A comparative study of field emission properties of carbon nanotube films prepared by vacuum filtration and screen-printing

A comprehensive comparative study of electron field emission properties of carbon nanotube (CNT) films prepared by vacuum filtration and screen-printing was carried out. Field emission performance of vacuum filtered CNT films with different filtered CNT suspension volumes was systematically studied, and the optimum electron emission was obtained with a low turn on field of ∼0.93 V/μm (at 1 μA/cm²) and a high field enhancement factor β of ∼9720. Comparing with screen-printed CNT films, vacuum filtered CNT films showed better electron emission performance, longer lifetime, and greater adhesive strength to substrates. This work reveals a potential use of vacuum filtered CNT films as field emission cathodes.     

Preparation and field emission properties of carbon nanotubes cold cathode using melting Ag nano-particles as binder

A new preparation process for carbon nanotubes (CNTs) cold cathode was studied through the replacement of traditional organic or inorganic binder with Ag nano-particles. This method has the advantages of low preparation temperature and fine electrical contact between CNTs paste and substrate. A mixture paste of CNTs, Ag nano-particles and other organic solvents was spreaded on Si substrate. By melting and connecting of Ag nano-particles after sintered 30 min at 250 °C, a flat CNTs films with good field emission properties was obtained. The measurements reveal that the turn on electric field and the threshold electric field of as-prepared CNTs cathode are 2.1 and 3.9 V/μm respectively and the field emission current density is up to 41 mA/cm² at an applied electric field of 4.7 V/μm.     

Improved emission stability of HfC-coated carbon nanotubes field emitters

Plasma-enhanced chemical vapor deposition (PECVD) method was employed to synthesize the Fe-catalyzed carbon nanotubes (CNTs). Hf films were deposited onto the synthesized CNTs, followed by heat treatment at 1200 °C which could form HfC. Field emission properties indicate that the HfC-coated CNTs have good emission current density due to low work function of HfC and also keep stable emission characteristics under poor vacuum owing to the chemical inertness of HfC. Consequently, field emission characteristics of the CNTs can be improved by the HfC-coated surface treatment compared with the synthesized CNTs.     

Effect of temperature on the electron field emission from aligned carbon nanofibers and multiwalled carbon nanotubes

Effect of temperature and aspect ratio on the field emission properties of vertically aligned carbon nanofiber and multiwalled carbon nanotube thin films were studied in detail. Carbon nanofibers and multiwalled carbon nanotube have been synthesized on Si substrates via direct current plasma enhanced chemical vapor deposition technique. Surface morphologies of the films have been studied by a scanning electron microscope, transmission electron microscope and an atomic force microscope. It is found that the threshold field and the emission current density are dependent on the ambient temperature as well as on the aspect ratio of the carbon nanostructure. The threshold field for carbon nanofibers was found to decrease from 5.1 to 2.6 V/μm when the temperature was raised from 300 to 650 K, whereas for MWCNTs it was found to decrease from 4.0 to 1.4 V/μm. This dependence was due to the change in work function of the nanofibers and nanotubes with temperature. The field enhancement factor, current density and the dependence of the effective work function with temperature and with aspect ratio were calculated and we have tried to explain the emission mechanism.     

Field emission properties of carbon nanotube film using a spray method

We fabricated carbon nanotube (CNT) emitters by a spray method using a CNT suspension with ethanol. Indium with a low melting pointing metal or indium tin oxide (ITO) was deposited on the glass substrate. The CNTs were sprayed on these layers and thermally annealed. The sprayed CNTs on an ITO were obtained a high emission current density, field enhancement factor, and a uniform emission pattern than the sprayed CNTs on an ITO layer. We found that the sprayed emitters on the indium layer had good field emission characteristics because of the strong adherence between the metal layer and CNTs.     

Effects of microwave plasma treatment on the field emission properties of printed carbon nanotubes/Ag nano-particles films

The effects of Ar microwave plasma treatment on field emission properties of the printed carbon nanotubes (CNTs) cathode films using Ag nano-particles as binder were investigated. The field emission J-E characteristics were measured at varied plasma treatment time. Significant improvement in emission current density, emission stability and uniformity were achieved for the Ar treated CNTs films, even though the plasma treatment increased the turn on electric field slightly. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy revealed the microstructural changes of CNTs after the plasma treatment. The improved field emission properties of CNTs film can be attributed to the generation of a high density of structural defects after treatment, which increased greatly the possible emission active sites. Besides, the formation of the sharpened and open-ended CNTs tips is all helpful for improving the field emission properties of the treated CNTs.     

