Carbon Nanotube Field Emission Research Articles

Enhancement of Emission Lifetime of CNT Emitters by Coating ZnO on the CNT Surface.

Carbon nanotubes (CNTs) have been investigated as field-emission sources owing to their high electrical conductivity and high aspect ratio. However, practical applications demand that the emission lifetime of CNTs be further improved. Since ZnO demonstrates impressive electrical and thermal conductivity, when coated on the surface of CNTs, it can allow the CNT field emitters to endure high electrical stress and high temperature. Moreover, ZnO nanostructures protect the CNT emitters from being bombarded by high-energy ions, which are accelerated by the high electric field. From the result of emission lifetime measurements at the emission current density of 100 mA/cm2, we found that the emission lifetime was increased by more than a factor of 2 when ZnO had been coated onto the CNT emitters. The observation registers as an important contribution to the practical application of CNT emitters with long-term emission stability, as well as with high emission currents. In this work, we elucidate the detailed mechanism of long-term stability that can be achieved by coating ZnO nanostructures on the surface of CNTs.

High-current field-emission of carbon nanotubes and its application as a fast-imaging X-ray source

The high-current field-emission (FE) characteristics of carbon nanotubes (CNTs) are reported. Powders of bismuth (Bi) were mixed with the CNTs and baked at 400°C, and because the melting point of Bi is low, the CNTs bonded strongly to the Bi blocks. In the FE characterization, the turn-on electric field was 1.54V/μm, with current density 1μA/cm2, and the threshold electric field was 4.24V/μm, with current density 1mA/cm2. Because the Bi blocks played a role in enhancing the adhesion of CNTs to the substrate and improving the FE performance, the stable FE current reached 88mA when the electric field was increased to 12.75V/μm; the current density was 11.2A/cm2. A fast-imaging X-ray tube with CNT field emitters was fabricated. The metal mesh was used as the gate electrode, which was designed to be cooled in oil in order to switch the electron beam rapidly. The X-ray image of a vibrating metronome was captured clearly when the exposure time was decreased to 20ms, showing that high-current CNT field emitters can be used to obtain X-ray images of moving objects.

Chemical ionization mass spectrometry using carbon nanotube field emission electron sources.

A novel chemical ionization (CI) source has been developed based on a carbon nanotube (CNT) field emission electron source. The CNT-based electron source was evaluated and compared with a standard filament thermionic electron source in a commercial explosives trace detection desktop mass spectrometer. This work demonstrates the first reported use of a CNT-based ion source capable of collecting CI mass spectra. Both positive and negative modes were investigated. Spectra were collected for a standard mass spectrometer calibration compound, perfluorotributylamine (PFTBA), as well as trace explosives including trinitrotoluene (TNT), Research Department explosive (RDX), and pentaerythritol tetranitrate (PETN). The electrical characteristics, lifetime at operating pressure, and power requirements of the CNT-based electron source are reported. The CNT field emission electron sources demonstrated an average lifetime of 320 h when operated in constant emission mode under elevated CI pressures. The ability of the CNT field emission source to cycle on and off can provide enhanced lifetime and reduced power consumption without sacrificing performance and detection capabilities. Graphical Abstract ᅟ.

Effect of substrate heating and microwave attenuation on the catalyst free growth and field emission of carbon nanotubes

Carbon nanotubes (CNTs) have been directly grown on Inconel 600 substrates by microwave plasma enhanced chemical vapor deposition without using any external catalyst. Grown CNTs were characterized by field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy and field emission measurements. Characterization results show that field emission current density increases from 200μA/cm2 at ∼5.5V/μm to 14.5mA/cm2 at ∼1.6V/μA when substrate is heat-treated and incident microwave is attenuated before reaching it. Detailed characterization reveals that heat-treatment results in migration of Cr and Fe oxides towards the top surface which completely changes substrate morphology also. Microwave attenuation reduces reflection of microwaves from the substrate and increases residence time of the precursor over the substrate promoting high density growth of CNTs. The combination of these two process parameters resulted in growth of long, dense CNTs with bamboo-like defects that contributes to enhanced current density at lower applied field.

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.

X-ray source using carbon-nanotube field emitter with side-gate electrode

A carbon-nanotube (CNT) field emitter with side-gate electrodes, which are placed in-plane with the CNT emitters, is one of the most promising structures to realize a large-area X-ray charge neutralizer with a cost-effective and easy process. An X-ray source using the screen-printed CNT field emitter with side-gate electrodes is developed. The field-emission and X-ray properties are evaluated with anode voltages of 5 and 10 kV. It is shown that the emission current can be controlled by the side-gate voltage even with such high anode voltages as 5 and 10 kV. It is reported that the highest anode current is approximately 1 µA, which is high enough for an X-ray charge neutralizer. The X-ray radiation is successfully controlled by the pulsed side-gate voltage with subsecond length.

