Free Surface Charge Research Articles

Graphite/h-BN van der Waals heterostructure as a gate stack for HgTe quantum wells

Two-dimensional topological insulators have attracted much interest due to their potential applications in spintronics and quantum computing. To access the exotic physical phenomena, a gate electric field is required to tune the Fermi level into the bulk band gap. Hexagonal boron nitride (h-BN) is a promising alternative gate dielectric due to its unique advantages such as flat and charge-free surface. Here we present a h-BN/graphite van der Waals heterostructure as a top gate on HgTe heterostructure-based Hall bar devices. We compare our results to devices with h-BN/Ti/Au and HfO2/Ti/Au gates. Devices with a h-BN/graphite gate show no charge carrier density shift compared to as-grown structures, in contrast to a significant n-type carrier density increase for HfO2/Ti/Au. We attribute this observation mainly to the comparable work function of HgTe and graphite. In addition, devices with h-BN gate dielectric show slightly higher electron mobility compared to HfO2-based devices. Our results demonstrate the compatibility between layered materials transfer and wet-etched structures and provide a strategy to solve the issue of significant shifts of the carrier density in gated HgTe heterostructures.

Polarization Saturation in Multilayered Interfacial Ferroelectrics.

Van der Waals polytypes of broken inversion and mirror symmetries have been recently shown to exhibit switchable electric polarization even at the ultimate two-layer thin limit. Their out-of-plane polarization has been found to accumulate in a ladder-like fashion with each successive layer, offering 2D building blocks for the bottom-up construction of 3D ferroelectrics. Here, it is demonstrated experimentally that beyond a critical stack thickness, the accumulated polarization in rhombohedral polytypes of molybdenum disulfide saturates. The underlying saturation mechanism, deciphered via density functional theory and self-consistent Poisson-Schrödinger calculations, point to a purely electronic redistribution involving: 1.Polarization-induced bandgap closure that allows for cross-stack charge transfer and the emergence of free surface charge; 2. Reduction of the polarization saturation value, as well as the critical thickness at which it is obtained, by the presence of free carriers. The resilience of polar layered structures to atomic surface reconstruction, which is essentially unavoidable in polar 3D crystals, potentially allows for the design of new devices with mobile surface charges. The findings, which are of general nature, should be accounted for when designing switching and/or conductive devices based on ferroelectric layered materials.

Variation of topological surface states of nodal line semimetal MgB2 resulting from adsorption of hydrogen, hydroxide, and water molecules.

Topological semimetals have garnered significant interest due to their intrinsic topological physics and potential applications in devices. A crucial feature shared by all topological materials is the bulk-boundary correspondence, indicating the presence of unique topologically protected conducting states at the edges when non-trivial band topology exists in the bulk. Previous studies on surface states of topological materials predominantly focused on pristine surfaces, leaving the exploration of surface states in topological semimetals with adsorbates relatively uncharted. This work, based on ab initio calculations, examines variations in the topological surface states of MgB2, a well-known conventional superconductor and topological nodal line semimetal. We employ a thick slab model with Mg/B atoms as surface terminations to simulate its topological surface states. Subsequently, we investigate the adsorption of hydrogen (H), hydroxide (OH), and water (H2O) on the surface slabs to observe changes in the surface states. The pristine slab model gives the drumhead-like surface states inside the surface projected nodal lines, while the topological surface states change differently after adsorbing H, OH, and H2O, which can be understood systematically by combining the surface adsorption Gibbs free energy ΔG, surface terminations, and surface charge density distributions. Especially, our findings suggest that the Bader charge transfer value of surface atoms providing topological states is a key indicator for evaluating the variation in topological surface states after adsorption. This study provides a systematic understanding of the topological surface states of MgB2 with different adsorbates, paving the way for future theoretical and experimental investigations in related fields and shedding light on the potential device applications of topological materials.

