A novel approach to improve the field emission characteristics of printed CNT films

A novel post-treatment method, including hard hairbrush and electrical treatment, is performed intentionally to improve the field emission capability and stability of screen-printed carbon nanotubes （CNTs）. Compared with untreated films, the field emission properties of the treated ones are greatly enhanced. Scanning electron microscopy （SEM） and Raman spectrum studies reveal that field emission properties are enhanced by two factors. Firstly, the improved field emission properties of CNT films can be attributed to the more active CNT surface by removing the organic material cover on the CNTs. Secondly, the gener- ation of a high density of structural defects and the lower resistance contact to the topside CNT emitters after treatment are all helpful to improving the field emission properties of the treated CNTs.     

Field emission characteristic of screen-printed carbon nanotube cathode

The fabrication of carbon nanotube emitters with excellent emission properties is described. The multi-walled carbon nanotubes (MWNTs) produced by chemical vapor deposition (CVD) method were purified with oxidation method and mixed with organic binding pastes and then screen-printed on glass substrates with ITO film. We applied anode voltage gradually to refine the emission behavior of the emitter by cleaning the top surface of screen-printed carbon nanotubes (CNTs). The density of the carbon nanotubes is about 2.5×10⁸/cm². Diode field emission experiments were performed in dynamic vacuum system to study the emission current, the emission uniformity, etc. Bright and stable character emission images were obtained in the diode structure and the emission current could approach 1 mA/cm².     

Enhanced field emission characteristics of zinc oxide mixed carbon nano-tubes films

A composite material of Zinc oxide and carbon nano-tubes (ZnO-CNTs) paste was synthesized by mixing multi-wall CNTs, ZnO nano-grains and organic vehicles. The microstructures and the morphologies of screen-printed films were characterized by field-emission scanning electron microscope. Results show that ZnO flakes geometrically matched with CNTs by filling into the interspaces of CNTs or directly covering upon CNTs. The field emission characteristics of films are found to be greatly effected by ZnO nano-grains. Especially, the turn-on electric field of ZnO-CNT film (1.17 V/μm) which is far lower than that of usual CNT films (1.70 V/μm). Furthermore, except that better emission stability is achieved, brightness and emission uniformity are notably enhanced as well. It can be speculated that the special microstructures of ZnO mixed CNT films dominate the enhanced electrical conductivity, thermal conductivity, and effective emitters.     

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.     

碳纳米管/膨润土复合膜的附着性能及场发射性能

碳纳米管(CNT)阴极膜与基底的附着性能严重影响场发射的均匀性和稳定性.提出一种制作CNT/膨润土(BP)复合膜的方法.通过添加纳米尺度BP作为填料,使BP与CNT间产生插层效应,进而减小CNT膜的间隙势牟,改善CNT与基底间的接触性能.测试结果显示,CNT/BP复合膜与基底的附着性能得到明显改善,并且使场发射均匀性和...     

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.     

Improved field emission stability and uniformity of printed carbon nanotubes prepared using high energy-milled glass frit

Stable and uniform electron field emitters were fabricated by the screen-printing of a carbon nanotube (CNT) paste. The CNT paste was prepared by mixing thin multi-walled CNTs, organic binder and glass frits in terpineol followed by three-roll milling. The paste was screen-printed onto an indium tin oxide (ITO)-coated glass substrate and exposed to firing. The emitters formed by the adhesive tape treatment of the printed pastes generally showed non-uniform heights of emitting tips due to the large-sized glass frit distributed in the paste. On the other hand, small glass frits with a uniform particle size distribution could be obtained by high energy milling. In addition, the CNT paste prepared using the high energy-milled glass frits showed very uniform tip heights. The uniform CNT tip height was found to play an important role in acquiring the long-term emission stability and uniformity of field emitters.     

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.     

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.     

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.     

Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.     

激光烧蚀对碳纳米管薄膜场发射性能的影响

采用丝网印刷工艺制作了碳纳米管(CNTs)薄膜阴极.经适当能量激光烧蚀后,相互粘连的CNTs随表面粘附有机物的蒸发而分散开,管间隙增加、屏蔽效应减小,使得场发射性能大幅度提高,开启场强降低、场倍增因子β增大.Raman光谱分析表明,随激光能量增加,CNTs表面缺陷增多,成为新的场发射点,对其β增大的贡献加强.相对于两电极结构,三电极中平栅极结构场发射性能经激光烧蚀有更显著的改善.这说明激光烧蚀是提高CNTs场发射性能的有效方法. 关键词： 碳纳米管薄膜 场发射 激光烧蚀 Raman光谱     

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.     

