Conductance Switching Behavior Research Articles

Synaptic and Gradual Conductance Switching Behaviors in CeO2/Nb–SrTiO3 Heterojunction Memristors for Electrocardiogram Signal Recognition

The synaptic properties of memristors have been widely studied. However, researchers are still committed to solving various challenges, including the study of highly reliable memristors with comprehensive synaptic functions and memristors that simulate highly complex neurological learning rules. In this work, we report a CeO2/Nb-SrTiO3 heterojunction memristor whose conductance could be gradually tuned under both positive and negative pulse trains. Due to the gradual conductance switching behavior and the high switching ratio (105), the CeO2/Nb-SrTiO3 heterojunction memristor could dutifully mimic biosynaptic functions, including excitatory/inhibitory postsynaptic current (EPSC/IPSC), paired-pulse facilitation and depression (PPF/PPD), spike amplitude-dependent plasticity (SADP), spike duration-dependent plasticity (SDDP), spike rate-dependent plasticity (SRDP), paired/triplet spiking-time-dependent plasticity (STDP), and Bienenstock-Cooper-Munro (BCM) rules. Moreover, a convolutional neural network based on the memristors is constructed to identify the electrocardiogram (ECG) data sets to realize the diagnosis of diseases with a recognition accuracy of 93%. Besides, the recognition accuracy of the handwriting digit reaches 96%. These studies broaden the research scope of high-level synaptic behavior and lay a foundation for the future full synaptic memristor networks.

Synthesis and memory properties of a series of novel asymmetric soluble polyimides

Abstract Two asymmetric diamines [1,1′-biphenyl]-4-yl(3,5-diaminophenyl) methanone (BPDAM) and (3,5-diaminophenyl)(4′-(naphthalen-1-yl)-[1,1′-biphenyl]-4-yl) methanone(DANPBPM) were synthesized by Suzuki coupling reaction from (4-bromophenyl)(3,5-diaminophenyl) methanone (BDAM) and corresponding arylboronic acid. A series of polyimides exhibiting organic solubility were prepared from 2,2′,3,3′-biphenyl tetracarboxylic dianhydride(BPDA) and these above three new diamines via a two-stage process. The obtained polymers showed outstanding organic solubility and high thermal stability. And studies have shown that the storage device with a sandwich type configuration of Al/polyimide/ITO was prepared by the traditional liquid spin coating technology, which showed the storage capacity of flash memory type. All the polyimide-based devices showed bistable conductivity switching and nonvolatile memory behavior that had long preservation period and high ON/OFF electric current, the rate of which was 104.

Tunable Thermal Switch via Order-Order Transition in Liquid Crystalline Block Copolymer.

Organic material-based thermal switch is drawing much attention as one of the key thermal management devices in organic electronic devices. This study aims at tuning the switching temperature (TS) of thermal conductivity by using liquid crystalline block copolymers (BCs) with different order-order transition temperature (Ttr) related to the types of mesogens in the side chain. The BC films with low Ttr of 363 K and high Ttr of 395 K exhibit reversible thermal conductivity switching behaviors at TS of ∼360 K and ∼390 K, respectively. The BC films also exhibit thermal conductivity variation originating from the anisotropy of the internal structures: poly(ethylene oxide) domains and liquid crystals. These results demonstrate that the switching behavior is attributed to an order-order transition between BC films with vertically arranged cylinder domains and the ones with ordered sphere domains. This highlights that BCs become a promising thermal conductivity switching material with tailored TS.

Controllable Nonclassical Conductance Switching in Nanoscale Phase-Separated (PbI2 )1- x (BiI3 )x Layered Crystals.

Layered 2D (PbI2 )1- x (BiI3 )x materials exhibit a nonlinear dependence in structural and charge transport properties unanticipated from the combination of PbI2 and BiI3 . Within (PbI2 )1- x (BiI3 )x crystals, phase integration yields deceptive structural features, while phase boundary separation leads to new conductance switching behavior observed as large peaks in current during current-voltage (I-V) measurements (±100V). Temperature- and time-dependent electrical measurements demonstrate that the behavior is attributed to ionic transport perpendicular to the layers. High-resolution transmission electron microscopy reveals that the structure of (PbI2 )1- x (BiI3 )x is a "brick wall" consisting of two phases, Pb-rich and Bi-rich. These brick-like features are 10s nm a side and it is posited that iodide ion transport at the interfaces of these regions is responsible for the conductance switching action.

