Bistable Characteristics Research Articles

Stable hybrid organic/inorganic multiple-read quantum-dot memory device based on a PVK/QDs solution

A thin-film memory device is proposed herein, based on quantum dots (QDs) with bi-stable characteristics under a positive voltage bias. The synthesized QDs had a CdSe/ZnS core/shell structure. The charge confinement effect within the QDs in the charge-storage layer was enhanced by adding (poly(9-vinylcarbazole)) (PVK). As the PVK concentration increases, the on/off ratio of the device increases. Noise was also reduced and stable I-V characteristics were demonstrated. Each thin film was fabricated by a spin-coating method, among solution process methods. The on/off ratio of the fabricated device was found to be maximum 378 × 103 at 1.5 wt% PVK concentration. The initial on/off state was maintained even when a negative voltage (commonly used for the “erase” function) was applied. In addition, the write voltage of the fabricated device using the conductive polymer poly-TPD was reduced from 2.8 to 1.7 V. By optimizing PVK concentration and forming the poly-TPD thin film, the fabricated memory device had an on/off ratio of about 4 × 103 at 0.5 V and the stored current maintained the initial value even after 200 h. Even with a single write process, the initially formed high state is maintained for more than 200 h, and it is possible to read repeatedly.

A Compact and Flexible Nonbeam-Type Vibrational Energy Harvesting Device With Bistable Characteristics

Conventional beam-type vibrational energy harvesting devices suffer from poor low-frequency performance, stress concentration, and limited compatibility with other device structures. This paper proposes a vibrational energy harvester based on a novel bellows structure together with permanent magnets that exploits nonlinear bistability to enhance its performance at low frequencies. The device uses a flexible bellows tube that is compact and can also be used as an elastic unit instead of springs in other vibration-based devices. First, the structural configuration of a prototype device is described, after which the equations of motion are established to allow a bistability analysis of the device. Simulations and experiments are carried out to investigate the effects of various parameters on the bistability, including the distance between magnets, the excitation amplitude, and the excitation frequency. The performance of the proposed device is investigated experimentally for excitation frequencies from 2 to 15 Hz and is compared with the performance of a device without external magnets. This compact nonbeam-type bistable energy harvesting device represents a new design of bistable vibration energy harvester, which has the potential to provide higher output power at lower working frequencies.

Preparation and multifunction of electrochromic polyamides containing flexible backbone chains with electrochemical, fluorescence and memory properties

A series of electroactive polyamides (PAs) containing alkyl chains in backbone and triarylamine unit as the functional group were designed and synthesized by condensation polymerization of N-(2-amino-ethyl)-N-(4-carbazol-9-yl-phenyl)-ethane-1, 2-diamine with various dicarboxyl acids. The characterization confirms the successful synthesis. The thermal properties of the polymers investigated by thermo gravimetric analysis (TGA) indicate the polymers are hear-resisting. The main absorption peaks of PAs appear at λmax = 283–296 nm and strong photoluminescence (PL) peaks at 363–404 nm exhibit in the NMP. Electrochromic (EC) characteristics of the PAs films reveal excellent stability together with multistep EC behaviors, which change from the almost colorless neutral form to the green, then dark green oxidized forms at applied potentials. The resistance memory with good electrical bistable resistive switching characteristics, which possesses low threshold voltage (VSET of −1.6 V and VRESET of 4.2 V). Conclusionally, the PAs can be used as hole transporting, electrochromic, electrofluor, and a nonvolatile memory materials.

Solitary waves in the Ablowitz–Ladik equation with power-law nonlinearity

We introduce a generalized version of the Ablowitz–Ladik model with a power-law nonlinearity, as a discretization of the continuum nonlinear Schrödinger equation with the same type of nonlinearity. In this model, we study the interplay of discreteness and generic nonlinearity features. We identify stationary discrete-soliton states for different values of nonlinearity power , and address changes of their stability as the frequency of the standing wave varies for given . Along with numerical methods, a variational approximation is used to predict the form of the discrete solitons, their stability changes, and bistability features by means of the Vakhitov–Kolokolov criterion (developed from first principles). Development of instabilities and the resulting asymptotic dynamics are explored by means of direct simulations.

