Surface Potential Gradient Research Articles

Enhancing the dipole ring of hexagonal boron nitride nanomesh by surface alloying

Surface templating by electrostatic surface potentials is the least invasive way to design large-scale artificial nanostructures. However, generating sufficiently large potential gradients remains challenging. Here, we lay the groundwork for significantly enhancing local electrostatic fields by chemical modification of the surface. We consider the hexagonal boron nitride (h-BN) nanomesh on Rh(111), which already exhibits small surface potential gradients between its pore and wire regions. Using photoemission spectroscopy, we show that adding Au atoms to the Rh(111) surface layer leads to a local migration of Au atoms below the wire regions of the nanomesh. This significantly increases the local work function difference between the pore and wire regions that can be quantified experimentally by the changes in the h-BN valence band structure. Using density functional theory, we identify an electron transfer from Rh to Au as the microscopic origin for the local enhancement of potential gradients within the h-BN nanomesh.

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.

Methylammonium Tetrel Halide Perovskite Ion Pairs and Their Dimers: The Interplay between the Hydrogen-, Pnictogen- and Tetrel-Bonding Interactions.

The structural stability of the extensively studied organic-inorganic hybrid methylammonium tetrel halide perovskite semiconductors, MATtX3 (MA = CH3NH3+; Tt = Ge, Sn, Pb; X = Cl, Br, I), arises as a result of non-covalent interactions between an organic cation (CH3NH3+) and an inorganic anion (TtX3-). However, the basic understanding of the underlying chemical bonding interactions in these systems that link the ionic moieties together in complex configurations is still limited. In this study, ion pair models constituting the organic and inorganic ions were regarded as the repeating units of periodic crystal systems and density functional theory simulations were performed to elucidate the nature of the non-covalent interactions between them. It is demonstrated that not only the charge-assisted N-H···X and C-H···X hydrogen bonds but also the C-N···X pnictogen bonds interact to stabilize the ion pairs and to define their geometries in the gas phase. Similar interactions are also responsible for the formation of crystalline MATtX3 in the low-temperature phase, some of which have been delineated in previous studies. In contrast, the Tt···X tetrel bonding interactions, which are hidden as coordinate bonds in the crystals, play a vital role in holding the inorganic anionic moieties (TtX3-) together. We have demonstrated that each Tt in each [CH3NH3+•TtX3-] ion pair has the capacity to donate three tetrel (σ-hole) bonds to the halides of three nearest neighbor TtX3- units, thus causing the emergence of an infinite array of 3D TtX64- octahedra in the crystalline phase. The TtX44- octahedra are corner-shared to form cage-like inorganic frameworks that host the organic cation, leading to the formation of functional tetrel halide perovskite materials that have outstanding optoelectronic properties in the solid state. We harnessed the results using the quantum theory of atoms in molecules, natural bond orbital, molecular electrostatic surface potential and independent gradient models to validate these conclusions.

The effect of the Madden–Julian Oscillation on the global electric circuit

The Madden–Julian Oscillation (MJO) is the most dominant component of the climate variability in the tropics on the timescale of tens of days. Occurring irregularly, the MJO consists of an eastward propagation of certain large-scale coupled structures in atmospheric circulation and deep convection. Here we investigate the effect of the MJO on the direct current global electric circuit (GEC), using both numerical simulations and the results of electric field measurements. We have reproduced the atmospheric dynamics for 1980–2020 with the help of the Weather Research and Forecasting model and meteorological reanalysis data; this allowed us to simulate 41 years of the GEC variation by parameterising contributions to the ionospheric potential (IP) in terms of precipitation and convection. Besides, we have analysed the results of surface potential gradient (PG) measurements at the Vostok station in Antarctica during 2006–2020. When averaged over many days, both the simulated IP and measured fair-weather PG show sinewave-like variations with the MJO phase, which manifest a statistically significant correlation with the MJO cycle. A more elaborate investigation involving an application of empirical orthogonal functions shows that the dynamics of contributions to the IP follows the patterns of variability of deep convection typical for the MJO. The variation of the IP on the MJO scale can largely be represented as a superposition of two basic oscillating patterns of convection, one of which is located over the Maritime Continent and its surrounding seas and the other is located over the Indian Ocean and part of South-East Asia. These oscillating patterns together describe the eastward progression of perturbations and can be naturally associated with the two components of the Real-time Multivariate MJO index, widely used to describe the MJO. Together with earlier results about links between the GEC and the El Niño—Southern Oscillation, the results concerning the MJO obtained in this study indicate that the GEC is an important part of the Earth system, which reflects climate variability on different timescales.

