Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Field emission properties and electronic structures of ultra small diameter carbon nanotubes

The field emission mechanisms of the ultra small armchair and zigzag single-walled carbon nanotubes with finite and infinite length are studied individually by using the first principle theory. For carbon nanotubes with finite length, the field emission is affected not only by the electronic behaviors related to the magnitude of diameter, but also by the geometrical conditions at the edge of finite nanotubes. On the other hand, for the ultra small carbon nanotube with infinite length, the work functions are enhanced significantly due to their remarkable curvature effects compared to the large one. For the open-ended carbon nanotubes, the no-bonding valence electrons induced at the mouth layer after geometrical relaxations are corresponding to the variation of work functions. The localized states at the mouth layer of the open-ended (3,0) and (5,0) exhibit the stable sp³-like and sp² structures, which will influence the occupied and the unoccupied states near the Fermi level and improve the field emission properties of carbon nanotubes.     

Field emission from carbon nanotubes on a graphitized carbon fabric

Multi-walled carbon nanotubes (MWCNTs) have been directly grown over a flexible graphitized carbon fabric by water assisted chemical vapor deposition. Field emission properties are compared with randomly oriented multi-walled and single walled carbon nanotube field emitters obtained by spin coating on to carbon fabric. The MWCNTs and single walled carbon nanotubes (SWCNTs) used in spin coating were characterized by X-ray diffraction (XRD) and Raman spectroscopy. High resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) were used to characterize the field emitters. The use of graphitized carbon fabric as substrate has brought in flexibility in the fabrication of carbon nanotube field emitters. The samples show good field emission properties with a fairly stable emission current. Analysis of field emission based on the Fowler-Nordheim theory reveals current saturation effects at high applied fields for all the samples.     

Field emission properties of N2 and Ar plasma-treated multi-wall carbon nanotubes

We modify multi-wall carbon nanotubes (MWCNTs) by plasma treatment with N₂ and Ar for varying durations and measure their field emission characteristics. The N₂ treated MWCNTs showed significant improvement in field emission properties, while the Ar treated MWCNTs displayed poorer field emission characteristics compared to untreated MWCNTs. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Raman spectroscopy and work function measurements are used to investigate the field emission mechanisms after plasma treatments.     

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 N₂. 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.     

Preparation and field emission properties of Er-decorated multiwalled carbon nanotubes

The field emission of multiwalled carbon nanotubes (MWCNTs) was improved after decorating their external surface with erbium (Er)-nanoparticles. The decoration was performed by liquid-phase reduction using ethylene glycol as the reducing agent. The oxidation of MWCNTs and the attachment of Er-nanoparticles on the surface of MWCNTs were confirmed by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Raman spectroscopy also revealed the oxidation and functionalization of the nanotubes. Thermogravimetric analysis showed that the decomposition temperature of the MWCNTs decreased gradually as a result of the oxidation process and sequential decoration with uniformly sized Er-nanoparticles (2-3 nm). This means that some of the defects formed by oxidation and decoration with Er-nanoparticles reduced the ignition temperature of the MWCNTs. After decoration with Er-nanoparticles, the MWCNTs showed a significantly better emission current density (3.45 mA/cm² at 3.98 V/μm) and turn-on field (1.8 V/μm) than the pristine MWCNTs.     

Patterned growth and field emission properties of vertically aligned carbon nanotubes

Vertically aligned carbon nanotubes were grown selectively on patterned Ni thin films by microwave plasma-enhanced chemical vapor deposition and their field emission properties were investigated using a diode-structure. Ni thin films patterned with a form of dot-arrays were prepared using a shadow mask having an array of holes. The nanotubes were found to be well-graphitized with multiwalled structures. The measurements of field emission properties revealed that the carbon nanotube tips emitted high current density at low macroscopic electric field. The Fowler–Nordheim (F–N) plot clearly showed two characteristic regions where the current saturates at the high electric field region. It was found that the saturation behavior was caused by the adsorbates-enhanced field emission mechanism. Eliminating the adsorbates resulted in no saturation behavior, increasing turn-on field, decreasing current, and increasing field enhancement factor. Using ZnS/Cu,Al phosphor, very bright and uniform emission patterns were obtained.     

Flame-synthesis of carbon nanotubes on silicon substrates and their field emission properties

We reported the flame-synthesis of patterned multiwalled carbon nanotubes (CNTs) on silicon substrate by a shadow mask and their field emission properties. It was found that CNTs with tangled and curved morphology were preferentially grown around the cracked edges of Ni dot pattern. A crack-induced catalyst-activation growth mechanism was proposed. The patterned CNTs fabricated by such a simple flame-synthesis method exhibited good field emission characteristics with uniform emission patterns and reproducible and stable emission behaviors, although the CNTs possessed many defective graphite layers and showed relatively higher turn-on and threshold field than other reported CNTs grown by chemical vapor deposition. Our results demonstrated that such a low-cost and scaleable CNT pattern fabrication process can be expected to have favorable applications in field emission devices.     

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 6 nm thickness of Ta decoration showed improved FE properties with a low turn-on field of 0.64 V/μm at 10 μA/cm², a threshold field of 1.06 V/μm at 1 mA/cm² and a maximum current density of 7.61 mA/cm² at 1.6 V/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 4.15 eV with increasing Ta thickness of 0–6 nm. 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.     

