Charged Walls Research Articles

Tuning Compressive Young's Moduli and Antibacterial Activities of Alginate/Poly(N-isopropylacrylamide) Hydrogels with Laponite Layers and Cerium Ions.

Hybrid hydrogels containing alginate (Alg) and poly(N-isopropylacrylamide) (PNIPAAm) chains as natural and synthetic components, respectively, were crosslinked using double and triple pairs of the crosslinkers Ce3+/Ce4+, laponite (LP) RD, and N,N’-methylenebisacrylamide (BIS). (Alg/PNIPAAm)-Ce3+ and (Alg/PNIPAAm-PNIPAAm)-Ce3+ double- and triple-network structures were prepared using multivalent cerium ions (Ce3+), multifunctional laponite layers (L), and/or neutral tetrafunctonal BIS molecules (B). Compressive Young’s moduli, E, were tuned by the type/concentration of crosslinkers and crosslinking procedures and the concentration of Alg chains. The antibacterial activity of positively charged ions and molecules is due to the electrostatic attraction with the negatively charged bacterial cell walls. In the current study, we report the antibacterial activity on Escherichia coli of Ce3+ ions in the absence and presence of gentamicin sulfate (GS) for double and triple networks. Nonbacterial areas, which are called inhibition zones, around the disks, and compressive E moduli of the single and double PNIPAAm and Alg/PNIPAAm networks crosslinked by LP RD and containing Ce3+/Ce4+ions in free and ionically bonded states, respectively, were higher than those of the ones crosslinked with BIS. Moreover, BIS- and LP RD-crosslinked single PNIPAAm hydrogels displayed larger inhibition zones than those of Alg/PNIPAAm hybrids, supporting the antibacterial activity of free Ce3+/Ce4+ ions diffused together with GS molecules. On the other hand, antibacterial activities of GS + Ce3+-loaded triple networks were much lower than those of their double counterparts because the increase in the structural complexity reduced the co-emission of antibacterial agents.

An Efficient Vircator With High Output Power and Less Drifting Electron Loss by Forming Multivirtual Cathodes

Multi virtual cathodes (MVCs) were created in an axial vircator utilizing two dielectric reflectors (DRs). The wall charge of drifting electrons on the surface of DRs led to the creation of VC2 and VC3 in the downstream area of drift tube, according to phase space simulations. The microwave generated by VC1 interacted with the charge of VC2 and VC3 to raise the power amplitude, resulting in an increase in output power from 284 MW to 1028 MW. The use of two dielectric reflectors was proven to have a remarkable impact in reducing electron loss, increasing output power, and improving efficiency. The suggested vircator’s efficiency with MVCs improved from 7.9 % to 28.6 %. The findings indicate important signs of progress in vircator research to decrease electron losses and boost beam–wave interaction, which might lead to the advancement of high-efficiency vircators.

Ferroelectric domain wall in two-dimensional GeS

Two-dimensional (2D) ferroelectrics have attracted extensive attention due to their rich variety of exquisite functionalities in novel nanoscale electronic devices. As domain walls (DWs) in ferroelectrics are topological defects separating domains with different orientations of the electric polarization, a detailed understanding of the energetic and atomistic characteristics of 2D ferroelectric DWs is a crucial issue due to its theoretical and technological importance. In the current study, using first-principles calculations, we provided a detailed investigation on the energy, variation of the atomic structure with applied strain, and the electronic properties of 180° and 90° DWs in 2D GeS including the uncharged and charged DWs. All types of DWs in 2D GeS were found to be atomically sharp. In addition, the 90° uncharged DW was more energetically favorable than the 180° DW, which is similar to DWs of perovskites. However, due to the effect of adverse electrostatic energy, the charged DW possessed higher energy than that of the uncharged DW. On the other hand, the polarization distortion of the domain region in all DWs is significantly strengthened by the biaxial strain. In addition, the density of states showed that the charged DW is conductive relative to the uncharged domain wall, because the uncompensated positive or negative charges exist at the charged domain wall. Our results provide necessary theoretical guidance to the future exploration and application of 2D ferroelectric materials.

The Third Dimension of Ferroelectric Domain Walls.

