Carbon Nanotube Field Emission Research Articles

Stabilized and Equalized Field-Emission Currents From Carbon Nanotube Emitters by Cascade Active-Current-Control

We have stabilized and equalized field-emission (FE) currents from carbon nanotube (CNT) emitters by a cascade active-current-control (ACC) using two transistors. Theoretical and experimental analyses on the cascade ACC operation, including single ACC with a single transistor, were done for both diode- and triode-configured CNT emitters. A fully vacuum-sealed CNT FE X-ray tube (FEXT) was used as a triode-configured test device. FE currents, in both configurations, by the cascade ACC were found to be stabilized to a standard deviation of 0.05% even in high-voltage and high-power circumstances while the single ACC could hardly regulate fluctuations in FE currents from CNT emitters. Furthermore, an array of CNT FEXTs successfully demonstrated a completely equalized emission current when the cascade ACC was employed. This result has important implications for FEXT current uniformity and other FEXTs applications, because the cascade ACC can equalize FE currents even through large variation of CNT emitters, thereby greatly enhancing the usability of FE devices.

Effect of different vacuum on field emission of carbon nanotube arrays

In this paper, the electron-molecule collision ionization is added to field emission under non-vacuum conditions, and the change of emission current caused by vacuum adjustment in field emission of carbon nanotubes is explained. The field emission current density equation under non-vacuum conditions is established. Through the theoretical analysis and the processing of experimental data, it can be concluded that when other variables are controlled unchanged, the change of pressure will affect the concentration of gas molecules in the air and the collision probability with electrons, then the density of emission current is changed. The study has a certain reference value for the application of field emission in low vacuum and atmospheric pressure.

Design and Fabrication of CNT-Based ${E}$ -Gun Using Stripe-Patterned Alloy Substrate for X-Ray Applications

We report the enhancement of carbon nanotube (CNT) field emitters by growing them directly on stripe-patterned alloy substrates. For a gate voltage of 1300 V, CNT field emitters grown on full and stripe-patterned alloy substrates achieved respective emission currents of 0.22 and 2.32 mA, corresponding to tenfold performance enhancement. Performance enhancement is attributed to the reduced screening effect (as a result of the spacing between strips) during field emission and a damage-free patterning process. To verify the ability of the stripe-patterned CNT field emitters to generate X-rays, we designed and assembled an electron gun through brazing. With the open type X-ray system, an X-ray image of human teeth was successfully obtained, verifying the potential of the stipe-patterned CNT emitters in X-ray applications.

On the quantum mechanics of how an ideal carbon nanotube field emitter can exhibit a constant field enhancement factor

Measurements of current-voltage characteristics from ideal carbon nanotube (CNT) field electron emitters of a small apex radius have shown that these emitters can exhibit a linear Fowler-Nordheim plot [e.g., Dean and Chalamala, Appl. Phys. Lett. 76, 375 (2000)]. From such a plot, a constant (voltage-independent) characteristic field enhancement factor (FEF) can be deduced. Over 15 years later, this experimental result has not yet been convincingly retrieved from first-principles electronic-structure calculations or, more generally, from quantum mechanics (QMs). On the contrary, several QM calculations have deduced that the characteristic FEF should be a function of the macroscopic field applied to the CNT. This apparent contradiction between the experiment and the QM theory has been an unexplained feature of CNT emission science and has raised doubts about the ability of existing QM models to satisfactorily describe experimental CNT emission behavior. In this work, we demonstrate, by means of a density functional theory analysis of single-walled CNTs “floating” in an applied macroscopic field, the following significant result. This is that the agreement between the experiment, classical-conductor CNT models, and QM calculations can be achieved if the latter are used to calculate (from the “real” total-charge-density distributions initially obtained) the distributions of “induced” charge-density, induced local fields, and induced local FEFs. A similar result was previously obtained for so-called vertically aligned CNT/graphene structures. The present work confirms, more reliably and in significantly greater detail than in earlier work on a different system, that this finding applies to the common “post-on-a-conducting plane” situation of CNT field electron emission. This finding also brings out various further theoretical questions that need to be explored.

