Field Emission Characteristics Of Carbon Nanotubes Research Articles

Impact of edge functionalization on electron field-emission characteristics of carbon nanotubes: A theoretical approach

A theoretical approach using first-principle calculations of the density of states (DOS) of functionalized carbon nanotubes (CNTs) is carried out to understand their enhanced electron field emission (FE) properties. The theoretical functionalization of CNTs is perceived using hydrogenation (H/O–H) and oxidation methods. In order to investigate the enhancement in the FE properties of functionalized CNTs, both single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) are studied. The work function is observed to be more decreased in case of zigzag SWCNT (φ = 3.32 for oxygenated SWCNT) as compared to others SWCNT. The dependence of DOS on zigzag/armchair edge functionalization has also been investigated to reveal the impact of DOS on FE. The observed results indicate that edge functionalization of CNTs leads to significant changes in the band structure, DOS, and work functions. The shifting of DOS away from the Fermi level and decrease in work function is evidenced owing to the reduction in the curvature effect of CNT. Also, due to enhanced DOS near the Fermi level, making them a better proposition for FE applications. Our calculations provide an in-depth insight into the functionalization mechanism to understand the enhanced FE properties from them.

Carbon nanotube field emitters on KOVAR substrate modified by random pattern

We investigated the field emission characteristics of patterned carbon nanotubes (CNTs) on KOVAR substrates with different surface morphologies. The substrate with a micro-sized random pattern was fabricated through chemical wet etching, whereas the substrate with a nano-sized random pattern was formed by surface roughening process of polymer and chemical wet etching. The field emission characteristics of these substrates were the compared with those of non-treated substrates. It was clearly revealed that the field emission characteristics of CNTs were influenced by the surface morphology of the cathode substrate. When the surface of cathode was modified by random pattern, the modified substrate provided a large surface area and a wider print area. Also, the modified surface morphology of the cathode provided strong adhesion between the CNT paste and the cathode. Particularly, the substrate with the nano-sized random pattern showed that the turn-on field value decreases and the field enhancement factor value improves as compared with non-treated substrate.

Carbon-Nanotube-Triggered Pseudospark Switch

Pseudospark switches are fast-closing low-pressure gas discharge switches with a hollow-cathode geometry. They can be triggered optically or by electron injection, where electrons emitted from a cold cathode emitter are accelerated in the electric field and initiate breakdown. The efficiency of charge carrier accumulation in the hollow-cathode structure determines the performance of the switch operation. The advantages of such switches include high hold-off voltage, high conduction current, fast current rise time, low delay and jitter time, and long lifetime. In this paper, the operating characteristics of a pseudospark switch, triggered by a carbon-nanotube (CNT)-coated electron emitter, are presented. The electron emitter is fabricated by coating randomly oriented CNTs on silicon substrate using chemical vapor deposition method. Field emission characteristics of CNTs are determined and then used as part of the trigger electrode which is integrated into an in-house constructed pseudospark switch. The operating characteristics of the switch are tested under different background pressures and different trigger voltages. Delay and jitter time under different working conditions are analyzed. Hold-off voltage and current rise time are also presented.

Synthesis of carbon nanotubes on silicon nanowires by thermal chemical vapor deposition

Carbon nanotubes (CNTs) were grown on a silicon nanowire substrate with Ni-catalyst layers of different thicknesses using thermal chemical vapor deposition. Scanning electron microscopy was used to observe the surface morphology, and Raman spectroscopy was used to investigate the structural changes in relation to the thicknesses of the catalyst deposited on the substrate. Field-emission characteristics of CNTs were also correlated with the catalyst thicknesses. The catalyst thickness was found to effectively change the field-emission characteristics of the CNTs. Obvious changes in the diameters, population density, and morphology of CNTs were found with differences in catalyst thickness. The field-emission characteristics of CNTs were dependent on their diameter.

