochemical reduction were investigated using W L3-edge X-ray absorption near-edge structure (XANES) spectra in high-energy-resolution fluorescence detection (HERFD) mode. W L3-edge HERFD-XANES spectra revealed the changes in the electronic state of W species and local structure of {WO6} units, which were negligible in conventional transmittance mode; the peak intensities of the split two white line peaks were switched by the electrochemical reduction accompanied by enlargement of the splitting width. Our findings suggest that the injected electron was delocalized on whole {WO6} units in [PW12O40]3− and that the improved symmetry of {WO6} units compensated for the change in valence due to the injected electron.

Polyvalent s-block elements: A missing link challenges the periodic law of chemistry for the heavy elements.

The Periodic Law of Chemistry is one of the great discoveries in cultural history. Elements behaving chemically similar are empirically merged in groups G of a Periodic Table, each element with G valence electrons per neutral atom, and with upper limit G for the oxidation and valence numbers. Here, we report that among the usually mono- or di-valent s-block elements (G = 1 or 2), the heaviest members (87Fr, 88Ra, 119E, and 120E) with atomic numbers Z = 87, 88, 119, 120 form unusual 5- or 6-valent compounds at ambient conditions. Together with well-reported basic changes of valence at the end of the 6d-series, in the whole 7p-series, and for 5g6f-elements, it indicates that at the bottom of common Periodic Tables, the classic Periodic Law is not as straightforward as commonly expected. Specifically, we predict the feasible experimental synthesis of polyvalent [RaL-n] (n = 4, 6) compounds.

Flattening the curve: COVID-19 induced a decrease in arousal for positive and an increase in arousal for negative words

AbstractIn this study, we compared affective ratings of emotional valence and arousal for 882 Serbian words at three points in time: before the onset of the COVID-19 pandemic (2018), during the COVID-19 lockdown (2020), and after the government measures were abandoned (2022). We did not observe a significant change in average valence or arousal ratings across time points. A more detailed look into the data revealed the change in arousal that was different across the valence values. An increase in their linear correlations and a decrease in the nonlinearity of the GAMM smooth demonstrated that, upon the onset of the COVID-19 pandemic, emotionally negative words elicited higher arousal ratings, whereas emotionally positive words elicited lower arousal ratings. It revealed that our participants became more sensitive to the negative content and less sensitive to the positive content. Our results add to the findings, which showed that the relationship between emotional valence and arousal is a function of contextual factors, which primarily influence the arousal of words.

NiTi-Layered Double Hydroxide@Carbon Nanotube as a Cathode Material for Chloride-Ion Batteries.

Chloride-ion batteries (CIBs) are one of the promising candidates for energy storage due to their low cost, high theoretical energy density and high safety. However, the limited types of cathode materials in CIBs have hindered their development. In this work, a NiTi-LDH@CNT composite is prepared using a reverse microemulsion method and applied in CIBs for the first time. The specific surface area and the pore volume of the obtained NiTi-LDH@CNT composites can reach 266 m2 g-1 and 0.42 cm3 g-1, respectively. Electrochemical tests indicate that the composite electrode delivers a reversible specific capacity of 69 mAh g-1 after 150 cycles at a current density of 100 mA g-1 in 0.5 M PP14Cl/PC electrolyte. Ni2+/Ni3+ and Ti3+/Ti4+ valence changes during electrochemical cycling are demonstrated by X-ray photoelectron spectroscopy (XPS), while reversible migration of Cl- is revealed by ex-situ EDS and ex-situ XRD. The stable layered structure and abundant valence changes of the NiTi-LDH@CNT composite make it an exceptional candidate as a cathode material for CIBs.

The Magnesium Insertion Effects into P2-Type Na2/3 Ni1/3 Mn2/3 O2.

A Mg-cell with P2-Na2/3 Ni1/3 Mn2/3 O2 layered oxide cathode provides novel reaction mechanism not observed in Na-cells. The sodium/vacancy ordering and Jahn-Teller effects are suppressed with the insertion of magnesium ion. The Mg-cell exhibits different features when operating between 4.5 and 0.15V and 3.9 and 0.15VversusMg2+ /Mg. To analyze the structural and chemical changes during Mg insertion,the cathode is first charged to obtain the Na1/3 Ni1/3 Mn2/3 O2 compound, which is formally accompanied by an oxidation from Ni2+ to Ni3+ . As structure models Mg1/6 Na1/3 Ni1/3 Mn2/3 O2 and Mg1/12 Na1/2 Ni1/3 Mn2/3 O2 are utilized with a large × supercell. On discharge, the Mg-cell exhibits a multistep profile which reaches ≈100mAhg-1 with the valence change from Ni3+ to Ni2+ . Such profile is quite different from its sodium counterpart (230mA h g-1 ) which exhibits the sodium/vacancy ordering and deleterious presence of Mn3+ . Depending on how the two interlayer spacings are filled by Na and Mg the "staged," "intermediated," and "average" models are analyzed for Mgy Na8 Ni8 Mn16 O48 supercell. This fact suggests differences in the cell performance when Mg is used as counter electrode providing some tips to improve the structure engineering on cathode materials.

