Influence of field evaporation treatment on the field emission properties of carbon nanotubes array

Field evaporation was used in the post-fabrication treatment of a carbon nanotubes (CNTs) array and effectively modified the CNTs morphology in favor of the field emission under a moderate field. After the field evaporation treatment, the uniformity of the emission site distribution improved but the onset voltage rose. Using the Fowler-Nordheim theory, the actual onset field and the evaporation field around the CNT were calculated to be −4.6-5 and 9-12 V/nm, respectively. These values are close to those obtained from the individual CNT samples. The above results have provided an alternative to modify the configuration of an array sample and demonstrated the feasibility of tackling the problem of the disparity in the field emission capability of different CNTs in an array.     

Modeling and simulation for the field emission of carbon nanotubes array

To optimize the field emission of the infinite carbon nanotubes (CNTs) array on a planar cathode surface, the numerical simulation for the behavior of field emission with finite difference method was proposed. By solving the Laplace equation with computer, the influence of the intertube distance, the anode–cathode distance and the opened/capped CNT on the field emission of CNTs array were taken into account, and the results could accord well with the experiments. The simulated results proved that the field enhancement factor of individual CNT is largest, but the emission current density is little. Due to the enhanced screening of the electric field, the enhancement factor of CNTs array decreases with decreasing the intertube distance. From the simulation the field emission can be optimized when the intertube distance is close to the tube height. The anode–cathode distance hardly influences the field enhancement factor of CNTs array, but can low the threshold voltage by decreasing the anode–cathode distance. Finally, the distribution of potential of the capped CNTs array and the opened CNTs array was simulated, which the results showed that the distribution of potential can be influenced to some extent by the anode–cathode distance, especially at the apex of the capped CNTs array and the brim of the opened CNTs array. The opened CNTs array has larger field enhancement factor and can emit more current than the capped one.     

Flame synthesis of carbon nanotubes for panel field emission lamp

Multi-walled carbon nanotubes (CNTs) were synthesized on the surfaces of Ni-alloy plated Fe-wires with the diameter of 2 mm using a conventional laboratory ethanol (C₂H₅OH) flame method at 560 °C. SEM showed that the product had bush-shaped micron-structures with diameters from 100 to 450 nm and lengths of over 1.0 μm. TEM revealed that the micron-structures were composed of multi-walled nanotube bundles with the diameters of about 50 nm. The test on the diode configuration field emission of the Fe-wire arrays was performed. The onset electric field was 2.95 V/μm and the emission current can reach 50 mA/cm² at an electric field of 9 V/μm. The average fluctuation of the emission current density was less than 7%. The result suggests that the field emission was uniform and the present technique was feasible to fabricate Panel Field Emission Lamp (PFEL) with arrays of carbon nanotubes. PFEL has the advantages of high luminescence as well as stability, and thus, it can be used to replace ordinary lights.     

Improve the field emission uniformity of carbon nanotubes treated by ball-milling process

Carbon nanotubes (CNTs) deposited by chemical vapor deposition (CVD) were treated by ball milling. The morphologies and field emission properties of the treated CNTs depending on milling time were studied. The emission turn-on field is increased, and the field emission current density is reduced, when the milling time increased from 0.5 to 3 h. The as-deposited long CNTs were cut to short CNTs (∼1 h) and micro-particles (>1 h) with increasing of the milling time. It is found that the optimized milling time is 0.5-1 h, the treated CNTs showed excellent field emission properties, such as low turn-on field, high emission current density and uniform luminescence spots distribution.     

Field emission characteristics of screen-printed carbon nanotubes cold cathode by hydrogen plasma treatment

Selective plasma etching and hydrogen plasma treatment were introduced in turn to improve field emission characteristics of screen-printed carbon nanotubes (CNTs) cold cathode, which was prepared by using slurry of mixture of multi-wall CNTs, organic vehicles and inorganic binder, i.e. silicon dioxide sol. The results show that selective plasma etching process could effectively remove parts of surface inorganic vehicle (SiO₂) layer and expose more smooth and clean CNTs on cathode surface, which could significantly decrease the operating field of CNTs cathode. There are some nanoparticles emerging on the out of CNTs wall after hydrogen plasma treatment, which are equivalent to increase field emission point of cathode. At the same time, these nanoparticles can increase the local electric field of CNTs, which can decrease operating voltage of CNTs cathode and improve uniformity field emission.     

