Field Emission Properties Of Carbon Nanotubes Research Articles

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.

Initiation of vacuum breakdown and failure mechanism of the carbon nanotube during thermal field emission**Project supported by the National Natural Science Foundation of China (Grant Nos. 11305263 and 61401484).

The carbon nanotube (CNT)-based materials can be used as vacuum device cathodes. Owing to the excellent field emission properties of CNT, it has great potentials in the applications of an explosive field emission cathode. The falling off of CNT from the substrate, which frequently appears in experiments, restricts its application. In addition, the onset time of vacuum breakdown limits the performance of the high-power explosive-emission-cathode-based diode. In this paper, the characteristics of the CNT, electric field strength, contact resistance and the kind of substrate material are varied to study the parameter effects on the onset time of vacuum breakdown and failure mechanism of the CNT by using the finite element method.

Low-Temperature Preparation of a Carbon Nanotube–ZnO Hybrid on Glass Substrate for Field Emission Applications

A new method for preparation of a carbon nanotube (CNT)–zinc oxide hybrid on a glass substrate at low temperature is introduced and the stability improvement of field emission of CNTs is reported. The emission current stability and substrate material are the two main commercial parameters for field emission applications of CNTs. The two alternative current (AC) and direct current (DC) plasma enhancement chemical vapor deposition (PECVD) is used to achieve low-temperature growth of vertically aligned CNTs and the hydrothermal processing is used for production of zinc oxide nanowires to improve the emission current stability and properties. The nanoscale junction between a semiconductor and conductive material are critical for electronic applications. The present study examined the field emission properties of CNTs near the crystalline structure of the zinc oxide hybrids and showed the improvement in the field enhancement factor and emission current stability of CNT– ZnO hybrid than the CNT emitters.

MEMS Carbon Nanotubes Field Emission Pressure Sensor With Simplified Design: Performance and Field Emission Properties Study

The pressure sensor application gained recently substantial interest in many fields of basic and applied research and applications. In this paper, microelectromechanical system (MEMS)-based pressure sensor contains nanostructured electrode consisting of carbon nanotube (CNT) array. CNTs are directly grown on such electrode by plasma-enhanced chemical vapor deposition method using microwave plasma torch at atmospheric pressure. This growth method enables us to use a simple electrode structure without need of buffer layer and time-consuming lithography process. Combination of CNTs field emission and MEMS membrane mechanical properties make possible to enhance sensitivity of the sensor. Field emission properties of CNTs are measured by newly developed system enabling us precise measurement of expecting properties, such as dependence on diaphragm (upper electrode) distance, applied voltage, and stability of the sensor. Measured values are compared with a numerical modeling of the membrane system in Coventor Ware software by finite-element method. We also suggest encapsulating the sensor using glass frit bonding because such method is more suitable for high vacuum requirements of the field emission operation.

Effect of Plasma Compositions on Growth and Field Emission Properties of a Spherical Carbon Nanotube (CNT) tip

The effect of plasma compositions (i.e., different concentration of participating ions) on growth, structure and field emission properties of spherical Carbon Nanotube (CNT) tip has been theoretically investigated. In plasma, two different kinds of positively charged ions with heavy to light ion mass ratio of 11.5 are considered and the effect of the different fractional concentrations of participating ions on the growth and field emission properties of CNT is studied. Numerical calculations of the radius of spherical CNT tip for different fractional light ion concentrations have been carried out for the typical glow discharge plasma parameters. It is found that on increasing fractional light ion concentration, the radius of spherical CNT tip decreases and consequently the field emission factor for spherical CNT tip increases.

Controlled surface damage of amorphous and crystalline carbon nanotubes for enhanced field emission

The effect of argon (Ar) ion treatment on the field-emission (FE) properties of amorphous carbon nanotubes (aCNTs) and crystalline multiwall carbon nanotubes (MWCNTs) is reported in this article. Under optimum Ar-ion irradiation conditions, the FE properties of carbon nanotubes (CNTs) have been found to improve. The turn-on field of MWCNTs downshifted from 2.81 to 0.85 V μm−1 and from 9.17 to 3.27 V μm−1 for aCNTs. Improvement of the FE behavior is associated with the morphological modifications of CNTs, as a result of Ar-ion irradiation. Ar-ion bombardment creates defect sites on the CNT surface as confirmed by various techniques such as field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). We have proposed that the defects caused modifications of the CNT surfaces resulting in an effective increment of the field-enhancement factor β.

Effects of Cs adsorption on the field emission characteristics of closed single-walled carbon nanotubes

Abstract

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.