Ferroelectric domain walls are quasi-2D systems that show great promise for the development of nonvolatile memory, memristor technology, and electronic components with ultrasmall feature size. Electric fields, for example, can change the domain wall orientation relative to the spontaneous polarization and switch between resistive and conductive states, controlling the electrical current. Being embedded in a 3Dmaterial, however, the domain walls are not perfectly flat and can form networks, which leads to complex physical structures. In this work, the importance of the nanoscale structure for the emergent transport properties is demonstrated, studying electronic conduction in the 3D network of neutral and charged domain walls in ErMnO3 . By combining tomographic microscopy techniques and finite element modeling, the contribution of domain walls within the bulk is clarified and the significance of curvature effects for the local conduction is shown down to the nanoscale. The findings provide insights into the propagation of electrical currents in domain wall networks, reveal additional degrees of freedom for their control, and provide quantitative guidelines for the design of domain-wall-based technology.

Application of Fluconazole-Loaded pH-Sensitive Lipid Nanoparticles for Enhanced Antifungal Therapy.

Cryptococcus neoformans is a yeast-like fungus that can cause the life-threatening disease cryptococcal meningitis. Numerous reports have shown increased resistance of this fungus against antifungal treatments, such as fluconazole (Fluc), contributing to an 80% global mortality rate. This work presents a novel approach to improve the delivery of the antifungal agent Fluc and increase the drug's targetability and availability at the infection site. Exploiting the acidic environment surrounding a C. neoformans infected site, we have developed pH-sensitive lipid nanoparticles (LNP) encapsulating Fluc to inhibit the growth of resistant C. neoformans. The LNP-Fluc delivery system consists of a neutral lipid monoolein (MO) and a novel synthetic ionizable lipid 2-morpholinoethyl oleate (O2ME). At neutral pH, because of the presence of O2ME, the nanoparticles are neutral and exhibit a liquid crystalline hexagonal nanostructure (hexosomes). At an acidic pH, they are positively charged with a cubic nanostructure (cubosomes), which facilitates the interaction with the negatively charged fungal cell wall. This interaction results in the MIC50 and MIC90 values of the LNP-Fluc being significantly lower than that of the free-Fluc control. Confocal laser scanning microscopy and scanning electron microscopy further support the MIC values, showing fungal cells exposed to LNP-Fluc at acidic pH were heavily distorted, demonstrating efflux of cytoplasmic molecules. In contrast, fungal cells exposed to Fluc alone showed cell walls mostly intact. This current study represents a significant advancement in delivering targeted antifungal therapy to combat fungal antimicrobial resistance.

Influence of viscoelectric effect on diffusioosmotic transport in nanochannel.

We have investigated the role of viscoelectric effect on diffusioosmotic flow (DOF) through a nanochannel connected with two reservoirs. The transport equations governing the flow dynamics are solved numerically using the finite element technique. We have extensively analyzed the variation of induced field due to electric double layer (EDL) phenomenon, relative viscosity as modulated by the viscoelectric effect as well as reservoir's concentration difference, and their eventual impact on the underlying flow characteristics. It is revealed that the induced electric field in the EDL enhances fluid viscosity substantially near the charged wall at a higher concentration. We have shown that neglecting viscoelectric effect in the paradigm of diffusioosmotic transport overestimates the net throughput, particularly at a higher concentration difference. Furthermore, we show that pertaining to chemiosmosis dominated regime, the average flow velocity modifies with the increase in concentration difference up to a critical value. In comparison, the rise in the strength of resistive electroosmotic actuation by the accumulation of anions in the upstream reservoir reduces the average flow velocity at a higher concentration difference. We have reported a reduction in critical concentration with the increase in viscoelectric effect. The inferences of this analysis are deemed pertinent to reveal the bearing of viscoelectric effect as a flow control mechanism pertaining to DOF at nanoscale.

Ultrahigh Carrier Mobilities in Ferroelectric Domain Wall Corbino Cones at Room Temperature.

Recently, electrically conducting heterointerfaces between dissimilar band insulators (such as lanthanum aluminate and strontium titanate) have attracted considerable research interest. Charge transport and fundamental aspects of conduction have been thoroughly explored. Perhaps surprisingly, similar studies on conceptually much simpler conducting homointerfaces, such as domain walls, are not nearly so well developed. Addressing this disparity, magnetoresistance is herein reported in approximately conical 180° charged domain walls, in partially switched ferroelectric thin-film single-crystal lithium niobate. This system is ideal for such measurements: first, the conductivity difference between domains and domain walls is unusually large (a factor of 1013 ) and hence currents driven through the thin film, between planar top and bottom electrodes, are overwhelmingly channeled along the walls; second, when electrical contact is made to the top and bottom of the domain walls and a magnetic field is applied along their cone axes, then the test geometry mirrors that of a Corbino disk: a textbook arrangement for geometric magnetoresistance measurement. Data imply carriers with extremely high room-temperature Hall mobilities of up to ≈3700 cm2 V-1 s-1 . This is an unparalleled value for oxide interfaces (and for bulk oxides) comparable to mobilities in other systems seen at cryogenic, rather than at room, temperature.

