Composition and Surface Morphology Invariant High On–Off Ratio from an Organic Memristor

Material composition plays a crucial role in the device performance; thus, nonvolatile memory devices from a small molecule named 5-mercapto-1-methyl tetrazole (MMT) in an insulating polymer matrix of poly­(4-vinyl pyridine) (PVP) were fabricated. The composition of the active material in the device was varied to observe its influence on the device’s electronic properties. The device with a more or less weight ratio of MMT has a much smoother surface morphology, whereas when the contributions of MMT and PVP were equal, the average surface roughness increased. However, the maximum on–off current ratio for all the devices is 105, suggesting that the MMT molecule does not show any change in its characteristic properties when surrounded by an insulating material. When the device was fabricated without the polymer matrix, the surface morphology of the device completely changed as it was filled with large holes. These holes provide short-circuited pathways for the current by forming the direct metal contact between the top and bottom electrodes. The carrier transport through these devices follows various conduction mechanisms. Some of the dominating conduction mechanisms are direct tunneling and trap-free and trap-assisted space–charge-limited conduction

Redox Switching Behavior in Resistive Memory Device Designed Using a Solution-Processable Phenalenyl-Based Co(II) Complex: Experimental and DFT Studies

We herein report a novel square-planar complex [CoIIL], which was synthesized using the electronically interesting phenalenyl-derived ligand LH2 = 9,9′-(ethane-1,2-diylbis­(azanediyl))­bis­(1H-phenalen-1-one). The molecular structure of the complex is confirmed with the help of the single-crystal X-ray diffraction technique. [CoIIL] is a mononuclear complex where the Co­(II) ion is present in the square-planar geometry coordinated by the chelating bis-phenalenone ligand. The solid-state packing of [CoIIL] complex in a crystal structure has been explained with the help of supramolecular studies, which revealed that the π···π stacking present in the [CoIIL] complex is analogous to the one present in tetrathiafulvalene/tetracyanoquinodimethane charge transfer salt, well-known materials for their unique charge carrier interfaces. The [CoIIL] complex was employed as the active material to fabricate a resistive switching memory device, indium tin oxide/CoIIL/Al, and characterized using the write-read-erase-read cycle. The device has interestingly shown a stable and reproducible switching between two different resistance states for more than 2000 s. Observed bistable resistive states of the device have been explained by corroborating the electrochemical characterizations and density functional theory studies, where the role of the CoII metal center and π-conjugated phenalenyl backbone in the redox-resistive switching mechanism is proposed

CdSe Quantum Dot-Based Nanocomposites for Ultralow-Power Memristors

The explosion in digital communication with the huge amount of data and the internet of things (IoT) led to the increasing demand for data storage technology with faster operation speed, high-density stacking, nonvolatility, and low power consumption for saving energy. Metal chalcogenide-based quantum dots (QDs) show excellent nonvolatile resistive memory behavior owing to their tunable electronic states and control in trap states by passivating the surface with different ligands. Here, we synthesized high-quality colloidal monodispersed CdSe QDs by the hot injection method. The CdSe QDs were blended with an organic polymer, poly­(4-vinylpyridine) (PVP), to fabricate an Al\CdSe-PVP\Al device. Our device shows excellent bipolar nonvolatile resistive random access memory (RRAM) switching behavior with a high current ON/OFF ratio (ION/OFF) of 6.1 × 104, and it consumes ultralow power. The charge trapping and detrapping in the potential well formed by the CdSe QD and PVP combination result in resistive switching. This CdSe-PVP-based resistive random access memory (RRAM) device with a high ION/OFF, ultrafast switching speed, high endurance, low operating voltage, and long retention period can be used as a high-performance and ultralow-power memristive device

Molecular Memory Switching Device Based on a Tetranuclear Organotin Sulfide Cage [(RSn^IV)₄(μ-S)₆]·2CHCl₃·4H₂O (R = 2‑(Phenylazo)phenyl): Synthesis, Structure, DFT Studies, and Memristive Behavior