Interfacial properties of protein nanofibrils with different morphology prepared using aqueous solvent with ethanol: Part I. preparation and characterization

β-lactoglobulin can self-assemble into nano-fibrils with high specific surface area and hydrophobic groups, resulting in excellent interfacial activity. In this paper, β-lactoglobulin fibrils with conformation from semi-flexible to worm-like were prepared by controlling the polarity of the self-assemble precursor solution. The effects of ethanol on the surface properties, microscopic interaction forces and secondary structure of the proteins in the precursor solution, and the morphology, secondary structure, mechanical properties, and surface properties of the formed fibrils were investigated, using techniques including atomic force microscopy, Fourier transform infrared spectroscopy, circular dichroism spectroscopy and molecular dynamic simulation, etc. It was found that with the increase of ethanol concentration in the aqueous solvent, both the fibril length and diameter became smaller, the content of β-sheet in the fibrils gradually decreased whereas the content of random curl increased, resulting in decrease of the modulus of the fibrils (5.47 ± 0.54 GPa for semi-flexible fibrils and 0.52 ± 0.20 GPa for worm-like fibrils). With increasing ethanol concentration, the polarity of the precursor solution decreased, and both hydrophobic interactions and disulfide bonds of the proteins decreased whereas the electrostatic interactions with solvent were enhanced, which resulted in the increase of particle size of the precursor proteins and thus facilitated the final formation of fibrils with worm-like morphology. However the fibrillation process mainly occurs in the heating stage but not in the precursor solution. The surface hydrophobicity of the formed fibrils increased, whereas the amount of free sulfhydryl groups and surface charge decreased slightly with the increase of ethanol concentration. Their effects on the interfacial functional properties will be investigated further.

Measurement of dc and ac Electric Fields inside an Atomic Vapor Cell with Wall-Integrated Electrodes

We present and characterize a Rb atomic vapor cell with silicon-ring electrodes embedded between borosilicate glass tubes. The ring electrodes can be externally connectorized for application of electric fields to the inside of the cell. An atom-based, all-optical, laser-spectroscopic field-sensing method is employed to measure electric fields in the cell. Here, the Stark effect of electric-field-sensitive rubidium Rydberg atoms is exploited to measure dc electric fields in the cell of approximately $5$ V/cm, with a relative uncertainty of 10%. Measurement results are compared with dc field calculations, allowing us to quantify electric field attenuation due to free surface charges inside the cell. We further measure the propagation of microwave fields into the cell, using Autler-Townes splitting of Rydberg levels as a field probe. Results are obtained for a range of microwave powers and polarization angles relative to the cell's ring electrodes. We compare the results with microwave-field calculations.

A METHOD OF WAVELET ANALYSIS OF TIME SERIES OF PARAMETERS OF DIELECTRIC ABSORPTION OF ELECTRICAL INSULATING STRUCTURES

Introduction: In the objects of control there are always a number of interfaces, for example, solid insulation – electrode. On contacting surfaces, free surface charges are transferred. Surface conductivity leads to fluctuations in the measured values of the capacitance and the tangent of the dielectric loss angle of solid insulation, the state of which is determined. The drain off of the surface charge does not lead to a decrease in the scatter of the measured dielectric absorption parameters. One of the main reasons for the significant time spread of the dielectric absorption parameters, and to a large extent (three orders of magnitude) of the dielectric loss tangent are tribo charges caused by triboelectrification of cable structural elements. Tribo charges cause internal noise in electrical insulating structures, masking processes in the polymer insulation itself. Purpose: Substantiation of a method for analyzing the time series of dielectric absorption parameters, which provides increased accuracy of control and diagnostics of solid polymer insulation of electrical insulation structures based on filtering experimental data using wavelet transform. Methodology: The inefficiency of filtering the spectra of time series using a low filter based on the direct Fourier transform is shown. Multilevel wavelet decomposition of the time series of parameters is presented, and the efficiency of applying wavelet transforms to identify high-frequency and low-frequency components in the measured values. Practical value: The method of analyzing the time series of dielectric absorption parameters using the wavelet transform, proposed for the first time, makes it possible to increase the accuracy of monitoring and diagnostics of solid polymer insulation both at the manufacturing stage and in the operation of electrical insulating structures. This method is the basis for creating a database of control results for assessing the state of solid polymer insulation of electrical insulation structures, in particular, power and information cables.

Potential of zero charge and surface charging relation of metal-solution interphases from a constant-potential jellium-Poisson-Boltzmann model

The potential of zero charge (pzc), a fundamental concept in interfacial electrochemistry, is revisited using a jellium-Poisson-Boltzmann model. Under constant-potential description of the metal-solution interphase, this model is able to calculate the surface charging relation (surface free charge density as a function of the electrode potential) and then to determine the pzc therefrom. The potential corresponding to the minimum of differential double-layer capacitance curve is shown to be lower than the pzc determined from surface charging relation, which is caused by free metal electrons entering the solution phase. The model further reveals that the pzc decreases when the vacuum gap between the solution phase and the metal surface, $d$, becomes narrower. This is consistent with the common observation that the pzc of metal-solution interphase is lower than that calculated from the work function of metal-vacuum interphase (the latter corresponds to $d=\ensuremath{\infty})$. Multifaceted roles played by the solvent, including electrostatic screening, polaron effect, and orthogonalizational repulsion, are analyzed. Also discussed are the effects of specific adsorption of ions and potential-dependent $d$ on the surface charging relation.

Surface charge transport behavior and flashover mechanism on alumina/epoxy spacers coated by SiC/epoxy composites with varied SiC particle size

Charge accumulation phenomenon on gas–solid interface greatly restricts the development of HVDC energy transmission system. In this study, the surface charge transport behavior and flashover performance on alumina/epoxy spacer coated by SiC/epoxy composites are experimentally investigated under DC stress. SiC/epoxy composites with varied SiC particle size are fabricated and deposited on spacer surface. Nearly a charge free surface is achieved especially at smaller SiC particle size, even when metallic wires are adhered on spacer surface and connected to high voltage electrode. The DC flashover voltage increases with the decrease of SiC particle size. On the one hand, smaller SiC particle size exhibits more outstanding nonlinear conductivity characteristics, contributing to accelerating charge dissipation, whereas on the other hand, it would introduce plenty of shallow traps due to the increased interfacial regions between SiC particle and epoxy matrix, resulting in aggravating charge accumulation. A theoretical model is proposed to reveal the control mechanism of SiC/epoxy coating with different SiC particle size on flashover performance. The validity of this model can be confirmed based on the surface trap distribution, carrier mobility and our previous investigations on gas–solid interface flashover development process.

Numerical modeling of partial discharges in a solid dielectric-bounded cavity: A review

Partial discharge (PD) taking place in a solid dielectric-bounded cavity involves physical processes such as free electron supply, discharge development and surface charge decaying, which bring about memory effects and become the main reasons for stochastic behavior of PDs. This paper reviews numerical modeling of cavity PD in the past 30 years. In the first place, physical processes relevant to PD activity are summarized, and modeling methods for discharge development are classified. Then some differences of PD modeling at AC and DC voltages are distinguished. Subsequently, reproducing methods from simulations to experiments are introduced, as well as their comparison under different conditions and with the emphasis on voltage frequency and PD aging. At last, some problems about current simulation models are discussed, and our suggestions for future work are proposed.

Analytical expression of top surface charge sensitivity in fully depleted semiconductor on insulator MOS transistor

An analytical expression of free top surface charge sensitivity in FDSOI MOS structure has been established for weak inversion region and validated by TCAD numerical simulation. The influence of various FDSOI stack parameters has been analyzed. The impact of the interface trap density has been particularly emphasized, leading to a strong undesired degradation of sensitivity. This indicates that top surface passivation is a key issue for efficient charge sensing. These expressions of top surface charge sensitivity and associated threshold voltage shift should be very useful for sensor design and electrical characterization purpose.

Polyhedral Oligomeric Silsesquioxane (POSS) Nano‐Composite Separation Membranes − A Review

Nano‐composite membranes are developed to overcome the limitation on trade‐off effect of permeability and selectivity from commercial membranes, however, careful selection of the nano‐fillers is required to match the selection criteria for specific applications. Polyhedral oligomeric silsesquioxane (POSS) nano‐particles are studied as the filler for nano‐composite membrane design for their unique 3‐dimensional building block property and flexible functionality toward the enhancement of separation. The review paper presents the benefits brought by POSS for nano‐composite membranes design, systematically compared to pristine or reference membranes. The scope of the review comprehensively includes liquid separation driven by pressure, desalination, solvent separation, gas separation, and proton exchange membranes. The free volume and surface charge modification as well as the structural enhancement from POSS incorporation are highlighted as keys to affect the trade‐off between permeation and selectivity and future developments for POSS nano‐composite membranes are discussed.

