Field Emission from Nanowires

This work reports the behavior of electron field emission from nanostructured CVD diamond films at elevated temperatures. The study is motivated by the possibility of using these structures in high-temperature electronics or direct energy conversion processes. Four nanostructured CVD diamond films were tested: nanocrystalline diamond, nitrogen-doped nanocrystalline diamond, peaked diamond microtips, and truncated diamond microtips. All samples displayed reasonably efficient field emission characteristics. For each, the onset of field emission decreased as the sample temperature increased. Truncated diamond microtips were found to provide the largest current-carrying capacity and effective emission current density. These values were 90μA and 12 1 1010A/cm2 respectively. At a temperature of 700 K, peaked diamond microtips showed the lowest turn-on field recorded at 38V μm. The highest turn-on field, at this same temperature, was found from the undoped nanocrystalline diamond and recorded at 90V μm. Address all correspondence to this author. NOMENCLATURE A effective emission area D tunneling transmission coefficient, see Eq. 2 E electron energy Ea conduction band energy EF Fermi level Ex axial electron energy F applied electric field h Planck constant h reduced Planck constant J current density kB Boltzmann constant m electron rest mass N electron supply function, see Eq. 5 p electron momentum q electron charge T temperature V voltage x axial coordinate β field enhancement factor φ work function μ chemical potential 1 American Institute of Aeronautics and Astronautics 8th AIAA/ASME Joint Thermophysics and Heat Transfer Conference 24-26 June 2002, St. Louis, Missouri AIAA 2002-3024 Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION Much attention has been paid to diamond thin films because of their potential applications in optics, high-speed electronics and thermal management devices. Diamond is chemically inert and has superior mechanical stability over typical materials used as electron field emission devices. Diamond’s sp3 tetragonal configuration consists of a central carbon atom with linear covalent bonds to neighboring carbon atoms 10. This particular arrangement facilitates phonon transport through the lattice of the structure, giving diamond a thermal conductivity four times that of pure copper 6. Furthermore, diamond is a wide band gap semiconductor. This material property results in diamond being an excellent electrical insulator. Diamond’s unique characteristic of being an excellent thermal conductor and electrical insulator make it ideal for nanoeletromechanical devices and electronic packaging. Nanocrystalline diamond films show strongly enhanced field emission properties that can be demonstrated reproducibly 9. The success of doping these films coupled with the absence of microscale features (i.e., using as-deposited films) make fabrication and implementation of nanocrystalline diamond relatively straightforward 13. Diamond tip arrays appear to be promising candidates for electron field emission devices (FEDs) because of the field enhancement produced by their geometry 1;8. This geometric enhancement effect occurs due to a deformation of the potential field. The altered potential field enhances the local electric field, enabling emission of electrons into vacuum at lower applied voltages. However, unlike nanocrystalline diamond, manufacturing diamond tips and gated tip arrays often requires many laborious post-fabrication etching processes. The present work reports the thermal effects of field emission for both nanocrystalline diamond and diamond tip arrays. High-temperature field emission experiments were performed on each sample. Current-voltage and FowlerNordheim plots are analyzed and discussed with corresponding theoretical formulations. Results show substantial differences between the electron field emission characteristics of the four samples. THEORETICAL FORMULATION A detailed discussion on the physics of field emission can be found from 5, and 4. The following provides an outline of the major physical concepts. The emission current density is defined as

Theoretical analyses have been performed of the quantum-size (QS) resonance tunneling in the field-emission (FE) phenomenon for different models of the emitting structures. Such experimentally observed peculiarities have been considered as the enhancement of the FE current, the deviation from the Fowler-Nordheim law, the appearance of sharp current peaks, and a negative resistance. Different types of FE cathodes with QS structures (quantized layers, wires, or dots) have been studied experimentally. Resonance current peaks have been observed, from which the values of the energy-level splitting can be estimated.

Effects of composition on field emission character of tetrahedral amorphous carbon

