Aliovalent Ta-Doping-Engineered Oxygen Vacancy Configurations for Ultralow-Voltage Resistive Memory Devices: A DFT-Supported Experimental Study.

Alteration of transport properties of any material, especially metal oxides, by doping suitable impurities is not straightforward as it may introduce multiple defects like oxygen vacancies (Vo) in the system. It plays a decisive role in controlling the resistive switching (RS) performance of metal oxide-based memory devices. Therefore, a judicious choice of dopants and their atomic concentrations is crucial for achieving an optimum Vo configuration. Here, we show that the rational designing of RS memory devices with cationic dopants (Ta), in particular, Au/Ti1-xTaxO2-δ/Pt devices, is promising for the upcoming non-volatile memory technology. Indeed, a current window of ∼104 is realized at an ultralow voltage as low as 0.25 V with significant retention (∼104 s) and endurance (∼105 cycles) of the device by considering 1.11 at % Ta doping. The obtained device parameters are compared with those in the available literature to establish its excellent performance. Furthermore, using detailed experimental analyses and density functional theory (DFT)-based first-principles calculations, we comprehend that the meticulous presence of Vo configurations and the columnar-like dendritic structures is crucial for achieving ultralow-voltage bipolar RS characteristics. In fact, the dopant-mediated Vo interactions are found to be responsible for the enhancement in local current conduction, as evidenced from the DFT-simulated electron localization function plots, and these, in turn, augment the device performance. Overall, the present study on cationic-dopant-controlled defect engineering could pave a neoteric direction for future energy-efficient oxide memristors.

Solution-processed light-induced multilevel non-volatile wearable memory device based on CsPb2Br5 perovskite.

Despite the recent advancements in memory devices, the quest for building materials with low power consumption is still on, with the ultimate focus on the durability of the system and reliability and reproducibility of its performance. Halide perovskites (HPs), which have several intriguing photoelectrical traits, have recently been utilized for memory applications; one of the highlights of these materials is the ionic-motion-based fast switching in their crystal structure. In this work, a CsPb2Br5 film is applied as a switching layer to implement memory devices with a flexible Al/CsPb2Br5/ITO-PET structure. The device exhibits a pronounced bipolar resistive switching (RS) characteristic at low operating voltage. The as-fabricated flexible device presented RS features with no initial forming process, concentrated distributions of high and low resistance states (HRS and LRS), uniform switching, endurance over 100 cycles, and a long retention time of 103 s with a high on/off ratio of around 102. Multilevel data-storage capability was also observed via subtle control of the compliance current (CC). Considering the current demand for smart, wearable, and flexible electronic gadgets, the current-voltage (I-V) characteristics of the as-fabricated all-inorganic halide-perovskite-based memory device were further explored under different bending conditions to determine its electrical reliability and mechanical stability. This flexible device exhibited no discernible difference in device performance under flat or bent conditions, and the performance remained nearly the same even after 500 bending cycles. In addition to control of the RS effect of the device using an electrical field, the performance of the device under light was also explored. Blue light modulates the resistive states by regulating the condition of photo-generated electron-hole pairs, and the SET and RESET voltages are changed from 2.34 to 2.14 V and from -2.04 to -1.90 V, respectively. The observed RS behavior is explained on the basis of the creation and partial annihilation of conductive multifilaments, which is dominated by the migration of bromine ions and their associated vacancies in the HP layer. We believe that this work will offer a new context to understand the intrinsic characteristics of HPs for RS applications at low voltage and validate their potential in the design of next-generation stable and nonvolatile memory devices for future flexible electronic systems.

Nonvolatile resistive switching and synaptic characteristics of lead-free all-inorganic perovskite-based flexible memristive devices for neuromorphic systems.

Recently, several types of lead halide perovskites have been actively researched for resistive switching (RS) memory or artificial synaptic devices due to their current-voltage hysteresis along with the feasibility of fabrication, low-temperature processability and superior charge mobility. However, the toxicity and environmental pollution potential of lead halide perovskites severely restrict their large-scale commercial prospects. In the present work, the environmentally friendly and uniform CsSnCl3 perovskite films are introduced to act as an active layer in the flexible memristors. Ag/CsSnCl3/ITO devices demonstrate bipolar RS with excellent electrical properties such as forming free characteristics, good uniformity, low operating voltages, a high ON/OFF ratio (102) and a long retention time (>104 s). The RS mechanism has been well explained in the outline of electric field-induced formation and rupture of Ag filaments in the CsSnCl3 layer. The metallic nature of the conducting filament has been further confirmed by temperature-dependent variation of low and high resistance states. Additionally, various pulse measurements have been carried out to mimic some of the basic synaptic functions including postsynaptic current, paired-pulse facilitation, long-term potentiation and long-term depression under normal as well as bending conditions. Our work provides the opportunity for exploring artificial synapses based on lead-free halide perovskites for the development of next-generation flexible electronics.

Incorporation of V2O5 nanorods into perovskite photodetectors as an alternative approach to enhance device performance: a step towards stability against ambient water species.