Surface Potential Gradients and NEXRAD Radar Reflectivities Before the Onset of Lightning at the KSC‐ER

AbstractMeasurements provided by Next Generation Weather Radar and operational thunderstorm monitoring instruments at the Kennedy Space Center and the Air Force Eastern Range have been examined to determine the initial electrical development of 13 isolated, air mass thunderstorms. The same instruments were used to examine the surface potential gradient prior to and following 13 long, horizontal discharges that propagated into the area. The motivation and primary objective for this work was to evaluate the safety of the existing lightning‐related launch constraints associated with surface potential gradient and precipitation radar measurements. The onset of cloud electrification as seen by a large‐area surface electric field mill network was detected 3.7–14.6 min before the first lightning discharge, with lead‐times that depended on proximity to the storm. In 11 of 13 cases, the first detectable field change was a positive excursion in potential gradient close to the storm, likely indicating initial development of lower positive charge. Lead‐times for the radar‐derived cloud tops (0 dBZ) reaching −20°C were longer than those for early electrification in all but two cases. Surface potential gradients above +500 V/m or below −100 V/m “warning thresholds” were exceeded before the first lightning flash in all cases. Radar reflectivities >35 dBZ above the −10°C level provided 3–14 min of lead time for lightning. Potential gradients just prior to and near long horizontal discharges exceeded 3 kV/m at most sites and were typically positive. Measurements at a single field mill site would not have provided adequate warning.

Audible noise evaluation for six‐phase overhead lines transformed from existing three‐phase double circuit infrastructures with uprated voltages

The drive to reduce carbon emissions and meet climate change targets set by governments worldwide has led to the growth of renewable energy sources and electric vehicles in recent years. Resulting intermittent generation and centralised demand require increased transmission capacities when operating associated power systems. Existing transmission lines are not capable of meeting planned needs. In many countries, obtaining a new right of way to build an overhead line (OHL) is increasingly difficult. There is therefore an urgent need to study techniques for transmission line uprating. Simply increasing voltages is not feasible because elevated surface potential gradient at conductor surfaces increases audible noise (AN) levels unacceptably. The feasibility of high phase order, especially six-phase, transmission techniques has been previously demonstrated, and it has been proved to be an effective way to uprate existing three-phase double circuit OHLs with increased voltages. This study analyses two typical overhead line structures, whose conductor surface potential gradient and AN level are calculated. The average and maximum surface potential gradient of the six-phase OHL are 18.5% and 18.6% lower than that of the three-phase ones. The AN of the six-phase single circuit OHL is predicted to be 6.33 and 6.45 dBA lower than that of the three-phase double circuit OHL after the voltage is increased by 45% and 50% for the lines considered. It is shown that six-phase OHLs can increase capacity by allowing elevating phase voltages without increasing acoustic noise levels.

The Response of a Baroclinic Anticyclonic Eddy to Relative Wind Stress Forcing

Abstract Including the ocean surface current in the calculation of wind stress is known to damp mesoscale eddies through a negative wind power input and have potential ramifications for eddy longevity. Here, we study the spindown of a baroclinic anticyclonic eddy subject to absolute (no ocean surface current) and relative (including ocean surface current) wind stress forcing by employing an idealized high-resolution numerical model. Results from this study demonstrate that relative wind stress dissipates surface mean kinetic energy (MKE) and also generates additional vertical motions throughout the whole water column via Ekman pumping. Wind stress curl–induced Ekman pumping generates additional baroclinic conversion (mean potential to mean kinetic energy) that is found to offset the damping of surface MKE by increasing deep MKE. A scaling analysis of relative wind stress–induced baroclinic conversion and relative wind stress damping confirms these numerical findings, showing that additional energy conversion counteracts relative wind stress damping. What is more, wind stress curl–induced Ekman pumping is found to modify surface potential vorticity gradients that lead to an earlier destabilization of the eddy. Therefore, the onset of eddy instabilities and eventual eddy decay takes place on a shorter time scale in the simulation with relative wind stress.

