Theoretical considerations on electron optical brightness for thermionic,field and T-F emissions

General theoretical considerations are made on electron optical brightness. The relation between the electron optical brightness and the energy distribution function of electrons emitted from the cathode surface is studied. A general expression, which is valid in various electron emission regions ranging from thermionic to field emission, is derived for the electron optical brightness for a general energy distribution at the emitting cathode. The result is then specialized to the case of the axial brightness, that is, the brightness on the axis in the axial direction. The axial brightness is found to be related to the tangential energy distribution function at zero tangential energy. The axial brightness is numerically evaluated over a wide range of the cathode temperature and field strength as well as work function, including the thermionic and field emission cases. The simplified analytical expressions of the axial brightness both for Schottky and field emission are also given together with the limits of their applicabilities. The useful numerical data for emission parameters necessary to evaluate the axial brightness are given. The theoretical values of the axial brightness are compared with the experimental ones in the case of the field emission gun.     

A new approach to gas field ion sources

A new approach to gas field ion sources is described. It is based on a structure made by inserting a field emission tip inside a small diameter tube. The tube supplies gas to the tip from a high-pressure chamber into a high-vacuum chamber where ionization takes place. Comparison of projection electron and ion micrographs shows that ionization results from a field ionization process taking place at the very end of the tip. Emission currents in the 10nA range, for a few kV emission voltages, are obtained with various gases including neon, air and hydrogen. Lifetime experiments with H(2) show stable emission for days.     

Electron spectrometer in adjustable triode configuration for photo-induced field emission measurements

We have constructed a new ultrahigh vacuum apparatus with a triode configuration for the systematic investigation of photo-induced field emission (PFE) from metallic or semiconducting cathodes. These are exposed to electric fields up to 400 MV∕m and laser irradiation by means of hole or mesh gates. Cathodes and gates are in situ exchangeable and adjustable with high precision to ensure a homogeneous extraction of electrons which are partially transmitted to the fixed electron spectrometer. Its hemispherical sector analyzer provides an energy resolution limit of 8 meV. The commissioning of the measurement system has been performed with a tungsten needle. Its temperature showed up in the high-energy tail of the electron spectrum, while its work function was derived from the spectral low-energy part combined with the integral current-voltage curve. First PFE measurements on B-doped Si-tip arrays yielded a small field emission current increase under green laser illumination. A shift and splitting of the energy spectra was observed which revealed different emission regimes as well as the photosensitivity of the cathode due to carrier excitation into the conduction band. For the full exploitation of the PFE system, a tunable laser over a wide eV-range is required.     

Effects of hot electron emission on a low-conductivity tetracyanoethylene polymer layer including studies of the corrugation of the film surface

The effect of strong field electron emission (FEE) on a tetracyanoethylene (TCNE) polymer layer was studied by Field Ion Microscopy (FIM) using TCNE and Ne as the imaging gases. The TCNE polymer was formed on each tungsten tip by radical polymerisation before FEE. The FIM images show field emission spots all over the surface of the tip. The FEM images show a random distribution of several field emission areas at the onset of FEE. After sometime at a current of about microA, there is a transition to higher currents at the same voltage, in which the electron emission pattern changes to have only one emitting area. After this transition, two different types of FIM images were observed, depending on the imaging gas that was used. Neon FIM images at low tip voltages show spots in the areas where the electron emission current was greatest, and at much higher voltages these images show emission from other areas with lower surface corrugation. However, the FIM images with TCNE as the imaging gas do not show any differences between the areas with and without electron emission. The FIM images remain as before FEE, which can be explained by the formation of a new polymer by the reaction of the surface layer with the imaging gas. It is assumed that chemically reactive fragments at the polymer/vacuum interface, which are needed for the polymerisation reaction, are formed by pyrolysis and sputtering processes during FEE.     

