Emission Properties Of Carbon Nanotubes Research Articles

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.

Effect of heating and resistance on emission properties of carbon nanotubes

We have studied the effect of the series resistance on the heating of the cathode, which is based on carbon nanotubes and serves to realize the field emission of electrons into the vacuum. The experiment was performed with the single multi-walled carbon nanotube (MCNT) that was separated from the array grown by CVD method with thin-film Ni-Ti catalyst (nickel 4 nm/Ti 10 nm). The heating of the cathode leads to the appearance of a current of the thermionic emission. The experimental voltage current characteristic exhibited the negative resistance region caused by thermal field emission. This current increases strongly with increasing voltage and contributes to the degradation of the cold emitter. The calculation of the temperature of the end of the cathode is made taking into account the effect of the phenomenon that warms up and cools the cathode. We have developed a method for processing of the emission volt-ampere characteristics of a cathode, which relies on a numerical calculation of the field emission current and the comparison of these calculations with experiments. The model of the volt-ampere characteristic takes into account the CNT’s geometry, properties, its contact with the catalyst, heating and simultaneous implementation of the thermionic and field emission. The calculation made it possible to determine a number of important parameters, including the voltage and current of the beginning of thermionic emission, the temperature distribution along the cathode and the resistance of the nanotube. The phenomenon of thermionic emission from CNTs was investigated experimentally and theoretically. The conditions of this type emission occurrence were defined. The results of the study could form the basis of theory of CNT emitter’s degradation.

Enhancement of Electron Emission Properties of Carbon Nanotubes by the Decoration with Low Work Function Metal Oxide Nanoparticles.

In the present report, the properties of the field emission devices of carbon nanotubes (CNTs) were remarkably improved by decorating their surface with magnesium oxide nanoparticles (MgO NPs). The MgO NPs were attached effectively on the surface of CNTs via thermal evaporation. The Raman spectra confirm the graphitic order of as-grown pristine CNTs with RBM (radial breathing mode), D band and G band peaks at the 282 cm-1, 1347 cm-1 and 1594 cm-1 respectively. The peak at 471 cm-1 indicates successful attachment of MgO NPs to the CNTs. The enhanced field emission properties of CNTs were mainly attributed to the MgO NPs which increased the field enhancement factor and the density of emission sites. The decreased work function and increased field enhancement factor were responsible for the improved FE properties of the CNTs. Our results indicate that the MgO decorated CNTs can be used as an effective field emitter for various electron emission devices. The turn-on field decrease from 1.6 V/μm to 1.3 V/μm and the maximum current density increases from 1.581 to 3.678 mA/cm2 after the decoration of CNTs with MgO NPs. The value of field enhancement factor (β) also increases from 2.814×10³ to 9.823×10³.

Mechanism of the Field Emission of Electrons in Single-Walled Carbon Nanotubes

A new interpretation of the emission properties of carbon nanotubes is presented using the theory of electron conjugation. The presence of emission molecular states in the energy spectrum of carbon nanotubes, characterized by specific localization of electron density on the end surfaces of carbon nanotubes, is explained from the viewpoint of the concepts of in-plane and ρ-electron conjugation. Based on an analysis of the behavior of emission molecular orbitals in a constant electric field, it is concluded that such localization of electron density is the physical basis for electron emission. It is established that this approach describes known experimental and theoretical results well and can be used to predict the emission properties of a wide range of objects.

