Cold Electron Emission Research Articles

Ionization processes of negative corona discharge. Part 2. Theory. Comparison with experiment

The purpose of the research is a theoretical analysis of ionization processes in the air, which are initiated by a high-voltage field generated by the negative needle-flat electrode electrode system. Methods. Fe, Cu, Al electrodes are used. Calculations are carried out using the methods of quantum mechanics and mathematical physics. Results. The processes of cold and thermoelectron emission of electrons, enhanced by an external high-voltage field (Schottky emission), are analyzed. The conditions under which the generation of charges from the cathode surface due to plasma-chemical reactions and the capture of surface electrons by electronegative oxygen molecules are decisive are investigated. Based on the PE theory, a theory of injection of negative ions from the cathode into an electronegative gas is constructed and the theory is compared with experiment. It has been shown that at small radii of curvature of needle tips, when E ≥ 106 V/cm, the generation of charges is due to cold emission of electrons. In average fields E ≈ 105 V/cm, electron emission is due to the Schottky effect. In fields E ≈ 104 V/cm and below, injection is caused by electrochemical processes and PE capture. A theory has been developed for PEs that appear at electric field intensities on the electrode of the order of several tens of kV/cm, that is, on flat and slightly curved negative electrodes. The capture of PE by electron-withdrawing molecules causes an injection current and, as a consequence, the ignition of a corona discharge. Analytical expressions for the surface density as a function of the local electric field strength, as well as an expression for the injection current, are obtained. The agreement between theory and experiment is satisfactory. Conclusion. An analysis of the development of CR on negative needles is given. A theory of PE at the cathode in strong electric fields was developed. Based on the results of measuring dark negative current-voltage characteristics, it is possible to determine the patterns of surface processes involving electronic transitions, in particular, determine the concentration of PE and injection currents.

Comprehensive Analysis of E478 Single-Component Epoxy Resin and Tungsten-E478 Interface for Metallic-Polymer Composite Electron Source Applications.

This study provides comprehensive elemental, optical, and energy gap characteristics of the E478 single-component epoxy resin. This type of epoxy resin has imperative applications in medium voltage insulation and cold field emission of electrons. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and hydrogen nuclear magnetic resonance (1H-NMR) were used to study the elemental and structural analyses, ultraviolet photoelectron spectroscopy (UPS) was used to obtain the local work function and the ionization potential energies, and ultraviolet/visible light spectroscopy (UV/VIS) was used to report the optical and energy gap characteristics of the epoxy resin being studied. Moreover, the UPS and UV/VIS analyses were merged to obtain the electron affinity of the E478 epoxy resin and to study the epoxy's energy band diagram and the tungsten-epoxy interface band structure. The results showed that the E478 epoxy resin is considered an n-type semiconductor of energy gap ∼3.94 eV, local work function ∼3.42 eV, ionization potential ∼6.10 eV, electron affinity ∼2.16 eV, and tungsten-epoxy Schottky contact barrier height ∼2.50 eV.

Coupled Monte Carlo density functional theory calculations of cold and laser-assisted electron field emission from graphene-coated copper cathode

Field emission is an important process with a variety of applications. Quantitative predictions of such electron emission need to include details of the internal potentials that shape the electronic wavefunctions (and hence the tunneling probability), predictive analysis of the work function barrier (ΦB), and knowledge of the electron distribution at the surface that constitutes the supply function. Here, these various factors were all collectively considered based on a combined Monte Carlo-density functional theory approach. Results were obtained for both the field-dependent cold electron emission current density as well as photoemission from a short laser pulse. The method also allows for calculations of field-dependent emittance. The technique is general and could be extended to include plasmon–polariton modes, different thicknesses of coatings, and role of surface adsorbates and defects.

