High Emission Current Density Research Articles

Synthesis and field emission property of carbon nanotubes with sharp tips

Carbon nanotubes (CNTs) with sharp tips were synthesized by a hydrogen arc discharge method. Three unusual morphologies, i.e., a cone-shaped tip, a suddenly-shrinking tip, and a pencil point-like tip were observed. These novel tip structures are considered to be related to the addition of a small amount of silicon powder in the raw material, which may introduce structural defects in the CNTs. The field emission property of the sharp-tip CNTs was investigated, and a low threshold electric field of 3.75 V/m, a high field emission current density of ∼1.6×10 5 A/cm 2, and a good emission stability were demonstrated. The superior field emission performance of the CNTs can be attributed to their good crystallinity and unique tip structures.

Fabrication of In-doped SnO2 nanowire arrays and its field emission investigations

The field emission of In-doped SnO2 wire array has been performed in parallel plate diode configuration. A maximum current density of 60 µA/cm2 is drawn from the emitter at an applied field of 4 V/µm. The nonlinearity in the Fowler–Nordheim plot, characteristics of semiconductor emitter has been observed and explained on the basis of electron emission from both the conduction and the valence bands. The current stability recorded at a preset value of 1 µA is observed to be good. The high emission current density, good current stability and mechanically robust nature of the wires offer unprecedented advantages as promising cold cathodes for many potential applications based on field emission.

High emission current density, vertically aligned carbon nanotube mesh, field emitter array

A vertically aligned carbon nanotube mesh emitter array has been fabricated and tested, giving a current density of up to 1.5 A/cm2, and a threshold field of 1.5 V/μm for a current density 1 mA/cm2. Low temperature carbon nanotube growth is used to fabricate the carbon nanotube mesh emitter arrays significantly reducing the cost of the fabrication of large area electron emitters. This system exhibits ultralong lifetime.

Growth of a three-dimensional complex carbon nanoneedle electron emitter for fabrication of field emission device

A three-dimensional complex carbon nanoneedle electron emitter film with high emission current density at low electric field has been developed by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibited novel film morphology. The scanning electron microscopy image taken from this film indicated a three-dimensional complex nanostructure emitter, the center of which was a carbon nanoneedle, and many small carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10 7/cm 2. The I– V characteristic addressed an emission current density of 251 mA/cm 2 at the electric field of 2.2 V/μm, and the field emission current was stable, making it possibly suitable for developing field emission device.

High-Quality Thin-Multiwalled Carbon Nanotubes Synthesized by Fe-Mo/MgO Catalyst Based on a Sol–Gel Technique: Synthesis, Characterization, and Field Emission

We synthesized thin-multiwalled carbon nanotubes (t-MWCNTs) using Fe-Mo/MgO catalyst by citrate precursor method based on a simple sol-gel technique. Synthesis of high-quality t-MWCNTs with uniform diameters was achieved in large-scale by catalytic decomposition of methane over Fe-Mo/MgO catalyst prepared under Ar-atmosphere. The produced t-MWCNTs had the outer diameters in the range of 4-8 nm, with a Gaussian average diameter of 6.6 +/- 0.1 nm and wall numbers in the range of of 4-7 graphenes. The product yield of the as-synthesized t-MWCNTs was over 450% relative to the used Fe-Mo/MgO catalyst, and TGA showed purity about 85%. The t-MWCNTs showed the low turn-on field about 1.24 V/microm, corresponding to the current density of 0.1 microA/cm2, and the high emission Current density of 10 mA/cm2 at the applied electric field of 2.6 V/microm. The t-MWCNTs indicate good emission stability without any fluctuation of emission current through a lifetime measurement for 20 h. It was confirmed that the field emission performance of t-MWCNTs was similar to DWCNTs and the field emission stability of t-MWCNTs was similar to MWCNTs.

Ultrathin single-crystal ZnO nanobelts: Ag-catalyzed growth and field emissionproperty

We report the growth of ultrathin single-crystal ZnO nanobelts by using a Ag-catalyzedvapor transport method. Extensive transmission electron microscopy and atomic forcemicroscopy measurements reveal that the thickness of the ultrathin ZnO nanobelts is ∼ 2 nm. Scanning electron microscopy and post-growth annealing studies suggest a ‘1D branchingand 2D filling’ growth process. Our results demonstrate the critical role of catalyst in thedeterministic synthesis of nanomaterials with the desired morphology. In addition, these ultrafinenanobelts exhibit stable field emission with unprecedented high emission current density of40.17 mA cm − 2. These bottom-up building blocks of ultrathin ZnO nanobelts may facilitate theconstruction of advanced electronic and photonic nanodevices.