Charge Transfer in the MoS2/Carbon Nanotube Composite

Composite MoS2/carbon nanotube material has been produced by hydrothermal decomposition of a mixture of multiwall carbon nanotubes (CNTs) and a water solution of ammonium molybdate and thiourea. Transmission electron microscopy and Raman spectroscopy showed formation of MoS2 layers on the CNT surface and MoS2 flakes. X-ray photoelectron spectroscopy revealed a downshift of C 1s peak of the composite as compared to the pristine CNT sample that was related to charge transfer between the components. This fact was confirmed by near-edge X-ray absorption fine structure spectroscopy which detected a decrease of intensity of π* resonance in the C K-edge spectrum after the MoS2 deposition. Quantum-chemical calculations of a CNT@MoS2 model showed a positive charging of the CNT surface. Comparison of field emission characteristics of CNTs and the composite indicated lowering of the voltage threshold in the latter sample.

Improvement of Field-Emission Characteristics of Carbon Nanotubes by Post Electrical Treatment

The field-emission characteristics of carbon nanotubes (CNTs), such as uniformity and brightness, were improved by electrical treatment using nonstationary electric fields between the cathode of screen-printed CNT emitters and the anode of a phosphor-coated indium-tin-oxide glass substrate in diode configuration. Dead or weak emission spots, where almost no emission of electrons was observed, started to emit electrons by applying an alternating-current voltage to the cathode electrode and a constant voltage to the anode electrode. The nonstationary electrical treatment was more effective than the direct-current (dc) and the square-pulsed electrical treatments for the emission uniformity and brightness. It was found that the nonstationary electrical treatment not only activated CNT emitters but also suppressed abnormally high emission spots without the drawbacks of electrical breakdown. Consequently, more than 1.8 and 1.3 times improvements of emission uniformity and brightness, respectively, were obtained after the treatment, when compared with the dc electrical treatment for the same amount of emission currents and the same duration of the treatments. Therefore, the method can be effectively applied to field-emission devices based on CNTs for the enhancement of emission properties.

Mechanism of field electron emission from carbon nanotubes

Field electron emission (FE) is a quantum tunneling process in which electrons are injected from materials (usually metals) into a vacuum under the influence of an applied electric field. In order to obtain usable electron current, the conventional way is to increase the local field at the surface of an emitter. For a plane metal emitter with a typical work function of 5 eV, an applied field of over 1 000 V/μm is needed to obtain a significant current. The high working field (and/or the voltage between the electrodes) has been the bottleneck for many applications of the FE technique. Since the 1960s, enormous effort has been devoted to reduce the working macroscopic field (voltage). A widely adopted idea is to sharpen the emitters to get a large surface field enhancement. The materials of emitters should have good electronic conductivity, high melting points, good chemical inertness, and high mechanical stiffness. Carbon nanotubes (CNTs) are built with such needed properties. As a quasi-one-dimensional material, the CNT is expected to have a large surface field enhancement factor. The experiments have proved the excellent FE performance of CNTs. The turn-on field (the macroscopic field for obtaining a density of 10 μA/cm2) of CNT based emitters can be as low as 1 V/μm. However, this turn-on field is too good to be explained by conventional theory. There are other observations, such as the non-linear Fowler-Nordheim plot and multi-peaks field emission energy distribution spectra, indicating that the field enhancement is not the only story in the FE of CNTs. Since the discovery of CNTs, people have employed more serious quantum mechanical methods, including the electronic band theory, tight-binding theory, scattering theory and density function theory, to investigate FE of CNTs. A few theoretical models have been developed at the same time. The multi-walled carbon nanotubes (MWCNTs) should be assembled with a sharp metal needle of nano-scale radius, for which the FE mechanism is more or less clear. Although MWCNTs are more common in present FE applications, the single-walled carbon nanotubes (SWCNTs) are more interesting in the theoretical point of view since the SWCNTs have unique atomic structures and electronic properties. It would be very interesting if people can predict the behavior of the well-defined SWCNTs quantitatively (for MWCNTs, this is currently impossible). The FE as a tunneling process is sensitive to the apex-vacuum potential barrier of CNTs. On the other hand, the barrier could be significantly altered by the redistribution of excessive charges in the micrometer long SWCNTs, which have only one layer of carbon atoms. Therefore, the conventional theories based upon the hypothesis of fixed potential (work function) would not be valid in this quasi-one-dimensional system. In this review, we shall focus on the mechanism that would be responsible for the superior field emission characteristics of CNTs. We shall introduce a multi-scale simulation algorithm that deals with the entire carbon nanotube as well as the substrate as a whole. The simulation for (5, 5) capped SWCNTs with lengths in the order of micrometers is given as an example. The results show that the field dependence of the apex-vacuum electron potential barrier of a long carbon nanotube is a more pronounced effect, besides the local field enhancement phenomenon.