New probes to surface free charge at electrochemical interfaces with platinum electrodes

Summary The free charge is a key variable dominating the properties of the interphase between a metallic electrode and an electrolytic solution. However, its determination in the case of platinum electrodes is not straightforward. Here, we review some of the experimental and computational strategies that can be used to gain information about the free charge in these interphases. First, we discuss the influence of the free charge on some reactions of technological interest. Then, we demonstrate the usefulness of peroxodisulfate (PDS) reduction as a probe reaction sensitive to the free charge on platinum electrodes. The comparison with other strategies, based on either CO displacement, measurements of the potential of maximum entropy of the interphase or N2O reduction, validates the consistency of this approach. Finally, it is remarked how the use of PDS reduction as a surface probe evidences the existence of a non-monotonic behavior in the variation of the charge as a function of potential on platinum electrodes.

Modulating capacitive response of MoS2 flake by controlled nanostructuring through focused laser irradiation

Unlike graphene nanostructures, various physical properties of nanostructured MoS2 have remained unexplored due to the lack of established fabrication routes. Herein, we have reported unique electrostatic properties of MoS2 nanostructures, fabricated in a controlled manner of different geometries on 2D flake by using focused laser irradiation technique. Electrostatic force microscopy has been carried out on MoS2 nanostructures by varying tip bias voltage and lift height. The analysis depicts no contrast flip in phase image of the patterned nanostructure due to the absence of free surface charges. However, prominent change in phase shift at the patterned area is observed. Such contrast changes signify the capacitive interaction between tip and nanostructures at varying tip bias voltage and lift height, irrespective of their shape and size. Such unperturbed capacitive behavior of the MoS2 nanostructures offer modulation of capacitance in periodic array on 2D MoS2 flake for potential application in capacitive devices.

Stimuli-Responsive Multilayers Based on Thiolated Polysaccharides That Affect Fibroblast Cell Adhesion.

Control of the biomaterial properties through stimuli-responsive polymeric platforms has become an essential technique in recent biomedical applications. A multilayer system of thiolated chitosan (t-Chi) and thiolated chondroitin sulfate (t-CS), consisting of five double layers ([t-Chi/t-CS]5), was fabricated here by applying a layer-by-layer coating strategy. To represent a novel class of chemically tunable nanostructures, the ability to cross-link pendant thiol groups was tested by a rise from pH 4 during layer formation to pH 9.3 and a more powerful chemical stimulus by using chloramine-T (ChT). Following both treatments, the resulting multilayers showed stimuli-dependent behavior, as demonstrated by their content of free thiols, wettability, surface charge, elastic modulus, roughness, topography, thickness, and binding of fibronectin. Studies with human dermal fibroblasts further demonstrated the favorable potential of the ChT-responsive multilayers as a cell-adhesive surface compared to pH-induced cross-linking. Because the [t-Chi/t-CS]5 multilayer system is responsive to stimuli such as the pH and redox environment, multilayer systems with disulfide bond formation may help to tailor their interaction with cells, film degradation, and controlled release of bioactive substances like growth factors in a stimuli-responsive manner useful in future wound healing and tissue engineering applications.

Role of a "surface wettability switch" in inter-fiber bonding properties.

The fiber surface wettability is one of the most important lignocellulosic fiber characteristics affecting the inter-fiber bonding properties of final bio-products. In this study, the surface wettability (evaluated by the surface free energy, surface lignin and surface charge) of mechanically refined fibers and the bonding properties of the fiber matrix (handsheets) were measured and correlated to each other. The results showed that the fiber surface charge increased from 48.38 mmol kg−1 to 60.38 mmol kg−1 and the surface lignin decreased from 87.1% to 77.5% during the fiber mechanical treatment, leading to the improvement of the fiber surface free energy from 46.63 mJ m−2 to 54.45 mJ m−2. As a result, the bonding strength index increased from 2.60 N m g−1 to 9.73 N m g−1 without significant loss of bulk properties. In a word, the fiber surface wettability could be adjusted to facilitate the inter-fiber bonding properties of the paper or paperboard products using lignin-rich fibers as raw materials.

Parameters characterizing the charge state of dielectrics

AbstractThis paper presents three physical sources of the electric field in dielectrics: excess free volume charges with the distribution qv(x,y,z), free surface charges with the distribution qs(x,y,z) and frozen polarization state in the dielectric. They have a deciding influence on the parameters of the electret, in particular they determine the total lifetime of the electret and technical components made of it. The indeterminacy related to the mutual proportions of the spatial and surface charges was discussed: one can find an infinite number of distributions of surface qse(x,y,z) and spatial qve(x,y,z) charges leading to the same distribution of the electric field E(x,y,z). A general case of electret was considered, where a coexistence of relaxation decay of frozen polarization and Maxwellian relaxation dependent on volume conductivity of the dielectric is assumed. An attempt to interpret the charge lifetime in real electrets was made.