Ionization Processes of Negative Corona Discharge Part 1. Review and Experiment

The purpose of the research is an experimental study of physical processes in the near-electrode zones of negative point electrodes in an electronegative gas (air) under the condition of a high-voltage field.Methods. Video image analysis, current-voltage characteristics measurements, and spectroscopy are used. The current-voltage characteristics of the corona discharge ignition voltage in the environment are measured, the problem of experimental identification of surface charges, the mechanism of negative corona discharge ignition, and the ionic structure of streamers is studied, degradation of electrodes during negative corona discharge, spectrum of corona discharge creation and electromagnetic sound.Results. It is shown that at small radii of curvature of needle electrodes, the development of a negative corona discharge is due to the cold emission of electrons, and on flat and slightly curved negative electrodes, due to the capture of surface electrons by electron-withdrawing molecules. The mechanism of ignition of a corona discharge in an electronegative gas with a negative needle, the structure of streamers from negative needles, the emission spectrum and ethonic degradation of electrodes have been studied.Conclusions. The ignition of a negative corona discharge at small radii of the tips of the pointed needles (ro less than tens of microns) is due to the cold emission of electrons followed by impact ionization of neutral molecules. With slightly curved electrodes, the appearance of a negative corona discharge is due to the capture of surface electrons by electronegative gas molecules with subsequent plasma-chemical reactions. The formation of surface electrons depends on many factors: the presence and structure of oxide films (roughness, defects, etc.), which determine the electron work function and form local fields at the tips of microtips. The appearance of a corona discharge leads to degradation of the corona electrodes, expressed in the spraying of the tips of the points and the melting of the needles. In the dark region of the negative corona discharge (U < U*), photons in the in the ultraviolet spectrum (UV spectrum) are emitted, and when U > U*, in addition to UV photons, photons in the visible region of the spectrum are emitted. Glow fluctuations are caused by the chaotic dynamics of the formation of injection centers (melted tubercles - ectons) on the surface of the corona electrode. The ignition of a corona discharge is accompanied by sound emission with a frequency of about 300 Hz.

Potential development and electron energy confinement in an expanding magnetic field divertor geometry

The formation of electrostatic potential in an expanding magnetic field divertor is numerically simulated using a kinetic model. As theoretically expected, the electrostatic potential is formed in the expanding magnetic field, which, in combination with the Debye potential near target walls, repels electrons back and balances electron and ion currents. Going beyond the existing theoretical description of the pre-sheath potential formation limited to the asymptotically low electron flow (ue≪vTe), we demonstrate the limit of applicability of asymptotic theory and study pre-sheath potential in practically important range of electron flow [0<Ie<2Isat, where Isat=en(Te+Ti)/mi is the ion saturation current]. Results of the asymptotic theory are fully reproduced at the low side of this range (Ie≪Isat), whereas at high electron current range Ie∼Isat, the pre-sheath potential substantially decreases. The formation of the pre-sheath potential minimizes the interaction of plasma electrons with the material walls and reduces the Debye sheath potential. Reducing the Debye potential forms favorable conditions for eliminating arcing and cold electron emission from the walls. In these favorable conditions, electron thermal losses at the wall could be reduced to minimal theoretical limit of ∼5−8Te per lost ion.

Особенности переноса заряда при холодной полевой эмиссии с катодов из углеродных нанотрубок

The conductivity of the emitter section of a nanodiode circuit with an emitter made of an individual closed carbon nanotube (CNT) and arrays of closed CNTs was studied. High values of cold field emission were observed, as well as nonlinearity and anomalies of the current-voltage characteristic (CVC), which manifested themselves in the form of peaks at low and high current values. Peaks in the CVC have distinct areas of negative differential conductivity. The anomalously high currents in the I–V characteristics of cold field emission of electrons from a nanoemitter made of closed CNTs can be associated with a sharp increase in the density of electron states at energies near the van Hove singularity.

First experience with He conditioning of a superconducting rf photoinjector

The recent achievements in the performance of superconducting rf (SRF) photoinjectors have opened a new era in the development of reliable high-brightness cw electron sources. While the SRF guns become one of the most promising technologies, the compatibility of the SRF environment with the complex photocathodes remains at the forefront of the modern accelerator science. The SRF cavities operate at cryogenic temperatures providing the ultrahigh vacuum environment highly beneficial for the photocathode performance. However, the necessity to keep the photocathodes at room temperature while being surrounded by the cavity walls that are kept at cryogenic temperatures creates an additional complexity for the SRF gun design and operation. The complex and volatile chemical compounds used for photocathodes have a high chance of contaminating the surfaces of the SRF cavity. When deposited, such compounds could create centers for cold electron emission which degrade the performance of the SRF guns. Such a circumstance would require the development of the in-situ processing techniques for restoring the SRF cavity performance. This paper presents the results of the successful implementation and application of the He conditioning method for cavity restoration using the existing SRF photoinjector at Brookhaven National Laboratory (BNL). The method has proven to be extremely effective and resulted in a dramatic improvement in the BNL gun performance.

