Download or read book Enhanced Field Emission Properties from Carbon Nanotube Emitters Grown on NiCr Alloy Surfaces with Grain Boundary Effect written by and published by . This book was released on 2008 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanotubes (CNTs) are known to have exceptional field emission properties such as low turn-on voltage and high current density. Nilsson et al. reported that field emission properties depend highly on the density of the CNT films. The CNTs must have low density in order to minimize electrostatic field screening from neighboring nanotubes. At the same time, the CNT films must have a high number of emitting sites to achieve a desirable current density. Recently, the direct growth of CNTs on metal alloys containing Ni, Cr, and Fe has been shown to be an attractive alternative for fabrication of CNT emitters without requiring a metal catalyst deposition step. Herein, we report the field emission properties of multi-walled carbon nanotube (MWNT) films grown on polished smooth 80/20 and 70/30 NiCr surfaces by thermal chemical vapor deposition. We show that 80/20 NiCr surfaces prohibit growth of MWNTs at the grain boundaries resulting in a lower turn-on field in comparison to a continuous MWNT film obtained with 70/30 NiCr substrates.

Download or read book Carbon Nanotube and Related Field Emitters written by Yahachi Saito and published by John Wiley & Sons. This book was released on 2010-10-01 with total page 551 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanotubes (CNTs) have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. These characteristics include extraordinary strength, unique electrical properties, and the fact that they are efficient heat conductors. Field emission is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields. CNT field emitters are expected to make a breakthrough in the development of field emission display technology and enable miniature X-ray sources that will find a wide variety of applications in electronic devices, industry, and medical and security examinations. This first monograph on the topic covers all aspects in a concise yet comprehensive manner - from the fundamentals to applications. Divided into four sections, the first part discusses the preparation and characterization of carbon nanotubes, while part two is devoted to the field emission properties of carbon nanotubes, including the electron emission mechanism, characteristics of CNT electron sources, and dynamic behavior of CNTs during operation. Part three highlights field emission from other nanomaterials, such as carbon nanowalls, diamond, and silicon and zinc oxide nanowires, before concluding with frontier R&D applications of CNT emitters, from vacuum electronic devices such as field emission displays, to electron sources in electron microscopes, X-ray sources, and microwave amplifiers. Edited by a pioneer in the field, each chapter is written by recognized experts in the respective fields.

