Novel ring structure for minimisation of screening effect in carbon nanotube based field emitters

Carbon Nanotubes (CNTs) are promising candidates for cold cathodes because of their high aspect ratio and robustness. However, the major hindrance in cold cathode based applications is the screening effect, which reduces the effective field at the tip and thereby the current density. The emission current can be improved by minimising the screening effect. The adverse effect of screening can be addressed by either controlling the growth density or by optimising the patterns of CNT cathodes. Here, novel patterns have been used to increase edge length per unit area in planar vertically aligned CNT bundles. Our motive was to increase the number of effective emitters, since the CNT at the edges are less screened by the proximal CNTs. By varying geometry and spacing of solid CNT dot patterns and by introducing the square ring structures; we could successfully enhance the effective emitters at the edges. It has been observed that an enhancement of edge length from 0.032 per micron to 0.2 per micron increases the current density from 0.71mA/cm² to 16.2 mA/cm² at a field of 4.5 V/μm. CNTs in dotted structure with high value of edge length per unit area emit very high current density as compared to other dotted structures with low value of edge length per unit area Simulation studies confirms our argument that CNTs at the corners are the least screened and have the maximum local electric field.     

Improvement and characteristics of conical silicon emitters employing wet-dry etching

A number of new technologies require conical and sharp tips to serve as electron emitters in the vacuum microelectronics. In this paper, we improved radius of curvature, height and cone angle of emitters in order to reach the enhancement result of field enhancement factor (β). We developed a fabrication process to improve geometry of emitter by employing isotropic dry etching in pure SF₆ and a mixture of SF₆ and O₂ followed by thermal oxidation technique. We successfully achieved excellent conical emitters with 5–10 nm radius of curvature, 4.4 μm height, and 30° cone angle. The conical silicon emitters current–voltage characteristics shows that E_to = 4.8 V/μm (turn-on electric field) with current density of 10 μA/cm², and maximum current density J = 60.4 μA/cm² at E = 8.14 V/μm. This study may provide a practical guideline for design and fabrication of a high-performance silicon emitter used in various industrial applications.     

Flexible Field Emission from Thermally Welded Chemically Doped Graphene Thin Films

Flexible field‐emission devices (FEDs) based on reduced graphene oxide (RGO) emitters are fabricated by the thermal welding of RGO thin films onto a polymeric substrate. The RGO edges are vertically aligned relative to the substrate as a result of cohesive failure in the RGO layer after thermal welding. Even at large bending angles, excellent electron emission properties, such as low turn‐on and threshold fields, a high emission current density, a high field enhancement factor, and long‐term stability of the emission properties of RGO emitters, arise from the uniform distribution and high density of the extremely sharp RGO edges, as well as the high interfacial strength between the RGO emitters and the substrate. Al‐ and Au‐doped RGO emitters are fabricated by introducing a dopant solution to the RGO emitters, and the resulting field‐emission characteristics are discussed. The proposed approach is straightforward and enables the practical use of high‐performance RGO flexible FEDs.     

Enhanced Field Emission of WS2 Nanotubes

Results on electron field emission from free standing tungsten disulfide (WS₂) nanotubes (NTs) are presented. Experiments show that the NTs protruding on top of microstructures are efficient cold emitters with turn‐on fields as low as 1 V/μm and field enhancement of few thousands. Furthermore, the emission current shows remarkable stability over more than eighteen hours of continuous operation. Such performance and long‐term stability of the WS₂ cathodes is comparable to that reported for optimized carbon nanotube (CNTs) based emitters. Besides this, it is found that the WS₂ cathodes prepared are less sensitive than CNTs in chemical reactive ambients. The high field enhancement and superior reliability achieved indicates a potential for vacuum nanoelectronics and flat panel display applications.     

