Carbon Nanotube Field Emission Research Articles

Effect of Crystallinity on the Field Emission Characteristics of Carbon Nanotube Grown on W-Co Bimetallic Catalyst

Carbon nanotube (CNT) is an excellent field emission material. However, uniformity and stability are the key issues hampering its device application. In this work, a bimetallic W-Co alloy was adopted as the catalyst of CNT in chemical vapor deposition process. The high melting point and stable crystal structure of W-Co helps to increase the grown CNT diameter uniformity and homogeneous crystal structure. High-crystallinity CNTs were grown on the W-Co bimetallic catalyst. Its field emission characteristics demonstrated a low turn-on field, high current density, stable current stability, and uniform emission distribution. The Fowler–Nordheim (FN) and Seppen–Katamuki (SK) analyses revealed that the CNT grown on the W-Co catalyst has a relatively low work function and high field enhancement factor. The high crystallinity and homogeneous crystal structure of CNT also reduce the body resistance and increase the emission current stability and maximum current. The result provides a way to synthesis a high-quality CNT field emitter, which will accelerate the development of cold cathode vacuum electronic device application.

A carbon nanotube x-ray source array designed for a new multisource cone beam computed tomography scanner

Cone beam computed tomography (CBCT) is known to suffer from strong scatter and cone beam artifacts. The purpose of this study is to develop and characterize a rapidly scanning carbon nanotube (CNT) field emission x-ray source array to enable a multisource CBCT (ms-CBCT) image acquisition scheme which has been demonstrated to overcome these limitations. A CNT x-ray source array with eight evenly spaced focal spots was designed and fabricated for a medium field of view ms-CBCT for maxillofacial imaging. An external multisource collimator was used to confine the radiation from each focal spot to a narrow cone angle. For ms-CBCT imaging, the array was placed in the axial direction and rapidly scanned while rotating continuously around the object with a flat panel detector. The x-ray beam profile, temporal and spatial resolutions, energy and dose rate were characterized and evaluated for maxillofacial imaging. The CNT x-ray source array achieved a consistent focal spot size of 1.10 ± 0.04 mm × 0.84 ± 0.03 mm and individual beam cone angle of 2.4° ± 0.08 after collimation. The x-ray beams were rapidly switched with a rising and damping times of 0.21 ms and 0.19 ms, respectively. Under the designed operating condition of 110 kVp and 15 mA, a dose rate of 8245 μGy s−1 was obtained at the detector surface with the inherent Al filtration and 2312 μGy s−1 with an additional 0.3 mm Cu filter. There was negligible change of the x-ray dose rate over many operating cycles. A ms-CBCT scan of an adult head phantom was completed in 14.4 s total exposure time for the imaging dose in the range of that of a clinical CBCT scanner. A spatially distributed CNT x-ray source array was designed and fabricated. It has enabled a new multisource CBCT to overcome some of the main inherent limitations of the conventional CBCT.

Imprinted CNTs field emission cathode for electron brushing

Microchannel plates (MCPs) are the key components for low-level light night vision systems. Electron brushing is a crucial processing for MCPs manufacturing. In this paper, an imprinted carbon nanotubes (CNTs) field emission cathode is fabricated and tested. And the electron brushing experiment based on this cathode is carried out. The result shows that the imprinted CNTs cathode has some advantages and can be used in electron brushing. This research lays the foundation for massive, low cost and highly efficient electron brushing applications in the future.

Boosting field electron emission of carbon nanotubes through small-hole-patterning design of the substrate

High field emission of carbon nanotube (CNT) cold cathodes is realized by printing modified CNT paste on small-hole-patterned substrates. The field emission characteristics and stability of samples under DC continuous and pulse driving modes have been investigated. The results show that the maximum emission current of CNT emitters can be up to 45 mA at an electric field of 2100 V (7.0 V μm−1), corresponding to a high current density of 643 mA cm−2 under continuous mode. The cathodes also demonstrate stable electron emission without obvious attenuation. In pulse (10 μs and 200 Hz) mode, the peak current can reach 250 mA and the corresponding current density is 3.57 A cm−2 under an electric field of 14.0 V μm−1. The hole-patterned CNT cathode presents unique advantages in field emission current, stability and especially endurance of high electric field. This work makes it possible to fabricate highly efficient emission CNT cold electron sources, which have broad application prospects in vacuum electronic devices requiring both large current and high current density.

