Conductive Filament Research Articles

Ultralow Operating Voltage Nb2O5-Based Multilevel Resistive Memory with Direct Observation of Cu Conductive Filament

Amorphous Nb2O5 is proposed as the resistive switching layer for conductive bridge random access memory. The Nb2O5 device is fabricated with the top Cu and bottom Au electrodes, which is fabricated at a fully room temperature process, exhibiting excellent resistive switching performance with an on/off ratio of up to 105 and ultralow operating voltage. The multilevel resistive switching characteristic can be rationally demonstrated by changing the compliance current in the Nb2O5 device. Each level of the resistance state is uniform with distinguishable read windows. The reset voltage reduces significantly as the compliance current decreases, which is merely 0.002 V when the compliance current is 10–5 A. An approach of the broken-down device is elaborately developed to firmly identify the migration of Cu and the existence of a Cu conductive filament and to explain the ultralow operating voltage. Amorphous Nb2O5 devices exhibit excellent resistive switching performance, proving it is a promising material for memory applications.

A versatile 3D printed multi-electrode cell for determination of three COVID-19 biomarkers

3D-printing has shown an outstanding performance for the production of versatile electrochemical devices. However, there is a lack of studies in the field of 3D-printed miniaturized settings for multiplex biosensing. In this work, we propose a fully 3D-printed micro-volume cell containing six working electrodes (WEs) that operates with 250 μL of sample. A polylactic acid/carbon black conductive filament (PLA/CB) was used to print the WEs and subsequently modified with graphene oxide (GO), to support protein binding. Cyclic voltammetry was employed to investigate the electrochemical behaviour of the novel multi-electrode cell. In the presence of K₃[Fe(CN)₆], PLA/CB/GO showed adequate peak resolution for subsequent label-free immunosensing. The innovative 3D-printed cell was applied for multiplex voltammetric detection of three COVID-19 biomarkers as a proof-of-concept. The multiple sensors showed a wide linear range with detection limits of 5, 1 and 1 pg mL−1 for N-protein, SRBD-protein, and anti-SRBD, respectively. The sensor performance enabled the selective sequential detection of N protein, SRBD protein, and anti-SRBD at biological levels in saliva and serum. In summary, the miniaturized six-electrode cell presents an alternative for the low-cost and fast production of customizable devices for multi-target sensing with promising application in the development of point-of-care sensors.

Sprayed FeWO4 thin film-based memristive device with negative differential resistance effect for non-volatile memory and synaptic learning applications

Resistive switching (RS) memories have attracted great attention as promising solutions to next-generation non-volatile memories and computing technologies because of their simple device configuration, high on/off ratio, low power consumption, fast switching, long retention, and significant cyclic stability. In this work, uniform and adherent iron tungstate (FeWO4) thin films were synthesized by the spray pyrolysis method with various precursor solution volumes, and these were tested as a switching layer for the fabrication of Ag/FWO/FTO memristive devices. The detailed structural investigation was done through various analytical and physio-chemical characterizations viz. X-ray diffraction (XRD) and its Rietveld refinement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. The results reveal the pure and single-phase FeWO4 compound thin film formation. Surface morphological study shows the spherical particle formation having a diameter in the range of 20 to 40 nm. The RS characteristics of the Ag/FWO/FTO memristive device demonstrate non-volatile memory characteristics with significant endurance and retention properties. Interestingly, the memory devices show stable and reproducible negative differential resistance (NDR) effects. The in-depth statistical analysis suggests the good operational uniformity of the device. Moreover, the switching voltages of the Ag/FWO/FTO memristive device were modeled using the time series analysis technique by utilizing Holt’s Winter Exponential Smoothing (HWES) approach. Additionally, the device mimics bio-synaptic properties such as potentiation/depression, excitatory post-synaptic current (EPSC), and spike-timing-dependent plasticity (STDP) learning rules. For the present device, the space-charge-limited current (SCLC) and trap-controlled-SCLC effects dominated during positive and negative bias I–V characteristics, respectively. The RS mechanism dominated in the low resistance state (LRS), and the high resistance state (HRS) was explained based on the formation and rupture of conductive filament composed of Ag ions and oxygen vacancies. This work demonstrates the RS in the metal tungstate-based memristive devices and demonstrates a low-cost approach for fabricating memristive devices.

