Opposite Charge Research Articles

Transient Response Mismatch: A Limiting Factor of Perovskite Photodetectors

AbstractDue to the excellent optoelectronic performance, perovskite photodetectors (PDs) have attracted a lot of attention in recent years. However, the response speed of such a device is still relatively low. To overcome this defect, the intrinsic mechanisms within the perovskite PDs have to be uncovered. Here a comprehensive optoelectronic simulation is implemented to examine the transient response of the perovskite PDs, where the dynamics of both carriers (electrons/hoes) and ions (anions/cations) are taken into account. The unmatched temporal responses between the photogenerated electrons and holes are found to severely limit the overall response speed. Unfortunately, solely manipulating the perovskite parameters (e.g., thickness, doping concentration, and ion migration) cannot effectively suppress such a detrimental effect. Further study indicates that the mismatch in temporal response originates from the different electrical properties of the opposite charge transport layers, and can only be alleviated by balancing the corresponding carrier extraction capabilities. A roadmap for improving the response speed of the perovskite PDs is finally proposed, which shows the greatly shortened response time from 118.21 to 7.45 ns.

Kinetics, isotherms, and mechanism of removing cationic and anionic dyes from aqueous solutions using chitosan/magnetite/silver nanoparticles

Modified magnetite chitosan with silver nanoparticles was synthesized and tested for removing cationic and anionic dyes in aqueous solutions. Initial dye concentration, pH, and contact time were examined. Results showed that pH (4.0) was optimal for removing anionic dyes (methyl orange) and pH 8.0 for removing cationic dyes (methylene blue). According to these results, zeta potentials were found to be 8.43 and − 39.17 mV at pH 4.0 and 8.0, respectively. So, it is attracted to positively charged cationic dyes in an alkaline medium and negatively charged anionic dyes in an acidic medium because of their opposite charges. Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), thermal gravimetric analysis (TGA), and zeta potential measurements were used to characterize the synthesized nanosorbents. A pseudo-second-order kinetic model is fitted with the Langmuir adsorption model, with an adsorption capacity of 417 and 476 mg/g for methyl orange and methylene blue, respectively. For both dyes, modified magnetite chitosan with silver nanoparticles showed high regeneration capability and recovery for up to four cycles without adsorption efficiency loss. Furthermore, modified magnetite chitosan with silver nanoparticles, as prepared in the present study, was demonstrated to be an effective adsorbent for organic pollutants in wastewater.

Phase-Change Perovskite Microlaser with Tunable Polarization Vortex.

Metasurfaces supporting optical bound states in the continuum (BICs) are emerging as simple and compact optical cavities to realize polarization-vortex lasers. The winding of the polarization around the singularity defines topological charges which are generally set by the cavity design and cannot be altered without changing geometrical parameters. Here, a subwavelength-thin phase-change halide perovskite BIC metasurface functioning as a tunable polarization vortex microlaser is demonstrated. Upon the perovskite structural phase transitions, both its refractive index and gain vary substantially, inducing reversible and bistable switching between distinct polarization vortexes underpinned by opposite topological charges. Dynamic tuning and switching of the resulting vector beams may find use in microscopy imaging, particle trapping and manipulation, and optical data storage.

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation.

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr1+ x Te2 with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr1.53 Te2 with a Curie temperature of 295K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106nΩ cm is observed at 290K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

An ameliorated anti-hTNF-α therapy for arthritis via carrier-free macromolecular nanoparticles consisted of infliximab

Infliximab (INF) is intravenously used for the clinical treatment of rheumatoid arthritis. However, it can cause serious side effects, which are mainly associated with systemic exposure and high doses. Here, we developed a modified hydrophobic ion-pairing complexes (INF HIPC) through the sequential introduction of bovine lactoferrin (BLF) and hyaluronic acid (HA) with opposite charges into the INF solution. INF and BLF were found to be not only integrally responsible for the structural integrity of HIPC but also were determined to have respective biological activities by binding human tumor necrosis factor-alpha (hTNF-α) or promoting the proliferation of osteoblasts. The INF HIPC had good stability, high drug-loading efficiency, and long-term retention effects. Whether via knee joint injection or intravenous injection, INF HIPC resulted in lower hTNF-α levels and less cartilage destruction than INFs in the transgenic mouse model. At the same time, INF HIPC could reduce toxicity based on body weight changes in transgenic mice. Our findings provide a simple and promising avenue to develop advanced delivery systems for other antibodies and macromolecules.