Effect of heating and resistance on emission properties of carbon nanotubes

We have studied the effect of the series resistance on the heating of the cathode, which is based on carbon nanotubes and serves to realize the field emission of electrons into the vacuum. The experiment was performed with the single multi-walled carbon nanotube (MCNT) that was separated from the array grown by CVD method with thin-film Ni-Ti catalyst (nickel 4 nm / Ti 10 nm). The heating of the cathode leads to the appearance of a current of the thermionic emission. The experimental voltage current characteristic exhibited the negative resistance region caused by thermal field emission. This current increases strongly with increasing voltage and contributes to the degradation of the cold emitter. The calculation of the temperature of the end of the cathode is made taking into account the effect of the phenomenon that warms up and cools the cathode. We have developed a method for processing of the emission volt-ampere characteristics of a cathode, which relies on a numerical calculation of the field emission current and the comparison of these calculations with experiments. The model of the volt-ampere characteristic takes into account the CNT’s geometry, properties, its contact with the catalyst; heating and simultaneous implementation of the thermionic and field emission. The calculation made it possible to determine a number of important parameters, among which the voltage and current of the beginning of thermionic emission, the temperature distribution along the cathode, the resistance of the nanotube. The phenomenon of thermionic emission from CNT’s was investigated experimentally and theoretically. The conditions of this type emission occurrence were defined. The results of the study could form the basis of theory of CNT emitter’s degradation.

Improved field emission properties of carbon nanotubes grown on stainless steel substrate and its application in ionization gauge

Vertically aligned carbon nanotube (CNT) arrays were fabricated by chemical vapor deposition (CVD) technique on different substrates. Microstructures and field emission characteristics of the as-grown CNT arrays were investigated systematically, and its application in ionization gauge was also evaluated preliminarily. The results indicate that the as-grown CNT arrays are vertically well-aligned relating to the substrate surfaces, but the CNTs grown on stainless steel substrate are longer and more crystalline than the ones grown on silicon wafer substrate. The field emission behaviors of the as-grown CNT arrays are strongly dependent upon substrate properties. Namely, the CNT array grown on stainless steel substrate has better field emission properties, including lower turn on and threshold fields, better emission stability and repeatability, compared with the one grown on silicon wafer substrate. The superior field emission properties of the CNT array grown on stainless steel substrate are mainly attributed to low contact resistance, high thermal conductivity, good adhesion strength, etc. In addition, the metrological behaviors of ionization gauge with the CNT array grown on stainless steel substrate as an electron source were investigated, and this novel cathode ionization gauge extends the lower limit of linear pressure measurement to 10−8Pa, which is one order of magnitude lower than the result reported for the same of gauge with CNT cathode.

Role of Plasma and Doping Elements on the Growth and Field Emission Properties of Metallic Carbon Nanotube (CNT) Tip Placed over Cylindrical Surface

Carbon nanotubes (CNTs) are currently attractive materials for a diverse range of applications because of their extra ordinary mechanical and electrical properties. Their application has already been demonstrated in field emission displays, nanoscale electronic devices, biosensors and hydrogen storage medium. The field emission properties of carbon nanotubes are an important field of study because they give very high values of current density as compared to the already existing field emission devices. The effect of various factors such as plasma parameters, substitutional atoms, dimensional effects, anode-cathode distance etc. on field emission from CNT has also been extensively studied. The present talk will cover the plasma production and effect of doping of hetero-atoms on the growth and field emission properties of Carbon Nanotubes (CNTs) tip placed over a cylindrical surface in complex plasma. A theoretical model incorporating kinetics of plasma species such as electron, ions and neutral atoms including doping elements like nitrogen (N), Boron (B), Potassium (K), Cesium (Cs) and energy balance of CNTs in complex plasma has been developed. The effect of doping elements of N, B, K, and Cs on the growth of CNTs namely the tip radius has been carried out for typical glow discharge plasma parameters. It is found that doping elements affect the radius of CNTs extensively. We obtained small radii for CNT doped with N and large radius for CNT doped with B. The field emission characteristics from CNTs have therefore been suggested on the basis of results obtained. Some of theoretical results are in compliance with the existing experimental observations.

Theoretical Investigations of the Effect of N-Doping on Growth and Field Emission Properties of Carbon Nanotubes (CNTs)

In this work, we investigate effects of doping of hetero-atoms such as Nitrogen (N) on the growth and field emission properties of Carbon Nanotubes (CNTs) in low temperature plasma. A theoretical model is developed to incorporate kinetics of electron, ions and neutral atoms with N as dop ing elements in complex and low temperature plasma. Results of numerical calculations of the radius of N-doped CNT are presented for typical glow discharge plasma parameters. It is found that the radius of CNT is reduced with N-doping. The field enhancement factor of CNT is also estimated from the obtained results.