A 46-inch diagonal carbon nanotube field emission backlight for liquid crystal display

A large area carbon nanotube field emission backlight that is built with a new cathode structure is reported. The approach involves unique gate insulator formation by glass etching, highly populated multi wall nanotube tips, and gate electrode assembly by anodic bonding. Impressive lamping performances are noted through dynamic control of the built in fine 4320 local dimming blocks as small as 1cm2. The liquid crystal display lit by the backlight demonstrates superior luminance uniformity 97%, and a native contrast ratio 400,000:1, whilst showing less than 10% decay in emission current of the sealed panel during continuous operation over 8000h.

A multifinger microtriode with carbon nanotubes field emission cathode operating at GHz frequency

Vacuum microelectronic devices play an important role in the field of micro- and nano-electronics and they have been strongly developed in recent decades. Vacuum microelectronics are mainly based on the field emission effect and the employment of electrons in vacuum in a device with dimensions from tenths to hundredths of a micrometer. In this work, we present the development of a carbon-nanotube-based multifinger microtriode operating from 0.5 to 2 GHz. In this frequency range, a minimum RF signal gain of 5 dB is achieved. Such a device represents an optimized alternative to the standard Spindt-type microtriode. The advantage of such multifinger architecture consists in the possibility to reduce the cathode-grid capacitance by reducing the overlap between the two electrodes using a parallel patterning. This approach allows increasing the cut-off frequency of the devices with respect to the Spindt-type triode. We realized a prototype of the multifinger triode and the field emission properties have been characterized. The frequency behavior has been measured, demonstrating the possibility to amplify RF signal.

High Field Emission Current Density from Patterned Carbon Nanotube Field Emitter Arrays with Random Growth.

High field emission (FE) current density from carbon nanotube (CNT) arrays grown on lithographically patterned silicon substrates is reported. A typical patterned field emitter array consists of bundles of nanotubes separated by a fixed gap and spread over the entire emission area. Emission performance from such an array having randomly oriented nanotube growth within each bundle is reported for different bundle sizes and separations. One typical sample with aligned CNTs within the bundle is also examined for comparison. It is seen that the current density from an array having random nanotube growth within the bundles is appreciably higher as compared to its aligned counterpart. The influence of structure on FE current densities as revealed by Raman spectroscopy is also seen. It is also observed that current density depends on edge length and increases with the same for all samples under study. Highest current density of -100 mA cm(-2) at an applied field of 5 V/μm is achieved from the random growth patterned sample with a bundle size of 2 μm and spacing of 4 μm between the bundles.

Low-Temperature Preparation of a Carbon Nanotube–ZnO Hybrid on Glass Substrate for Field Emission Applications

A new method for preparation of a carbon nanotube (CNT)–zinc oxide hybrid on a glass substrate at low temperature is introduced and the stability improvement of field emission of CNTs is reported. The emission current stability and substrate material are the two main commercial parameters for field emission applications of CNTs. The two alternative current (AC) and direct current (DC) plasma enhancement chemical vapor deposition (PECVD) is used to achieve low-temperature growth of vertically aligned CNTs and the hydrothermal processing is used for production of zinc oxide nanowires to improve the emission current stability and properties. The nanoscale junction between a semiconductor and conductive material are critical for electronic applications. The present study examined the field emission properties of CNTs near the crystalline structure of the zinc oxide hybrids and showed the improvement in the field enhancement factor and emission current stability of CNT– ZnO hybrid than the CNT emitters.

High performance field emission of carbon nanotube film emitters with a triangular shape

We report novel two-dimensional (2D) shaped carbon nanotube (CNT) field emitters using triangular-shaped CNT films and their field emission properties. Using the 2D shaped CNT field emitters, we achieved remarkable field emission performance with a high emission current of 22mA (equivalent to an emission current density >105A/cm2) and long-term emission stability at 1mA for 20h. We also discuss the field emission behavior of the 2D shaped CNT field emitter in detail.

High current field emission from individual non-linear resistor ballasted carbon nanotube cluster array

Field emission (FE) electron sources based on carbon nanotubes (CNTs) have the potential to serve as cold cathodes for various vacuum microelectronic and nanoelectronic devices. Emission currents are extremely sensitive to variation in emitter geometry and local surface states, both of which are difficult to synthesize uniformly when fabricating a CNT field emission array (FEA). Such non-uniformities cause unstable emission, limiting the current output. Here, we propose a method for simulating and fabricating a high performance CNT-FEA with emission units that are individually connected to a single crystalline silicon pillar (SP), which acts as an non-linear ballast resistor. Results showed that the driving field for this CNT-FEA was greatly reduced relative to CNT-FEAs on a flat silicon substrate. This improvement was due to the high aspect ratio of the CNT clusters combined with SPs. The FE behavior demonstrated that the emission current was limited by the non-linear resistors (NLRs). Emitted currents density over 1.65A/cm2 at a low extraction field of 5.8V/μm were produced by a 1mm2 emmiting area. The proposed technology may be used to fabricate cathodes capable of reliable, uniform, and high current emission.

