Surface Potential Measurements Research Articles

Charge interaction behaviors at interfacial domains in DC GIL insulators

Long-term operation of high voltage direct current at elevated temperatures can result in the accumulation of surface charges in DC gas-insulated transmission line (GIL) insulators. Such a phenomenon leads to localized electric field distortion, increasing the risk of surface discharge. The analysis of interaction behaviors between surface charge and space charge at interfacial domains of GIL insulators is a complex task, which requires a comprehensive understanding of physical mechanisms of the gas–solid interface charging. In this work, a two-dimensional bipolar charge transport and interaction (2D BCTI) model is established, with the consideration of both surface and space charge dynamics. Pulsed electroacoustic tests and surface potential measurements are conducted on DC GIL insulator materials under different electrical-thermal coupling conditions. Experimental results exhibit great consistency with the predictions from the 2D BCTI model. The local accumulation of space charge near interfaces has certain effects on surface potential distribution, which in turn influences charge injection behavior from electrodes. In comparison to traditional surface charge simulation models, the consideration of space charge–surface charge interaction behaviors proves to be essential for estimating the polarity and amplitude of surface potential distribution. This model holds promise for assessing charge characteristics in electrical equipment where direct measurement is challenging.

Type-dependent hot carrier behavior in photoelectrochemical reduction and oxidation of Au/GaN junction photoelectrodes

In junctions with semiconductors, plasmonic metal nanoparticles (NPs) can be utilized to efficiently harness solar energy by collecting hot carriers. However, the behavior of hot carriers at the interfaces between semiconductors with opposite doping types is not fully elucidated. Here, Au NPs are attached to n-doped (Au/n-GaN) or p-doped gallium nitrides (Au/p-GaN) to examine the photoelectrochemical performance regarding hot carrier behavior. Direct surface potential measurements revealed that electrons are collected in n-type GaN (n-GaN) when illuminated, while hot holes remain in Au NPs for oxidation reactions via the plasmonic process. Conversely, holes are collected in p-type GaN (p-GaN), thereby promoting the reduction reaction by the electrons left in Au NPs. Specifically, the change in surface potential difference induced by green light illumination in Au/p-GaN is four times greater than that observed in Au/n-GaN. Conversely, the changes in open-circuit potential and photocurrent density under light illumination are approximately six times more significant in Au/n-GaN compared to Au/p-GaN. This difference in efficiency can be attributed to the more favorable oxidation reaction occurring in Au NPs at the interface with GaN materials, as opposed to the reduction reaction, due to the difference in band alignment between the Au/GaN junction systems.

SQUID magnetoneurography: an old-fashioned yet new tool for noninvasive functional imaging of spinal cords and peripheral nerves.

We are engaged in the development and clinical application of a neural magnetic field measurement system that utilizes biomagnetic measurements to observe the activity of the spinal cord and peripheral nerves. Unlike conventional surface potential measurements, biomagnetic measurements are not affected by the conductivity distribution within the body, making them less influenced by the anatomical structure of body tissues. Consequently, functional testing using biomagnetic measurements can achieve higher spatial resolution compared to surface potential measurements. The neural magnetic field measurement, referred to as magnetoneurography, takes advantage of these benefits to enable functional testing of the spinal cord and peripheral nerves, while maintaining high spatial resolution and noninvasiveness. Our magnetoneurograph system is based on superconducting quantum interference devices (SQUIDs) similar to the conventional biomagnetic measurement systems. Various design considerations have been incorporated into the SQUID sensor array structure and signal processing software to make it suitable for detecting neural signal propagation along spinal cord and peripheral nerve. The technical validation of this system began in 1999 with a 3-channel SQUID system. Over the course of more than 20 years, we have continued technological development through medical-engineering collaboration, and in the latest prototype released in 2020, neural function imaging of the spinal cord and peripheral nerves, which could also be applied for the diagnosis of neurological disorders, has become possible. This paper provides an overview of the technical aspects of the magnetoneurograph system, covering the measurement hardware and software perspectives for providing diagnostic information, and its applications. Additionally, we discuss the integration with a helium recondensing system, which is a key factor in reducing running costs and achieving practicality in hospitals.