Modulation of the Dirac Point Band Gap in the Antiferromagnetic Topological Insulator MnBi2Te4 due to the Surface Potential Gradient Change

Modulation of the Dirac Point Band Gap in the Antiferromagnetic Topological Insulator MnBi2Te4 due to the Surface Potential Gradient Change

Visualization of electrochemical behavior in carbon steel assisted by machine learning

Identification of microstructures in steel has been extensively studied to improve the understanding of corrosion behavior. However, identification by expert eyes could be subjective, and most previous works on identification are solely based on morphological features. Furthermore, it is more difficult to identify local microstructures on a small scale-length. In this study, we developed a method for differentiating local microstructures on low carbon steel based on multiple physical properties at the nanoscale combined with machine learning techniques. Machine learning techniques were applied to the atomic force microscopy images of multiple physical properties, that is, not only of morphological features but also of the surface potential and capacitance gradient. Thereafter, we analyzed the corrosion behavior according to the concentration of the NaCl solution in which the samples were immersed, on the basis of the identified local microstructures as well as obtained physical properties. This study, which is based on these multiple physical properties, potentially provides a powerful tool for identifying and visualizing features of data. It could be further extended to electrochemical systems with more complex microstructures.

Measuring Global Signals in the Potential Gradient at High Latitude Sites

Previous research has shown that the study of the global electrical circuit can be relevant to climate change studies, and this can be done through measurements of the potential gradient near the surface in fair weather conditions. However, potential gradient measurements can be highly variable due to different local effects (e.g., pollution, convective processes). In order to try to minimize these effects, potential gradient measurements can be performed at remote locations where anthropogenic influences are small. In this work we present potential gradient measurements from five stations at high latitudes in the Southern and Northern Hemisphere. This is the first description of new datasets from Halley, Antarctica; and Sodankyla, Finland. The effect of the polar cap ionospheric potential can be significant at some polar stations and detailed analysis performed here demonstrates a negligible effect on the surface potential gradient at Halley and Sodankyla. New criteria for determination of fair weather conditions at snow covered sites is also reported, demonstrating that wind speeds as low as 3 m/s can loft snow particles, and that the fetch of the measurement site is an important factor in determining this threshold wind speed. Daily and seasonal analysis of the potential gradient in fair weather conditions shows great agreement with the “universal” Carnegie curve of the global electric circuit, particularly at Halley. This demonstrates that high latitude sites, at which the magnetic and solar influences can be present, can also provide globally representative measurement sites for study of the global electric circuit.

Computational analysis of short‐range surface‐directed polymerization‐induced phase separation

AbstractThe conventional off‐critical polymerization‐induced phase separation (PIPS) of binary mixtures leads to the fabrication of droplet‐type structure. However, the presence of a surface preferably attracting one of the components while phase separation by spinodal decomposition is occurring can change the configuration of surface energy and result in the formation of wetting layer(s) adjacent to the wall. Farther from the wall, droplets can still form. The method is called surface‐directed spinodal decomposition. The method is applicable in the fabrication of materials with layered morphology and may lead to the enhancement of the surface physical characteristics of mixtures. The current paper theoretically studied short‐range surface‐directed PIPS of a solvent/monomer mixture. The results showed the average diameter of particles in the bulk in the intermediate stage of phase separation grew with time by power‐law function <dave > ∝ t*α, and the exponent (αave ≈ 0.3) was found close to the Lifshitz‐Slyozov growth exponent (1/3) for the diffusivities studied in this paper. The time evolution of the thickness of the wetting layer was found to be logarithmic. The effects of the parameters such as diffusion coefficient, surface potential, and temperature gradient on the morphology development by the short‐range surface‐directed PIPS technique are discussed in this paper.