Bias-driven conductance switching in encapsulated graphene nanogaps

Feedback-controlled electric breakdown of graphene in air or vacuum is a well-established way of fabricating tunnel junctions, nanogaps, and quantum dots. We show that the method is equally applicable to encapsulated graphene constrictions fabricated using hydrogen silsesquioxane. The silica-like layer left by hydrogen silsesquioxane resist after electron-beam exposure remains intact after electric breakdown of the graphene. We explore the conductance switching behavior that is common in graphene nanostructures fabricated via feedback-controlled breakdown and show that it can be attributed to atomic-scale fluctuations of graphene below the encapsulating layer. Our findings open up routes for fabricating encapsulated room-temperature single-electron nanodevices and shed light on the underlying physical mechanism of conductance switching in these graphene nanodevices.

Role of V-V dimerization in the insulator-metal transition and optical transmittance of pure and doped VO2 thin films

Insulator to metal (IMT) transition (T$_t$ $\sim$ 341 K) in the VO2 accompanies transition from an infrared (IR) transparent to IR opaque phase. Tailoring of the IMT and associated IR switching behavior can offer potential thermochromic applications. Here we report on effects of the W and the Tb doping on the IMT and associated structural, electronic structure and optical properties of the VO2 thin film. Our results show that the W doping significantly lowers IMT temperature ($\sim$ 292 K to $\sim$ 247 K for 1.3\% W to 3.7\% W) by stabilizing the metallic rutile, $\it{R}$, phase while Tb doping does not alter the IMT temperature much and retains the insulating monoclinic, $\it{M1}$, phase at room temperature. It is observed that the W doping albeit significantly reduces the IR switching temperature but is detrimental to the solar modulation ability, contrary to the Tb doping effects where higher IR switching temperature and solar modulation ability is observed. The IMT behavior, electrical conductivity and IR switching behavior in the W and the Tb doped thin films are found to be directly associated with the spectral changes in the V 3$\it{d_{\|}}$ states.

Colossal change in thermopower with temperature-driven p–n-type conduction switching in LaxSr2−xTiFeO6 double perovskites

Double perovskite materials have been studied in detail by many researchers, as their magnetic and electronic properties can be controlled by the substitution of alkaline earth metals or lanthanides in the A site and transition metals in the B site. Here we report the temperature-driven, p–n-type conduction switching assisted, large change in thermopower in La3+-doped Sr2TiFeO6-based double perovskites. Stoichiometric compositions of LaxSr2−xTiFeO6 (LSTF) with 0 ⩽ x ⩽ 0.25 were synthesized by the solid-state reaction method. Rietveld refinement of room-temperature XRD data confirmed a single-phase solid solution with cubic crystal structure and space group. From temperature-dependent electrical conductivity and Seebeck coefficient (S) studies it is evident that all the compositions underwent an intermediate semiconductor-to-metal transition before the semiconductor phase reappeared at higher temperature. In the process of semiconductor–metal–semiconductor transition, LSTF compositions demonstrated temperature-driven p–n-type conduction switching behavior. The electronic restructuring which occurs due to the intermediate metallic phase between semiconductor phases leads to the colossal change in S for LSTF oxides. The maximum drop in thermopower (ΔS ~ 2516 µV K−1) was observed for LSTF with x = 0.1 composition. Owing to their enormous change in thermopower of the order of millivolts per kelvin, integrated with p–n-type resistance switching, these double perovskites can be used for various high-temperature multifunctional device applications such as diodes, sensors, switches, thermistors, thyristors, thermal runaway monitors etc. Furthermore, the conduction mechanisms of these oxides were explained by the small polaron hopping model.