Stable electron field emission from carbon nanotubes emitter transferred on graphene films

Carbon nanotubes (CNTs) arrays grown by microwave plasma enhanced chemical vapor deposition (MPCVD) method was transferred onto the substrate covered with graphene layer obtained by thermal chemical vapor deposition (CVD) technology. The graphene buffer layer provides good electrical and thermal contact to the CNTs. The field emission characteristics of this hybrid structure were investigated in this study. Compared with the CNTs arrays directly grown on the silicon substrate, the hybrid emitter shows better field emission performance, such as high emission current and long-term emission stability. The presence of this graphene layer was shown to improve the field emission behavior of CNTs. This work provides an effective way to realize stable field emission from CNTs emitter and similar hybrid structures.     

Effects of Cs treatment on field emission properties of capped carbon nanotubes

Within the methodology [M. Khazaei, A.A. Farajian, Y. Kawazoe, Phys. Rev. Lett. 95 (2005) 177602] based on first-principles electronic structure calculations, the effects of Cs treatment on current emissions and emission patterns of capped carbon nanotubes are considered at low deposition densities when the nanotubes are under an electric field 0.2 V/Å. The results show that the current emission from the cap with one adsorbed Cs is 3.4 times larger than the cap without any Cs. It is 9.6 times larger in the cap with two adsorbed Cs atoms. After Cs deposition the emission patterns become asymmetric (current emission from the carbon atoms located at the topmost pentagon ring close to Cs atoms is larger than the other atomic sites). There are very few localized states on Cs atoms. Hence, although the tunneling probability of electron emission from Cs atoms is significant, there is no current from Cs atoms. In addition, the effect of Cs on work function reduction of nanotubes is explained in terms of Cs deposition densities and the surface dipole moments.     

Field electron emission images of multi-walled carbon nanotubes

Field electron emission microscope images from multi-walled carbon nanotubes can typically be characterized by the presence of five pentagons surrounding a sixth central pentagon. The observations of bright line centered interference patterns between adjacent pentagons in the field electron emission microscope images of multi-walled carbon nanotubes have been reported in the literature. We have observed a shift from bright to dark line centered interference patterns and associated this with the presence of surface adsorption. In order to identify the origin of the contaminant, multi-walled carbon nanotubes were dosed with H₂, H₂O, CO and O₂ and then imaged in the field electron emission microscope. Only the samples exposed to O₂ showed a shift from a bright line centered pattern between adjacent pentagons of a clean surface to a dark line centered pattern when one pentagon was contaminated or a bright line centered pattern when both adjacent pentagons become contaminated. The results of the experimental studies and the modeling of the changes in the field emission pattern as phase shifts in the wave function of the tunneling electrons due to modifications in the surface work function are presented.     

Effects of total CH4/Ar gas pressure on the structures and field electron emission properties of carbon nanomaterials grown by plasma-enhanced chemical vapor deposition

The effects of total CH₄/Ar gas pressure on the growth of carbon nanomaterials on Si (1 0 0) substrate covered with CoO nanoparticles, using plasma-enhanced chemical vapor deposition (PECVD), were investigated. The structures of obtained products were correlated with the total gas pressure and changed from pure carbon nanotubes (CNTs) through hybrid CNTs/graphene sheets (GSs), to pure GSs as the total gas pressure changed from 20 to 4 Torr. The total gas pressure influenced the density of hydrogen radicals and Ar ions in chamber, which in turn determined the degree of how CoO nanoparticles were deoxidized and ion bombardment energy that governed the final carbon nanomaterials. Moreover, the obtained hybrid CNTs/GSs exhibited a lower turn-on field (1.4 V/μm) emission, compared to either 2.7 V/μm for pure CNTs or 2.2 V/μm for pure GSs, at current density of 10 μA/cm².     

