Electrical Conductivity Switching Research Articles

Modulation of a Molecular π-Electron System in a Purely Organic Conductor that Shows Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism.

New important aspects of the hydrogen-bond (H-bond)-dynamics-based switching of electrical conductivity and magnetism in an H-bonded, purely organic conductor crystal have been discovered by modulating its tetrathiafulvalene (TTF)-based molecular π-electron system by means of partial sulfur/selenium substitution. The prepared selenium analogue also showed a similar type of phase transition, induced by H-bonded deuterium transfer followed by electron transfer between the H-bonded TTF skeletons, and the resulting switching of the physical properties; however, subtle but critical differences due to sulfur/selenium substitution were detected in the electronic structure, phase transition nature, and switching function. A molecular-level discussion based on the crystal structures shows that this chemical modification of the TTF skeleton influences not only its own π-electronic structure and π-π interactions within the conducting layer, but also the H-bond dynamics between the TTF π skeletons in the neighboring layers, which enables modulation of the interplay between the H-bond and π electrons to cause such differences.

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.

Hydrogen-bond-dynamics-based switching of conductivity and magnetism: a phase transition caused by deuterium and electron transfer in a hydrogen-bonded purely organic conductor crystal.

A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

Nonvolatile transparent manganese oxide thin film resistance random access memory devices

In this study, the electrical conduction and bipolar switching properties in transparent manganese oxide thin films were investigated and discussed. The as-deposited manganese oxide thin films were prepared by the rf magnetron sputtering method with the rf power of 130 W, chamber pressure of 10 mTorr, substrate temperature of 550 °C, and different oxygen concentrations. To discuss the resistive switching properties of the manganese oxide thin films, the conventional thermal annealing treatment of the manganese oxide thin film resistive random access memory (RRAM) devices were investigated. Finally, the current–voltage characteristics were determined by the ohmic contact and the Poole–Frankel emission conduction method in low/high-voltage regions, respectively.

Fine structures and switching of electrical conductivity in labyrinth silver films on sapphire

Changes in electrical resistance of silver films were measured in the range from 1013 to 103 Ω during thermal deposition on sapphire in a high vacuum, after the deposition over time, and under an applied voltage. The dependences of the electrical resistance of the films on their thickness and deposition rate were determined. It was established that, with an increase in the film thickness from 2 to 10 nm during the deposition at rates of 0.6 and 0.1 A/s, the resistance decreases by 7.5 and 4 orders of magnitude, respectively. The measured dependences of the resistance on the deposition time were found to be close to exponential. The room-temperature resistance of 10-nm-thick films deposited at different rates changed spontaneously by 3–4 orders of magnitude in different ways: the resistance of the slowly deposited films spontaneously increased, whereas in the rapidly deposited films, it spontaneously decreased. After fine annealing, the steady-state resistance changed also differently: it increased by 2 orders of magnitude in the former case and by 9 orders of magnitude in the latter case. Under voltages above 5 V, the resistance of the rapidly deposited films abruptly decreased from ∼1012 to ∼106 Ω, and these films became ohmic. After fine annealing, they became again high-ohmic. Under voltages above 5 V, the high-ohmic films thus obtained became again low-ohmic. This cycle of electrical conductivity switching was reproduced many times. The observed phenomena were explained in the framework of the hypothesis of the formation of fine metastable structures in channels of labyrinth films, namely, protrusions and bridges that bring together the boundaries of islands and connect them into conducting clusters, respectively.

Multifunctional π-Expanded Macrocyclic Oligothiophene 6-Mers and Related Macrocyclic Oligomers