Download or read book Fabrication Field Emission Properties and Theoretical Simulation of Triode type Carbon Nanotube Emitter Arrays written by Jianfeng Wu and published by . This book was released on 2010 with total page 143 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanotubes exhibit excellent field emission properties and will likely be prime candidates as electron sources in future vacuum electronic applications. Recent research has focused on enhancing field emission from traditional diode-type emitters by adding a gate electrode between the anode and the cathode. Since the gate to cathode (emitter) distance in this triode-type structure is small relative to the anode to cathode distance, this structure allows relatively small gate voltages to significantly enhance or dampen field emission. The key challenge for this research is: synthesizing vertically aligned carbon nanotube field emitters inside arrays of triode-type devices. The most common "top-down", etch-deposit-synthesis method of synthesizing carbon nanotubes inside gated cavities is discussed here, and a novel "bottom-up" method is presented. This new approach bypasses the lithography and wet chemistry essential to the etch-deposit-synthesis method, instead using a dual-beam focused ion beam (FIB) system to mill cavities into a multi-layered substrate. Here the substrate is designed such that the act of milling a hole simultaneously creates the gate structure and exposes the catalyst from which carbon nanotubes can then be grown. Carbon nanotubes are synthesized using plasma enhanced chemical vapor deposition (PECVD) rather than thermal chemical vapor deposition, due to the superior alignment of the PECVD growth. As dual-beam FIB and PECVD can both be largely computerized, this synthesis method is highly reproducible. The dual-beam FIB also permits a high degree of controllability in gate radius, cavity depth and emitter spacing. The effects of a host of PECVD growth parameters (initial catalyst thickness, gas concentration, growth temperature, temperature ramping rate, chamber pressure, and plasma voltage) were characterized so that the morphology of the carbon nanotube emitters could be controlled as well. This "bottom-up" method is employed to construct functional, large area carbon nanotube field emitter arrays (CNT FEAs). The role of the gate layer in field emission is examined experimentally as well as through theoretical models. Field emission testing revealed that increasing gate voltage by as little as 0.3 V had significant impact on the local electric fields, lowering the turn-on and threshold fields by 3.6 and 3.0 V/um, respectively, and increasing the field enhancement factor from 149 to 222. A quantum mechanical model of such triode-type field emission indicates that the local electric field generated by a negatively or positively biased gate directly impacts the tunneling barrier thickness and thus the achievable emission current. However, the geometry of triode-type devices (gate height, gate radius, emitter density) can influence the degree to which the gate voltage influences field emission. I demonstrate here an effective method of analytically calculating the effect of various such geometric parameters on the field emission. Results show that gate type (the height of the gate relative the emitter tip) can significantly impact the local electric field and hence the type of applications a device is suitable for. Side gates (gate height emitter height) induced the highest local electric field, while top gates (gate height emitter height) provided the greatest controllability. For all gate types, increasing the size of the gate opening increased the local electric field by diminishing the gate-emitter screening effect. However, gate voltages were able to enhance or inhibit the local electric field much more readily with smaller gate radii. Due to the strength of gate-emitter field screening in the triode-type structure, the spacing between emitters had virtually no impact on the local electric field, allowing relatively high emitter densities. These theoretical results, combined with a highly controllable synthesis method, provide valuable information and methodology for those designing and optimizing triode-type devices targeted at specific applications.

Download or read book Field Emission Properties of Carbon Nanotube Pillar Arrays Patterned Directly on Metal Alloy Surfaces written by and published by . This book was released on 2008 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanotube pillar arrays (CPAs) for cold field emission were fabricated using a conventional photolithography process, and the geometry of these arrays was studied and the effect of pillar height on field emission was quantified. Our CPA samples achieved turn-on fields as low as 0.9 V/micrometer and stable current densities of 10 mA/cm(2) at applied field lower than 6V/micrometer.

Download or read book Synthesis and Field Emission Properties of Carbon Nanotube Films written by and published by . This book was released on 2002 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In this study, vertically aligned multiwalled carbon nanotube films were grown by microwave plasma chemical vapor deposition (MWCVD). Through controlling the thickness of the iron thin film, carbon nanotubes with different diameters were obtained. When the thickness of the iron layer was reduced to 0.3-0.5 nm, single-wall and double-wall nanotubes were obtained with a high areal density (~ 1012/cm2) and vertical alignment. Scanning electron microscopy, Raman spectroscopy and high resolution transmission electron microscopy were employed to characterize the as-deposited nanotubes. In addition, a systematic study of the internal structure transition of the carbon nanotubes has been conducted and a growth model was proposed in terms of carbon surface and bulk diffusion. The field emission of the carbon nanotube films has also been explored in this study. Different measurement systems including a variable distance field emission system, a field emission imaging system, and a field electron emission system (FEEM) were employed. The effects of the diameter (multi-wall vs single- and double- wall), the adsorbates, and the temperature on the field emission properties of carbon nanotubes have been exhibited. Finally, two processes including hydrogen plasma etching and re-growth were used to treat the as-deposited film, and an increased emission site density was observed for the re-grown carbon nanotube film.

