Ionic Regime Research Articles

Persistent polarization effects and memory properties in ionic-liquid gated InAs nanowire transistors

Iontronics exploits mobile ions within electrolytes to control the electronic properties of materials and devices' electrical and optical response. In this frame, ionic liquids are widely exploited for the gating of semiconducting nanostructure devices, offering superior performance compared to conventional dielectric gating. In this work, we engineer ionic liquid gated InAs nanowire-based field effect transistors and adopt the set-and-freeze dual gate device operation to probe the nanowires in several ionic gate regimes. We exploit standard back-gating at 150 K, when the ionic liquid is frozen and any crosstalk between the ionic gate and the back gate is ruled out. We demonstrate that the liquid gate polarization has a persistent effect on the nanowire properties. This effect can be conveniently exploited to fine-tune the properties of the nanowires and enable new device functionalities. Specifically, we correlate the modification of the ionic environment around the nanowire to the transistor threshold voltage and hysteresis, on/off ratio and current level retention times. Based on this, we demonstrate memory operations of the nanowire field effect transistors. Our work shines a new light on the interaction between electrolytes and semiconducting nanostructures, providing useful insights for future applications of nanodevice iontronics.

Studying ion transport dynamics in electrochemical measurements of lateral flow assays

Lateral Flow Assays (LFAs) are among the most widely used biosensors. Conventional colorimetric LFAs yield binary, non-quantifiable results. Electrochemical LFAs (eLFAs) aim to overcome this limitation. However, the reported disadvantage of extreme sensitivity to experimental conditions poses a significant challenge in the development of new eLFAs. In this work, impedimetric measurements were performed to probe the impact of experimental factors such as electrolyte conductivity/ionic strength on the eLFA outcomes. The time-dependent evolution of non-Faradaic impedance measurements coupled with flow velocity computations were employed to investigate ion transport across different LFA configurations and buffer compositions. The analysis of the Péclet number, a characteristic dimensionless parameter of flow dynamics within the LFA, revealed the coexistence of diffusive and convective ionic transport regimes over the LFA operational time. In addition, our findings underscore the critical role of absorbent pad dead volume adjustments in governing the capacity to maintain sample flow within the membrane over extended durations. These electro-fluidics phenomena are essential considerations for conducting electrochemical measurements within LFAs. Overall, this study offers insights into key design parameters for the integration of electrochemistry into LFAs.

Modelling dielectric behaviour and anomalous relaxation in chromonic liquid crystals

ABSTRACT We discuss the feasibility of applying a frequency-dependent diffusive model to study the dielectric response of complex fluids. The model considers the effect of free and bound particles by solving a non-exponential relaxation behaviour and anomalous diffusion phenomena into a coupled system of differential equations and boundary conditions. To illustrate the potentialities of the model in the data interpretation, we used the experimental results that provide the frequency spectra of a system formed by an aqueous solution of sunset yellow, a chromonic liquid crystal. Our model yields a considerably better adjustment of the experimental data compared to the simple Havriliak-Negami model and provides the relaxation frequency due to the director mode associated with the stack fluctuations about their short axis. Furthermore, we obtained parameters associated with the ion motion nature of the sample, including diffusion coefficient, Debye length and the ionic diffusive regime, which present a similar subdiffusive behaviour found in nematic thermotropic liquid crystals.

Emission and performance characterization of ionic liquids for an externally wetted electrospray thruster

Ionic liquids are the base for electrospray propulsion, yet few have been explored. The performance of an externally wetted electrospray thruster is characterized for seven different ionic liquids: 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), dicyanamide (EMI-DCA), bistrifluoromethylsulfonylimide (EMI-Im), thiocyanate (EMI-SCN), ethylsulphate (EMI-EtOSO3), triflate (EMI-OTf), and trifluoromethanesulfonate (EMI-TFA). This research marks the first time that the latter four liquids are reported for externally wetted emitters. A complete thruster assembly has been tested with each of the propellants. Using time-of-flight mass spectrometry and a Faraday cup for beam divergence, the performance of these ionic liquids has been characterized, assessing thrust, specific impulse and thrust efficiency. EMI-BF4, EMI-DCA and EMI-SCN operated in pure ionic regime while EMI-Im, EMI-EtOSO3, EMI-OTf and EMI-TFA operated in mixed regime.