Effect of catalyst thickness and plasma pretreatment on the growth of carbon nanotubes and their field emission properties

We demonstrated that the diameter and the density of carbon nanotubes (CNTs) which had a close relation to electric-field-screening effect could be easily changed by the control of catalytic Ni thickness combined with NH3 plasma pretreatment. Since the diameter and the density of CNTs had a tremendous impact on the field-emission characteristics, optimized thickness of catalyst and application of plasma pretreatment greatly improved the emission efficiency of CNTs. In the field emission test using diode-type configuration, well-dispersed thinner CNTs exhibited lower turn-on voltage and higher field enhancement factor than the densely-packed CNTs. A CNT film grown using a plasma-pretreated 25 angstroms-thick Ni catalyst showed excellent field emission characteristics with a very low turn-on field of 1.1 V/microm @ 10 microA/cm2 and a high emission current density of 1.9 mA/cm2 @ 4.0 V/microm, respectively.     

Photolithography-based carbon nanotubes patterning for field emission displays

Carbon nanotubes (CNTs) emitters were successfully patterned in small pixels (50×50 μm²) by using photolithography process on a hard metal electrode for field emission displays (FEDs) application. The CNTs particles in the patterned pixels were uniformly distributed on 2-inch diagonal substrates. The maximum diameter of CNTs particles could be controlled less than 20 μm. After patterning and heat treatment process below 300°C, most of CNTs bundles on the cathode electrode were aligned perpendicular to the substrates. The threshold electric field of emission for patterned CNTs was about 4.2 V μm⁻¹ and the field enhancement factor derived from the Fowler–Nordheim plots of the electron emissions was about 100 000 in the high voltage region. This newly developed process can be applicable to field emitter arrays for high resolution FEDs.     

石墨烯含量对碳纳米管电极发射器结构和运行稳定性的影响

利用石墨烯电学特性与碳纳米管场发射特性,加入不同含量的石墨烯浆料并对比了各试样场发射特性与器件运行稳定性.实验测试研究结果表明:石墨烯对碳纳米管实现了良好的分隔作用,形成了更大的碳纳米管间隙,降低了电场屏蔽的程度.碳纳米管周围被石墨烯紧密填充,起到了良好的固定作用,能够使碳纳米管获得多级场发射效应.随着石墨烯加入量由2...     

Study of electrophoretic deposition of ZnS:Ag/CNT composites for luminescent applications

In present work, electrophoretic deposition of novel photoluminescence (PL) composites of ZnS:Ag/carbon nano tube (CNT) on the surface of Al substrates was investigated. In deposition process, CNT concentration and applied coating voltage were studied as the effective parameters. Deposition weight showed the reverse relationship with the amount of concentration and direct dependence to the applied voltages. Furthermore, current densities were decreased with increasing CNT concentrations up to 12.5 wt%, and increased strongly with further CNT concentrations. Moreover, applied voltage and current density show the same positive trends. Other results revealed that PL emission intensities were significantly quenched with increasing the CNT concentration. Nevertheless, PL intensities were improved with increasing applied voltage up to 300 V, but reduced with further voltage increase. Morphological studies of ZnS:Ag/CNT composites confirmed that the intertwined architecture was formed by wrapping of CNTs on the surfaces of ZnS microsize particles.     

Effect of patterned and aligned carbon nanotubes on field emission properties

The field emission (FE) properties of carbon nanotubes (CNTs) films with different morphologies were simulated and examined. Based on the FE mechanism of aligned CNTs’ emitter the theoretical analysis exhibited the following relations: the FE enhancement factor with the distance between the emitters, the electric field with work function, and the work function with FE enhancement factor. Using the structure-induced CNTs growing method and theoretical results the direction of aligned CNTs could be controlled and the special morphology with different aligned CNTs’ film could be fabricated. Comparing to the experimental results (the medium density and patterned CNTs’ emitters) with the theoretically calculating results the I-V curves had the same trend with only 0.05 mA deviation. Based on the better experimental methods and means the accurate of formula could be further improved by modified the FE properties (Φ, β).     

Effect of anodization voltage on electron field emission from carbon nanotubes in anodized alumina template

In this work, electron field emission from AAO-CNT structure is studied as a function of anodizing voltage. It is found that the turn-on electric field of AAO-CNTs reduces from 5 V/microm to 4 V/microm as anodization voltage increase from 20 to 30 V. On the other hand, CNTs the turn-on electric field of AAO-CNTs increases from 4 V/microm to 6 V/microm as anodization voltage increase from 30 to 40 V. Thus, anodization voltage of 30 V provides an optimal AAO-CNTs structure for electron field emission. The emission data have been analyzed based on the Fowler Nordhiem (F-N) model. AAO template prepared with 30 V anodization voltage is found to yield CNT nanoarray with optimum alignment and spacing that increase field enhancement factor by the lowering of field screening effect without significant lowering of CNTs density.     