Corrugation-Induced Active Sites on Pristine Graphene for H2 Activation

Pristine graphene is widely considered to be chemically inert, but recent experimental studies have suggested that it can provide frustrated Lewis pairs (FLPs) for activating H2 under mild conditions. Using density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations, we explore in this work the possibility that corrugation in pristine graphene is responsible for the formation of FLPs and thus its observed catalytic activity. Our DFT results demonstrate that the ease of H2 activation is proportional to the degree of corrugation of graphene. For corrugated graphene, the two catalytic para-carbon sites show clear signs of transient Lewis acid/base properties along the reaction coordinate, a pre-requisite for FLPs. Furthermore, the two dissociating H atoms carry opposite charges, suggesting heterolytic activation of H2. This behavior is confirmed by AIMD simulations at room temperature, which show that fluctuations of the corrugated graphene lead to C sites with diverse distances and varying charges. These observations offer a plausible rationalization of the experimental observations and possible design principles for FLP catalysts using homogeneous two-dimensional materials.

An investigation on adsorption of carbamazepine with adsorbents developed from flax shives: Kinetics, mechanisms, and desorption

In this work, adsorbents based on hydrochars and steam activated hydrochars developed from flax shives were used to adsorb carbamazepine (CBZ) from simulated contaminated water. The adsorption kinetics was investigated at various temperatures (293–313 K) and times (2–96 h). Four different kinetic models were used to analyze the experimental CBZ adsorption kinetic data. Among the tested various models, the pseudo-second-order kinetic model provided the best simulation to the kinetic data of both adsorbents with R2 of being 0.98 and above, and activation energy of 31.8 and 16.45 kJ/mol, respectively. The determined adsorption rate constant of the steam activated hydrochars ((5.88–9.09) × 10−5) was much higher than that of the hydrochars ((2.81–6.44) × 10−6). Further, X-ray photoelectron spectra and Near-Edge X-Ray Absorption Fine Structure (NEXAFS) Spectroscopy analysis were done before and after loading with CBZ. The results show π-π electron donor-acceptor interaction played an important role on the adsorption mechanism of CBZ on the above-mentioned adsorbents. Hydrophobic interaction also contributed while the roles of hydrogen bonding and electrostatic attraction between CBZ and adsorbents with opposite charges is insignificant. Pore filling may contribute. Desorption of CBZ from CBZ-loaded adsorbents was investigated with various organic solvents, and water at different pH (2, 6 and 10), among which ethanol was the most effective solvent to desorb CBZ. Overall, the results demonstrated that flax shives are promising feedstocks to be made into adsorbents for treatment of wastewater contaminated by antibiotics and additional pharmaceuticals.

Polystyrene Sulfonate Particles as Building Blocks for Nanofiltration Membranes.

Today the standard treatment for wastewater is secondary treatment. This procedure cannot remove salinity or some organic micropollutants from water. In the future, a tertiary cleaning step may be required. An attractive solution is membrane processes, especially nanofiltration (NF). However, currently available NF membranes strongly reject multivalent ions, mainly due to the dielectric effect. In this work, we present a new method for preparing NF membranes, which contain negatively and positively charged domains, obtained by the combination of two polyelectrolytes with opposite charge. The negatively charged polyelectrolyte is provided in the form of particles (polystyrene sulfonate (PSSA), d ~300 nm). As a positively charged polyelectrolyte, polyethyleneimine (PEI) is used. Both buildings blocks and glycerol diglycidyl ether as crosslinker for PEI are applied to an UF membrane support in a simple one-step coating process. The membrane charge (zeta potential) and salt rejection can be adjusted using the particle concentration in the coating solution/dispersion that determine the selective layer composition. The approach reported here leads to NF membranes with a selectivity that may be controlled by a different mechanism compared to state-of-the-art membranes.

