Local Charge Density Research Articles

Dust shedding from a dielectric surface in plasma as a result of charge fluctuations

If a body is in a plasma and/or is exposed to streams of charged particles with different signs, or is under UV electromagnetic radiation, etc., then the distribution of the different sign charges on the surface is random. As a result, the surface of the dielectric will be covered by microscopic “spots” with different signs and values of charges, continuously changing in shapes and sizes. A Coulomb force acting on a dust particle lying near the center of such a spot is proportional to the square of the local surface charge density. A stochastic differential equation describing the dynamics of an ensemble of these spots was derived and solved in the article. A solution was obtained also for the dependence of the average spot size, its lifetime, and the standard deviation of the charge density as a function of the plasma species fluxes incident on the surface. It is shown that for normal values of plasma parameters, the Coulomb force repelling a submicron dust particle from a charge spot of somewhat larger size can reach a few tenths of pN, which is comparable to the value of the adhesive van der Waals force that holds dust particles on the surface. The possibility of improving the cleaning efficiency with changes in surface treatment conditions is analyzed as well.

Electronic switching by metastable polarization states in BiFeO3 thin films

We present an approach to control resistive switching in metal-ferroelectric contacts using a radially symmetric electric field. In ferroelectrics with significant polarization along the corresponding field lines, the field above a critical threshold will induce polarization discontinuity, a corresponding nanoscale volume of space charge, and a conducting junction. We demonstrate this principle using nanoscale polarization switching of a conventional (001)-oriented thin film of $\mathrm{BiFe}{\mathrm{O}}_{3}$. Without any optimization, the conducting state created in this regime of resistive switching exhibits local currents of $\ensuremath{\sim}1\text{--}10\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$, approaching the $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$ threshold required for device implementation [J. Jiang et al., Nat. Mater. 17, 49 (2018)]. The corresponding electronic function is that of a volatile resistive switch, which is directly compatible with neuristor functionality that encodes the functioning basis of an axon [M. D. Pickett et al., Nat. Mater. 12, 114 (2013)]. Phase-field modeling further reveals that in the strongly charged local configuration, $\mathrm{BiFe}{\mathrm{O}}_{3}$ locally undergoes a rhombohedral-tetragonal (R-T) phase transition, in part due to substantial piezoelectric expansion of the lattice. The estimated local charge density can be as high as $\ensuremath{\sim}{10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, which would be extremely difficult to achieve by conventional doping approaches without altering other material properties. Therefore, this method for creating stable and reproducible strongly charged ferroelectric junctions enables more systematic studies of their physical properties, such as the possibility of structural and electronic phase transitions, and it can lead to new ferroelectric devices for advanced information functions.

Time-reversal invariant resonant backscattering on a topological insulator surface driven by a time-periodic gate voltage

We study the scattering of the Dirac electrons by a point-like nonmagnetic impurity on the surface of a topological insulator, driven by a time-periodic gate voltage. It is found that, due to the doublet degenerate crossing points of different Floquet sidebands, resonant backscattering can happen for the surface electrons, even without breaking the time-reversal (TR) symmetry of the topological surface states (TSSs). The energy spectrum is reshuffled in a way quite different from that for the circularly polarized light, so that new features are exhibited in the Friedel oscillations of the local charge and spin density of states. Although the electron scattering is dramatically modified by the driving voltage, the 1/ρ scale law of the spin precession persists for the TSSs. The TR invariant backscattering provides a possible way to engineer the Dirac electronic spectrum of the TSSs, without destroying the unique property of spin-momentum interlocking of the TSSs.

Reevaluation of radiation reaction and consequences for light-matter interactions at the nanoscale.

In the context of electromagnetism and nonlinear optical interactions, damping is generally introduced as a phenomenological, viscous term that dissipates energy, proportional to the temporal derivative of the polarization. Here, we follow the radiation reaction method presented in [Phys. Lett. A157, 217 (1991)], which applies to non-relativistic electrons of finite size, to introduce an explicit reaction force in the Newtonian equation of motion, and derive a hydrodynamic equation that offers new insight on the influence of damping in generic plasmas, metal-based and/or dielectric structures. In these settings, we find new damping-dependent linear and nonlinear source terms that suggest the damping coefficient is proportional to the local charge density and nonlocal contributions that stem from the spatial derivative of the magnetic field. We discuss the conditions that could modify both linear and nonlinear electromagnetic responses.