Fully 3D-printed carbon nanotube field emission electron sources with in-plane gate electrode

We report the design, fabrication, and experimental characterization of the first fully additively manufactured carbon nanotube (CNT) field emission electron sources. The devices are created via direct ink writing (DIW)—one of the least expensive and most versatile additive manufacturing methods, capable of creating monolithic multi-material objects. The devices are 2.5 cm by 2.5 cm glass substrates coated with two imprints, i.e. a trace made of a CNT ink (the emitting electrode), symmetrically surrounded on both sides by a trace made of Ag microparticle ink (the in-plane extractor gate). The CNT ink is a mixture of (–COOH)-functionalized multiwalled CNTs (MWCNTs), N,N-Dimethylformamide, and ethyl cellulose. Optimization of the formulation of the CNT ink resulted in a MWCNT concentration equal to 0.82 wt% and in imprints with an electrical resistivity equal to 0.78 Ω cm. 3D-printed devices having CNT imprints with active length equal to 25 mm (a single, straight trace with 174.5 μm gap between adjacent Ag microparticle imprints) and 135 mm (a square-loop spiral with 499 μm gap between Ag microparticle adjacent imprints) were characterized in a triode configuration (i.e. using an external anode electrode) at ∼2.5 × 10–7 Torr, yielding emission currents as large as 120 μA (60 μA cm−2), start-up voltages as low as 62 V and gate transmission as high as 99%. The low-cost cold cathode technology is compatible with compact applications such as miniaturized mass spectrometry, handheld x-ray generation, and nanosatellite electric propulsion.

Residual Gas Adsorption and Desorption in the Field Emission of Titanium-Coated Carbon Nanotubes.

Titanium (Ti)-coated multiwall carbon nanotubes (CNTs) emitters based on the magnetron sputtering process are demonstrated, and the influences of modification to CNTs on the residual gas adsorption, gas desorption, and their field emission characteristic are discussed. Experimental results show that Ti nanoparticles are easily adsorbed on the surface of CNTs due to the “defects” produced by Ar+ irradiation pretreatment. X-ray photoelectron spectroscopy (XPS) characterization showed that Ti nanoparticles contribute to the adsorption of ambient molecules by changing the chemical bonding between C, Ti, and O. Field emission of CNTs coated with Ti nanoparticles agree well with the Fowler–Nordheim theory. The deviation of emission current under constant voltage is 6.3% and 8.6% for Ti-CNTs and pristine CNTs, respectively. The mass spectrometry analysis illustrated that Ti-coated CNTs have a better adsorption capacity at room temperature, as well as a lower outgassing effect than pristine CNTs after degassing in the process of field emission.

Restoring Observed Classical Behavior of the Carbon Nanotube Field Emission Enhancement Factor from the Electronic Structure

Experimental Fowler-Nordheim plots taken from orthodoxly behaving carbon nanotube (CNT) field electron emitters are known to be linear. This shows that, for such emitters, there exists a characteristic field enhancement factor (FEF) that is constant for a range of applied voltages and applied macroscopic fields $F_\text{M}$. A constant FEF of this kind can be evaluated for classical CNT emitter models by finite-element and other methods, but (apparently contrary to experiment) several past quantum-mechanical (QM) CNT calculations find FEF-values that vary with $F_\text{M}$. A common feature of most such calculations is that they focus only on deriving the CNT real-charge distributions. Here we report on calculations that use density functional theory (DFT) to derive real-charge distributions, and then use these to generate the related induced-charge distributions and related fields and FEFs. We have analysed three carbon nanostructures involving CNT-like nanoprotrusions of various lengths, and have also simulated geometrically equivalent classical emitter models, using finite-element methods. We find that when the DFT-generated local induced FEFs (LIFEFs) are used, the resulting values are effectively independent of macroscopic field, and behave in the same qualitative manner as the classical FEF-values. Further, there is fair to good quantitative agreement between a characteristic FEF determined classically and the equivalent characteristic LIFEF generated via DFT approaches. Although many issues of detail remain to be explored, this appears to be a significant step forwards in linking classical and QM theories of CNT electrostatics. It also shows clearly that, for ideal CNTs, the known experimental constancy of the FEF value for a range of macroscopic fields can also be found in appropriately developed QM theory.