Simulating anti-skyrmions on a lattice

Magnetic skyrmions are meta-stable spin structures that naturally emerge in magnetic materials. While a vast amount of effort has gone into the study of their properties, their counterpart of opposite topological charge, the anti-skyrmion, has not received as much attention. We aim to close this gap by deploying Monte Carlo simulations of spin-lattice systems in order to investigate which interactions support anti-skyrmions, as well as skyrmions of Bloch and Néel type. We find that the combination of ferromagnetic exchange and Dzyaloshinskii–Moriya (DM) interactions is able to stabilize each of the three types, depending on the specific structure of the DM interactions. Considering a three-dimensional spin lattice model, we provide a finite-temperature phase diagram featuring a stable anti-skyrmion lattice phase for a large range of temperatures. In addition, we also shed light on the creation and annihilation processes of these anti-skyrmion tubes and study the effects of the DM interaction strength on their typical size.

Propagation of Gaussian vortex beams in electromagnetically induced transparency media.

Electromagnetically induced transparency (EIT) is an important phenomenon in quantum optics, and has a wide range of applications in the fields of quantum information processing and quantum precision metrology. Recently, with the rapid progress of the generation and detection of structured light, the EIT with structured light has attracted enormous interests and offers new and novel functionalities and applications. Here, we theoretically study the propagation and evolution of Gaussian vortex beams, a typical type of structured light, in an EIT medium with Λ-type three-level atoms. Based on the generalized Huygens-Fresnel principle, we derive the analytical expressions of fully and partially coherent Gaussian vortex beams propagating in the EIT medium, and study the evolution of the intensity and phase distributions of the beams and their dependencies on parameters such topological charge, coherence length, Rabi frequency, etc. It is shown that both the fully and partially coherent Gaussian vortex beams undergo focusing and diverging periodically during propagation. The phase singularity of the fully coherent beam keeps unchanged, while the phase singularity of the partially coherent beam experiences splitting and recombination periodically. In addition, new phase singularities with opposite topological charge are generated in the latter case. Our results not only advance the study of the interaction between structured light and coherent media, but also pave the avenue for manipulating structured light via EIT.

Choice of DLVO approximation method for quantifying the affinity between latex particles and membranes

The Derjaguin-Landau-Verwey-Overbeek (DLVO) model, as well as the extended model (XDLVO), is popularly employed to quantify the interfacial interactions underlying membrane fouling. However, disagreements between membrane fouling extent and the interaction energies derived from DLVO or XDLVO models have been reported, which suggests gaps in the understanding. This study demonstrates that the DLVO approximation methods for predicting the interfacial foulant-membrane interaction are sensitive to the boundary condition assumptions (e.g., constant charge versus constant potential). In particular, while both the Poisson-Boltzmann (P–B) and linear superposition approximation (LSA) equations can quantify the electrostatics (EL) interaction energy component, the former assumes constant potential, while the latter additionally considers constant charge scenarios. The relative accuracy of these two equations were evaluated here. For dead-end filtration tests, flux decline trends, OCT analysis results and fouling model parameters were obtained. Regarding fouling of pristine PCTE membrane by latex particles of opposite charge signs, both the P–B and LSA equations contribute to predict relative fouling extents correctly. However, for the case of the BPEI-coated membrane, the P–B equation failed whereas the LSA equation gave good agreements. For cross-flow filtration tests in organic solvents, LSA also out-performed the P–B equation in providing more accurate predictions of membrane fouling. The results here highlight the shortcomings in the commonly used P–B equation and are expected to be potentially valuable in the development of better approximations for quantifying interfacial interaction energies.