Method for Measuring Surface Charge on Insulating Materials Based on the Vibrating Capacitor Method

The phenomenon of surface charging, known as contact electrification or tribocharging, has wide-ranging applications but also notable hazards. Precisely measuring surface charge density in insulating materials is crucial for optimizing tribocharging and mitigating adverse effects. Although the vibrating capacitor method is commonly used for this purpose, its principle, designed for conductive materials, limits direct application to insulating surfaces, leaving the relationship between measured surface potential and surface charge density unclear. To address this issue, this study simulated the process of measuring the surface potential of insulating materials using Comsol simulations. It analyzed the effects of charged area size, probe height, and probe position, and utilized the spatial distribution of potential measurement values of surface point charges to derive an integral relationship between the distribution of measured surface potential values and the distribution of surface charge density. The integral relationship of surface potential distribution under different forms of surface charge density distributions calculated from this formula largely matches the numerical simulation results. Based on this, a relationship between the distribution of surface charge density and surface potential measurement values was further derived. This relationship can be used for measuring the surface charge density of insulating materials.

Charging Properties of Electrospun Poly(l‐lactic acid) Submicrofiber Mat and Its Electrical Applications

Wearable pressure sensors have attracted significant attention owing to their potential applications in health monitoring and connectivity to internet‐based apps. Polymers such as poly(vinylidene fluoride) have been used in sensors. However, being petroleum‐derived materials, they do not decompose and remain in the soil when disposed. Poly(l‐lactic acid) (PLLA) is a promising material because of its biodegradable nature and its derivation from plant‐based materials. In addition, the electrospun PLLA fiber mat contains real charges and exhibits electromechanical properties. However, the detailed charging properties of the PLLA fiber mats remain unclear. Herein, the charge distribution of these fiber mat is presented, and a charging model of the fiber mat and a numerical model of the output charges from the fiber mats with electrodes are proposed. Additionally, the retention properties of the stored charges are determined using surface potential measurements at different temperatures. In addition, a self‐power‐generating touch sensor and mask‐type sensor are developed using biodegradable materials produced from biomass. These studies contribute to the improvement in the charge properties of PLLA fiber mats and the resulting wearable biodegradable sensors.

Spatially Resolved High Voltage Kelvin Probe Force Microscopy: A Novel Avenue for Examining Electrical Phenomena at Nanoscale

AbstractKelvin probe force microscopy (KPFM) is a well‐established scanning probe technique, used to measure surface potential accurately; it has found extensive use in the study of a range of materials phenomena. In its conventional form, KPFM frustratingly precludes imaging samples or scenarios where large surface potential or surface potential gradients exist outside the typical ±10 V window. If the potential regime measurable via KPFM can be expanded, to enable precise and reliable metrology, through a high voltage KPFM (HV‐KPFM) adaptation, it can open up pathways toward a range of novel experiments, where the detection limit of regular KPFM has so far prevented the use of the technique. In this work, HV‐KPFM is realized and shown to be capable of measuring large surface potential and potential gradients with accuracy and precision. The technique is employed to study a range of materials (positive temperature coefficient of resistivity ceramics, charge storage fluoropolymers, and pyroelectrics) where accurate, spatially resolved mapping of surface potential within high voltage regime facilitates novel physical insight. The results demonstrate that HV‐KPFM can be used as an effective tool to fill in existing gaps in surface potential measurements while also opening routes for novel studies in materials physics.

Formation of one-dimensionally self-aligned Si-QDs and their local electron discharging properties

Self-aligned Si-quantum-dots (Si-QDs) with an areal density as high as ∼1011 cm−2 have been fabricated on ultrathin SiO2 by using a ∼4.5 nm thick poly-Si on insulator (SOI) substrate, and controlling low-pressure CVD using monosilane (SiH4), and followed by thermal oxidation. By controlling the thermal oxidation processes of Si-QDs and the poly-Si layer, we have successfully demonstrated the vertical alignment of Si-QDs, where the Si-QDs are also used as a shadow mask of the underlying poly-Si layer. We also demonstrated in-plane alignment of the one-dimensionally self-aligned Si-QDs on line-patterned SiO2. In addition, from surface potential measurements by using atomic force microscopy/Kelvin probe force microscopy, we confirmed that the initial surface potential change caused by valence electron extraction from the dots to the tip was stably maintained until ∼120 min, implying the quantum confinement effect at discrete energy levels of the upper and lower-QDs.