Multifunctional π-expanded macrocyclic oligothiophene 6-mer 1, as well as 9- (2) and 12-mers (3), was synthesized using a McMurry coupling reaction as the key step. The 6-mer 1 was converted to cyclo[6](2,5-thienylene-ethynylene) (4) by using a bromination-dehydrobromination procedure. From X-ray analysis, the crystal structures of nonplanar 1 and round-shaped 2 and 4 were elucidated. STM showed that 4 formed a self-assembled monolayer at the liquid/solid interface to produce a hexagonal porous network. Chemical oxidation of 1, 2, and 4 with 1 and 2 equiv of Fe(ClO4)3 produced 1(•+) and 1(2+), 2(•+) and 2(2+), and 4(•+) and 4(2+), respectively. Although oligothiophene radical cations containing β,β-disubstituted thiophenes usually do not form π-dimers, 4(•+) clearly formed a π-dimer owing to its planar, round shape. As for the dications of 1, 2, and 4, 1(2+), which has 34π-electrons, exhibited a large diatropic ring current effect, whereas 34π dication 4(2+) only showed a medium diatropic ring current effect. In contrast to 1(2+) and 4(2+), 52π dication 2(2+) had biradical cationic character instead of Hückel-type cyclic conjugation. Interestingly, 6-mer 1 showed polymorphism and unusual melting point behavior due to the number of stable conformations in the solid state. Single crystals of 1 melted at 176 °C, whereas an amorphous film of 1 crystallized in the temperature range of 80-83 °C to form a lamellarly stacked microcrystalline film, which melted at 139 °C. The polymorphism of 1 was applied to either fluorescence switching or switching of field effect transistor (FET) activity and electrical conductivity.

Bipolar Switching Characteristics of RRAM Cells with CaBi4Ti4O15Film

The electrical conduction and bipolar switching properties of resistive random access memory (RRAM) cells with transparent calcium bismuth titanate (CaBi4Ti4O15—CBTi144) thin films were investigated. Experimentally, the (119)-oriented CBTi144 thin films were deposited onto the ITO/glass substrates by RF magnetron sputtering followed by rapid thermal annealing (RTA) at a temperature range of 450–550°C. The surface morphologies and crystal structures of the CBTi144 thin films were examined by using field-emission scanning electron microscopy and X-ray diffraction measurements. The on/off ratio and switching behaviors of the transparent Al/CBTi144/ITO/glass RRAM devices were further discussed in this work.

An influence of bottom electrode material on electrical conduction and resistance switching of TiOxthin films

We investigated the electrical conduction and resistance switching mechanisms of TiOx thin films grown on three kinds of bottom electrode at room temperature (an inert Pt, an active Ti and fluorine tin oxide FTO electrodes). The bottom electrode materials strongly affect the I-V characteristics and switching parameters. The I-V characteristic is explained through the presence of interface states in the metal electrode devices (Pt and Ti) and the work function in the metal oxide device (FTO). The Pt device has the smallest VSET and largest switching ratio, while the Ti device shows the largest VSET and smallest switching ratio. XPS data shows non-lattice oxygen in TiOx films. Therefore, the proposed bipolar resistance switching arises from formation and rupture of filament paths, generated by the movement of oxygen vacancies. All devices depict the same electrical conductions, trap-controlled space-charge-limited, FN tunneling and Ohmic conductions for a high resistance state and a low resistance state, respectively. In this study, the rarely reported FN tunneling conduction in published TiOx-based ReRAM device was found, which can be attributed to an influence of the bottom electrode on the electronic distribution in devices.

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.

Imprint electric field controlled electronic transport in TlGaSe2 crystals

The effect of built–in electric field onto the dc electrical conductivity, photoconductivity, and electrical switching phenomenon were investigated in TlGaSe2 layered semiconductor within the temperature range of 77–300 K. We have used different types of electrodes for different TlGaSe2 samples in both parallel and perpendicular directions to the plane of layers. The effect of electric field was investigated by cooling the samples from the room temperature under the electric field and then removing it at ∼80 K. After the procedure, it was found that a built-in internal electric field which strongly affects transport properties appears in TlGaSe2 crystals. Substantial increasing of both dark currents and photo-conductivities were observed predominantly at low temperatures, where hopping was the main conductivity mechanism. The anomalous decrease of the activation energy in the low temperature region and the switching effect are also the main experimental findings of the present work. Such behavior can be understood by assuming that the built-in electric field greatly increases the contribution of the hopping conductivity at low temperatures. Obtained results are discussed on the basis of the models widely used for disordered semiconductors. It was shown that TlGaSe2 crystal demonstrates the peculiar behavior that is typical to such type of semiconductors.

