Crystallographic Structure

2.1 Crystallographic Structure as Seen by HRTEM

For fundamental studies and for some applications, one is looking for "perfect" nanotube structures, which means well-graphitized nanotube walls, with a level of defects as low as possible. High-Resolution Transmission Electron Microscopy (HRTEM) provides valuable information about the nanotube quality prepared by different synthesis methods. The MWNTs grown by the arc discharge method have generally the best structures, presumably due to the high temperature of the synthesis process. It is likely that during

Fig. 3. Scanning Electron Micrographs (SEM) of a MWNT deposit (top) and MWNTs after purification (bottom right) and as a 'side-product' of onion-like carbon nanoparticles (bottom left) [7,8]

the growth process, most of the defects are annealed. In the HRTEM image these rolled-up graphene sheets appear as straight lines (Fig. 4). Each nested graphite cylinder has a diameter which is given by two integers m and n, the coefficients of the lattice vectors a1 and a2 in the chiral vector Ch = na1 + ma2 which describes the way the graphene sheet is rolled-up, and relates to the helicity of the nanotube [26,27], and is described elsewhere in this volume [28,29]. The diameter dt of the (n, m) nanotube is given by dt = a(m2 + mn + n2)1/2/n. (1)

The interlayer spacing in MWNTs (0.34 nm) is close to that of turbostratic graphite [16,30]. Since nanotubes are composed of nearly coaxial cylindrical layers, each with different helicities, the adjacent layers are generally non-commensurate, i.e., the stacking cannot be classified as AA or AB as in graphite [16,30]. The consequence of this interplanar stacking disorder is a decreased electronic coupling between the layers relative to graphite. MWNTs also have structural features which are different from the well-ordered graphitic side-walls: the tip, the internal closures, and the internal tips within the central part of the tube, called the "bamboo" structures

Fig. 4. HRTEM images of MWNTs grown by the arc-discharge method, showing the well graphitized wall, the tip and the "bamboo" structure [9,10,11]

(Fig. 4). The structure of the tips is closely related to that of the icosahedral fullerenes [31], where the curvature is mediated by introducing pentagons (and higher polygons) into the structure, while maintaining essentially the sp2 electronic structure (i.e., 3 bonds) at each carbon atom site. A variety of tip structures are commonly observed [32], and an usual feature is that the tubules within the MWNT often close in pairs at their tips. The tips play an especially important role in the electronic and field emission properties of nanotubes. There has been considerable debate concerning the structure of MWNTs, i.e., whether in fact the tubules close on themselves or if nano-tubes sometimes are scrolled [21] (i.e., whether one or more of the sheets of graphite that are rolled have two edges). This interpretation followed analysis of HRTEM and electron diffraction studies of nanotubes which revealed that the number of different chiral angles which are observed in a MWNT are usually less than the number of tubules. Anomalous layer spacings are also observed (at least for catalytically grown tubes). This would be the case if one sheet produced several cylindrical layers in the tube. However, the consensus viewpoint from many HRTEM studies appears to favor the Russian doll model, since there has been little evidence for the edges which should result from graphene scrolls. However, nanotubes morphologies may vary considerably with the production method. This is especially true for catalytically grown nanotubes. If the growth conditions are not adjusted properly, then poorly graphitized walls result, and tubes with so-called coffee-cup structures form (Fig. 5).

There can be other types of defects which are more difficult to visualize like point defects in planar graphite, consisting of vacant sites (Schottky defects) and displacements of atoms to interstitial positions (Frenkel defects).

Fig. 5. HRTEM images of MWNTs produced by the decomposition of acetylene using a Co/silica catalyst, (i) at 720°C and (ii) at 900°C. All scale bars are 10 nm [7]

These are quite costly in energy due to the strength of the broken bonds, but these defects can be introduced by high energy electron irradiation [33]. Ultrasonic treatment may also produce defects in MWNTs [34] and SWNTs [35], which become more susceptible to chemical attack after being subjected to high intensity ultrasound treatment. Defective structures also result after severe chemical attack (i.e., in hot nitric acid and oxidation at elevated temperature) which has the effect of destroying the tips and hence provides a method to open the tubes.

2.2 Filling of Carbon Nanotubes

The hollow interior of carbon nanotubes naturally presents us with an opportunity to fill the nanotube core with atoms, molecules, or metallic wires, thereby forming nanocomposites with new electronic or magnetic properties. In early approaches, electrodes composed of carbon impregnated with the filling material were used [36,37]. This resulted in a large variety of filled graphitic structures, metallic carbides, but with relatively low yield. Loiseau and collaborators [38] realized that a minute quantity of sulfur present in the graphite/metal mixture helps to wet the surface of the cavity, so that the tubes fill up over micron distances with a high yield [20]. Nanowires of various elements (or their sulfides) were formed by this method, including transition metals (Cr, Ni, Co, Fe), rare earth metals (Gd, Sm, Dy) and co-valent elements (Ge, Se, Sb). Figure 6 displays a nanotube with a crystalline wire of Ge inside the core [38,20].

Fig. 6. HRTEM image of a MWNT filled with crystalline Ge. [38,20]

Other approaches to filling the hollow core involved the exploitation of nanotube capillarity [39]. This procedure for filling nanotubes may involve a chemical method, using wet chemistry [40]; or (b) a physical method, where capillarity forces induce the filling by a molten material [41,42,43]. In the wet-chemistry method, the nanotubes are refluxed in a nitric acid bath in order to open their tips. When a metal salt is simultaneously used in the bath, it is possible to obtain oxide or pure metal particles by a subsequent annealing process. Also, the metal/salt solvent bath can also be used on previously opened tubes. However, the drawback of this method is that it requires chemical manipulation over a long time to open nanotubes containing various tip structures, "bamboo" closures and various numbers of SWNT constituent layers. Furthermore, the reduction of the metal salts inside the tubes does not give a continuous wire, but rather yields metallic blobs. This can be observed in Fig. 7 in the case of a nanotube filled with silver [43].

In the physical method, the nanotubes are filled by first opening them, for example, by heating them in air to oxidize the nanotube tips preferentially, and the opened tubes are then submerged in molten material [41,42,43]. Studies of filling nanotubes with liquid metals have shown that only certain metals will enter the nanotubes and that the metal surface tension is the determining factor [37,43]. Further studies along these lines have demonstrated that the capillary action is related to the diameter of the inner cavity of the nanotube. This was explained in terms of the polarizability of the cavity wall, which is related to the radius of curvature, through the pyramidalization angle (bond-bending angle) and the polarizability of the filling material [43]. The filled nanotubes with different metals offer exciting structures for future electronic studies. In the following sections, we focus on a review of electrical and magnetic measurements that have been performed on arc-discharge

Fig. 7. HRTEM of a carbon nanotube whose inner cavity is filled with silver particles (lattice fringes correspond to [111] planes of Ag, with a lattice spacing of 0.23 nm). The tube cavity is first filled with molten silver nitrate that is subsequently reduced to metal by electron irradiation, in situ, in the electron microscope [43]

Fig. 7. HRTEM of a carbon nanotube whose inner cavity is filled with silver particles (lattice fringes correspond to [111] planes of Ag, with a lattice spacing of 0.23 nm). The tube cavity is first filled with molten silver nitrate that is subsequently reduced to metal by electron irradiation, in situ, in the electron microscope [43]

grown MWNTs, which show good structural order and their purity is well controlled.

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