Compositional modification of Sr2TiCoO6 double perovskites by Mo and La for high temperature thermoelectric applications

Double perovskites (A2B/B//O6) are an interesting family of materials, which are amenable to compositional modification. Recently double perovskites have shown promising thermoelectric properties especially at high temperature. In this report, we investigated the combination of two different cubic double perovskites with similar lattice constant but one (Sr2TiCoO6) showing good Seebeck coefficient and the other (Sr2TiMoO6) with high electrical conductivity. Dense ceramic samples of Sr2TiCo0.5Mo0.5O6 and La0.2Sr1.8TiCo0.5Mo0.5O6 were synthesized by solid state reaction method. Rietveld refinement of XRD data confirmed the cubic structure with Pm3¯m space group in these ceramics. Doping of Mo in Sr2TiCoO6 helped in increasing its conductivity; however Seebeck coefficient showed the n-type behavior unlike the p-type conductivity observed in Sr2TiCoO6 ceramics. Aliovalent doping of La in Sr2TiCo0.5Mo0.5O6 further increased its conductivity and Seebeck coefficient demonstrated temperature driven p-type to n-type conduction switching behavior. Conductivity mechanism of these ceramics was found to be governed by small polaron hopping model. Temperature driven p-n switching observed in the thermopower (S) measurement of La0.2Sr1.8TiCo0.5Mo0.5O6 was further explained by an analytical model.

Light-induced Conductance Switching in Photomechanically Active Carbon Nanotube-Polymer Composites

Novel, optically responsive devices with a host of potential applications have been demonstrated by coupling carbon nanomaterials with photochromic molecules. For light-induced conductance switching in particular, we have recently shown that carbon nanotube-polymer nanocomposites containing azobenzene are very attractive and provide stable and non-degradable changes in conductivity over time at standard laboratory conditions. In these composites, the photoswitching mechanisms are based on light-induced changes in electronic properties and related to the Pool-Frenkel conduction mechanism. However, no link between conductivity switching and the molecular motion of azobenzene chromophores could be found due to application of high elastic modulus polymer matrices. Here we report on single wall carbon nanotube-polymer nanocomposites with a soft polycaprolactone polymer host. Such a system clearly shows the transfer of light-induced, nano-sized molecular motion to macroscopic thickness changes of the composite matrix. We demonstrate that these photomechanical effects can indeed overshadow the electronic effects in conductivity switching behavior and lead to a reversion of the conductivity switching direction near the percolation threshold.

Visualizing Local Morphology and Conductivity Switching in Interface-Assembled Nanoporous C60 Thin Films.

Carbon materials promise a revolution in optoelectronics, medical applications, and sensing provided that their morphology can be controlled down to the nanometer scale. Nanoporous materials are particularly appealing as they offer a drastically enlarged interfacial area compared to the corresponding planar materials. Entire fields such as organic solar cells, catalysis, or sensing may profit from an enlarged interface and facilitated molecular interaction between a carbon material and the environment. Nanoporous fullerene thin films obtained by the deposition of suspended nanoclusters of fullerene were already reported but suffered from the limitation of the size of these particles to over 100 nm. We study here a complementary method based on interfacial self-assembly forcing C60 clusters to spontaneously form 2D percolating monolayers with most morphological features in the 5-20 nm range. Analysis of these films by means of electron microscopy and scanning probe microscopy proved their morphology to be a nanocomposite of crystalline beads embedded in an amorphous matrix of fullerenes. When contacted between two gold electrodes, these films show an intrinsic conductivity switching behavior. Their electrical conductivity could be reversibly switched on by applying a threshold electrical current and switched off by exposure to oxygen. Interestingly, the on-state exhibits an astonishing conductivity of over 10-3 S/m. Kelvin probe force microscopy (KFM) was used to observe local changes in the distribution of electrical potential upon switching, on the relevant length scale of a few nanometers.

Synthesis and optical and electrochemical memory properties of fluorene–triphenylamine alternating copolymer

A fluorene–triphenylamine copolymer (PF–TPA) was designed and synthesized under Suzuki coupling reaction conditions in this work. It exhibited a typical electrical conductance switching behavior and non-volatile flash memory effects.