Intravacuolar network may act as a mechanical support for Toxoplasma gondii inside the parasitophorous vacuole

The intravacuolar network inside the parasitophorous vacuole of Toxoplasma gondii consists of an intricate system of membrane-limited tubules of uncertain role in parasite development. We propose that it is an important structural support to the maintenance of the parasites in the characteristic rosette arrangement of parasites inside the vacuole, rather than being associated with the nutrient acquisition from the host cell, as previously suggested. We based our assumptions on observations made by field emission scanning electron microscopy of an epithelial cell line (LLCMK2) infected at various time intervals. Scraping the surface of infected monolayers with Scotch tape exposed the inner organization of the parasitophorous vacuole. Ultrathin sections and freeze-fracture replicas of analogous samples were correlated with field emission observations and added new data on tubular membranes and general organization of the parasitophorous vacuole.     

On the application of quantum transport theory to electron sources

Electron sources (e.g., field emitter arrays, wide band-gap (WBG) semiconductor materials and coatings, carbon nanotubes, etc.) seek to exploit ballistic transport within the vacuum after emission from microfabricated structures. Regardless of kind, all sources strive to minimize the barrier to electron emission by engineering material properties (work function/electron affinity) or physical geometry (field enhancement) of the cathode. The unique capabilities of cold cathodes, such as instant ON/OFF performance, high brightness, high current density, large transconductance to capacitance ratio, cold emission, small size and/or low voltage operation characteristics, commend their use in several advanced devices when physical size, weight, power consumption, beam current, and pulse repletion frequency are important, e.g., RF power amplifier such as traveling wave tubes (TWTs) for radar and communications, electrodynamic tethers for satellite deboost/reboost, and electric propulsion systems such as Hall thrusters for small satellites. The theoretical program described herein is directed towards models to evaluate emission current from electron sources (in particular, emission from WBG and Spindt-type field emitter) in order to assess their utility, capabilities and performance characteristics. Modeling efforts particularly include: band bending, non-linear and resonant (Poole-Frenkel) potentials, the extension of one-dimensional theory to multi-dimensional structures, and emission site statistics due to variations in geometry and the presence of adsorbates. Two particular methodologies, namely, the modified Airy approach and metal-semiconductor statistical hyperbolic/ellipsoidal model, are described in detail in their present stage of development.     

Development and Significance of the Phosphorus Emission Index of Engine Oils

This paper reports the author's effort to verify some of the present assumptions regarding causes and relationships in the volatility of engine oil phosphorus. Using Selby‐Noack volatility data from 1300 engine oils collected by the Institute of Materials in three areas of the world, volatilised phosphorus showed very low statistical dependence on either oil volatility or phosphorus concentration in the fresh oil. Rather, the data seemed to indicate that the chemistries of the phosphorus‐containing additives and their formulation with other additives were the controlling cause of phosphorus volatility and, by extension, emission level. The study permitted the development of a phosphorus emission index that predicts the emission potential of a formulated oil based on the amount of phosphorus found in the volatilised oil collected in the Selby‐Noack test.     

Transient analysis of gaseous electron-ion recombination in the environmental scanning electron microscope

Most of the work carried out in relation to contrast mechanisms and signal formation in an environmental scanning electron microscope has yet to consider the time dependent aspects of image generation at a quantitative level. This paper quantitatively describes gaseous electron‐ion recombination (also known as ‘signal scavenging’) in an environmental scanning electron microscope at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector images of alumina (Al₂O₃) immediately after a region of enhanced secondary electron emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron‐ion recombination that takes into consideration transients caused by the time constant of the gaseous secondary electron detector electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks are then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients and (iii) electron drift velocities, as well as absolute values of the total time constant of the gaseous secondary electron detection system and external circuitry, to be determined as a function of microscope operating parameters such as gaseous secondary electron detector bias, sample‐electrode separation, imaging gas pressure, and scan speed. The results revealed, for the first time, the exact dependence that the effects of secondary electron‐ion recombination on signal formation has on reduced electric field and time in an environmental scanning electron microscope. Furthermore, the model implicitly demonstrated that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought.     

