Low Ion Density Research Articles

Experimentally verified organic electrochemical transistor model

The Bernards–Malliaras model, published in 2007, is the primary reference for the operation of organic electrochemical transistors (OECTs). It assumes that, as in most transistors, the electronic transport is drift only. However, in other electrochemical devices, such as batteries, the charge neutrality is accompanied by diffusion-only transport. Using detailed 2D device simulations of the entire structure while accounting for ionic and electronic conduction, we show that high ion density (>1019 cm−3) results in Debye screening of the drain–source bias at the electrodes’ interface. Hence, unlike the drift-only current in standard FETs or low ion density OECTs, the current in high ion density OECTs is diffusion only. Also, we show that since in OECTs, the volumetric capacitor and the semiconductor are one, the threshold voltage has a different meaning than that in FETs, where the semiconductor and the gate-oxide capacitor are distinct entities. We use the above insights to derive a new model useful to experimentalists. Lastly, we fabricated PEDOT:PSS fiber-OECTs and used the results to verify the model.

Electrolytes in conducting nanopores: Revisiting constant charge and constant potential simulations.

Simulating electrolyte-electrode systems poses challenges due to the need to account for the electrode's response to ion movements in order to maintain a constant electrode potential, which slows down the simulations. To circumvent this, computationally more efficient constant charge (CC) simulations are sometimes employed. However, the accuracy of CC simulations in capturing the behavior of electrolyte-electrode systems remains unclear, especially for microporous electrodes. Herein, we consider electrolyte-filled slit nanopores and systematically analyze the in-pore ion structure and diffusivity using CC and constant potential simulations. Our results indicate that CC simulations provide comparable pore occupancies at high bulk ion densities and for highly charged pores, but they fail to accurately describe the ion structure and dynamics, particularly in quasi-2D (single-layer) pores and at low ion densities. We attribute these results to the superionic state emerging in conducting nanoconfinement and its interplay with excluded volume interactions.

Numerical modelling of sawteeth and sawtooth-free regime

To better understand the sawteeth physics and the sawtooth-free regime associated with the hybrid scenario in tokamak experiments, numerical calculations up to quasi-steady state have been carried out for realistic middle-size tokamak plasma parameters, including the bootstrap current perturbation and basing on both the single- and two-fluid equations with the large aspect ratio approximation. Two types of the sawtooth crash are found in multiple sawteeth simulations: (1) For a low equilibrium bootstrap current fraction, the crash is caused by the internal kink mode, as expected; (2) When the bootstrap current density fraction is larger than 10% in the core region, however, the crash is caused by the non-ideal double kink mode, in contrary to the conventional understanding. In this case, a non-monotonic radial profile of the safety factor q with two q=1 surfaces emerges before the crash, caused by the bootstrap current density and plasma resistivity perturbations, although the original equilibrium has only single q = 1 surface. In both types of sawtooth crashes, the crash time in two-fluid simulations is tens of microseconds, as observed in experiments. Furthermore, for a relatively low ion density and finite bootstrap current density fraction, a transition from the sawtooth to the sawtooth-free regime is found, in which flat q profiles with the q value being about unity in the central region, similar to that observed in hybrid scenario experiments, are maintained by the dynamo effect. To enter into the sawtooth-free regime in two-fluid simulations, a much larger Alfvén velocity than that in single-fluid simulations is required due to the diamagnetic drift.

Assessment of runaway electron beam termination and impact in ITER

The vertical motion and shrinking of the cold plasma column after a tokamak disruption leads to a natural decrease in the edge safety factor when most of the current is carried by runaway electrons (REs). Reaching a low edge safety factor can potentially cause a strong plasma instability. We present magnetohydrodynamic simulations of the termination of a post-disruption plateau-phase RE beam in ITER when the edge safety factor falls close to two. Growth of instabilities is observed to result in stochastization of the magnetic field and a prompt loss of REs. As RE impact must be mitigated in ITER, the effect of parameters that influence the final termination have been assessed. Higher background plasma resistivity is seen to cause larger mode magnitudes and stronger stochastization, leading to less remnant REs after the termination event. Lower ion-densities also project a qualitatively similar behavior although weaker in effect. Using computations from a wall collision model, the ensuing load distribution on the first-wall is also presented.