Density functional theory for field emission from carbon nano-structures

We report the synthesis of two-dimensional porous ZnO nanosheets, CuSCN nanocoins, and ZnO/CuSCN nano-heterostructure thin films grown on fluorine-doped tin oxide substrates via two simple and low-cost solution chemical routes, i.e., chemical bath deposition and successive ionic layer adsorption and reaction methods. Detail characterizations regarding the structural, optoelectronic, and morphological properties have been carried out, which reveal high-quality and crystalline synthesized materials. Field emission (FE) investigations performed at room temperature with a base pressure of 1 × 10–8 mbar demonstrate superior FE performance of the ZnO/CuSCN nano-heterostructure compared to the isolated porous ZnO nanosheets and CuSCN nanocoins. For instance, the turn-on field required to draw a current density of 10 μA/cm2 is found to be 2.2, 1.1, and 0.7 V/μm for the ZnO, CuSCN, and ZnO/CuSCN nano-heterostructure, respectively. The observed significant improvement in the FE characteristics (ultralow turn-on field of 0.7 V/μm for an emission current density of 10 μA/cm2 and the achieved high current density of 2.2 mA/cm2 at a relatively low applied electric field of 1.8 V/μm) for the ZnO/CuSCN nano-heterostructure is superior to the isolated porous ZnO nanosheets, CuSCN nanocoins, and other reported semiconducting nano-heterostructures. Complementary first-principles density functional theory calculations predict a lower work function for the ZnO/CuSCN nano-heterostructure (4.58 eV), compared to the isolated ZnO (5.24 eV) and CuSCN (4.91 eV), validating the superior FE characteristics of the ZnO/CuSCN nano-heterostructure. The ZnO/CuSCN nanocomposite could provide a promising class of FE cathodes, flat panel displays, microwave tubes, and electron sources.

Enhanced electron field emission (EFE) behavior of a core-shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR-UNCDN core-shell heterostructures show a high emission current density of 5.5 mA cm(-2) at an applied field of 4.25 V μm(-1) , and a low turn-on field of 2.08 V μm(-1) compared to the 1.67 mA cm(-2) emission current density (at an applied field of 28.7 V μm(-1) ) and 16.6 V μm(-1) turn-on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR-UNCDN core-shell heterostructures in the enhancement of electron emission.

Electrochemical synthesis of Polypyrrole (PPy) thin films on Sn substrates has been carried out under cyclic voltammetry (CV) mode. The structural, morphological and chemical properties of the as-synthesized PPy films were investigated using various characterization techniques like SEM, UV-Visible and FTIR. Furthermore, field emission (FE) behaviour of the PPy thin film emitter were carried out at base pressure of ~ 1×10-8 mbar and found to be interesting. The threshold field, required to draw emission current density of 1 μA/cm2, is observed to be 0.90 V/μm and very high emission current density of 12.87 mA/cm2 has been drawn at applied field of ~ 2.8 V/μm. The emission current stability investigated at preset values of 1, 10 and 100 μA/cm2 is observed to be fairly good over duration of more than three hours. The simplicity of the synthesis route coupled with the capability to deliver very high emission current density at relatively lower applied field make the PPy thin field emitter as a potential candidate for practical applications in field emission based devices

Single-crystal LaB6 field emission tips have strong anti-poisoning ability, good emission stability, high emission current density and mature applications. As such, they have attracted much attention in field emission research. In this paper, an electrochemical corrosion device is designed to enable the simple and efficient preparation of single-crystal-LaB6 tips with different radius of curvature values. Computer simulation technology is used to study the field emission behavior of the single-crystal LaB6 tip and provides the theoretical basis for the calculation of field emission current density. The results show that the preparation process is automated, more efficient and more controllable than previous methods. When the applied voltage was 2250 V, the single-crystal LaB6 tip achieved maximum emission current density of 2.05 × 104 A/cm2 at a radius of curvature of 0.5 μm.

Carbon Nanotubes (CNTs) grown on substrates are potential electron sources in field emission applications. Several studies have reported the use of CNTs in field emission devices, including field emission displays, X-ray tube, electron microscopes, cathode-ray lamps, etc. Also, in recent years, conventional cold field emission cathodes have been realized in micro-fabricated arrays for medical X-ray imaging. CNT-based field emission cathode devices have potential applications in a variety of industrial and medical applications, including cancer treatment. Field emission performance of a single isolated CNT is found to be remarkable, but the situation becomes complex when an array of CNTs is used. At the same time, use of arrays of CNTs is practical and economical. Indeed, such arrays on cathode substrates can be grown easily and their collective dynamics can be utilized in a statistical sense such that the average emission intensity is high enough and the collective dynamics lead to longer emission life. The authors in their previous publications had proposed a novel approach to obtain stabilized field emission current from a stacked CNT array of pointed height distribution. A mesoscopic modeling technique was employed, which took into account electro-mechanical forces in the CNTs, as well as transport of conduction electron coupled with electron–phonon induced heat generation from the CNT tips. The reported analysis of pointed arrangements of the array showed that the current density distribution was greatly localized in the middle of the array, the scatter due to electrodynamic force field was minimized, and the temperature transients were much smaller compared to those in an array with random height distribution. In the present paper we develop a method to compute the emission efficiency of the CNT array in terms of the amount of electrons hitting the anode surface using trajectory calculations. Effects of secondary electron emission and parasitic capacitive nonlinearity on the current-voltage signals are accounted. Field emission efficiency of a stacked CNT array with various pointed height distributions are compared to that of arrays with random and uniform height distributions. Effect of this parasitic nonlinearity on the emission switch-on voltage is estimated by model based simulation and Monte Carlo method.

