Carbon nanotubes controlled fabrication

Although random and irregular type GaN nanorods have been prepared by using transition metal nanoparticles, such as Ni, Co, and Fe as catalysts and carbon nanotubes as the template, the preparation of controllable regular array of strai t GaN nanorods has not yet been reported. Fabrication of well-ordered nano-structures with high density is very important for the application of nano-structures to practical devices. 

As the analytical, synthetic, and physical characterization techniques of the chemical sciences have advanced, the scale of material control moves to smaller sizes. Nanoscience is the examination of objects—particles, liquid droplets, crystals, fibers—with sizes that are larger than molecules but smaller than structures commonly prepared by photolithographic microfabrication. The definition of nanomaterials is neither sharp nor easy, nor need it be. Single molecules can be considered components of nanosystems (and are considered as such in fields such as molecular electronics and molecular motors). So can objects that have dimensions of >100 nm, even though such objects can be fabricated—albeit with substantial technical difficulty—by photolithography. We will define (somewhat arbitrarily) nanoscience as the study of the preparation, characterization, and use of substances having dimensions in the range of 1 to 100 nm. Many types of chemical systems, such as self-assembled monolayers (with only one dimension small) or carbon nanotubes (buckytubes) (with two dimensions small), are considered nanosystems. 

Carbon nanotubes inevitably contain defects, whose extent depends on the fabrication method but also on the CNT post-treatments. As already seen, oxidizing treatments, such as acid, plasma or electrochemical, can introduce defects that play an important role in the electrochemical performance of CNT electrodes. For instance, Collins and coworkers have published an interesting way to introduce very controlled functionalization points or defects on individual SWNTs by electrochemical means [96]. Other methodologies to introduce artificial defects comprise argon, hydrogen and electron irradiation. Under this context, a number of recent works have appeared with the goal of tailoring the electrochemical behavior of CNT surfaces by the controlled introduction of defects [97, 98]. 

Moreover, the carbon nanotube density is low. Thus, there is a considerable interest in using nanotubes to fabricate composite materials from the point of view of mechanical reinforcement. However, the above models assume a perfect adhesion of the nanotubes to the matrix. In practice, the interface can fail. Of course, this lowers the stress transfer and the reinforcement by the nanotubes. This is why controlling the surface chemistry of the nanotubes and their interactions with a polymer matrix are also critical challenges. 

This paper represents an overview of investigations carried out in carbon nanotube / elastomeric composites with an emphasis on the factors that control their properties. Carbon nanotubes have clearly demonstrated their capability as electrical conductive fillers in nanocomposites and this property has already been commercially exploited in the fabrication of electronic devices. The filler network provides electrical conduction pathways above the percolation threshold. The percolation threshold is reduced when a good dispersion is achieved. Significant increases in stiffness are observed. The enhancement of mechanical properties is much more significant than that imparted by spherical carbon black or silica particles present in the same matrix at a same filler loading, thus highlighting the effect of the high aspect ratio of the nanotubes. 

In this context, nanoporous carbons are extremely interesting materials which can be used either as electrodes of supercapacitors or hydrogen reservoir. They are commercially available at a low cost and under various forms (powder, fibers, foams, fabrics, composites) [3]. They can be obtained with well-developed and controlled porosity [4,5] and with a rich surface functionality [6,7], As far as electrochemistry applications are concerned, very important advantages of carbons are a high electrical conductivity, a good chemical stability in various electrolytic media and the possibility to control wettability by the nature of the surface functionality. When they are not playing the role of active material for the storage process, carbons may be also useful as additive in a composite to improve its physical properties. Particularly carbon nanotubes are able to improve the electrical conductivity and mechanical properties of electrodes [8],... 

J.Y. Kim, K.H. Kim, and K.B. Kim, Fabrication and electrochemical properties of carbon nanotube/polypyrrole composite film electrodes with controlled pore size, J. Power Sources, 176(1), 396 02 (2008). 

It has been proposed that nanotube-based transistors can operate at frequencies in the terahertz region as generators, frequency multipliers and detectors [84] though carbon nanotube transistors are good candidates for RF and opto electronics [85], fabrication of nanotube arrays with controllable chirality and diameters is stiU a challenge for large scale fabrication of the devices. 

Carbon nanotubes are typically synthesized with poor control of length and chirality [24, 25]. After carbon nanotubes have been selected and deposited on a substrate, an important step in the fabrication of carbon nanodevices is the patterning to reach an optimal size or dimensions. A common tool to pattern carbon nanostmcffires is the selective exposure to ion beams, with this technique it has been possible reaching the 10 nm range [26, 27]. 

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