Characteristics of inductively coupled plasma radio-frequency ion source of ion implanters for high number density dopant generation

In this study, we developed an inductively coupled plasma ion source that can be applied to implanters in semiconductor production. We employed an infrared camera and thermocouples to assess the temperature properties of the ion source operated at temperatures below 500 °C. This reduced temperature is expected to facilitate the adoption of various materials as the ion source. Ion densities of the direct current ion source measured using a double Langmuir probe were found to range from 1.66 × 1016 to 5.06 × 1016 m−3 within an input power range of 682–895 W. In contrast, the ion densities of a radio-frequency ion source ranged from 7.86 × 1016–9.58 × 1016 m−3 within an input power range of 700–900 W. This proposed ion source can serve as a next-generation solution because of its low operating temperature and high ion density.

STEVE Events With FUV Emissions

AbstractSTEVE, Strong Thermal Emission Velocity Enhancement, was often observed by ground‐based imagers in visible wavelengths and rarely detected by global FUV imagers. We present a new event, and revisit two reported STEVE events, that were observed by the DMSP/SSUSI FUV imager at O 135.6 nm, N2 LBHS, and LBHL emissions. Coincident particle and plasma drift observations showed that the events were associated with high ion drift speed, low ion density and no energetic particle precipitation. This is consistent with earlier findings (e.g., MacDonald et al., 2018, https://doi.org/10.1126/sciadv.aaq0030; Gallardo‐Lacourt et al., 2018a, https://doi.org/10.1029/2018ja025368, 2018b, https://doi.org/10.1029/2018gl078509; Archer et al., 2019). In this paper, several new features are identified: (a) Detection rates of FUV STEVE are much lower than that of visible STEVE; (b) The STEVE on 27 March 2008 covers 3‐hr local time with a length up to 2,700 km around 60° magnetic latitudes; (c) Different widths of STEVE observed in O 135.6 nm and N2 LBH images indicate a large altitude range in the STEVE FUV emissions; (d) The true ion (assuming O+) drift speeds during the 27 March 2008 STEVE event could be up to 20 km/s, well above the DMSP SSIES sensor limit. The kinetic energy of the high speed ions is larger than the excitation potential of the observed FUV emissions; (e) The requirement of such extreme high ion drift speed explains why FUV STEVE have been rarely observed, compared to the visible STEVE; (f) The three FUV STEVE events occurred during moderate geomagnetic activity; (g) Theses events were conjugate in both hemispheres.

The Surface Properties of Implant Materials by Deposition of High-Entropy Alloys (HEAs).

High-entropy alloys (HEAs) contain more than five alloying elements in a composition range of 5-35% and with slight atomic size variation. Recent narrative studies on HEA thin films and their synthesis through deposition techniques such as sputtering have highlighted the need for determining the corrosion behaviors of such alloys used as biomaterials, for example, in implants. Coatings composed of biocompatible elements such as titanium, cobalt, chrome, nickel, and molybdenum at the nominal composition of Co30Cr20Ni20Mo20Ti10 were synthesized by means of high-vacuum radiofrequency magnetron (HVRF) sputtering. In scanning electron microscopy (SEM) analysis, the coating samples deposited with higher ion densities were thicker than those deposited with lower ion densities (thin films). The X-ray diffraction (XRD) results of the thin films heat treated at higher temperatures, i.e., 600 and 800 °C, revealed a low degree of crystallinity. In thicker coatings and samples without heat treatment, the XRD peaks were amorphous. The samples coated at lower ion densities, i.e., 20 µAcm-2, and not subjected to heat treatment yielded superior results in terms of corrosion and biocompatibility among all the samples. Heat treatment at higher temperatures led to alloy oxidation, thus compromising the corrosion property of the deposited coatings.

Experimental investigation of ion recombination in a locally made low-voltage ionization chamber

In this study, the weak alpha radiation sources and the beta radiation by the effect of them on the electrical conductivity of air inside the ionization chamber are studied by measuring the produced current. The aim of this study is to make an ionization chamber to measure the ionization caused by weak radioactive sources which produce the electrical current at the order of femto- to pico-amperes and try to estimate the ion recombination coefficient at low ion densities. A parallel-plate ionization chamber with proper shielding and noise reduction circuits are made which measure data and sent them to a personal computer by USB port. The current-voltage characteristics of the ion chamber are measured when different radioactive sources are placed inside the chamber at the different separation of electrodes. The recombination constant obtained is estimated equal to 1.18 × 10−6 cm3/s. It is shown that current-voltage characteristic for ion chamber with different electrode separation lies on a unique curve by a proper normalization in the pico- and femto-ampere current range.

Particle-in-cell techniques for the study of space charge effects in an electrostatic ion beam trap.

