Surface Charge Fluctuations Research Articles

Computational Studies of Auto-Active van der Waals Interaction Molecules on Ultra-Thin Black-Phosphorus Film.

Using the van der Waals density functional theory, we studied the binding peculiarities of favipiravir (FP) and ebselen (EB) molecules on a monolayer of black phosphorene (BP). We systematically examined the interaction characteristics and thermodynamic properties in a vacuum and a continuum, solvent interface for active drug therapy. These results illustrate that the hybrid molecules are enabled functionalized two-dimensional (2D) complex systems with a vigorous thermostability. We demonstrate in this study that these molecules remain flat on the monolayer BP system and phosphorus atoms are intact. It is inferred that the hybrid FP+EB molecules show larger adsorption energy due to the van der Waals forces and planar electrostatic interactions. The changes in Gibbs free energy at different surface charge fluctuations and temperatures imply that the FP and EB are allowed to adsorb from the gas phase onto the 2D film at high temperatures. Thereby, the results unveiled beneficial inhibitor molecules on two dimensional BP nanocarriers, potentially introducing a modern strategy to enhance the development of advanced materials, biotechnology, and nanomedicine.

General Model and Equivalent Circuit for the Chemical Noise Spectrum Associated to Surface Charge Fluctuation in Potentiometric Sensors

This paper firstly reports a general and powerful approach to evaluate the power spectral density (PSD) of the surface charge fluctuations, so-called “chemical noise”, from a generic set of reactions at the sensing surface of potentiometric sensors such as, for instance, Ion-Sensitive Field Effect Transistors (ISFETs). Starting from the master equation, the spectral noise signature of a reaction set is derived as a function of the reaction kinetic parameters and of the interface concentration of the ionic species. Secondly, we derive an equivalent surface admittance, whose thermal noise PSD produces a noise PSD equal to that of the surface charge fluctuations. We also show how to expand this surface admittance into stair-case RC networks, with a number of elementary cells equal to the number of surface reactions involved. This admittance can be included in circuit simulations coupled with a SPICE compact model of the underlying FET, to enable the physically based modelling of frequency dispersion and noise of the sensing layer when simulating the sensor and the read-out. Validation with existing models and literature results as well as new application examples are provided. The proposed methodology to compute the PSD from rate equations is amenable to use in different contexts where fluctuations are generated by random transitions between discrete states with given exchange rates.

One-dimensional Coulomb system in a sticky wall confinement: Exact results.

This work investigates a one-component one-dimensional Coulomb system in sticky wall confinement. Sticky wall is introduced as an alternative and intuitive depiction of charge regulation, the notion that a surface charge is not a fixed but a fluctuating quantity in dynamic equilibrium with its immediate environment. Emphasis is placed on intuitive derivation and expressions are obtained by observing that the partition function of a charge regulated system can be decomposed into a collection of independent equilibriums with different fixed surface charges. Adsorbed particles behave as ideal-gas particles in a one-dimensional box whose length corresponds to the parameter of stickiness. Among various scenarios considered are a single- and two-wall confinement as well as the case of sticky counterions capable of associating into pairs. Exact solutions provide a view of the role and behavior of surface charge fluctuations, which is an important step in the "beyond-mean-field" analysis. Consequently, the model serves as a simple paradigm of the mechanism that gives rise to the Kirkwood-Shumaker interactions detected in real systems.

Jeans surface instability of an electron-ion plasma

The dispersive properties of Jeans surface instability at a charged plane interface of weakly coupled, classical self-gravitating, magnetized electron-ion plasma and vacuum, are investigated. The general dispersion relation is formulated by employing one fluid magnetohydrodynamic (MHD) model and Poisson’s equations for both electrostatic and gravitational potentials. It is shown that both the surface charge and mass density fluctuations are coupled together effectively, to enhance the growth rate of surface Jeans instability, whereas an increase in magnetic field and surface tension tends to stabilize Jeans surface instability. The modified Jeans criterion of gravitational instability is obtained by formulating a new definition of Jeans wavelength. These results may be useful to examine the gravitational collapse of self-gravitating electron-ion plasma, leading to star formation in astro-cosmic environments.