Resistive random access memories fabricated by using solution-processed AlZnSnO semiconductor films and indium ball electrodes

Resistive random access memories (ReRAMs) using solution-processed AlZnSnO (AZTO) oxide semiconductor films as resistive switching layers were fabricated. Electrodes were prepared by applying Ag paste or by pressing indium balls onto the AZTO surface. Vacuum processes were not required. The resistive switching behavior was not found for the ReRAM using a Ag paste electrode. However, a significant resistive switching characteristic could be obtained when using an In ball as the electrode. In addition, by adjusting compliance current from 80 mA to 20 mA, a stable resistance difference between high and low resistance state of 1839 Ω could be obtained over 500 operations. The ReRAM also exhibited good data retention capability and read disturb immunity for at least 103 s. Physical mechanisms of current conduction and resistance switching behavior were also investigated.

Conductance switching behavior of GeTe/Sb2Te3 superlattice upon hot-electron injection: a scanning probe microscopy study

ABSTRACTTopological (GeTe)/(Sb2Te3) superlattices (SL) are of practical interest for memory applications because of different mechanism of electric conductance switching in the crystalline phase. In the work, electrical switching behavior of individual SL grains was examined employing a multimode scanning probe microscope (MSPM) in a lithography mode at room temperature. Using programmed bias voltage with different amplitude and pulse duration, we observed the position-dependent variations of the switching voltage and the current injection delay for [(GeTe)2 (Sb2Te3)]4 SLs on Si(100). The results shed a light on the role of electric field and hot-electron injection on the SL conductance switching.

Negative differential resistance and stable conductance switching behaviors of salicylideneaniline molecular devices sandwiched between armchair graphene nanoribbon electrodes

By applying nonequilibrium Green's functions in combination with the density-functional theory, we investigate the electron transport properties of the salicylideneaniline molecule sandwiched between two armchair graphene nanoribbon (AGNR) electrodes. It shows that the width of the AGNR electrodes plays a significant role in the transport properties of the salicylideneaniline molecular junctions. The current–voltage characteristics of the salicylideneaniline molecule sandwiched between 8AGNR electrodes can perform a stable conductance switching behavior at the bias region [1.2 V, 1.6 V] when the molecule translates between the trans-enol form and the trans-keto form. When the electrodes change to 7AGNR, a remarkable negative differential resistance behavior can be found in the salicylideneaniline molecular device at the trans-keto form. That means the salicylideneaniline molecule can perform different functions when it connects to the different AGNR electrodes.

Bistable electrical switching and nonvolatile memory effect in carbon nanotube-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite films.

PSS and single-wall carbon nanotubes (SWCNTs) composite thin films have been investigated. The effect of doping level on electrical conductance switching behavior of an indium tin oxide/ PSS + SWCNTs/aluminum (ITO/PEDOT:PSS + SWCNTs/Al) sandwich structure has been reported. The ITO/(PEDOT:PSS + SWCNTs)/Al devices show current bistability. Moreover, the ON/OFF state current ratio is in an extent of 10(2)-10(4). The OFF state current increased with the decreasing SWCNTs content in the composite film, and the ON/OFF state current ratio of the device increases with the increase in SWCNTs content of the composite film. Thus, by varying the doping level of SWCNTs in PSS composite thin films, the electrical conductance switching behavior of ITO/PEDOT:PSS + SWCNTs/Al can be modulated in a controlled manner.

Current Conduction Mechanism of Nitrogen-Doped ${\rm AlO}_{x}$ RRAM

To eliminate the forming process in resistive random access memory (RRAM), we introduced nitrogen into ${\rm AlO}_{x}$ layer to generate a moderate amount of traps inside the film. The composition of nitrogen-doped ${\rm AlO}_{x}$ (N- ${\rm AlO}_{x}$ ) RRAM is revealed by X-ray photoelectron spectroscopy and X-ray diffraction analyses. To understand the current conduction mechanism of N- ${\rm AlO}_{x}$ RRAM, we investigated field and temperature dependence of the charge transport. Based on the extracted trap levels of high-resistance state and various low-resistance states, the current conduction is found to be governed by Frenkel–Poole emission. An energy band model is also proposed to clarify the current conduction mechanism and switching behavior of N- ${\rm AlO}_{x}$ RRAM.