Field electron emission in the atmospheric pressure range

Field emission is a well-known technology for generating electrons under vacuum conditions. Here we assess whether gated silicon field emitter microstructures, originally developed for use in space, can ionise air for electroaerodynamic propulsion and other applications. Electron range in air is compared to breakdown voltage over the atmospheric pressure range, to evaluate whether an operational region exists for which the geometry permits electrons to escape and ionise, whilst keeping the voltage low enough to avoid breakdown. An operational region is identified between 500–100 V for pressures corresponding to 0–20 km altitude. This offers low-power and low-mass operation in comparison to existing ionisation technologies. Carbon nanotube field emitters may offer enhanced performance over the silicon emitters. Field electron emission represents a new air ionisation technology which may be appropriate for light-weight, high-altitude aircraft propulsion.

Volumetric computed tomography with carbon nanotube X-ray source array for improved image quality and accuracy

Cone beam computed tomography (CBCT) is widely used in medical and dental imaging. Compared to a multidetector CT, it provides volumetric images with high isotropic resolution at a reduced radiation dose, cost and footprint without the need for patient translation. The current CBCT has several intrinsic limitations including reduced soft tissue contrast, inaccurate quantification of X-ray attenuation, image distortions and artefacts, which have limited its clinical applications primarily to imaging hard tissues and made quantitative analysis challenging. Here we report a multisource CBCT (ms-CBCT) which overcomes the shortcomings of the conventional CBCT by using multiple narrowly collimated and rapidly scanning X-ray beams from a carbon nanotube field emission source array. Phantom imaging studies show that, the ms-CBCT increases the accuracy of the Hounsfield unit values by 60%, eliminates the cone beam artefacts, extends the axial coverage, and improves the soft tissue contrast-to-noise ratio by 30–50%, compared to the CBCT configuration.

X-ray source arrays for volumetric imaging during radiotherapy treatment

This work presents a novel hardware configuration for radiotherapy systems to enable fast 3D X-ray imaging before and during treatment delivery. Standard external beam radiotherapy linear accelerators (linacs) have a single X-ray source and detector located at ± 90° from the treatment beam respectively. The entire system can be rotated around the patient acquiring multiple 2D X-ray images to create a 3D cone-beam Computed Tomography (CBCT) image before treatment delivery to ensure the tumour and surrounding organs align with the treatment plan. Scanning with a single source is slow relative to patient respiration or breath holds and cannot be performed during treatment delivery, limiting treatment delivery accuracy in the presence of patient motion and excluding some patients from concentrated treatment plans that would be otherwise expected to have improved outcomes. This simulation study investigated whether recent advances in carbon nanotube (CNT) field emission source arrays, high frame rate (60 Hz) flat panel detectors and compressed sensing reconstruction algorithms could circumvent imaging limitations of current linacs. We investigated a novel hardware configuration incorporating source arrays and high frame rate detectors into an otherwise standard linac. We investigated four potential pre-treatment scan protocols that could be achieved in a 17 s breath hold or 2–10 1 s breath holds. Finally, we demonstrated for the first time volumetric X-ray imaging during treatment delivery by using source arrays, high frame rate detectors and compressed sensing. Image quality was assessed quantitatively over the CBCT geometric field of view as well as across each axis through the tumour centroid. Our results demonstrate that source array imaging enables larger volumes to be imaged with acquisitions as short as 1 s albeit with reduced image quality arising from lower photon flux and shorter imaging arcs.

Proof-of-Concept Vacuum Microelectronic NOR Gate Fabricated Using Microelectromechanical Systems and Carbon Nanotube Field Emitters.

This paper demonstrates a fully integrated vacuum microelectronic NOR logic gate fabricated using microfabricated polysilicon panels oriented perpendicular to the device substrate with integrated carbon nanotube (CNT) field emission cathodes. The vacuum microelectronic NOR logic gate consists of two parallel vacuum tetrodes fabricated using the polysilicon Multi-User MEMS Processes (polyMUMPs). Each tetrode of the vacuum microelectronic NOR gate demonstrated transistor-like performance but with a low transconductance of 7.6 × 10-9 S as current saturation was not achieved due to a coupling effect between the anode voltage and cathode current. With both tetrodes working in parallel, the NOR logic capabilities were demonstrated. However, the device exhibited asymmetric performance due to differences in the CNT emitter performance in each tetrode. Because vacuum microelectronic devices are attractive for use in high radiation environments, to test the radiation survivability of this device platform, we demonstrated the function of a simplified diode device structure during exposure to gamma radiation at a rate of 45.6 rad(Si)/second. These devices represent a proof-of-concept for a platform that can be used to build intricate vacuum microelectronic logic devices for use in high-radiation environments.