Enhanced Field‐Emission Behavior of Layered MoS2 Sheets

Field emission studies are reported for the first time on layered MoS₂ sheets at the base pressure of ～1 × 10^?8 mbar. The turn‐on field required to draw a field emission current density of 10 μA/cm² is found to be 3.5 V/μm for MoS₂ sheets. The turn‐on values are found to be significantly lower than the reported MoS₂ nanoflowers, graphene, and carbon nanotube‐based field emitters due to the high field enhancement factor (～1138) associated with nanometric sharp edges of MoS₂ sheet emitter surface. The emission current–time plots show good stability over a period of 3 h. Owing to the low turn‐on field and planar (sheetlike) structure, the MoS₂ could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.     

有机质掺杂碳纳米管阵列场发射性质的研究

碳纳米管(CNT)作为理想的场发射阴极材料,在场发射阴极阵列中的密度与阴极的场发射性能有着非常密切的关系。通过解拉普拉斯方程,得到碳管阵列密度对场增强因子的影响,同时通过对不同阵列密度的碳纳米管阵列进行有机质的填充解决了碳纳米管阵列在制备过程中的定向排列问题,并对所需碳管与有机质的质量比进行了理论计算,结果表明,阵列密度和管间距对碳管与有机质的质量比有很大影响。     

Mechanical and Metallurgical Studies in Double Shielded GMAW of Dissimilar Stainless Steels

In the present work, a simple arrangement is made to provide double layer shielding gas supply in addition to primary shielding during gas metal arc welding (GMAW) of two dissimilar stainless steels, i.e., AISI 316 and duplex 2205. Influences of double layer shielding in addition to five more process parameters like welding current, voltage, material of the electrode wire, the type of primary shielding gas, and flow rate on joint tensile strength and fusion zone microhardness are studied. An experimental design technique is used to design the experimental conditions and the results are analyzed to observe the influences of each process parameter and their interactions. The tensile strength is more influenced by the electrode material and the type of shielding, whereas current, interaction between current × voltage and current × flow rate significantly influence microhardness. Welding voltage influences both tensile strength and microhardness. Double layer shielding with CO₂ as an outer shielding layer helps in controlling the cooling rate which improves the tensile strength and microhardness. Microstructural observations by scanning electron microscopy reveal that moderate to low heat input with a single layer of shielding results in poor joint strength and severe damage or lack of fusion, and the duplex 2205 filler gives the maximum joint strength due to the presence of a ferrite structure.     

Growth of vertically aligned arrays of carbon nanotubes for high field emission

Vertically aligned multi-walled carbon nanotubes have been grown on Ni-coated silicon substrates, by using either direct current diode or triode plasma-enhanced chemical vapor deposition at low temperature (around 620 °C). Acetylene gas has been used as the carbon source while ammonia and hydrogen have been used for etching. However densely packed (∼ 10⁹ cm^− 2) CNTs were obtained when the pressure was ∼ 100 Pa. The alignment of nanotubes is a necessary, but not a sufficient condition in order to get an efficient electron emission: the growth of nanotubes should be controlled along regular arrays, in order to minimize the electrostatic interactions between them. So a three dimensional numerical simulation has been developed to calculate the local electric field in the vicinity of the tips for a finite square array of nanotubes and thus to calculate the maximum of the electron emission current density as a function of the spacing between nanotubes. Finally the triode plasma-enhanced process combined with pre-patterned catalyst films (using different lithography techniques) has been chosen in order to grow regular arrays of aligned CNTs with different pitches in the micrometer range. The comparison between the experimental and the simulation data permits to define the most efficient CNT-based electron field emitters.     

Well-aligned open-ended carbon nanotube architectures: an approach for device assembly

To circumvent the high carbon nanotube (CNT) growth temperature and poor adhesion with the substrates that currently plague CNT implementation, we proposed using CNT transfer technology enabled by open-ended CNTs. The process is featured with separation of CNT growth and CNT device assembly. Field emission testing of the as-assembled CNT devices is in good agreement with the Fowler-Nordheim (FN) equation, with a field enhancement factor of 4,540. This novel technique shows promising applications for positioning CNTs on temperature-sensitive substrates and for the fabrication of field emitters, electrical interconnects, and thermal management structures in microelectronics packaging.     