Impact of propellant temperature on the emission regime of an externally wetted electrospray system using time-of-flight mass spectrometry

Electrospray thrusters operate in different regimes depending on the ejected particles: pure ionic regime, characterized by the presence of only solvated ions; droplet regime; and mixed regime, where both ion and droplets are emitted. The electrospray emission regime is driven by the physical properties of the liquid, the flow rate to the emission site, and the electric field applied. Because of the temperature dependence of the ionic liquid, the emission regime and hence the propulsive performance are sensitive to the liquid temperature. Additionally, in externally wetted electrospray emitters, where the flow rate is passively controlled by viscous forces, temperature plays a crucial role as it drives the hydraulic impedance of the emitter due to the variation of the liquid’s viscosity, and therefore the emitter flow rate. The research described in this work is focused on the effect of the propellant temperature on the emission regime of externally wetted emitters. By means of a time-of-flight mass spectrometer the plume composition has been analyzed under different conditions of applied voltage and operating temperature, finding that voltage increases the relative amount of ion species and temperature increases the relative amount of droplets.

Inferring electrospray emission characteristics from molecular dynamics and simulated retarding potential analysis

In this work, we present coordinated molecular dynamics, ion cluster acceleration, and retarding potential analysis simulations to determine cluster fragmentation behavior in a realistic emitter geometry for electrosprays operating in the pure ionic regime. Molecular dynamics simulations are used to determine the fragmentation rates of ionic liquid clusters as a function of internal energy, electric field strength, and cluster size. A simplified model of electrospray cluster acceleration is developed from previous electrohydrodynamic emission models and used to simulate retarding potential analysis curves. Fragmentation rates and beam composition are inferred for experimental data based on the molecular dynamics and cluster acceleration simulations. We find that for these experimental data, temperatures of EMI-BF4 dimers likely range between 590 and 687 K while trimer temperatures are larger between 989 and 1092 K. The percentage of monomers, dimers, and trimers in the beam is approximately 45%, 30%–43%, and 13%–25%, respectively. Both ionic liquid cluster temperatures and beam composition agree with previous analysis of this experimental work, supporting the use of coordinated molecular dynamics and retarding potential analysis as a method of inferring electrospray beam parameters. Insights gained from this simulation process are discussed in the context of currently unexplained electrospray emitter behavior and experimental results including the presence of tetramers and trimers in the beam and fragmentation rates in high electric field regions.

Solution-Processed Synaptic Memristors Based on Halide Perovskite Nanocrystals.

Exploring new materials and structures to construct synaptic devices represents a promising route to fundamentally approach novel forms of computing. Nanocrystals (NCs) of halide perovskites possess unique charge transport characteristics, i.e., ionic-electronic coupling, holding considerable promise for energy-efficient and reconfigurable artificial synapses. Herein, we report solution-processed thin-film memristors from all-inorganic CsPbBr3 perovskite NCs, functioning as an electrically programmable analog memory with good stability. The devices are demonstrated to successfully emulate a number of essential synaptic functions with low power consumption, including reversible potentiation and depression, short-term plasticity (STP), paired-pulse facilitation (PPF), and long-term plasticity (LTP), such as spike-number-dependent plasticity (SNDP), spike-rate-dependent plasticity (SRDP), spike-timing-dependent plasticity (STDP), and spike-voltage-dependent plasticity (SVDP). It is proposed that a coupled capacitive and inductive phenomenon originating from charge trapping and ion migration in CsPbBr3 NC films, controlled by the amplitude and timing of the programming pulses, defines the degree of synaptic plasticity. A transition emerges from the fast trap-related capacitive regime to a slow ionic inductive regime, which enables continuous change of the film resistance and the magnitude of the electronic current, analogous to the synaptic weight modulation in biological synapses.

Multiscale modeling of electrospray ion emission

A multi-scale approach to electrospray ion source modeling has been developed. The evolution of a single-emitter electrospray plume in a pure ionic regime is simulated with a combination of electrohydrodynamic fluids and n-body particle modeling. Simulations are performed for the ionic liquid, EMI-BF4, firing in a positive pure-ion mode. The metastable nature of ion clusters is captured using an ion fragmentation model informed by molecular dynamics simulations and experimental data. Results are generated for three operating points (120, 324, and 440 nA) and are used to predict performance relevant properties, such as the divergence angle and the extractor surface impingement rate. Comparisons to experimental data recorded at similar operating points are provided.

Multiple Emission Sites on Porous Glass Electrospray Propulsion Emitters Using Dielectric Propellants

Spatially resolved emission current density maps are presented as a function of extraction voltage for two near-conical porous borosilicate glass electrospray emitters using a 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imid ionic liquid. The current measurements are complimented with time-of-flight measurements to characterize the charge-to-mass ratio of the beams. The spatial current probe measurements resolve individual beamlets associated with separate emission sites, allowing the observation of the growth of the number of emission sites as the extraction voltage is increased. The emitters are found to operate near the pure ionic regime, and no evidence of the transition to unstable emission is observed up to above onset. An empirical model is developed for the onset of emission sites that accounts for the changes in site hydraulic resistance with the number of emission sites. The model satisfactorily reproduces the experimental voltage–current curves and average beam charge-to-mass ratios after the adjustment of several empirical parameters.

Aerosol-Printed MoS2 Ink as a High Sensitivity Humidity Sensor.

Molybdenum disulfide (MoS2) is attractive for use in next-generation nanoelectronic devices and exhibits great potential for humidity sensing applications. Herein, MoS2 ink was successfully prepared via a simple exfoliation method by sonication. The structural and surface morphology of a deposited ink film was analyzed by scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM). The aerosol-printed MoS2 ink sensor has high sensitivity, with a conductivity increase by 6 orders of magnitude upon relative humidity increase from 10 to 95% at room temperature. The sensor also has fast response/recovery times and excellent repeatability. Possible mechanisms for the water-induced conductivity increase are discussed. An analytical model that encompasses two ionic conduction regimes, with a percolation transition to an insulating state below a low humidity threshold, describes the sensor response successfully. In conclusion, our work provides a low-cost and straightforward strategy for fabricating a high-performance humidity sensor and fundamental insights into the sensing mechanism.

Comparison of computational algorithms for simulating an electrospray plume with a n-body approach

In order to better evaluate the trade-offs between different simulation options for an electrospray thruster plume, we have developed a multi-scale n-body code to compute the evolution of a single emitter electrospray plume in the pure ionic regime. The electrostatic force computations in the simulation are captured through the use of three different computational algorithms with various degrees of approximation. The results of the simulations for a simple test case are compared in terms of computational speed and accuracy. The test case utilizes a single operating point (323nA) for a stable meniscus solution for the ionic liquid EMI-BF4 firing in the positive pure ion mode. Complex species and probabilistic fragmentation processes are neglected. An overview is provided of the trade-off between accuracy and computational speed for the three algorithms in the context of simulating the electrostatic interactions between particles. For a large number of particles, the faster algorithms show a significant reduction in computational time while maintaining a high level of accuracy with a proper choice of tuning parameters.

Second Harmonic Scattering Reveals Ion-Specific Effects at the SiO2 and TiO2 Nanoparticle/Aqueous Interface

Ion-specific effects play a crucial role in controlling the stability of colloidal systems and regulating interfacial processes. Although mechanistic pictures have been developed to explain the electrostatic structure of solid/water colloidal interfaces, ion-specific effects remain poorly understood. Here we quantify the average interfacial water orientation and the electrostatic surface potential around 100 nm SiO2 and TiO2 colloidal particles in the presence of NaCl, RbCl, and CaCl2 using polarimetric angle-resolved second harmonic scattering. We show that these two parameters can be used to establish the ion adsorption mechanism in a low ionic strength regime (<1 mM added salt). The relative differences between salts as a function of the ionic strength demonstrate cation- and surface-specific preferences for inner- vs outer-sphere adsorption. Compared to monovalent Rb+ and Na+, Ca2+ is found to be preferentially adsorbed as outer-sphere on SiO2 surfaces, while a dominant inner-sphere adsorption is observed for Ca2+ on TiO2. Molecular dynamics simulations performed on crystalline SiO2 and TiO2 surfaces support the experimental conclusions. This work contributes to the understanding of the electrostatic environment around colloidal nanoparticles on a molecular level by providing insight into ion-specific effects with micromolar sensitivity.

