Electrostatic Potential Barrier Research Articles

Reversible assembly of gold nanoparticles confined in an optical microcage

As optical trapping by a focused laser beam is applied for nanoparticles, multiple particles are grasped in the focal spot and make an assembly. Gold nanoparticles confined in such a submicrometer optical cage show a characteristic extinction spectrum depending on laser power. The spectral change can be induced reversibly and repeatedly by tuning the laser power, demonstrating that the assembly of gold nanoparticles can be controlled by the gradient force. This is attributed to the soft confinement of nanoparticles dressed in electrostatic potential barriers.

Electron Tunneling at the TiO2/Substrate Interface Can Determine Dye-Sensitized Solar Cell Performance

The electric potential distribution in dye-sensitized solar cells plays a major role in the operation of such cells. Models based on a built-in electric field which sets the upper limit for the open circuit voltage (Voc) and/or the possibility of a Schottky barrier at the interface between the mesoporous wide band gap semiconductor and the transparent conducting substrate have been presented. We show that I−V characteristics in the dark and upon illumination are very well explained by electron tunneling, rather than transport over a Schottky barrier, at this interface. Our calculations, based on tunnel currents, show that a discontinuity of the conduction band at the TiO2/FTO interface, rather than a built-in electric field, suffices for efficient electron transfer through this interface, and, thus, for efficient operation of this type of solar cell. Clearly, this will hold only if the photoinduced electrostatic potential barrier between the transparent conducting substrate and the mesoporous wide band ga...

Current–Voltage Characteristics of Cobalt‐Doped Inversion Boundaries in Zinc Oxide Bicrystals

Undoped and cobalt‐doped basal inversion boundaries were fabricated in ZnO bicrystals to investigate their current–voltage characteristics. High‐resolution transmission electron microscopy observations and energy‐dispersive X‐ray spectroscopy analyses for a cobalt‐doped bicrystal revealed that the boundary was highly coherent and free from intergranular phases and precipitates, but a certain amount of cobalt was present near the boundary. The cobalt‐doped bicrystals exhibited nonlinear characteristics that depended on cooling rates from annealing temperature, in contrast to linear characteristics of the undoped bicrystals. It is suggested that the presence of cobalt impurities enhances the formation of acceptor‐like native defects near the boundaries to generate electrostatic potential barriers.

Nanoscale Tuning of Grain Boundary Potential Barriers in Doped Strontium Titanate

Perovskite oxides are systems exhibiting a broad spectrum of technologically important properties. In particular, there has been a substantial interest in exploiting their functionality for superconductivity, ferromagnetism, magneto-resistance, ionic conductivity, and as next generation dielectrics [1]. Over the years it has become very clear that nanoscale features of interfaces in such materials often influence bulk phenomena and interface mediated properties. The electrical activity of such interfaces originates from a convoluted yet tractable interplay among numerous interfacial variables, including local atomic structure, defect chemistry and resultant localized changes in electronic structure. This is the basis for formation of interfacial trapped charge, and space-charge potential, which modify substantially under applied bias. An excellent example of this observed in electroceramics, where, the trapping of charge at interfaces has a decisive influence on the electrical transport properties through the formation of electrostatic potential barriers [2].

Chemical and structural features of the grain boundaries in semiconducting BaTiO 3 ceramics prepared from surface-coated powders

The chemical and structural features of the grain boundaries of the BaTiO 3 ceramics, which were synthesized by hot-press sintering Mn-coated semiconducting powders, were investigated, and the relations with their electrical characteristics were discussed. The distribution of coating materials near the grain boundaries appears to produce about 1.2% strain within the region approximately 20 nm from the grain boundary plane, as well as particular electrical features; the electrostatic potential barrier heights of the BaTiO 3 ceramics increased from 0.18 to 0.24 eV when the amount of coating materials increased from 0 to 1.0 at.%.