The role of spatial scale in organism\u2013environment positive feedback

Organism–environment positive feedback (i.e., ecosystem self-modification or facilitation) will incur bistability, which is often disadvantageous to biological conservation and ecosystem restoration. Using a spatially correlated equation based on pair approximation and simulation, we found that the feature of bistability in the positive feedback system strongly depends on spatial scale of organism–environment feedback. It will mitigate and even disappear when the interaction between organisms and environment is localized spatially, while the population will lessen globally when dispersal colonization is limited. As the spatially local influence is a basic property of real ecological systems (especially for sessile organisms), but classical ecological models based on mean-field assumption (or mass action) does not consider the impact of spatial scale. This implies that spatial configuration and local facilitation could be essential process for the stability and maintenance of ecosystem.

Frequency-induced polarization switching and bistability in a 1550 nm VCSEL subject to parallel optical injection

We experimentally investigate frequency-induced polarization switching (PS) and bistability (PB) characteristics in a 1550 nm vertical-cavity surface-emitting laser (1550 nm VCSEL) subject to parallel optical injection, for which the polarization direction of the injection light is along with the direction of dominant polarization mode in the solitary 1550 nm VCSEL. The results show that, for a 1550 nm VCSEL under parallel optical injection with frequency detuning ∆ν (= νinj − ν0, where νinj is the frequency of injection light and ν0 is the frequency of dominant polarization mode of the solitary 1550 nm VCSEL), type I PS (the dominant polarization mode switches from Y-polarization mode with a shorter wavelength to X-polarization mode with a longer wavelength) and type II PS (the dominant polarization mode switches from X-polarization mode to Y-polarization mode) can be observed through continuously scanning the frequency of injection light. Moreover, the values of ∆ν required for generating PSs are dependent on the variation route, namely frequency-induced PB phenomenon occurs. In the region with negative ∆ν, there exist two hysteresis loops, whose widths are seriously influenced by the injection power and the biased current.

A two-dimensional modular deployable truss structure with bistability

In this article, a new concept of two-dimensional modular deployable truss structure is proposed. To eliminate additional needs of containment mechanism, the proposed deployable structure is designed to have bistable characteristics so that it can stay robust both in the stowed and deployed states. The deployment of each module is triggered by the actuation of shape memory alloys. Static and dynamic models of the proposed deployable structure are established to analyze the characteristics and behavior of the system. The packaging ratio of the proposed structure is analyzed in terms of the design parameters. The results of static and packaging efficiency analysis show that the proposed concept has operational flexibility and high packaging efficiency. Verification of the dynamic model is performed based on a comparison of the response of the dynamic model and the actual deployment process of an experimental model; the deployment process is captured using a stereo pattern recognition camera system. It is shown that the established dynamic model can accurately describe the deployment behavior of the proposed structure.

Direct measurement of foldover in cavity magnon-polariton systems

An approach to directly measure the nonlinear foldover effect for cavity magnon polaritons is demonstrated by placing a nonlinear medium (yttrium iron garnet) into a Fabry-Perot-like microwave cavity. The resulting bistability features can exhibit clockwise, counterclockwise, and butterflylike hysteresis loops and are solely dependent on the relative weight of the magnonlike and photonlike components. In addition to accurately describing our experimental observations, our model also allows us to calculate the crucial system conditions required to produce photonlike foldover effects. Though the photon subsystem has no nonlinear components of its own, these photonlike foldover effects are produced through light-matter interactions with a nonlinear magnetic subsystem. Our model's description of these nonlinear light-matter interactions and their effects is not limited to ferromagnetic systems and should be generally applicable to other coherent coupled systems.