MEMS Carbon Nanotubes Field Emission Pressure Sensor With Simplified Design: Performance and Field Emission Properties Study

The pressure sensor application gained recently substantial interest in many fields of basic and applied research and applications. In this paper, microelectromechanical system (MEMS)-based pressure sensor contains nanostructured electrode consisting of carbon nanotube (CNT) array. CNTs are directly grown on such electrode by plasma-enhanced chemical vapor deposition method using microwave plasma torch at atmospheric pressure. This growth method enables us to use a simple electrode structure without need of buffer layer and time-consuming lithography process. Combination of CNTs field emission and MEMS membrane mechanical properties make possible to enhance sensitivity of the sensor. Field emission properties of CNTs are measured by newly developed system enabling us precise measurement of expecting properties, such as dependence on diaphragm (upper electrode) distance, applied voltage, and stability of the sensor. Measured values are compared with a numerical modeling of the membrane system in Coventor Ware software by finite-element method. We also suggest encapsulating the sensor using glass frit bonding because such method is more suitable for high vacuum requirements of the field emission operation.

Effect of Ar-ion Irradiation on the Field-emission Properties of Screen-printed Carbon-nanotube Field Emitters

Effect of Ar-ion Irradiation on the Field-emission Properties of Screen-printed Carbon-nanotube Field Emitters

Operation of a Carbon Nanotube Field Emitter Array in a Hall Effect Thruster Plume Environment

The Hall effect thruster is an electric propulsion device for space applications that efficiently reduces the propellant mass requirements on a spacecraft in comparison with chemical rockets. To date, the Hall effect thruster technology relies on the thermionic cathodes that consume up to 10% of the total propellant used in the system to neutralize the ion beam of the thruster; however, such propellant usage does not contribute to thrust generation. An array of thin-film, carbon nanotube field emitters that emit electrons through field emission can potentially neutralize the ion beam without consuming any propellant. This paper examines the effects of 40 min exposure of carbon nanotube field emitter arrays to the plasma environment in the exit plane of a Hall effect thruster. The physical structures that enable field emission appear largely unaffected by placement in the plasma as well as operation in the plasma. This indicates that a refined design of this carbon nanotube field emitter array may potentially provide an alternative to the thermionic cathode used on contemporary Hall effect thrusters and verifies that no fundamental incompatibilities exist between these two technologies.

Stationary chest tomosynthesis using a carbon nanotube x-ray source array: a feasibility study

Chest tomosynthesis is a low-dose, quasi-3D imaging modality that has been demonstrated to improve the detection sensitivity for small lung nodules, compared to 2D chest radiography. The purpose of this study is to investigate the feasibility and system requirements of stationary chest tomosynthesis (s-DCT) using a spatially distributed carbon nanotube (CNT) x-ray source array, where the projection images are collected by electronically activating individual x-ray focal spots in the source array without mechanical motion of the x-ray source, detector, or the patient. A bench-top system was constructed using an existing CNT field emission source array and a flat panel detector. The tube output, beam quality, focal spot size, system in-plane and in-depth resolution were characterized. Tomosynthesis slices of an anthropomorphic chest phantom were reconstructed for image quality assessment. All 75 CNT sources in the source array were shown to operate reliably at 80 kVp and 5 mA tube current. Source-to-source consistency in the tube current and focal spot size was observed. The incident air kerma reading per mAs was measured as 74.47 uGy mAs−1 at 100 cm. The first half value layer of the beam was 3 mm aluminum. An average focal spot size of 2.5 × 0.5 mm was measured. The system MTF was measured to be 1.7 cycles mm−1 along the scanning direction, and 3.4 cycles mm−1 perpendicular to the scanning direction. As the angular coverage of 11.6°–34°, the full width at half maximum of the artifact spread function improved greatly from 9.5 to 5.2 mm. The reconstructed tomosynthesis slices clearly show airways and pulmonary vascular structures in the anthropomorphic lung phantom. The results show the CNT source array is capable of generating sufficient dose for chest tomosynthesis imaging. The results obtained so far suggest an s-DCT using a distributed CNT x-ray source array is feasible.