RSnCl3 (R = 2-phenylazophenyl) on reaction with Na2S·9H2O in a 1:1 mixture of acetone and methanol afforded a tetranuclear monoorganotin sulfide cage [(RSnIV)4(μ-S)6]·2CHCl3·4H2O (R = 2-phenylazophenyl) (1). Complex 1 crystallizes in the monoclinic space group P2/n. The molecular structure of 1 contains five-coordinate tin centers in distorted trigonal bipyramidal geometry. Complex 1 is monoorganotin sulfide derivative having a tetranuclear double-decker cage-like structure. In 1, four tin centers are bridged by a μ2-S unit affording a ubiquitous Sn–S–Sn motif among monoorganotin sulfide compounds. In addition, each tin also has intramolecular coordination to a nitrogen atom of a 2-phenylazophenyl substituent (N → Sn). The DFT calculation suggests that the complex 1 involves mainly ligand based transitions. The complex 1 based device was studied for its electrical behavior and was found to show stable, reproducible memristive behavior with an on–off ratio of 103, which suggests that the complex 1 is a promising material for memory device applications

Scanning Tunneling Microscopy Investigation of Synaptic Behavior in AgInS₂ Quantum Dots: Effect of Ion Transport in Neuromorphic Applications

Scanning tunneling microscopy (STM) is a powerful technique for investigating the nanoscale properties of functional materials. Additionally, scanning tunneling spectroscopy (STS) facilitates the determination of the local density of states (LDOS) within the material. In this study, we present an exploration of the resistive switching (RS) properties and neuromorphic computing capabilities of individual AgInS2 quantum dots, utilizing STM and STS techniques. By examining the material’s bandgap and its temperature dependence, we uncover a nonlinear variation below the Debye temperature and a linear trend at higher temperatures. Moreover, STS measurements demonstrate changes in the conducting states induced by localized pulses, further confirming the unique characteristics of the quantum dots. The experimental devices constructed by using these quantum dots effectively replicate the RS properties observed at the nanoscale. To assess the neuromorphic application of the devices, pulse transient measurements simulating the learning and forgetting processes were conducted. The gradual set and reset processes successfully mimic the information retention and erasure capabilities essential for neuromorphic computing. Notably, the resistive switching mechanism in these devices is attributed to localized ionic transport, which highlights the significant involvement of ionic species in the observed RS behavior. The outcomes of this study contribute to the fundamental understanding of RS properties in single AgInS2 quantum dots and offer valuable insights into their potential applications in neuromorphic computing

Anomalous Seebeck Coefficient in Sulfur-Substituted Bismuth Telluride

Seebeck coefficient is determined by the asymmetry of density of states across Fermi level and their occupancy. These parameters are, in turn, essentially governed by the band structure and carrier concentration of the materials. In the present study, sulfur was substituted in versatile bismuth telluride thermoelectric material in subatomic ratio. The successful substitution in the resulting compounds, viz., Bi2Te2.85S0.15 and Bi2Te2.7S0.3, was confirmed using X-ray diffraction with Rietveld refinement and X-ray photoelectron spectroscopy. High resolution-transmission electron microscopy showed an ultrathin platy morphology in nanopowder and a randomly oriented high grain density at nanometer scale in the consolidated pellet. The sulfur substitution resulted in polarity reversal and enhanced value of the Seebeck coefficient at subzero temperatures in the bismuth telluride matrix. This anomaly in the sign and value of the Seebeck coefficient was analyzed using temperature-dependent Hall measurement, scanning tunneling spectroscopy, and first-principle computation using density functional theory coupled with Boltzmann transport theory. The analysis showed that the band-structure variations due to the localized polarity induced by the higher-electronegativity sulfur atom at TeI sites and suppression of antisite defects may be the prominent reasons for causing this anomaly. This study may also pave the way for the design of thermoelectric materials for enhanced performance in subzero temperature range for solid-state refrigeration application