Electrical Bistability Research Articles

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.

Resistive switching in silver/polystyrene/silver nano-gap devices

In this paper, we demonstrate reversible resistive switching in silver/polystyrene/silver nano-gap devices comprising Ag nano-strips separated by a nanoscale gap and encapsulated in polystyrene (PS). These devices show highly reversible switching behavior with high on-off ratios (>103) during cyclic switching tests over many cycles. We also observe evolution of the gap after extensive testing, which is consistent with metal filament formation as the switching mechanism in these Ag/PS/Ag nano-gap devices. The reversible electrical bistability demonstrated here was accomplished with an electrically inactive polymer, thereby extending the range of polymers suitable for organic digital memory applications.

Ferroelectric Polarization Induces Electric Double Layer Bistability in Electrolyte-Gated Field-Effect Transistors

The dense surface charges expressed by a ferroelectric polymeric thin film induce ion displacement within a polyelectrolyte layer and vice versa. This is because the density of dipoles along the surface of the ferroelectric thin film and its polarization switching time matches that of the (Helmholtz) electric double layers formed at the ferroelectric/polyelectrolyte and polyelectrolyte/semiconductor interfaces. This combination of materials allows for introducing hysteresis effects in the capacitance of an electric double layer capacitor. The latter is advantageously used to control the charge accumulation in the semiconductor channel of an organic field-effect transistor. The resulting memory transistors can be written at a gate voltage of around 7 V and read out at a drain voltage as low as 50 mV. The technological implication of this large difference between write and read-out voltages lies in the non-destructive reading of this ferroelectric memory.

Electrical bistability, negative differential resistance and carrier transport in flexible organic memory device based on polymer bilayer structure

Bistable nonvolatile memory devices containing two different layers of polymers, viz. MEH-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenyl vinylene]) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) has been fabricated by a simple spin-coating technique on flexible polyimide (PI) substrates with a structure Al/MEH-PPV/PEDOT:PSS/Ag-Pd/PI. The current–voltage measurements of the as-fabricated devices showed a nonvolatile electrical bistability with electric field induced charge transfer through the polymer layers and negative differential resistance (NDR) which is attributed to the charge trapping in the MEH-PPV layer. The current ON/OFF ratio between the high-conducting state (ON state) and low-conducting state (OFF state) is found to be of the order of 103 at room temperature which is comparable to organic field effect transistor based memory devices. We propose that such an improvement of rectification ratio (ON/OFF ratio) is caused due to the inclusion of PEDOT:PSS, which serves as a conducting current path for carrier transport; however, NDR is an effect of the trapped charges in the MEH-PPV electron confinement layer. The device shows excellent stability over 104s without any significant degradation under continuous readout testing in both the ON and OFF states. The carrier transport mechanism of the fabricated organic bistable device has been explained on the basis of different conduction mechanisms such as thermionic emission, space-charge-limited conduction, and Fowler–Nordheim tunneling. A band diagram is proposed to explain the charge transport phenomena. These bilayer structures are free from the drawbacks of the single organic layer based memory devices where the phase separation between the nanoparticles and polymers leads to the degradation of device stability and lifetime.

Azobenzene-Functionalized Gold Nanoparticles as Hybrid Double-Floating-Gate in Pentacene Thin-Film Transistors/Memories with Enhanced Response, Retention, and Memory Windows

Gold nanoparticles (Au-NPs) with surfaces covered with a self-assembled monolayer of azobenzene derivatives were prepared at the interface of dielectric insulator SiO2 and pentacene thin film. Transistors constructed with these composite channel materials exhibited electric bistability upon different gate biases, with the monolayer serving as a barrier layer, a work function modulator, as well as additional charge trapping sites at the Au-NPs/semiconductor interface at the same time. In comparison with simple alkanethiol monolayer-covered Au-NPs, the CH3-substituted azobenzene-functionalized Au-NPs result in a transistor memory device with about 70% more charges trapped, much faster response time as well as higher retention time. Besides, depending on the substituent on the azobenzene moieties (CH3, H, or CF3) and the tethering alkyl chain length, the speed at which the carriers are trapped (affecting switching response) and the stability of the carriers that are trapped (affecting memory retention) can be modulated to improve the device performance. The structural characterization and electronic characteristics of these devices will be detailed.