Growth and integration of aligned carbon nanotube-based field emission cathode for electron gun device-level fabrication

This study elucidates on growth and integration of aligned carbon nanotube-based field emission cathode for electron gun-device fabrication. Particularly, it emphasises on field emission (FE) and thermal transport (TT) properties of carbon nanotube (CNT) arrays with controlled atomistic defects (AD) to design a cold cathode (CC) for electron gun. To assess the influence of AD on FE and TT, aligned undoped (pure CNT) and N-doped herringbone (doped CNT) were fabricated using droplet-assisted solvent-thermolysis to use them as CC. Results demonstrated that doped CNT with higher degree of AD exhibited a lower turn-on field in FE in comparison to pure CNT could be ascribed to N addition in carbon lattice. Ambiguously, doping-mediated AD in doped CNT helps to achieve the best FE but lowers TT with significant decrease in thermal conductivity (111 to 5.1 W/m.k at room temperature) in comparison to pure CNT. Based on the FE behavior, a doped CNT arrays-based cathode was integrated into an electron gun, which exhibited maximum emission current of 81.3 mA/cm2 at a grid voltage of 2 kV.

The effects of Ar + N2 plasma power-based attachment of metal nanoparticles on the electron field emission properties of carbon nanotubes

An ideal vacuum-based electron emitter requires a low turn-on field and quick emission processes. Carbon nanotubes (CNTs) have great potential for emission applications, but have some limitations that affect their performance. Consequently, they can benefit from the alteration of their surface morphology upon decoration with plasma-based metal nanoparticles. In this work, we have attached iron nanoparticles on the CNTs surface via a radio frequency (rf) sputtering process using Ar + N2 gas with various plasma power in steps of 50 W. Field emission scanning electron microscopy (FESEM) micrographs have confirmed the attachment of the iron nanoparticles and etching of the nanotubes surface at high plasma power. The Raman spectra strongly confirm the formation of defects in the carbon sheet of the nanotubes. In the plasma treatment process, the increase in the Dband intensity enhanced the degree of disorder and may be beneficial for enhanced emission properties. The field electron emission properties are significantly enhanced after the Fe nanoparticles attached with plasma incorporated in terms of the reduction in Eto (1.64 from 2.02 V/μm) &Eth (1.98 from 2.64 V/μm), and increase in J (391.11–1179.24 μA/cm2) and β (1419–2900), respectively. The field emission parameters of the field emitters are modified due to incorporation of nitrogen species, attached Fe nanoparticles, and reduction in the nanotube density. The as-prepared field emitters also qualified the orthodox field emission hypothesis test by exhibiting fextr values in an acceptable range. Therefore, we can use the Ar + N2 plasma-based Fe nanoparticles decorated CNT field emitters in vacuum electronic device applications.

Fabrication of high quality X-ray source by gated vertically aligned carbon nanotube field emitters

We fabricated a cold cathode-driven x-ray source with vertically aligned carbon nanotubes (VACNTs). Dose and spatial resolution characteristics are compared to commercially available portable x-ray sources, and our system outperformed its counterparts. At the same 1.0 mAs condition, our x-ray source represented a dose rate of 0.37 mGy/s, which is 7.8 and 2.4 times greater than that of the thermionic emitter and paste carbon nanotubes based commercial x-ray sources, respectively. In addition, our x-ray source represented better image resolution by achieving a nominal focal spot size of 0.35 mm. We believe that high quality x-ray properties were attained, thanks to the narrow electron beam divergence and high reduced brightness of the electrons from VACNTs, and that this will open up advanced x-ray applications.

Influence of power-dependent Argon gas plasma treatment on the electron field emission properties of carbon nanotube field-emitters

Influence of power-dependent Argon gas plasma treatment on the electron field emission properties of carbon nanotube field-emitters

Mini-type low pressure helium detection from adsorption enhanced carbon nanotube field emission

The miniature low pressure helium detection technique is of great interest for some important industrial and scientific fields. A novel low pressure helium sensing method from carbon nanotube (CNT) field emission has been developed. The low field emission currents from multi-walled carbon nanotubes (MWNTs) with low crystallinities increase in helium pressure range of 10−7-10−3 Pa. The field emission analysis and first-principles simulation revealed that work functions of CNT emitters remain consistent with helium adsorptions. The simulation showed that the adsorption of He atom results in extra charge aggregation on carbon atoms at the defect, enhancing the field emission current. The helium sensing MWNT emitter normally doesn’t present good hydrogen, oxygen, and nitrogen sensing properties, benefiting the leak detection applications. The mini-type helium sensor in millimeter scale was demonstrated in various gas backgrounds with leak rates down to 10−8 Pa·l/s, very promising for helium detection and leak checking in wide types of in-situ measurement applications, including vacuum electronic devices, low power consumption instruments, and compact space facilities.

