Inelastic Deformations in Multi-Walled Carbon Nanotubes
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Broken tubes are abundant in the ultrasonicated materials. Fig. (I a) shows an STM micrograph of a tube in which the outer shells of the tube have been segmented. This tube is
Figure 1: (a) STM image showing outer shell segmented MWNT laying on AU substrate. (b) 3D image of the same tube showing the diameter difference at the various sections along the tube. Supported on a clean gold. The difference in the diameters of the tube at section B and C is about 2.4 nm. If we assume that the same interlayer distance exists in MWNT as in graphite. then the broken segment is about ten shells thick. As shown on the 3D image of the tube, Fig. (lb), diameter difference also exists between the cross at A and B. The difference in diameter between A and B is about 1.6nm, which suggests that the cross-section of the tube at A is about five shells thicker than the cross-section at B. A variety of kinked, or inelastically deformed, nanotubes can also be found in the sample as shown in Fig. 2. The kinked tubes generally occur in broken bundles, however, occasionally an isolated tube can be found as shown here. Notice in the STM micrograph the kinked region appears "folded"' but continuous.
Figure 2: STM image kinked showing MWNT bundle on HOPG. Notice that a isolated relatively kinked nanotube exists.
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Kinked MWNTs are also observed in most TEM micrographs of the material. Fig.3 shows a TEM (Hitachi H-7000) micrograph of the MWNTs after agitation. Generally, we find that the smaller diameter tubes (< 5 nm) appear kinked while the larger diameter tubes remain straight. This varies with ultrasonic power.
Figure 3: Kinked MWNTs are also observed in TEM micrograph. Kinking is mostly observed on the small nanotubes (less than 5nm in diameter). Tunneling spectroscopy, I (V) measurements, were taken on isolated tubes as shown in figure (4). We note that the topological profile of the kink area is approximately the same height as the other parts of the kink, suggesting that there is no adsorbate attached here as might be expected from polymer-nanotube composite studies where the polymer is found to wet at the tube bends.
Figure 4: I-V measurement was taken on an isolated 1.8nm kinked nanotube at A and B. Figure (5) compares the local density of electronic states (LDOS) of the tube far from the kink area (A) and on the kink area (B). These LDOS were derived numerically from the tunneling spectra using standard methods [9]. Clearly, these spectra are different particularly in the conduction band. The energy band gap that exists away from the kink (A) tends to close on the
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kink (B). This change in conductivity could be explained in terms of a re-hybridization that took place on the kink. The deformation causes some local reordering of the tube lattice. This local reordering in turn brings a change in the local electronic structure of the tubes. Classically, the kink area has a higher charge density than areas away from the kink, which gives the kink area metallic property. 0.3
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