Ternary Łukasiewicz logic using memristive devices

Memristive devices based on the Valence Change Mechanism (VCM) are promising devices for storage class memory, neuromorphic computing and logic-in-memory (LIM) applications. They are suited for such a wide range of applications, due to their possibility for extreme dense integration, low power consumption and multilevel capabilities. Through LIM concepts, Boolean logic operations can be performed directly in memory. In many of these concepts, the resistance state of the device is interpreted as the logical input and output of the logic function, which is why these concepts are called ‘stateful’ logic. Most of the proposed ideas, however, are defined based on only binary switching VCM devices and neglect their multi-level capabilities. Extending LIM concepts towards multinary logic, e.g. a ternary logic, would increase the data density inside the memory array and reduce the number of devices required to perform a certain operation. In this work, we discuss two possibilities of realizing a ternary logic based on the analog switching in the RESET or in the SET direction. For both directions we verify the logic functionality by showing the basic operations of implication, negation and false operation, which together form a functionally complete logic. Additionally, for both switching directions, we discuss a 41-Trit ( 64-Bit) addition. For all investigations the physics-based compact model JART VCM v1b is used, which has been verified on the RESET direction multilevel properties of the TaO devices.

Correlation between Electronic Structure, Microstructure, and Switching Mode in Valence Change Mechanism Al2O3/TiOx‐Based Memristive Devices

AbstractMemristive devices with valence change mechanism (VCM) show promise for neuromorphic data processing, although emulation of synaptic behavior with analog weight updates remains a challenge. Standard filamentary and area‐dependent resistive switching exhibit characteristic differences in the transition from the high to low resistance state, which is either abrupt with inherently high variability or gradual and allows quasi‐analog operation. In this study, the two switching modes are clearly correlated to differences in the microstructure and electronic structure for Pt/Al2O3/TiOx/Cr/Pt devices made from amorphous layers of 1.2 nm Al2O3 and 7 nm TiOx by atomic layer deposition. For the filamentary mode, operando spectromicroscopy experiments identify a localized region of ≈50 nm in diameter of reduced titania surrounded by crystalline rutile‐like TiO2, highlighting the importance of Joule heating for this mode. In contrast, both oxide layers remain in their amorphous state for the interfacial mode, which proves that device temperature during switching stays below 670 K, which is the TiO2 crystallization temperature. The analysis of the electronic conduction behavior confirms that the interfacial switching occurs by modulating the effective tunnel barrier width due to accumulation and depletion of oxygen vacancies at the Al2O3/TiOx interface. The results are transferable to other bilayer stacks.

Correlated microstructure and magnetic properties of Ce-substituted sintered Nd-Fe-B magnets: Role of various phases at triple junctions and grain boundaries

We report the investigation of the microstructure and magnetic properties of a sintered (Nd,Pr)30.3-xCexFebalM1.4B0.92 (in wt.%, xCe = 0) magnet with varying Ce substitutions (xCe = 2, 4, 6, 8). Structure analysis revealed a lattice contraction with Ce addition of up to xCe < 6, and the further increase in the Ce content to 6 wt.% led to a suppression in lattice contraction. The magnetic properties showed a notable deviation from the linear trend in coercivity and remanent magnetization for the magnets with Ce content of xCe > 4. The detailed microstructure analysis revealed that the magnets with xCe > 4 exhibit the REFe2 phase at the expense of a reduced mass fraction of a REOx phase, suggesting that the generation of the REFe2 phase suppressed the formation of the REOx phase. The site occupancy and valence state of Ce, analyzed using atomic-resolution energy dispersive spectroscopy and electron energy loss spectrometry, established that the suppressed lattice contraction is accompanied by a preferential site occupation of Ce from a random case to a 4f-centric site, along with a valence change from a mixed state to a trivalent state, inferring that Ce3+ ions are rejected from the 4g site of the RE2Fe14B phase to occupy the 4f site with a subsequently reduced number of Ce4+ ions due to the magneto-volume and chemical bonding effects. These findings display a new phase formation pathway of the REFe2 phase from a valence state perspective, i.e., the part of Ce4+ ions is considered to be rejected from the RE2Fe14B phase to nucleate the REFe2 phase, which is later found to separate chemically into RE-rich and Fe-rich regions. Further, a bi-layer interfacial structure, composed of Fe-rich and RE-rich layers, is formed in between the RE2Fe14B and the REFe2 phases due to the weak wettability of the REFe2 phase and a slightly positive mixing enthalpy between Ce and Fe. The deviation in magnetic properties is discussed based on the structural and microstructural results. These findings provide structural insight for developing low-cost and high-performance Ce-substituted Nd-Fe-B magnets.