Additive manufactured microfluidic device for electrochemical detection of carbendazim in honey samples

The use of pesticides is one of the primary means of protecting crops. However, this class of compounds can be highly toxic to the environment and humans. In this aspect, developing analytical devices for monitoring pesticides such as carbendazim in food sources is of paramount importance. Thus, the present work presents the application of a paper-based microfluidic device coupled to an additive manufactured platform and electrochemical sensors (produced from lab-made conductive filaments based on carbon black) for the sustainable detection of carbendazim in honey samples. The microfluidic system presented satisfactory results for the analysis of carbendazim, in the linear range from 0.5 to 40.0 µmol L − 1 with a LOD of 0.09 µmol L − 1. The recovery test performed in honey samples showed values ranging between 92.4 and 108.8%. According to the results, the proposed microfluidic device demonstrated a good potential for detecting carbendazim in real samples, with the advantages of employing sustainable and renewable materials.

Microbial nanowires: type IV pili or cytochrome filaments?

A dynamic field of study has emerged involving long-range electron transport by extracellular filaments in anaerobic bacteria, with Geobacter sulfurreducens being used as a model system. The interest in this topic stems from the potential uses of such systems in bioremediation, energy generation, and new bio-based nanotechnology for electronic devices. These conductive extracellular filaments were originally thought, based upon low-resolution observations of dried samples, to be type IV pili (T4P). However, the recently published atomic structure for the T4P from G. sulfurreducens, obtained by cryo-electron microscopy (cryo-EM), is incompatible with the numerous models that have been put forward for electron conduction. As with all high-resolution structures of T4P, the G. sulfurreducens T4P structure shows a partial melting of the α-helix that substantially impacts the aromatic residue positions such that they are incompatible with conductivity. Furthermore, new work using high-resolution cryo-EM shows that conductive filaments thought to be T4P are actually polymerized cytochromes, with stacked heme groups forming a continuous conductive wire, or extracellular DNA. Recent atomic structures of three different cytochrome filaments from G. sulfurreducens suggest that such polymers evolved independently on multiple occasions. The expectation is that such polymerized cytochromes may be found emanating from other anaerobic organisms.

Understanding the Resistive Switching Mechanism of 2-D RRAM: Monte Carlo Modeling and a Proposed Application for Reliability Research

The 2-D materials have become promising candidates for resistive random access memory (RRAM) devices as more unique resistive switching (RS) characteristics have recently been revealed. However, endurance is a major challenge for industrialization. Unlike the well-developed and recognized conductive filament (CF) model for oxide-based RRAM, the RS mechanism for 2-D RRAM is not well understood. In this article, we first review the dissociation–diffusion–adsorption (DDA) model and the cluster model proposed in previous works on monolayer 2-D RRAM devices. The use of a Monte Carlo (MC) simulator for multilayer 2-D RRAM devices to expand the application of DDA and cluster models is then discussed. A simulator was designed to provide an intuitive physical view by visualizing the stochastic behaviors of the RS process in multilayer 2-D RRAM devices. By comparing the simulated results with experimental data, the endurance characteristic was found to be mainly determined by the formation and collapse of an effective switching layer. We also found that the thickness of the effective switching layer is independent of the total thickness of the multilayer 2-D material and the initial status of the device, which is consistent with the experimental observations. The model and results discussed in this work provide additional insights and guidance for improving the reliability of 2-D RRAM devices.