Aluminum-enhanced sharpening of silicon nanocones

Here we report on the fabrication of high-density aligned Si nanotip arrays by chemical vapor deposition followed by dry oxidation and an etching treatment. The dry oxidation investigations indicated that Al-catalyst particles located at the tip of Si nanocones enhance their sharpening. This oxidation behavior is quite different from that of Au-catalyzed Si nanowires and is more favorable to form very sharp Si nanotips. Field emission from an individual Si nanotip showed good field-emission characteristic with a high emission current density of 1×104 A/cm2 because of its sharp tip geography, suggesting their potential application for field emitters. Our work provides an effective approach to fabricate high-density Si nanotip arrays, which overcomes some problems in the conventional fabrication approaches, such as high cost, poor controllability, and complicated process.

Excellent Field Emitters: Onion-Shaped Tipped Carbon Nanotubes

Large-scale synthesis of highly graphitic carbon nanotubes (with a diameter of ∼5−8 nm) from a thermal reaction of activated carbon and Bi2O3 powders, which was grown via a carbon-thermal reduction and Bi catalyzed vapor−liquid−solid process has been obtained. Individual carbon nanotubes were terminated with an onion-shaped carbon tip, which has never been observed in growth of a carbon nanotube. Field emitters made from the present carbon nanotubes exhibit excellent emission properties: they can be operated at a very large current density of ∼145 mA/cm2 at a low voltage of 5.5 V/μm, and possess a long period (>32 h) of high emission stability at a high emission current density of ∼9 mA/cm2 and under a constant applied voltage. These properties are equally important for high image quality in field emission display fabrication. Hence, as-fabricated CNTs could act as excellent electron emitters and have great potential application in field emission panel displays.

Enhanced field emission properties from titanium-coated carbon nanotubes

The effect of titanium (Ti) coating over the surface of carbon nanotubes (CNTs) on field emission characteristics was investigated. Vertically aligned CNTs were grown by inductively-coupled plasma-enhanced chemical vapor deposition (ICP-CVD). In order to reduce the screening effect of electric field due to densely packed CNTs, as-grown CNTs were partly etched back by DC plasma of N 2. Ti with various thicknesses from 5 nm to 150 nm was coated on CNTs by a sputtering method. Since thick Ti coating with thickness of 100 nm or more resulted in the shape of a metal post by merging an individual CNT in a bundle, it was inadequate to a field emission application. On the other hand, thin Ti-coated CNTs with thickness of 10 nm or less showed a lower turn-on field, a higher emission current density, and improved emission uniformity compared with pristine CNTs. The improved emission performance was mainly attributed to the low work function of Ti and the reliable and lower resistance contact between CNTs and substrates.

Multi-walled carbon nanotube/ribonucleic acid hybrid field emitters fabricated by spray deposition

Multi-walled carbon nanotubes (MWCNTs) are shown to disperse in water, and be functionalized using RNA so that MWCNT/RNA hybrids can be fabricated on substrates by spray method. The material has strong adhesion on glass substrates or metal wires. A liquid elastomer surface treatment is used to make the field emitter tips protrude from the cathode. Possible methods of realizing field emitter sources, including X-ray sources, are discussed. The MWCNT/RNA hybrids have a higher emission current density and more uniform emission image than the MWCNTs on their own, since the RNA coated MWCNTs attach more strongly to the substrate. A diode configuration field emission X-ray source using the MWCNT/RNA hybrids on tungsten wire tip was tested and found to provide clear X-ray images.

Emission site density depending on surface area and morphology of nanotube film emitters

The influence of emitter surface morphology on emission site density (ESD) is investigated for carbon nanotube films. The authors show that the ESD varies with cathode-anode distance for rough surface emitters while it is almost invariant for smooth ones. In addition, it has been revealed that the ESD decreases with the increase in the film area, resulting in low emission current density for large-area films. The present study suggests that the high ESD is more important to achieve high emission current density and long lifetime for film emitters than the high field enhancement factor.

Enhancement mechanism of field electron emission properties in hybrid carbon nanotubes with tree- and wing-like features

In this work, the tree-like carbon nanotubes (CNTs) with branches of different diameters and the wing-like CNTs with graphitic-sheets of different densities were synthesized by using plasma enhanced chemical vapor deposition. The nanostructures of the as-prepared hybrid carbon materials were characterized by scanning electron microscopy and transmission electron microscopy. The structural dependence of field electron emission (FEE) property was also investigated. It is found that both of the tree- and wing-like CNTs exhibit a lower turn-on field and higher emission current density than the pristine CNTs, which can be ascribed to the effects of branch size, crystal orientation, and graphitic-sheet density.

The stability of the CNT/Ni field emission cathode fabricated by the composite plating method

A novel composite plating method has been developed for the fabrication of carbon nanotube/Ni (CNT/Ni) field emission cathode. The field emission properties of the initial CNT/Ni field emitter show a low turn-on electric field E on of about 1.1 V/μm with an emission current density of 1 μA/cm 2, and a low threshold electric field E th of about 1.7 V/μm with an emission current density of 1 mA/cm 2. After performing a stability test with a high emission current density in high vacuum, the corresponding microstructure and the degree of graphitization of the CNT/Ni field emitter were measured by using scanning electron microscopy and Raman spectroscopy. We found that the degree of graphitization slowly decreases with the duration time t FE of the stability test, the size of small rod-like CNT/Ni composite structures in the film increases with t FE, and obvious cracks appear in the film as t FE is larger than 60 h. The degradation of the field emission properties may be explained by the Joule heating effect on the CNT/Ni field emitter under high emission current density.