Summary form only given. Zinc oxide (ZnO), a wide band gap semiconductor has been widely exploited for its application in field emission based devices. It offers various advantages like strong excitonic binding energy (60 meV), negative electron affinity, and high mechanical strength making it a good candidate for field emitter arrays of flat panel display devices. However, most of the reports in literature deal with the field emission studies carried out in close proximity geometry, subject to a narrow range of applied voltage. Such type of experimental arrangement forbids the measurement at relatively higher field, as it may lead to an arc formation. The study of the field emission characteristics over a wide applied field range is critical to understand the physics at low dimensions. Moreover, the reason for the higher field enhancement and the appropriate relation of the geometry with the field emission characteristics is important and desirable. Accordingly, we have studied the field emission properties of various ZnO structures in both the configuration to elucidate the relation between morphology and geometry of emitter on the field emission behavior.

LaB6 nanofilm is coated on the top surface of silicon nanopillars forming a field emission array (FEA). Such a structure will be referred to subsequently as a ‘LaB6 hat-coated Si-FEA’. The electron emission mechanism of the LaB6 hat-coated Si-FEA is analyzed by OPERA simulation software. It reveals that the electron emission mainly depends on the LaB6 film edge. Moreover, the LaB6 hat-coated Si-FEA sample is prepared by microsphere lithography, and the field emission properties are tested. The results show that when the anode voltage is 850 V, the field emission current density reaches 458 mA cm−2, which is in good agreement with the simulation results. This work demonstrates that the field emission cathodes with low cost, high integration and high current can be realized by using LaB6 as emitter material combined with microsphere lithography technology.

Study of luminous phenomena observed on contaminated metallic surfaces submitted to high RF fields

Two types of graphene cathodes were constructed by doctor blade and ultrasonic atomization spray respectively. The effects of cathodic film morphology and thickness on graphene cathodes' field emission performance were investigated. Ultrasonic atomization spray coated graphene cathodes possess a much lower turn-on and threshold field but much higher emission current density than graphene cathodes coated by doctor blade. The enhanced field emission properties can be ascribed to the suppression of field-screening effect by roughened surface geometry rendered by ultrasonic atomization spray. For ultrasonic atomization sprayed graphene cathodes at a film thickness of 116 μm, an emission current density as large as 29.6 mA cm−2 was achieved at 5 V μm−1, along with a turn-on field and threshold field as low as 1.52 V μm−1 and 2.65 V μm−1, respectively. Emission stability examination shows no visible emission current density fluctuation or decline over a 10 hour operation at ∼3.72 mA cm−2, demonstrating excellent field emission stability for ultrasonic atomization sprayed graphene cathodes. The luminance test also indicates good uniformity of electron emission from ultrasonic atomization sprayed graphene cathodes. The above experiment results indicate that the ultrasonic atomization spray method is suitable for large-area fabrication of high-performance graphene field emitters and holds great potential for applications in field emission displays.

The purpose of this paper is to investigate the applicability of field emission (FE) cathode arrays for a cathodoluminescent display device. We proposed a tetrode structure which has a grid inserted between the Mo emitter FE cathode arrays and the anode. The focusing properties of electron beams were measured using the Y2SiO5:Tb phosphor, and a green emission of about 66,700 cd/m2 was achieved at the anode current of about 90 mA/cm2 and the anode voltage of 8 kV in a vacuum chamber kept at 1×10-9 Torr, in which 14,700 Spindt-type cathode tips were used. The experimental results showed that the dispersion of electron beams can be controlled to a considerable extent by the shape, distance and voltage of the grid. An effect of SO2 gas on the emission property of FE cathode arrays was measured, too.

The mechanism of field emission from a metal surface was well explained based on the quantum mechanics in early 20th century. Since then, various materials have been studied for field emitters. However, so far, we have been using only limited materials as a field emitter and an application in some area requires further scientific understandings and technological advancements. In this paper, we review the current status of researches in field emission and emission phenomena of carbon nanotubes (CNTs). This may include current saturation induced by gas adsorbates, screening effects, high current emission, degradation of emitter, and field enhancement factor. We also introduce the present status in the development of various CNT-based field emission devices and discuss their performances. In this part, various potential applications such as field emission display, ionization gauge, X-ray gun, and lamp will be presented.