Rational Design of Molecular Ferroelectrics with Negatively Charged Domain Walls.

Ferroelectric domains and domain walls are unique characteristics of ferroelectric materials. Among them, charged domain walls (CDWs) are a special kind of peculiar microstructure that highly improve conductivity, piezoelectricity, and photovoltaic efficiency. Thus, CDWs are believed to be the key to ferroelectrics' future application in fields of energy, sensing, information storage, and so forth. Studies on CDWs are one of the most attractive directions in conventional inorganic ferroelectric ceramics. However, in newly emerged molecular ferroelectrics, which have advantages such as lightweight, easy preparation, simple film fabrication, mechanical flexibility, and biocompatibility, CDWs are rarely observed due to the lack of free charges. In inorganic ferroelectrics, doping is a traditional method to induce free charges, but for molecular ferroelectrics fabricated by solution processes, doping usually causes phase separation or phase transition, which destabilizes or removes ferroelectricity. To realize stable CDWs in molecular systems, we designed and synthesized an n-type molecular ferroelectric, 1-adamantanammonium hydroiodate. In this compound, negative charges are induced by defects in the I- vacancy, and CDWs can be achieved. Nanometer-scale CDWs that are stable at temperatures as high as 373 K can be "written" precisely by an electrically biased metal tip. More importantly, this is the first time that the charge diffusion of CDWs at variable temperatures has been investigated in molecular ferroelectrics. This work provides a new design strategy for n-type molecular ferroelectrics and may shed light on their future applications in flexible electronics, microsensors, and so forth.

Constructing fast ion-transport channels for reversible zinc metal anodes enabled by self-concentration effect

The zinc (Zn) metal batteries suffer from poor plating/stripping behaviors due to the severe dendrite growth and side reactions stemming from the sluggish ion migration at the electrolyte/electrode interface. Despite effectiveness by constructing artificial ion-diffusion layer to alleviate these issues, the intrinsic mechanism of zinc ion (Zn2+) diffusion within the interfacial layer is not well elucidated yet. Here, inspired by the ability of vermiculite (VRM) to promote the self-concentration kinetics of metal ions, this intrinsic issue can be tackled by elaborately constructing VRM with fast ion-transport channels on the surface of Zn metal anodes. This unique ion channel with a natural negatively charged wall can accelerate Zn2+ transport and reduce the presence of water molecules in Zn2+ solvated shell, which can be demonstrated by molecular dynamic simulation. As a proof of concept, the symmetric cell with VRM@Zn anode achieves dendrite-free plating/stripping with a stable cycling life (580 h) even at a high rate of 25 mA cm−2 with a capacity of 12.5 mAh cm−2. Meanwhile, the Zn-carbon supercapacitor with VRM@Zn anode exhibits an extended stability over 5000 cycles at 1 A g−1. This work reveals the internal mechanism of Zn2+ diffusion within the interfacial layer, and provides an avenue for constructing highly reversible Zn metal anode.

Electrostatic-Gated Kinetics of Rapid Ion Transfers at a Nano-liquid/Liquid Interface

Charge (ion and electron)-transfer reactions at a liquid/liquid interface are critical processes in many important biological and chemical systems. An ion-transfer (IT) process is usually very fast, making it difficult to accurately measure its kinetic parameters. Nano-liquid/liquid interfaces supported at nanopipettes are advantageous approaches to study the kinetics of such ultrafast IT processes due to their high mass transport rate. However, correct measurements of IT kinetic parameters at nanointerfaces supported at nanopipettes are inhibited by a lack of knowledge of the nanometer-sized interface geometry, influence of the electric double layer, wall charge polarity, etc. Herein, we propose a new electrochemical characterization equation for nanopipettes and make a suggestion on the shape of a nano-water/1,2-dichloroethane (nano-W/DCE) interface based on the characterization and calculation results. A theoretical model based on the Poisson-Nernst-Planck equation was applied to systematically study how the electric double layer influences the IT process of cations (TMA+, TEA+, TPrA+, ACh+) and anions (ClO4-, SCN-, PF6-, BF4-) at the nano-W/DCE interface. The relationships between the wall charge conditions and distribution of concentration and potential inside the nanopipette revealed that the measured standard rate constant (k0) was enhanced when the polarity of the ionic species was opposite to the pipette wall charge and reduced when the same. This work lays the right foundation to obtain the kinetics at the nano-liquid/liquid interfaces.