Simultaneously elevating the resistive switching performance and thermal/irradiative stabilities of biomemorizer based on twisted carboxylated multi-walled carbon nanotube-chitosan composites

Bio-resistive memory devices with good environmental robustness are important for the next generation of biodegradable electronic devices. In this field, the facile achievement of both resistive switching performance and environmental robustness is still challengeable. Herein, resistive switching active oxidized multi-walled carbon nanotube (O-MWCNT) has been doped into chitosan (CS) to produce twisted O-MWCNT-CS biocomposites based on strong -COO-/NH3+ electrostatic interaction. The FTO/O-MWCNT-CS(8%)/Ag biomemorizer can exhibit good resistive switching performance with high ON/OFF ratio (104.51) and low setting voltage (0.86 V). Besides, this biomemorizer possesses good environmental robustness with thermal (200 °C) and ionizing irradiation tolerance (UV exposure for 72 h). Ag conductive filament formation and rupture account for the resistive switching behavior. Specially, the conduction pathways provided by O-MWCNT and its good radical scavenging ability are the reason for the facile realization of resistive switching performances even under large layer thickness. The facile carboxylation on MWCNT and robust O-MWCNT-CS biocompoiste together with good environmental robustness render the promising applications as green, stable and biodegradable electronic devices.

Self-Compliant Threshold Switching Devices with High On/Off ratio by Control of Quantized Conductance in Ag Filaments

Threshold switches based on conductive metal bridge devices are useful as selectors to block sneak leakage paths in memristor arrays used in neuromorphic computing and emerging nonvolatile memory. We demonstrate that control of Ag-cation concentration in Al2O3 electrolyte and Ag filament size and density play an important role in the high on/off ratio and self-compliance of metal-ion-based volatile threshold switching devices. To control Ag-cation diffusion, we inserted an engineered defective graphene monolayer between the Ag electrode and the Al2O3 electrolyte. The Ag-cation migration and the Ag filament size and density are limited by the pores in the defective graphene monolayer. This leads to quantized conductance in the Ag filaments and self-compliance resulting from the formation and dissolution of the Ag conductive filament.

Electrochemical Ionic Synapses: Progress and Perspectives.

Artificial neural networks based on crossbar arrays of analog programmable resistors can address the high energy challenge of conventional hardware in artificial intelligence applications. However, state-of-the-art two-terminal resistive switching devices based on conductive filament formation suffer from high variability and poor controllability. Electrochemical ionic synapses are three-terminal devices that operate by electrochemical and dynamic insertion/extraction of ions that control the electronic conductivity of a channel in a single solid-solution phase. They are promising candidates for programmable resistors in crossbar arrays because they have shown uniform and deterministic control of electronic conductivity based on ion doping, with very low energy consumption. Here, the desirable specifications of these programmable resistors are presented. Then, an overview of the current progress of devices based on Li+ , O2- , and H+ ions and material systems is provided. Achieving nanosecond speed, low operation voltage (≈1V), low energy consumption, with complementary metal-oxide-semiconductor compatibility all simultaneously remains a challenge. Toward this goal, a physical model of the device is constructed to provide guidelines for the desired material properties to overcome the remaining challenges. Finally, an outlook is provided, including strategies to advance materials toward the desirable properties and the future opportunities for electrochemical ionic synapses.

Composite filament with super high effective thermal conductivity

The low thermal conductivity of the clothing filament constrains the efficiency of the thermal management garments. In this paper, a super thermal conductive composite filament (STCCF) was proposed for body thermal management, which was fabricated by hollow Teflon filaments incorporated with hollow copper filaments in the heating and cooling sections. By introducing R134a into the hollow filament, a thermally driven flow was generated due to the pressure imbalance caused by the evaporation and condensation of R134a, which enhanced the heat transport capability significantly. The apparent thermal conductivity of the STCCF increased with the increasing heat power, being up to 5042 W/(m·K) at the heating temperature of 49 °C. Furthermore, bending only had a small effect on the thermal performance of STCCF due to the flow pattern transition in the curved section. Moreover, the heat transfer capacity could be enhanced by a lower temperature difference between the hot and cool sections under constant heat power. Therefore, the proposed STCCF has great potential for human body thermal management, which could contribute to human health and thermal comfort as well as building energy saving.