Miniature vacuum sensor based on gas adsorptions from carbon nanotube field emitters

A mini-type vacuum sensor based upon current enhancement from gas adsorption for multi-walled carbon nanotube (MWNT) field emitters is developed. The current increase rates in low emission state are proportional to gas pressures in nitrogen, hydrogen, and mixed gas environments. The field emission energy distribution (FEED) measurement demonstrated the reductions of electron tunneling energy barriers in H2 and N2 ambiences. The first-principles simulation indicates that the energy barrier reduction, which is related to the surface energy well from nitrogen adsorption, is attributed to the sensing effect for nitrogen. Both experiments and first-principles simulation affirmed that the high MWNT crystallinity benefits the sensing performance. The miniature sensor in millimeter dimensions based on MWNT field emitter with simple diode configuration can operate in a wide vacuum range from 10−7 to 10−2 Pa. The sensor was assembled inside the X-ray tube to measure the in-situ device vacuum, showing great potentials for vacuum electronic device applications.

High-Performance Cold Cathode X-ray Tubes Using a Carbon Nanotube Field Electron Emitter.

A cold cathode X-ray tube was fabricated using a carbon nanotube (CNT) field electron emitter made by a free-standing CNT film which is composed of a highly packed CNT network. A lot of CNT bundles with a sharp tip are vertically aligned at the edge of the thin CNT film with a length of 10 mm and a thickness of 7 μm. The cold cathode X-ray tube using the CNT field emitter presents an extremely high tube current density of 152 A/cm2 (corresponding to tube current of 106.4 mA), the electron beam transmittance of 95.2% and a small focal spot size (FSS) of 0.5 mm. In addition, the cold cathode X-ray tube also shows stable lifetime during 100 000 shots. High emission current density of the cold cathode X-ray tube is mainly attributed to a lot of electron emission sites at an edge of the CNT film. The small FSS is caused by an ensemble of the CNT field electron emitter made by a free-standing thin CNT film and the optimized curve-shape elliptical focusing lens. Based on obtained results, the cold cathode X-ray tube can be widely used for various X-ray applications such as medical diagnosis systems and security check systems in the future.

A dual-functional micro-focus X-ray source based on carbon nanotube field emission

A micro-focus X-ray source with miniature pressure sensor was developed based on the multi-walled carbon nanotube (MWNT) field emission. The field emitters for both X-ray cathode and the pressure sensor were synthesized by direct chemical vapor deposition growth of MWNTs on catalytic substrates. MWNTs exhibited excellent field emission properties with the threshold field at 10 mA/cm2 of 2.30 V/μm, and the high current density of 10 A/cm2 could be reached at the field of 4.36 V/μm. The field emission electron gun was constructed with the aid of electrostatic simulations on the electron beam trajectories and the focal spot size (FSS) on the target with the demagnification factor of 5. The practical objects, including the semiconductor chip and the ceramic heating element, were tested by the X-ray source with the spatial resolution of less than 35 μm, and the fracture of the ceramic element inside metal case could be clearly detected. The pressure sensor monitored the in-situ pressures inside the source during the processing and after the sealing-off. The pressures in the sealed sources were generally in 10−4 Pa level and could rise to 10−2 Pa level during the operation.

Field Emission of Multi-Walled Carbon Nanotubes from Pt-Assisted Chemical Vapor Deposition

Multi-walled carbon nanotubes (MWNTs) were grown directly on a metal substrate with the assistance of Pt using a chemical vapor deposition method. In addition, the growth mechanism of Pt-assisted catalytic CNT was discussed. MWNTs were characterized by SEM, TEM, AFM, Raman, and EDS, and the field emission (FE) properties were investigated, comparing with the direct grown MWNTs. The results showed that CNTs could not been synthesized by Pt particles alone under the experimental condition, but Pt may accelerate the decomposition of the carbon source gas, i.e., assisting MWNT growth with other catalysts. The Pt-assisted MWNTs were longer with larger diameters of around 80 nm and possessed better structural qualities with very few catalyst particles inside. Improved field emission properties were demonstrated for the Pt-assisted MWNTs with lower turn-on fields (for 0.01 mA·cm−2 current density) of 2.0 V·μm−1 and threshold field (for 10 mA·cm−2 current density) of 3.5 V·μm−1, as well as better stability under a long-term test of 80 h (started at 3.0 mA for the Pt-assisted emitter and 3.25 mA for the direct grown emitter). This work demonstrated a promising approach to develop high performance CNT field emitters for device applications.

