Presence Of Surface Charges Research Articles

ZnO/NiFe2O4 heterostructure on nickel foam for the electrochemical detection of uric acid

In this study, heterostructures containing hydrothermally derived ZnO and combustion derived NiFe2O4 were co-deposited on nickel foam (NF) substrate. The modification of NF based substrate with the synthesised electrocatalysts is capable of increasing the electrochemically active surface area (ECSA) due to its porous network. The fabricated ZnO@NF, NiFe2O4@NF and ZnO/NiFe2O4@NF electrodes were tested in phosphate buffer saline (PBS) (pH = 7.4 ± 0.2) to understand their chemical kinetics which reveals a diffusion-controlled process. A comparative study reveals that the presence of surface charges on ZnO/NiFe2O4@NF electrode, influences the selectivity by enhancing the sensitivity towards UA which is attributed to the low charge transfer resistance and its higher ECSA of 1.487 cm2. The linear response current was observed between 0.52 mM and 1.48 mM with a detection limit (LOD) of 8.22 μM and limit of quantification (LOQ) of 27.4 μM. The sensitivity was found to be 2.31 mA.mM−1cm−2. The main advantages of ZnO/NiFe2O4@NF electrode is its simple fabrication technique which is inexpensive, stability over a period of 13 days with current loss of ∼19.2 %. The proposed ZnO/NiFe2O4 based non-enzymatic UA sensor is capable of selectively detecting UA in the presence of various interferents. UA acts as a biomarker for the diagnosis of different stages of chronic kidney disease (CKD). Hence, the validity of the developed heterostructure was investigated for real time UA sensing using spot urine samples. The recovery percentage was found to be between 95.7 % and 103.5 % further paving way for the use of this electrode for diagnosis of CKD.

Ion concentration polarization causes a nearly pore-length-independent conductance of nanopores.

There has been a great amount of interest in nanopores as the basis for sensors and templates for preparation of biomimetic channels as well as model systems to understand transport properties at the nanoscale. The presence of surface charges on the pore walls has been shown to induce ion selectivity as well as enhance ionic conductance compared to uncharged pores. Here, using three-dimensional continuum modeling, we examine the role of the length of charged nanopores as well as applied voltage for controlling ion selectivity and ionic conductance of single nanopores and small nanopore arrays. First, we present conditions where the ion current and ion selectivity of nanopores with homogeneous surface charges remain unchanged, even if the pore length decreases by a factor of 6. This length-independent conductance is explained through the effect of ion concentration polarization (ICP), which modifies local ionic concentrations, not only at the pore entrances but also in the pore in a voltage-dependent manner. We describe how voltage controls the ion selectivity of nanopores with different lengths and present the conditions when charged nanopores conduct less current than uncharged pores of the same geometrical characteristics. The manuscript provides different measures of the extent of the depletion zone induced by ICP in single pores and nanopore arrays, including systems with ionic diodes. The modeling shown here will help design selective nanopores for a variety of applications where single nanopores and nanopore arrays are used.

What surfaces in the operation of noble liquids dark matter detectors

Though noble element dual-phase detectors have a long application history in dark matter searches, some uncertainties and differences in backgrounds persist. We compare effects caused by unextracted electrons on the liquid-gas interface in Xe and Ar dual-phase detectors with a large family of phenomena at the liquid helium surface. We pose that electron and ion accumulation on the liquid surface in detectors can lead to the formation of ordered surface states, charged liquid surface instabilities in an electric field, electrospraying, interactions with surface waves, and other effects. Not only delayed electron emission signals can be generated, but the extraction efficiency for electrons produced below the liquid surface can be altered by the presence of surface charges. Several factors lead to surface electron accumulation, and problems can become more severe with the increased detector size. We discuss possible experiments to reveal surface electron effects and design changes to alleviate electron accumulation. We conclude that studies of these effects are desirable before making final design decisions for the new multi-ton liquid Xe dark matter detector projects like DARWIN, XLZD, and large Ar dual-phase detectors.

Ion filling of a one-dimensional nanofluidic channel in the interaction confinement regime.