Carbon nanotube pillar arrays for achieving high emission current densities

We introduce an innovative geometry carbon nanotube (CNT) field emitter array capable of achieving stable and high current densities. Arrays of toroid CNT pillars were grown directly on bulk metal alloy substrates and on patterned metal catalyst on silicon substrates. Compared to a solid CNT pillar array (CPA), this toroid CPA (tCPA) provides a larger edge area for achieving a higher stable current density of 50 mA/cm2 at an applied dc field of less than 8 V/μm. Electrostatic simulation data confirming the field enhancement at the inner and outer edges of the tCPA are also presented.

Space charge effects in field emission nanodevices

Electron field emission from a single nanoemitter is a barrier tunneling, quantummechanical process that can, therefore, be described by the well-knownFowler–Nordheim (FN) equation. At high emission current densities, however,the space charge caused by the cathode may affect the current density–voltage(J–V) characteristics predicted by the FN theory. In this study, we theoretically investigated theeffect of space charge on FE nanodevices, including diode and triode structures. TheJ–V characteristics of FE nanodevices were obtained by analytically (diode structures) ornumerically (triode structures) solving the coupled FN equation and Poisson’s equation.We discuss the behavior of FE nanodiodes and nanotriodes displaying different geometries,dimensions and work functions of their emitter materials. In the high current densityregion, space charge plays an important role in FE nanodevices; the threshold currentdensity of space–charge limitation is related to the electric field distributions. Besides, ourtheoretical results are in good agreement with the experimental results reported previously.

Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB3 (MmNi3, MmFe1.5Ni1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm2 at 0.9 V/μm.

Track technology for creating the arrays of nickel microtips, micronozzles, and microtubes

A method for obtaining surface arrays of nickel objects (microtips, micronozzles, and microtubes) with preset geometric parameters by means of a track technology has been developed using the data of scanning electron microscopy. It is shown that Ni microtips with a radius as small as 10–25 nm can be obtained, which allows these microobject arrays to be used as multiple-tip field emission cathodes capable of providing high emission current densities.

Improved emission uniformity and stability of printed carbon nanotubes in electrolyte

In this paper, we studied the effect of NaCl electrolyte as a surface treatment on improving the uniformity and stability of field emission of screen-printed carbon nanotubes (CNTs). A short period of the electrolyte treatment of CNT films remarkably increase emission uniformity and stability. Furthermore, the field emission characteristics of screen-printed carbon nanotubes (CNTs) such as low turn-on field, high emission current density and strong adhesion of the CNT film on the substrate were also reinforced after post-treated. SEM, TEM and Raman spectrum study revealed that uniformity and stability of field emission is enhanced by two factors. Firstly, the electrolyte treatment appeared to render the CNT surfaces more actively by exposing more CNTs form the CNT paste, which dominates initial uniformity and stability of field emission. Secondly, the number of opened CNTs and defects CNTs of CNT film were increased by electrical current energy.

Dependence of RF power on the phase transformation for boron nitride films deposited on graphite at room temperature

Cubic boron nitride (cBN) thick films deposited on mainly c-axis-oriented graphite substrate at room temperature and zero bias by radio frequency (RF) magnetron sputtering were studied. In the growth process, RF power plays a key role in determining the content of cubic phase in films, while the conventional substrate heating and biasing have been neglected. With increase in RF power, the dominated content of films converts from explosion boron nitride (eBN) to cBN. The transformation mechanism has been discussed. The unique structural properties of the “soft” graphite are favorable to propose simple conditions for growing “hard” cBN films. Furthermore, the optical band gap of BN films having ∼90% cubic phase is of ∼5.8 eV obtained from ultraviolet–visible optical transmission measurement. The electron field emission examination shows that cBN film on graphite has a high emission current density of 2.8×10 −5 A/cm 2 at an applied field of ∼30 V/μm.

The field emission properties of nonpolar a -plane n-type GaN films grown on nano-patterned sapphire substrates

Si-doped nonpolar a -plane GaN films were grown on nano-patterned sapphire substrates by a low-pressure metal organic chemical vapor deposition (MOCVD) system. The structure, morphology and field emission properties of the sample were studied by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and field emission measurement. The XRD analysis shows that the sample is a nonpolar a -plane (110) GaN film. The field emission measurement shows that the nonpolar GaN films exhibit excellent field emission properties with a threshold emission field of as low as 10 V/μm at a current density of 0.63 μA/cm2, and a high field emission current density of 74 mA/cm2 at an applied field of 24 V/μm. Moreover, the Fowler–Nordheim plot of the sample fits a near linear relation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)