Doping exohedral

Fig. 34.11. Ground triplet (d D2 ) and excited singlet (s C2a) states of (Cgo) with two views (for the alkali cation doped exohedral fullerenes), and spin-containing endohedral fullerenes [128,148].

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Modifications of the conduction properties of semiconducting carbon nanotubes by B (p-type) and N ( -type) substitutional doping has also been dis-cussed[3l] and, in addition, electronic modifications by filling the capillaries of the tubes have also been proposed[32]. Exohedral doping of the space between nanotubes in a tubule bundle could provide yet an-... 

Carbon nanostructures can be doped by the introduction/interaction of foreign atoms. Different doping categories include (a) exohedral doping or intercalation, (b) endohe-dral doping or encapsulation or (c) in-plane or substitutional doping. 

It was shown how both endohedral and exohedral fullerenes can be inserted in nanotubes. In the peapods containing endohedral fullerenes (for instance Ce Cg2), HRTEM images showed interesting rotation and translation motion of the trapped spheroids. Exohedral metallofullerenes, CsC io have been synthesized and successfully encapsulated into SWCNTs via a new chemical reduction of Cgo molecules into anions. The addition of iodine to already prepared peapods allowed the coalescence of Cgo directly inside the nanotubes. Indeed, after heating at 550 C, iodine-doped peapods, inside the Cgo molecules molecules have been transformed in a tubular structure. Khlobystov et al. were able to perform reactions on the inner surface of carbon nanotubes in the presence of catalyti-cally active atoms of rhenium and monitor the whole process via HRTEM. ... 

The first exohedral metallofullerenes to be synthesized and isolated were if complexes of C6oPt(PPh3)2 and C6oPd(PPh3)2 [7-9]. This work was later extended to monoatomic coordination by a whole range of metals including Ti, Nb, Re, Fe, Ru and Ni, and also multiple metal doping [4, 10-13]. 

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