Efficient fabrication of carbon nanotube micro tip arrays by tailoring cross-stacked carbon nanotube sheets

Carbon nanotube (CNT) micro tip arrays with hairpin structures on patterned silicon wafers were efficiently fabricated by tailoring the cross-stacked CNT sheet with laser. A blade-like structure was formed at the laser-cut edges of the CNT sheet. CNT field emitters, pulled out from the end of the hairpin by an adhesive tape, can provide 150 μA intrinsic emission currents with low beam noise. The nice field emission is ascribed to the Joule-heating-induced desorption of the emitter surface by the hairpin structure, the high temperature annealing effect, and the surface morphology. The CNT emitters with hairpin structures will greatly promote the applications of CNTs in vacuum electronic devices and hold the promises to be used as the hot tips for thermochemical nanolithography. More CNT-based structures and devices can be fabricated on a large scale by this versatile method.     

碳纳米管复合材料场发射性能研究现状

碳纳米管具有管径小、长径比高的结构以及物理化学性能稳定等优良特性,被认为是真空冷阴极场发射电子源和场发射平板显示理想的阴极材料。加之碳纳米管兼具有机械强度高、韧性好等出众的力学性能,使其成为复合材料的理想添加相,将其与其他材料复合,可以制备出具有更加出众性能的复合材料。近年来有关碳纳米管及其复合材料场发射研究已成为一个备受关注的热点。概述了阴极场发射理论以及与碳纳米管场发射相关的几种场发射物理机制,介绍了碳纳米管复合场发射阴极的研究现状及制备方法,最后对碳纳米管复合阴极场发射的发展前景进行了展望。     

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.     

Hot-filament CVD synthesis and application of carbon nanostructures

We have reported the catalyst-assisted hot-filament chemical vapor deposition (HFCVD) of various carbon nanostructures, such as carbon nanotube (CNT), nanoparticle (CNP), and nanofiber (CNF), and their application examples. In the case of CNT, vertically aligned high-density growth has been investigated in detail using in-situ optical reflectivity measurement. For CNF showing large field-emission currents, adsorbate-related emission behavior and the emission from patterned cathodes have been studied. Regarding CNP, the performances of a series of triode field emitters with CNP cathodes have been compared, and the application of CNP triode to miniature time-of-flight mass spectrometer has been discussed.     

A study on the mechanical and electrical reliability of individual carbon nanotube field emission cathodes

Individual carbon nanotube (CNT) field emission characteristics present a number of advantages for potential applications in electron microscopy and electron beam lithography. Mechanical and electrical reliability of individual CNT cathodes, however, remains a challenge and thus device integration of these cathodes has been limited. In this work, we present an investigation into the reliability issues concerning individual CNT field emission cathodes. We also introduce and analyze the reliability of a novel individual CNT cathode. The cathode structure is composed of a multi-walled carbon nanotube (MWNT) attached by Joule heating to a nickel-coated Si microstructure. The junction of the CNT and the Si microstructure is mechanically and electrically robust to withstand the strong electric field conditions that are typical for field emission devices. An optimal Ni film coating of 25?nm on the Si microstructure is required for mechanical and electrical stability. Experimental current-voltage data for the new cathode structure definitively demonstrates carbon nanotube field emission. Additionally, we demonstrate that our new nanofabrication method is capable of producing sophisticated cathode structures that were previously not realizable, such as one consisting of two parallel MWNTs, with highly controlled CNT lengths with 40?nm accuracy and nanotube-to-nanotube separations of less than 10?μm.     

Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm² and 7.0 mA/cm², respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.     

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.     

Carbon nanotubes as field emitter

Carbon nanotubes (CNTs) have recently emerged as a promising material of electron field emitters. They exhibit extraordinary field emission properties because of their high electrical conductivity, high aspect ratio "needle like" shape for optimum geometrical field enhancement, and remarkable thermal stability. In this Review, we emphasize the estimation and influencing factors of CNTs' emission properties, and discuss in detail the emission properties of macroscopic CNT cathodes, especially fabricated by transplant methods, and describe recent progress on understanding of CNT field emitters and analyze issues related to applications of CNT based cold cathodes in field emission display (FED). We foresee that CNT-FED will take an important place in display technologies in the near future.     