碳、碳氮和硼碳氮纳米管场发射性能的比较研究

采用高温热解法在860℃分别制备出了碳、碳氮和硼碳氮纳米管，提纯后利用丝网印刷工艺分别将它们制备成薄膜，并测试了它们的场发射性能.结果表明：碳纳米管、碳氮纳米管和硼碳氮纳米管薄膜的开启电场分别为2.22，1.1和4.4V/μm，当电场增加到5.7V/μm时，它们的电流密度分别达到1400，3000μA/cm²和小于50μA/cm².碳和碳氮纳米管薄膜的场增强因子分别为10062和11521.可见，碳氮纳米管的场发射性能优于碳纳米管，而硼碳氮纳米管的场发射性能比前两者要差.解释了这三种纳米管场发射性能差别的原因. 关键词： 碳纳米管 碳氮纳米管 硼碳氮纳米管 场发射     

Influence of growth time on field emission properties from carbon nanotubes deposited on arrayed nanoporous silicon pillars

We investigated the influence of growth time on field emission properties of multi-walled carbon nanotubes deposited on silicon nanoporous pillar array (MWCNTs/Si-NPA), which were fabricated by thermal chemical vapour deposition at 800 °C for 5, 15 and 25 min respectively, to better understand the origins of good field emission properties. The results showed that the MWCNTs/Si-NPA grown for 15 min had the highest field emission efficiency of the three types of samples. Morphologies of the products were examined by field-emission scanning electron microscope, and the excellent field emission performance was attributed not only to the formation of a nest array of multi-walled carbon nanotubes, which would largely reduce the electrostatic shielding among the emitters and resulted in a great enhancement factor, but also to the medium MWCNTs density films, there was an ideal compromise between the emitter density and the intertube distance, which also could effectively avoid electrostatic shielding effects, along with a high emitter density.     

Field emission properties of carbon nanotubes film grown on NiCr alloy films

Carbon nanotubes (CNTs) are prepared on NiCr alloy films by low pressure thermal chemical vapor deposition at 600 °C. NiCr alloy films are deposited by magnetic co-sputtering method, and the various thickness and Ni/NiCr ratios are controlled by sputtering power. The diameter and length of CNTs, as well as the roughness of the CNTs films, mainly depend on the Ni/NiCr ratio. The field emission current density of the CNTs film increases with the increasing Ni/NiCr ratio from 65 wt% to 83 wt%, and decreases when the Ni/NiCr ratio is more than 87 wt% in the alloy film.     

Influence of high-energy electron irradiation on field emission properties of multi-walled carbon nanotubes (MWCNTs) films

The effect of very high energy electron beam irradiation on the field emission characteristics of multi-walled carbon nanotubes (MWCNTs) has been investigated. The MWCNTs films deposited on silicon (Si) substrates were irradiated with 6 MeV electron beam at different fluence of 1×10¹⁵, 2×10¹⁵ and 3×10¹⁵ electrons/cm². The irradiated films were characterized using scanning electron microscope (SEM) and micro-Raman spectrometer. The SEM analysis clearly revealed a change in surface morphology of the films upon irradiation. The Raman spectra of the irradiated films show structural damage caused by the interaction of high-energy electrons. The field emission studies were carried out in a planar diode configuration at the base pressure of ∼1×10⁻⁸ mbar. The values of the threshold field, required to draw an emission current density of ∼1 μA/cm², are found to be ∼0.52, 1.9, 1.3 and 0.8 V/μm for untreated, irradiated with fluence of 1×10¹⁵, 2×10¹⁵ and 3×10¹⁵ electrons/cm². The irradiated films exhibit better emission current stability as compared to the untreated film. The improved field emission properties of the irradiated films have been attributed to the structural damage as revealed from the Raman studies.     

A comparative study of thermionic emission from vertically grown carbon nanotubes and tungsten cathodes

Thermionic emission from vertically grown carbon nanotubes (CNTs) by water-assisted chemical vapor deposition (WA-CVD) is investigated. I-V characteristics of WA-CNT samples exhibit strong Schottky effect leading to field proportionality factor β ∼ 10⁴ cm⁻¹in contrast to β ∼ 200 cm⁻¹ for the bare tungsten substrate. Non-contact atomic force microscopy imaging of CNT samples show propensity of nanoasperities over a scale of micron size over which the tungsten surface is seen to be atomically smooth. The values of root mean-square roughness for CNTs and W were found to be 24.2 nm and 0.44 nm respectively. The Richardson-Dushman plots yield work function values of Φ_CNT ? 4.5 and Φ_W ? 4.3 eV. Current versus time data shows that CNT cathodes are fifteen times noisier than tungsten cathode presumably due to increased importance of individual atomic events on the sharp CNT tips of bristle like structures. Power spectral density of current exhibited 1/f^ξ behavior with ξ ? 1.5, and 2 for W and CNTs. The former suggests surface diffusion whereas the latter indicates adsorption/desorption of atomic/molecular species as a dominant mechanism of noise generation.     