Plume Composition Measurements of a High-Emission-Density Electrospray Thruster

An electrospray thruster designed for nanosatellites is introduced in this paper. The thruster has a compact size featuring a passive propellant transport method through the use of porous materials. Porous electrospray emitters with 1, 25, and 100 emission tips were fabricated using conventional machining techniques. The geometrical features of the emission tips were measured using a high-precision laser profilometer. Thruster prototypes fitted with different emitters were tested in a bipolar operation mode. The emission currents and voltage characteristics were studied, which demonstrated a wide range of operational voltage with relatively high emission currents. The maximum overall emission currents were and achieved using a 100-tip emitter at , and the maximum per-tip emission currents were and achieved using a 25-tip emitter at , respectively. Time-of-flight systems were used to characterize the mass and velocity distribution of the charged particles in the thruster plume, and the results suggest that the electrospray emission were highly ionic, containing mostly monomer ions and dimer ions. The thruster performance was conservatively estimated, with the thrust ranging from approximately 0.92 to , and the specific impulse ranging from 2733 to 5550 s. The maximum thrust per emission tip was approximately in a purely ionic emission regime, which is considerably high in this type of technology.

Efficiency Mechanisms in Porous-Media Electrospray Thrusters

The overall propulsion efficiency for ion-mode electrospray thrusters has been predicted to be as high as 90%; however, experimental measurements currently fall far short of these predictions. Further complicating this is that for passively fed electrospray thrusters, the mass flow rate, which is required to obtain the propulsion efficiency and specific impulse, is not directly controlled or measured, and so this parameter is typically estimated by assuming all mass loss is due to the emitted ion current. Presented here is a detailed investigation into the efficiencies associated with a porous-media-based electrospray thruster operated in the purely ionic regime using the ionic-liquid propellant 1-ethyl-3-methylimidazolium tetrafluoroborate in both positive and negative ion emission modes. Measurements of performance metrics that affect thruster efficiency are discussed, including the transmission, angular, polydispersive, energy, and mass utilization efficiencies, in order to determine their impact on overall efficiency. The overall propulsion efficiency and specific impulse are also calculated using a variety of methods to better investigate how the assumptions made to estimate the mass flow rate affect these parameters. These results suggest that the efficiency of these devices may primarily be limited by the presence of additional mass loss mechanisms other than ion emission occurring during thruster operation.

ASSESSMENT OF HYDROPHYSICAL AND HYDROCHEMICAL FEATURES OF WATER BODIES: A CASE STUDY OF LAKE IMANTAU, KAZAKHSTAN

The lakes of “Kokshetau” State National Natural Park (SNNP) are scanty and have a mosaic, fragmented character due to the present ecological state. In current work, the chemistry and degree of pollution in this lake is studied. The present research aimed to analyse the hydrophysical and hydrochemical parameters of lake Imantau of “Kokshetau” SNNP. This assessment includes dynamics of the hydrochemical water composition and benthal deposits, heavy metals content, and morphometric indicators of the lake using Earth’s remote sensing technique. This technique is based on Earth’s retrospective multichannel satellite images Landsat. Ionic water composition, total mineralization, hydrogen index, gas regime, and nutrient content (nitrates, nitrites) are determined. To assess the geochemical state of benthal deposits, the parameters like concentration coefficient (Cc), maximum allowable concentration (MAC) of pollutants in the soil, and total pollution index (Zc) are calculated. The results of this work is measured in terms of changes takes place in lake depth, water mass volume, water hardness, chemical concentrations.

ASSESSMENT OF HYDROPHYSICAL AND HYDROCHEMICAL FEATURES OF WATER BODIES: A CASE STUDY OF LAKE IMANTAU, KAZAKHSTAN

The lakes of “Kokshetau” State National Natural Park (SNNP) are scanty and have a mosaic, fragmented character due to the present ecological state. In current work, the chemistry and degree of pollution in this lake is studied. The present research aimed to analyse the hydrophysical and hydrochemical parameters of lake Imantau of “Kokshetau” SNNP. This assessment includes dynamics of the hydrochemical water composition and benthal deposits, heavy metals content, and morphometric indicators of the lake using Earth’s remote sensing technique. This technique is based on Earth’s retrospective multichannel satellite images Landsat. Ionic water composition, total mineralization, hydrogen index, gas regime, and nutrient content (nitrates, nitrites) are determined. To assess the geochemical state of benthal deposits, the parameters like concentration coefficient (Cc), maximum allowable concentration (MAC) of pollutants in the soil, and total pollution index (Zc) are calculated. The results of this work is measured in terms of changes takes place in lake depth, water mass volume, water hardness, chemical concentrations.