Reverse electrodialysis through nanochannels with inhomogeneously charged surfaces and overlapped electric double layers

Modeling of electro-chemo-mechanical transport phenomena in simple (nanochannel) or complex (nanoporous media) geometries with inhomogeneous surface charge and overlapped electric double layers remains challenging. This bottleneck originates from lack of a comprehensive model to predict the local surface charge density based on the variable local solution properties. This work aims to propose a model, so-called representative bulk layer (RBL), which makes the chemically non-isolated solid-liquid interfaces (due to the electric double layers interaction) as isolated interfaces by introducing a local effective bulk ion concentration. Using RBL together with the electrical triple layer model to provide boundary conditions for the multi-physio-chemical transport equations (PNP + NS), we investigate the reverse electrodialysis (RED) in nanochannels. Our modeling results indicate that the length of an ion-selective membrane not only influences the ionic current but also the logarithm of the slope of current-voltage curve increases linearly with the ratio of nanochannel length to height. This interesting finding inspires us to propose a dimensionless relation for the current-voltage curve that is independent of the nanochannel dimensions. The present contribution numerical framework could shed light on the electro-chemo-mechanical transport mechanism through nanofluidic devices and membranes.

Investigation of the phase transformation and its effects on the microwave performance in 3ZnO-Nb2O5-GeO2 system

The phase composition, crystal structure and microwave dielectric properties of (1-x) (3ZnO-Nb2O5)-xGeO2 were investigated systematically. The phase composition and phase transformation of (1-x) (3ZnO-Nb2O5)-xGeO2 ceramics were confirmed by XRD refinement, SEM micrographs and EDS analysis. All sintered samples were composed of Zn3Nb2O8 and ZnNb2O6 phases. Phase transition between Zn3Nb2O8 and ZnNb2O6 was closely related the introduction of GeO2. When the doping content was low (x = 0–0.007), the phase transformation from ZnNb2O6 to Zn3Nb2O8 occurred, which was discovered for the first time. To clarify the phase transition, the local charge density and band structures were calculated using the CASTEP program. With the substitution of Ge4+, part of the electrons transferred from the oxygen atom to the middle of 'niobium-oxygen' and 'zinc-oxygen'. According, the covalency of 'niobium-oxygen' and 'zinc-oxygen' was enhanced. The shrinkage of ZnO6 octahedron resulted in a decrease in the coordination number of zinc atoms. At last, the relationships between phase transition, crystal structure and the microwave performance were established. The system of 2.73ZnO-0.91Nb2O5-0.09GeO2 sintered at 1150 °C for 6 h had excellent microwave dielectric properties: εr = 21.3, Qf = 99,140 GHz, τf = −54 ppm/°C, which, to our best knowledge, the best performance with relative lower sintering temperature.

Influence of Cr3+ doping on the enhanced dielectric and nonlinear optical features of pyroelectric Pb5Ge3O11 single crystals

It was shown a possibility to enhance the output laser stimulated second harmonic susceptibility for Pb5Ge3O11 single crystals in their ferroelectric phase using an external laser light and appropriate doping by Cr3+ ions. The improvement of the Nonlinear Optical effect is achieved due to local enhancement of the local charge density non-centrosymmetry (acentricity), changes of the effective excitation of the occupied states and additional photo-polarization of the 3d-localized Cr3+ states. The addition of Cr3+ ions leads to additional enhancement of hyperpolarizability. To clarify the origin of the observed effect, and to understand the origin of its interesting properties, the calculations of the band structure of Pb5Ge3O11 were performed using, for the first time, the first-principles DFT calculations. These reveal that Pb5Ge3O11 has an indirect energy band gap Eg equal to about 2.6 eV and that the highest occupied valence band is very flat and is separated from the remaining valence bands by a small energy gap equal to about 0.3–0.4 eV. This discovery could explain many unexplained puzzling experimental results and especially the different photo-activated effects found previously in Pb5Ge3O11. The possible origin of the DFT calculated band structure is discussed. The enhancement of the second order optical susceptibility was explained within a framework of additional charges of the particular dopants during the coherent laser illumination.

A Numerical Method Charactering the Electromechanical Properties of Particle Reinforced Composite Based on Statistics.