Improved performance of stretchable piezoelectric energy harvester based on stress rearrangement

With the development of wearable devices and soft electronics, the demand for stretchable piezoelectric energy harvesters (SPEHs) has increased. Energy harvesting can provide energy when large batteries or power sources cannot be employed, and stretchability provides a user-friendly experience. However, the performance of SPEHs remains low, which limits their application. In this study, a wearable SPEH is developed by adopting a kirigami structure on a polyvinylidene fluoride film. The performance of the SPEH is improved by rearranging the stress distribution throughout the film. This is conducted using two approaches: topological depolarization, which eliminates the opposite charge generation by thermal treatment, and optimization of the neutral axis, which maximizes the stress applied at the surface of the piezoelectric film. The SPEH performance is experimentally measured and compared with that of existing SPEHs. Using these two approaches, the stress was rearranged in both the x–y plane and z-direction, and the output voltage increased by 21.57% compared with that of the original film with the same stretching motion. The generated energy harvester was successfully applied to smart transmittance-changing contact lenses.

Hydrophilic Versus Hydrophobic Coupling in the Pressure Dependence of the Chemical Potential of Alkali Metal and Halide Ions in Water.

We computed the chemical potential for some alkali metal ions (K+, Rb+, and Cs+) and two halide ions (Br- and I-) in aqueous solution at ambient T and various pressures in the range 1-8000 atm. Results were obtained from classic Monte Carlo simulations in the NPT ensemble by means of the free energy perturbation method. Here, the chemical potential is computed as the sum of a term relative to a Lennard-Jones solute and a term relative to the process in which this solute is transformed into the ion. Hydrophobic and hydrophilic features of these two components of the chemical potential show opposite behaviors under isothermal compression. The increase in pressure determines an increase in the hydrophobic component, which becomes more positive with a stronger effect for larger ions. Correspondingly, the values of the hydrophilic component become more negative for alkali ions, whereas they are only slightly affected by compression for halide ions. Hydrophobic-hydrophilic quasi-compensation in the slopes is observed for Rb+. For a smaller ion, such as K+, the dependence on pressure of the hydrophilic component is slightly dominant. For a larger ion, as observed in the cases of Cs+, Br-, and I-, the hydrophobic component assumes the determinant role. Pressure dependence of the chemical potential is little affected by corrections introduced for molecular potential truncation. This view can change for possible boundary artifacts that could have affected the static electrostatic potential. Some inference is obtained from comparison with experimental data at 1 atm on the free energy of hydration. Discrepancies show the characteristic asymmetry between cations and anions. The further addition of a correction based on the static potential significantly reduces these discrepancies with important error cancellation on the sum of chemical potentials of ions of opposite charge. The correction is applied also at higher pressures, and results are compared with those obtained by adding an alternative correction that is based on the water number density. Regardless of the ion, changes of the chemical potential induced by an increase in pressure appear to be dominated by the hydrophobic component, in particular when using the alternative correction. For bromide and iodide electrolytes, the two corrections give chemical potentials in good agreement.

Pickering Emulsions Stabilized by Binary Mixtures of Colloidal Particles: Synergies between Contrasting Properties.

Pickering emulsions that are stabilized by colloidal particles have attracted substantial research attention because of their potential applications in various industries. Previously, single colloidal particles have usually been used to fabricate Pickering emulsions and to investigate the stabilization mechanism. However, surface modification of the colloidal stabilizer is normally required to adjust the particle wettability, which often involves chemical modification, the adsorption of a surfactant or polymer, and the addition of an electrolyte. Such a modification is expensive, time-consuming, and thus only partially effective. In this Perspective, we describe an alternative approach that uses binary mixtures of particles as stabilizers and could be an effective solution to the above-described problems with Pickering emulsions. We introduce various types of Pickering emulsions stabilized by binary mixtures of particles with different functional groups, opposite charges, or opposite wettabilities (i.e., they are hydrophilic or hydrophobic). Examples of stabilizing mechanisms are discussed, showing that compared with emulsions stabilized by single colloidal particles, emulsions stabilized by binary mixtures of particles are generated via simpler particle-pretreatment processes and have higher stability and customizable properties and thus can enable the exploration of the next generation of Pickering emulsions.