Download or read book Nanostructured Carbon Electron Emitters and Their Applications written by Yahachi Saito and published by CRC Press. This book was released on 2022-01-27 with total page 374 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon forms a variety of allotropes due to the diverse hybridization of s- and p-electron orbitals, including the time-honored graphite and diamond as well as new forms such as C60 fullerene, nanotubes, graphene, and carbyne. The new family of carbon isotopes—fullerene, nanotubes, graphene, and carbyne—is called “nanostructured carbon” or “nanocarbon.” These isotopes exhibit extreme properties such as ultrahigh mechanical strength, ultrahigh charge–carrier mobility, and high thermal conductivity, attracting considerable attention for their electronic and mechanical applications as well as for exploring new physics and chemistry in the field of basic materials science. Electron sources are important in a wide range of areas, from basic physics and scientific instruments to medical and industrial applications. Carbon nanotubes (CNTs) and graphene behave as excellent electron-field emitters owing to their exceptional properties and offer several benefits compared to traditional cathodes. Field emission (FE) produces very intense electron currents from a small surface area with a narrow energy spread, providing a highly coherent electron beam—a combination that not only provides us with the brightest electron sources but also explores a new field of electron beam–related research. This book presents the enthusiastic research and development of CNT-based FE devices and focuses on the fundamental aspects of FE from nanocarbon materials, including CNTs and graphene, and the latest research findings related to it. It discusses applications of FE to X-ray and UV generation and reviews electron sources in vacuum electronic devices and space thrusters. Finally, it reports on the new forms of carbon produced via FE from CNT.

Download or read book Field Emission Characteristics of Carbon Nanotube Emitters Using Screen Printing Technique written by Cheng-Chung Lee and published by . This book was released on 2000 with total page 10 pages. Available in PDF, EPUB and Kindle. Book excerpt: A carbon nanotube emitter with high uniformity, adhesion and emission current has been formed by screen printing. The raw carbon nanotubes (CNTs) chunk were crushed, purified, dispersed, mixed with organic additives and then screen-printed on Ag electrode. After burning out the organic additives under two thermal cycles, the surface of CNTs emitters were treated. The treated emitters showed a better emission characteristic than untreated emitters.

Download or read book Fabrication and Analysis of Carbon Nanotube Based Emitters written by Vladimir Mancevski and published by . This book was released on 2011 with total page 270 pages. Available in PDF, EPUB and Kindle. Book excerpt: We have advanced the state-of-the-art for nano-fabrication of carbon nanotube (CNT) based field emission devices, and have conducted experimental and theoretical investigations to better understand the reasons for the high reduced brightness achieved. We have demonstrated that once the CNT emitter failure modes are better understood and resolved, such CNT emitters can easily reach reduced brightness on the order of 109 A m−2 sr−1 V−1 and noise levels of about 1%. These results are about 10% better than the best brightness results from a nanotip emitter archived to date. Our CNT emitters have order of magnitude better reduced brightness than state-of-the-art commercial Schottky emitters. Our analytical models of field emission matched our experimental results well. The CNT emitter was utilized in a modified commercial scanning electron microscope (SEM) and briefly operated to image a sample. We also report a successful emission from a lateral CNT emitter element having a single suspended CNT, where the electron emission is from the CNT sidewall. The lateral CNT emitters have reduced brightness on the order of 108 A m−2 sr−1 V−1, about 10X less than the vertical CNT emitters we fabricated and analyzed. The characteristics of the lateral field emitter were analyzed for manually fabricated and directly grown CNT emitters. There was no significant difference in performance based on the way the CNT emitter was fabricated. We showed that the fabrication technique for making a single CNT emitter element can be scaled to an array of elements, with potential density of 106-107 CNT emitters per cm2. We also report a new localized, site selective technique for editing carbon nanotubes using water vapor and a focused electron beam. We have demonstrated the use of this technique to cut CNTs to length with 10s of nanometers precision and to etch selected areas from CNTs with 10s of nanometers precision. The use of this technique was demonstrated by editing a lateral CNT emitter. We have conducted investigations to demonstrate the effects of higher local water pressure on the CNT etching efficiency. This was achieved by developing a new method of localized gas delivery with a nano-manipulator.