Effect of a Blocking Layer on the Decrease in the Leakage Current in Organic Bistable Devices

The electrical bistabilities and the memory stabilities of organic bistable devices (OBDs) based on multi-core-shell CdSe/CdS/ZnS nanoparticles embedded in a polystyrene (PS) layer fabricated by using a spin-coating method were investigated. The current density-voltage (J-V) curves for the Al/multi-core-shell CdSe/CdS/ZnS nanoparticles embedded in PS layer/WO3/indium-tin-oxide (ITO) devices showed current bistability with a maximum ON/OFF ratio of 1 x 10(3), which was much larger than that of a device without a WO3 layer. The leakage current of the OBDs was decreased by insertion of the WO3 layer between the PS layer containing nanoparticles and the ITO electrode, resulting in a decrease in the current deviation between the experimental and the simulated currents in the low-voltage region. The effects of the WO3 blocking layer on the electrical characteristics of the OBDs were investigated, and the carrier transport mechanisms for the OBDs were described on the basis of the J-V experimental data and theoretical results.

Electrical bistability in monolayers of cobalt-doped ZnO nanocrystals: Effect of alignment of their magnetic domains

We grow a range of cobalt-doped ZnO nanocrystals, form a monolayer through electrostatic assembly approach, and characterize the nanostructures with a scanning tunneling microscope tip. The scanning tunneling spectroscopy results show electrical bistability, that is, presence of a low- and a high-conducting state, in such monolayers under application of a voltage pulse. The bistability depends on the content of cobalt in the nanocrystals. With the use of such ferromagnetic nanocrystals, we align the magnetization vector of the nanocrystals in a monolayer in order to study the effect of alignment of their magnetic domains on the electrical bistability. We observe that the transport gap of the nanocrystals decreases upon conductance switching. The gap decreases also due to alignment of the magnetic domains of the cobalt-doped nanocrystals. When the magnetically aligned nanocrystals undergo a conductance switching, the transport gap further decreases evidencing a correlation between the electrical bistability and the (change in) transport gap.

(Invited) Electrical Conductivity Bistability in Nano-Composite

Nano-composite polymer memory devices are fabricated by depositing a blend (an admixture of organic polymer, small organic molecules and nanoparticles) between two metal electrodes. These devices show two electrical conductance states (“1” and “0”) when voltage is applied, thus rendering the structures suitable for data retention. These two states can be viewed as the realisation of non-volatile memory. Nano-composite polymer memory devices comprising of a blend of a polymer and small molecules and/or nanoparticles are investigated. This study is aimed at further understanding the electrical bistability observed in such devices. This work also investigates if an electrical charge can be transferred to gold nano-particles and, between small molecule complexes.

Electrical Bistability around Room Temperature in an Unprecedented One-Dimensional Coordination Magnetic Polymer

The synthesis, crystal structure, and physical properties of an unprecedented one-dimensional (1D) coordination polymer containing [Fe2(S2C6H2Cl2)4](2-) entities bridged by dicationic [K2(μ-H2O)2(THF)4](2+) units are described. The magnetic properties show that the title compound presents pairwise Fe-Fe antiferromagnetic interactions that can be well reproduced with a S = 1/2 dimer model with an exchange coupling, J = -23 cm(-1). The electrical conductivity measurements show that the title compound is a semiconductor with an activation energy of about 290 meV and two different transitions, both with large hysteresis of about 60 and 30 K at 260-320 K and 350-380 K, respectively. These two transitions are assumed to be due to slight structural changes in the cation-anion interactions. Differential Scanning Calorimetry confirms the presence of both transitions. This compound represents the first sample of a coordination polymer showing electrical bistability.

Self-heating, bistability, and thermal switching in organic semiconductors.

We demonstrate electric bistability induced by the positive feedback of self-heating onto the thermally activated conductivity in a two-terminal device based on the organic semiconductor C(60). The central undoped layer with a thickness of 300 nm is embedded between thinner n-doped layers adjacent to the contacts, minimizing injection barriers. The observed current-voltage characteristics follow the general theory for thermistors described by an Arrhenius-like conductivity law. Our findings include hysteresis phenomena and are of general relevance for the entire material class since most organic semiconductors can be described by a thermally activated conductivity.