Electric Noise Spectra of a Near-Surface Nitrogen-Vacancy Center in Diamond with a Protective Layer

Surface noise is a detrimental issue for sensing devices based on shallow nitrogen vacancy (NV) color center diamonds. A recent experiment indicates that electric field noise is significant compared to magnetic field noise. They also found that the electric field noise can be reduced with a protective surface layer, though the mechanism of noise reduction is not well understood. We examine the effect of a protective surface layer on the noise spectrum, which is caused by surface charge fluctuations. We use the fluctuation-dissipation theorem to calculate and analyze the noise spectrum for six different surface layer materials typically used for NV center diamond devices. We find that four parameters largely affect the noise spectrum: effective relaxation time, effective loss tangent, power law exponent of the noise spectrum, and layer thickness. Our results suggest that a surface covering layer is indeed useful for decreasing surface noise, but which material is most suitable depends on the device operational frequency range.

High Performance Biosensor Based on Electronic Transduction of Refractive Index Using 3D Nanostructured Biogenic Silica

Various nanostructured materials have been deployed in developing biosensors. Nevertheless, achieving sub-attomolar detection limit by label free method is still challenging owing to the different noises during the sensing process. In this paper, we introduce an impedance immunosensor on freely suspended 3D nanostructured biogenic silica block, commonly called Tabashir. The working principle of this sensor is based on the electrical transduction of variations in refractive index after antigen capture. Due to the variation in refractive index, the Tabashir sensor changes the solution lining on the surface and within the pores, which has been converted to a significant variation in bulk conductivity. This screens the impedance measurement from major noise interference like the surface charge fluctuations and antibody-antigen association dissociation transients, resulting in a remarkable steady state impedance sensitivity of around 120% along with distinct shape and cut-off frequencies in the noise power spectral density (PSD) for a trace concentration of 0.1 aM Hep-B virus even in a complex mixture containing high concentration of non-specific antigen. This detection limit is an improvement by almost two orders of magnitude compared to the most sensitive label free electrical biosensor.

Ultralow-Noise Atomic-Scale Structures for Quantum Circuitry in Silicon

The atomically precise doping of silicon with phosphorus (Si:P) using scanning tunneling microscopy (STM) promises ultimate miniaturization of field effect transistors. The one-dimensional (1D) Si:P nanowires are of particular interest, retaining exceptional conductivity down to the atomic scale, and are predicted as interconnects for a scalable silicon-based quantum computer. Here, we show that ultrathin Si:P nanowires form one of the most-stable electrical conductors, with the phenomenological Hooge parameter of low-frequency noise being as low as ≈10(-8) at 4.2 K, nearly 3 orders of magnitude lower than even carbon-nanotube-based 1D conductors. A in-built isolation from the surface charge fluctuations due to encapsulation of the wires within the epitaxial Si matrix is the dominant cause for the observed suppression of noise. Apart from quantum information technology, our results confirm the promising prospects for precision-doped Si:P structures in atomic-scale circuitry for the 11 nm technology node and beyond.

Decoherence of Near-Surface Nitrogen-Vacancy Centers Due to Electric Field Noise

We show that electric field noise from surface charge fluctuations can be a significant source of spin decoherence for near-surface nitrogen-vacancy (NV) centers in diamond. This conclusion is based on the increase in spin coherence observed when the diamond surface is covered with high-dielectric-constant liquids, such as glycerol. Double-resonance experiments show that improved coherence occurs even though the coupling to nearby electron spins is unchanged when the liquid is applied. Multipulse spin-echo experiments reveal the effect of glycerol on the spectrum of NV frequency noise.

Substrate-induced band structure and electronic properties in graphene/Al2O3(0001) interface

Band structure investigation results of two-dimensional (2D) graphene (SLG) on Al2O3(0001) using the density functional theory (DFT) method as a possible element base for spintronics are presented. Regularities of a band structure change in the order three-dimensional (3D) Al2O3→2D Al2O3(0001)→2D SLG/Al2O3(0001) as well as features of a chemical bond between SLG and sapphire on the basis of DFT calculations have been studied. Analysis of the band structure and interatomic spacing in the interface for both models allows speaking about physical SLG adsorption on the (0001)-surface sapphire constrained by aluminum atoms. Energy distribution features of surface states in 2D SLG/Al2O3(0001) interface are discussed. Analysis of effective atomic charge in the interface revealed surface charge fluctuations on the substrate in the presence of SLG, which can be explained by a decrease of the energy of occupied subsurface Al2O3 states relatively to the Fermi level.

