Spatial Charge Distribution Research Articles

The Electronic State and Spatial Distribution of Excess Charge Created by Oxygen Vacancies on Titanium Dioxide Surfaces

We have investigated the atomic-site resolved distribution of the excess electronic charge density created by the oxygen vacancies on the rutile TiO2(110) surface. Contrary to the conventional model where the excess charge is localized at the defect, scanning tunneling microscopy and density functional theory show that the charge is delocalized over more than 10 proximate unit cells. According to the theory, the lattice distortion and accompanying rehybridization between the Ti 3d and O 2p orbitals displace the charge from the defect site.

Corrosion resistance of amorphous aluminium–molybdenum alloys in an acidic chloride environment

Pure aluminium corrodes readily in 1 M HCl solution while its supersaturated alloys with Mo, explored in this work, exhibit excellent corrosion properties due to the oxide barrier film formation on the alloy surface. The influence of Mo content in the alloy on the potential and charge distribution across the system alloy |oxide film| solution was studied in situ using dc and ac electrochemical methods. Passivity, a sudden change in electrode impedance, and the onset of secondary passivity and pitting were discussed in terms of the solute segregation, according to the solute-rich mechanism, change in the alumina structure and solid state processes connected with the transport of point defects and spatial distribution of surface charge.

Imaging of charge spatial density on insulating materials

We describe the measurement of spatial charge distribution, using a new non-invasive technique. This measurement, based on a 16-element array of ultra-high impedance electric field sensors, is capable of producing both quantitative results for the total amount of surface charge present, as well as imaging the charge to produce plots representing spatial charge distribution. We calibrate the measurement against a conventional induction field meter charge measurement which discharges the sample. Further to this, we show that our technique has no discharging effect on the sample and that therefore it is possible to observe the discharging of insulating materials over periods of several days.

Characterization of complexation of poly (N-isopropylacrylamide-co-2-(dimethylamino) ethyl methacrylate) thermoresponsive cationic nanogels with salmon sperm DNA

Thermoresponsive cationic nanogel (TCNG) networks based on N-isopropylacrylamide (NIPAM), 2-(dimethylamino)ethyl methacrylate (DMAEMA), and quaternary alkyl ammonium halide salts of DMAEMA (DMAEMAQ) were synthesized by dispersion polymerization technique. The thermoresponsive properties of TCNGs and TCNG-salmon sperm DNA (sasDNA) polyplexes were characterized in aqueous media of various pH and ionic strength. P[NIPAM] and P[NIPAM/DMAEMA] TCNGs exhibited sharp volume phase transition (VPT) in water at critical temperatures (T(c)) of 32 degrees C and 36 degrees C, respectively. Quaternized P[NIPAM/DMAEMAQ] TCNGs did not undergo sharp VPT up to 50 degrees C. The VPT of uncomplexed TCNGs were sensitive to the ionic composition and ionic strength of salts in solution, but were insensitive to pH in the range 5.0 to 7.4. The VPT of P[NIPAM/DMAEMAQ]/sasDNA diminished in magnitude with increasing W(p)/W(d) suggesting greater compaction of the polyplexes. The distinct phase-transition properties of P[NIPAM/DMAEMA]/sasDNA and P[NIPAM/DMAEMAQ]/sasDNA polyplexes were attributed to the condensing capability of polycations and to differences in the spatial distribution of structural charges in quaternized and nonquaternized networks. The findings demonstrate that stable TCNGs can be prepared with controllable responsive properties determined by the nature of the cationic charge incorporated and may have potential as vehicles for DNA delivery.

Representation of a Lightning Return-Stroke Channel as a Nonlinearly Loaded Thin-Wire Antenna

Distributed nonlinear resistive elements are incorporated into an antenna theory (AT) model with fixed inductive loading (ATIL-F) to realistically model the current wave propagation along the lightning return-stroke channel (RSC). Resistive elements are considered as a nonlinear distributed load whose resistance is a function of both current and time. This is adopted from the numerical models of a spark channel and consequent shockwave from a lightning discharge, yielding a changing value of the channel radius from the base to the cloud along the RSC. It is demonstrated that the proposed model is more consistent with the nonlinear behavior of a lightning channel inferred from experimental physical observations. The model also satisfies most of the well-known characteristics of RSC, including the temporal and spatial current and charge distributions, some important features observed in radiated electromagnetic fields, and wave propagation speed profiles along the channel when compared with those predicted by the ATIL-F model.