Enhancement of Field Emission Properties of Carbon Nanotubes by ECR-Plasma Treatment

We report a significant improvement in electron field emission property of carbon nanotubes film by using an electron cyclotron resonance plasma treatment. Our research results reveal that plasma treatment can modify the surface morphology and enhance the field emission characteristics of carbon nanotubes. Raman spectra indicate that plasma treated CNTs sample has lesser defects. Before plasma treatment, low current density of 6.5 mA/cm2 at 3.0 V/μm and at a high turn-on field Eto of 2.4 V/μm was observed. ECR plasma treated CNTs showed a high current density of 20.0 mA/cm2 at 3.0 V/μm and at a low Eto of 1.6 V/μm. The calculated enhancement factors are 694 and 8721 for ECR-plasma untreated and treated carbon nanotubes, respectively. We found an increase in the enhancement factor and emission current after the ECR-plasma treatment. This may be attributed to creation of geometrical features through the removal of amorphous carbon and catalyst particles.

Effect of doping on growth and field emission properties of spherical carbon nanotube tip placed over cylindrical surface

Theoretical investigations to study the effect of doping of hetero-atoms on the growth and field emission properties of Carbon Nanotubes (CNTs) tip placed over a cylindrical surface in complex plasma have been carried out. A theoretical model incorporating kinetics of plasma species such as electron, ions, and neutral atoms including doping elements like nitrogen (N) and boron (B) and energy balance of CNTs in a complex plasma has been developed. The effect of doping elements of N and B on the growth of CNTs, namely, the tip radius has been carried out for typical glow discharge plasma parameters. It is found that N and B as doping elements affect the radius of CNTs extensively. We obtain small radii of CNT doped with N and large radius of CNT doped with B. The field emission characteristics from CNTs have therefore been suggested on the basis of results obtained. Some of theoretical results are in compliance with the existing experimental observations.

Improved field emission properties of carbon nanotubes decorated with Ta layer

The field emission (FE) properties of vertically aligned carbon nanotube (CNT) arrays having a surface decorated with Ta layer were investigated. The CNTs with 6nm thickness of Ta decoration showed improved FE properties with a low turn-on field of 0.64V/μm at 10μA/cm2, a threshold field of 1.06V/μm at 1mA/cm2 and a maximum current density of 7.61mA/cm2 at 1.6V/μm. After Ta decoration, the increased emission centres and/or defect sites on the surface of CNTs improved the field enhancement factor. The work function of CNTs with Ta decoration measured with ultraviolet photoelectron spectroscopy decreased from 4.74 to 4.15eV with increasing Ta thickness of 0–6nm. The decreased work function and increased field enhancement factor were responsible for the improved FE properties of the vertically aligned CNTs. Moreover, a significant hysteresis in the cycle-testing of the current density with rising and falling electric field process was observed and attributed to the adsorption/desorption effect, as confirmed by the photoelectron spectrum.

Improved field emission properties of carbon nanotubes by dual layer deposition

In this paper, we tried to increase the current density of carbon nanotubes (CNTs) by depositing double layer of CNTs instead of single layer. Both the layers of CNTs are deposited by the low pressure chemical vapour deposition technique on silicon substrate with Fe catalyst. Scanning electron microscopic images show the surface morphology of single and double layer of CNTs. Dual layer deposition of CNTs is a very simple and easy method to increase the current density of CNTs based field emitters than other conventional methods. Excellent field emission properties of double layer of CNTs are exhibited with large field enhancement factor and low turn-on voltage as compared to those for single layer of CNTs. High current density of CNTs is required for field-emission-based display devices associated with field enhancement factor and number of emitting electrons. Therefore, we may say that dual layer deposition of CNTs can be utilised as an alternative approach to improve the current density for field emitters. Stability measurement of the samples was also performed for 3 h (180 min) with current at constant applied voltage, and it was found that the stability of dual layer of CNTs is remarkable than that of single layer of CNTs.

