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materials. The first approach involves inserting the selected materials, either physically or chemically, into the opened carbon nanotubes [31, 32]. The as-produced carbon nano-tubes, however, normally have closed caps at their ends. Filling of nanotubes, therefore, always relies on a method to open the ends of the nanotubes. Oxidation and acid treatment are two efficient routes for opening the tips of carbon nanotubes [33-35]. Open nanotubes can be filled via capillary action using either a low melting or low surface tension material (i.e., material with a surface tension lower than the threshold value of 100-200 millinewtons per meter) [36]. The first filling experiment reported by Ajayan et al. used an oxidation method to simultaneously open and fill multi-walled carbon nanotubes (MWCNTs) [31]. Metallic Pb was first deposited onto samples containing MWCNTs produced by arc-discharge method. The samples were then heated in air to about 400 °C which is above the melting point of Pb. High-resolution electron microscopy (HREM) images taken after the heat treatment show that MWCNTs are opened at their tips and filled with solid materials (Fig. 3). Using this approach, metal (Pd, Ag, Au, Bi), metal oxides (oxides of Mo, Sn, Ni, U, Co, Fe, Nd, Sm, Eu, La, Ce, Y, Cd), metal chlorides (AuCl, UCl4), FeBiO3, HReO4, MoO2, and SnO, have all been successfully filled into carbon nanotubes [33, 37-46]. This approach is also useful for filling single-walled carbon nanotubes (SWCNTs or SWNTs) with halides, oxides, carbides, and metals [47-63]. One good example is KI filled SWCNT prepared by heating KI with SWCNT inside an evacuated silica ampoule at the temperature of 954 K for 2 hr. Figure 4 shows a phase image reconstructed from a 20-member focal series of a 1.6-nm-diameter SWCNT containing a KI single crystal [49]. In cross section, the filled crystal can be regarded as a single KI unit cell viewed along the (110) direction of the parent rock salt structure, with the (001) direction parallel to the tube axis. The crystal is well resolved in regions 1 and 3 of the restoration, whereas in region 2, the crystal is rotated about wmm-^

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Figure 3. HREM images of MWNTs filled with Pb compound (dark contrast). Reprinted with permission from [31], P. M. Ajayan and S. Iijima, Nature 361, 333 (1993). © 1993, Macmillan Magazines Ltd.

Figure 4. Phase image showing a (110) projection of KI incorporated within a 1.6-nm-diameter SWCNT, reconstructed from a focal serials of 20 images. The upper left inset shows an enlargement of region 1. The lower right inset shows the surface plot of the region 1. Reprinted with permission from [49], R. R. Meyer et al., Science 289, 1324 (2000). © 2000, American Association for the Advancement of Science.

Figure 3. HREM images of MWNTs filled with Pb compound (dark contrast). Reprinted with permission from [31], P. M. Ajayan and S. Iijima, Nature 361, 333 (1993). © 1993, Macmillan Magazines Ltd.

Figure 4. Phase image showing a (110) projection of KI incorporated within a 1.6-nm-diameter SWCNT, reconstructed from a focal serials of 20 images. The upper left inset shows an enlargement of region 1. The lower right inset shows the surface plot of the region 1. Reprinted with permission from [49], R. R. Meyer et al., Science 289, 1324 (2000). © 2000, American Association for the Advancement of Science.

its axis. In the (110) projection, each white spot corresponds to a column of pure I or pure K that is exactly one, two, or three atoms in thickness, as can be seen in the enlargement of region 1 (upper left inset). The different phase shifts arising from different atomic columns are visible when region 1 is displayed as a surface plot (lower right inset) [49]. SWCNT filled with polystyrene and styrene-isoprene is also reported [64].

In 1998, a new type of self-assembled hybrid structures consisting of fullerene arrays inside SWCNTs, so-called "peapods," were reported [65, 66]. Recent progress in the high yield synthesis opens up a huge possibility for the pea-pod research. Various types of fullerenes and endohedral metallofullerenes, such as C60, [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], can now be fully inserted inside SWCNTs [67-80]. Potential applications of peapods range from data storage to quantum cascade laser, quantum computing [81, 82], field-effect transistor [75], and hydrogen storage [83].

The second approach is to simultaneously encapsulate the materials during the formation of carbon nanotubes.

Such encapsulation has been observed in MWCNTs synthesized by arc-discharge [84-95], electrolysis [96-98], fieldanode activation [99], laser ablation [100], chemical vapor deposition [101-103], and pyrolysis [104] with metals, oxides, carbide, nitrides, chlorides, and sulfides. Among these processes, arc-discharge produces the best-graphitized sheathing MWCNTs, which is also true in synthesizing hollow carbon nanotubes. Arc-discharge is also more versatile for encapsulation of different elements. In this process, a graphite anode is drilled and filled with a mixture of graphite and the chosen element powders. One then proceeds with the conventional Kratschmer-Huffman arc deposition technique. Encapsulation abilities of more than 41 elements have been systematically investigated [39, 85, 87, 89]. This approach is also useful for filling single-walled carbon nano-tubes [105, 106].

The third approach is a two-step process. First, nanowires are synthesized and then used as templates to deposit graphite layers on their surface [107, 108]. Zhang et al. deposited thick graphitic layers on the surface of Si nanowires by using a hot filament chemical vapor deposition method. When the temperature was as high as 1000 °C, Si cores tended to transform into SiC cores [107]. Han et al. reported the synthesis of GaN nanorods coated with graphitic carbon layers (usually less than 5), including single carbon layers, on preproduced GaN nanorods using a conventional thermal chemical vapor deposition method. The graphitic carbon layers conform to the shape of the GaN nanorods [108].

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