Electron field emitters based on multi-walled carbon nanotubes coated with conducting polymer/metal/metal-oxide composites

The present work describes the field emission properties of multi-walled nanotubes (MWNTs)-based conducting polymer/metal-oxide/metal/MWNTs composites (polyaniline (PANI)/SnO₂/Sn/MWNTs). MWNTs were synthesised by chemical vapour deposition technique. SnO₂/Sn/MWNTs were prepared by using chemical reduction followed by calcination. By in situ polymerisation method, surface of SnO₂/Sn/MWNTs were coated with PANI. PANI/SnO₂/Sn/MWNTs field emitters were fabricated over flexible graphitised carbon fabric substrate by spin coating technique. High-resolution transmission electron microscopy and scanning electron microscopy were used to characterise the field emitters. Field emission properties have been studied using an indigenously made facility. The fabricated PANI/SnO₂/Sn/MWNTs field emitters exhibited excellent field emission properties with a turn on field of 1.83 V µm^?1 and a field enhancement factor of 4800.     

Forms and behaviour of vacuum emission electronic devices comprising diamond or other carbon cold cathode emitters

Nanocarbon-derived electron emission devices, specifically nanodiamond lateral field emission (FE) diodes and gated carbon nanotube (CNT) triodes, are new configurations for robust nanoelectronic devices. These novel micro/nanostructures provide an alternative and efficient means of accomplishing electronics that are impervious to temperature and radiation. For example, nitrogen-incorporated nanocrystalline diamond has been lithographically micropatterned to use the material as an electron field emitter. Arrays of laterally arranged 'finger-like' nanodiamond emitters constitute the cathode in a versatile diode configuration with a small interelectrode separation. A low diode turn-on voltage of 7V and a high emission current of 90 microA at an anode voltage of 70V (electric field of approx. 7V microm(-1)) are reported for the nanodiamond lateral device. Also, a FE triode amplifier based on aligned CNTs with a low turn-on voltage and a small gate leakage current has been developed.     

电泳沉积制备平行栅碳纳米管场发射阴极的研究

利用磁控溅射、光刻、湿法刻蚀和电泳技术在玻璃基片上成功制备平行栅场发射阴极阵列,用光学显微镜、场发射扫描电镜和拉曼光谱观察了碳纳米管的形貌和结构,并测试所制备的平行栅碳纳米管阴极的场发射性能.光学显微镜和场发射电子显微镜测试表明,平行栅结构阴极和栅极交替地分布,同一个平面内,CNTs有选择性地沉积在平行栅结构中的阴极表...     

碳纳米管表面化学镀银及场发射性能研究

利用化学镀方法对碳纳米管(carbon nano-tubes,CNTs)表面金属化镀银,研究表面化学镀银碳纳米管的场发射性能。碳纳米管经氧化处理后,表面存在一些羰基(CO)、羧基(—COOH)和羟基(—OH)等活性基团,经敏化、活化处理后,形成金属钯活化中心,进而还原金属银离子,从而获得表面化学镀银的碳纳米管。表面化学镀银碳纳米管阴极的开启电场约为0.19V/μm,当电场强度为0.37V/μm时,最大发射电流达6mA/cm2,场增强因子约为25565。实验结果表明,化学镀银层可以提高碳纳米管的电子传输和热传输能力,提高碳纳米管的场发射电流和发射稳定性,有利于碳纳米管在场发射平板显示领域的应用。     

Field Electron Emission from Single‐Walled Carbon Nanotube Layers

Abstract

Field emission characteristics of single‐walled carbon nanotube layers have been investigated at room and low temperatures. For these layers the emission current density of 10 mA/cm² was obtained at the average field E _av = 1.6–3.8 V/µm. Current–voltage characteristics in Fowler–Nordheim coordinates have a break at emission current about 10^?8 A. Cooling of samples only insignificantly changed the form of current–voltage characteristics. This indicates, that investigated single‐walled nanotubes have the metal type conductivity.