(Invited) Electrical Conductivity Bistability in Nano-Composite

not Available.

Electrical Bistabilities of Write-Once-Read-Many-Times Memory Devices Fabricated Utilizing Indium Tin Oxide Nanoparticles Embedded in a Poly 4-Vinyl Phenol Layer

Electrical Bistabilities of Write-Once-Read-Many-Times Memory Devices Fabricated Utilizing Indium Tin Oxide Nanoparticles Embedded in a Poly 4-Vinyl Phenol Layer

Nanostructural Anisotropy Underlies Anisotropic Electrical Bistability

Regular arrays of nanorods having asymmetric cross-sections are fabricated by a combination of electrodeposition and glancing-angle deposition (GLAD). When these nanorods are embedded in a polymer matrix, they give rise to composite materials in which the structural anisotropy at the nanoscale translates into functional anisotropy in the form of direction-dependent electrical bistability. The degree of this directional bistability depends on and can be controlled by the spacing between the nearby nanorods.

Effects of the acceptor conjugation length and composition on the electrical memory characteristics of random copolyimides

AbstractResistive‐switching memories based on copolyimides (coPIs), PI‐NTCDIX and PI‐BTCDIX, with different compositions of 4,4′‐diamino‐4″‐methyltriphenylamine (AMTPA), 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride, and N,N′‐bis‐(4‐aminophenyl)‐1,8:4,5‐naphthalenetetracarboxydiimide (NTCDI) or N,N′‐bis‐(4‐aminophenyl)‐1,2:4,5‐benzenetetracarboxydiimide (BTCDI) have been developed. By varying the feed ratio of monomers, PI‐NTCDIX and PI‐BTCDIX showed tunable optical and electronic properties through the charge transfer (CT) between AMTPA and NTCDI or BTCDI. The memory devices based on PI‐NTCDIX exhibited the tunable electrical bistability from the volatile dynamic random access memory to nonvolatile write once read many memory characteristics as the NTCDI composition increased. The OFF/ON electrical switching transition was mainly attributed to the CT mechanism for the charge separated high conductance, based on the analysis of model compounds and density functional theory calculation. Also, the volatility of the memory device depended on the stability of CT complex. The long conjugation and high electron affinity of the NTCDI moiety stabilized the radical anion generated in the CT complex and prevented the recombination of segregated radical species even through applying the high positive or negative voltage. On the other hand, the memory devices based on PI‐BTCDIX showed a rather unique behavior compared with those based on PI‐NTCDIX. At the low BTCDI composition, the device exhibited volatile memory property. However, no switching behavior was observed at the high BTCDI composition due to the low highest occupied molecular orbital energy level of BTCDI. Combining these results and our previous study on perylenebisimide (PBI), we concluded that memory characteristics could be tailored by changing the conjugation length (PBI > NTCDI > BTCDI) and the acceptor composition in random coPIs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Dependencies of Donor–Acceptor Memory on Molecular Levels

This study reports the variation in the switch-on voltages of resistive switching devices using an electron-donating conjugated polymer, poly(3-hexylthiophene), P3HT, and hexaazatriphenylene-derivative acceptors. Memory devices fabricated using the spin-coated blends demonstrated electrical bistability with a conductivity modulation of ∼2 orders in magnitude and with negligible data loss after a stress test of 500 consecutive readouts conducted at 0.2 V. The switch-on voltage of the resistive switching device can be altered by using molecular acceptors with different LUMO levels. The charge transfer between the donor and acceptor was shown by quenching the fluorescence emission from P3HT after adding the acceptors. UV–vis absorption and Raman spectroscopic characterization revealed electrostatic doping of P3HT after switching the device from its OFF to ON state. The charge injection mechanisms of the two different conductive states were also analyzed and were found to be due to thermionic emission and Poole–Frenkel tunneling when the device is in the low and high conductive states, respectively.