Direct Observation of Contact Electrification Effects at Nanoscale Using Scanning Probe Microscopy

AbstractIn the last decade, contact electrification has gained attention for its potential in energy harvesting, addressing fundamental physics. Scanning probe microscopy (SPM) has evolved as a powerful platform, enabling the in situ characterization/manipulation of a sample's tribological/electrical properties at nanoscale. However, although both the sliding and tapping modes are available in the energy harvesting, the lateral sliding using contact mode SPM has predominantly been employed in triboelectric study. In this work, contact electrification on polydimethylsiloxane is investigated, using peak force tapping atomic force microscopy (PF‐AFM). As PF‐AFM quasi‐discretely offers vertical tapping motions of the probe with a regulated force amplitude, resembling a vertical‐type triboelectric nanogenerator, while minimizing lateral forces. The subsequent surface potential measurements reveal that the generated tribocharge is influenced by both the effective work function difference and energy dissipation at the interface. Furthermore, the accumulation of transferred charges is explored by measuring tip‐sample current during the PF‐AFM operation, showing the comparable results with the surface potential measurement. The results can be attributed to the contact potential difference assisted by the energy dissipation at the interface. This study offers an advanced opportunity to understand and study charge generation behavior based on surface properties without damaging the sample.

Quantitative characterization of built-in potential profile across GaAs p–n junctions using Kelvin probe force microscopy with qPlus sensor AFM

The electrostatic potential distribution in materials and devices plays an important role in controlling the behaviors of charge carriers. Kelvin probe force microscopy (KPFM) is a powerful technique for measuring the surface potential at a high spatial resolution. However, the measured surface potential often deviates from the potential deep in the bulk owing to certain factors. Here, we performed KPFM measurements across the p–n junction, in which such factors were eliminated as much as possible by selecting the sample, force sensor, and measurement mode. The measured surface potential distribution agrees well with the line shape of the simulated bulk potential. Our results demonstrate that KPFM is capable of quantitatively characterizing potential distributions whose changes occur on the order of 10 nm.

Kelvin Probe Characterization of Nanocrystalline Diamond Films with SiV Centers as Function of Thickness

Optically active color centers in diamonds have been intensively studied due to their potential in photonics, energy harvesting, biosensing, and quantum computing. Silicon vacancy (SiV) center offers an advantage of suitable emission wavelength and narrow zero‐phonon line at room temperature. Measurement of surface potential and photovoltage can provide better understanding of the physics and control of SiV light emission, such as charge states and charging effects. Herein, optoelectronic properties of nanocrystalline diamond films with SiV centers at different layer thicknesses (10–200 nm, controlled by the growth time) under ambient conditions are studied. Time‐dependent measurements are performed in the light–dark–light cycle. Positive photovoltage arises on samples with SiV layer thicknesses below 55 nm on both H‐ and O‐terminated surfaces. Above 55 nm the photovoltage switches to negative. This layer thickness thus represents a halfway boundary between surface‐controllable and bulk SiV centers dominant contribution. A band diagram scheme explaining the photovoltage switching mechanism is provided.

Universal Digital Probe Electrometer for Testing Semiconductor Wafers

Non-contact electrical methods are widely used for research and control of semiconductor wafers. The methods are usually based on surface potential measurement (CPD) in combination with illumination and/or deposition of charges on the sample using a corona discharge, and are also based on the measurement of surface photo-emf. By photo-EMF (SPV) it is possible to determine the lifetime of minor charge carriers, their diffusion length and detect traces of heavy metals on the surface. In addition, using photo-EMF it is possible to determine the surface resistance of the plate, some parameters of the dielectric layer on the surface and barrier photo-EMF (JPV). Electrical performance results reflect the influence of near-surface characteristics on the final performance of devices. The aim of the work was to develop a universal digital probe electrometer that implements various non-contact electrical methods for analyzing semiconductor wafers, in which the change in operating modes and configuration, transmission of the received data, remote testing and calibration are carried out via digital local control channels. This paper describes a universal digital probe electrometer developed by the authors, which implements the above-described non-contact electrical methods for analyzing semiconductor wafers (CPD, SPV and JPV), in which the change in operating modes and configuration, transmission of the received data, remote testing and calibration are carried out via digital local control channels. Due to their high speed, electrical characterization methods are suitable for inspecting semiconductor wafers during production. The results of testing the developed probe electrometer in CPD, SPV and JPV modes are presented, which reflect the effectiveness of the proposed approaches.