Numerical simulation of electrohydrodynamic convection induced by fast oscillating field emission

The paper presents a mathematical model of electrohydrodynamic convection in a mixture of benzene and chlorobenzene subjected to highly inhomogeneous alternating electric field. The field inhomogeneity is determined by the shape of the electrodes, of which one is a flat electrode, and the other is a tip oriented perpendicular to the plane of the first electrode. Recent experiments carried out with a field oscillating at a frequency of 50 Hz have shown that the excitation of an averaged convective flow at a given maximum voltage between the electrodes (8 kV) is observed only at a mole fraction of chlorobenzene of at least 0.4. The flow arises in a threshold manner and is a narrow intense jet directed from the sharp electrode towards the plane closed by an axisymmetric return flow. We propose a model that explains this phenomenon by analyzing the interaction between two fields of charged particles. The sharp electrode injects electrons every half period due to cold field emission. We assume that a chlorobenzene molecule has an uncompensated dipole moment, i.e., in a strong electric field, a chlorobenzene molecule gains an electron and forms a negative ion. However, an electron can exist in a polar dielectric solution for a long time due to the effect of its salvation by the medium. Therefore, the ionization process occurs both at the injector electrode and in the bulk of the medium. The mathematical model includes the Navier–Stokes equation, the transport equations for electrons and ionized chlorobenzene molecules, and the equation of alternating electric field. The sharp electrode boundary conditions are formulated to take into account the cold emission of electrons, which is described by a piecewise linear approximation of the Fowler–Nordheim law. The study of the performance of the proposed model was carried out by the method of direct numerical simulations of non-stationary physical fields. It is shown that if the presence of electrons in the volume of the medium is ignored, the averaged electrohydrodynamic convection does not occur, even if the sharp electrode continues to inject chlorobenzene molecules. The proposed model explains most of the experimental facts. As in the experiment, the excitation of convection critically depends on the mole fraction of chlorobenzene in the binary mixture. The calculated and experimentally observed structures of the convective flow are in good qualitative and quantitative agreement. The results of numerical experiments provide a good basis for the development of an averaged theory of electrohydrodynamic convection.

Nanostructured SiC as a promising material for the cold electron emitters

In this paper, the novel cold electron emitters based on nanostructured SiC layers covering the Si(001) substrate have been proposed. Their main advantage is an extremely simple and cost-effective manufacturing process based on the standard microelectronics-grade silicon wafers with no ultra-high vacuum required and no complicated chemical deposition processes or toxic chemicals involved. It integrates within a single technological step both the SiC growth and nanostructuring the surface in the form of nanosized protrusions, which is extremely beneficial for cathode applications. A simple mathematical model predicts field emission current densities and turn-on electric fields, which would allow practical device applications. According to our estimations, emission currents in the milli-Amp range can be harvested from one square centimeter of the cathode surface with electric field close to 107 V/m. So, the nanostructured SiC can be the promising material for the cold electron emitters.

Centers of cold electron emission from molybdenum thin films

Thin-film semiconductor devices and functional metal-oxide-semiconductor structures have drawn attention as being applicable for on-chip electronics. Similar structures, however, were also reported to be promising as efficient sources of electrons. This paper presents the results of scanning electron microscopy analysis of molybdenum thin films capable of low-macroscopic-field electron emission. Supposedly, the images show the centres of cold-field emission at different stages of their life cycles from activation to destruction. It is assumed that initially continuous Mo films experience dewetting under heating and ion bombardment, which are caused by the emission current flow. The results contribute to understanding the mechanism of low-field emission from thin metal films, which seems to be different from those previously proposed for thin carbon coatings.