Graphene/MoS2/SiOx memristive synapses for linear weight update

Memristors for neuromorphic computing have gained prominence over the years for implementing synapses and neurons due to their nano-scale footprint and reduced complexity. Several demonstrations show two-dimensional (2D) materials as a promising platform for the realization of transparent, flexible, ultra-thin memristive synapses. However, unsupervised learning in a spiking neural network (SNN) facilitated by linearity and symmetry in synaptic weight update has not been explored thoroughly using the 2D materials platform. Here, we demonstrate that graphene/MoS2/SiOx/Ni synapses exhibit ideal linearity and symmetry when subjected to identical input pulses, which is essential for their role in online training of neural networks. The linearity in weight update holds for a range of pulse width, amplitude and number of applied pulses. Our work illustrates that the mechanism of switching in MoS2-based synapses is through conductive filaments governed by Poole-Frenkel emission. We demonstrate that the graphene/MoS2/SiOx/Ni synapses, when integrated with a MoS2-based leaky integrate-and-fire neuron, can control the spiking of the neuron efficiently. This work establishes 2D MoS2 as a viable platform for all-memristive SNNs.

Evaluation of Cellulose-MXene Composite Hydrogel Based Bio-Resistive Random Access Memory Material as Mimics for Biological Synapses.

We report a memory device based on organic-inorganic hybrid cellulose-Ti3C2TX MXene composite hydrogel (CMCH) as a switching layer sandwiched between Ag top and FTO bottom electrodes. The device (Ag/CMCH/FTO) was fabricated by a simple, solution-processed route and exhibits reliable and reproducible bipolar resistive switching. Multilevel switching behavior was observed at low operating voltages (±0.5 to ±1 V). Furthermore, the capacitive-coupled memristive characteristics of the device were corroborated with electrochemical impedance spectroscopy and this affirmed the filamentary conduction switching mechanism (LRS-HRS). The synaptic functions of the CMCH-based memory device were evaluated, wherein potentiation/depression properties over 8 × 103 electric pulses were observed. The device also exhibited spike time-dependent plasticity-based symmetric Hebbian learning rule of a biological synapse. This hybrid hydrogel is expected to be a potential switching material for low-cost, sustainable, and biocompatible memory storage devices and artificial synaptic applications.

Transition between resistive switching modes in asymmetric HfO2-based structures

We claim that the evolution of conductive filaments (CF) in bipolar resistive switching (BRS) and complementary resistive switching (CRS) is similar in HfO2-based structures with one active layer. The difference in the observed I-V characteristics is explained by the presence of a non-Ohmic contact at the interface with the injection electrode, with the higher barrier at the BRS. In order to describe the transition between BRS and CRS in the asymmetric structure, we form Pt/HfO2(2 nm)/HfOXNY (4 nm)/TiN structure. The active layer of the structure and part of the active electrode were formed via plasma-enhanced atomic layer deposition. Top Pt-electrode was formed via sputtering throughout a shadow mask and area of top electrodes was equal to 0.0360 ± 0.0015 mm2. Pulse and DC measurements of I-V characteristics were carried out. A self-compliance RS with a current lower than 10 mA was found. The primary conduction mechanism in the low-resistance state for both RS modes was estimated as space-charge limited current (SCLC). For BRS, the unusual transition from trap-limited conduction to charge-carrier saturation (I ∼ V1.14) is found to be at 0.35 V in positive bias polarity. Charge-carrier saturation was not found at negative bias polarity. We attribute this to the higher injection barrier on the active electrode side (TiN). We attribute the increase in the barrier height to the accumulation of positive vacancies near the electrode interface.