Ion transport measurements are widely used as an indirect probe for various properties of confined electrolytes. It is generally assumed that the ion concentration in a nanoscale channel is equal to the ion concentration in the macroscopic reservoirs it connects to, with deviations arising only in the presence of surface charges on the channel walls. Here, we show that this assumption may break down even in a neutral channel due to electrostatic correlations between the ions arising in the regime of interaction confinement, where Coulomb interactions are reinforced due to the presence of the channel walls. We focus on a one-dimensional channel geometry, where an exact evaluation of the electrolyte's partition function is possible with a transfer operator approach. Our exact solution reveals that in nanometer-scale channels, the ion concentration is generally lower than in reservoirs and depends continuously on the bulk salt concentration, in contrast to the conventional mean-field theory that predicts an abrupt filling transition. We develop a modified mean-field theory taking into account the presence of ion pairs that agrees quantitatively with the exact solution and provides predictions for experimentally relevant observables, such as the ionic conductivity. Our results will guide the interpretation of nanoscale ion transport measurements.

Molecular Approach for Understanding the Stability, Collision, and Coalescence of Bulk Nanobubbles

Though long-lived nanobubbles (NBs) have been reported by multiple researchers, the underlying reason behind their stability is still obscure. Some of the conjectured reasons include diffusive shielding, the presence of surface charges, and stability due to contamination. Still, the stability of NBs against coalescence and Ostwald ripening is not confirmed. Using molecular dynamics simulations, the present study aims to understand the stabilization effects due to diffusive shielding and the presence of an electrical double layer at the surface of NBs. Accumulation of charges on NBs for different concentrations of ions is discussed. Also, the collision of equal-sized NBs with different approach velocities and offset distances is simulated. A regime map is predicted on the basis of initial approach velocity and offset distance. The transition in regime obtained upon increasing the offset distance is discussed, which differs from the collision characteristics of macroscopic bubbles and drops. The merging of NBs is initiated through the bridge formation, for which the temporal evolution rate along with the scaling argument is presented. The stress terms involved and the corresponding regimes are predicted based on the fluid properties. For all the cases where merging is observed, the estimated probability is observed to be low, which suggests the stability of NBs against coalescence.

Dielectric Passivation and Edge Effects in Planar GaN Schottky Barrier Diodes

The influence of passivation on the edge effects (EEs) present in the capacitance-voltage ( C- V) characteristics of GaN Schottky barrier diodes (SBDs) with realistic geometry is analyzed by means of Monte Carlo simulations. The enhancement of the performance of SBDs as frequency multipliers is based on the optimization of the nonlinearity of the C- V curve, where EEs, strongly influenced by the dielectric passivation of the diode, play a significant role and must be carefully considered. The extra capacitance associated with EEs is affected by the presence of surface charges at the semiconductor/dielectric interface, which is considered by means of a self-consistent model in which the local value of the surface charge is updated according to the surrounding electron density. Our results indicate that, in realistic SBD geometries, a higher dielectric constant of the passivation material leads to more pronounced EEs. The thickness of the dielectric and the lateral extension of the epilayer are also important parameters to be taken into account when dealing with EEs.

Stochastic inference of surface-induced effects using Brownian motion

Brownian motion in confinement and at interfaces is a canonical situation, encountered from fundamental biophysics to nanoscale engineering. Using the Lorenz-Mie framework, we optically record the thermally-induced tridimensional trajectories of individual microparticles, within salty aqueous solutions, in the vicinity of a rigid wall, and in the presence of surface charges. We construct the time-dependent position and displacement probability density functions, and study the non-Gaussian character of the latter which is a direct signature of the hindered mobility near the wall. Based on these distributions, we implement a novel, robust and self-calibrated multifitting method, allowing for the thermal-noise-limited inference of diffusion coefficients spatially-resolved at the nanoscale, equilibrium potentials, and forces at the femtoNewton resolution.