We developed a simulation technique to study the effect of space charge interaction between trapped ions in the electrostatic ion beam trap (EIBT). The importance of space charge is demonstrated in both the dispersive and the self-bunching regime of the ion trap. The simulation results provide an estimate for the space charge effect on the trapping efficiency. They also allow for a better understanding of the enhanced diffusion and the self-bunching effect and provide a better characterization of the EIBT as a mass spectrometer, where peak coalescence is important. The numerical results reproduce all experimental data, demonstrating the critical importance of including space charge effects, even at low ion density, to understand the ion trap dynamics.

A Study on the Etching and Surface Characteristics of SiOC Thin Films After C6F12O Plasma Etching.

Silicon oxycarbide (SiOC) film was etched using a CF₄/C6F12O/O₂ mixed gas plasma through an inductively coupled plasma etcher. Changes in the dielectric constant and surface chemical bonding properties were investigated using ellipsometry and Fourier transform infrared spectroscopy. Plasma diagnosis was carried out using a double Langmuir probe, ultraviolet detector, and residual gas analyzer. The physical and chemical plasma properties of CHF₃ and C6F12O exhibited similar trends. However, the C6F12O mixed plasma exhibited a smaller change in dielectric constant compared to that of a conventional CHF₃ mixed plasma, because of the lower ion density, ion energy flux, and UV intensity and thinner fluorocarbon-based polymer formation. Therefore, the liquefied C6F12O gas can substitute for the existing etching process gas and reduce the change in dielectric constant.

Gate Alignment of Liquid Water Molecules in Electric Double Layer.

The behavior of liquid water molecules near an electrified interface is important to many disciplines of science and engineering. In this study, we applied an external gate potential to the silica/water interface via an electrolyte-insulator-semiconductor (EIS) junction to control the surface charging state. Without varying the ionic composition in water, the electrical gating allowed an efficient tuning of the interfacial charge density and field. Using the sum-frequency vibrational spectroscopy, we found a drastic enhancement of interfacial OH vibrational signals at high potential in weakly acidic water, which exceeded that from conventional bulk-silica/water interfaces even in strong basic solutions. Analysis of the spectra indicated that it was due to the alignment of liquid water molecules through the electric double layer, where the screening was weak because of the low ion density. Such a combination of strong field and weak screening demonstrates the unique tuning capability of the EIS scheme, and would allow us to investigate a wealth of phenomena at charged oxide/water interfaces.

Atomic frequency comb optical memory in EuCl<sub>3</sub>·6H<sub>2</sub>O crystal

Quantum memory is a crucial component for the large-scale quantum networks. Rare-earth-ion doped crystals have been a promising candidate for the practical quantum memory because of its very long coherence time. However, doped ions cause unwanted lattice distortion, and consequently reduce the optical depth and the storage efficiency. The stoichiometric rare-earth crystals have low lattice distortion and high rare earth ion density, and thus are expected to enable high-efficiency storage. EuCl<sub>3</sub>·6H<sub>2</sub>O is a promising material for quantum memory applications because its optical inhomogeneous broadening can be smaller than its hyperfine splitting and the theoretically predicted spin coherence time is up to 1000 seconds. Despite the numerous efforts in solid-state quantum memory based on rare-earth ion doped crystals, optical memory and quantum memory have not been implemented with stoichiometric rare-earth crystals yet. Here, we report the atom frequency comb optical storage using a EuCl<sub>3</sub>·6H<sub>2</sub>O crystal. A coherence time of 55.7 μs is obtained by photon echo measurements on <inline-formula><tex-math id="M2">\begin{document}$^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210648_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210648_M2.png"/></alternatives></inline-formula> transition. The two-level atomic frequency comb storage is demonstrated with a storage efficiency of 1.71% at a storage time of 1 μs, showing the potential capability of optical quantum storage of this material. Based on the analysis of the line shift of <inline-formula><tex-math id="M3">\begin{document}$^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210648_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210648_M3.png"/></alternatives></inline-formula> depending on the sample temperature, we highlight the challenge of achieving high-efficiency optical pumping in this material, which imposes a limit on the achievable efficiency.

Probing the ionic defect landscape in halide perovskite solar cells

Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance; however, many open questions remain unanswered. In this work, we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide (MAPbI3) perovskites in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results highlight the profound influence of defects on the electronic landscape, exemplified by their impact on the device built-in potential, and consequently, the open-circuit voltage. Even low ion densities can have an impact on the electronic landscape when both cations and anions are considered as mobile. Moreover, we find that all measured ionic defects fulfil the Meyer–Neldel rule with a characteristic energy connected to the underlying ion hopping process. These findings support a general categorization of defects in halide perovskite compounds.