PH-Dependent Aggregation and Disaggregation of Native β-Lactoglobulin in Low Salt

The aggregation of β-lactoglobulin (BLG) near its isoelectric point was studied as a function of ionic strength and pH. We compared the behavior of native BLG with those of its two isoforms, BLG-A and BLG-B, and with that of a protein with a very similar pI, bovine serum albumin (BSA). Rates of aggregation were obtained through a highly precise and convenient pH/turbidimetric titration that measures transmittance to ±0.05 %T. A comparison of BLG and BSA suggests that the difference between pHmax (the pH of the maximum aggregation rate) and pI is systematically related to the nature of protein charge asymmetry, as further supported by the effect of localized charge density on the dramatically different aggregation rates of the two BLG isoforms. Kinetic measurements including very short time periods show well-differentiated first and second steps. BLG was analyzed by light scattering under conditions corresponding to maxima in the first and second steps. Dynamic light scattering (DLS) was used to monitor the kinetics, and static light scattering (SLS) was used to evaluate the aggregate structure fractal dimensions at different quench points. The rate of the first step is relatively symmetrical around pHmax and is attributed to the local charges within the negative domain of the free protein. In contrast, the remarkably linear pH dependence of the second step is related to the uniform reduction in global protein charge with increasing pH below pI, accompanied by an attractive force due to surface charge fluctuations.

Current Fluctuation Analysis to Study Mg2+-Binding in the Bacterial Porin OmpF

OmpF is a protein that forms water-filled, wide channels in the outer membrane of Escherichia coli. Previous studies have shown that the narrowest part of OmpF aqueous pore is largely responsible for its characteristic transport properties. To a large extent the pore eyelet controls the conductance and selectivity of the channel. In addition, the channel crystal structure obtained from a MgCl2 solution of the protein showed that Mg2+ ions bind between two acidic residues located at the channel constriction. Subsequent studies have demonstrated that this binding causes OmpF selectivity inversion. Here, we aim to get further insight into the effects of MgCl2 binding on OmpF ion conduction through the analysis of current fluctuations. We use planar lipid bilayer electrophysiology to measure time-resolved single-channel currents and analyze the associated fluctuations. We first evaluate the high-frequency regime for different concentrations of KCl and identify a contribution to the spectrum that seems to arise from surface charge fluctuations. We then compare these results with those obtained in MgCl2. In addition, we analyze the low-frequency current noise, which shows the characteristic Lorentzian profile, and study its dependence on electrolyte concentration in the presence of monovalent and divalent cations.

Mineralogical characteristics and related surface charge fluctuations of some selected soils of temperate regions of northern Iran

AbstractThe majority of investigations of soil electrical characteristics are concerned with tropical and subtropical regions and hence there is little information about soils from other regions. The present study attempts to discover the most important constituents affecting soil electrical charge characteristics in a temperate zone. The study area is located on southern coastal zones of the Caspian Sea in northern Iran (Guilan province). The soil moisture and temperature regimes of the region determined by means of Newhall software were udic and thermic, respectively. Six representative pedons developed on granite, andesite, basalt, phyllite and limestone as their parent rocks are described and their physicochemical, mineralogical and electrical properties studied. The results indicate that the point of zero charge (PZC) values are small in all samples and smaller at the surface than in subsurface horizons. There is a positive correlation betweenpH0(PZC) values, organic carbon percentage and crystalline Fe (Fed) content. The points of zero net charge (ZPNC) values are <2.5 in all the pedons studied, which refers to large amounts of negative charge in these soils. The results also show that the differences in negative charge development in a pH range from 2.5 to 6 were largest for horizons rich in organic C and least for those with significant amounts of layer silicate minerals. Therefore, the most important variable charge component in all of the soils studied is organic matter which controls the negative charge at the soil surface. The permanent charge (бp) of the soils studied is also large and negative, which agrees with the amount of clay and the mineralogy of these soils.

Erratum: Probing Surface Charge Fluctuations with Solid-State Nanopores [Phys. Rev. Lett.102, 256804 (2009)

Received 9 June 2010DOI:https://doi.org/10.1103/PhysRevLett.105.039902©2010 American Physical Society

Probing surface charge fluctuations with solid-state nanopores.

We identify a contribution to the ionic current noise spectrum in solid-state nanopores that exceeds all other noise sources in the frequency band 0.1-10 kHz. Experimental studies of the dependence of this excess noise on pH and electrolyte concentration indicate that the noise arises from surface charge fluctuations. A quantitative model based on surface functional group protonization predicts the observed behaviors and allows us to locally measure protonization reaction rates. This noise can be minimized by operating the nanopore at a deliberately chosen pH.

Low-frequency noise in solid-state nanopores

Low-frequency ionic current noise in solid-state nanopores imposes a limitationon the time resolution achieved in translocation experiments. Recently, this1/f noise was described as obeying Hooge’s phenomenological relation, where the noise scalesinversely with the number of charge carriers present. Here, we consider an alternativemodel in which the low-frequency noise originates from surface charge fluctuations. Wecompare the models and show that Hooge’s relation gives the best description forthe low-frequency noise in solid-state nanopores over the entire salt regime from10−3 to1.6 M KCl.