Design and simulation of 3C-SiC vertical power MOSFETs

ABSTRACT This paper reports on the design and simulation of 3C-SiC low-doped drain power MOSFETs, including key parameters such as the avalanche impact ionisation model and its relation to the breakdown voltage, the doping dependence of the bulk mobility, and the relationship between the on-resistance and breakdown voltage. A set of MOSFETs with different blocking layer doping were designed while scaling the parasitic JFET. A stepped doping profile is used to minimise localised breakdown at the p-well/n-type drift layer interface. The characteristics of the MOSFETs were obtained using a commercially available 2D simulation program. 2D Simulation results show good agreement with 1D model of the on-state resistance and current-voltage characteristics. The deviation from the experimental data may be due to using different designs and or may be due to the presence of surface charges excluded from 2D simulations and 1D quadratic model. Simulation results indicate that, for the chosen material parameters, a 600 V, 3C-SiC MOSFET with a thinned substrate, containing a drift-layer of 7 μm and a blocking layer doping density of 1 × 1016 cm−3, can have an on-state resistance of 0.8 mΩ-cm2.

Effect of accumulated surface charges on DC flashover of SiR insulators under pollution and aging conditions

In this paper, the measurements and modeling of the DC flashover behaviors of silicone rubber (SiR) insulators under pollution and aging conditions in the presence of surface charges have been presented. For this purpose, four different types of standard HTV-SiR insulators were selected, and then, DC flashover performance was carried out on the studied insulators in the three states including non-aged, aged, and contaminated conditions in the presence surface charges. The surfaces of the insulators were charged by an external corona source in the experimental tests, and then, the surface electric charges were measured. The experimental results of DC flashover voltage showed that the positive and negative surface charges affected flashover performance in all the states. Based on obtained empirical models of the DC flashover performance, it can be concluded that the increased aging time, positive electric charges, pollution, and average diameter reduced DC flashover performance, while the ratio of shed spacing to shed depth and negative charges enhanced the flashover voltage level.

Chitin and Chitinases: Biomedical And Environmental Applications of Chitin and its Derivatives

Disposal of chitin wastes from crustacean shell can cause environmental and health hazards. Chitin is a well known abundant natural polymer extracted after deproteinization and demineralization of the shell wastes of shrimp, crab, lobster, and krill. Extraction of chitin and its derivatives from waste material is one of the alternative ways to turn the waste into useful products. Chitinases are enzymes that degrade chitin. Chitinases contribute to the generation of carbon and nitrogen in the ecosystem. Chitin and chitinolytic enzymes are gaining importance for their biotechnological applications. The presence of surface charge and multiple functional groups make chitin as a beneficial natural polymer. Due to the reactive functional groups chitin can be used for the preparation of a spectrum of chitin derivatives such as chitosan, alkyl chitin, sulfated chitin, dibutyryl chitin and carboxymethyl chitin for specific applications in different areas. The present review is aimed to summarize the efficacy of the chitinases on the chitin and its derivatives and their diverse applications in biomedical and environmental field. Further this review also discusses the synthesis of various chitin derivatives in detail and brings out the importance of chitin and its derivatives in biomedical and environmental applications.

Trapping of Gas Bubbles in Water at a Finite Distance below a Water-Solid Interface.

Gas bubbles in a water-filled cavity move upward because of buoyancy. Near the roof, additional forces come into play, such as Lifshitz, double layer, and hydrodynamic forces. Below uncharged metallic surfaces, repulsive Lifshitz forces combined with buoyancy forces provide a way to trap micrometer-sized bubbles. We demonstrate how bubbles of this size can be stably trapped at experimentally accessible distances, the distances being tunable with the surface material. By contrast, large bubbles (≥100 μm) are usually pushed toward the roof by buoyancy forces and adhere to the surface. Gas bubbles with radii ranging from 1 to 10 μm can be trapped at equilibrium distances from 190 to 35 nm. As a model for rock, sand grains, and biosurfaces, we consider dielectric materials such as silica and polystyrene, whereas aluminium, gold, and silver are the examples of metal surfaces. Finally, we demonstrate that the presence of surface charges further strengthens the trapping by inducing ion adsorption forces.