Vertically-Ordered Mesoporous Silica Films for Electrochemical Detection of Hg(II) Ion in Pharmaceuticals and Soil Samples.

Rapid and simple determination of mercury ion (Hg2+) in pharmaceuticals and soil samples is vital for human health and the environmental monitoring. Vertically-ordered mesoporous silica films (VMSF) supported by the indium tin oxide (ITO) electrode surface were prepared by electrochemically assisted self-assembly method and utilized for electrochemical detection of Hg2+. Owing to the negatively charged channel walls and ultrasmall pore diameter, VMSF displays obvious cationic selectivity and has highly electrostatic interaction for Hg2+, giving rise to the strong electrochemical signals. By recording the anodic stripping signals of adsorbed Hg2+ using differential pulse voltammetry, quantitative detection of Hg2+ was achieved with a wide linear range (0.2 μM–20 μM) and a low limit of detection (3 nM). Furthermore, considering the anti-fouling and anti-interference capacity of VMSF, the proposed VMSF/ITO sensor has been successfully applied to detect Hg2+ in pharmaceuticals and soil samples without tedious pretreatment processes of samples.

Study of ferroelectric thin films by ising model with a depolarizing field

In this paper, thin films are considered within the framework of the Ising model with a depolarizing field. In the existing works, the depolarizing field is considered to be proportional to the polarization, but it is not suitable for polydomain films. According to our approach, the depolarizing field energy is calculated as the difference between the energy of dipole-dipole interactions in a bulk sample and a thin film. Stripe domains with neutral and charged walls can appear in the considered model. We also studied domain structures arising in films of different thicknesses with increasing temperature and depolarizing field value.

Manipulation of charged domain walls in geometric improper ferroelectric thin films: A phase-field study

Using phase-field simulations, we show how interfaces acting on the geometric-improper ferroelectric polarization of hexagonal manganite and ferrite thin films can be used to control the formation of charged domain walls. We modify the Landau expansion of the free energy valid in bulk to emulate interface effects known from previous cross-sectional experiments, and we verify our model by comparing our results with images obtained in these experiments. We then show how the interface affects the orientation of ferroelectric domain walls in the fully three-dimensional case. Furthermore, we demonstrate that interface effects combined with an external electric field enable us to specifically choose the dominant domain-wall type (head-to-head, tail-to-tail, or neutral). We also find that an electric field can stabilize a novel domain-wall type which only emerges in the improper ferroelectric order but not in the primary structural distortion. Since the domain walls have a conductivity that is different from the interior of the domains, the influence of the interfaces of a thin film on the type and distribution of the walls gives us the possibility to control the transport properties of a material by appropriate thin-film engineering.

Ferroelectric domain-wall logic units

The electronic conductivities of ferroelectric domain walls have been extensively explored over the past decade for potential nanoelectronic applications. However, the realization of logic devices based on ferroelectric domain walls requires reliable and flexible control of the domain-wall configuration and conduction path. Here, we demonstrate electric-field-controlled stable and repeatable on-and-off switching of conductive domain walls within topologically confined vertex domains naturally formed in self-assembled ferroelectric nano-islands. Using a combination of piezoresponse force microscopy, conductive atomic force microscopy, and phase-field simulations, we show that on-off switching is accomplished through reversible transformations between charged and neutral domain walls via electric-field-controlled domain-wall reconfiguration. By analogy to logic processing, we propose programmable logic gates (such as NOT, OR, AND and their derivatives) and logic circuits (such as fan-out) based on reconfigurable conductive domain walls. Our work might provide a potentially viable platform for programmable all-electric logic based on a ferroelectric domain-wall network with low energy consumption.

Current Flow in a Cylindrical Nanopore with an Object-Implications for Virus Sensing.

Interest is growing in nanopores as real-time, low-cost, label-free virus size sensors. To optimize their performance, we evaluate how external electric field and ion concentrations and pore wall charges influence currents and object (disk) radius-current relationship using simulations. The physics was described using the Poisson-Nernst-Planck and Navier–Stokes equations. In a charged cylindrical nanopore with a charged disk, elevated external electric field produces higher (and polarity independent) ion concentrations and greater ion current (largely migratory). Elevated external ion concentrations also lead to higher concentrations (mainly away from the pore wall), greater axial electric field especially in the disk-pore wall space, and finally larger current. At low concentrations, current is disk radius independent. The current rises as concentrations increase. Interestingly, the rise is greater for larger disks (except when the pore is blocked mechanically). Smaller cross-sectional area for current flow or volume exclusion of electrolyte by object thus cannot be universally accepted as explanations of current blockage. Ion current rises when pore wall charge density increases, but its direction is independent of charge sign. Current-disk radius relationship is also independent of pore wall charge sign. If the pore wall and disk charges have the same sign, larger current with bigger disk is due to higher counter-ion accumulation in the object-pore wall space. However, if their signs are opposite, it is largely due to elevated axial electric field in the object-pore wall space. Finally in uncharged nanopores, current diminishes when disk radius increases making them better sensors of virus size.