Flexible memristive devices based on polyimide:mica nanosheet nanocomposites with an embedded PEDOT:PSS layer

Flexible memristive devices with a structure of Al/polyimide:mica/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/indium-tin-oxide/polyethylene glycol naphthalate showed electrical bistability characteristics. The maximum current margin of the devices with mica nanosheets was much larger than that of the devices without mica nanosheets. For these devices, the current vs. time curves showed nonvolatile characteristics with a retention time of more than 1 × 104 s, and the current vs. number-of-cycles curves demonstrated an endurance for high resistance state/low resistance state switchings of 1 × 102 cycles. As to the operation performance, the “reset” voltage was distributed between 2.5 and 3 V, and the “set” voltage was distributed between −0.7 and −0.5 V, indicative of high uniformity. The electrical characteristics of the devices after full bendings with various radii of curvature were similar to those before bending, which was indicative of devices having ultra-flexibility. The carrier transport and the operation mechanisms of the devices were explained based on the current vs. voltage curves and the energy band diagrams.

A polymer microsphere-filled cholesteric-liquid crystal film with bistable electro-optical characteristics

In this paper, we promote a new polymer microsphere-filled cholesteric-liquid crystal (PFLC) system sandwiched between two flexible substrates for making a robust composite film with both bistable optical performance and high mechanical strength. The effects of the size of the polymer microspheres (PMs) and the pitch of the cholesteric liquid crystals (ChLCs) on the electro-optical performance of the as-made PFLC films were systematically investigated. Results show that the optical appearance of the optimized PFLC film can be reversibly changed between transparent and light-scattering states according to electric field by switching the ChLCs between planar (P) and focal conic (FC) states. Moreover, both P and FC states are stable for more than one year after removing the electric field, which is due to the great reduced polymer specific surface area of 1.78 m2/g and the spatial hindrance from the in situ thermally polymerized PMs, respectively. Aside from the long-term optical stability, the high content of the densely packed PMs also endows the PFLC film with a satisfactory shearing strength of 206.0 kPa between the top and bottom substrates, showing great potential applications in a wide range of uses, such as smart windows, electronic paper or writing tablets.

Bistability in a multiferroic composite resonator

Bistable characteristics of a nonlinear multiferroic composite resonator containing ferromagnetic and piezoelectric layers are investigated. The resonator was a borosilicate glass substrate of 25 mm × 2 mm dimensions and 150 μm thickness with a 2 μm thick amorphous ferromagnetic FeCoSiB layer and a 2 μm thick piezoelectric AlN layer deposited on its sides by magnetron sputtering. The resonator was excited by ac voltage at a frequency of 156 kHz, matching its longitudinal acoustic resonance frequency. The bistability loops were observed with increasing and decreasing frequency at constant excitation voltage and with increasing and decreasing voltage at constant frequency. With increasing excitation voltage, the resonator frequency first decreases by ∼0.7 kHz and then increases again to the initial value. A bistability model is suggested that uses Lorentzian shape resonance line and measured dependences of the resonance frequency and transmission coefficient on the output signal, which quantitatively describes experimental data. It is shown that bistability in a multiferroic resonator arises due to the nonlinearity of the ferromagnetic layer.

Application of an adaptive bistable power capture mechanism to a point absorber wave energy converter

This work proposes a novel adaptive bistable power capture mechanism, which is applied to a point absorber wave energy converter in regular waves. The adaptive bistable mechanism is realized by two symmetrically oblique main springs (responsible for the bistable characteristics) together with two auxiliary springs (making the bistable system adaptive) and it can adjust the potential function automatically to lower the potential barrier near the unstable equilibrium position. The “adaptive” feature helps solve the low-energy-absorption problem of a conventional bistable wave energy device (whose potential function is invariable in time) caused by the intrawell oscillation in low-amplitude excitation. With a suitable choice of physical and geometric parameters, an adaptive bistable wave energy converter outperforms its linear counterpart while a conventional bistable wave energy device has a poorer performance in low amplitude waves. Even for relatively large waves, an adaptive bistable wave energy converter has a comparable power capture capacity with its conventional bistable counterpart, both of which perform better than a linear wave energy converter especially in the low wave frequency region.

