History and State of the Art of CVD of Carbon Nanotubes

In the CVD process, in general, catalytic metal particles are exposed to a medium containing hydrocarbon gases, and the formation of nanotubes is catalyzed. During growth, good uniformity in the size of the tubes is achieved (controlled by the size of the seeded catalyst particles). In some cases, when the catalysts are prefabricated into patterned arrays, well-aligned nanotube assemblies can be readily produced. Similarly, template-based approaches are also in use, where the aligned pores of a nanoporous membrane (such as electrodeposited porous alumina, zeolite, or porous silicon) are filled with carbon species through vapor deposition and later graphitized to produce nanotubes.

In the early years of study of chemical vapor deposition, different parameter ranges for distinguishing multi- and single-walled growth, as well as differentiating the product from carbon nanofibers, were outlined [13-15]. These results basically were the establishment of chemical vapor deposition as a route for carbon nanotube production. After receiving the first batches of carbon nanotubes by this new technique, a wide parameter range opened up for investigation; namely, it turned out that the reaction temperature, catalyst type, geometry, and substrate can change the properties of the product dramatically. Logically, one needs to optimize the process for purity of the product and also for the yield, where both of these optimizations are still a challenge for scientists. Parallel with this problem, new challenges have arisen: further tailoring the properties of the product distinguishing not only between MWNTs and SWNTs but predefining the diameter, length, and number of walls, and eventually chirality of the tubes needs to be controlled. Success has been achieved in almost all of the directions of tailoring these parameters, although open questions still exist; for example, growth of nanotubes with predefined chirality has not been solved.

Among the processes developed for mass production of carbon nanotubes (SWNTs), we need to emphasize the HiPCO process, optimized for relatively high yield and production rate by scientists at Rice University [16]. The quantity of the product is still behind the amount predicted [17] and its price has not dropped as was expected. Nowadays, this is the most widely available and used SWNT product on the market produced by the CVD technique.

Parallel with the experimental and theoretical work done to explain the role of different parameters in the growth process, it turned out that the mechanism of the growth may be different for every group [10,18], which means that solving the problem of predefined carbon nanotube growth is equivalent to solving the same problem for a series of independent processes. By now, good progress has been shown in the tailoring of the carbon nanotube length and diameter [19,20], and the number of nanotube walls also has became a well-controlled parameter. Along with SWNTs and MWNTs, double-walled carbon nanotubes (DWNTs) were also prepared via the CVD method, and their purification was reported [21]. Catalyst material and particle size have been widely investigated [22-28] given their critical role in growth.

Beyond the properties of individual nanotubes, their collective behavior (synergy) is also important for a wide range of applications. First, the most obvious question was investigated: what are the methods to grow carbon nanotubes parallel to each other, and thus produce them in an aligned layer? The first papers reported the growth of blanket nanotube films [29] using a porous template and catalyst layer, or tailoring the location of the growth by focusing the laser beam in order to etch the substrate/catalyst [30,31].

To think further along these lines, there is a need to control the location of growth while keeping some level of alignment. Most of the methods are based on catalyst deposition onto planar substrates, so patterning the catalyst on the sample surface was evidently a way to control the growth location: a nanotube layer was developed only on that part of the sample covered by catalyst. To prepare the pattern into the catalyst layer, several methods were applied. Dai et al. [32] prepared organized nanotube towers by CVD growth involving catalyst patterning and rational design of the substrate to enhance catalyst-substrate interactions and to control the catalyst particle size. The substrate was planar porous silicon, and the catalyst was a thin iron film evaporated through a shadow mask. The resulting structure consisted of multiwalled carbon nanotubes bound to each other by van der Waals interactions. Similar structures were achieved when the shadow mask was applied in the CVD process itself [33] to prevent carbon nanotube growth on unwanted locations, and also to keep the shape of the grown nanotube towers. Furthermore, microcontact printing is a simple but powerful method to prepare a large area of lower-resolution catalyst patterns. A stamp may be applied to print catalyst-containing ink onto the substrate, and later carry out selective nanotube growth only on the printed areas [34].

A higher-level control was established in the CVD process when Duesberg et al. produced small vertical bundles of carbon nanotubes in lithographically defined, submicrometer-sized holes of the template [35,36]. Similarly high-level control was achieved in the structures when Dai et al. [32] presented a method for controlled synthesis for free-standing single-walled and multiwalled nanotubes with directed orientations. Their process starts with silicon/silica tower fabrication, followed by catalyst deposition from the liquid phase using a block copolymer providing the necessary catalyst parameters. Extensive calcination and subsequent CVD growth with the substrate yielded SWNTs emanating from the towers. Directed freestanding SWNT networks are formed by nanotubes growing to adjacent towers and suspended above the surface.

In summary, we list the main parameters of the chemical vapor deposition of carbon nanotubes: the temperature of the growth, the carbon source, the materials and size of the catalyst, and the materials and surface properties of the substrate.

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