Growth of carbon nanofibers on aligned zinc oxide nanorods and their field emission properties

Carbon nanofibers were grown by electrodeposition technique onto aligned zinc oxide (ZnO) nanorods deposited by hybrid wet chemical route on glass substrates. X-ray diffraction traces indicated very strong peak for reflections from (0 0 2) planes of ZnO. The Raman spectra were dominated by the presence of G band at about 1597 cm⁻¹ corresponding to the E_2g tangential stretching mode of an ordered graphitic structure with sp² hybridization and a D band at about 1350 cm⁻¹ originating from disordered carbon. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peak at ∼511 cm⁻¹ for Zn-O stretching mode. Photoluminescence spectra indicated band edge luminescence of ZnO at ∼3.146 eV along with a low intensity peak at ∼0.877 eV arising out of carbon nanofibers. Field emission properties of these films and their dependence on the CNF coverage on ZnO nanorods are reported here. The average field enhancement factor as determined from the slope of the FN plot was found to vary between 1 × 10³ and 3 × 10³. Both the values of turn-on field and threshold field for CNF/ZnO were lower than pure ZnO nanorods.     

Field emission properties and synthesis of carbon nanotubes grown by rf plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition (PECVD) method was employed to grow the Fe-catalyzed carbon nanotubes (CNTs). The grown CNTs with a uniform diameter in the range of about 10-20 nm and the typical lengths beyond 1 μm resulted in a very high aspect ratio. The Raman and TEM results showed that the grown CNTs contained a large amount of carbonaceous particles and crystal defects, such as pentagon-heptagon pair defects. XPS measurement indicated that the CNTs had CH covalent bonds. Field emission characteristics exhibited the low turn-on threshold field of 2.75 V/μm and the maximum emission current density of 7.75 mA/cm² at 6.5 V/μm. The growth mechanism of CNTs and the effects of hydrogen plasma on their structure were discussed.     

Efficient field emission from coiled carbon nano/microfiber on copper substrate by dc-PECVD

Crystalline coiled carbon nano/micro fibers in thin film form have been synthesized via direct current plasma enhanced chemical vapor deposition (PECVD) on copper substrates with acetylene as a carbon precursor at 10 mbar pressure and 750 °C substrate temperature. The as-prepared samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD pattern as well as selected area electron diffraction (SAED) pattern showed that the samples were crystalline in nature. SEM and HRTEM studies showed that as synthesized coiled carbon fibers are having average diameter ∼100 nm and are several micrometers in length. The as-prepared samples showed moderately good electron field emission properties with a turn-on field as low as 1.96 V/μm for an inter-electrode distance 220 μm. The variation of field emission properties with inter-electrode distance has been studied in detail. The field emission properties of the coiled carbon fibrous thin films are compared with that of crystalline multiwalled carbon nanotubes and other carbon nanostructures.     

Field emission and current-voltage properties of boron nitride nanotubes

We have measured electrical transport properties of boron nitride nanotubes using an in situ manipulation stage inside a transmission electron microscope. Stable currents were measured in a field emission geometry, but in contact the nanotubes are insulating at low bias. At high bias, the nanotubes show stable, reversible breakdown current.     

Thinning and diluting aligned carbon nanotube films for uniform field emission

Aligned carbon nanotube (CNT) films are potential field emitters for large-area flat panel displays. However, the distribution of emission areas in the CNT films is quite non-uniform because of inhomogeneous nanotube growth, which is hard to avoid using the conventional chemical vapor deposition (CVD) method. Here we show that the emission uniformity of CNT films can be improved simply by reducing the film thickness (thinning) or the nanotube density (diluting). The thickness and density of CNT films could be controlled by controlling the CNT growth time and temperature. Received: 12 June 2001 / Accepted: 27 October 2001 / Published online: 23 January 2002     

Enhancement of field emission properties of graphite flakes by producing carbon nanotubes on above using thermal chemical vapor deposition

In order to improve the field emission properties of the graphite flakes, the carbon nanotubes (CNTs) are produced on above without the metallic catalyst using mixtures of C₂H₂ and H₂ gases by thermal chemical vapor deposition. We spin the graphite solution on the silicon wafer and dry it, then synthesize the CNTs on the graphite flakes. We change the synthetic time to obtain the optimal conditions for enhancement of field emission properties of graphite flakes. The experimental results show that the density and quality of the CNTs could be controlled significantly by the synthetic time. Besides, the field emission properties of the treated graphite flakes are also affected greatly by it. The emission current density of the treated graphite flakes reaches to 0.5 mA/cm² at 3 V/μm, and the turn-on field is decreased from 7.7 to 1.9 V/μm after producing the CNTs on above.     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