MEMS ion source for ion mobility spectrometry

This paper describes a miniature, silicon-glass, electron-impact ion source, in which a field emission carbon nanotube cathode is separated from the external atmosphere by a semitransparent Si3N4 membrane. High vacuum necessary for the proper operation of the ion source is generated on a chip by an integrated micropump. The experiment revealed that it is possible to obtain and maintain high vacuum inside the structure; however, some problems associated with outgassing occurred during field emission measurements. Despite this fact, both the transmission of electrons through the thin membrane and the possibility of gas ionization have been confirmed.

Highly Crystalline Single-Walled Carbon Nanotube Field Emitters: Energy-Loss-Free High Current Output and Long Durability with High Power

Power consumption reduction and energy savings for field-emission (FE) electron sources for the loading of high current output, long durability, and high power are considerably challenging in the field of materials. Here we show a new approach using a simple structure comprising highly crystalline single-walled carbon nanotubes (SWCNTs) in the cathode for meeting these goals of power consumption and energy efficiency. Efficacy and applicability were successfully demonstrated by revealing the ideal FE properties of SWCNTs via the control of their crystallinity. The FE fluctuation and emission lifetime of highly crystalline SWCNTs exhibited good stability for greater than 1300 h at 30 mA/cm2 and durability with an FE high current density of 10 A/cm2 via the application of a direct current constant voltage in the cathodic planar field emitter. Moreover, field emitters using highly crystalline SWCNTs permitted the use of new vacuum power switches for the loading power operation of greater than 27 kW, with a h...

Cathodoluminescence of a 2 inch ultraviolet-light-source tube based on the integration of AlGaN materials and carbon nanotube field emitters

High efficiency and mass-scale production ultraviolet (UV) light sources have become a basic requirement for various applications, and as such have attracted considerable technological interest.

Low-cost, additively manufactured electron impact gas ionizer with carbon nanotube field emission cathode for compact mass spectrometry

We report the design, fabrication, and experimental characterization of the first additively manufactured electron impact gas ionizer for compact mass spectrometry in the literature. The device occupies a total volume equal to 2.3 cm3 and is composed of a carbon nanotube (CNT) field emission electron source and a multi-material, 3D-printed multi-electrode ion-generating structure. The ionizer’s electron source is a silicon chip coated with a forest of plasma-enhanced chemical vapour deposited CNTs as emitting substrate and a 100 µm thick stainless-steel stencil with thousands of 100 µm diameter apertures at 150 µm aperture pitch as extraction gate. The device’s ion-generating structure is a set of 3D-printed, finely featured dielectric and metallic parts that kindle interaction between electrons and neutrals and shepherd ions outside the structure; its dielectric components were printed in polymer via high-resolution digital light projection stereolithography (25 µm pixels), while its metallic pieces were binder inkjet-printed in SS 316L with features as small as 340 µm (thickness of the wires of the ion cage). The CNT field emission electron sources were experimentally characterized in high vacuum (2.0 × 10−7 Torr), emitting up to 1.4 mA @ 700 V with 31.5% gate transmission. The ionizers were characterized in air at pressures as high as 5 mTorr, generating up 38 µA ion current with 8.5% ionization efficiency. The ion current is linear with pressure, in agreement with the electron impact ionization model.

Fabrication of Stable Carbon Nanotube Cold Cathode Electron Emitters with Post-Growth Electrical Aging.

We fabricated carbon nanotube (CNT) cold cathode emitters with enhanced and stable electron emission properties and long-time stability with electrical aging as a post-treatment. Our CNT field emitters showed improved electrical properties by electrical aging. We set the applied bias for effective electrical aging, with the bias voltage defined at the voltage where Joule heating appeared. At the initial stage of aging, the electron emission current started to increase and then was saturated within 3 h. We understood that 5 h aging time was enough at proper aging bias. If the aging bias is higher, excessive heating damages CNT emitters. With the electrical aging, we obtained improved electron emission current from 3 mA to 6 mA. The current of 6 mA was steadily driven for 9 h.