Hierarchical nickel sulphide microstructures for controlled water disinfection and cold cathode emission

Hierarchical nickel sulphide microtubes were prepared by cost effective sol-gel technique with tuning of morphology and applied for photo-induced hazardous dye segregation and cold electron emission. The synthesized samples were characterized for crystalline phase, morphology and composition via X-ray diffractometry, electron microscopy and energy dispersive X-ray studies respectively. The band gaps were estimated from UV–vis data. The samples were found to be effective as a dye degrading agent under UV irradiation. The pollutant removal efficiency of the nickel sulphide (NS) microstructures was found to be governed by dimension and compositional parameters jointly. The sample showing highest degradation efficiency was found to degrade phenol effectively. Scavenger test was carried out to identify the active radicals involved in the catalysis process. To ensure long term action of the catalyst sample, recyclability of the same was investigated thoroughly. The NS microstructures also showed field emission behaviour which is reported for the first time. The growth of different shapes and hierarchical units of NS structures was correlated with stoichiometric alteration which may also be extended for synthesis of other nickel chalcogenides.

Mathematical modelling of the electric field of carbon nanotube arrays used in cold cathode electron emission devices

Influence of the ratios of the carbon nanotube arrays geometrical parameters (height to radius – H/R, distance between the individual nanotube to height – S/H) on their properties to enhance the electric field strength at their tips and to accumulate electrostatic charge have been investigated. It was done with the help of their electric field mathematical modelling. Such modelling has shown that dependences of the emission current density on S/H ratio have maximums that are determined both by H/R and the applied electric field strength levels.

Increased Efficiency and Duration of Emission from Carbon Nanotubes Processed in Ammonia Plasma

A technique for obtaining carbon nanotubes (CNTs) doped with nitrogen in ammonia plasma has been developed. In this technique, CNTs synthesized by a method excluding their doping with nitrogen are post-treated in ammonia plasma. This treatment leads to a more than fivefold increase in the concentration of nitrogen (from 0.7 to 3.6 at %), a sixfold decrease in the resistance, and a 10% decrease in the work function of CNTs, resulting in stabilization of the process of cold electron emission.

V doped BaSnO3 nanocubes as a field emitting material: Experimental and theoretical investigation

In search of a multipurpose perovskite system pure and vanadium doped BaSnO3 nanocubes were prepared using facile solid-state reaction method. Theoretical simulation via ANSYS Maxwell software was employed to predict whether doping in nano-BaSnO3 can lead to cold electron emission from the samples. The work function, a crucial factor for potential cold cathodes, was determined using density functional theory and the outcome was used to calculate the field enhancement factor. Encouraged by theoretical outcome, experimental investigation for field emission capability of the samples was carried out in a high vacuum chamber. Several factors like impurity doping, emitter dimension, decrement of work function were identified as tuning factors. The emission current density and the field enhancement factor of 5% doped sample were enhanced ~3 and 4.3 times respectively compared to the pure one with 30% reduction of the turn on field. The first ever investigation of field emission properties of pure and doped BaSnO3 in this work, showed that the novel perovskite can serve as an environment-friendly cold cathodes in field emission display unit, electron microscopes etc. along with its traditional application as solar energy harvesting material.

Electric field emission characteristics of a triode-structured metallic edge emitter from geometric conditions

Spindt-tip type cold electron emission has potential in various applications; however, it has a limited current capacity that prevents the emitters from being used for higher current sources, and the fabrication of the emitters is not easy. In this study, we investigate the electric field emission characteristics of a triode-structured metallic edge emitter from geometric conditions. We perform a systematic simulation based on Fowler-Nordheim theory to analyze the feasibility of using the proposed emitter. The dependences of the emission current, electron transmittance, and region of the triode mode on specific geometrical parameters, including the anode-gate vertical distance, gate-cathode vertical distance, gate-cathode horizontal distance, anode voltage, and gate voltage, are analyzed. The results show that the electric field for a narrow gate-cathode vertical distance is superior to the electric field of the gate electrode for a narrow gate-cathode horizontal distance, while the electric field for a wide gate-cathode vertical gap is more dominant than the electric field of the anode electrode for a wide gate-cathode horizontal gap. Moreover, the electron transmittance gradually increases with a low gate voltage, wide anode-gate vertical distance, and high anode voltage, and the region of the triode mode also increases with a high gate voltage, wide anode-gate vertical distance, and low anode voltage. Consequently, our results appear to be effective for an optimization in an experiment and are expected to improve present electric field emission techniques.