First-principles calculations of an oxygen deficient Σ = 3(111)[10] grain boundary in strontium titanate

An oxygen vacancy in a Σ = 3(111)[101̄]grain boundary (GB) in SrTiO3 is modelled using DFT plane-wave pseudopotentialmethods. The formation energy of the vacancy in the boundary is found todepend on its charge state and on the electron chemical potential. There is astrong driving force for segregation when the vacancy is in the charge neutralstate and this results in an electrostatic potential barrier at the GB. The vacancyis found to act as a donor and the GB becomes n-type. The defect inducessmall atomic relaxations in an otherwise highly ordered GB structure.

Grain Boundaries of Semiconducting SrTiO3 and BaTiO3 Ceramics Synthesized from Surface‐Coated Powders

The chemical and electrical features of the grain boundaries in polycrystalline SrTi0.99Nb0.01O3 (ST) and BaTiO3 (BT) ceramics, which were synthesized by hot‐press sintering Na‐ and Mn‐coated semiconducting ST and BT powders, respectively, were investigated. Because of the excess negative electric charges formed near grain boundaries, electrostatic potential barriers were formed near the grain boundaries. The electrical features of the grain boundaries in ceramics are very sensitive to the amount of the coating material. When the amount of the coating material was increased from 0 to 5 wt%, the threshold voltage of the ST ceramics and the resistivity jump ratio of the BT ceramics increased from 0.7 to 81.0 V/cm and from 1.0 to 2.0 × 103, respectively. The electrical features of the grain boundaries are related to their chemical characteristics.

Predictive Structure-Property Correlations for SrtiO3 Grain Boundaries

In polycrystalline materials the trapping of charge at interfaces has a decisive influence on the electrical transport properties through the formation of electrostatic potential barriers. This can either be an intrinsic phenomenon or can be related to impurity segregation leading to complex defect centers. This plays a key role in technologically important systems, especially those having electrically active interfaces, e.g.,p-n junctions in semiconductors and grain boundaries in electroceramics.Electroceramic oxides such as ZnO and SrTiO3 are common systems that exhibit the tendency of current control by internal potential barriers. While bulk measurements, either electrical (e.g. P-E, C-V, I-V), or optical (e.g. Raman) have contributed significantly to the understanding of charged interfaces, there are a very few direct observations of electrical activity at a nanometer level. For instance, it has been recognized that space charge and dopant segregation at the grain boundary are inter-related.

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3

The dynamics of electrostatic potential barriers at grain boundaries (GBs) in Nb-doped SrTiO3 bicrystals is investigated using a unique combination of bulk and in-situ TEM electrical measurements across isolated GBs, coupled with electron holography under in-situ applied bias. The Nb bulk-doped bicrystals exhibit a positive GB potential that suppresses reversibly under applied bias greater than the nonlinearity threshold in the current-voltage curve. This suppression is interpreted as break-down of the potential barrier to current transport. The results on Nb bulk-doped bicrystals have been compared to those in which Mn has been added as a grain boundary specific dopant. This acceptor doping of the grain boundary causes an appreciable increase in the grain boundary resistance and extension of the nonlinear regime. A preliminary account of static electron holography shows a relatively flat potential profile across the GB, indicating probable compensation of donor states at the GB core with Mn-acceptors. Interestingly, the phase profile under applied bias in this case exhibits a reversible “dip” at the GB which is interpreted as an activation of GB trap states due to Mn-acceptor dopants trapping extra electrons (the majority charge carriers) at the GB core, inducing a negative GB potential, and diminishing current transport until the threshold bias is exceeded. The synergistic combination of nanoscale TEM measurements coupled with traditional macroscopic electrical measurements is emphasized.

Permeability of small hydrophilic solutes in small unilamellar vesicle (SUV)

Permeabilities of hydrophilic solutes with and without net charge in small unilamellar vesicles (SUV) were determined. Their permeabilities showed strong dependence on molecular weight. The permeabilities of net zero charged molecule (glucose and alanine) in SUV consisting of dioreoylphosphatidylcholine (DOPC) or soybean phosphatidylcholine (SoyPC) were larger than those in SUV consisting of dimyristoylphosphatidylcholine (DMPC). The permeabilities of net charged molecules (lysine and glutamate), on the other hand, in SUV consisting of DOPC or SoyPC were lower than those in SUV consisting of DMPC. These results suggest that the electrostatic potential barrier of lipid bilayer has significant effects on the permeabilities of hydrophilic solutes in the liposomes.