A Fully Closed Nano-Focus X-Ray Source With Carbon Nanotube Field Emitters

We developed a fully closed nano-focus X-ray source using carbon nanotube (CNT) field emitters. The nano-focus X-ray source includes a fully vacuum-sealed CNT Electron-gun (E-gun) tube and E-beam-focusing modules consisting of electrostatic and magnetic lenses, facilitating very compact and high-resolution X-ray imaging. The paste-printed CNT emitters were prepared in a small pattern as an electron source, and the entire components, including the CNT emitter module, Cu tubes, collimators, and transmissive anode target, were specially brazed at an elevated temperature, resulting in a very compact E-gun tube. The CNT emitter showed a stable and reliable emission current with its density of over 300 mA/cm2 even in the vacuum-sealed E-gun tube. With the anode grounded and the CNT emitter module biased to negative high voltages, we achieved a considerably magnified x-ray image of a several hundred nanometer resolution. The fully vacuum-sealed CNT E-gun tube along with the magnetic lens module can offer much more compact, easily maintainable, high-resolution X-ray imaging compared with conventional open-type thermionic X-ray sources.

Development of an CNT field emission UHV ionization gauge with customized electronics

An ionization vacuum gauge with carbon nanotube (CNT) cathode was designed for measurement of ultra high vacuum (UHV). The sensor was constructed based on Bayard-Alpert type gauge. The key index of the control unit is determined by theoretical analysis and experimental results. A customized detection circuit was designed considering the characteristics of the output ion current of the sensor, the performance of which, such as linearity, noise, bandwidth and gain, were determined by a series of tests and calibrations. At last, the vacuum measurement range and uncertainty of this gauge were determined. The test results showed that the lower limit of the CNT-cathode ionization vacuum gauge enters 10-8Pa, The measurement uncertainty is about 4.0% in the pressure range of 10-4Pa-10-7Pa. This work can provide a new solution for UHV measurement.

Theoretical study of extended interaction frequency‐locking oscillator based on carbon nanotube cold cathodes

An extended interaction frequency-locking oscillator based on carbon nanotube (CNT) cold cathode is proposed to overcome locked-frequency limits of the conventional oscillator. Compared with the conventional oscillators, the oscillation frequency is locked by a modulation electron beam, which can be obtained in a field emission CNT cold cathode electron gun. The frequency-locking signal does not enter the high-frequency (HF) system but imposes an additional HF electric field on the cathode surface by a microstrip structure, which consumes considerably less power to lock the oscillation frequency. A ladder structure extended interaction oscillator operating in 2π mode is numerically investigated by three-dimensional Particle-In-Cell simulation code. By analysing the impacts of different frequency-locking power on the locked ranges, the results show that the average output power of 30.6 W is achieved at 35.11 GHz when the frequency-locking power consumption is 460 mW. The 3-dB bandwidth of a frequency-locking region reaches 100 MHz.

Temperature Comparison of Looped and Vertical Carbon Nanotube Fibers during Field Emission

Carbon nanotube (CNT) fiber-based emitters have shown great potential to deliver stable, high current beams for various potential applications. Because of joule heating, CNT field emitters are heated to high temperatures during field emission. It is important to improve the thermal management of emitters to increase their reliability and prevent premature failure. This paper compares the field emission characteristics and the temperature distribution of a new configuration of a looped CNT fiber emitter with a traditional single vertical CNT fiber emitter. It is found that the maximum temperature of the looped fiber emitter (~300 °C) is significantly reduced compared to that of the vertical fiber (~600 °C) at the same emission current of 3 mA. The experimentally measured temperature distribution is compared with a recent theory on joule heating of a one-dimensional conductor. This study provides new insights into the design of high performance field emitters.

Carbon Nanotube Field Emitters Synthesized on Metal Alloy Substrate by PECVD for Customized Compact Field Emission Devices to Be Used in X-Ray Source Applications.