Carbon nanotube field emitter

Recently, carbon nanotubes (CNTs), possessing excellent properties as field emitters, are attracting considerable attention as electron emitters of a cold cathode. In this review article, field emission phenomena of carbon nanotubes with various morphologies and surfaces (clean surface or adsorbed molecules on it) revealed by field emission microscopy are first described. Then, the main subject of this article, application of CNTs as electron sources in display devices is reviewed. Other electric devices utilizing CNT-field emitters are also presented.     

Improvement of emission reliability of carbon nanotube emitters by electrical conditioning

We have investigated the effect of electrical aging to improve emission reliability of carbon nanotube (CNT) emitter. The CNT emitters were prepared by the screen-printing of the CNT paste. The electrical aging treatment was carried out in a high vacuum chamber by applying the pulsed dc voltage. The field emission measurements and lifetime tests were performed on the CNT emitters depending on the electrical aging time and current density. After the electrical aging treatment, emission properties of CNT emitter were suppressed but the half lifetime was dramatically improved.     

Tip cooling effect and failure mechanism of field-emitting carbon nanotubes

The cooling effect accompanying field electron emission has been considered for a single carbon nanotube (CNT) used as a field emission (FE) electron source. An improved model for the failure mechanism of field emitting CNTs has been proposed and validated. Our model predicts a maximum temperature (T-max) located at an interior point rather than the tip of the CNTs, and the failure of the CNT emitters tends to take place at the T-max point, inducing a segment by segment breakdown process. A combination of Joule heating and electrostatic force effect is proposed responsible for initiating the failure of the field emitting CNT and validated by in situ FE observation.     

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.     

Gas ionization sensors with carbon nanotube/nickel field emitters

Gas ionization sensors based on the field emission properties of the carbon nanotube/nickel (CNT/Ni) field emitters were first developed in this work. It is found that the breakdown electric field (E(b)) slightly decreases from 2.2 V/microm to 1.9 V/microm as the pressure of H2 gas increases from 0.5 Torr to 100 Torr. On the contrary, E(b) obviously increases from 2.9 V/microm to 6.5 V/microm as O2 gas pressure increases from 0.5 Torr to 100 Torr. This may be explained by the depression of the electron emission that caused by the adsorption of the O2 gas on the CNT emitters. The Raman spectra of the CNT/Ni emitters also show that more defects were generated on the CNTs after O2 gas sensing. The Joule heating effect under high current density as performing H2 sensing was also observed. These effects may contribute the pressure dependence on the breakdown electric field of the CNT/Ni gas ionization sensors.     

纳米碳管阵列

在概括纳米碳管阵列特异的场发射效应及在场发射器方面应用前景的基础上，介绍了合成纳米碳管阵列的研究历程以及化学气相沉积法在纳米碳管阵列合成方面的重要意义，就当前纳米碳管阵列的快速合成与低温合成两个发展方向进行了概述，并指出等离子体化学气相沉积法能有效地用于纳米碳管阵列的低温合成。     

Geometry dependence of the electrostatic and thermal response of a carbon nanotube during field emission

In this paper we present an analysis to simulate heating within an isolated carbon nanotube (CNT) attached to an etched tungsten tip during field emission of an electron beam. The length, radius, wall thickness and shape of the tip (closed with a hemispherical shape or open and flat) of the CNT and its separation distance from the flat surface are considered as variables. Using a finite element method, we predict the field enhancement, emission current and temperature of the CNT as a function of these parameters. The electrostatic and transient thermal analyses are integrated with the field-emission models based on the Fowler-Nordheim approximation and heating/cooling due to emitting energetic electrons (the Nottingham effect). These simulations suggest that the main mechanism responsible for heating of the CNT is Joule heating, which is significantly larger than the Nottingham effect. Results also indicate that the electrostatic characteristics of CNTs are very sensitive to the considered parameters whereas the transient thermal response is only a function of the CNT radius and wall thickness. Further, the thermal response of the CNT is independent of its geometry, meaning that, as long as a given set of geometrical conditions are present that result in a given emission current, the maximum temperature a CNT attains will be the same.     