Contribution of surface charges on high‐voltage DC silicon rubber insulators to DC flashover performance

In this study, the results of measurement for DC flashover characteristics of silicon rubber insulators in the presence of surface charges and in terms of geometric characteristics have been examined and analysed. In the carried out experimental tests, the surface of different insulators was charged by an external corona source and both the metal end fittings were kept grounded while the needles have been connected to a certain applied negative and positive DC voltages and then surface electric charge measured. A series of flashover tests have been carried out on the charged insulators under positive DC voltages to investigate the effects of surface charges. The experimental results revealed that positive electric charges reduced flashover performance while negative charges increased the flashover voltage level. Also, the experimental modelling of DC flashover of charged insulators regarding the ratio of shed spacing to shed depth and specific leakage distance in terms of surface electric charges, revealed that flashover voltage gradient of the charged insulator is most affected by specific leakage.

Molecular dynamics investigation on enhancement of heat transfer between electrified solid surface and liquid water

Interfacial nanolayer acting as a bridge linking a bulk solid phase and a bulk liquid phase has a crucial impact on the heat transfer from the solid to the liquid. In this work, we proposed a promising method to enhance the heat transfer between a Cu surface and liquid water by applying surface charges to the surface, which was expected to change the structure of the interfacial nanolayer. We employed molecular dynamics simulations to verify the effectiveness of the method. Our simulations showed that applying surface charges led to an orientational alignment of water molecules, reducing the spacing between the solid surface and the interfacial nanolayer and increasing the number of water molecules located at the interface nanolayer. Meanwhile, the presence of surface charges also improves the mismatch of vibrational density of states between solid atoms and water molecules. As a results, the heat transfer was enhanced as compared with that without surface charges and the enhancement becomes more remarkeble when applying more surface charges to the Cu surface.

Electrohydrodynamic channeling effects in narrow fractures and pores.

In low-permeability rock, fluid and mineral transport occur in pores and fracture apertures at the scale of micrometers and below. At this scale, the presence of surface charge, and a resultant electrical double layer, may considerably alter transport properties. However, due to the inherent nonlinearity of the governing equations, numerical and theoretical studies of the coupling between electric double layers and flow have mostly been limited to two-dimensional or axisymmetric geometries. Here, we present comprehensive three-dimensional simulations of electrohydrodynamic flow in an idealized fracture geometry consisting of a sinusoidally undulated bottom surface and a flat top surface. We investigate the effects of varying the amplitude and the Debye length (relative to the fracture aperture) and quantify their impact on flow channeling. The results indicate that channeling can be significantly increased in the plane of flow. Local flow in the narrow regions can be slowed down by up to 5% compared to the same geometry without charge, for the highest amplitude considered. This indicates that electrohydrodynamics may have consequences for transport phenomena and surface growth in geophysical systems.

Lipid Monolayer Formation and Lipid Exchange Monitored by a Graphene Field-Effect Transistor.

Anionic and cationic lipids are key molecules involved in many cellular processes; their distribution in biomembranes is highly asymmetric, and their concentration is well-controlled. Graphene solution-gated field-effect transistors (SGFETs) exhibit high sensitivity toward the presence of surface charges. Here, we establish conditions that allow the observation of the formation of charged lipid layers on solution-gated field-effect transistors in real time. We quantify the electrostatic screening of electrolyte ions and derive a model that explains the influence of charged lipids on the ion sensitivity of graphene SGFETs. The electrostatic model is validated using structural information from X-ray reflectometry measurements, which show that the lipid monolayer forms on graphene. We demonstrate that SGFETs can be used to detect cationic lipids by self-exchange of lipids. Furthermore, SGFETs allow measuring the kinetics of layer formation induced by vesicle fusion or spreading from a reservoir. Because of the high transconductance and low noise of the electrical readout, we can observe characteristic conductance spikes that we attribute to bouncing-off events of lipid aggregates from the SGFET surface, suggesting a great potential of graphene SGFETs to measure the on-off kinetics of small aggregates interacting with supported layers.

Numerical Simulation and Experimental Study of Electrostatic Field near Man with Protective Polymeric Clothing

In the article the mathematical model and the method of numerical calculation, based on the method of finite elements of three-dimensional electrostatic field near-by a man at presence of surface charge on his protective clothing, made from polymeric material, were considered. Experimental researches were conducted to determine the value of the surface charge. The results of numeral calculations of the distribution of electric potential and field intensity, as well as integral value of electric energy, stored in this field, were presented.