Branched SL(2,ℤ) duality

We investigate how SL(2,ℤ) duality is realized in nonrelativistic type IIB superstring theory, which is a self-contained corner of relativistic string theory. Within this corner, we realize manifestly SL(2,ℤ)-invariant (p, q)-string actions. The construction of these actions imposes a branching between strings of opposite charges associated with the two-form fields. The branch point is determined by these charges and the axion background field. Both branches must be incorporated in order to realize the full SL(2,ℤ) group. Besides these string actions, we also construct D-instanton and D3-brane actions that manifestly realize the branched SL(2,ℤ) symmetry.

An efficient new method for electrospinning chitosan and heparin for the preparation of pro‐angiogenic nanofibrous membranes for wound healing applications

AbstractChronic skin wounds and surgical sutures need critical care and fast recovery, robust connection of blood vessels and effective restoration of circulation is necessary in progressive wound healing. Heparin is well known for its' anticoagulant properties, VEGF activation and antithrombosis action. Chitosan aid in the development of the vascular grafts due to its ECM like properties of blood vessels. For electrospinning of Heparin negatively charged and low molecular weight positively charges chitosan, the homogenous solution is required, they precipitate out during mixing due to opposite charges. In the current study, a an efficient strategy is developed for the electrospinning of chitosan and heparin in the presence of lysozyme. The insolubility/non‐homogenous solution formation for electrospinning from charged chitosan and heparin fast acting was solved by using a small amount of N‐cetyl‐N,N,N‐trimethyl ammonium bromide (CTAB) and the resulting solution produced very smooth nanofibers. Polycaprolactone (PCL) was found to be a suitable polymer for the electrospinning of chitosan and heparin using organic and inorganic solvents. Surface morphology of the synthesized fibers was investigated by scanning electron microscopy (SEM) and the presence of functional groups was investigated by FTIR. Degradation studies were performed which revealed that lysozyme loaded materials were degraded much faster as compared to other materials. The angiogenic and biocompatible potential of heparin with 1 mg/ml and 4 mg/ml lysozyme concentration was demonstrated by chorionic allantoic membrane (CAM) assay and it was estimated that 1 mg/ml (lysozyme) loaded CS/HA material was found to be an efficient biomaterial to stimulate angiogenesis.

AC Kelvin Probe Force Microscopy Enables Charge Mapping in Water.

Mapping charged chemical groups at the solid-liquid interface is important in many areas, ranging from colloidal systems to biomolecular interactions. However, classical methods to measure surface charges either lack spatial resolution or─like Kelvin-probe force microscopy (KPFM)─cannot be applied in aqueous solutions because a DC bias voltage is used. Here, we show that using AC Kelvin probe force microscopy (AC-KPFM), in which the DC bias is replaced with an AC voltage of sufficiently high frequency, the surface potential of spatially fixated, charged surface groups can be mapped in aqueous solution. We demonstrate this with micropatterned, functionalized alkanethiol layers which expose ionized amino- and carboxy-groups. These groups are representative of the charged groups of most biomolecules such as proteins. By adjusting the pH of the solution, the charge of the groups was reversibly altered, demonstrating the electrostatic nature of the measured signal. The influence of the electric double layer (EDL) on the measurement is discussed, and we, furthermore, show how charged, micropatterned layers can be used to spatially direct the deposition of nanoparticles of opposite charge.

Effect of Cationic Modified Microcrystalline Cellulose on the Emulsifying Properties and Water/Oil Interface Behavior of Soybean Protein Isolate.