Influence of the initial charge on the triboelectrification of millimeter size set of particles in an inclined tube

In this paper, the triboelectric charge buildup on a set of initially charged particles is investigated. During the study, 30 g of charged polypropylene particles of 2–3 mm size were repeatedly passed through a 1 m inclined copper or polyvinylchloride (PVC) tube. At the tube output, the charge is measured after each passage, allowing the cumulative charge evolution over time to be measured. The obtained curves are fitted to an exponential model to extract the time constant of the process. For each experience, particles are first charged at different levels and polarities relatively to the presumed saturation charge using both corona discharge and triboelectricity. The obtained results show that the initial charge value and polarity strongly affect the subsequent triboelectricity variations as well as the process speed. The triboelectric charge evolution is mainly related to the ratio between initial charge (Q0) and saturation charge value (Qs). Therefore, the particle's charge will increase if Q0 < Qs, decrease if Q0 > Qs, or be maintained at the same level when Q0 ≈ Qs. The particles can change polarity and be recharged with the opposite sign according to the tube and the initial charge level. This triboelectric charge behavior is explained based on the surface energy state in the electron transfer model. Furthermore, increasing the particles quantity from 5 g to 60 g reduces the saturation charge from 11 nC/g to 2.5 nC/g, which is approximately 77% less. Surface potential distribution measurements show an uneven distribution with less charge in large amounts compared to lower amounts.

A Spontaneously Electrical State of Matter.

ConspectusMolecular deposition on solid surfaces forms crystalline or amorphous/glassy thin solid films. Intermolecular interactions govern the packing and dynamics of these films. The connection between molecular structure and intermolecular interactions is based on understanding electrostatic forces, dispersion forces and hydrogen bonding. Recently, an entire class of dipolar molecular species have demonstrated counterintuitive self-organization such that the dipole moments of individual molecules are oriented in thin films. This leads to the spontaneous generation of polarized molecular films manifesting a polarization charge equivalent to tens to hundreds of volts in strength at the film-vacuum interface, relative to the film-substrate interface. These voltages, and the corresponding electric fields present in such films, result from a collective and spontaneous orientation of molecular dipoles throughout the film during film growth and represent a metastable state of polarized material. The existence of these materials should encourage reconsideration of the importance of solid-state intermolecular electrostatic interactions.This account will detail observations of the spontaneous electric fields in molecular solids, provide insights into the dynamics and structure of molecular materials that the emergence of these electric fields can facilitate, and present a dipole-alignment based mean-field model that reproduces the temperature dependence of the electric field strength. Species as diverse as carbon monoxide, nitrous oxide, freons, simple alcohols, and cis-methyl formate have been demonstrated to spontaneously generate electric fields. We have reported electric fields more than 108 V m-1, have shown how field strength varies with the film deposition temperature, and have reported temperature-dependent Stark shifts observable in both infrared and ultraviolet absorption spectra. The latter has led to the reporting of large Wannier-Mott excitons in wide band gap molecular materials, such as solid carbon monoxide and ammonia.Changes in the field strength with time, at specific temperatures, can be related to the structural dynamics of glassy molecular solids. Measurement of surface potentials is a very sensitive technique by which to observe the rotation and translation of molecular species buried in thin films. This is particularly true for polarized, supercooled molecular glasses, where surface potentials have been used to report on secondary relaxation processes that have hitherto been hidden from measurement.Characterizing spontaneously electric molecular films, and understanding their behavior, requires the inclusion of nonlocal and nonlinear effects. The mean-field model that we present describes the data by connecting the energy of interaction of an average dipole with the mean effective field in the film, where this field is itself a function of the degree of polarization. This feedback loop produces a smooth function with a nonintuitive, discontinuous differential. The condensation of thin molecular films is an important means by which molecular solids are generated in the interstellar medium and represents a key pathway for the generation of organic optically and electrically active materials. It may also be possible to manipulate chemistry with the intense, localized electric fields acting as or acting on catalysts. The repercussions of the spontaneous generation of bound surface charges and the presence of electric fields in molecular solids will be discussed in these contexts.