Multiscale Surfaces and Amontons’ Law of Friction

The theories of Archard, Greenwood, and more recently Persson all predict that the area of actual contact between two rough surfaces will be approximately proportional to the applied normal force, though these authors make different assumptions and approach the problem from different points of view. Here we discuss the nature of these approximations and show that the common conclusion follows from the multiscale nature of the surface profile. In particular, it is shown that whatever assumption is made about the nature of friction on the microscale, the macroscale frictional behaviour will always approximate Amontons’ law if the surface has sufficient multiscale content.     

Characterization of a high voltage flat panel display unit using nanotube-based emitters.

Details are given of an experimental investigation carried out to study the field electron emission characteristics of a field emission flat panel display unit using a carbon nanotube-epoxy composite as electron emission material. These include: (i) dependence of direct emission current-voltage characteristic on vacuum gap spacing, (ii) the variation of the proportion of emission current passing through an aperture hole of a gate electrode with changing structural parameters of the device, and (iii) the uniformity and display characteristics of a typical display unit. Our findings indicate that it is very likely for one to produce a near-market prototype high voltage field emission flat panel display, if more sophisticated fabrication and assembly technique is adapted.     

Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy. An introduction to the proceedings of the Second International Symposium and Workshop on Emission microscopy and Related Techniques

Emission microscopes and related instruments comprise a specialized class of electron microscopes that have in common an acceleration field in combination with the first stage of imaging (i.e., an immersion objective lens, also called a cathode lens or emission lens). These imaging techniques include photoelectron emission microscopy (PEEM or PEM), electron emission induced by heat, ions, or neutral particles, mirror electron microscopy (MEM), and low-energy electron microscopy (LEEM), among others. In these instruments the specimen is placed on a flat cathode or is the cathode itself. The low-energy electrons that are emitted, reflected, or backscattered from the specimen are first accelerated and then imaged by means of an electron lens system resembling that of a transmission electron microscope. The image is formed in a parallel mode in all of the above instruments, in contrast to the image in scanning electron microscopes, where the information is collected sequentially by scanning the specimen. A brief history and introduction to emission microscopy, MEM, and LEEM is presented as a background for the Proceedings of the Second International Symposium and Workshop on this subject, held in Seattle, Washington, August 16-17, 1990. Current trends in this field gleaned from the presentations at that meeting are discussed.     

Exoelectron Emission for the Study of Surface Fatigue Wear

An apparatus has been constructed for studying exoelectron emission from steel bearing balls which have been run in a Barwell four-ball surface fatigue wear tester. The test ball is periodically removed from the tester, cleaned to remove its oil film, transferred to a vacuum chamber, and illuminated by ultraviolet light. The emitted exoelectrons originating at new surface area generated on the ball during fatigue testing are detected by an electron multiplier. By rotating the ball and measuring the exoelectron emission as a function of position, the source or sources of the emission can be localized. The resolution of the research apparatus is presently limited to 0.27 mm, the width of the scanning spot. Exoelectron emission decay results indicate a relatively slow drop in emission with time. Preliminary results indicate that the fatigue failure occurs at one of several sites which have given enhanced electron emission.     

Beam transfer characteristics of a commercial environmental SEM and a low vacuum SEM

Two commercial instruments that permit a gaseous environment in their specimen chamber have been investigated, namely, a 'FEI Quanta 600 FEG' environmental scanning electron microscope and a 'LEO SUPRA 35VP FESEM' low vacuum scanning electron microscope. The gas flow field is first computed by the direct simulation Monte Carlo method and the gas density gradient, speed, Mach number and temperature are found in the transition region from high pressure to vacuum. The electron beam transfer characteristics are then derived for different accelerating voltages and pressures and a comparison is made among different situations and with some published works. Certain physical parameters are analysed and discussed to establish a figure of merit that can be used as a standard performance specification for commercial environmental scanning electron microscope and low vacuum scanning electron microscope.     