Development of low pressure sensor based on carbon nanotube field emission

A newly developed low pressure detection technology using carbon nanotube (CNT) field emission effect has been designed and manufactured. The fabricated pressure sensor is of a triode type, consisting of a cathode (CNTs field emitter arrays), a grid and a collector. The principle described here is that, for a constant number of electrons available for ionization emitted from CNT arrays by a grid potential, a constant fraction of gas molecules will be ionized and the number of ions collected in a collector will be proportional to the number of gas molecules in the chamber traversed by the electrons. Due to the excellent field emission characteristics of CNT, it is possible to make a cost-effective cold cathode type ion gauge. A screen-printing method has been used to make the CNT cathode. A glass grid with Cr deposited by an e-beam has been put on the cathode with a gap of 200 µm between two electrodes. Due to the voltage applied to the grid, the electrons emitted from the CNTs ionize gas molecules in the chamber and the ionized molecules are gathered to the collector. Then, the collector voltage is made lower than the grid voltage to obtain a large ionization ratio. The current detected in the collector is proportional to the pressure in the chamber. The ionization characteristics are dependent on the gas and the voltage applied to the grid and the collector. In this paper we will show the various metrological characteristics of the home-made pressure sensor utilizing CNTs.

Ionization gauge by carbon nanotube field emission

A newly developed ionization gauge using carbon nanotube (CNT) field emission effect has been designed and manufactured. The fabricated ionization gauge is of a triode type, consisting of a cathode (carbon nanotube field emitter arrays), a grid and a collector. The principle involved here is that for a constant number of electrons available for ionization emitted from carbon nanotube arrays by the grid potential, a constant fraction of gas will be ionized and the number of ions collected in the collector will be proportional to the number of gas molecules in the chamber traversed by the electrons. Due to the excellent field emission characteristics of CNT, it is possible to make a cost effective cold cathode ionization gauge. A screen-printing method has been used to make the CNT cathode. The glass grid with Cr deposited by E-beam has been put on the cathode with a gap of 200 μm between the two electrodes. Using the voltage applied to the grid, the electrons emitted from the carbon nanotube ionize gas molecules in the chamber and the ionized molecules are gathered in the collector. At this time, the collector voltage is maintained at a lower level than that of the grid voltage to obtain a large ionization ratio. The current detected in the collector is proportional to the pressure in the chamber. The ionization characteristics are dependent on the gas and the voltage applied to the grid and collector. In this paper we have shown the various metrological characteristics of the simple pressure sensor utilizing carbon nanotube.

Application of carbon nanotube field emission effect to an ionization gauge

Using the field emission effect of a carbon nanotube (CNT), we characterized a new type of technology for detecting low pressure. The fabricated low pressure sensor is of a triode type, consisting of a cathode (carbon nanotubes field emitter arrays), a grid, and a collector. The gauge described here has a triode configuration similar to that of a conventional hot cathode ionization gauge but also has a cold emission source. Due to the excellent field emission characteristics of CNT, it is possible to make a cost effective cold cathode type ionization gauge. For an effective CNT cathode, we used the screen-printing method and also, we controlled the collector and the grid potentials in order to obtain a high ionization current. We found that the ratio of the ionization current to the CNT cathode current changes according to the pressure in the chamber. In short, we elaborate the various metrological characteristics of a home-made pressure sensor that uses CNTs.