Current conduction and resistive switching characteristics of Sm2O3 and Lu2O3 thin films for low-power flexible memory applications

In this article, the current conduction and resistive switching (RS) behavior in flexible Sm2O3 and Lu2O3 resistive random access memories (ReRAM) are investigated. Amorphous Sm2O3 and Lu2O3 thin films were deposited at room temperature by radio-frequency magnetron sputtering on flexible polyethylene terephthalate substrate. The structural morphologies of the Sm2O3 and Lu2O3 thin films strongly depend on the lattice energy of the oxides. The dominant current conduction mechanism in the oxide layer changes from electrode control Schottky emission in Sm2O3 to bulk controlled space-charge-limited-current in Lu2O3. The barrier height extracted from Schottky emission model is 0.96 eV in Sm2O3 thin film, while the activation energy of traps calculated from the Arrhenius plots is about 0.23 eV in Lu2O3 thin film. Additionally, the Ni/Sm2O3/ITO flexible memory device shows promising RS behavior with very low power of operation (∼30 μW) and small distribution of switching parameters. The memory reliability characteristics of switching endurance, data retention, good flexibility, and mechanical endurance show promising for future memory applications. The filament conduction model is adopted to describe the RS behavior in the Sm2O3 and Lu2O3 ReRAM devices. The improved RS performance in Sm2O3 thin film is attributed to the different physical properties of the thin films.

Structural effect on controllable resistive memory switching in donor–acceptor polymer systems

Controllable bistable electrical conductivity switching behavior and resistive memory effects have been demonstrated in Al/polymer/indium-tin oxide (ITO) sandwich structure devices, constructed from non-conjugated vinyl copolymers of PTPAnOXDm with pendant donor–acceptor chromophores. The observed electrical bistability can be attributed to the field-induced intra- and intermolecular charge transfer interaction between triphenylamine electron donor (D) and oxadiazole electron acceptor (A) entities, and is highly dependent on the chemical structure of the copolymers. The vinyl copolymers showed different memory behaviors, which depended on the loading of D/A ratios. The polymers containing only donor or acceptor moieties showed as insulators, the polymers containing both donor and acceptor moieties showed as WORM, flash and DRAM as D/A ratio increased. The structural effect on the physicochemical and electronic properties of the PTPAnOXDm copolymers, viz surface morphology, thermal stability, optical absorbance and photoluminescence, and molecular orbital energy levels, were investigated systematically to study the factors that influence the memory characteristics of the devices.

Electrical Conduction and Resistance Switching Mechanisms of Ag/ZnO/Ti Structure

We investigated electrical conduction and resistance switching behavior of the Ag/ZnO/Ti structures for random access memory devices. These films were prepared on glass substrate by dc sputtering technique at room temperature. The resistance switching follows unipolar switching mode with small switching voltages (0.4 V - 0.6 V). Two electrical conduction mechanisms dominating the LRS and HRS are Ohmic and trap-controlled space charge limited current, respectively. These both conductions are consistent with the filamentary model. Based on the filamentary model, the switching mechanism was also interpreted.

Single-Molecule Electrochemical Gating in Ionic Liquids

The single-molecular conductance of a redox active molecular bridge has been studied in an electrochemical single-molecule transistor configuration in a room-temperature ionic liquid (RTIL). The redox active pyrrolo-tetrathiafulvalene (pTTF) moiety was attached to gold contacts at both ends through -(CH(2))(6)S- groups, and gating of the redox state was achieved with the electrochemical potential. The water-free, room-temperature, ionic liquid environment enabled both the monocationic and the previously inaccessible dicationic redox states of the pTTF moiety to be studied in the in situ scanning tunneling microscopy (STM) molecular break junction configuration. As the electrode potential is swept to positive potentials through both redox transitions, an ideal switching behavior is observed in which the conductance increases and then decreases as the first redox wave is passed, and then increases and decreases again as the second redox process is passed. This is described as an "off-on-off-on-off" conductance switching behavior. This molecular conductance vs electrochemical potential relation could be modeled well as a sequential two-step charge transfer process with full or partial vibrational relaxation. Using this view, reorganization energies of ~1.2 eV have been estimated for both the first and second redox transitions for the pTTF bridge in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIOTf) ionic liquid environment. By contrast, in aqueous environments, a much smaller reorganization energy of ∼0.4 eV has been obtained for the same molecular bridge. These differences are attributed to the large, outer-sphere reorganization energy for charge transfer across the molecular junction in the RTIL.