Characterization and field emission characteristics of carbon nanotubes modified by titanium carbide

Carbon nanotubes (CNTs) were modified by depositing a thin layer of titanium film on the surface using magnetron sputtering method, followed by vacuum annealing at 900 °C for 2 h. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed that the as-deposited thin titanium film reacted with carbon atoms to form titanium carbide after annealing. The experiment results show that the thickness of sputter-deposited titanium film has significant effect on the field emission J-E characteristic of modified CNTs film. The titanium carbide-modified CNTs film obtained by controlling the titanium sputtering time to 2 min showed an improved field emission characteristics with a significant reduction in the turn-on electric field and an obvious increase in the emission current density as well as an improvement in emission stability. The improvement of field emission characteristics achieved is attributed to the low work function and good resistance to ion bombardment of titanium carbide.     

Field emission properties of electrophoretic deposition carbon nanotubes film

The field emission properties of electrophoretic deposition(EPD) carbon nanotubes (CNTs) film have been improved by depositing CNTs onto the titanium (Ti)-coated Si substrate, followed by vacuum annealing at 900 °C for 2 h, and the enhanced emission mechanism has been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. Field emission measurements showed that the threshold electric field was decreased and the emission current stability was improved compared to that of EPD CNTs film on bare Si substrate. XRD and Raman spectroscopy investigations revealed that vacuum annealing treatment not only decreased the structural defects of CNTs but made a titanium carbide interfacial layer formed between CNTs and substrate. The field emission enhancement could be attributed to the improved graphitization of CNTs and the improved contact properties between CNTs and substrate including electrical conductivity and adhesive strength due to the formed conductive titanium carbide.     

Effect of ion impinging on the microstructure and field emission of carbon nanotubes

Effects of ion impinging on the microstructure and field electron emission properties of screen-printed carbon nanotube films were investigated. We observed that the plasma treatment modified the microstructure of CNTs along with the remarkable increase of emission site density. With the prolongation of ion impinging time, the emission current falls down first, and then rises up to higher than that of the untreated films. It is proposed that the change of emission characteristics is due to the different emission mechanisms. After the treatment, electrons are emitted predominantly from the nano-nodes on the tube wall instead from the nanotube tips.     

Work function of single-walled carbon nanotubes determined by field emission microscopy

The field emission characteristics of a single micro-bundle of single-walled carbon nanotubes (SWCNTs) were investigated using field emission microscopy (FEM). Fowler–Nordheim plots revealed that the work function of the SWCNTs was reduced with increasing heating temperature, and reached a minimum value around 1000 °C, assuming that the β factor was constant during the heating process. Field emission patterns also demonstrated fine structures that were believed to be images of the cap of a SWCNT, which was in a clean state. The radius of the SWCNT micro-bundle was measured by transmission electron microscopy (TEM), and the β factor was calculated using two empirical formulae. Then, the work function of the SWCNT was determined from the slope, K, of its Fowler–Nordheim plot. The work function values were Φ₁=4.76 eV and Φ₂=4.88 eV, respectively. Received: 26 October 2001 / Revised version: 19 February 2002 / Published online: 6 June 2002     

Electrically conducting ion tracks in diamond-like carbon films for field emission

Electrically conducting channels in an insulating carbon matrix were produced by 140-MeV Xe ion irradiation. The high local energy deposition of the individual ions along their pathes causes a rearrangement of the carbon atoms and leads to a transformation of the insulating, diamond-like (sp³-bonding) form of carbon into the conducting, graphitic (sp²-bonding) configuration. The conducting ion tracks are clearly seen in the current mapping performed with an atomic force microscope (AFM). These conducting tracks are of possible use in field emission applications. Received: 4 May 1999 / Accepted: 5 May 1999 / Published online: 1 July 1999     

The influence of H2 plasma treatment on the field emission of amorphous GaN film

The influence of H₂ plasma treatment on the field emission properties of amorphous GaN (a-GaN) films is studied. It is found that the treatment makes little change to the surface morphology. The current density of the treated film decreases from 400 to 30 μA/cm² at the applied field of about 30 V/μm. The treatment can reduce the defects in a-GaN films, and therefore the treatment results in the weakening of the tunneling emission of the a-GaN film at the high field region. The treatment also seems to change the conduction mechanism of the a-GaN film.