Reconstruction of the ferroelectric domain structure morphology in BaTiO3 single crystals using Čerenkov-type second harmonic generation microscopy

Domain structure arising as a result of surface polishing in the bulk of barium titanate BaTiO3 single crystal was imaged by polarized optical microscopy and Čerenkov-type second harmonic generation (ČSHG) microscopy, which allowed distinguishing between 90° and 180° domain walls. 3D-morphology of different domain patterns in the bulk was recovered using ČSHG images. The domain structure represented the stack of the intermitting needlelike a-domains with charged domain walls rarely crossed by the zigzag domain patterns with 180° walls. The used experimental technique can be used for understanding the domain kinetics in the multiaxial crystals.

Decay of domains created by local switching on non-polar cut of MgO doped LiNbO3 single crystals

Decay of isolated wedge-like domains with charged domain walls during local switching by a biased SPM tip on non-polar cut of lithium niobate doped by MgO was studied using piezoresponse force microscopy. It was found that domain imaging led to domains length reduction, whereas domain width did not change. Domain decay without scanning was also revealed. All the obtained effects were attributed to influence of injected charges, residual depolarization field, and AC voltage. Spatial distribution of electric field near SPM tip was simulated numerically. The obtained results provide new understanding of domain decay problem for nanodomain engineering.

Shape of charged domain walls in bidomain lithium tantalate plates with composition gradients

Lithium tantalate LiTaO3 (LT) plates with various composition gradient along the polar axis were produced by partial vapor transport equilibration (VTE) with annealing temperature above phase transition. The proper annealing parameters allow creating a charged domain wall (CDW) in the center of the plate. The CDW morphology depends on the spatial distribution of stoichiometry deviation. The bidomain actuators manufactured from the LT plates after partial VTE procedure possess a linear voltage dependence of the displacement of the free end without any hysteresis. It is shown that increase in CDW roughness leads to a noticeable decrease in the effective piezoelectric coefficient.

A square superlattice pattern formed through complex interactions among volume discharges and surface discharge in dielectric barrier discharge

We report a square superlattice pattern with two interleaving grids [(line-grid) and (rod-grid)] and three lattices composed of discrete spots [spot, halo, and spot(w)] in dielectric barrier discharge. The spatiotemporal dynamics is measured by intensified charge-coupled device, photomultiplier tubes, and high-speed video camera. It is found that the line-grid is composed of direction-selective surface discharges, which are induced by wall charge of spot, compressed by wall charge of spot(w), and guided by wall charge of random spots in rod. The rod-grid and the following halo consist of random volume discharges, which are affected by the distribution of wall charges of spot(w), spot, and line-grid. The pattern is formed through a series of complex interactions among volume discharges and surface discharge. These results will promote the study on interaction between volume discharge and surface discharge in dielectric barrier discharge.

Simulation study on the flow behaviors of weak electrolyte solutions in nano-sized and micron-sized pores considering electric double layer interactions

ABSTRACT The interactions between the ions in the weak electrolyte solutions and the charged solid wall result in the formation of an electric double layer (EDL) in nano-sized and micron-sized pores. However, in nano-sized pores, EDLs may be overlapping. Herein, a widely applicable Poisson-Nernst-Planck equation combined with the Navier–Stokes equation was adopted for the flow of weak electrolyte solutions under pressure-driven conditions. Meanwhile, the finite element method was utilized to solve the governing equations and investigate the influence of EDL interactions on flow behaviors in different pore radii (25 nm~5 μm). The results showed that as the pore radius increased, the influence of EDL interactions on fluid flow behaviors first increased and then decreased. The influence of EDL interactions on fluid flow behaviors was the most noticeable when the pore radius was close to the EDL thickness. Furthermore, the influence of EDL interactions appeared to be negligible in the pore radius greater than 5 μm. When EDL interactions were ignored, the volume flow in the pores of the circular, square, and equilateral triangle was increased by about 13%. This study provides new insights into the flow behaviors of weak electrolyte solutions in nano-sized and micron-sized pores.