Theory of Carbon Nanotube (CNT)-Based Electron Field Emitters.

Theoretical problems arising in connection with development and operation of electron field emitters on the basis of carbon nanotubes are reviewed. The physical aspects of electron field emission that underlie the unique emission properties of carbon nanotubes (CNTs) are considered. Physical effects and phenomena affecting the emission characteristics of CNT cathodes are analyzed. Effects given particular attention include: the electric field amplification near a CNT tip with taking into account the shape of the tip, the deviation from the vertical orientation of nanotubes and electrical field-induced alignment of those; electric field screening by neighboring nanotubes; statistical spread of the parameters of the individual CNTs comprising the cathode; the thermal effects resulting in degradation of nanotubes during emission. Simultaneous consideration of the above-listed effects permitted the development of the optimization procedure for CNT array in terms of the maximum reachable emission current density. In accordance with this procedure, the optimum inter-tube distance in the array depends on the region of the external voltage applied. The phenomenon of self-misalignment of nanotubes in an array has been predicted and analyzed in terms of the recent experiments performed. A mechanism of degradation of CNT-based electron field emitters has been analyzed consisting of the bombardment of the emitters by ions formed as a result of electron impact ionization of the residual gas molecules.

Tailoring of structural and electron emission properties of CNT walls and graphene layers using high-energy irradiation

Structural and electron emission properties of carbon nanotubes (CNTs) and multilayer graphene (MLG) are tailored using high-energy irradiation by controlling the wall thickness and number of layers. Ion irradiation by 100 MeV Ag+ ions at different fluences is used as an effective tool for optimizing defect formation in CNTs and MLGs, as analysed by micro-Raman spectroscopy. It is found that the cross section for defect formation (η) is 3.5 × 10−11 for thin-walled CNTs, 2.8 × 10−11 for thick-walled CNTs and 3.1 × 10−11 for MLGs. High-resolution transmission electron microscopy results also show that thin-walled CNTs and MLGs are more defective in comparison with thick-walled CNTs. Carbon atoms rearrange at a fluence of 1 × 1012 ions cm−2 in thick-walled CNTs to heal up the damage, which aggravates at higher fluences. The observed electron emission parameters of the modified thin-walled CNTs and MLGs are confirmed with the changes in the structures and are optimized at a fluence of 1 × 1011 ions cm−2. However, the electron emission properties of thick-walled CNTs are modified at a fluence of 1 × 1012 ions cm−2. The enhancement in the electron emission properties is due to the rearrangement of bonds and hence modified tips due to irradiation.

Field Emission Properties of Carbon Nanotubes with Boron Doping and H2O Adsorption

Gas adsorption and atom doping usually present when preparing carbon nanotubes (CNTs) and can affect the field emission properties of carbon nanotubes. H2O molecule and boron atom are the most important adsorbates, respectively. Using ab-initio calculations, we have investigated the electron field emission performance of CNTs simultaneously adsorbed with one H2O molecule and doped with one boron atom (BCNT+H2O) in this paper. The results indicate that the electrons localize at the top of BCNT+H2O and the electronic density of states (DOS) around the Fermi level is enhanced obviously. It is expected that BCNT+H2O will be more propitious to the field emission of electrons based on the calculations of DOS, HOMO/LUMO, and Mulliken charge population.