多壁碳纳米管阵列场发射研究

研究了Ar离子束轰击及温度对多壁碳纳米管阵列场发射性能的影响.经Ar离子轰击35min后，发现阵列顶端的Fe催化剂颗粒明显减少，弯曲的顶部被轰击掉，使碳纳米管的场发射电流明显减小而场发射像无明显改变.温度的增加引起碳纳米管的场发射电流也随之增加.还研 究了在透明阳极技术中涂在阳极的荧光粉对场发射电流的影响.对同一碳纳米管阵列样品，发现涂有荧光粉的透明阳极使测量到的场发射电流大幅度减小，只是未涂荧光粉阳极电流的 1/30左右.直接用二氧化锡导电膜作阳极时，测得样品的开启场强为1.0V/μm.沉积了荧光粉的二 关键词： 多壁碳纳米管 场发射     

碳纤维阴极的场致发射特性实验研究

 研究了具有一定导电性能的碳纤维的直流场致发射电子束特性,实验分别在大气环境、低真空（10^-1 Pa）及高真空(10^-5 Pa)环境下进行。实验结果表明,碳纤维具有一定的场致发射能力,并且发射特性和发射环境的真空度密切相关。在大气环境下,发射电子束流与所施加的电压符合Fowler-Nordheim关系,当电压为7 kV,电流为61.4 μA时,根据Fowler-Nordheim定理推算出碳纤维场致发射场增强因子为3.75×10⁵。在低真空条件下阳极只收集到微弱的电流;在高真空条件下,阴极发射明显,在较低电压下就能观测到阳极电流,放电前阳极最高电流是大气条件下的3~4倍,发射的束流大小和所施加在尖端的电压关系接近Child-Langmiur定律。     

锥顶碳纳米管的结构稳定性与场致发射性能

运用密度泛函理论研究了锥顶碳纳米管的结构稳定性与电子场致发射性能.结果表明:在外电场作用下,该体系的结构稳定性明显优于碳纳米锥体、C₃₀半球封口的碳纳米管,且电子发射性能与锥角大小、锥顶构型密切相关,特别是锥角38.9°及棱脊型顶部的cone₁@(6,6)综合性能最优,用其作为场致发射源的阴极时可显著提高发射电流密度并延长器件的使用寿命. 关键词： 锥顶碳纳米管 电子场致发射 结构稳定性 密度泛函理论     

Effects on the field emission properties by variation in surface morphology of patterned photosensitive carbon nanotube paste using organic solvent

Photosensitive carbon nanotube (CNT) paste was prepared by 3-roll milling of multi-walled carbon nanotubes (MWNTs), UV-sensitive binder solution, and Ag as filler additives. Arrays of MWNT dots with a diode structure were fabricated by a combination of screen printing method and photolithography using these paste, and acetone utilized as the developer. The MWNT dots were well-defined and the organic binder materials in the dots were partially removed. The MWNT film without a heat treatment showed a high current density of 1.35 mA/cm² at 3.25 V/μm and low turn-on field of 2.2 V/μm at 100 μA/cm². Acetone can be used as an efficient developer to form patterns and to remove the organic residues in patterns, simultaneously.     

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I-V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm² at 2.5 V/μm.     

Water vapor-induced structure modification of vertically-aligned carbon nanotube arrays and successive thin film coating for enhanced field emission properties

Vertically-aligned carbon nanotubes (VCNTs) are used as electron source in various field emission applications owing to its high aspect ratio, chemical inertness, mechanical strength and electrical conductivity. Here, we demonstrate that surface structure modification along with thin film coating enhances the field emission performance, such as turn-on voltage, emission site density, and stability. In the present study, VCNTs with different heights were grown on silicon wafers by thermal chemical vapor deposition followed by the structure modification of VCNTs using capillarity-driven water vapor condensation. We obtained various surface morphologies by varying the water vapor exposure time and heating temperature. In addition, the structure-modified VCNTs surfaces were coated with W and SiO₂ thin films using electron-beam evaporation. It was observed that W-coated VCNTs with modified surface morphology results in the best field emission performance.