Frequency Diversity in a Linear Inversion Algorithm for GPR Prospecting

A tomographic approach is presented for processing GPR data under a multifrequency/multistatic/multiview measurement configuration. The approach exploits the Born Approximation so that a linear inversion problem is achieved. This allows to analyze the reconstruction capabilities of the solution algorithm and also discuss about the requirements of the number and the spacing of the frequencies needed in order to achieve a nonredundant set of data. It is stressed their dependence on the extent of the observation and investigation domains and the dielectric permittivity of the host medium. The investigation is first performed in the simplifying assumptions of lossless environment so to gain guidelines. Then numerical reconstruction results with model and exact scattered field data, for more realistic geometries, are presented with the aim of confirming the theoretical expectations.     

Field electron and ion emission from charged surfaces: a strategic historical review of theoretical concepts

The field-electron (FE) and field-ion techniques directly observe and measure atomic-level surface processes that occur in very high electric fields. In theoretical terms, the high fields put large additional terms into Hamiltonians and free energies, and significantly modify many aspects of the surface physics and chemistry, as compared with the field-free situation.This paper presents a strategic review of the fundamental science of some of these high-field surface effects and processes, as developed in the context of the field electron and ion emission techniques. It outlines the main theoretical concepts developed, notes some twists of scientific history, and suggests useful contributions made to mainstream science. Topics covered are basic aspects of FE emission, surface field ionisation, localised field adsorption, charged surfaces theory, field-ion image contrast theory and associated imaging-gas kinetics, field evaporation, and aspects of the thermodynamics of charged surfaces. Despite many years of effort, important aspects of the theory remain incomplete. Some theoretical challenges are noted.     

A momentum imaging microscope for dissociative electron attachment

We describe an experimental approach to image the three-dimensional (3D) momentum distribution of the negative ions arising from dissociative electron attachment (DEA). The experimental apparatus employs a low energy pulsed electron gun, an effusive gas source and a 4π solid-angle ion momentum imaging spectrometer consisting of a pulsed ion extraction field, an electrostatic lens, and a time- and position-sensitive detector. The time-of-flight and impact position of each negative ion are measured event by event in order to image the full 3D ion momentum sphere. The system performance is tested by measuring the anion momentum distributions from two DEA resonances, namely H(-) from H(2)O(-) ((2)B(1)) and O(-) from O(2)(-) ((2)Π(u)). The results are compared with existing experimental and theoretical data.     

Ion induced electron emission from polycrystalline copper

The total yield of secondary electrons emitted from a polycrystalline copper target during bombardment by protons, noble gas ions and copper ions has been studied in the energy range 10–400 keV.Comparison is made with a new theory for secondary electron emission in which the energy dependence of the yield is explained in terms of energyy transferred to the electrons in a surface layer of the target. Both the energy transferred from the ion to the target electrons and the energy transferred from the recoiling target atoms to the electrons are taken into account. Within the validity region of the theory the agreement between the theory and the experiment is good. The experimental equipment and procedure are described.     

Spatial distribution of the charged particles and potentials during beam extraction in a negative-ion source

We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H(-)) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H(-) density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H(-) density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H(-) charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H(-) density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.     

Study of the frequency response of the thin film cold cathode electron source of a lighting element.

Details of the recent experimental and theoretical studies of the frequency characteristic of the field emission electron source of a lighting element are given. The response times at different frequencies (0.02-200 kHz) and applied gap fields, acting on cathode surface, have been studied. A correlation has been found to exist between the response time and the frequency of applied voltage pulse. The response time remains almost constant within a range of frequencies between 1 and 30 kHz, and it remains nearly constant with increasing applied gap field higher than the threshold field. Finally, the cutoff frequency of the electron source is found, and with the current design of the electron source, it can be as high as 40 kHz. An equivalent circuit model is proposed, and theoretical results based on this model agree well with experimental findings.