Filling Inorganic Nanotubes

Analogous to filled carbon nanotubes, inorganic nanotubes, such as BN [17], BxCyNz [25], and sulfide nanotubes [15], have also been filled with different materials. This category of nanocable is usually synthesized through a simultaneous phase separation process during the synthesis of nanotubes and nanowires.

The first sophisticated coaxial nanocable structure with insulated shells was produced via laser ablation of a composite target containing BN, graphite, and a small amount of SiO and Li3N at high temperature [10]. Figure 5 shows the transmission electron microscope (TEM) images of the fabricated coaxial nanocables. They are normally a few tens of micrometers long and 10-100 nm in their diameters. The diameters are quite uniform throughout the whole length of the nanocable (Fig. 5A). High-resolution TEM images in Figure 5B and C indicate that the core of the nanowire has a crystalline phase whose electron diffraction pattern (left bottom inset of Fig. 5B) and lattice image (right bottom inset of Fig. 5C) fit well with that of cubic SiC. The intermediate layer shows an amorphous phase. The outermost layer consists of graphite multiwalls. A detailed elemental profile analysis across the nanocable by STEM-PEELS reveals the Si:C ratio in the core wire and Si:O ratio in the intermediate layer is 1:1 and 1:2, respectively. The outer layer of the nanocable is composed of C and BN with signature of intershell phase separation similar to that of composite nanotubes described in the previous section. The solid phases in the core and intermediate layer region have been

Figure 5. HREM images and selected-area electron diffraction pattern of SiC-SiO2-(BN)I Cy coaxial nanocables. Reprinted with permission from [10], Y. Zhang et al., Science 281, 973 (1998). © 1998, American Association for the Advancement of Science.

further confirmed by comparing the Si L-edge fine structure with those of SiC and SiO2 reference spectra (Fig. 6). The characterization therefore gives a well-defined composite nanowire structure with SiC-SiO2-(BN)xCy multilayers forming a semiconductor-insulator-(semi)conductor coaxial nanocable.

BN nanotubes filled with SiC were synthesized by using multiwall carbon nanotubes as templates [109]. This method combines both carbon nanotubes (CNTs)-substitution reaction [110] and confined reaction [111, 112]. Through the CNT-substitution reaction, CNTs react with boron oxide vapor in the presence of nitrogen gas to form BN-NTs, whose diameters and lengths are similar to those of the starting CNTs. The formation of the SiC filling proceeds by the penetration of SiO vapor into the cavity of the nanotubes. The subsequent reaction of SiO vapor with the inner carbon layers or volatile carbon mono-oxide in the interior forms SiC nanowires. The entire length of the nanotubes can be filled. SiC filled (BN)XCy nanotubes, which are a metal(C)-insulator(BN)-semiconductor (SiC) (MIS)

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