Solution to the ion feedback problem in hybrid photon detectors and photomultiplier tubes

A general solution to the positive ion feedback problem in Hybrid Photon Detectors (HPD), photomultipliers and other similar detectors was found in the insertion of a permanent electrostatic potential barrier which prevents drift of positive ions from the anode, or the first dynode, towards the photocathode. In this paper, we present the method as applied to the Intevac HPD.

Are predicted superluminal tunneling times an artifact of using the nonrelativistic Schrödinger equation?

AbstractFor many approaches to timing electrons scattering from an electrostatic potential barrier there are simple realistic situations, involving barrier heights and incident electron kinetic energies both many orders of magnitude less than the rest energy mc2, for which the calculated mean transmission times imply superluminal velocities. The question of whether or not this is an artifact of using the nonrelativistic Schrödinger equation is investigated for the Bohm trajectory and Salecker‐Wigner quantum clock approaches.

Self‐consistent determination of the electrostatic potential barrier due to the photoelectron sheath near a spacecraft

A series of scientific spacecraft either in orbit or to be launched in the very near future carries ion emitters for active control of the electrostatic surface potential. The ion beam emission changes, via the associated reduction of the positive surface potential, the photoelectron sheath around the spacecraft which may result in the formation of a potential barrier. In this study, a spherically symmetric model of the photoelectron sheath around a spacecraft (with conductive surface) is analyzed. An algebraic equation is set up to describe the current equilibrium between the escaping photoelectrons, incoming ambient plasma electrons, and the artificially emitted charged particles at the surface of the spacecraft. The height of the potential barrier can be given by solving the algebraic equation, if the electrostatic surface potential of the spacecraft, the density and temperature of the ambient plasma electrons are known. The position of the potential barrier is estimated with the assumption that the electrostatic potential distribution is of a Debye form/The method has been applied to the data gained from the Geotail spacecraft at times when the ion emitter was operational. From the calculation, it is shown that the height of the potential barrier grows from 0.27(Tph/e) to 0.91(Tph/e), as the ion emitter current increases from 9.2(μA) to 38.3(μA). The maximum barrier height appears to be about 2 V, when the ion emitter operates at its maximum current about 50 μA.

Landau levels and persistent currents in nonuniform magnetic fields.

The single-particle energy spectra are calculated for a two-dimensional electron gas under perpendicularly applied nonuniform magnetic fields. It is found that the spatial modulation of magnetic fields in an annular geometry can lift the degeneracy of bulk Landau levels, and accordingly results in current-carrying states that are distinguished from the edge states. The prominent features of the calculated energy spectrum are anticrossing and the repulsion of levels. The net current does not vanish in the ground state due to the presence of the lower-lying current-carrying states below the Fermi energy. Quantum structures confined by electrostatic potential barriers are also investigated within an independent electron approximation. Consequently, a rich structure of persistent currents is obtained at zero temperature as a function of the electron number and magnetic field variation. \textcopyright{} 1996 The American Physical Society.

A Simple Electron Trap with Perpendicular Injection

A simple electron trap with a uniform axial magnetic field was designed and constructed for stationary operation. Electrons from a small filament placed near the outer wall were continuously carried inside between electrostatic potential barriers by a positive bias to a center conductor.

Interaction of field‐aligned cold plasma flows with an equatorially‐trapped hot plasma: Electrostatic shock formation

Effects of equatorially trapped hot plasma on the highly supersonic cold‐plasma flow occurring during early stage plasmaspheric refilling are studied by means of numerical simulations. It is shown that the equatorially trapped hot ions set up a potential barrier for the cold ion beams and facilitate formation of electrostatic shocks by reflecting them from the equatorial region. Simulations with and without the hot plasma show different flow properties; the formation of electrostatic shocks occur only in the former case. The simulation with the hot plasma also reveals that the magnetic trapping in conjunction with the evolution of the electrostatic potential barrier produces ion velocity distribution functions consisting of a cold core and a hot ring in the perpendicular velocity. Such a distribution function provides a source of free energy for equatorial waves. The corresponding electron population is warm and field aligned.