Optical signal processing in nano nonlinear Fabry-Perot resonator containing negative index material

This paper investigates the issue of signal processing in a bistable Fabry-Perot (FP) resonator. FP resonator includes two one-dimensional photonic crystals (1DPCs) containing periodic structure of positive and negative index material (PIM-NIM) considering as two semi-transparent reflectors in nano scale which sandwiched a nanocavity. The transmitted intensity versus input intensity curve is obtained using the transfer matrix method which incorporates the bistability characteristics in some convenient conditions and parameters. We consider the bright soliton as an input pulse with nearly constant frequency and by changing its amplitude we investigate the output pulse in quasi-pulse regime. Such devices can practically be used as nano signal processing systems where it is illustrated that light can be controlled absolutely by light.

Hygroscopic influence on bistable characteristics of antisymmetric composite cylindrical shells: An experimental study

This paper investigates the effect of moisture on bistable characteristics of antisymmetric composite cylindrical by using experimental and finite element method. The bistable characteristics are characterized by the curvatures of antisymmetric composite cylindrical shells in different stable states and snap processes between the two stable states which are indicated by the load–displacement curve and snap load. The manufactured specimens after dried in the oven are immersed in distilled water to full saturation and the saturated salt solutions (MgCl2) to full saturation. The specimen achieves different moisture that is immersed in distilled water at the different period until full saturation and in the saturated salt solution (MgCl2) with the same period of saturation in distilled water. Specimens with different moisture are then mechanically loaded on a testing machine to transform between two stable states. Load–displacement curves are recorded in the computer, from which the snap loads can be found. After the test, the principal and twisting curvatures are captured by a digital image processing. The results are contrasted with hygroscopic influence on another kind of bistable composite structure (asymmetric cross-ply laminates) in this paper. The results show that the shapes and snap loads of antisymmetric composite cylindrical shells are influenced by the moisture increasing.

Bistable metallic materials produced by nanocrystallization process

Through a localized nanocrystallization process using surface mechanical attrition treatment (SMAT), a new method is proposed to build bistable and multistable metallic shells. The impacts from randomly fast-moving balls during the nanocrystallization process accumulate plastic deformations, which induce nanotwins and mesh grains into nanoscales significantly increasing elastic behavior range of the treated shells. The in-plane self-equilibrium residual stress field, which is induced from the stretching plastic deformations in the treated region under the constraint of the untreated region, renders nanostructured shells bistable characteristics. An effective numerical modelling is carried out to analyze the bistable behavior and predict their stable configurations. A flat plate with multiple nanostructured regions is manufactured and numerically studied, which is capable of holding multiple stable configurations. In addition, the developed bistable and multistable shells can be further mechanically processed to modify their stable configurations.

Effect of thickness on bistable switching characteristics of a pentacene based memory device

In this paper, the effect of thicknesses on vacuum deposited pentacene (C22H14) thin films on ITO coated glass substrate for use as write once read many (WORM) memory devices are reported. RV characteristics of pentacene devices deposited for different thicknesses of 50 nm, 100 nm and 200 nm at a deposition rate of 20 Å/s were studied. Results revealed that the ON/OFF ratio was more for the device with active material deposited at a thickness of 100 nm. This device showed a stable switching with an ON/OFF current ratio as high as nearly 108 with a switching threshold voltage of 1.14 V. The irreversible switching of all the devices makes it suitable for a write once read many memory device applications. The structural studies of these pentacene thin films on glass substrates were also done and the dependence of the grain size of the films with thickness is also reported.

Model-based design of bistable cell factories for metabolic engineering.