Measurement of charge distribution in actin bundles by surface potential microscopy

Bundles of filamentous actin (F-actin) deposited on a gold-plated surface were concurrently imaged using atomic force microscopy (AFM) and surface potential microscopy (SPM). The surface potential was mapped as a function of tip distance to surface using a constant bias potential. There was an uneven spatial distribution of charges detected by SPM, consistent with the segmented topological features shown by AFM of the actin bundles. SPM analysis showed localized changes in surface potential between the axial and transversal sections of the bundles, which are consistent with nonuniform charge distributions of adsorbed salt ions on F-actin.

The Complete Semiconductor Transistor and Its Incomplete Forms

This paper describes the definition of the complete transistor. For semiconductor devices, the complete transistor is always bipolar, namely, its electrical characteristics contain both electron and hole currents controlled by their spatial charge distributions. Partially complete or incomplete transistors, via coined names or/and designed physical geometries, included the 1949 Shockley p/n junction transistor (later called Bipolar Junction Transistor, BJT), the 1952 Shockley unipolar ‘field-effect’ transistor (FET, later called the p/n Junction Gate FET or JGFET), as well as the field-effect transistors introduced by later investigators. Similarities between the surface-channel MOS-gate FET (MOSFET) and the volume-channel BJT are illustrated. The bipolar currents, identified by us in a recent nanometer FET with 2-MOS-gates on thin and nearly pure silicon base, led us to the recognition of the physical makeup and electrical current and charge compositions of a complete transistor and its extension to other three or more terminal signal processing devices, and also the importance of the terminal contacts.

Impact of the field induced polarization space‐charge on the characteristics of AlGaN/GaN HEMT: Self‐consistent simulation study

AbstractUsing self‐consistent numerical simulation we have studied the impact of the field induced polarization on the characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). The strain induced by external electric fields is on top of the built‐in strain due to lattice mismatch. It has been suggested that this additional strain can play a role in device degradation and failure (J. Joh and J. del Alamo, Mechanisms for Electrical Degradation of GaN High‐Electron Mobility Transistors, IEDM Tech. Dig., pp. 415–418, Dec. (2006) [1]). The study is carried out using commercial TCAD tools carefully calibrated against the measured characteristics of 0.25 μm physical gate length transistors. A coupled model for piezoelectric materials, including the impact of the field, is used to determine the strain and thus the polarization in the device. The spatial charge distribution is derived from the gradient of the polarization and is fed back self‐consistently to the simulator. Results with and without the field‐induced polarization are compared and discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron beam probing of silica surface layers on alumina

The electron beam induced self-consistent charge injection and transport in a layered insulator SiO 2 –Al 2 O 3 is described by means of an electron–hole flight–drift model FDM and an iterative computer simulation. Thermal and field-enhanced detrapping are included by the Poole–Frenkel effect. The surface layer with a modified electric surface conductivity is included which describes the surface leakage currents. Furthermore, it will lead to particular charge incorporation at the interface between the surface layer and the bulk substrate. As a main result the time-dependent spatial distributions of currents j ( x , t ), charges ρ ( x , t ), field F ( x , t ), and potential V ( x , t ) are obtained. The spatial charge distribution with depth shows a quadro-polar plus–minus–plus–minus structure in nanometer dimension.

Attraction between like-charged surfaces induced by orientational ordering of divalent rigid rod-like counterions: theory and simulations

We consider the interaction between two equally charged surfaces in an electrolyte solution composed of long divalent rigid rod-like counterions of arbitrary length. Further, we study the influence of orientational ordering of rigid rod-like counterions on the interaction between charged surfaces. Density functional theory is introduced, where the spatial distribution of charge within the divalent rod-like counterions is represented by two effective charges at a fixed distance. The result of variational procedure gives an integral differential equation for the electrostatic potential which was solved numerically. From the electrostatic potential and the concentration of counterions, the free energy of two charged surfaces interacting in a solution of rod-like counterions is calculated. For large surface charge densities and for long enough divalent rod-like counterions the minimum of the free energy is obtained at a distance between the surfaces which equals the counterion length. This indicates that a bridging mechanism might be responsible for the attraction between like-charged surfaces. The analysis of the orientational distribution function confirms that, at the minimum of the free energy, the rod-like counterions are oriented perpendicularly and thus connect the like-charged surfaces. Finally, canonical Monte-Carlo simulations confirm the theoretical calculations of the osmotic pressure between like-charged surfaces for long enough rod-like counterions.