Wave Propagation in Piezoelectric Medium with the Flexoelectric Effect Considered

AbstractWith the consideration of the flexoelectric effect, the dispersion feature of plane waves propagating in infinite piezoelectric medium and the reflection amplitude ratio at a mechanically traction-free and dielectrically charge-free surface of semi-infinite piezoelectric half-space are studied in this paper. The flexoelectric effect is essentially the microstructure effect of heterogeneous piezoelectric medium. Besides, the strain gradient effect and the micro-inertial effect are also introduced to reflect the influences of the microstructure. Different from the classical piezoelectric medium, there are five kinds of coupled elastic waves in the microstructured piezoelectric medium. The influences of the flexoelectric effect, the strain gradient effect and the micro-inertial effect upon the dispersion feature of the coupled elastic waves and the reflection amplitude ratio are studied numerically. The numerical results are shown graphically and are validated by the energy conservation law. It is found that these microstructure effects have obvious influences upon the dispersion feature of the coupled elastic waves and therefore also affect the reflection energy partition at the mechanically free surface. Especially, the micro-inertial effect has fundamental influences on the dispersion properties of the coupled elastic waves.

A visco-hyperelastic model characterizing the electromechanical behavior of nonhomogeneous soft material

In this work, a model is proposed extending the basic concept of particle of continuum mechanics combining the distribution of polymer chain. Particularly, the general mechanical behavior of complex system, including complex geometry of interface and material defect, can be described by the local distribution of polymer chain. Furthermore, the local density of the polymer chain derived from the Gaussian statistics has been introduced and related to the electromechanical behavior. When the system is in thermodynamic equilibrium, the free surface charge density is modified according to the external voltage and deformation. The total stress field, which includes the electrical effect, has emerged as a function of deformation and time history. Eventually, the model is fitted to experimental data available in the literature.

Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li-S Batteries by Li Ion and Electron Beams.

We report the characterization of multiscale 3D structural architectures of novel poly[sulfur-random-(1,3-diisopropenylbenzene)] copolymer-based cathodes for high-energy-density Li-S batteries capable of realizing discharge capacities >1000 mAh/g and long cycling lifetimes >500 cycles. Hierarchical morphologies and interfacial structures have been investigated by a combination of focused Li ion beam (LiFIB) and analytical electron microscopy in relation to the electrochemical performance and physicomechanical stability of the cathodes. Charge-free surface topography and composition-sensitive imaging of the electrodes was performed using recently introduced low-energy scanning LiFIB with Li+ probe sizes of a few tens of nanometers at 5 keV energy and 1 pA probe current. Furthermore, we demonstrate that LiFIB has the ability to inject a certain number of Li cations into the material with nanoscale precision, potentially enabling control of the state of discharge in the selected area. We show that chemical modification of the cathodes by replacing the elemental sulfur with organosulfur copolymers significantly improves its structural integrity and compositional homogeneity down to the sub-5-nm length scale, resulting in the creation of (a) robust functional interfaces and percolated conductive pathways involving graphitic-like outer shells of aggregated nanocarbons and (b) extended micro- and mesoscale porosities required for effective ion transport.

Measurement and Study of Electromagnetic Emission Generated by Tensile Fracture of Polymers and Carbon Fibres

Electromagnetic emission (EME) signals generated by tensile fracture of four different types of polymers and three different carbon fibre types are presented and discussed. A suitable set-up for the detection of the electric field component of EME generated by fracture of solids is proposed. Basic theoretical considerations are made about the coupling between these field components and the capacitive sensors used to directly measure the short ranged and low frequency (kHz–MHz) electric fields emitted by the generation of free surface charges and their spatial movement as dictated by the vibrational motion of the crack walls. Special focus is put on solids with low conductivity, where the influences of the material on the emitted fields is small and the detected electric signals almost solely depend on the source dynamics and the sensor characteristics. Analysis of the influence of the acquisition circuit is presented. The discussion of the electric signals emitted by tensile fracture of carbon fibres and polymer specimens comprises the influences of the material properties on the signals as well as correlations between the signals and the crack dynamics, including the crack propagation velocities.