Colloidal stability of electrostatically stabilized sol particles. Part I: The role of hydration in coagulation and repeptization of ferric hydroxide sol

FeO(OH)·0.5 H2O powders were prepared by drying portions of a FeO(OH)·0.5 H2O sol at different relative vapor pressures, and the adsorption of water on the powders was determined. The magnitude of electrostatic potential barrier for the sol is of about 9 mJ·m−2. The reduction in the immersion enthalpy and in the surface free-energy in a range of the relative vapor pressures of 0.0–0.9 is as high as 140 mJ·m−2 and 130 mJ·m−2, respectively.

On impulsive penetration of solar wind plasmoids into the geomagnetic field

The idea that solar wind plasma-field irregularities, i.e. plasmoids with an excess momentum density penetrate deeper into the geomagnetic field was introduced in 1976 by Lemaire and Roth at an EGS meeting. It was based on the observation that the solar wind is most of the time patchy over distances smaller than the diameter of the magnetosphere. In this early paper about “impulsive penetration”, the authors did not attend to give a detailed physical description of the underlying mechanism. When Lemaire was more informed about some relevant laboratory plasma experiments carried out by Bostick, Baker and Hammel or Demidenko et al., he published in 1985 [Lemaire, J. (1985) Plasma Phys. 33, 425] a physical description of the mechanism, based on a theory first proposed by Schmidt in 1960 [Schmidt, G. (1960) Phys. Fluids 3,961]. Transient and impulsive interaction processes between the solar wind and the magnetosphere have now become an important and highly debated topic. In particular, Heikkila's argument claiming that the effects of induced electric fields are the primary cause for impulsive penetration has been shown by Owen and Cowley to be erroneous. Although the conclusions reached by Owen and Cowley [Owen C.J. and Cowley S. W. H. (1991) J.geophys. Res. 96, 5565] are correct, at least within the framework contrived by Heikkila (i.e. that of ideal MHD) [Heikkila W. J. (1982) Geophys. Res. Lett. 9,159], they do not demonstrate that real plasmoids can not penetrate impulsively onto closed geomagnetic field lines. Indeed, non-ideal MHD processes, like collective polarization effects, formation of electrostatic potential barriers, adiabatic and non-adiabatic brakings or collective diamagnetic effects, have to be taken into account in the “real world”. Account of the theory of “impulsive penetration” both for weakly and strongly diamagnetic plasmoids is given, emphasizing in which respect the entry mechanism differs from ideal entry mechanisms like those proposed by Schindkr in 1979 [Schindler, G. (1979) J. geophys. Res. 84, 7257] and by Heikkila in 1982.

The solution of the Dirac equation for a high square barrier

The transmission of a Dirac particle through a high (V0>E+m) square electrostatic potential barrier is investigated further. As is well known, the transmission coefficient is nonzero even as V0 goes to infinity. As well as reproducing the usual wave mechanics approach, a detailed fermion–antifermion multiple scattering calculation and an S-matrix calculation are examined to provide additional insight into the problem. The origin of the curious behavior is found to be the result of pair creation and annihilation events.

Effects of copper precipitation in Σ=25 silicon bicrystals by deep-level transient spectroscopy

The electrical properties of grain boundaries have been investigated by means of capacitance measurements such as deep-level transient spectroscopy and thermally stimulated capacitance. This paper describes the evolution of the electrical properties of n-type Σ=25 silicon bicrystals as a function of heat treatments. It results from this work, that after annealing and quenching, the Σ=25 bicrystal presents an electrical activity and that metallic impurities are responsible for the grain boundary electrostatic potential barrier. The investigations by transmission electron microscopy have shown that the grain boundary activity is in relationship with the presence of colonies of precipitates at the interface, these precipitates being silicides. The results of microanalysis experiments made elsewhere on other similar bicrystals indicate the presence of copper and nickel in the precipitates forming the colonies. The importance of the heating and cooling rates of the samples about the energy distribution of the interface states display the kinetic aspect of the mechanisms involved (segregation-precipitation).