Synthesize carbon nanotube field emitters by local heating chemical vapor deposition
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Abstract:
The effects of growth time on synthesizing of carbon nanotubes were reported. Prepared different forms of carbon nanotubes on silicon substrates by local heating chemical vapor deposition. The results showed that most of carbon nanotubes grew out with small diameter (60-80nm) and thinner in the case of short growth time (30s); when the growth time increased to 10 min or even longer, most of carbon nanotubes had large diameter(over 500nm) and there were many small CNTs attached to the wall of big carbon nanotubes; long growth time (10min) carbon nanotubes had good performance on field emission current and stability, but the shorter growth time (30s) one was much worse.Keywords:
Carbon fibers
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Vertically aligned carbon nanotubes, embedded in titanium oxide, have been fabricated and their electron emission behavior has been characterized. The growth of carbon nanotubes was achieved using a dc plasma enhanced chemical vapor deposition method with a mixture of acetylene and hydrogen, and nickel was used as the catalyst layer. Titanium oxide, deposited using an atmospheric pressure chemical vapor deposition, was used to encapsulate the grown carbon nanotubes and the physical properties of the as-grown carbon nanotubes as well as the encapsulated structures have been investigated using scanning electron microscopy. By a sequential polishing and plasma ashing, it is possible to open up the top side of the encapsulated carbon nanotubes. Also by means of a reactive ion etching, carbon nanotubes are exposed with an inherent gate surrounding each one. This technique allows the evolution of individually processed nanotubes with no need of nanolithography. The emission of electrons from carbon nanotubes was examined and a preliminary field emission display was prepared.
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Acetylene
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Carbon black can act as catalysts to grow carbon nanotubes or carbon nanofibers through a metal-catalyst-free thermal chemical vapor deposition.
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The field emission properties of multiwall carbon nanotube films with and without a coating of tetrahedrally bonded amorphous carbon (ta-C) were investigated. Voltage thresholds of 2.4 V/μm for uncoated films and 1.5 V/μm for ta-C coated films were found. The results for the uncoated films are in good agreement with previous measurements of field emission from carbon nanotubes. The effect of the ta-C coating on the emission properties is discussed in light of current field emission models.
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Due to their extraordinary electrical, thermal and mechanical properties, carbon nanotubes have been foreseen as potential materials for electronics devices in the future. To integrate carbon nanotubes in electronic applications, carbon nanotubes would need to be grown on different metal layer. In this study, carbon nanotubes growth with Ni as catalyst on three different support layers, Cu, Al and Cr, by hot filament chemical vapor deposition (HFCVD) is reported. The nanotubes were grown using C2H2 acetylene as carbon feedstock, in a hydrogen and nitrogen atmosphere. The catalyst layers and their support layers were deposited by magnetron sputtering technique. Deposited films were annealed at 600 °C for 10 minutes before exposing to C2H2 for the growth of nanotubes at same temperature for another 10 minutes. The effects of the support layer have been investigated with reference to nanotubes formation. The morphology and microstructure of the films were measured and analyzed by scanning electron microscopy (SEM) and Raman spectrometer. It was found that reaction of the catalyst with its supporting layer has significant effects on the growth of nanotubes. For Cu or Cr as support layer, its effect on the nanotubes growth was minimal. However Al support layer prevented the growth of carbon nanotubes. The possible mechanisms for the observed results are proposed.
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Due to their unique electronic and structural properties, carbon nanotubes have attracted substantial attention as field emitters in vacuum microelectronics. Field emission from carbon nanotubes with high current densities at low applied electric fields has been demonstrated by several groups. The mechanism behind the field emission remains unclear. Not only are most samples mixtures of single-walled and multi-walled nanotubes with widely varying electrical properties, but the emission behavior reported for nanotubes varies widely. To get a better perspective on these basic issues, we studied the thermal field emission characteristics of single walled carbon nanotubes (SWNT) using a atomic resolution field emission microscope (FEM).
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Field emission from carbon nanotubes grown on carbon cloth has been studied. An extremely low electric field of less than 0.4V∕μm is required to reach an emission current density of 1mA∕cm2. This ultralow operating electric field of carbon nanotubes grown on carbon cloth is mainly due to a very high field enhancement factor of 1.882×104, which is the result of geometrical configuration of the carbon nanotubes and the substrate. In addition to the field enhancement, the highly disordered microstructure of carbon nanotubes grown on carbon cloth plays an important role to field emission. This unexpected result indicates that the roughness of the substrates on which carbon nanotubes grow is very important. This result also brings us significantly closer to practical applications such as highly efficient lamps, field emission displays, micro vacuum electron sources, etc.
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Carbon fibers
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We have synthesized by chemical vapor deposition (CVD) single-, double-, and multiwalled carbon nanotubes without magnetic impurities. In particular, we have applied a rhenium-based CVD technique yielding nonmagnetic carbon nanotubes with diamagnetic Re particles. In addition, carbon nanotubes prepared with iron as catalyst particles are annealed at very high temperatures in which the catalyst material is completely vaporized, while the carbon nanotubes are structurally preserved. Detailed magnetic studies show for both approaches a clear diamagnetic behavior typical for pure carbon nanotubes but no indication of ferromagnetic or paramagnetic material.
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Carbon nanotubes and Ga-doped carbon nanotubes were synthesized by pyrolysis and then purified. Thin films of the purified samples were fabricated by a screen-printing method. Field emission properties of these films were studied. It was shown that the turn-on field of carbon nanotubes and Ga-doped carbon nanotubes was 2.22V/μm and 1.0V/μm, and the current densities were 400μA/cm2 and 4000μA/cm2 for carbon nanotubes and Ga-doped carbon nanotubes at applied fields 2.4V/μm. The electron field emission properties of the gallium-doped nanotubes were much better than those of carbon nanotubes. Mechanisms of field emission of gallium-doped nanotubes were explained.
Frit compression
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