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

Using first-principles density-functional theory investigate (5, 5) capped single-walled carbon nanotubes adsorbed at one end by hydrogen molecule with and without an applied electric field. It is found that the structure of carbon nanotubes with hydrogen molecules is stable under field-emission conditions. The induced dipole moments is the direction of the applied electric field，and the LUMO-HOMO energy gaps decrease. 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.

Improving the field-emission properties of carbon nanotubes by magnetically controlled nickel-electroplating treatment

A novel magnetically controlled Ni-plating method has been developed to improve the field-emission properties of carbon nanotubes (CNTs). The effect of the magnetic field and Ni-electroplating on CNT field-emission properties was investigated, and the results are demonstrated using scanning electron microscopy, J—E and the duration test. After treatment, the turn-on electric field declines from 1.55 to 0.91 V/μm at an emission current density of 100 μA/cm2, and the emission current density increases from 0.011 to 0.34 mA/cm2 at an electric field of 1.0 V/μm. Both the brightness and uniformity of the CNT emission performance are improved after treatment.

Effects of Potassium Hydroxide Post-Treatments on the Field-Emission Properties of Thermal Chemical Vapor Deposited Carbon Nanotubes

In this study, a simple potassium hydroxide treatment was applied to functionalize the surface and to modify the structure of multi-walled carbon nanotubes grown on silicon substrates by thermal chemical vapor deposition. Scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and energy dispersive spectrometry were employed to investigate the mechanism causing the modified field-emission properties of carbon nanotubes. From our experimental data, the emitted currents of carbon nanotubes after potassium hydroxide treatment are enhanced by more than one order of magnitude compared with those of untreated carbon nanotubes. The emitted current density of carbon nanotubes increases from 0.44 mA/cm2 to 7.92 mA/cm2 after 30 minutes KOH treatment. This technique provides a simple, economical, and effective way to enhance the field-emission properties of carbon nanotubes.

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.

Electron field emission from carbon nanotubes in gas

Field emission (FE) properties of carbon nanotubes (CNTs) in vacuum, Ar gas or air were investigated by scanning electron microscopy. The gas pressures were changed from 0·2 to 1 kPa. The results showed that when the gap was smaller than 20 μm, the FE properties of CNTs were the same in vacuum and in Ar. The FE properties in original vacuum and in air were the same, which were different from that in vacuum after FE measurement in air. When the gap was changed from 20 to 100 μm, there was a discharge at 200–300 V. After the Ar discharge, the FE properties were the same in vacuum. After the air discharge, there was damage on the surface of the CNTs and the turn-on voltage in vacuum was higher than that in original vacuum. The stability of the CNTs experiments results showed that the CNTs were more stable in Ar than that in air during 300 s measurement.

Field emission characteristics of carbon nanotubes post-treated with high-density Ar plasma

The field emission characteristics of carbon nanotubes (CNTs) grown by thermal chemical vapor deposition (CVD) and subsequently surface treated by high-density Ar plasma in an inductively coupled plasma reactive ion etching (ICP-RIE) with the various plasma powers were measured. Results indicate that, after treated by Ar plasma with power between 250 and 500 W, the emission current density of the CNTs is enhanced by nearly two orders of magnitude (increased from 0.65 to 48 mA/cm 2) as compared to that of the as-grown ones. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to investigate the structural features relevant to the modified field emission properties of CNTs. The SEM images of CNTs subjected to a 500 W Ar plasma treatment exhibit obvious damages to the CNTs. Nevertheless, the turn-on fields decreased from 3.6 to 2.2 V/μm, indicating a remarkable field emission enhancement. Our results further suggest that the primary effect of Ar plasma treatment might be to modify the geometrical structures of the local emission region in CNTs. In any case, the Ar plasma treatment appears to be an efficient method to enhance the site density for electron emission and, hence markedly improving the electric characteristics of the CNTs.

Field-emission properties of multiwalled carbon nanotubes in gas

Ar gas effects on the field-emission (FE) properties of multiwalled carbon nanotubes (CNTs) were investigated. The FE measurement was carried out in vacuum and in Ar gas using a scanning-electron microscope (SEM) chamber with a removable gas cell fitted in the SEM chamber. The gap between electrodes was set by observing a SEM image. The electrodes were characterized in situ before and after FE measurements by SEM observation. The FE properties of CNTs in Ar gas at 80kPa and lower pressures were the same as those in vacuum before and after the measurements in gas. At pressures more than 80kPa, the FE threshold voltages in Ar gas were higher than those in vacuum and increased with increasing Ar gas pressure. However, the FE properties in vacuum after the measurement in gas were the same as before. When the FE threshold voltages became higher than the voltage for gas breakdown, a gas discharge was ignited before the FE appearance. After the gas discharge, the carbon nanotube cathode was damaged.