Visualizing the role of applied voltage in non-metal electrocatalysts.

Understanding how applied voltage drives the electrocatalytic reaction at the nanoscale is a fundamental scientific problem, particularly in non-metallic electrocatalysts, due to their low intrinsic carrier concentration. Herein, using monolayer molybdenum disulfide (MoS2) as a model system of non-metallic catalyst, the potential drops across the basal plane of MoS2 (ΔVsem) and the electric double layer (ΔVedl) are decoupled quantitatively as a function of applied voltage through in-situ surface potential microscopy. We visualize the evolution of the band structure under liquid conditions and clarify the process of EF keeping moving deep into Ec, revealing the formation process of the electrolyte gating effect. Additionally, electron transfer (ET) imaging reveals that the basal plane exhibits high ET activity, consistent with the results of surface potential measurements. The potential-dependent behavior of kf and ns in the ET reaction are further decoupled based on the measurements of ΔVsem and ΔVedl. Comparing the ET and hydrogen evolution reaction imaging results suggests that the low electrocatalytic activity of the basal plane is mainly due to the absence of active sites, rather than its electron transfer ability. This study fills an experimental gap in exploring driving forces for electrocatalysis at the nanoscale and addresses the long-standing issue of the inability to decouple charge transfer from catalytic processes.

Evolution Characteristics of DC Breakdown for Biaxially Oriented Polypropylene Films

Biaxially oriented polypropylene (BOPP) films are the preferred storage dielectric for dry-type film capacitors due to excellent dielectric properties. However, the evolution characteristics of DC Breakdown of BOPP film in air remain unclear. This study focuses on the influence of curvature, contact area and material of electrode on the DC breakdown in BOPP films. The on-line/off-line measurements of surface potential and characterizations of trap distribution are achieved. And the simulations of surface charge, electric field and energy injection are accomplished. The breakdown path and degradation traces in BOPP films are also analyzed. The experimental results show that the effective insulation distance is the key to determine the actual performance of the film, affected by the trap distribution and density. This paper has a reference for the optimization of insulation structure for dry-type film capacitors in the field of high-voltage direct current (HVDC).

Stimuli-Responsive Langmuir Films Composed of Nanoparticles Decorated with Poly(N-isopropyl acrylamide) (PNIPAM) at the Air/Water Interface.

The nanotechnology shift from static toward stimuli-responsive systems is gaining momentum. We study adaptive and responsive Langmuir films at the air/water interface to facilitate the creation of two-dimensional (2D) complex systems. We verify the possibility of controlling the assembly of relatively large entities, i.e., nanoparticles with diameter around 90 nm, by inducing conformational changes within an about 5 nm poly(N-isopropyl acrylamide) (PNIPAM) capping layer. The system performs reversible switching between uniform and nonuniform states. The densely packed and uniform state is observed at a higher temperature, i.e., opposite to most phase transitions, where more ordered phases appear at lower temperatures. The induced nanoparticles' conformational changes result in different properties of the interfacial monolayer, including various types of aggregation. The analysis of surface pressure at different temperatures and upon temperature changes, surface potential measurements, surface rheology experiments, Brewster angle microscopy (BAM), and scanning electron microscopy (SEM) observations are accompanied by calculations to discuss the principles of the nanoparticles' self-assembly. Those findings provide guidelines for designing other adaptive 2D systems, such as programable membranes or optical interfacial devices.