Effects of Co doping on electronic structure and electric/magnetic properties of La0.1Bi0.9FeO3 ceramics

In this work, we report the influence of Co-doping on the electronic band structure, dielectric and magnetic properties of La0.1Bi0.9Fe1−xCoxO3 ceramics. X-ray photoelectron spectroscopy investigation shows that Co dopant can shift the valence band spectrum and core-level lines of constituent elements towards higher bind energy regions simultaneously increase the concentration of oxygen vacancies in ceramics. The effects of dopant are discussed with focus given to the Co-doping induced enhancement of electrical conductivity and resistive switching phenomena.

Study of electrical conductivity and memory switching in the zinc–vanadium–phosphate glasses

Vanadium zinc phosphate glasses were prepared by the conventional melt quenching technique and effect of V2O5 concentration on d.c. conductivity of prepared samples were investigated. X-ray diffraction patterns confirmed the glassy character of the samples. The d.c. conductivity increased with increase in V2O5 content. Results showed that activation energy has a single value in the investigated range of temperature, which can be explained in accordance with Mott small pollaron hopping model. I–V characteristics at high electric field showed that switching in these glasses was memory type. The threshold field of switching was found to decrease with increase in V2O5 content. Non-linear behavior and switching phenomenon was explained by Pool–Frenkel effect and thermal model.

Electrical conduction and bipolar switching properties in transparent vanadium oxide resistive random access memory (RRAM) devices

In this study, the electrical conduction and bipolar switching properties in transparent vanadium oxide thin films are investigated and discussed. (110)-oriented vanadium oxide thin films were well deposited onto transparent ITO substrates for the possible development of applications in the structure of system-on-panel devices. For the as-deposited vanadium oxide thin films, they were prepared for 1 h by a rf magnetron sputtering method of rf power 130 W, chamber pressure 10 mTorr, substrate temperature 550 °C, and different oxygen concentrations. In addition, the Al/V2O5/ITO device presents reliable and bipolar switching behavior. The on/off ratio and switching cycling of two stable states are found and discussed. We suggest that the current–voltage characteristics are governed by ohmic contact and Poole−Frankel emission transport model mechanisms in low- and high-voltage regions, respectively.

Electrical Bistability and WORM Memory Effects in Donor–Acceptor Polymers Based on Poly(N‐vinylcarbazole)

AbstractBistable electrical conductivity switching behavior and write‐once, read‐many‐times (WORM) memory effects have been demonstrated in Al/polymer/ITO sandwich devices. These devices were constructed from two poly(N‐vinylcarbazole) derivatives with pendant donor–trap–acceptor (D‐T‐A) structures. The observed electrical bistability can be attributed to the field‐induced charge‐transfer interaction between the carbazole electron‐donor unit and the terminal electron‐acceptor unit, and subsequent charge trapping at the intermediate azobenzene chromophores. The charge‐transfer and trapping processes are further stabilized by the conformational relaxation of the total energy of the D‐T‐A system through donor–acceptor electrostatic interaction. The proposed switching and conduction mechanism is supported by density functional theory calculations, UV/Visible absorption spectra, core‐level X‐ray photoelectron spectra, and high‐resolution transmission electron microscopy images of the polymer thin films. The influence of the charge‐trapping effect of the azobenzene mediator is further explored by studying the electronic properties of two other poly(N‐vinylcarbazole) derivatives as the control samples, in which nitro or cyano acceptor groups are directly bonded to the carbazole electron‐donor moieties.

Wettability and switching of electrical conductivity in UV irradiated graphene oxide films

Graphene oxide (GO) flakes were assembled into a thin film that was subsequently exposed to UV light. It was found that pristine hydrophilic and insulating GO films present a poorer wettability together with a decrease of their surface resistivity under UV irradiation. It was observed that the pristine hydrophilic and insulating properties can be restored after few hours from irradiation. Using infrared spectroscopy and micro-Raman spectroscopy, we study the relationship between surface chemistry and structural distortion of GO films before and after UV irradiation. The obtained results open a solvent free as well as easy route for the integration of GO based materials into optoelectronic devices.

