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A carbon nanotube (CNT) is a tubular structure made of carbon atoms, having diameter of nanometer order but length in micrometers. Right from its discovery, we have been listening exciting quotations about CNT, viz. “CNT is 100 times stronger than stainless steel and six times lighter...” “CNT is as hard as diamond and its thermal capacity is twice that of pure diamond...” “CNT’s current-carrying capacity is 1000 times higher than that of copper...” “CNT is thermally stable up to 4000K...” “CNT can be metallic or semiconducting, depending on their diameter and chirality...” However, it is important to note that all those superlative properties were predicted for an atomically-perfect ideal CNT which is far from the CNTs we are practically producing today. Despite a huge progress in CNT research over the years, we are still unable to produce CNTs of well-defined properties in large quantities by a cost-effective technique. The root of this problem is the lack of proper understanding of the CNT growth mechanism. There are several questions at the growth level awaiting concrete answer. Till date no CNT growth model could be robustly established. Hence this chapter is devoted to review the present state of CNT synthesis and growth mechanism. There are three commonly-used methods of CNT synthesis. Arc-discharge method, in which the first CNT was discovered, employs evaporation of graphite electrodes in electric arcs that involve very high (~4000°C) temperatures (Iijima, 1991). Although arc-grown CNTs are well crystallized, they are highly impure; about 60–70% of the arc-grown product contains metal particles and amorphous carbon. Laser-vaporization technique employs evaporation of high-purity graphite target by high-power lasers in conjunction with high-temperature furnaces (Thess et al., 1996). Although laser-grown CNTs are of high purity, their production yield is very low (in milli gram order). Thus, it is obvious that these two methods score too low on account of efficient use of energy and resources. Chemical vapor deposition (CVD), incorporating catalyst-assisted thermal decomposition of hydrocarbons, is the most popular method of producing CNTs; and it is truly a low-cost and scalable technique for mass production of CNTs (Cassell et al., 1999). That is why CVD is the most popular method of producing CNTs nowadays. Here we will review the materials aspects of CNT synthesis by CVD and discuss the CNT growth mechanism in the light of latest progresses in the field.
Heterojunction structures of nanocrystalline materials are of great importance in scientific and industrial research for their potential applications in nanoscale electronics and photonics. Here, we report a simple wet-chemical method of nitrogen-mediated growth of ZnO nanocrystals on carbon nitride (CNx) nanotubes. SEM and TEM analyses show self-organized ZnO nanoflowers on CNx stems. PL spectra exhibiting a blue emission at 449 nm confirms the junction formation between CNx and ZnO. The field emission (FE) properties of CNx-ZnO film are greatly improved over those of pristine CNx. The turn-on and threshold fields for bare CNx film are 1.70 and 2.95 V/μm, whereas those for CNx-ZnO hybrid are found to be 0.75 and 1.3 V/μm, respectively. This significant downshift in the turn-on and threshold fields is believed to occur via lowering of the Schottky barrier at the metal−semiconductor interface. Three-dimensionally blossomed ZnO nanopetals with multiple sharp tips effectively enhance the FE performance. Moreover, this heterojunction reinforces the electron emission lifetime and protects the CNx tubes against thermal degradation.
Vertically grown planar ZnO nanowalls, with typical dimensions of 40–80 nm thickness and several micrometers wide, were electrodeposited on an indium–tin-oxide (ITO)–glass substrate at 70 °C. X-ray photoelectron spectroscopy (XPS) studies reveal that the nanowalls consist of ZnO covered with a Zn(OH)2 overlayer. An x-ray diffraction (XRD) study shows that these nanowalls have the wurtzite structure and are highly crystalline. The corresponding Raman and photoluminescence spectra further indicate the presence of oxygen deficiency. These ZnO nanowalls exhibit excellent field emission performance, with not only a considerably lower turn-on field of 3.6 V µm−1 (at 0.1 µA cm−2) but also a higher current density of 0.34 mA cm−2 at 6.6 V µm−1 than most ZnO nanowires and other one-dimensional nanostructures reported to date.
This review article deals with the growth mechanism and mass production of carbon nanotubes (CNTs) by chemical vapor deposition (CVD). Different aspects of CNT synthesis and growth mechanism are reviewed in the light of latest progresses and understandings in the field. Materials aspects such as the roles of hydrocarbon, catalyst and catalyst support are discussed. Many new catalysts and new carbon sources are described. Growth-control aspects such as the effects of temperature, vapor pressure and catalyst concentration on CNT diameter distribution and single- or multi-wall formation are explained. Latest reports of metal-catalyst-free CNT growth are considered. The mass-production aspect is discussed from the perspective of a sustainable CNT technology. Existing problems and challenges of the process are addressed with future directions.
One-dimensional ZnO nanopillars of diameter 80−120 nm and two-dimensional nanowalls of thickness 100−300 nm are electrochemically grown at 70 °C on a flexible polyester substrate. The low turn-on electric fields measured at a current density of 1 μA/cm2 for nanopillars (1.2 V/μm) and nanowalls (2.2 V/μm) illustrate their superior field emission properties to most of the reported ZnO nanostructures. The present method of direct electrodeposition of ZnO on plastic without the need of template or catalyst offers a low-cost technique for fabricating field emission devices on flexible substrates for large-area display and other technologies.
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