A novel model for a network of polymer chains is proposed considering the distribution of polymer chains inside the composite in this work. Some factors that influence the distribution of polymer chains are quantitatively investigated, such as external surface geometry, internal filler, and local deformation. Furthermore, the Maxwell stress induced by an electric field is characterized by the statistics of local charge density, as the basic analyzing electromechanical properties of materials. In particular, taking the non-uniform distribution of polymer chains into account, the electromechanical properties of two materials—VHB 4910 and CaCu3Ti4O12-polydimethylsiloxane (CCTO-PDMS)—are investigated to validate the applicability of the proposed model. The comparison between simulation results and experimental results from existing literature shows that the model was successfully employed to predict the electromechanical properties of polymer composites.

Effect of anti-site point defects on the mechanical and thermodynamic properties of MgZn2, MgCu2 Laves phases: A first-principle study

A theoretical investigation on the effect of anti-site point defects on mechanical and thermodynamic properties of MgZn2 and MgCu2 based on the first-principles calculations has been implemented. The results show that Mg anti-site defect on Zn or Cu site plays a reinforcing role, while exhibiting a more brittle behavior. However, the defect phase of Cu anti-sites on Mg sublattice shows a ductile tendency. The temperature-dependent thermodynamic properties are also predicted along with Debye-Grüneisen model including Debye temperature, thermal expansion coefficient and vibrational heat capacity as well as vibrational entropy. Taking into account the contribution of the lattice vibration and thermal electronic excitations, the Helmholtz free energy F can be calculated, suggests that anti-site defects caused by the occupancy of Mg at the Zn or Cu site are thermodynamically unstable compared to the defect-free phases. In addition, the total amount of charge transfer, the overall and the local difference charge densities are further discussed to analyze the mechanism of mechanical properties.

Numerical Simulation of Ion Transport in a Nano-Electrospray Ion Source at Atmospheric Pressure.

Understanding ion transport properties from the ion source to the mass spectrometer (MS) is essential for optimizing device performance. Numerical simulation helps in understanding of ion transport properties and, furthermore, facilitates instrument design. In contrast to previously reported numerical studies, ion transport simulations in a continuous injection mode whilst considering realistic space-charge effects have been carried out. The flow field was solved using Reynolds-averaged Navier-Stokes (RANS) equations, and a particle-in-cell (PIC) method was applied to solve a time-dependent electric field with local charge density. A series of ion transport simulations were carried out at different cone gas flow rates, ion source currents, and capillary voltages. A force evaluation analysis reveals that the electric force, the drag force, and the Brownian force are the three dominant forces acting on the ions. Both the experimental and simulation results indicate that cone gas flow rates of ≤250 slph (standard liter per hour) are important for high ion transmission efficiency, as higher cone gas flow rates reduce the ion signal significantly. The simulation results also show that the ion transmission efficiency reduces exponentially with an increased ion source current. Additionally, the ion loss due to space-charge effects has been found to be predominant at a higher ion source current, a lower capillary voltage, and a stronger cone gas counterflow. The interaction of the ion driving force, ion opposing force, and ion dispersion is discussed to illustrate ion transport mechanism in the ion source at atmospheric pressure. Graphical Abstract.

Probing the oxidation state of transition metal complexes: a case study on how charge and spin densities determine Mn L-edge X-ray absorption energies.

Transition metals in inorganic systems and metalloproteins can occur in different oxidation states, which makes them ideal redox-active catalysts. To gain a mechanistic understanding of the catalytic reactions, knowledge of the oxidation state of the active metals, ideally in operando, is therefore critical. L-edge X-ray absorption spectroscopy (XAS) is a powerful technique that is frequently used to infer the oxidation state via a distinct blue shift of L-edge absorption energies with increasing oxidation state. A unified description accounting for quantum-chemical notions whereupon oxidation does not occur locally on the metal but on the whole molecule and the basic understanding that L-edge XAS probes the electronic structure locally at the metal has been missing to date. Here we quantify how charge and spin densities change at the metal and throughout the molecule for both redox and core-excitation processes. We explain the origin of the L-edge XAS shift between the high-spin complexes MnII(acac)2 and MnIII(acac)3 as representative model systems and use ab initio theory to uncouple effects of oxidation-state changes from geometric effects. The shift reflects an increased electron affinity of MnIII in the core-excited states compared to the ground state due to a contraction of the Mn 3d shell upon core-excitation with accompanied changes in the classical Coulomb interactions. This new picture quantifies how the metal-centered core hole probes changes in formal oxidation state and encloses and substantiates earlier explanations. The approach is broadly applicable to mechanistic studies of redox-catalytic reactions in molecular systems where charge and spin localization/delocalization determine reaction pathways.