Electric Bistability Induced by Incorporating Self-Assembled Monolayers/aggregated Clusters of Azobenzene Derivatives in Pentacene-Based Thin-Film Transistors

Composite films of pentacene and a series of azobenzene derivatives are prepared and used as the active channel material in top-contact, bottom-gate field-effect transistors. The transistors exhibit high field-effect mobility as well as large I-V hysteresis as a function of the gate bias history. The azobenzene moieties, incorporated either in the form of self-assembled monolayer or discrete multilayer clusters at the dielectric surface, result in electric bistability of the pentacene-based transistor either by photoexcitation or gate biasing. The direction of threshold voltage shifts, size of hysteresis, response time, and retention characteristics all strongly depend on the substituent on the benzene ring. The results show that introducing a monolayer of azobenzene moieties results in formation of charge carrier traps responsible for slower switching between the bistable states and longer retention time. With clusters of azobenzene moieties as the trap sites, the switching is faster but the retention is shorter. Detailed film structure analyses and correlation with the transistor/memory properties of these devices are provided.

Conjugated Polymer with On‐Chain Pt(II) Complex for Resistive Random‐Access Memory Device

AbstractA novel conjugated polymer, containing carbazole which acted as an electron‐donor moiety, and a Pt(II) complex which acted as an electron‐acceptor moiety, was synthesized and characterized. Electrical characterizations for the sandwiched polymer memory device (ITO/polymer/Al) indicate that the polymer possesses electrical bistability and the device exhibits resistive random‐access memory. The memory mechanism has been investigated through theoretical calculations and attributed to the formation and dissociation of a charge‐transfer state under the applied voltage. The device exhibits excellent memory performances, such as low threshold voltage, high ON/OFF current ratio, and good stability.

Particle size dependence of resonant-tunneling effect induced by CdS nanoparticles in a poly(N-vinylcarbazole) polymer matrix

CdS nanoparticles of different sizes were synthesised in poly(N-vinylcarbazole) and studied in device structures glass/indium tin oxide (ITO)/PVK:CdS/Al. Electrical bistability and negative differential resistance (NDR) effects were observed in the current-voltage characteristics. In addition, the devices showed a considerable enhancement of the current magnitude. A dependence of the current conduction on the nanoparticle size and size distribution in the polymer was studied through electrical impedance measurements. The study revealed the importance of the charge effects of the nanoparticles resulting in a bistable behavior. A resonant tunneling current model was proposed to explain the NDR and its relation with the nanoparticle size and size distribution.

Organic bistable devices utilizing carbon nanotubes embedded in poly(methyl methacrylate)

The electrical and memory behavior of organic bistable memory devices in the form of metal-embedded insulator-metal (MIM) structure are described. The devices utilize layer-by-layer (LbL) deposited single walled carbon nanotubes (SWCNTs) as charge traps embedded between two polymethylmethacrylate (PMMA) insulating layers. The stack was sandwiched between two aluminium electrodes to form an Al/PMMA/SWCNTs/PMMA/Al structure. The current-voltage (I-V) characteristics of the devices exhibit electrical bistability and non-volatile memory characteristics in terms of switching between high conductive (ON) and low conductive (OFF) states. The different conductive states were programmed by application of a positive and negative voltage pulse for the ON and OFF states, respectively. A maximum ON/OFF ratio of 2 × 105 is achieved at low reading voltage of 1 V. Space-charge-limited-current (SCLC) conduction model was used to describe the carriers transport and the electrical bistability in the devices, which was attributed to the trapping and detrapping of electrons inside the SWCNTs.

Carrier Transport Mechanisms of Organic Bistable Devices Fabricated Utilizing Hybrid C60/Poly(methyl methacrylate) Nanocomposites

Organic bistable devices (OBDs) based on nanocomposites consisting of C60 embedded in the poly(methyl methacrylate) (PMMA) layer were fabricated by using a spin coating method. The current density–voltage (J–V) curves of the Al/C60 embedded in PMMA layer/indium–tin-oxide devices exhibited an electrical bistability with a low-conductivity state, a transition state, and a high-conductivity state. The J–V curves for OBDs containing hybrid C60 and PMMA except a low voltage range were reasonably fitted by using the space-charge-limited current conduction model. The trapped electron densities of the OBDs were attributed to the space charge relative to the internal electric field.