Plasma Activation of Polyethylene Powder

Polyethylene powder of average particle diameter of 160 µm was activated in a plasma reactor made from aluminum of volume 64 dm3 at the pressure 100 Pa. Dense oxygen plasma was sustained with a microwave discharge powered by a pulsed magnetron source of power 1 kW mounted onto the top flange of the plasma reactor. Polymer powder was treated in a batch mode with 0.25 kg/batch. The powder was placed into a stainless-steel dish mounted in the center of the reactor where diffusing plasma of low ion density, and the O-atom density of 2 × 1021 m−3 was sustained. The powder was stirred in the dish at the rate of 40 rpm. The evolution of powder wettability versus treatment time was measured using the Washburne method, and the surface composition was determined by X-ray Photoelectron Spectroscopy (XPS). The wettability versus the oxygen concentration assumed a parabolic behavior. The maximal oxygen concentration, as revealed by XPS, was 17.5 at.%, and the maximal increase of wettability was 220%. The efficiency of O-atoms utilization in these experimental conditions was about 10% taking into account the spherical geometry of dust particles and perfectly smooth surface. The method is scalable to large industrial systems.

A combination of plasma diagnostics and Raman spectroscopy to examine plasma-graphene interactions in low-pressure argon radiofrequency plasmas

Graphene films were exposed to low-pressure capacitively coupled (E-mode) and inductively coupled (H-mode) argon radio frequency plasmas to investigate damage formation by very-low-energy ion irradiation. In the H-mode, plasma parameters were assessed by a Langmuir probe and plasma sampling mass spectrometry to determine the conditions of fixed ion fluence but with different average ion energies. The populations of argon metastable and resonant argon atoms were also measured by optical absorption spectroscopy to determine their contribution to the total energy flux during plasma treatment. In the H-mode, in which plasma-graphene interactions are dominated by ion irradiation effects, Raman spectroscopy reveals a significant rise in the D/G ratio and full width at half maximum of the G peak as well as the onset of graphene amorphization, even at very low ion energies (between 7 and 13 eV). In the E-mode characterized by comparable ion energy but much lower ion density, significant damage is also observed, a feature ascribed to the additional energy flux linked to the de-excitation of metastable argon species on the graphene surface.

Tunable optical delay with low intensity loss in a cascade structure Er3+-doped optical fiber amplifier

In this paper, we propose a new pumping scheme-a dual-frequency laser-pumped cascade structure of Er3+-doped optical fiber based on the theory of coherent population oscillation. While realizing slow light transmission, the loss of output signal light power is reduced. Using numerical simulation, we compare the effects of a single fiber without pumping, a 980nm single pumping single fiber, and a dual-frequency laser-pumped cascade structure of Er3+-doped optical fiber on signal light intensity loss coefficient τ and maximum time delay with the increase of input signal light power. The results show that the signal light intensity loss coefficient τ of the three structures decreases gradually with the increase of input signal light power. However, the transmission of lower signal light intensity loss coefficient τ=0.0823 and a larger time delay of 0.15ms can be obtained under the dual-frequency laser-pumped cascade structure of Er3+-doped optical fiber. On the basis of the dual-frequency laser-pumped cascade structure of Er3+-doped optical fiber, the signal light intensity loss coefficient τ decreases gradually with the increase of pump ratio M (M=P1480∶P980) and the decrease of length ratio N (N=L2∶L1; L1 and L2 represent the length of the first and second cascaded optical fibers, respectively). Moreover, we compare the effects of ion density of the Er3+-doped optical fiber on the time delay of three structures. The dual-frequency laser-pumped cascade structure of Er3+-doped optical fiber can provide a larger time delay and smaller signal light intensity loss coefficient τ at low ion density of the Er3+-doped optical fiber.

Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model

We present a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini–Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini–Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to 116 and 74 u, respectively. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N2 as well as new photodissociation branching ratios for CH4 and C2H2. Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions.Main reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan’s atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH+) in the upper ionosphere, (ii) the low mass negative ion densities (CN−,C3N−/C4H−) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO2, H2O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.