Stabilizing emulsion using complex biopolymers is a common strategy. It would be very interesting to characterize the impact of charge density on the emulsifying properties of complex polyelectrolytes carrying opposite charges. In this study, cationic modified microcrystalline celluloses (CMCC) of different charge densities were prepared and mixed with soy protein isolate (SPI) for emulsion applications. CMCC-1 to 3 with various cationic charge values were successfully prepared as characterized by zeta-potential and FTIR. The positive charge density’s effects on solubility, thermogravimetric properties, and rheological properties were studied. Complexes of SPI-CMCC with various zeta-potential values were then obtained and used to stabilize soybean oil emulsions. The results show that emulsions stabilized by complexes of SPI and CMCC-3 at a ratio of 1:3 had the best emulsification ability and stability. However, the interfacial tension-reducing ability of complexes decreased continuously with increasing cationic charge value, while the rheological results show that complexes of SPI-CMCC-3 at a ratio of 1:3 formed a stronger viscoelastic network than other complexes. Our results indicate that this SPI-CMCC complex formula showed excellent emulsification performance, which could be adjusted and promoted by changing the charge density. This complex formula is promising for fabrication of emulsion-based food and cosmetic products.

Strategies That Utilize Ion Pairing Interactions to Exert Selectivity Control in the Functionalization of C-H Bonds.

Electrostatic attraction between two groups of opposite charge, typically known as ion-pairing, offers unique opportunities for the design of systems to enable selectivity control in chemical reactions. Catalysis using noncovalent interactions is an established and vibrant research area, but it is noticeable that hydrogen bonding interactions are still the main interaction of choice in system design. Opposite charges experience the powerful force of Coulombic attraction and have the ability to exert fundamental influence on the outcome of reactions that involve charged reagents, intermediates or catalysts. In this Perspective, we will examine how ion-pairing interactions have been used to control selectivity in C–H bond functionalization processes. This broad class of reactions provides an interesting and thought-provoking lens through which to examine the application of ion-pairing design strategies because it is one that encompasses great mechanistic diversity, poses significant selectivity challenges, and perhaps most importantly is of immense interest to synthetic chemists in both industry and academia. We survey reactions that proceed via radical and ionic mechanisms alongside those that involve transition metal catalysis and will deal with control of site-selectivity and enantioselectivity. We anticipate that as this emerging area develops, it will become an ever-more important design strategy for selectivity control.

Two-dimensional intrinsic ferrovalley Janus 2H-VSeS monolayer with high Curie temperature and robust valley polarization

Inspired by the successful synthesis of two-dimensional (2D) V-based Janus dichloride monolayers with intrinsic ferromagnetism and high Curie temperature (T$_{c}$), the electronic structure, spin-valley splitting and magnetic anisotropy of Janus 2H-VSeS monolayers are investigated in detailed using first-principles calculations. The results show that the Janus 2H-VSeS monolayer exhibits a large valley splitting of 105meV, high T$_{c}$ of 278K and good magnetocrystalline anisotropy (0.31meV) contributed by the in-plane d$_{x^{2}-y^{2}}$/d$_{xy}$ orbitals of V atoms. The biaxial strain ($-$8%<$\varepsilon$<8%) can effectively tune the magnetic moments of V atom, valley splitting $\Delta$E, T$_{c}$ and MAE of Janus 2H-VSeS monolayer. The corresponding $\Delta$E and T$_{c}$ are adjusted from 72meV to 106.8meV and from 180K to 340K, respectively. The electronic phase transition from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), and half-metal (HM) is also observed due to the change of V 3d-orbital occupation. Due to the broken space- and time-reversal symmetry, the opposite valley charge carriers carry opposite Berry curvature, which leads to prominent anomalous Hall conductivity at the K and K$^{\prime}$ valleys. The maximum modulation of Berry curvature can reach to 45% and 9.5% by applying the biaxial strain and charge carrier doping, respectively. The stable in-plane magnetocrystalline anisotropy and large spontaneous valley polarization make the ferromagnetic Janus 2H-VSeS monolayer a promising material for achieving the spintronics and valleytronics devices.