Effect of microfillers on the concrete structure formation

Physical and mechanical properties of concrete as a composite material are determined by structure formation processes. This paper examines the effect of dispersed mineral fillers of anthropogenic origin on concrete structure formation processes and physical and mechanical properties of fine concrete. The presence of surface charge and electrosurface potential leads to the formation of electrically heterogeneous contacts between oppositely charged surfaces which determine the strength of the material as a whole. The introduction of steel microfillers leads to the formation of strong electrically heterogeneous contacts, hardening of concrete structure and increase in its strength.

Monte Carlo Study of 2-D Capacitance Fringing Effects in GaAs Planar Schottky Diodes

Nanometer scale planar Schottky barrier diodes (SBDs) with realistic geometries have been studied by means of a 2-D ensemble Monte Carlo simulator. The topology of the devices studied in this paper is based in real planar GaAs SBDs used in terahertz applications, such as passive frequency mixing and multiplication, in which accurate models for the diode capacitance are required. The intrinsic capacitance of such small devices, which due to edge effects strongly deviates from the ideal value, has been calculated. In good agreement with the classical models, we have found that the edge capacitance is independent of the properties of the semiconductor beneath the contact and, as novel result, that the presence of surface charges at the semiconductor dielectric interface can reduce it almost 15%. We have finally provided a compact model for the total capacitance of diodes with arbitrary shape that could be easily implemented in design automation software such as Advance Design System (ADS).

Two competing interpretations of Kelvin probe force microscopy on semiconductors put to test

Kelvin probe force microscopy (KPFM) is a popular tool for studying properties of semiconductors. However, the interpretation of its results is complicated by the possibility of so-called band bending and the presence of surface charges. In this work we study two different interpretations for KPFM on semiconductors: the contact potential difference (CPD) interpretation, which interprets the measured potential as the work function difference between the sample and the probe, and a newer, alternative, interpretation proposed by Baumgart, Helm and Schmidt (BHS). By performing model calculations we demonstrate that these models generally lead to very different results. Hence it is important to decide which one is correct. We demonstrate that BHS predictions for the Kelvin voltage difference between the $p$ and $n$ parts of a $pn$-junction are inconsistent with a set of experimental results from the literature. In addition, the BHS interpretation predicts an independence from the probe material as well as from surface treatments, which we both find to disagree with experiment. On the other hand, we present a theoretical argument for the validity of the CPD interpretation and we show that the CPD interpretation is able to accommodate all of these experimental results. Thus we posit that the BHS interpretation is generally not suitable for the analysis of KPFM on semiconductors and that the CPD interpretation should be used instead.

Structure of water at charged interfaces: a molecular dynamics study.

The properties of water molecules located close to an interface deviate significantly from those observed in the homogeneous bulk liquid. The length scale over which this structural perturbation persists (the so-called interfacial depth) is the object of extensive investigations. The situation is particularly complicated in the presence of surface charges that can induce long-range orientational ordering of water molecules, which in turn dictate diverse processes, such as mineral dissolution, heterogeneous catalysis, and membrane chemistry. To characterize the fundamental properties of interfacial water, we performed molecular dynamics (MD) simulations on alkali chloride solutions in the presence of two types of idealized charged surfaces: one with the charge density localized at discrete sites and the other with a homogeneously distributed charge density. We find that, in addition to a diffuse region where water orientation shows no layering, the interface region consists of a "compact layer" of solvent next to the surface that is not described in classical electric double layer theories. The depth of the diffuse solvent layer is sensitive to the type of charge distributions on the surface and the ionic strength. Simulations of the aqueous interface of a realistic model of negatively charged amorphous silica show that the water orientation and the distribution of ions strongly depend on the identity of the cations (Na(+) vs Cs(+)) and are not well represented by a simplistic homogeneous charge distribution model. While the compact layer shows different solvent net orientation and depth for Na(+) vs Cs(+), the depth (~1 nm) of the diffuse layer of oriented waters is independent of the identity of the cation screening the charge. The details of interfacial water orientation revealed here go beyond the traditionally used double and triple layer models and provide a microscopic picture of the aqueous/mineral interface that complements recent surface specific experimental studies.