Field emission properties of as-grown multiwalled carbon nanotube films

Multiwalled carbon nanotubes have been produced by ethylene catalytic chemical vapor deposition and used to fabricate thick and dense freestanding films (“buckypapers”) by membrane filtering. Field emission properties of buckypapers have been locally studied by means of high vacuum atomic force microscopy with a standard metallic cantilever used as anode to collect electrons emitted from the sample. Buckypapers showed an interesting linear dependence in the Fowler–Nordheim plots demonstrating their suitability as emitters. By precisely tuning the tip-sample distance in the submicron region we found out that the field enhancement factor is not affected by distance variations up to 2 μm. Finally, the study of current stability showed that the field emission current with intensity of about 3.3 × 10^?5 A remains remarkably stable (within 5% fluctuations) for several hours.     

Field emission property of arrayed nanocrystalline diamond

Arrays of nanocrystalline diamond (NCD) stripes were fabricated by plasma etching of a NCD film. Electron field emission (EFE) of NCD arrays with 100-μm-wide stripes separated by different spacings was analyzed. The NCD arrays had higher EFE efficacy than the non-patterned blanket NCD film. The turn-on electric field (E_on) decreased from 5.4 V/μm^-1 for the blanket NCD film to 4.2, 4.4 and 4.7 V/μm^− 1 for the NCD arrays with 100, 500 and 1000 μm of spacing, respectively. Both the effective emitting area and the field enhancement factor for the NCD emitters were increased by patterning. The enhanced EFE from arrayed NCD stripes was possibly attributed to the edge effect and reduction of electrostatic screening.     

碳纳米管场发射测试方法对其性能的影响

采用丝网印刷技术在金属基片上制备碳纳米管列阵薄膜,应用二极管结构对其场发射性能进行测试,并分析影响碳纳米管阴极场发射性能的因素。结果表明,随着测试次数的增加,碳纳米管阴极场发射性能会有一定程度的提高,同时场发射性能的稳定性增加;阴阳极距离15μm时,碳纳米管具有最好的场发射性能。     

碳纳米管场发射测试方法对其性能的影响

采用丝网印刷技术在金属基片上制备碳纳米管列阵薄膜,应用二极管结构对其场发射性能进行测试,并分析影响碳纳米管阴极场发射性能的因素。结果表明,随着测试次数的增加,碳纳米管阴极场发射性能会有一定程度的提高,同时场发射性能的稳定性增加;阴阳极距离15μm时,碳纳米管具有最好的场发射性能。     

Synthesis of carbon nanotubes on single crystal diamond

Vertically aligned multiwalled carbon nanotubes (MWCNTs) have been synthesised on Ni coated single crystal diamond substrates using a glow-discharge technique. A mixture of gases including CH₄, H₂ and N₂ has been used for growth. The effect of the CH₄/H₂ gas mixture and growth temperature on the structure and yield of the MWCNTs has been studied. Atomic force microscopy is used to characterise the annealed Ni film prior to growth. Scanning electron microscope studies have also been carried out to observe the yield, height and diameter of MWCNTs produced under various experimental conditions. Raman spectroscopy has been performed to provide quantitive information on the crystallinity of our as-grown MWCNTs. It has been shown that highly adherent, vertically aligned MWCNTs can be grown on type Ib diamond (100) substrates with an interface free from metal catalyst.     

Improved field emission properties of double-walled carbon nanotubes decorated with Ru nanoparticles

The field emission properties of double-walled carbon nanotubes (DWCNTs) were remarkably improved by decorating their surface with ruthenium (Ru) metal nanoparticles. The Ru nanoparticles were attached effectively on the surface of DWCNTs via a chemical procedure. The Ru-decorated DWCNTs showed lower turn-on voltage, higher emission current density, and improved emission uniformity compared with pristine DWCNTs. The effect of Ru nanoparticles on the work function and density of states was evaluated by the first-principles calculation. The enhanced field emission properties of Ru-DWCNTs were mainly attributed to the Ru nanoparticles which increased the field enhancement factor and the density of emission sites. Our results indicate that the Ru-decorated DWCNTs can be used as an effective field emitter for various field emission devices.     

Thermionic emission properties and the work function determination of arrays of conical carbon nanotubes

Thermionic emission properties of conical carbon nanotubes (CCNTs) grown on platinum wires and planar graphite foils were investigated. The work function (Φ) values extracted from the thermionic emission data range from 4.1 to 4.7 eV. The range of Φ values is attributed to the morphological characteristics, such as tip radius, aspect ratio, density, and wall structure of CCNTs. The observed lower values for Φ are significantly smaller than that of multi-walled carbon nanotubes (MWNTs). The reduced Φ values are attributed to field penetration effect as a result of the local field enhancement from these structures having high aspect ratio and an excellent field enhancement factor. The high amplification of the external field at the apex of the nanostructures is capable of reducing both the barrier height and the width, in turn contributing to the improved emission current at lower temperatures. The ultraviolet photoemission spectroscopy data of CCNTs grown on Pt wires are in reasonable agreement with the thermionic emission data. The conical carbon nanotubes may be potential candidates for thermionic cathodes with superior performance over conventional cathodes.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Growth of high-quality carbon nanotubes on free-standing diamond substrates

Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH₄ as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C₂H₂ as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra.