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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.
The electromagnetic characterization of carbon nanotube films (CNT) grown by the surface decomposition of silicon carbide (SiC) has been performed. The CNT films formed on the carbon and silicon terminated face of the SiC substrate were uncapped by an annealing process at a temperature of 4000 C with dwelling time up to 60 minutes in oxygen or carbon dioxide atmosphere. X-Y scans of the quality factor were used to deduce the local conductive properties of the films measured by evanescent microwave microscopy. Real and imaginary permittivity values, as determined by these electromagnetic measurements, provided valuable information for future field emission testing on these films. A theoretical model, adapted from the literature, was used to find the real and imaginary component of the permittivity of the CNT films. The results showed improvement in the surface conductivity of the samples after the annealing treatment.
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.
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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.
Recently, carbon nanosheets (CNS), a novel nanostructure, were developed in our laboratory as a field emission source for high emission current. To characterize, understand and improve the field emission properties of CNS, a ultra-high vacuum surface analysis system was customized to conduct relevant experimental research in four distinct areas. The system includes Auger electron spectroscopy (AES), field emission energy spectroscopy (FEES), field emission I-V testing, and thermal desorption spectroscopy (TDS). Firstly, commercial Mo single tips were studied to calibrate the customized system. AES and FEES experiments indicate that a pyramidal nanotip of Ca and O elements formed on the Mo tip surface by field induced surface diffusion. Secondly, field emission I-V testing on CNS indicates that the field emission properties of pristine nanosheets are impacted by adsorbates. For instance, in pristine samples, field emission sources can be built up instantaneously and be characterized by prominent noise levels and significant current variations. However, when CNS are processed via conditioning (run at high current), their emission properties are greatly improved and stabilized. Furthermore, only H2 desorbed from the conditioned CNS, which indicates that only H adsorbates affect emission. Thirdly, the TDS study on nanosheets revealed that the predominant locations of H residing in CNS are sp2 hybridized C on surface and bulk. Fourthly, a fabricating process was developed to coat low work function ZrC on nanosheets for field emission enhancement. The carbide triple-peak in the AES spectra indicated that Zr carbide formed, but oxygen was not completely removed. The Zr(CxOy) coating was dispersed as nanobeads on the CNS surface. Although the work function was reduced, the coated CNS emission properties were not improved due to an increased beta factor. Further analysis suggest that for low emission current (10 uA), thermal, ionic or electronic transition effects may occur, which differently affect the field emission process.
Carbon nanotubes, with their extraordinary mechanical and unique electronic properties, have garnered much attention in the past five years. With a broad range of potential applications including nanoelectronics, composites, chemical sensors, biosensors, microscopy, nanoelectromechanical systems, and many more, the scientific community is more moti
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.
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.