Metallic SWCNTs

As mentioned above, the electronic properties of SWCNTs depend on their chirality and may be semiconducting or metallic. There is still no satisfying way to produce just one sort of SWCNTs, which would require the exact control of catalyst particle size at elevated temperature. Hence, the separation of semiconducting from metallic SWCNTs is of paramount importance for their application in, for example, electric devices, field emission and photovoltaics etc. 

Fig. 17.7 Current-voltage curves (a) and calculated electron diffusion lengths (b) from three DSSCs with Ti02 nanoparticles only, with 0.2 wt% pure semiconducting SWCNT, and with 0.2 wt% pure metallic SWCNT. L is the film thickness and Ln is the electron diffusion length.

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... 

SAMFETs have also been used in chemical sensing. The a-substituted quincpiethiophene SAMFETs were covered with a 10-nm pinhole-riddled iron tetraphenylporphyrin chloride layer, that acts as a receptor to nitric oxide (NO), an important biomarker [74]. The threshold voltage, measured by the FET transfer characteristics with the porphyrin receptor shifts upon increased exposure to NO. Annealing the monolayer FET in vacuum restores the initial FET behavior. Also, in the single monolayer HBC assembled FETs between metallic SWCNT source and drain electrodes increased current levels were measured in /d-Fds and Aj-Fg characteristics (Fig. 9) upon exposure to solutions of the electron acceptor TCNQ [68]. While the mechanism of response is not known, TCNQ has an affinity for coronene, and likely gives rise to charge transfer between electron-deficient TCNQ... 

Attaching chemical functionalities to CNTs can improve their solubility and allow for their manipulation and processability [24]. The chemical functionalization can tailor the interactions of nanotubes with solvents, polymers and biopolymer matrices. Modified tubes may have physical or mechanical properties different from those of the original nanotubes and thus allow tuning of the chemistry and physics of carbon nanotubes. Chemical functionalization can be performed selectively, the metallic SWCNTs reacting faster than semiconducting tubes [25]. 

Figure 1. Charge transfer Aq/e from the zigzag SWCNTs (p,0) with p = 5-15 to the F atom adsorbed on their surfaces as a function of the tube radius R. The heavy dots and triangles refer to semiconducting and metallic SWCNTs, respectively. The solid lines are intended as a guide to the eye.

Due to the complexity of the transport properties of an SWCNT network, the physical mechanisms at stake during exposure to gas constitute an important source of debate. The percolation theory is invoked to account for the influence of SWCNT density on the percolation threshold of metallic SWCNTs and on the transport properties of SWCNT networks (Topinka et al, 2009). As a result, the SWCNT network exhibits a metallic or a semiconducting character. 

This combination of features might be useful in applications where high-electric field and high-frequency operations are essential [128]. The ballistic electronic properties of metallic SWCNTs lend this material to use as a contact to molecules. Calculations suggest that alignment of the Fermi level of m-SWCNTs and the energy levels of coimected molecules may enable the use of such systems in a new generation of nanoscale devices [129—131]. 

To further improve the performance of the counter electrode in DSSCs, it is highly desirable to use enriched long and smaller diameter metallic SWCNTs. The long tubes should reduce the inter-tube connections, increasing the conductivity of the cathode and small diameter to enhance the chemical reactivity. The use of these high conductive metallic SWCNTs would improve the rate transfer of the electrons arriving from the external circuit to the redox system. 

---