Long-lived charge traps in functionalized pentacene and anthradithiophene studied by time-resolved electric force microscopy

Charge trapping in functionalized pentacene and anthradithiophene transistors was studied by time-resolved electric force microscopy (EFM). The spatial distribution of long-lived trapped charge was examined, in TIPS pentacene, for transistors prepared by four different solution deposition techniques. The distribution is markedly different in the four samples, establishing that charge trapping in this material is at least as sensitive to morphology as mobility is. The rate of trap formation in TIPS pentacene depends strongly on the initial free hole concentration, consistent with the view that charge traps in this material should not be viewed as static defect states but as states that are slowly created by reactions of free holes at a localized defect. In one TIPS pentacene sample, the rate of formation and steady-state concentration of trapped charge was found to be independent of the initial free hole concentration, allowing us to estimate the concentration of the impurity giving rise to the trapped charge. Transistors of functionalized anthradithiophene, prepared by spin casting and solvent annealing, showed evidence of grain boundary trapping. In both materials, regions can be found that exhibit essentially no long-lived traps, indicating that the cations of the two materials are not inherently prone to trapping and degradation in an unilluminated film. We find that electrons can be injected from untreated gold electrodes into both materials, and in both materials we observe a finite electron mobility.

Formation of calcite thin films by cooperation of polyacrylic acid and self-generating electric field due to aligned dipoles of polarized substrates

We demonstrated the formation of calcite thin films on positively and negatively charged surfaces of a hydroxyapatite (HAp) electret coexisting with polyacrylic acid (PAA) and self-generating surface electric fields due to HAp electrets with electrically aligned dipoles. The cooperation of PAA and the self-generating surface electric field due to the electrets favored the formation of calcite thin films and acted remarkably on the negatively charged surface. Calcite thin films, 4–10 μm thick, with a shell-like microstructure were produced on the negatively charged surfaces with a small amount of PAA. In contrast, under other reaction conditions, calcite thin films with a fan-like structure in the cross section formed on the polarized substrates, and their thickness ranged from 2 to 7 μm. The films were composed of hemispheric- or flat island-shaped aggregates that were made of the calcite crystals that elongated along the c-axis. The morphology of the PAA–Ca 2+ complex assembly, which adsorbed onto the polarized HAp substrates, was controlled by the balance of the spatial charge distribution in its structure and the properties of the self-generating surface electric field, which led to the different morphologies of the calcite thin films. We proposed that the formation mechanism of the films formed coexisting with PAA and the self-generating electric fields.

Surface charge behaviour of corona-charged thin polymer films -simultaneous LIPP and TSDC measurement

Simultaneous T hermally Stimulated Disc harge Cu rrents (TSDCs-) a nd LIPP -(Laser-Induced Press ure Puls e) signal m easuring system was co nstructed t o id entity th e charge behaviour in/on the dielectric film. For 50 or 100 μm thick corona-charged PTFE sheet, the correlation b etween the spatial charge distribution change with the temperature and TSDC peaks was examined. Corona-charging temperature dependence on that is also examined.

Injection of charge nanostructures into insulators

The electron beam induced selfconsistent charge transport in insulators is described by means of an electron-hole flight–drift model FDM and an iterative computer simulation. Ballistic secondary electrons and holes, their attenuation and drift, as well as their recombination, trapping, and detrapping are included. Thermal and field-enhanced detrapping are described by the Poole–Frenkel effect. As a main result the time dependent secondary electron emission rate σ ( t ) and the spatial distributions of currents j ( x , t ) , charges σ ( x , t ) , field F ( x , t ) , and potential V ( x , t ) are obtained. The spatial charge distributions with depth show a quadropolar plus–minus–plus–minus structure in nanometer dimensions.

Nonspecific interactions in spontaneous assembly of empty versus functional single-stranded RNA viruses

We investigate the influence of salt concentration, charge on viral proteins and the length of single-stranded RNA (ssRNA) molecule on the spontaneous assembly of viruses. Only the nonspecific interactions are assumed to guide the assembly, i.e., we exclude any chemical specificity that may lock the viral proteins and ssRNA in preferred configurations. We demonstrate that the electrostatic interactions screened by the salt in the solution impose strong limits on viral composition that can be achieved by spontaneous assembly. In particular, we show that viruses whose ssRNA carries more than twice the amount of charge that is located on the viral proteins, cannot be assembled spontaneously. We find that the spatial distribution of protein charge is important for the energetics of the assembly. We also show that the pressures that act on the viruses as a result of attractive protein-ssRNA electrostatic interactions are at least an order of magnitude smaller than is the case with bacteriophage viruses that contain double-stranded DNA molecule.