Enhanced resistive switching performance of TiO2 based RRAM device with graphene oxide inserting layer

In this work, graphene oxide (GO)/TiO2 heterostructures for resistive random access memory devices were fabricated, and the composition and microstructure of TiO2 and GO were characterized by x-ray diffraction, Raman spectroscopy, scanning electronic microscopy, and transmission electron microscopy. The resistive characteristics of the fabricated devices were investigated, and the remarkable improvement in cycle-to-cycle uniformity and high ON/OFF ratio of the TiO2 thin film-based memory device were realized by introducing a thin GO layer. The formation/rupture of the conductive filament through the migration of oxygen vacancies in the TiO2 substrate was responsible for the resistive switching. Owing to the different activation energies of reduction and oxidation of the GO, the set voltage became larger than the reset voltage. According to the linear fitting of double logarithm I–V plots, the conduction mechanism in low and high resistance states was governed by the ohmic mechanism and trap-controlled space charge limited current, respectively. The oxygen migration-induced oxidation/reduction in GO rendered it a good oxygen vacancy reservoir, which is responsible for the enhanced cycle-to-cycle uniformity and high ON/OFF ratio.

Preparation of Thermoplastic Polyurethane/Multi-Walled Carbon Nanotubes Composite Foam with High Resilience Performance via Fused Filament Fabrication and CO2 Foaming Technique.

Wearable flexible sensors with high sensitivity and wide detection range are applied in motion detection, medical diagnostic result and other fields, but poor resilience and hysteresis remain a challenge. In this study, a high-resilience foam sensor was prepared through a combination of additive manufacturing and green physical foaming method. The conductive filaments were prepared by using MWCNTs-modified TPU by the physical method of melt blending. Samples were prefabricated using the FFF printer and then saturated with CO2 in an autoclave before being removed and heated to foam. The composite foam effectively reduced residual strain, demonstrating the high resilience of the 3D-printed composite materials with a foam porous structure. The residual strain of the sample before foaming was >6% after a single cycle, and then gradually increased. The residual strain of the foamed samples is less than 5%. In addition, composite foam has high sensitivity and can monitor subtle pressure changes (0~40 kPa). The sensing performance of the composite foam was evaluated, and the current signal remained stable under different loading rates and small compression strains (2~5%). By using this highly resilient conductive composite material, a hierarchical shoe insole was designed that successfully detected human walking and running movements.

Effect of Y-doping on switching mechanisms and impedance spectroscopy of HfO x -based RRAM devices

Y-doping can effectively improve the performance of HfO x -based resistive random-access memory (RRAM) devices, but the underlying physical mechanism of Y-doping affecting the performance of HfO x -based memristors is still missing and unclear. Although impedance spectroscopy (IS) has been widely used to investigate impedance characteristics and switching mechanisms of RRAM devices, there is less IS analysis on Y-doped HfO x -based RRAM devices as well as devices at different temperatures. Here, the effect of Y-doping on the switching mechanism of HfO x -based RRAM devices with a Ti/HfO x /Pt structure were reported using current–voltage characteristics and IS. The results indicated that doping Y into HfO x films could decrease the forming/operate voltage and improve the RS uniform. Both doped and undoped HfO x -based RRAM devices obeyed the oxygen vacancies (V O) conductive filament model along the grain boundary (GB). Additionally, the GB resistive activation energy of the Y-doped device was inferior to that of the undoped device. It exhibited a shift of the V O trap level towards the conduction band bottom after Y-doping in the HfO x film, which was the main reason for the improved RS performance.

Tin Oxide Nanorod Array-Based Photonic Memristors with Multilevel Resistance States Driven by Optoelectronic Stimuli