In this study, a simple, efficient, and economical process is reported for the direct synthesis of carbon nanotube (CNT) field emitters on metal alloy. Given that CNT field emitters can be customized with ease for compact and cold field emission devices, they are promising replacements for thermionic emitters in widely accessible X-ray source electron guns. High performance CNT emitter samples were prepared in optimized plasma conditions through the plasma-enhanced chemical vapor deposition (PECVD) process and subsequently characterized by using a scanning electron microscope, tunneling electron microscope, and Raman spectroscopy. For the cathode current, field emission (FE) characteristics with respective turn on (1 μA/cm2) and threshold (1 mA/cm2) field of 2.84 and 4.05 V/μm were obtained. For a field of 5.24 V/μm, maximum current density of 7 mA/cm2 was achieved and a field enhancement factor β of 2838 was calculated. In addition, the CNT emitters sustained a current density of 6.7 mA/cm2 for 420 min under a field of 5.2 V/μm, confirming good operational stability. Finally, an X-ray generated image of an integrated circuit was taken using the compact field emission device developed herein.

Use of tapered Pyrex capillary tubes to increase the mechanical stability of multiwall carbon nanotubes field emitters

In this study, NanocylTM NC 7000 Thin Multiwall Carbon Nanotubes (MWCNTs) were used with a high aspect ratio (>150) made by the process of catalytic chemical vapor deposition (CCVD). The field emitter tips were prepared by inserting these MWCT into fine glass capillary tubes that were pulled at high temperatures and then cut. Measurements were carried out under ultra-high vacuum (UHV) conditions with a base pressure of 10−9 mbar. The data show the effects of initial conditioning of MWCNT and hysteresis. Compression of the MWCNT by the capillary tubes appears to provide adequate mechanical support without requiring the use of a low-melting point electrically-conductive binder as has been used previously. Emission currents in excess of 1 μA were obtained so this technique shows promise as a reliable, stable, powerful electron source.

Fine tuning of the color of light emitted from Sr1-xCaxS:Mn2+ phosphors for CNT-FEDs

Sr1-xCaxS:Mn2+ phosphors with calcium ion mole fraction (x) values of 0, 0.2, 0.4, 0.6, 0.8, and 1 were synthesized from CaS, SrS, and MnS by carrying out solid-state reactions. Cathodoluminescence (CL) spectra of these phosphors were acquired using moderate voltages in the range 4–10 kV in order to assess their applicability to carbon nanotube field emission displays (CNT-FEDs). The wavelength at the maximum intensity of the CL spectra of the phosphors increases gradually from 549 to 602 nm, and then slightly decreases to 599 nm, as x is increased. This increase can be interpreted in terms of the crystal structures of the phosphors. The ability to tune the color of light emitted from the Sr1-xCaxS:Mn2+ phosphors from greenish yellow to red confirms the promise of these phosphors for use in CNT-FEDs.

Field emission properties of SiO2-wrapped CNT field emitter

Carbon nanotubes (CNTs) exhibit unstable field emission (FE) behavior with low reliability due to uneven heights of as-grown CNTs. It has been reported that a mechanically polished SiO2-wrapped CNT field emitter gives consistent FE performance due to its uniform CNT heights. However, there are still a lack of studies on the comparison between the FE properties of freestanding and SiO2-wrapped CNTs. In this study, we have performed a comparative study on the FE properties of freestanding and SiO2-wrapped CNT field emitters. From the FE measurements, freestanding CNT field emitter requires lower applied voltage of 5.5 V μm−1 to achieve FE current density of 22 mA cm−2; whereas SiO2-wrapped field emitter requires 8.5 V μm−1 to achieve the same current density. This can be attributed to the lower CNT tip electric field of CNTs embedded in SiO2, as obtained from the electric field simulation. Nevertheless, SiO2-wrapped CNTs show higher consistency in FE current than freestanding CNTs. Under repeated FE measurement, SiO2-wrapped CNT field emitter achieves consistent FE behavior from the 1st voltage sweep, whereas freestanding field emitter only achieved consistent FE performance after 3rd voltage sweep. At the same time, SiO2-wrapped CNTs exhibit better emission stability than freestanding CNTs over 4000 s continuous emission.