Semiconducting SWCNTs

As described above, metallic CNTs are of great interest because they possess molecular orbitals which are highly delocalised. However, metallic CNTs are very difficult to use in actual devices because they require very low temperatures to control their carrier transfer. On the contrary, even at room temperature, the nonlinear /-V jas curve and the effective gate voltage dependence have been presented by using individual semiconducting SWCNTs [29]. 

Fig. 9./-Tbias curve of an individual semiconducting SWCNT with different gale voltages measured at room temperature [29].

Information obtained Chirality of semiconducting SWCNTs (and thus diameter), band gap, bundling. 

Common separation methods can be divided into chemical and physical routes. Chemical approaches rely on the interaction of the surface of different CNT types with surfactant molecules. Early work has shown that octadecylamine [94] and agarose gel [95] adsorb preferably on semiconducting SWCNTs, while diazonium reagents [96] and DNA [97, 98] show preference with metallic tubes. The assemblies with adsorbed molecular species are considerably larger and heavier than the indi-... 

The physical approach uses alternating current (ac-) dielectrophoresis to separate metallic and semiconducting SWCNTs in a single step without the need for chemical modifications [101]. The difference in dielectric constant between the two types of SWCNTs results in an opposite movement along an electric field gradient between two electrodes. This leads to the deposition of metallic nanotubes on the microelectrode array, while semiconducting CNTs remain in the solution and are flushed out of the system. Drawbacks of this separation technique are the formation of mixed bundles of CNTs due to insufficient dispersion and difficulties in up-scaling the process [102]. 

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.

Semiconductive SWCNT FET transistors capable of operating at room temperature were constructed several years ago and their operation in the terahertz frequency range was predicted [160]. These early devices had p-type characteristics (hole... 

Here 1 labels the electronic states of the SWCNT with the chiral index (p,0), which are described by a simple two-band k p model based on an effective mass approximation [4], p being equal to 3M + v with integer M and v = 0( 1) for metallic (semiconducting) SWCNTs. The energy bands in Eq.(3) are given by... 

Besides semiconducting SWCNTs, properly doped diamonds can also be nsed as the channel material for fabricating FETs for biosensing as demonstrated by Song et al. [75]. DNA probes are immobilized directly on the aminated snr-face of a p-type polycrystalline diamond film which serves as the conduction channel between the source and drain electrodes. Since a diamond surface is chemically stable and presents a wide potential window, it permits direct contact of biomolecules with the channel surface, eliminating the needs of a polymer coating (in CNT FETs) or dielectric encapsulation (in Si-FETs). As a result, diamond FET is potentially a much more sensitive and faster biosensor, which can operate in solution. Hybridization with complementary and 3-mer mismatched DNA targets in 10 pM can be discriminated in cyclically repeated hybridization and denatnre experiments. 

The commonly accepted physico-chemical gas sensing mechanisms of SWCNT-based gas sensors are discussed in detail by several authors (Bradley 2003 Pengetfli,2009 Battie eta/.,2012b).These mechanisms,which will be described in Section 10.3, are the charge transfer between adsorbed gas molecules and SWCNTs, and the modulation of the Schottky barrier established between semiconducting SWCNTs and metallic electrodes. 

Kong et al (2000) have compared the sensitivity to NO2 and NH3 of (a) a CNTFET based on individual semiconducting carbon nanotubes with the sensitivity of (b) a CNTFET-based SWCNT network. The SWCNTs which composed this second device were synthesised by laser ablation. The resistance of device (a) increases and the resistance of device (b) decreases when exposed, respectively, to NH3 and N02.The responses of a CNTFET based on an SWCNT network to 1% of NH3 and 200 ppm of NO2 were approximately 50%. Compared with a CNTFET based on individual SWCNTs, the sensitivity is dramatically reduced. In the case of SWCNT networks, the response of semiconducting SWCNTs is averaged by the small response of metallic carbon nanotubes. Furthermore, due to the van der Waals... 

In the absence of a gate voltage, the hole carrier injection through the Schottky barrier formed at the interface between metallic electrodes and semiconducting SWCNTs is dominated by thermal emission at room temperature. Considering this effect, the vs characteristics are described by the analytical Equation [10.5] ... 

Hie Hiales group has patented an original sensor architecture based on an array of CNTFETs with various metals for the electrodes (BondavaUi et al., 2008, 2010). As described in Section 10.3.3, the sensing mechanism of CNTFETs is mainly attributed to the modulation of the Schottky barrier built at the contacts between metallic electrodes and semiconducting SWCNTs. Assuming that the adsorption of gas at the contacts depends on its chemical affinity with the electrodes, each gas-metal couple should induce different modulation of the Schottky barrier. 

It has been found that nanosized carbons, such as single-waUed carbon nanotubes and graphenes, can be used as SAs for ML of solid-state lasers, as long as these carbon materials are semiconductive. For example, semiconducting SWCNTs with a high content of 80 % have been produced by using a polymer extraction technique. These SWCNTs exhibited a saturation intensity of 1.7 MW cm, modulation depth of 6 %, and nonresidual loss of 5.5 %, which ensured to generate stable pulses of 8.3 ps, with a repetition rate of 90 MHz, when combined with a 0.5 at.% Nd YAG ceramic laser pumped with a CW 808-nm diode [164]. 

Such a combination of semiconducting SWCNTs coated with an ordered P3HT layer is expected to enhance the charge separation and transport in the devices active layer, which is a key towards efficiency enhancement. Electrical characteristics (J and V ) of devices were found to strongly dependent on the SWCNT loading (Figure 5.14c), such that both the... 

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