Fabrication of Carbon Nanotube Field-Emission Cathodes by Laser-Induced Transfer of Carbon Nanotubes and Silver Paste

A new laser material-transfer scheme for transfer printing both the silver-paste and the carbon-nanotube (CNT) thin film in pattern simultaneously is reported. The transfer mechanism is discussed and demonstrated to be unlike to those conventional laser-transfer approaches which only the material on the laser-irradiated region is transferable, the present scheme is capable of transferring the materials on the unexposed locations as well. The transferred CNTs are directly on the top of the silver paste which is a suitable configuration for application to the CNT field-emission cathodes. Measurements demonstrate that the fabricated CNT field-emission cathodes exhibit a low turn-on field of 2.5 V/μm at current density of 10 μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A lifetime test also demonstrates that the device is stable. By a single laser shot, this study successfully demonstrates that the transferred CNTs are preserved on the silver paste's surface that requires no subsequent post-treatment processes. Moreover, all the fabrication steps are executable at room temperature and in the ambient environment.

Great improvement in adhesion and uniformity of carbon nanotube field emitters through reactive nanometer-scale SiC fillers

We developed highly adhesive uniform field emitters (FEs) from carbon nanotube (CNT) pastes by reacting nanometer-scale silicon carbide (SiC) fillers on a Kovar substrate at a high temperature in vacuum. The reaction of SiC on Kovar results in significant morphological changes at the substrate–composite interface along with moderate Si diffusion into the substrate, enhancing adhesion of CNT FEs to the substrate greatly. Furthermore, a post surface-treatment after the reaction of SiC fillers results in very uniform CNT FEs over an entire emitter pattern of several hundred micrometers. The strongly enhanced adhesion and uniformity of the CNT FEs, in turn, give stable and reliable field emission even at a high current density. The applicability of the SiC/CNT FEs was evaluated in super-miniature X-ray tubes where any detachment of materials including outgassing from the inner side of the tubes should be avoided. The smallest super-miniature CNT X-ray tube to date, with an outer diameter of 2.0mm, shows good operation with X-ray imaging at an anode voltage of above 25kV, confirming almost no outgassing and strong substrate adhesion of the CNT FEs.

Study on the Driving Circuit of Carbon Nanotube Field Emission Display Based on Luminance Control

Carbon nanotube field emission display (CNT-FED) is one of the most significant subjects due to its unique qualities and perfect performance. But there are still some problems in FED, for example, the modulation of each pixel unit of field emission display device is discrete, and the traditional voltage pulse-width modulation driving mode cannot solve luminance non-uniformity and non-linearity of FED. So a novel driving circuit based on cathode current source is proposed. The current driving circuit can be fabricated on Si substrate in advance, and then carbon nanotube is grown at room temperature, carbon nanotube and constant current source circuits are integrated on the same Si substrate. Current source circuit and cathode emission part are integrated together, which not only can solve the FED luminance problem, but also can meet FED thin design.

Excellent field emission properties from carbon nanotube field emitters fabricated using a filtration-taping method

Abstract A filtration-taping method was demonstrated to fabricate carbon nanotube (CNT) emitters. This method shows many good features, including high mechanical adhesion, good electrical contact, low temperature, organic-free, low cost, large size, and suitability for various CNT materials and substrates. These good features promise an advanced field emission performance with a turn-on field of 0.88 V/mm at a current density of 0.1 mA/cm 2 , a threshold field of 1.98 V/mm at a current density of 1 mA/cm 2 , and a good stability of over 20 h. The filtration-taping technique is an effective way to realize low-cost, large-size, and high-performance CNT emitters. Key words: field emission properties, carbon nanotube, filtration-taping method 1. Introduction Due to their unique geometry and physical properties, carbon nanotubes (CNTs) are currently being investigated as next-generation field emitter materials for cold cathodes. Numerous papers can be found on field emission applications such as field emission displays, lamps, X-ray sources, high-resolution electron-beam instruments, and microwave amplifiers [1-5]. Up to now, various techniques have been developed for fabricating CNT field emitters using as-grown, spraying, electrophoresis, screen-printing, and attaching individual CNTs on tungsten probe methods, etc. [6-10]. However, these techniques suffer from weak me-chanical adhesion between CNT emitters and the cathode, or severe degradation of CNT emitters due to the organic binders used in the process, which lead to a short lifetime. There-fore, an efficient and organic binder-free technique for fabricating CNT emitters is still a great challenge. In this letter, we demonstrate an advanced organic-free technique named the filtration-taping method. The filtration-taping method is suitable to fabricate simple, large, and cost-effective CNT emitters that can ensure a high mechanical adhesion, good electri-cal contact, and damage-free CNT tips, resulting in enhanced field emission properties and excellent stability.