The Effect of Water on Ionic Conductivity in Olivine

AbstractHigh‐temperature ionic conductivity in olivine single crystals has been measured in the [100], [010], and [001] crystallographic orientations as a function of pressure from 2 to 10 GPa, temperature from 1450 to 2180 K, and H2O content from 20 to 580 wt. ppm using multianvil presses with in situ impedance analyses. The experimental results yield an activation energy, activation volume, and H2O content exponent of 250–405 kJ/mol, 3.2–5.3 cm3/mol, and 1.3 ± 0.2, respectively, for the high‐temperature ionic conduction regime. Olivine ionic conductivity has negative pressure and positive temperature dependences and is significantly enhanced by H2O incorporation. The [001] direction is more conductive than the [100] and [010] directions. The H2O‐enhanced ionic conductivity may contribute significantly to the electrical conductivity profile in the asthenosphere, especially in the regions under relatively high‐temperature and low‐pressure conditions.

Redox-Driven Reversible Gating of Solid-State Nanochannels.

The design of an electrochemically addressable nanofluidic diode is proposed, which allows tunable and nanofluidic operations via redox gating under electrochemical control. The fabrication process involves the modification of an asymmetric gold-coated solid-state nanopore with a thin layer of a redox polymer, poly(vinylferrocene) (PVFc). The composite nanochannel acts as a gate electrode by changing the electrochemical state and, consequently, the conversion/switching of ferrocene into ferricenium units upon the application of different voltages. It is shown that the electrochemical input accurately controls the surface charge density of the nanochannel walls with a predictable concomitant effect on the rectification properties. PVFc-based nanofluidic devices are able to discriminate the passage of anionic species through the nanochannel in a qualitative and quantitative manner by simply switching the redox potential of the PVFc layer. Experimental data confirmed that a rapid and reversible modulation of the ionic transport regimes can be easily attained by changing the applied potential. This applied potential plays the role of the gate voltage (Vg) in field-effect transistors (FET), so these nanofluidic channels behave as ionic FETs. Depending on the Vg values, the iontronic behavior can be switched between ohmic and diode-like regimes. We believe that this system illustrates the potential of redox-active polymers integrated into nanofluidic devices as plausible, simple, and versatile platforms to create electrochemically addressable nanofluidic devices for multiple applications.

Carrier conduction mechanisms of mesoporous titania thin films assessed by impedance spectroscopy

The electrical characterization of porous titania thin films remain a subject of study due to the material interest in widespread device applications. Highly ordered mesoporous titania thin films prepared by sol-gel method under different post-synthesis annealing temperatures were prepared and characterized by impedance spectroscopy at different humidity conditions. Slow ionic and fast electronic processes have different responses to the humidity and are carefully studied by assessing their frequency dependence. Specific ionic conduction regimes are identified for different water content that strongly depend on the structural properties of the material such as porosity and pore size. Also, the analysis of the Warburg coefficient describes the response of the mass transport for the different materials characteristics. Highly porous materials obtained for elevated annealing temperatures do not saturate with water even at high humidities and present inferior ionic transport mechanism. By analyzing the impedance response at different bias voltage it was possible to discriminate the electronic conduction for the mesoporous layer and observe the electronic defects by reproducing a space charge limited current behavior.

Combined Experimental and Theoretical Study of Weak Polyelectrolyte Brushes in Salt Mixtures.

The swelling behavior of a hydrophobic poly(2diisopropylamino)ethyl methacrylate (PDPA) brush immersed in aqueous solutions of single and mixed salts has been investigated using ellipsometry and numerical self-consistent field (nSCF) theory. As a function of solution ionic strength, the osmotic and salted brush regimes of weak polyelectrolyte brushes as well as substantial specific anion effects in the presence of K+ salts of Cl–, NO3–, and SCN– are found. For solutions containing mixtures of NO3– and Cl–, the brush swelling is the same as one would expect on the basis of the concentration-weighted average of the brush behavior in the single salt solutions. However, in mixtures of SCN– and Cl–, the swelling response is more complicated and substantial divergence from ideal behavior is observed. Mean-field theory shows excellent qualitative agreement with the ellipsometry findings. nSCF reveals that for the SCN–/Cl– cases the swelling behavior of the PDPA brush most likely arises from the predominant localization of the weakly hydrated SCN– within the brush compared to the more strongly hydrated Cl–.