Electrical conductivity switching and memory effects in poly(N-vinylcarbazole) derivatives with pendant azobenzene chromophores and terminal electron acceptor moieties

Bistable electrical conductivity switching and non-volatile memory effects were demonstrated in Al/polymer/indium-tin oxide sandwich structure devices, constructed from two poly(N-vinylcarbazole) derivatives with pendant azobenzene chromophores and terminal electron acceptor moieties (–NO2 or –CN). Both polymers (PVK-AZO-NO2 and PVK-AZO-2CN) exhibit write-once read-many-times (WORM) type memory effects, with low switching threshold voltages below −2 V and high ON/OFF current ratios of ∼105–106. The observed electrical bistability in both polymers can be attributed to the field-induced charge transfer interaction between carbazole electron donor and terminal electron acceptor (–NO2 or –CN) moieties, and subsequent charge trapping at the intermediate azobenzene chromophores. The proposed switching mechanism is supported by the changes in the UV-visible absorption spectra of the polymer film upon transition from the OFF to the ON state.

Plasmon-induced optical switching of electrical conductivity in porous anodic aluminum oxide films encapsulated with silver nanoparticle arrays

We report on plasmon induced optical switching of electrical conductivity in two-dimensional (2D) arrays of silver (Ag) nanoparticles encapsulated inside nanochannels of porous anodic aluminum oxide (AAO) films. The reversible switching of photoconductivity greatly enhanced by an array of closely spaced Ag nanoparticles which are isolated from each other and from the ambient by thin aluminum oxide barrier layers are attributed to the improved electron transport due to the localized surface plasmon resonance and coupling among Ag nanoparticles. The photoconductivity is proportional to the power, and strongly dependent on the wavelength of light illumination. With Ag nanoparticles being isolated from the ambient environments by a thin layer of aluminum oxide barrier layer of controlled thickness in nanometers to tens of nanometers, deterioration of silver nanoparticles caused by environments is minimized. The electrochemically fabricated nanostructured Ag/AAO is inexpensive and promising for applications to integrated plasmonic circuits and sensors.

Enhanced Electrical Switching and Electrochromic Properties of Poly(p‐phenylenebenzobisthiazole) Thin Films Embedded with Nano‐WO3

AbstractThe electrical switching and electrochromic phenomena of a novel nanocomposite comprising poly(p‐phenylenebenzobisthiazole) (PBZT) and tungsten oxide (WO3) nanoparticles are investigated as a function of the nanoparticle loading. Both dissolving PBZT and doping PBZT backbone structure with acid are achieved by one simple step. Chlorosulfonic acid (CSA) is used as a solvent and spontaneously transformed to sulfuric acid upon exposure to moisture. The formed sulfuric acid serves as doping agent to improve the electrical conductivity of PBZT. The most significant enhancement of electrical switching is observed in the nanocomposites with low weight fraction (5%). The electrical conductivity of 5% WO3/PBZT nanocomposite thin film is increased by about 200 times and 2 times, respectively, as compared to those of the as‐received PBZT and PBZT/CSA thin films. As the nanoparticle loading increases to 20% and 30%, the nanocomposites follow an ohmic conduction mechanism. Stable electrical conductivity switching is observed before and after applying a bias on the pristine PBZT and WO3/PBZT nanocomposite thin films. Electrochromic phenomena of both PBZT and WO3/PBZT nanocomposite thin films with high contrast ratio are observed after applying a bias (3 V). The mechanisms of the nanoparticles in enhancing the electrical switching and electrochromic properties are proposed.

Organo- and Water-Dispersible Graphene Oxide−Polymer Nanosheets for Organic Electronic Memory and Gold Nanocomposites

A solution-processable graphene oxide (GO)−polymer complex, consisting of GO nanosheets with covalently grafted poly(tert-butyl acrylate) (PtBA) brushes, was prepared via surface-initiated atom transfer radical polymerization (ATRP). The grafted hydrophobic polymer brushes substantially enhance the solubility of GO in organic solvents, and the GO-g-PtBA nanosheets can form a uniform and stable dispersion in toluene. The functionalized GO nanosheets can also be integrated into an electroactive polymer matrix. Bistable electrical conductivity switching behavior and a nonvolatile electronic memory effect were demonstrated in a composite thin film of poly(3-hexylthiophene) (P3HT) containing 5 wt % GO-g-PtBA in an Al/GO-g-PtBA+P3HT/ITO sandwich structure (ITO = indium−tin oxide). Hydrolysis of GO-g-PtBA produces water-dispersible GO-g-PAAc (PAAc = poly(acrylic acid)) nanosheets and allows the preparation of gold nanoparticle-decorated GO-g-PAAc nanofilms from aqueous dispersions.