Growth of High-Quality Carbon Nanotubes by Grid-Inserted Plasma-Enhanced Chemical Vapor Deposition for Field Emitters

The effect of a grid inserted between the anode and the cathode in plasma-enhanced chemical vapor deposition (PECVD) on carbon nanotube (CNT) growth was studied. The optimum grid-cathode distance and grid-cathode voltage were 4 mm and 2–10 V, respectively. Long small-diameter nanotubes were grown under these conditions. The smallest nanotube diameter obtained was 3 nm. However, when the grid-cathode voltage was higher than 10 V, short large-diameter nanotubes were grown. The field emission characteristics of CNTs with a low ON voltage and a high current density were explained by Fowler-Nordheim (FN) tunneling.

Growth behavior and interfacial reaction between carbon nanotubes and Si substrate

The effect of the interfacial reaction of Co catalyst with a Si substrate on growth of carbon nanotubes (CNTs) was investigated. Well-aligned multiwall CNTs (MWCNTs) were synthesized and applied barrier layers by microwave plasma-enhanced chemical vapor deposition (MPECVD). Growth proceeded in a flowing mixture of H2, CH4, and N2 as precursors at a temperature of 600 °C and a −200 V substrate bias; Co was sputtered as the catalytic material. Transmission electron microscopy (TEM) and x-ray diffraction were employed to examine the growth behavior of CNTs on Si (100) substrates on which Co had been deposited by MPECVD. The TEM results indicate that discrete conical CoSi2 layers with {111} and (100) faceted interfaces were formed on a Si (100) substrate during CNTs growth. Direct evidence that the growth is by tip growth and base growth is presented. The results show that well-aligned CNTs exhibit a significant emission current. The field emission characteristics of CNTs are contributed to the relationship between the application of different barrier layers and the growth mode of CNTs.

Field-emission characteristics of carbon nanotubes and their applications in photonic devices

Based upon the field-emission investigation of carbon nanotubes, several prototype devices have been suggested that operate with low swing voltages with sufficiently high current densities. Characteristics that allow improved current stability and long operating lifetime as electrical and opto-electronic devices are presented. The aim of this paper is to illustrate the useful characteristics of carbon nanotubes and their possible applications.

Field emission characteristics of carbon nanotubes synthesized by C3H4 and NH3 gases

AbstractUsing a gas mixture of propyne (C3H4) and ammonia (NH3) as a carbon precursor, we have successfully synthesized multiwalled carbon nanotubes (CNTs) by the direct current (dc) plasma enhanced chemical vapor deposition (PECVD) onto Co-sputtered glass at 550°C. As the flow ratio of NH3 to C3H4 in the mixture gas increased, the crystallinity and alignment of CNTs were improved. In addition, the field emission characteristics of CNTs were also improved. the turn-on voltage became lower, and the current density and the field enhancement factor were more increasing. Raman spectroscopy and scanning electron microscopy were utilized to confirm the effect of the gas flow ratio on CNTs. Therefore, the gas flow ratio was found to be one of important factors to govern the crystalline and field emission characteristics of CNTs. The growth mechanism of CNTs using a C3H4 gas is under investigation with the possibility that three carbon atoms in a C3H4 molecule is converted directly to a hexagon of a CNT by combining two molecules.

Field emission characteristics of CNTs synthesized by bias-assisted ICPHFCVD and ICPCVD techniques

Carbon nanotubes (CNTs) were vertically well-grown on Ni/Cr-deposited glass substrates at 580 °C by ICPCVD and bias-assisted ICPHFCVD techniques. The vertically well-aligned CNTs showed multi-walled type with hollow structure. The measured critical current density on CNTs grown by the ICPCVD technique was 1.0×10−6 A cm−2 at 5 V μm−1 of turn-on field and 7.7×10−5 A cm−2 at 7.8 V μm−1 of the critical field. On the other hand, the critical current density on CNTs grown by the bias-assisted ICPHFCVD technique was 3.7×10−7 A cm−2 at 3 V μm of turn-on field and 3.3×10−4 A cm−2 at 6.8 V μm−1 of the critical field, respectively. On comparing the two processes, it can be concluded that CNTs grown by bias-assisted ICPHFCVD are more suitable than those grown by ICPCVD for the possible application of field emission displays (FEDs).