Download or read book Field Emission Properties of Carbon Nanotube Fibers and Sheets for a High Current Electron Source written by Larry A. Christy and published by . This book was released on 2014 with total page 79 pages. Available in PDF, EPUB and Kindle. Book excerpt: Field emission (FE) properties of carbon nanotube (CNT) fibers from Rice University and the University of Cambridge have been studied for use within a high current electron source for a directed energy weapon. Upon reviewing the performance of these two prevalent CNT fibers, cathodes were designed with CNT fibers from the University of Cincinnati Nanoworld Laboratory. Cathodes composed of a single CNT fiber, an array of three CNT fibers, and a nonwoven CNT sheet were investigated for FE properties; the goal was to design a cathode with emission current in excess of 10 mA. Once the design phase was complete, the cathode samples were fabricated, characterized, and then analyzed to determine FE properties. Electrical conductivity of the CNT fibers was characterized with a 4-probe technique. FE characteristics were measured in an ultra-high vacuum chamber at Wright-Patterson Air Force Base. The arrayed CNT fiber and the enhanced nonwoven CNT sheet emitter design demonstrated the most promising FE properties. Future work will include further analysis and cathode design using this nonwoven CNT sheet material to increase peak current performance during electron emission.

Download or read book Characterization of the Field Emission Properties of Carbon Nanotubes Formed on Silicon Carbide Substrates by Surface Decomposition written by and published by . This book was released on 2006 with total page 180 pages. Available in PDF, EPUB and Kindle. Book excerpt: Dense arrays of vertically aligned carbon nanotubes (CNTs) form on the surface of silicon carbide wafers during high temperature anneals under moderate vacuum conditions. The novelty of this growth method is that the CNTs form without the aid of a metal catalyst, allowing for potentially impurity-free CNTs to form. In this study, CNT films were grown by the surface decomposition of silicon carbide substrates. The associated field emission characteristics were investigated to determine if films grown using this method possessed advantages over films grown using metal-catalyzed methods. The associated turn-on and threshold voltages, maximum current density, and emission current stability of the CNT films were measured using a standard vacuum tube diode test configuration. Although the samples tested did not demonstrate improved field emission characteristics when compared to values found in the literature for catalyst-grown CNT films, the data collected compares well with data in the literature and shows that further investigation is warranted regarding the emission capabilities of CNT films formed on SiC by surface decomposition. From the measured CNT data, the lowest turn-on electric field was found to be lower than 3.0 V/micrometers, while exhibiting a high maximum current density of 4.25 mA/sq cm at 6.7 V/micrometers.

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Download or read book Dynamic Properties of Individual Carbon Nanotube Emitters for Maskless Lithography written by and published by . This book was released on 2008 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: The stable chemical structure, high current density, and low turn-on fields of carbon nanotubes make them highly attractive for cold field emission applications. The individual CNT's low electron beam energy spread and high brightness values make it particularly desirable for advanced applications such as electron microscopy and electron beam lithography at 20-nm and below critical dimension regime. A fundamental understanding of the individual carbon nanotube electron source is essential for implementation of such applications. We previously demonstrated an improved fabrication technique for individual CNT emitters based on MEMS technology. Also, our group presented an experimental and simulation investigation of the influence of cathode support structure geometry on the field emission properties of a single CNT emitter. In this paper, we present an empirical study of dynamic behavior of an electron source system which incorporates an individual CNT. We propose a representative circuit model that is simple yet particularly valuable for emission current control for each CNT emitter in an array to facilitate high throughput maskless lithography.

Download or read book Conductive Effect of Increased Crystallinity of Single Walled Carbon Nanotubes as Field Emitter written by Norihiro Shimoi and published by . This book was released on 2018 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanotubes (CNTs) exhibit chemical stability, thermal conductivity, mechanical strength, and unique properties as a quasi-one-dimensional material with nanoscale needle shape. Field-emission (FE) electron sources appear to be the most promising industrial application for CNTs, and their deployment is approaching practical utilization. So far, efforts to construct an FE cathode with single-walled carbon nanotubes (SWCNTs) have only managed to average out the large FE current fluctuations in a nonhomogeneous electron emitter plane and the short emission lifetime because the crystal defects in the carbon network in CNTs prevent the realization of a stable emission current. The utilization of CNTs to obtain an effective electronic device, one with stable emission and low FE current fluctuations, relies on the high crystallization of CNTs, a task that can be fulfilled by using highly crystalline SWCNTs (hc-SWCNTs). The author could succeed in developing a model of the flow of electrons through the inside of the hc-SWCNTs and SWCNTs with crystal defects to the outside using the fluctuations of the tunneling current. Therefore, we expect that the hc-SWCNTs are used as field emitters with stable emission and low power consumption for saving energy.