Direct measurement of surface charge distribution in phase separating supported lipid bilayers.

The local surface charge density of the cell membrane influences regulation and localization of membrane proteins. The local surface charge density could, until recently, not be measured directly under physiological conditions, and it was largely a hypothetical yet very important parameter. Here we use unsaturated lipids of a distinct charge (DOTAP, DOPC, and DOPG) and a neutral fully saturated lipid (DPPC) to create model membranes with phase separating domains of a defined charge. We then apply quantitative surface charge microscopy (QSCM) to investigate the local surface charge density; this is a technique based on a scanning ion conductance microscope (SICM) capable of measuring surface charge density with nanoscale lateral resolution. We are able to clearly distinguish lipid domains from charge and topography in all three model membranes. The measured surface charge densities furthermore reveal that disordered domains formed by charged lipids are in fact not only impure, but also incorporate uncharged saturated lipids. We estimate that at least 30% of disordered domains in DOPG : DPPC and DOTAP : DPPC will be DPPC. These ratios could present a limit for the formation of charged domains in lipid membranes.

Sequence length dependence in arginine/phenylalanine oligopeptides: Implications for self-assembly and cytotoxicity

We present a detailed study on the self-assembly and cytotoxicity of arginine-rich fragments with general form [RF]n (n=1–5). These highly simplified sequences, containing only two l-amino acids, provide suitable models for exploring both structure and cytotoxicity features of arginine-based oligopeptides. The organization of the sequences is revealed over a range of length scales, from the nanometer range down to the level of molecular packing, and their cytotoxicity toward C6 rat glioma and RAW264.7 macrophage cell lines is investigated. We found that the polymorphism is dependent on peptide length, with a progressive increase in crystalline ordering upon increasing the number of [RF] pairs along the backbone. A dependence on length was also found for other observables, including critical aggregation concentrations, formation of chiral assemblies and half maximum inhibitory concentrations (IC50). Whereas shorter peptides self-assemble into fractal-like aggregates, clear fibrillogenic capabilities are identified for longer sequences with octameric and decameric chains exhibiting crystalline phases organized into cross-β structures. Cell viability assays revealed dose-dependent cytotoxicity profiles with very similar behavior for both glioma and macrophage cell lines, which has been interpreted as evidence for a nonspecific mechanism involved in toxicity. We propose that structural organization of [RF]n peptides plays a paramount role regarding toxicity due to strong increase of local charge density induced by self-assemblies rich in cationic groups when interacting with cell membranes.

Effects of polysaccharide charge pattern on the microstructures of β-lactoglobulin-pectin complex coacervates, studied by SAXS and SANS

Protein and polysaccharide complex coacervates have been widely used in developing complex food products, the function as well as the physicochemical properties of these food materials are closely related to the microstructure of the coacervates. To this end, the effects of the total amount and distribution of charges carried by pectic polysaccharides on the microstructure of complex coacervates formed through interactions with the protein, bovine β-lactoglobulin variant A (βLgA) were investigated. Small angle X-ray and neutron scattering reveal the major building blocks of complex coacervates are compact primary particles. These primary particles are composed of overlapping protein-binding pectin chains, bridged by βLgA-rich clusters. The overall and local charge densities of pectin influence the size and distribution of βLgA-rich clusters within the primary particles. Moreover, depending on the spatial arrangements of the primary particles, the resulting structure could be either a complex coacervate or a precipitate. Results represented in this study imply the possibility of controlling the microstructure of protein-polysaccharide complex coacervates using polysaccharides with various charge patterns.

Atomically Resolved Band Bending Effects in a p-n Heterojunction of Cu2O and a Cobalt Macrocycle.

We present a hetero junction based on macrocyclic hydrogen evolution catalysts (HEC) physisorbed on a single crystalline Cu2O(111) surface. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) provides the spatial resolution of the band bending within the first nanometer of the subsurface region. Oxygen vacancies on the Cu2O(111) surface cause a downward band bending which is conserved upon adsorption of HEC layers of various thicknesses. This allows photoexcited electrons to be directed toward the surface where they can be made available for the reduction of protons by the HEC. Furthermore, Poisson's equation relates more subtle changes in the measured ARXPS spectra to the local charge density profile within the first 7 Å away from the surface and with atomic resolution. All observations are consistent with a polarization of the molecular layer in response to the electrical field at the oxide surface, which should be a general phenomenon at such organic-oxide heterointerfaces.