Probing interfacial water structure induced by charge reversal and hydrophobicity of silica surface in the presence of divalent heavy metal ions using sum frequency generation spectroscopy

HypothesisAdsorption of divalent heavy metal ions (DHMIs) at the mineral–water interfaces changes interfacial chemical species and charges, interfacial water structure, Stern (SL), and diffuse (DL) layers. These molecular changes can be detected by probing changing orientation and hydrogen-bond network of interfacial water molecules in response to changing local charges and hydrophobicity. ExperimentsSum-frequency generation (SFG) spectroscopy was used to probe changes in vibrational resonances of interfacial OH vs. DHMI concentration and pH. SFG spectra were deconvoluted using the measured surface potential and maximum entropy method in conjunction with the electrical double-layer theory for the SL and DL structures and correlated by hydrophobicity. FindingsThree surface charge reversals (CRs) were detected at low (CR1), medium (CR2), and high (CR3) pHs. Unlike CR1, SFG signals were minimized at CR2 and CR3 for DHMIs-silica systems highlighting considerable alterations in the structure of interfacial waters due to the inner-sphere sorption of metal hydroxo complexes. SFG results showed “hydrophobic-like” stretching modes at > 3600 cm−1 for Pb-, Cu-, and Zn-treated silica. However, contact angle measurements revealed the hydrophobization of silica only in the presence of Pb(II), as confirmed by an in-depth SFG analysis of the hydrogen-bond network of the interfacial water molecules in the SL.

High-accuracy characterization of pyroelectric materials: A noncontact method based on surface potential measurements

The characterization of pyroelectric materials is essential for the design of pyroelectric-based devices. Pyroelectric current measurement is the commonly employed method, but can be complex and requires surface electrodes. Here, we present noncontact electrostatic voltmeter measurements as a simple but highly accurate alternative, by assessing thermally-induced pyroelectric surface potential variations. We introduce a refined model that relates the surface potential variations to both the pyroelectric coefficient and the characteristic figure of merit (FOM) and test the model with square-shaped samples made from PVDF, LiNbO3 and LiTaO3. The characteristic pyroelectric coefficient for PVDF, LiNbO3 and LiTaO3 was found to be 33.4, 59.9 and 208.4 [Formula: see text]C m[Formula: see text]K[Formula: see text], respectively. These values are in perfect agreement with literature values, and they differ by less than 2.5% from values that we have obtained with standard pyroelectric current measurements for comparison.

Effects of Fabrication Conditions on the Partial Discharge Mitigation Performance of Electrets for Power Electronic Driven Systems

In our previous studies, we have shown that electrets can be designed to mitigate partial discharge (PD) in power electronic converter (PEC)-driven systems by neutralizing locally-enhanced harmful electric fields. Surface charge density is one of the main factors that determines the PD mitigation performance of electrets. In this study, we fabricate electrets from polyvinylidene fluoride (PVDF) films under various charging conditions including voltage magnitude, charging duration, voltage polarity, and temperature using triode-corona discharge method. The surface charge density and PD mitigation performance of the fabricated electret films are assessed by surface potential measurements and PD experiments. <styled-content style="color:black">The effects of fabrication temperature on electret surface charge stability, and the impacts of atmospheric conditions on electret surface charge density and PD magnitude are discussed.</styled-content> This study provides knowledge that is essential for establishing the fabrication conditions of electrets that can effectively mitigate PD in PEC-driven systems.

Van der waals epitaxial growth of mixed-dimensional 1D/2D heterostructures with tellurium nanowires and transition metal dichalcogenide nanosheets for nonlinear optical applications

One-dimensional (1D) tellurium (Te) nanowires (NWs) have triggered intense attention due to their remarkable properties in electronics, optoelectronics and nonlinear optics. However, the saturable absorption properties of Te NWs and the ability to be bleached by strong pulsed energy need to be further improved. Van der Waals (vdW) heterostructures based on two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit prominent nonlinear optical properties. Direct epitaxial growth of large-area 1D/2D Te-TMDC vdW heterostructures is still rarely investigated. Herein, we demonstrate the controllable epitaxial growth of large-scale 1D/2D vertical Te-TMDC heterostructures via two-step vapor phase deposition method. Our experimental results have confirmed that the saturable absorption properties of as-prepared Te-TMDC heterostructures can be obviously enhanced. The first principle calculations and surface potential measurements demonstrate that effective interlayer charge transfer between Te NWs and TMDC nanosheets is beneficial for promoting the saturable absorption properties. Systematic investigation of vdW Te-TMDC heterostructures with nonlinear optical properties is vital for the next generation of nanophotonics and integrated photonics, which paves the way for the development of saturable absorbers and light modulators based on Te-TMDC vdW heterostructures.