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Download or read book Carbon Nanotube Field Emitter Array and Its Application to Novel X ray Systems written by Yunhan Li and published by . This book was released on 2018 with total page 151 pages. Available in PDF, EPUB and Kindle. Book excerpt: With the emergence of ever more demanding X-ray imaging applications, including radiotherapy, radiography, computed tomography and tomosynthesis, there is a tendency to use field emission (FE) to take the place of thermionic emission in X-ray generation. Carbon nanotube (CNT) field emitter array (FEA) is able to provide enhanced current and low energy dispersion for X-ray generation. The low operating temperature, instantaneous response, and small size of CNT FEA enable efficient and controllable FE performance for high spatial and temporal resolution X-ray imaging and instantaneous switching among multiple X-ray sources. Challenges are still remaining for CNT FEA study and development. The first is the insufficient understanding of FE uniformity of CNT FEA and CNT emitter failure mechanism, which limits the development of stable and reliable CNT FEA. The second is the lack of applications of CNT FEA for the emergence of novel X-ray imaging applications that require dose adjustments, fluence field modulation, and shape variation. In this thesis, recent advances in CNT FE and field emitters are comprehensively reviewed. Issues related to the principles, characteristics, and applications of CNT based FE are discussed. The research starts with a fundamental study of understanding the FE uniformity and emitter failure mechanism of CNT FEA by designing and demonstrating a simple and reliable field emission microscopy method using PMMA (Poly (methyl methacrylate)) thin film with microscopic camera. This novel approach is able to resolve the challenges of observing FE uniformity of CNT FEA and CNT field emitter failure behaviour. The unique phenomenon of light emission and Coulomb explosion of CNT field emitter failure induced by FE is represented and systematically characterized, which is a combined effect of Joule heating and excessive charging. Then, individual ballast of vertically aligned CNT FEA using silicon current limiters is designed and fabricated, which exhibits improved reliability and stability. The CNT FEA is applied to a low-cost self-contained and self-focusing compact X-ray source integrated with resonant transformer. The X-ray source can be powered by low voltage to generate pulsed X-ray and achieve sub-millimetre resolution. Using finite element analysis and Monte Carlo simulation, CNT FEA based two dimensional multi-pixel X-ray source is designed and experimentally verified, which is feasible for X-ray fluence field modulation in next generation X-ray imaging systems.