Atmospheric electromagnetic pulse propagation effects from thick targets in a terawatt laser target chamber

Generation and effects of atmospherically propagated electromagnetic pulses (EMPs) initiated by photoelectrons ejected by the high density and temperature target surface plasmas from multiterawatt laser pulses are analyzed. These laser radiation pulse interactions can significantly increase noise levels, thereby obscuring data (sometimes totally) and may even damage sensitive probe and detection instrumentation. Noise effects from high energy density (approximately multiterawatt) laser pulses (approximately 300-400 ps pulse widths) interacting with thick approximately 1 mm) metallic and dielectric solid targets and dielectric-metallic powder mixtures are interpreted as transient resonance radiation associated with surface charge fluctuations on the target chamber that functions as a radiating antenna. Effective solutions that minimize atmospheric EMP effects on internal and proximate electronic and electro-optical equipment external to the system based on systematic measurements using Moebius loop antennas, interpretations of signal periodicities, and dissipation indicators determining transient noise origin characteristics from target emissions are described. Analytic models for the effect of target chamber resonances and associated noise current and temperature in a probe diode laser are described.

Understanding polyelectrolyte solutions: Macroion condensation with emphasis on the presence of neutral co-solutes

The multi-faceted applications of polyelectrolyte solution systems to a kaleidoscope of technological and biological processes make the understanding of these systems important and of interest. The highly relevant issue of instabilities that may occur in a polyelectrolyte solution and the ensuing macroion condensation constitute the premise of this review. An abundance of experimental and numerical simulation results in recent years provide evidence that a net electrostatic attractive force may exist between macroions and may lead to a phase separation. Specifically, in this review, three different types of instability involving macroions of spherical geometry are discussed. (i) The instability arising out of strong Coulomb correlations between counterions in the solution; this is most likely to occur in solutions containing multivalent counterions and/or in the presence of solvents of low relative permittivity. (ii) The instability caused by the macroion surface-charge fluctuations; the resultant charge correlations may induce an effective attraction between the weakly charged macroions. (iii) The instability due to the combined effect of electrostatic and crowding interactions when an inert co-solute is added to the solution. A sufficient increase in the concentration of the neutral species leads to a gradual change in the nature of the interaction between two macroions, from being repulsive to less repulsive and ultimately attractive. The structural features and thermodynamics in these complex systems are shaped by the collective and often competing effects of the species.

Classical Coulomb systems: Screening and correlations revisited

From the laws of macroscopic electrostatics of conductors (in particular, the existence of screening), taken as given, one can deduce universal properties for the thermal fluctuations in a classical Coulomb system at equilibrium. The universality is especially apparent in the long-range correlations of the electrical potentials and fields. The charge fluctuations are derived from the field fluctuations. This is a convenient way to study the surface charge fluctuations on a conductor with boundaries. Explicit results are given for simple geometries. The potentials and the fields have Gaussian fluctuations, except for a short-distance cutoff.

Effect of surface charge fluctuations on the spectral density of chemisorbed atoms

Basing our analysis on the Anderson-Newns model for H chemisorption, we develop a systematic method of calculating the adatom self-energy arising from the effect of substrate charge fluctuations. Using the method of functional derivatives, we generate a Dyson equation for the adatom Green's function and evaluate the self-energies due to the hopping and Coulomb interactions between metal and atom to second order. The latter involves the substrate density response function, and this allows us to include the effect of surface plasmons, bulk plasmons, and the particle-hole continum of the metal surface on an equal footing. The latter become increasingly important close to the surface, where simple image-potential arguments are no longer adequate. We formulate our theory treating the adatom Coulomb repulsion $U$ perturbatively (Newns, Hertz and Handler) as well as in the atomic limit (Brenig and Sch\"onhammer, Bell and Madhukar). Keeping only surface plasmons, we discuss the relaxation shifts and surface-plasmon satellites of the H-atom spectral density within the context of the Brenig-Sch\"onhammer approximation. In particular, we show how the effective Coulomb repulsion $U$ is reduced to $U\ensuremath{-}2{V}_{I}$. We also prove that the substrate-induced self-energy term which gives rise to the above changes on the spectral density has no effect on the net chemisorption energy. The small image-potential energy shift of the main resonance is canceled by the large shift associated with the weak surface-plasmon satellite. Much of our theoretical analysis is applicable to the core-level spectrum, and not just the valence electrons involved in chemisorption.