Semi-conductivity intrinsic

Fig. 6.5 Some important examples of the basic structures of intrinsically (semi)conducting...

To further probe the SWCNTs purity, their intrinsic conductivities were measured by preparing buckypapers from the respective dispersions. Conductivities of buckypapers prepared from HiPCO and Carbolex SWCNTs were approximately 1 x 10 and 1 x 10 S m, respectively. The significant difference in intrinsic conductivities indicates that there is certainly a difference in the electronic properties of the SWCNTs. The lower conductivity of the Carbolex SWCNTs could be attributed to more wall defects, a larger fraction of semi-conducting SWCNTs or a carbonaceous coating on the SWCNT wall. To determine the main cause of these lower conductivities would require further investigation. For the application intended here, this difference is of utmost importance. One further prerequisite is that the aspect ratio of the two SWCNT batches is not vastly different. 

Intrinsic semi-conductivity Under effect of temperature only ... 

We refer to the intrinsic semi-conductivity. On closer examination of the values of Eg for a certain number of semi-conductor oxides or carbides (see Table 11.1) we can see that the concentrations of electrons and holes may be significant only at veiy high temperatures, unlike what we see for more traditional semi-conductors like silicon... 

A GaN substrate would be a help in this respect but it would need to be semi-insulating. In addition, GaN has a poor thermal conductivity and is not very suitable due to this negative material property. Aluminum nitride substrates may become the substrate of choice for GaN high-frequency applications. It has a reasonable thermal conductivity and is intrinsically semi-insulating but only time will tell. 

Semi-conductors have conducting properties between those of insulators and conductors. The band structures of conductors, intrinsic semiconductors and insulators are represented schematically in Figure 1.4. The widths and the separations of the bands are dependent upon the intemuclear spacings of the constituents, so that the band structure may be modified in the vicinity of the surfaces of the crystal, by the occurrence of surface reactions, or by interactions with gases, liquids or other solids. 

Through a comprehensive review of the recent conductive polymer literature, it has been demonstrated that photoelectron spectroscopy provides a very unique and powerful tool for analyzing the intrinsic structure, the charge transfer interaction, and the stability and degradation behaviour of electroactive polymers. It is further demonstrated that photoelectron spectroscopy is also ideal for investigating the chemistry and electronic structure of the electroactive polymer interface with other polymers, semi-conductors, and metals. The surface and interfacial analytical capability of photoelectron spectroscopy can be further extended to include molecular specificity when coupled with the SIMS technique. Finally, the imaging XPS technique is fast becoming widely available [368]. 

Conducting liquid crystals. Several approaches have been tried, more or less successfully, based also on a 1-D stacking. One of the firsts has been to prepare a liquid crystal based on a phthalocyanine as an intrinsic semi-conductor [83]. One other attempt has been to use discotic liquid crystals, based on the triphenylene molecule oxidized by bromine [84] or by aluminium trichloride [8] in the latter, the conductivity changes by six orders of magnitude (from 10" cm- to 10 cm-i), while the mesomorphic character is conserved. 

A mathematical model which can predict mobilization of entrapped ganglia under the influence of both gravity and flow has been recently developed (8). The work, which was based on suitable force balances and included the effects of contact angle, was supported by measurements performed using a fluid-fluid-solid system and a simple cubic geometry. This model differed from that of Ng et al., in that their analysis of blob movement was semi-quanti-tative for what was assumed to be a uniformly wetted system (13). Ganglion mobilization experiments which included various intrinsic or smooth surface contact angles were conducted by Morrow, but were not quantitatively assessed (9). 

ILs are excellent solvents that can support many types of solvent-solute interactions (Welton 1999). They also offer other advantages such as decontamination, product recovery, and recyclable properties. Other pertinent properties include high intrinsic ionic conductivity, nonflammability, wide electrochemical stable window, broad liquid range and excellent heat transfer properties, and most importantly their hydrophobic nature (Ahmad 2009). The hydrophobic nature of ILs and the possibility to form various composites with polymers (Scott et al. 2002) are also important features for sensor applications (Wang et al. 2004). These composite materials can be used as conductive materials, semi-permeable membranes, and electrodes. 

Figure 2.8 Semi-logarithmic plot of the conductivity of the SWCNT /PmPV composite for various mass fractions of CNT powder. The sharp increase of the conductivity is partly due to the intrinsic conductivity of the polymer matrix and not only to the formation of a connective CNT network in the nanocomposite polymer matrix. (From Ref. 79. Reprinted with permission of the American Physical Society]...

The results on MWCNT-polymer nanocomposites reported in this chapter demonstrate the versatility of the latex concept to prepare nanocomposites with a broad range of "home-made" or industrially manufactured polymers, namely, amorphous, semi-crystalline, and blended polymer matrixes. Note that blending can further be done in very different fashions, i.e., in situ, while the emulsion polymerization proceeds, by mixing of two different polymer latexes synthesized independently from each other, or by a "masterbatch approach." This study confirms that the CNT-polymer interactions are of major importance to influence the percolation behavior of the nanocomposites, as well as the viscosity, morphology, and the intrinsic conductivity of the polymer matrix. 

The heavily -doped AlGaAs donates the electrons that are forced into the semi-insulating (near intrinsic) GaAs to form an n-channel depletion FET. Since the ionized donors stay in the AlGaAs layer, they do not scatter the electrons in the n-channel which permits the conducting electron to have very high mobilities. The high electron mobility allows the transistor to operate at frequencies in excess of 300 GHz. 

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