716,033 research outputs found

    Double layer effects in voltammetric measurements with scanning electrochemical microscopy (SECM)

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    Scanning electrochemical microscopy (SECM) operating as a variable gap ultra-thin layer twin-working electrode cell, has long been recognised as a powerful technique for investigating fast kinetics (heterogeneous electron transfer and homogeneous reactions coupled to electron transfer) as a consequence of high mass transport rates between the working electrodes when biased to promote redox shuttling. Recently, SECM has advanced technically and nanogap cells with dimensions on the 10s of nm scale have been reported. In this paper, we consider double layer effects on voltammetric measurements in this configuration, outlining a comprehensive model that solves the Nernst-Planck equation and Poisson equation with charged interfaces. For supporting electrolyte concentrations that have been used for such measurements (50 mM and 100 mM), it is shown that for typical electrode charges and charge on the glass insulator that encases the ultramicroelectrode (UME) tip used in SECM, there are profound effects on the voltammetric wave-shapes for redox reactions of charged redox couples, in the common modes used to study electron transfer kinetics, namely the tip-voltammetry (feedback) mode and substrate-voltammetry (substrate-generation/tip-collection and competition) modes. Using the reduction and oxidation of a singly charged redox species to a neutral and doubly charged species, respectively, as exemplar systems, it is shown that the charge on the electrodes can greatly distort the voltammetric wave-shape, while charge on the glass that surrounds the UME tip can affect the limiting current. Analysis of SECM voltammograms using methods that do not account for double layer effects will thus result in significant error in the kinetic values derived and tip-substrate distances that have to be estimated from limiting currents in SECM. The model herein provides a framework that could be developed for further studies with nanogap-SECM (e.g. consideration of alternative models for the electrical double layer, other supporting electrolyte concentrations, potential of zero charge on the electrodes and charges on the redox couples). The model results presented are shown to qualitatively match to SECM voltammetric features from experimental data in the literature, and are further supported by experimental data for redox processes of tetrathiafulvalene (TTF), namely the TTF/TTF+ and TTF+/TTF2+ redox couples. This serves to demonstrate the immediate practical application of some of the ideas presented herein. For future applications of SECM, the use of different supporting electrolyte concentrations and a range of tip-substrate separations may allow the determination of both electron transfer kinetics and double layer properties

    Investigation on influential factors on chloride concentration index of cement-based materials by pore solution expression method

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    In this study, the effects of different factors on chloride concentration index (N-c) of cement paste were studied. The factors including chloride concentration in soaking solution, slag replacement, external applied voltage and cation ions of soaking solution were all studied from the electrical double layer (EDL) properties point of view. Zeta potential and proton Nuclear Magnetic Resonance (H-1 NMR) measurements were conducted to investigate the properties of electrical double layer for cement paste specimens and their effects on the value of chloride concentration index. The results showed that these factors all impacted effects on chloride concentration in electrical double layer and chloride concentration index. The properties of electrical double layer including chloride distribution and thickness of electrical double layer mainly controlled the phenomenon of "chloride concentrate" and value of chloride concentration index. As the increase of zeta potential and electrical double layer thickness, the content of chloride ions in electrical double layer and the value of chloride concentration index gradually increased. (C) 2019 Elsevier Ltd. All rights reserved

    Josephson Effects in Double-Layer Quantum Hall States

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    Under quite plausible assumptions on double-layer quantum Hall states with strong interlayer correlation, we show in general framwork that coherent tunneling of a single electron between two layers is possible. It yields Josephson effects with unit charge tunneling. The origin is that Halperin states in the quantum Hall states are highly degenerate in electron number difference between two layers in the absence of electrons tunneling.Comment: 9 Pages, Revtex Inpress Int.J.Mod.Phys.

    Capillary wave dynamics on supported viscoelastic films: Single and double layers

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    We study the capillary wave dynamics of a single viscoelastic supported film and of a double layer of immiscible viscoelastic supported films. Using both simple scaling arguments and a continuum hydrodynamic theory, we investigate the effects of viscoelasticity and interfacial slip on the relaxation dynamics of these capillary waves. Our results account for the recent observation of a wavelength-independent decay rate for capillary waves in a supported polystyrene/brominated polystyrene double layer [X. Hu {\em et al.}, Phys. Rev. E {\bf 74}, 010602 (R) (2006)].Comment: 14 pages, 9 figure

    Impact of diffusion layers in strong electrolytes on the transient current

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    Transient currents of electrolytes in response to a voltage step can reveal a lot about the behavior of charges present in an electrolyte. In this paper, electrolytes with high ionic strength are considered. In the limit of small voltage steps, the interpretation is straightforward as the equations describing the transient can be linearized. However, when high ion concentrations and voltage steps of the order of kT/q are considered, we find higher-order effects that occur simultaneously with the diffuse double layer charging. In this case, the diffuse double layer and the transient diffusion layer are coupled because of the screening of the field, leading to a -3/2 power law for the transient current

    Self-Powered, Highly Sensitive, High Speed Photodetection Using ITO/WSe2/SnSe2 Vertical Heterojunction

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    Two dimensional transition metal di-chalcogenides (TMDCs) are promising candidates for ultra-low intensity photodetection. However, the performance of these photodetectors is usually limited by ambience induced rapid performance degradation and long lived charge trapping induced slow response with a large persistent photocurrent when the light source is switched off. Here we demonstrate an indium tin oxide (ITO)/WSe2_2/SnSe2_2 based vertical double heterojunction photoconductive device where the photo-excited hole is confined in the double barrier quantum well, whereas the photo-excited electron can be transferred to either the ITO or the SnSe2_2 layer in a controlled manner. The intrinsically short transit time of the photoelectrons in the vertical double heterojunction helps us to achieve high responsivity in excess of 11001100 A/W and fast transient response time on the order of 1010 μ\mus. A large built-in field in the WSe2_2 sandwich layer results in photodetection at zero external bias allowing a self-powered operation mode. The encapsulation from top and bottom protects the photo-active WSe2_2 layer from ambience induced detrimental effects and substrate induced trapping effects helping us to achieve repeatable characteristics over many cycles

    A fluid description of plasma double-layers

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    The space-charge double-layer that forms between two plasmas with different densities and thermal energies was investigated using three progressively realistic models which are treated by fluid theory, and take into account four species of particles: electrons and ions reflected by the double-layer, and electrons and ions transmitted through it. The two plasmas are assumed to be cold, and the self-consistent potential, electric field and space-charge distributions within the double-layer are determined. The effects of thermal velocities are taken into account for the reflected particles, and the modifications to the cold plasma solutions are established. Further modifications due to thermal velocities of the transmitted particles are examined. The applicability of a one dimensional fluid description, rather than plasma kinetic theory, is discussed. Theoretical predictions are compared with double layer potentials and lengths deduced from laboratory and space plasma experiments

    Effects of Fermion Flavor on Exciton Condensation in Double Layer Systems

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    We use fermionic path integral quantum Monte Carlo to study the effects of fermion flavor on the physical properties of dipolar exciton condensates in double layer systems. We find that by including spin in the system weakens the effective interlayer interaction strength, yet this has very little effect on the Kosterlitz-Thouless transition temperature. We further find that, to obtain the correct description of screening, it is necessary to account for correlation in both the interlayer and intralayer interactions. We show that while the excitonic binding cannot completely surpress screening by additional fermion flavors, their screening effectiveness is reduced leading to a much higher transition temperatures than predicted with large-N analysis.Comment: 4 pages, 3 figure
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