Thin carbon films: Correlation between morphology and field-emission capability

The paper presents a new study of correlation between morphology of thin carbon films deposited on Si substrates, their electronic properties and capability of cold electron emission in electric field with macroscopic magnitude of the order of 1 V/μm. Comparative experiments have demonstrated that substrate parameters have little, if any, direct effect on emission properties of the films: the observed variations of the emission threshold field value in the range from several percent up to two times depending on the substrate species was attributed to substrate-orientation-sensitive mechanism of the film growth leading to notable difference in topography of the compared films (indirect effect). Tunneling spectroscopy study showed that the films with the best emission properties comprise sp2 carbon islands (or carbon dots, CDs) having electron systems separated from each other and from the substrate. This conclusion has been supported by the presence of a photoluminescence component in Raman spectra of the emissive films, which is often associated with electron confinement effects in CDs. In combination, the new experimental data witness in favor of hot-electron mechanism of low-threshold electron emission from the studied films.

Narrow-band pulsed electron source based on near-threshold photoionization of Cs in a magneto-optical trap

The newly developed method of time-of-flight (ToF) momentum microscopy was used to analyse the cold electron emission from a Cs 3D magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states (6P3/2 pumped with 852 nm laser and 7S1/2 with 1470 nm) and a tuneable femtosecond Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs-laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field induced ionization of highly-excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. ToF analysis allowed us to disentangle the ionization paths and the dependence of the spatio-temporal distribution of the cold electrons on the polarization of the ionizing laser.

A Novel Stationary CT Scheme Based on High-Density X-Ray Sources Device

With the introduction of cold electron sources such as nanotubes, X-ray sources became small in size so that many sources are installed in array to form stationary CT, which brings benefits of simplicity in mechanism and fast response. However, the nanotube-based X-ray source is still singly packaged and hence density of X-ray sources is limited, which introduces severe sparse-view artifacts and could not be used in practice. With the invention of X-ray sources device based on ZnO nanowires as cold electron emitters, a large amount of X-ray sources is first integrated in a flat panel device. In this study, we design a stationary CT architecture using high-density sources devices, called HD-SCT, and develop corresponding reconstruction algorithm. Specifically, several flat panel detectors and X-ray sources devices are spliced together to form a source-detector plate. Multiple source-detector plates are organized as a polygon that encloses object to complete CT scanning. In HD-SCT, X-ray sources are close to object which can significantly decrease CT device volume. The X-ray beam emitted by each source just cover part of object and the spot sources alternately light to completely scan object. Simultaneously, we propose a reconstruction method for the specific configuration which adopts an iterative algorithm to resolve the reconstruction based on alterative part projections, as well as a novel correction process of projection completion for the shadowed area of placing X-ray source devices. To validate the design of the proposed HD-SCT, we execute a series of simulation experiments, which show that the proposed HD-SCT and corresponding reconstruction method effectively remove the artifacts caused by the embed X-ray source device. The results also disclose that with the increase of density of X-ray sources, the resolution of reconstruction has been dramatically improved, which demonstrates the practical value of introducing high-density X-ray sources device into stationary CT design.

Cold cathode electron emission with ultralow turn-on fields from Au-nanoparticle-decorated self-organized Si nanofacets

Fabrication of highly dense conical nanostructures and their subsequent controlled metallization make them ideal candidates for enhancing cold cathode electron emission efficiency.

Simulations of emission properties of carbon nanobelts

The electronic structure of conjugated hydrocarbons and nitrogen-containing nanobelts cyclacene and cyclophenacene types of different diameter method was investigated by DFT B3LYP/6-31G. The two molecular states with a specific localization of the electron density on the structure fragments of nanobelts (emission orbitals EMO) was found in the energy spectra of molecules as a result of the new types of p-electron conjugation (in-plane and p-electron conjugation). The simulation of a constant electric field influence on the energy spectra of nanobelts shown that the energy of EMO substantially depends on the magnitude of the field strength, namely an increase in field strength translates one of the EMO in the region of the occupied molecular states. This fact is the physical basis for the realization of cold field emission of electrons in nanobelts, which corresponds to the experimental data.