碳纳米管薄膜场蒸发效应

在超高真空系统中对基于丝网印刷方法制备的碳纳米管薄膜的场蒸发效应进行实验研究. 实验发现, 碳纳米管薄膜样品存在场蒸发现象, 蒸发阈值场在10.0-12.6 V/nm之间, 蒸发离子流可以达到百皮安量级; 扫描电子显微镜分析和场致电子发射测量结果表明, 场蒸发会使碳纳米管分布变得更加不均匀, 会导致薄膜的场致电子发射开启电压上升(240→300V)、场增强因子下降(8300→4200)、蒸发阈值场上升(10→12.6V/nm), 同时使得薄膜场致电子发射的可重复性明显变好. 场蒸发也是薄膜自身电场一致性修复的表现, 这种修复并非表现在形貌上, 而是不同区域场增强因子之间的差距会越来越小, 这样薄膜场致电子发射的可重复性和稳定性自然会得到改善.     

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.     

Carbon nanotube field emission cathodes fabricated with chemical displacement plating

A new approach for making field emission cathodes consisting of carbon nanotubes (CNTs) is discussed. The authors used a chemical displacement technique to fabricate field emission cathodes by co-depositing CNTs/nickel composite onto the surface of a zinc-coated soda-lime glass. There are several advantages of this displacement method for preparing field emission cathodes such as the uniform distribution of CNTs in the composite cathodes, low cost of consumed CNTs, low cost of instrument and equipment, feasibility of large-area mass production, and stability of plating solution, which can be used for many times and still remain useful after a long-time storage. The results show that, after the CNT purification and dispersion processes, a CNT content of 1.0 g/L, a pH value of 7.0, and a temperature of 50 ± 3 °C are the optimal process conditions which give better CNT distribution in the CNTs/Ni composite emitter and better field emission performance. The CNTs/Ni composite deposited with a plating solution which has been used for tens of times has an emission effect similar to those deposited with a new solution.     

沉积工艺参数对碳纳米管薄膜场发射性能的影响

利用微波等离子体化学气相沉积(MWPCVD)方法,在不锈钢衬底上直接沉积碳纳米管膜。通过SEM、拉曼光谱和XRD表征,讨论了制备温度和甲烷浓度对碳纳米管膜场发射的影响。结果表明:不同条件下制备的碳纳米管膜的场发射性能有很大差异,保持氢气的流量(100sccm)、生长时间(10min)、反应室压力不变,当甲烷流量为8sccm、温度为700~800℃时,场发射性能最好,开启场强仅为0.8V/μm,发射点分布密集、均匀。     

Effect of sputtered Cu film’s diffusion barrier on the growth and field emission properties of carbon nanotubes by chemical vapor deposition

Growth of carbon nanotube (CNT) films with good field emission properties on glass is very important for low cost field emission display (FED) applications. In addition to Ni, Co and Fe, Cu can be a good catalyst for CNT growth on glass, but due to diffusion into SiO₂ it is difficult to control the CNTs density and uniformity. In this paper, four metal barrier layers (W, Ni, Cr, Ti) were deposited by dc magnetron sputtering on glass to reduce the Cu diffusion. As-grown CNT films showed various morphologies with the use of different barrier metals. CNTs with uniform distribution and better crystallinity can be synthesized only on Ti/Cu and W/Cu. Voltage current measurements indicate that better field emission properties of CNT films can be obtained on titanium and tungsten barriered Cu, while chromium and nickel are not suitable barrier candidates for copper in CNT-FED applications because of the reduced emission performance. PACS 81.05.Uw; 61.46.Fg; 85.45.Db; 66.30.-h     

基于丝网印刷技术的碳纳米管场发射冷阴极制作

制备了一种新型碳纳米管浆料,并总结了一套阴极制作工艺.实验表明,质量比约为10%的纯化碳纳米管、5%的纳米金属粉末和有机材料混合形成的印刷浆料,其阴极具有较好的发射均匀特性.在浆料制备过程中通过加入表面活性剂使碳纳米管分散更加均匀.用丝网印刷技术制作阴极,并用机械刀刻的办法制作阴、栅两极之间的沟槽,经烧结后分析阴极膜的电阻率和粘附性随阴极材料组分和制作工艺的变化关系,确定较为合适的升温曲线及丝网目数,阴极碳纳米管的均匀性、导电性的提高改善了发射均匀性.该阴极开启场为2.5 V/μm,在电场强度为3.3 V/μm下,阳极电流为5.6 μA,场发射电流稳定,波动小于5%.     

LPCVD法制备碳纳米管薄膜及其场发射性能的研究

采用低压化学气相沉积法(LPCVD)在镍片上直接制备碳纳米管(CNTs)薄膜,系统地研究了生长温度(500~800℃)对碳纳米管薄膜形貌、结构及场发射性能的影响,并对此方法的生长机理进行了分析。当温度从500℃升高到650℃时,碳纳米管的生长速率随着温度升高而增大,而温度继续上升,速率则明显减小。利用扫描电镜(SEM)和拉曼光谱仪表征和检测了碳纳米管薄膜的形貌和结构。碳纳米管的管径、长度、一致性和晶化程度随温度都有明显的变化。同时还对碳纳米管薄膜的场发射特性进行了测试,对其场发射机理进行了深入地探讨,表明温度对碳纳米管的性能有很大影响,并存在最优化的温度条件。实验结果表明碳纳米管薄膜的形貌、结构及其场发射性能可通过生长温度进行一定范围的控制。