Carbon Nanotubes for Cold Electron Sources

Technological advances in the field of microelectronic fabrication techniques have triggered a great interest in vacuum microelectronics. In contrast to solid‐state microelectronics, which entails scattering‐dominated electron transport in semiconducting solids, vacuum microelectronics relies on the scattering‐free, ballistic motion of electrons in vacuum. Since the first international conference on vacuum microelectronics substantial progress in this field has been made. The first technological devices using micrometer‐sized electron emitting structures are currently being commercialized. Field‐emission flat‐panel displays (FED) seem to be an especially promising competitor to LCD displays. Today there is only one mature technology for producing micro‐gated field‐emission arrays: the Spindt metal‐tip process. The drawbacks of this technology are expensive production, critical lifetime in vacuum, and high operating voltage. Carbon nanotubes (CNT) can be regarded as the potential second‐generation technology to the Spindt metal micro‐tip. In this review we show that the field emission (FE) behavior of CNT can be accurately described by Fowler–Nordheim tunneling and that the field‐enhancement factor β is the most prominent factor. Therefore the FE properties of a CNT thin film can be understood in terms of local field enhancement β(x,y), which can be determined with scanning anode field emission microscopy (SAFEM). To characterize the FE properties of an ensemble of electron emitters we used a statistical approach (as for thin film emitters), where f(β)dβ gives the number of emitters on a unit area with field‐enhancement factors within the interval [β,β + dβ]. We show that the field‐enhancement distribution function f(β) gives an almost complete characterization of the FE properties.     

Electron emission from individual indium arsenide semiconductor nanowires

A procedure was developed to mount individual semiconductor indium arsenide nanowires onto tungsten support tips to serve as electron field-emission sources. The electron emission properties of the single nanowires were precisely determined by measuring the emission pattern, current-voltage curve, and the energy spectrum of the emitted electron beam. The two investigated nanowires showed stable, Fowler-Nordheim-like emission behavior and a small energy spread. Their morphology was characterized afterward using transmission electron microscopy. The experimentally derived field enhancement factor corresponded to the one calculated using the basic structural information. The observed emission behavior contrasts the often unstable emission and large energy spread found for semiconductor emitters and supports the concept of Fermi-level pinning in indium arsenide nanowires. Indium arsenide nanowires may thus present a new type of semiconductor electron sources.     

Field emission in doped nanotubes

Doping into nanotubes is an attractive and significant way to tailor their electron transport and emission properties. This article reviews some recent experimental and theoretical advances in the studies of doping behaviors in carbon nanotubes and gallium nitrogen nanotubes, and doping effects on their field electron emission properties. The general theory for field emission mechanism of one-dimensional nanosystems is presented to provide an overall picture of the field emission process and doping behavior. Potential applications of doped nanotubes as diverse nanoscale emitters, microscopy probes, electronic guns and nanoelectronic devices are discussed.     

A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

A newly developed vacuum sensor using carbon nanotube (CNT) field-emission has been designed. The fabricated device is an ionization gauge with a silk-like CNT yarn cathode, and the vacuum is indicated by the ratio of the ion current to the electron-emission current. The metrological characteristics of the sensor were studied in a dynamic vacuum system. It showed good linearity ranged from 10⁻⁴ to 10⁻¹ Pa. Taking advantage of the field-emission cathode, the power consumption is only about 5.5 mW. Moreover, comparing it to the conventional thermionic cathode, the CNT yarn cathode is more miniature and a cold cathode with no obvious thermal outgassing effect. Due to these features, the sensor described here could have potential applications in measuring vacuum inside sealed and miniaturized devices.     

碳纳米管场发射电子源

从实验和理论上研究单根碳纳米管(CNT)场发射电子源的稳定问题.利用透射电镜/扫描探针显微镜(TEM/SPM)和场发射显微镜/场离子显微镜(FEM/FIM)对CNTs的场发射特性进行了实验研究.同时从密度泛函理论出发,利用相关程序模拟计算了吸附对单壁碳纳米管(SWNTs)场发射的影响.发现SWNTs荷电体系的总能量与荷电电荷数量关系具有抛物线形式,先减小,达到最小值,之后增加.通常荷4个电子时达到最小值,即体系处于最稳定状态.表明SWNTs有很大的电负性,是容易发生凝聚和吸附分子的根源.进而计算了对氢、氧和水的吸附特性,讨论了吸附对场发射的影响.这些结果对CNTs的场发射特性和作为新型电子源的应用都是有重要意义的.