Enhanced electron field emission of carbon nanotubes by Si ion beam irradiation

The electron field emission of carbon nanotubes (CNTs) has been enhanced by Si ion beam irradiation. The defects generated by ion beam irradiation and the absorbed molecules on the CNTs were responsible for the enhancement of electron field emission. This investigation provides a possible way to improve the electron field emission properties of CNTs and understand the mechanism of electron field emission enhancement of CNTs.

Electron Field Emission from Carbon Nanotubes in Air for Excitation of Atmospheric Pressure Microplasma

Electron field emission (FE) from carbon nanotubes (CNTs) in air for excitation of atmospheric pressure microplasma was investigated in a scanning electron microscope (SEM). The results showed that, the FE properties of CNTs at 10 kPa and lower pressures were the same as those in vacuum. At pressures more than 10 kPa, the FE threshold voltages in air were higher than those in vacuum and increased with increasing air pressure. When the FE threshold voltage became higher than the gas breakdown voltage, microplasma was ignited before FE appearance. FE properties of CNTs were stable in air before appearing gas discharges.

Field-emission properties of carbon nanotubes grown using Cu–Cr catalysts

In addition to Ni, Co, and Fe, the authors show that a copper (Cu) chromium (Cr) alloy can be a good catalyst for controlled growth of carbon nanotubes (CNTs). A thermal chemical vapor deposition was used at 600°C to deposit the CNTs, and a dual-target magnetron sputtering system was used to control the ratio of the Cu∕Cr alloy. The material properties of the CNTs, such as concentration, diameter, and density, are directly affected by the ratio of Cu∕Cr alloy, which subsequently affect the field-emission properties. Their results showed that Cr can be used to effectively control the catalytic effects of the Cu catalysts, where the quality of the CNTs could vary while the density could be controlled from 4×107to5×1010tubes∕cm2. The field-emission current density of the CNT film increases with Cu∕Cr ratio from 1∕6.4 to 4.6∕1 and decreases when the Cu∕Cr ratio is more than 4.8∕1 in the alloy film. When the Cu∕Cr ratio is at 4.6∕1, the average current density peaks at 5129μA∕cm2 at the applied field of 3.53V∕μm, showing an optimized field-emission property.

Enhancement of Field Emission Characteristics for Multi-Walled Carbon Nanotubes Treated with a Mixed Solution of Chromic Trioxide and Nitric Acid

A simple acid treatment method was applied to functionalize the surface and to modify the structures of multi-walled carbon nanotubes (CNTs) grown on silicon substrates using a mixed solution of chromic trioxide (CrO 3) and nitric acid (HNO 3). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and energy dispersive spectrometer (EDS) were employed to investigate the mechanism causing the modified field emission (FE) properties of the CNT films. After 20 min of CrO 3+HNO 3 treatment, the emitted currents were enhanced by more than one order of magnitude compared with those of the untreated CNTs. This large increase in emitted current can be attributed to the favorable surface morphologies, open-ended structures, and highly curved CNT surfaces in the CNT films. These factors altogether caused an increase in the field enhancement factors of CNTs. We also demonstrated that using a mixed solution of CrO 3+HNO 3 post-treatment exhibited a higher emission current and a lower turn-on electric field than in the CNTs treated with HNO 3. The method provides a simple, economical, and effective way to enhance the CNT field emission properties.

Field-emission properties of carbon nanotubes grown on a submicron-sized tungsten tip in terms of various buffer layers

Submicron-sized conical tungsten (W) field emitters based on carbon nanotubes (CNTs) are fabricated with the configuration of CNTs/catalyst/buffer/W tip by using various buffer layers such as TiN, Al, Al/TiN, and Al/Ni/TiN. This study focuses on elucidating how the buffer layers affect the structural properties of CNTs and the electron-emission characteristics of CNT emitters. Field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) are used to study the nanostructures, surface morphologies, chemical bonds of the catalysts, and grown CNTs. Further, the crystalline structure of CNTs is characterized by Raman spectroscopy. Furthermore, the measurement of field-emission characteristics of all the fabricated field emitters shows that the emitter that employs the Al/Ni/TiN stacked buffer exhibits excellent performances, e.g., a maximum emission current of 202 μA, threshold field of 2.08 V/μm, and long-term (up to 24 h) stability of the emission current.