Individual CNT Electrodes

We will discuss, on the one hand, the spaghetti-like CNT electrodes on conductive surfaces or randomly dispersed in a polymer matrix and on the other hand, CNT arrays grown in situ on substrates with large-scale control of location and orientation. Finally, individual CNT electrodes will be briefly discussed. Figure 3.10 illustrates these different CNT electrode arrangements. 

FigureS.IO Illustration of different CNT electrode configurations (a) randomly dispersed on a surface, (b) vertically aligned CNTs, (c) randomly dispersed CNT composites, (d) oriented CNTs embedded in a polymer matrix, (e) individual CNT electrode.

The first individual CNT electrode was created by Campbell et al., who attached 80-200 nm diameter CNTs to a Pt tip and electrodeposited a polyphenol layer on the CNT. The electrode surface was exposed by applying a bias to the tip in solution (which exposes the CNT) or at a specific site along the CNT in air, which cuts the CNT/polyphenol at the desired point. The fabrication of another individual CNT nanoelectrode was done by Kawano et al., where CNTs made through Joule heating on a silicon support structure were subsequently coated with parylene in a core-shell-type arrangement, shown in Figure 9.33. ... 

The schematic model is depicted in Fig. 8. As the bias voltage increases, the number of the molecular orbitals available for conduction also increases (Fig. 8) and it results in the step-wise increase in the current. It was also found that the conductance peak plotted vs. the bias voltage decreases and broadens with increasing temperature to ca. 1 K. This fact supports the idea that transport of carriers from one electrode to another can take place through one molecular orbital delocalising over whole length of the CNT, or at least the distance between two electrodes (140 nm). In other words, individual CNTs work as coherent quantum wires. 

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]. 

Electrochemical polymerization provides a convenient approach to fabricate CNTs/CP nanocomposites [46-53], Using such a strategy, the morphology and properties of the nanocomposites can be controlled by the electropolymerization conditions, such as the applied potential or current density. Ajayan and co-workers have reported the electrochemical oxidation of aniline in H SO on the CNTs electrode to fabricate CNT/PANl composites [46]. Chen et al. fabricated CNT/PPy nanocomposites, the first example of anionic CNTs acting as the dopant of a CP [47]. Their results showed that PPy was xmiformly coated on the surface of individual CNTs by electrolysis at a low apphed potential for a short time, rendering them potential applications in nanoelectronic devices. Another kind of CNTs/CP composite nanostructures, e.g., CNTs as inorganic fillers in CP matrices [54] can be prepared by a template-directed electropolymerization method. (Figure 13.3)... 

As for the coherent length in CNTs, a very interesting paper has been published from the group at the Georgia Institute of Technology about the conductance of individual MWCNTs [34], They have observed the quantisation of conductance by changing the distance between the two electrodes. This result indicates ballistic conduction in a CNT, which suggests the formation of stationary waves of electrons inside CNTs. 

The conductive properties of SWCNTs were predicted to depend on the helicity and the diameter of the nanotube [112, 145]. Nanotubes can behave either as metals or semiconductors depending upon how the tube is rolled up. The armchair nanotubes are metallic whereas the rest of them are semiconductive. The conductance through carbon nanotube junctions is highly dependent on the CNT/metal contact [146]. The first measurement of conductance on CNTs was made on a metallic nanotube connected between two Pt electrodes on top of a Si/Si02 substrate and it was observed that individual metallic SWCNTs behave as quantum wires [147]. A third electrode placed nearby was used as a gate electrode, but the conductance had a minor dependence on the gate voltage for metallic nanotubes at room temperature. The conductance of metallic nanotubes surpasses the best known metals because the... 

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