Numerous reports have been made in the literature on the usability of a halide perovskite, namely methyl ammonium lead iodide (CH3NH3PbI3), as a light harvesting material. Suitable optimization in material composition and synthesis has led to an improvement in device performance. However, the susceptibility of CH3NH3PbI3 towards atmospheric water is an alarming issue which hinders its long-term application in day-to-day life. Herein, low temperature synthesized V2O5 nanorods are incorporated into the CH3NH3PbI3 matrix to facilitate carrier dynamics by interface engineering. The optimized hybrid sample exhibits a much improved and stable photo-response behaviour than the pristine building blocks. Additionally, the modified perovskite shows improved hydrophobicity in the form of small-scale surface undulations due to the incorporation of V2O5 nanorods into it. This way, we have solved two problems with one single action, firstly by improving the device performance and secondly by stabilizing the sample against ambient water species.

Age related and hypothyroidism related changes on the stoichiometry of neurofilament subunits in the developing rat brain.

Hypothyroidism in the developing brain results in progressive intraneuronal accumulation of neurofilament (NF) proteins in the proximal hillock regions of axons, analogous to the pathological intraneuronal accumulation of NF in common neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis. A preferential decline in the expression of the light chain of NF occurs in all the three diseases leading to an absolute change in stoichiometry of the NF subunits. Using the developing hypothyroid rat cerebra as a model, we have tried to elucidate if age or hypothyroidism causes a change in the stoichiometry or molar ratio of the NF subunits which could be responsible for their aberrant intraneuronal accumulation. Western blotting and chemiluminescence assay of cytoskeletal preparations from normal and hypothyroid developing rats at postnatal days 5 (PND5), PND15 and PND25 shows that in the normal cerebra, the expression of NFL and NFM were abundant during the first 2 weeks, corresponding to the onset of axonal outgrowth and synaptogenesis, whereas that of NFH was predominant during the second and third weeks corresponding to the period of maturation of synapses, axonal caliber and transport processes. These results show that consistent with the requirement for neuronal differentiation during synaptogenesis, the molar ratios NFH:NFM:NFL changed significantly from 1:3:9 at PND5 to 1:2:6 at PND25. Hypothyroidism caused a 40-60% decline in the expression of all three subunits. However, at all three ages examined, differences in the molar ratios of the NF subunits between normal and hypothyroid cerebra were insignificant suggesting that factors other than alteration in the stoichiometry of NF subunits are associated with their aberrant intraneuronal aggregation.

Thyroid hormone promotes glutathione synthesis in astrocytes by up regulation of glutamate cysteine ligase through differential stimulation of its catalytic and modulator subunit mRNAs.

To elucidate how thyroid hormone (TH) modulates glutathione (GSH) biogenesis in developing brain, the effect of the hormone on the activity of glutamate cysteine ligase (GCL), previously known as gamma-glutamyl synthetase (gamma-GCS), has been investigated. Hypothyroidism in developing rat brain declined the activity of GCL. Conversely, administration of TH to hypothyroid rats elicited an increase in the activity of the enzyme. TH treatment of astrocytes resulted in a rapid increase in the level of GSH and this up regulation was completely inhibited by L-buthionine S,R-sulfoximine. Kinetics of induction of GCL by TH in astrocytes were closely parallel to that of GSH and the induction was sensitive to both cycloheximide and actinomycin D. Quantitative RT-PCR analysis revealed that astrocytes contained a basal excess of GCLC (catalytic subunit of GCL) mRNA, relative to GCLM (modulator subunit of GCL) mRNA, the ratio being 4:1. TH treatment led to a differential increase in the expression of these two mRNAs, which resulted in a decline in the stoichiometric ratio of GCLC:GCLM mRNA that may favor holoenzyme formation with enhanced catalytic efficiency. TH treatment improved the antioxidative defense in astrocytes by enhancing their hydrogen peroxide scavenging ability with a decrease in peroxide half-life from 7.4 to 4.2 min. The overall results suggest that TH plays a positive role in maintaining GSH homeostasis in astrocytes and in protecting the brain from oxidative stress.

Thyroid hormone stimulates gamma-glutamyl transpeptidase in the developing rat cerebra and in astroglial cultures.

Hypothyroidism in the developing rat brain is associated with enhanced oxidative stress, one of the earliest manifestations of which is a decline in the level of glutathione (GSH). To investigate the role of thyroid hormone (TH) on GSH homeostasis, the effect of TH on gamma-glutamyl transpeptidase (gammaGT), the key enzyme involved in the catalysis of GSH, was studied. Hypothyroidism declined the specific activity of cerebral gammaGT at all postnatal ages examined (postnatal day 1-20) with a maximum inhibition of 42% at postnatal day 10. Intraperitoneal injection of TH to 15-day-old rat pups increased the specific activity of gammaGT by 25-30% within 4-6 hr. Treatment of primary cultures of astrocytes by TH also enhanced the specific activity of gammaGT by 30-40% within 4-6 hr. The induction of gammaGT by TH was blocked by actinomycin D or cycloheximide. gammaGT is an ectoenzyme that is normally involved in the catabolism of GSH released by astrocytes. In the presence of the gammaGT-inhibitor, acivicin, GSH released in the culture medium of astrocytes increased linearly for at least 6 hr and TH had no effect on this accumulation pattern. In the absence of acivicin, GSH content of the medium from TH-treated cells was significantly lower than that of untreated controls due to activation of gammaGT by TH and a faster processing of GSH. Because the products of gammaGT reaction are putative precursors for neuronal GSH, the activation of gammaGT by TH may be conducive to GSH synthesis in neurons and their protection from oxidative stress.