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

Model charged cylindrical nanopore in a colloidal dispersion: charge reversal, overcharging and double overcharging

Using the hypernetted-chain/mean spherical approximation (HNC/MSA) integral equations we study the electrical double layer inside and outside a model charged cylindrical vesicle (nanopore) immersed into a primitive model macroions solution, so that the macroions are only present outside the nanopore, i.e., the vesicle wall is impermeable only to the external macroions. We calculate the ionic and local linear charge density profiles inside and outside the vesicle, and find that the correlation between the inside and outside ionic distributions causes the phenomena of overcharging (also referred to as surface charge amplification) and/or charge reversal. This is the first time overcharging is predicted in an electrical double layer of cylindrical geometry. We also report the new phenomenon of double overcharging. The present results can be of consequence for relevant systems in physical-chemistry, energy storage and biology, e.g., nanofilters, capacitors and cell membranes.

Classical density functional theory and Monte Carlo simulation study of electric double layer in the vicinity of a cylindrical electrode

We have performed extensive Monte-Carlo simulations and classical density functional theory (DFT) calculations of the electrical double layer (EDL) near a cylindrical electrode in a primitive model (PM) modified by incorporating interionic dispersion interactions. It is concluded that (i) in general, an unsophisticated use of the mean field (MF) approximation for the interionic dispersion interactions does not distinctly worsen the classical DFT performance, even if the salt ions considered are highly asymmetrical in size (3:1) and charge (5:1), the bulk molar concentration considered is high up to a total bulk ion packing fraction of 0.314, and the surface charge density of up to 0.5 C m−2. (ii) More specifically, considering the possible noises in the simulation, the local volume charge density profiles are the most accurately predicted by the classical DFT in all situations, and the co- and counter-ion singlet distributions are also rather accurately predicted; whereas the mean electrostatic potential profile is relatively less accurately predicted due to an integral amplification of minor inaccuracy of the singlet distributions. (iii) It is found that the layered structure of the co-ion distribution is abnormally possible only if the surface charge density is high enough (for example 0.5 C m−2); moreover, the co-ion valence abnormally influences the peak height of the first counter-ion layer, which decreases with the former. (iv) Even if both the simulation and DFT indicate an insignificant contribution of the interionic dispersion interaction to the above three ‘local’ quantities, it is clearly shown by the classical DFT that the interionic dispersion interaction does significantly influence a ‘global’ quantity like the cylinder surface-aqueous electrolyte interfacial tension, and this may imply the role of the interionic dispersion interaction in explaining the specific Hofmeister effects. We elucidate all of the above observations based on the arguments from the liquid state theory and at the molecular scale.

Feasibility study of TPC at electron positron colliders at Z pole operation

TPC is a promising technology for the future electron positron colliders. However, its application might be limited at high event rate and high hit occupancies. In this paper, we study the feasibility of using TPC at the circular electron positron collider (CEPC) at Z pole using full simulated Z→ qq̄ samples, by evaluating the local charge density and voxel occupancy at different TPC parameters. Our study shows that the TPC could be applied to the CEPC Z pole operation if back flow ion is controlled to per mille level. We also find that the distortion is considerable for the FCC-ee Z pole operation. And a few approaches are proposed to reduce the distortion.

Rational Design of Lithium-Sulfur Battery Cathodes Based on Experimentally Determined Maximum Active Material Thickness.

Rational design of conductive carbon hosts for high energy density lithium-sulfur batteries requires an understanding of the fundamental limitations to insulating active material loading. In this work, we investigate the electrochemistry of lithium sulfide films ranging in thickness from 30 to 3500 nm. We show that films thicker than approximately 40 nm cannot be charged at local charge densities above 1 μA cm-2, and by implication, the maximum useful pore diameter is near 60 nm in a practical cathode. "Activation" overpotentials for Li2S are identified in thicker films, resulting from polysulfide generation, but are shown not to improve the fundamental areal charge limitations. We develop a model for filling of conductive pores with active material to rationally design composites based on local charge density. For low-electrolyte applications, the importance of matching micropore volume to sulfide loading and cycling rate is emphasized.