Improvement of the field emission of carbon nanotubes-metal nanocomposite

Carbon nanotubes (CNT) decorated with Ni and Ag performed by electroless plating, and the effect of Ni and Ag nanoparticles and coating distribution on field emission of CNT are studied. The chemical composition, microstructure of CNT/Ni and CNT/Ag nanocomposites are characterized by an energy dispersion X-ray spectroscope (EDS), a transmission electron microscope and a scanning electron microscope. The field emission properties of CNT/Ni and CNT/Ag cathodes are measured using a diode structure under a pressure of 10−5 Pa. The experimental results show that fine and well-dispersed metallic nanoparticles and discontinuous coating of Ni and Ag on the CNT surface can be obtained by electroless plating. Moreover, the enhanced field emission properties of CNT decorated with Ni and Ag can be obtained by lowering the work function of emitters and reducing the contact resistance between cathode and substrate. The field enhancement factors as high as 24264 of CNT/Ni and 25565 of CNT/Ag emitters can be improved by the distributed nano-sized Ni and Ag formed on the CNT surface.

Field emission properties of carbon nanotubes in a stretchable polydimethylsiloxane matrix

The stretchable conducting films are fabricated by screen-printing method combined with polydimethylsiloxane (PDMS) infiltration. This film is a hybrid composite film formed by embedding carbon nanotubes in a PDMS matrix. The sheet resistance is measured with the tensile strain, and the film maintains electric conductivity at the strain up to 55%. The composite film is used as cathode to measure the field emission properties under varying stretching conditions, which is sensitive to the strain. Through the images of field emission scanning electron microscope, the formed pinholes on the surface are found to be contributed to the change in field emission performance.

Improvement of the field emission properties of carbon nanotubes by CNT/Fe3O4 composite electrophoretic deposition

A simple CNT/Fe3O4 composite electrophoretic deposition method to improve the field emission cathode properties of carbon nanotubes (CNTs) is proposed. It is found that CNT/Fe3O4 composite electrophoretic deposition leads to better field emission performance than that of single CNT electrophoretic deposition. The result is investigated using SEM, J-E and FE. After the process, the turn-on electric field decreases from 0.882 to 0.500 V/μm at an emission current density of 0.1 mA/cm2, and the latter increases from 0.003 to 1.137 mA/cm2 at an electric field of 0.64 V/μm. CNT/Fe3O4 composite electrophoretic deposition is an easy and effective cathode preparation for field emission display applications.

Density Functional Theory Study of Influence of Hydrogen Absorption on the Field Emission Properties of Carbon Nanotubes

The field-emission properties of capped(5, 5) single-walled carbon nanotubes with hydrogen adsorbed on the tip with and without an applied electric field have been investigated using first-principles density-functional theory. It is found that the structure of carbon nanotubes with hydrogen molecules is stable under field-emission conditions. The local density of states at the Fermi level increases with the adsorption of hydrogen molecules. These results elucidate that the field-emission properties of carbon nanotubes can be enhanced by the adsorption of hydrogen molecules, and are consistent with the experimental results.

Effect of O2+, H2++ O2+, and N2++ O2+ ion-beam irradiation on the field emission properties of carbon nanotubes

The effect of O2+, H2++ O2+, and N2++ O2+ ion-beam irradiation of carbon nanotubes (CNTs) films on the chemical and electronic properties of the material is reported. The CNTs were grown by the chemical vapor deposition technique (CVD) on silicon TiN coated substrates previously decorated with Ni particles. The Ni decoration and TiN coating were successively deposited by ion-beam assisted deposition (IBAD) and afterwards the nanotubes were grown. The whole deposition procedure was performed in situ as well as the study of the effect of ion-beam irradiation on the CNTs by x-ray photoelectron spectroscopy (XPS). Raman scattering, field-effect emission gun scanning electron microscopy (FEG-SEM), and field emission (FE) measurements were performed ex situ. The experimental data show that: (a) the presence of either H2+ or N2+ ions in the irradiation beam determines the oxygen concentration remaining in the samples as well as the studied structural characteristics; (b) due to the experimental conditions used in the study, no morphological changes have been observed after irradiation of the CNTs; (c) the FE experiments indicate that the electron emission from the CNTs follows the Fowler-Nordheim model, and it is dependent on the oxygen concentration remaining in the samples; and (d) in association with FE results, the XPS data suggest that the formation of terminal quinone groups decreases the CNTs work function of the material.