Surface-Charge Lithography for Direct PDMS Micro-Patterning

Direct patterning of PDMS films is achieved by modulating the wettability of polar dielectric substrates. Periodic array structures of microbumps can be madeup by functionalizing periodically poled lithium niobate crystals. The modulation of surface wettability is obtained through the spatial distribution of the surface electric charges generated by the pyroelectric effect under electrode-less configuration. An appropriate thermal treatment of the substrates assures both the wettability patterning and the fast cross-linking of the PDMS film.

Impact of surface electric properties of carbon-based thin films on platelets activation for nano-medical and nano-sensing applications

Electric surface properties of biomaterials, playing key role to various biointerfacial interactions, were related to hemocompatibility and biosensing phenomena. In this study, the examination of surface electric properties of amorphous hydrogenated carbon thin films (a-C:H) was carried out by means of electrostatic force microscope (EFM) and observation of differences in spatial charge distribution on the surface of the examined films during platelets adhesion was made. The thrombogenic potential of a-C:H thin films developed by magnetron sputtering with approximately 42% sp(3) content and hydrogen partial pressure during deposition was evaluated, by in situ observation with atomic force microscope (AFM) of platelets' activation and their subsequent adhesion. Platelet-rich plasma drawn from healthy donors was used and semi-contact mode of AFM was applied. Platelets behavior and their correlation with the electric surface properties of the examined a-C:H films by EFM was made for hemocompatibility enhancement and sensing platelets that are less electrical negatively charged and with higher tendency to aggregate and form thrombus. The results are discussed in view of the effect of different deposition conditions of hydrogenated carbon films on their structural and morphological characteristics, surface roughness and electrical properties attributing to different hemocompatibility and sensing aspects.

Endurance Reliability of Multilevel-Cell Flash Memory Using a <formula formulatype="inline"><tex>$ \hbox{ZrO}_{2}/\hbox{Si}_{3}\hbox{N}_{4}$</tex></formula> Dual Charge Storage Layer

The mechanisms of programming/erasing (P/E) and endurance degradation have been investigated for multilevel-cell (MLC) Flash memories using a (NROM) or a dual charge storage layer (DCSL). Threshold-voltage -level disturbance is found to be the major endurance degradation factor of NROM-type MLCs, whereas separated charge storage and step-up potential wells give rise to a superior -level controllability for DCSL MLCs. The programmed levels of DCSL MLCs are controlled by the spatial charge distribution, as well as the charge storage capacity of each storage layer, rather than the charge injection. As a result, DCSL MLCs show negligible -level offsets (< 0.2 V) that are maintained throughout the P/E cycles, demonstrating significantly improved endurance reliability compared to NROM-type MLCs.

Effects of fatigue and damage on the hysteresis loops of ferroelectric ceramics

Electric fatigue and radiation damage may induce pinched polarization hysteresis loops and asymmetric strain hysteresis loops of ferroelectric ceramics, respectively. Based on a gradual domain switching model, this paper formulates a novel constitutive model to investigate the hysteresis deformation behavior of ferroelectric ceramics by radiation damage and electric fatigue. First, two important physical mechanisms, namely, alignment of the defect dipoles and domain-wall pinning due to diffusion of charged defects, are considered in irradiated ferroelectric ceramics. It is found that the alignment of the defect dipoles acts as an internal field and increases the coercive field, while the domain-wall pinning clamps the domain and causes the vanishment of remanent polarization. Second, the effects of point defects and domain pinning by space charges are taken into account in ferroelectric fatigue model. The results of simulations reveal that the spatial distribution and electric property of space charges play a dominant role in the asymmetry of strain hysteresis loops. The proposed model can elucidate experimental phenomena reported in the literature.

Tubulin electrostatics and isotype specific drug binding

We present the results of molecular dynamics computations based on the atomic structures of tubulin. The values of the net charge, the spatial charge distributions, and the dipole moment components are reported for tubulin alpha–beta dimers. The physical consequences of these results and subsequent computations are discussed for microtubules in terms of the effects on test charges, test dipoles, and neighbouring microtubules. Our calculations indicate typical distances over which electrostatic effects can be felt by biomolecules, ions, and other microtubules. We also demonstrate the importance of electrostatics in the interaction between microtubules and drugs such as taxanes and colchicine. PACS Nos.: 82.35.Rs, 87.14.Ee, 87.15.Aa, 87.15.By, 82.39.–h, 77.84.Jd