Correlating morphology to thermal and electrical properties in imidazolium-poly(ethylene glycol) copolyesters

Ionic liquid-based copolyesters containing an imidazolium cation backbone and PF6− or Tf2N− counter-anion are synthesized from N,N′-bis(ω-hydroxyalkyl)imidazolium salts (PF6− or Tf2N−) (0.5 equiv.), poly(ethylene glycol) (PEG; number average molecular weight: 1000 or 2000) (0.5 equiv.), and sebacoyl chloride (1.0 equiv.). The imidazolium monomers are room-temperature ionic liquids. In the ionic copolymers, the 1:1 ratio between imidazolium and PEG, and number average molecular weights were confirmed using 1H NMR. Crystallization of the PEG segments was affected by the alkyl spacer length of the ionic units and by the anion type. Softer Tf2N− can prevent copolyester crystallization during cooling when the anions are close to the PEG segments; moreover, some Tf2N− copolymers show crystallization during a heating scan. The crystalline phase of the PEG-2000 segments gives distinct X-ray diffraction peaks at 2θ ≈ 19.0 and 23.1°, and the lateral chain packing of the C11-(imidazolium PF6)-C11 structures is shown at 2θ ≈ 21.5°. These crystalline domains are shown as Tc peaks during a cooling scan. The room-temperature ionic conductivities of the Tf2N− copolyesters are above 10−5 S cm−1; however, the ionic conductivity drops significantly below the melting temperatures of the PEG copolyesters. In contrast, the predominantly amorphous polymers that contain PF6− and Tf2N− counter-anions do not show a sudden conductivity drop over the entire temperature range. The imidazolium-PEG copolyesters, which have a low ion density, show a higher ionic conductivity than a reported poly(imidazolium-sebacate) without PEG segments because of the lower activation energy of the isolated conducting anions.

Influence of grid resolution in fluid-model simulation of nanosecond dielectric barrier discharge plasma actuator

Two-dimensional numerical simulation of a surface dielectric barrier discharge (SDBD) plasma actuator, driven by a nanosecond voltage pulse, is conducted. A special focus is laid upon the influence of grid resolution on the computational result. It is found that the computational result is not very sensitive to the streamwise grid spacing, whereas the wall-normal grid spacing has a critical influence. In particular, the computed propagation velocity changes discontinuously around the wall-normal grid spacing about 2 μm due to a qualitative change of discharge structure. The present result suggests that a computational grid finer than that was used in most of previous studies is required to correctly capture the structure and dynamics of streamer: when a positive nanosecond voltage pulse is applied to the upper electrode, a streamer forms in the vicinity of upper electrode and propagates along the dielectric surface with a maximum propagation velocity of 2 × 108 cm/s, and a gap with low electron and ion density (i.e., plasma sheath) exists between the streamer and dielectric surface. Difference between the results obtained using the finer and the coarser grid is discussed in detail in terms of the electron transport at a position near the surface. When the finer grid is used, the low electron density near the surface is caused by the absence of ionization avalanche: in that region, the electrons generated by ionization is compensated by drift-diffusion flux. In contrast, when the coarser grid is used, underestimated drift-diffusion flux cannot compensate the electrons generated by ionization, and it leads to an incorrect increase of electron density.

Characterization of the low latitude plasma density irregularities observed using C/NOFS and SCINDA data

Complex electrodynamic processes over the low latitude region often result in post sunset plasma density irregularities which degrade satellite communication and navigation. In order to forecast the density irregularities, their occurrence time, duration and location need to be quantified. Data from the Communication/Navigation Outage Forecasting System (C/NOFS) satellite was used to characterize the low latitude ion density irregularities from 2011 to 2013. This was supported by ground based data from the SCIntillation Network Decision Aid (SCINDA) receivers at Makerere (Geographic coordinate 32.6°E, 0.3°N, and dip latitude −9.3°N) and Nairobi (Geographic coordinate 36.8°E, −1.3°N, and dip latitude −10.8°N). The results show that irregularities in ion density have a daily pattern with peaks from 20:00 to 24:00 Local Time (LT). Scintillation activity at L band and VHF over East Africa peaked in 2011 and 2012 from 20:00 to 24:00 LT, though in many cases scintillation at VHF persisted longer than that at L band. A longitudinal pattern in ion density irregularity occurrence was observed with peaks over 135–180°E and 270–300°E. The likelihood of ion density irregularity occurrence decreased with increasing altitude. Analysis of C/NOFS zonal ion drift velocities showed that the largest nighttime and daytime drifts were in 270–300°E and 300–330°E longitude regions respectively. Zonal irregularity drift velocities over East Africa were for the first time estimated from L-band scintillation indices. The results show that the velocity of plasma density irregularities in 2011 and 2012 varied daily, and hourly in the range of 50–150ms−1. The zonal drift velocity estimates from the L-band scintillation indices had good positive correlation with the zonal drift velocities derived from VHF receivers by the spaced receiver technique.