Download or read book Preparation of Nanostructures for Electron Field Emission Applications written by Pejman Hojati Talemi and published by . This book was released on 2011 with total page 356 pages. Available in PDF, EPUB and Kindle. Book excerpt: Carbon nanostructures are a very promising option as new materials in field emission (FE) devices. In this thesis, a number of current challenges in the application of such materials in FE devices are addressed. A new microwave-based method was developed to improve the FE properties of commercial carbon nanotubes (CNT), and methods to produce macro-scale ensembles of CNTs as either networks or composites, were developed. Research was also carried out to develop a simple and economic method for preparation of carbon nanofibers and graphene sheets. At each step, a combination of analytical techniques including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, electron microscopy, thermogravimetry, as well as FE analyses, were used to determine the resultant material properties. In order to modify the FE properties of CNTs, two new microwave-based, treatments were developed. XPS and Raman results of samples prepared by conventional acid treatments or after thermal treatments were compared to the microwave-treated materials. It was found that the microwave-based methods resulted in less damage to the CNT structure and more uniformity in chemical functional groups on CNTs, compared to other treatments. It was also found that the microwave-plasma method was able to produce a "sheet-like" graphitic material between CNTs. Since the only carbon source in the MW-plasma process was the CNTs, a mechanism based on unzipping of CNTs was proposed and confirmed by XPS and Raman spectroscopy results. The formation of this new nanostructure was found to show considerably enhanced FE properties, compared to those of the CNTs. The unzipped nanotubes have additional sharper and thinner emitting tips (edges) with sp3 hybridisation of the carbon atoms on the edges, which is the likely reason for the significant improvement of FE properties in these post-treated samples. A number of methodologies aimed at fabrication of efficient CNT-based FE arrays were examined. In the first approach, nanocomposites based on CNTs or graphene nanosheets and conductive polymers were prepared. The most stable CNT/ polypyrrole nanocomposites were achieved by direct electropolymerisation on the surface of a membrane acting as spacer in the FE process, with the functional groups on the surface of CNTs acting as the necessary counter-ions. The FE measurement of these samples showed that limiting the emitting surface caused the turn on electrical field to increase, however the stability of emission was also improved. The other approach for the preparation of the macro-scale emitters was based on forming a network of CNTs. Webs based on spinnable CNT forests were drawn over a graphitic substrate. The FE studies of these samples showed a direct relationship between the density and thickness of the webs and FE properties, and an inverse relationship was found between the length of CNTs and their FE performance. By defining a tip number parameter, a direct linear relationship was observed between tip number and emission current. It was found that increasing the number of layers of CNT webs did not result in any improvement in FE performance, whilst double-layered samples with the two layers vertical to each other was found to significantly improve the FE performance. The final approach for producing macro-scale samples involved producing CNT networks by molecularly-fusing CNTs using the high temperature, spark plasma sintering (SPS) process. The networks prepared by this method showed that by increasing the sintering temperature from 1000C to 2000C, the CNT network became more packed and dense with better graphitic structure. However, improved fusion of the nanotubes at high temperatures led to the loss of freedom of CNT tips, and reduced FE properties. In the last aspect of this research, the microwave-based method previously used for modification of CNTs, was employed for synthesis of carbon-based nanomaterials themselves. It was found that synthesis of nanofibers could be catalysed by the presence of metallic catalyst nanoparticles which can be produced by the microwave-plasma process itself, from an incorporated solid metallic coupon. It was found that changing the carbon source from polystyrene to polyethylene (with higher hydrogen to carbon ratio) in the absence of a catalytic trigger can also be used for preparation of graphene nanosheets. FE studies of these samples showed that samples containing nanofibres exhibit better FE properties than graphene-based samples, although thermal treatment of the graphene samples did result in an improvement in field emission properties. It was also shown that depositing of the nanostructures during the production process results in better FE properties, in comparison to samples prepared by coating the substrate by solution casting from a dispersion of nanoparticles, due to the vertical alignment of the directly-coated nanostructures.

Download or read book Growth and Characterization of Carbon Nanomaterials written by Jay Patel and published by . This book was released on 2011 with total page 87 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this thesis, single and multi-layered graphene films were epitaxially grown on either Si-face or C-face of SiC single crystal substrates. The film growth conditions, such as decomposition temperatures and pressures, and their surface morphologies were optimized. These films were then characterized by using surface analysis tools including SEM, TEM, AFM evanescent wave microscopy and electron educed spectroscopy. In addition to studying graphene decomposed from SiC crystals, carbon nanotube material was fabricated using a floating catalyst technique. These carbon nanotube material was then studied for potential cathode applications in this thesis. Field emission properties of these cathodes was measured and compared between carbon nanotubes grown by the floating catalyst technique and carbon nanotube material fabricated from a super acid solution spinning process. The result found that carbon nanotube material produced from the floating catalyst method supported the highest emission currents. As a result of this research, carbon nanotube field emitters fabricated from this method are now being studied in a wide range of vacuum electronic applications.