Feasibility of dual-energy CBCT by spectral filtration of a dual-focus CNT x-ray source.

Cone beam computed tomography (CBCT) is now widely used in dentistry and growing areas of medical imaging. The presence of strong metal artifacts is however a major concern of using CBCT especially in dentistry due to the presence of highly attenuating dental restorations, fixed appliances, and implants. Virtual monoenergetic images (VMIs) synthesized from dual energy CT (DECT) datasets are known to reduce metal artifacts. Although several techniques exist for DECT imaging, they in general come with significantly increased equipment cost and not available in dental clinics. The objectives of this study were to investigate the feasibility of developing a low-cost dual energy CBCT (DE-CBCT) by retrofitting a regular CBCT scanner with a carbon nanotube (CNT) x-ray source with dual focal spots and corresponding low-energy (LE) and high-energy (HE) spectral filters. A testbed with a CNT field emission x-ray source (NuRay Technology, Chang Zhou, China), a flat panel detector (Teledyne, Waterloo, Canada), and a rotating object stage was used for this feasibility study. Two distinct polychromatic x-ray spectra with the mean photon energies of 66.7keV and 86.3keV were produced at a fixed 120kVp x-ray tube voltage by using Al+Au and Al+Sn foils as the respective LE and HE filters attached to the exist window of the x-ray source. The HE filter attenuated the x-ray photons more than the LE filter. The calculated post-object air kerma rate of the HE beam was 31.7% of the LE beam. An anthropomorphic head phantom (RANDO, Nuclear Associates, Hicksville, NY) with metal beads was imaged using the testbed and the images were reconstructed using an iterative volumetric CT reconstruction algorithm. The VMIs were synthesized using an image-domain basis materials decomposition method with energy ranging from 30 to 150keV. The results were compared to the reconstructed images from a single energy clinical dental CBCT scanner (CS9300, Carestream Dental, Atlanta, GA). A significant reduction of the metal artifacts was observed in the VMI images synthesized at high energies compared to those from the same object imaged by the clinical dental CBCT scanner. The ability of the CNT x-ray source to generate the output needed to compensate the reduction of photon flux due to attenuation from the spectral filters and to maintain the CT imaging time was evaluated. The results demonstrated the feasibility of DE-CBCT imaging using the proposed approach. Metal artifact reduction was achieved in VMIs synthesized. The x-ray output needed for the proposed DE-CBCT can be generated by a fixed-anode CNT x-ray source.

Investigation of the Pulsing Characteristic of a Carbon Nanotube Emitter.

The carbon nanotube (CNT) field emitter is suitable for the high frequency pulsing of X-ray. Pulsing reduces 49% of the dose in grid-controlled fluoroscopy and improves the image of moving objects. Various structures and manufacturing processes are being studied. However, more studies on the dynamic characteristic of a pulsing CNT and its application are needed. In this study, the combined dynamics including the field emission, MOSFET, and modified gate driver for MOSFET have been analyzed. In this configuration, between the cathode of the tube and ground, there is a MOSFET switch that turns the tube current on/off and a shunt resistor that measures the tube current. Due to the high impedance of the vacuum between the gate and cathode of the tube, about 85% of the gate voltage is still exerted between the Gate and cathode of the tube during the off-state of the MOSFET. Therefore, space charges are built during the off-state and then released at the beginning of the on-state of the MOSFET. The modified gate driver structure for MOSFET that we propose in this paper can limit the amount of current flow through the cathode. Tube current (boosted current) can be accurately controlled through a modified gate driver structure. Combining the boosted current and pulse control of MOSFET, the dynamic current performance of a CNT tube can be enhanced and the average tube current or dose can be accurately controlled. Experiments, simulation, and analysis have been conducted to study the combined dynamics and its applications.

High-Performance Field Electron Emitters Fabricated Using a Free-Standing Carbon Nanotube Film

The carbon nanotube (CNT) field emitter was fabricated using a thin free-standing CNT film, indicating a line-shape CNT field emitter. Field emission properties of the CNT field emitter were investigated in both diode and triode configurations. The CNT field emitter showed a low turn-on electric field of 1.8 V/μm and a high emission current of 40.3 mA, corresponding to the emission current density of 96 A/cm in the diode configuration. It also exhibited a high anode current of 40 mA, corresponding to the anode current density of 95.2 A/cm in the triode configuration. In addition, the CNT field emitter showed a good electron beam transmittance of 86.4% and excellent emission stability without degradation for 15 h. The main reason for the high performance of our CNT field emitter is caused by the high density of emission sites at the edge of the CNT film.