Microscale surface potential gradient disturbances observed in bilayer graphene

Van der Waals mono- and heterostructures have received great attention in recent years due to their new physics and electronic properties. Several novel device structures have been developed based on their properties, including tunnelling transistors, light emitting diodes and barristors. However, the construction of such structures usually involves transfer techniques which could induce damage and modify the electronic properties of the structures and degrade device performance. Here we show that a multi-step transfer process for graphene may disturb its transport properties at microscale. Bilayer graphene was transferred, using PMMA as supporting material, onto an Al2O3/Si substrate with pre-patterned chromium-palladium electrodes. Using scanning tunnelling potentiometry, we observed that the local potential gradient (and resulting current flow) significantly changed compared with the potential gradient arising from the electrodes’ position and biasing. This could be attributed to local material ad-layers, unspecified contact or additives present between sub-layers. This finding represents an important step in understanding the effect of transfer techniques on the quality of multilayer van der Waals mono- and heterostructures.

Evaluation Model for the Scope of DC Interference Generated by Stray Currents in Light Rail Systems

Electrochemical corrosion caused by stray currents reduces the lifespan of buried gas pipelines and the safety of light rail systems. Determining the scope of stray current corrosion will help prevent the corrosion of existing buried pipelines and provide an effective reference for new pipeline siting. In response to this problem, in this paper the surface potential gradient was used to evaluate the scope of stray current corrosion. First, an analytical model for the scope of the stray current corrosion combined with distributed parameters and the electric field generated by a point current source was put forward. Second, exemplary calculations were conducted based on the proposed model. Sensitivity of the potential gradient was analyzed with an example of the transition resistance, and the dynamic distribution of surface potential gradient under different locomotive operation modes was also analyzed in time-domain. Finally, the scope was evaluated at four different intervals with the parameters from the field test to judge whether the protective measures need to be taken in areas with buried pipelines and light rail systems nearby or not.

AFM-Based Characterization of Electrical Properties of Materials.

Capabilities of atomic force microscopy (AFM) for characterization of local electrical properties of materials are presented in this chapter. At the beginning the probe-sample force interactions, which are employed for detection of surface topography and materials properties, are described theoretically in their application in different AFM modes and electrical techniques. The electrical techniques, which are based on detection of electrostatic probe-sample forces, are outlined in AFM contact and oscillatory resonant modes. The basic features of the detection of surface potential and capacitance gradients are explained. The applications of these techniques are illustrated on metals, surfactant compounds, semiconductors, and different polymers. Practical recommendations on use of the AFM-based electrical methods and the related challenges are given in the last section.

Surface aging effects of long-term energized conductors on spectrum characteristics of audible noise

The impact of surface aging level of long-term energized conductors on spectrum characteristics of audible noise is studied in this paper. It is found that the average roughness and root-mean-square roughness of conductors increase with the operating time, namely the aging degree, through the quantitative measurement. A-weighted equivalent continuous sound pressure level (LAeq) is found to increase with the aging level. As the aging aggravation, the LAeq and its high-frequency components (>1 kHz) increase substantially and high-frequency components are mainly contributing to the increasing of LAeq. The high-frequency components (>1 kHz) of the new and aged stranded conductors have a marked increasing trend as the surface potential gradient increases, while the low-frequency components are mainly environment noise.

Investigation on the interfacial chemical state and band alignment for the sputtering-deposited CaF2/p-GaN heterojunction by angle-resolved X-ray photoelectron spectroscopy

The interfacial chemical state and the band alignment of the sputtering-deposited CaF2/p-GaN hetero-structure were investigated by angle-resolved X-ray photoelectron spectroscopy. The dependence of Ga 3p core-level positions on the collection angles proves that the downward band bending of p-GaN is reduced from 1.51 to 0.85 eV after the deposition of CaF2, which may be due to the reduction of Mg-Ga-O-related interface states by the oxygen-free deposition of CaF2. The band gap of sputtering-deposited CaF2 is estimated to be about 7.97 eV with a potential gradient of 0.48 eV obtained by the variation of the Ca 2p3/2 position on different collection angles. By taking into account the p-GaN surface band bending and potential gradient in the CaF2 layer, large valence and conduction band offsets of 2.66 ± 0.20 and 1.92 ± 0.20 eV between CaF2 and p-GaN are obtained. These results indicate that CaF2 is a promising gate dielectric layer on the p-GaN for the application of metal-insulator-semiconductor devices.