Metabolism can exhibit dynamic phenomena like bistability due to the presence of regulatory motifs like the positive feedback loop. As cell factories, microorganisms with bistable metabolism can have a high and a low product flux at the two stable steady states, respectively. The exclusion of metabolic regulation and network dynamics limits the ability of pseudo-steady state stoichiometric models to detect the presence of bistability, and reliably assess the outcomes of design perturbations to metabolic networks. Using kinetic models of metabolism, we assess the change in the bistable characteristics of the network, and suggest designs based on perturbations to the positive feedback loop to enable the network to produce at its theoretical maximum rate. We show that the most optimal production design in parameter space, for a small bistable metabolic network, may exist at the boundary of the bistable region separating it from the monostable region of low product fluxes. The results of our analysis can be broadly applied to other bistable metabolic networks with similar positive feedback network topologies. This can complement existing model-based design strategies by providing a smaller number of feasible designs that need to be tested in vivo. http://lmse.biozone.utoronto.ca/downloads/. krishna.mahadevan@utoronto.ca. Supplementary data are available at Bioinformatics online.

Viscoelastic bistable behaviour of antisymmetric laminated composite shells with time-temperature dependent properties

Due to the bistable characteristics, antisymmetric laminated ([+α/−α]n) composite bistable shells have been used as novel morphing structures in many engineering fields. Previous theoretical analyses were mainly based on the hypothesis that the composite shell is elastic, which leads to unexpected calculation and prediction errors. In this paper, the fiber reinforcements are assumed to be elastic while the matrix is treated as a viscoelastic material. The resulting viscoelastic material properties of the composite shell are obtained through the experimental test and numerical modeling. Based on the classical lamination theory, together with the principle of minimum potential energy and Maxwell viscoelasticity model, a theoretical model is developed to predict the bistable behaviour of those composite shells with viscoelastic material properties. Subsequently, the influences of applied temperature and relaxation time on the second stable configuration of bistable composite shells are analytically investigated. The results are then compared with those obtained from the experiments and numerical modeling. Comprehensive results show that the principal curvature of the shell's second stable state increases as the applied temperature and relaxation time increase. In contrast, the twisting curvature of the second stable shape generally decreases with the relaxation time increasing but increases with the applied temperature rising.

Study of gain-coupled distributed feedback laser based on high order surface gain-coupled gratings

Single-longitudinal-mode, gain-coupled distributed feedback (DFB) lasers based on high order surface gain-coupled gratings are achieved. Periodic surface metal p-contacts with insulated grooves realize gain-coupled mechanism. To enhance gain contrast in the quantum wells without the introduction of effective index-coupled effect, groove length and depth were well designed. Our devices provided a single longitudinal mode with the maximum CW output power up to 48.8 mW/facet at 971.31 nm at 250 mA without facet coating, 3dB linewidth (<3.2 pm) and SMSR (>39 dB). Optical bistable characteristic was observed with a threshold current difference. Experimentally, devices with different cavity lengths were contrasted on power–current and spectrum characteristics. Due to easy fabrication technique and stable performance, it provides a method of fabricating practical gain-coupled distributed feedback lasers for commercial applications.

Highly-reproducible nonvolatile memristive devices based on polyvinylpyrrolidone: Graphene quantum-dot nanocomposites

Abstract The properties of nonvolatile memristive devices (NMD) fabricated utilizing organic/inorganic hybrid nanocomposites were investigated due to their superior advantages such as mechanical flexibility, low cost, low-power consumption, simple technological process in fabrication and high reproducibility. The current-voltage (I-V) curves for the Al/polyvinylpyrrolidone (PVP): graphene quantum-dot (GQD)/indium-tin-oxide (ITO) memristive devices showed current bistability characteristics at 300 K. The window margins corresponding to the high-conductivity (ON) state and the low-conductivity (OFF) state of the devices increased with increasing concentration of the GQDs. The ON/OFF ratio of the optimized device was 1 × 104, which was the largest memory margin among the devices fabricated in this research. The endurance number of ON/OFF switching was above 1 × 102 cycles, and the retention time was relatively constant, maintaining a value above 104 s. The devices showed high reproducibility with the writing voltage being distributed between −0.5 and −1.5 V and the erasing voltage being distributed between 2 and 3 V. The ON state currents remained between 0.02 and 0.03 A, and the OFF state currents stayed between 10−6 and 10−4 A. The carrier transport mechanisms are illustrated by using both the results obtained by fitting the I-V curves and the energy band diagrams of the devices.