One-dimensional (1D) metal oxide-based photonic memristors, combining information storage and optical response, have shown great potential for the design and development of high-density and high-efficient computing systems beyond the era of von-Neumann architecture and Moore's law. Here, the functional memristive devices based on SnOx slanted nanorod arrays are demonstrated; wherein both the optical and electrical stimuli have been used to modulate the switching characteristics to achieve multilevel cell operations. The switching characteristics of Al/SnOx/FTO devices include low operating voltages (0.7 V/-0.6 V), moderate ON/OFF ratio (>10), and longer endurance (>102 cycles) and retention (>103 s) with a self-compliance effect in the dark. Under illumination, ranging from ultraviolet (254 and 365 nm) to visible light (405 and 533 nm), an unusual negative photo response with an enlarged ON/OFF ratio of >107 and a faster response time of <8 ms is observed. Additionally, multiple low and high resistance states have been achieved by modulating the programming current and the optical stimulus, respectively. The optoelectronic resistive memory behavior is attributed to the electric field-induced formation and light-stimulated dissolution of oxygen vacancies. Comprehensively, the results suggest that the optical illumination reduces the oxygen ion migration barrier, leading to the dissolution of conductive filaments and thereby locally increasing the OFF state resistance. The fabricated photonic memristors demonstrate the potential applications of metal oxide-based 1D nanostructures for artificial visual memory and optoelectronic applications.

Design and Testing of 3D-Printed Stackable Plant-Microbial Fuel Cells for Field Applications

The prevalence of non-renewable energy has always been a problem for the environment that needs a long-term solution. Plant-Microbial Fuel Cells (PMFCs) are promising bioelectrochemical systems that can utilize plant rhizodeposition to generate clean electricity on-site, without harming the plants, paving the way for simultaneous agriculture and power generation. However, one of the biggest hurdles in large-scale PMFC application is the diffused nature of power generation without a clear path to consolidate or amplify the small power of individual cells. In this study, stacking configurations of 3D-printed PMFCs are investigated to determine the amplification potential of bioelectricity. The PMFCs designed in this study are made of 3D-printed electrodes, printed from 1.75 mm Proto-pasta (ProtoPlant, USA) conductive PLA filament, and a terracotta membrane acting as the separator. Six cells were constructed with the electrodes designed to tightly fit with the ceramic separator when assembled. An agriculturally important plant (S. Melongena) was utilized as the model plant for testing purposes. Stacking of cells in series had resulted in severe voltage loss while stacking of cells in parallel preserved the voltage and current of the cells. Cumulative stacking verified the increasing voltage losses as more cells are connected in series, while voltage and current were generally supported well as more cells were connected in parallel. Combination stacks were also investigated, but while 2 sets of 3 cells in parallel stacked in series generated proportionately larger power and power density compared to individual cells, the drop in current density suggests that pure parallel stacks are still more attractive for scaling up, at least for the proposed stake design in this study. The results of this study indicated that the scale up of PMFC technology is possible in field applications to continuously generate electricity while growing edible plants.

A physics-based compact model of thermal resistance in RRAMs

In this paper, we present a physics-based compact model of thermal resistance in Resistive Random-Access Memory (RRAM) devices considering (a) thermal properties of electrode materials, (b) temperature-induced variations in material thermal conductivity, and (c) parasitic heat losses through the oxide surrounding the filament. Fully coupled numerical simulation data obtained from the self-consistent solution of the drift-diffusion equation (for vacancy migration), carrier continuity equation (for electronic conduction), and the Fourier heat diffusion equation (accounting for electro-thermal heating) has been used for developing and validating the proposed model. Excellent agreement is observed between the model and the numerical simulation results. The proposed model is validated with experimentally measured conductive filament temperature obtained from two different temperature sensing methods. Finally, we present a modeling framework to estimate the potential thermal-cross talk in RRAM crossbar arrays. Our results illustrate that a power-dependent thermal resistance model significantly improves the accuracy in estimating the electro-thermal effects in RRAMs.

Thermal Characterization of Conductive Filaments in Unipolar Resistive Memories.

A methodology to estimate the device temperature in resistive random access memories (RRAMs) is presented. Unipolar devices, which are known to be highly influenced by thermal effects in their resistive switching operation, are employed to develop the technique. A 3D RRAM simulator is used to fit experimental data and obtain the maximum and average temperatures of the conductive filaments (CFs) that are responsible for the switching behavior. It is found that the experimental CFs temperature corresponds to the maximum simulated temperatures obtained at the narrowest sections of the CFs. These temperature values can be used to improve compact models for circuit simulation purposes.