Rashba-type spin splitting and the electronic structure of ultrathin Pb/MoTe2 heterostructure

The spin-polarized band structures of the Pb(111)/MoTe2 heterostructure are studied by the first-principles calculations. Due to strong spin–orbit coupling and space inversion asymmetry, large Rashba spin splitting of electronic bands appears in this hybrid system. The spin splitting is completely out-of-plane and opposite at and points. Rashba spin splitting also appears along the in-plane momentum direction around the point due to the existence of surface potential gradient induced by charge transfer at interface. Furthermore, our calculations show that the spin-polarized bands closely approach the Fermi level in Pb/MoTe2 heterostructure, showing that this heterostructure may be a good candidate in valleytronics or spintronics.

Conductor Surface Conditions Effects on Audible Noise Spectrum Characteristics of Positive Corona Discharge

Conductor surface conditions of high voltage overhead transmission lines have a large influence on corona characteristics. Both audible noise and corona current is highly depended on the corona discharge characteristics. However, relatively few investigations have previously been made on conductor surface conditions effects on audible noise spectrum characteristics. In the present work, the artificial pollution experiments were carried out to simulate the various surface conditions. At the same time, the audible noise spectra and the corona current of the polluted conductor are recorded. From the analysis of the experiment results, it was found that the audible noise spectra level (>1 kHz) and corona current increased obviously as the relative permittivity grew and the amount of pollution increased. Furthermore, from the results of the long-term statistical measurement about A-weighted equivalent continuous sound level (LAeq) of the polluted conductor, the close correlation between the surface potential gradient and the audible noise spectra were obtained.

Diamond nanoparticles as a way to improve electron transfer in sol–gel l-lactate biosensing platforms

In the present work, we have included for the first time diamond nanoparticles (DNPs) in a sol–gel matrix derived from (3-mercaptopropyl)-trimethoxysilane (MPTS) in order to improve electron transfer in a lactate oxidase (LOx) based electrochemical biosensing platform. Firstly, an exhaustive AFM study, including topographical, surface potential (KFM) and capacitance gradient (CG) measurements, of each step involved in the biosensing platform development was performed. The platform is based on gold electrodes (Au) modified with the sol–gel matrix (Au/MPTS) in which diamond nanoparticles (Au/MPTS/DNPs) and lactate oxidase (Au/MPTS/DNPs/LOx) have been included. For the sake of comparison, we have also characterized a gold electrode directly modified with DNPs (Au/DNPs). Secondly, the electrochemical behavior of a redox mediator (hydroxymethyl-ferrocene, HMF) was evaluated at the platforms mentioned above. The response of Au/MPTS/DNPs/LOx towards lactate was obtained. A linear concentration range from 0.053 mM to 1.6 mM, a sensitivity of 2.6 μA mM−1 and a detection limit of 16 μM were obtained. These analytical properties are comparable to other biosensors, presenting also as advantages that DNPs are inexpensive, environment-friendly and easy-handled nanomaterials. Finally, the developed biosensor was applied for lactate determination in wine samples.

Study of the Polarization Behavior of Ce0.9Gd0.1O2-δ Single Crystals below 350°C to Room Temperature

Single crystalline ceria samples with the composition Ce0.9Gd0.1O2-δ were pre-polarized with ±5 V for up to 300 s using a Pt coated AFM tip as working electrode. The direct contact zone had a diameter of <50 nm. Subsequently, the effect of the polarization on the surface potential of the samples was investigated by mapping the introduced defect gradient and its decay with time using Kelvin probe force microscopy. The generated surface potential gradients were found to have a diameter of up to 1 μm, which is explained by the local ionization of defect associates by the applied high electric field. Measurements were performed at room temperature and 50°C. The polarization behavior of the Ce0.9Gd0.1O2-δ single crystals was compared to cyclovoltammetry and polarization-relaxation experiments at T ≤ 350°C and in dry air or nitrogen which were performed using a specially suited AFM (Controlled Atmosphere High Temperature Scanning Probe Microscope CAHT-SPM by Semilab).