Twisting of Single-Walled Carbon Nanotube Bundles
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higher structural stability, it is natural to conceive that twisted rope-like bundles would better suit the purpose than raft-like parallel bundles, much the same as in the case of making macroscopic ropes from fibers and yams. In this paper, a rope-like morphology of single-walled carbon nanotubes is reported and analyzed. This configuration is explained with a model that relates the morphology to the helicities of the constituent nanotubes. It is suggested that this rope-like twisting is favored due to the minimization of the total formation energy when the neighboring nanotubes have different helicities. EXPERIMENTAL Single-walled carbon nanotubes were produced by the Nd-YAG single-beam laser evaporation method as described in [9]. Carbon nanotubes were collected from the inside wall of the quartz tube where soot-like carbon was deposited during the evaporation. Pristine material was then dispersed in ethanol before it was placed on the microgrids for transmission electron microscopy observations. A Topcon-002B transmission electron microscope operated at 200 kV was used for this study. RESULTS AND DISCUSSION Figure 1 shows a typical high-resolution electron micrograph of the nanotubes made for the present investigation. The nanotube bundles are often seen bent and tangled together. In addition, some amounts of amorphous carbon, fullerenes, graphite flakes as well as metallic catalyst particles were present in the pristine nanotube samples.
Figure 1. High-resolution electron microscope image of single-walled carbon nanotubes produced by single-beam laser evaporation showing a typical morphology of the product. Carbon nanotubes and nanotube bundles are tangled together. 34
Figure 2 shows the electron microscope image of a nanotube bundle that has been twisted. Letters A, B, and C indicate the areas in which the bundles are imaged along a close-packed row of nanotubes. The images reveal clearly the numbers of nanotube rows seen in the imaging direction. The measured distance between the adjacent nodes is about 65 nm, after which a twisting of 60' has been completed. The enlarged images of these areas are given in Fig. 2 (b) which shows that the numbers of nanotube rows are 4, 5, and 3, at the areas marked with A, B, and C, respectively. Basing on these projection images, the three-dimensional structure can be reconstructed algebraically [11] and the reconstructed structure is illustrated in Fig. 2(c) in the end-on orientation. The three directions along which the image is formed are also marked with letters A, B, and C, respectively.
(c)
(b)
Figure 2 (a) A twisted bundle of single-walled carbon nanotubes forming a rope-like structure. The letters indicate the locations where the images reveal the numbers of the nanotube rows along the electron beam direction; (b) Enlarged images of the portions marked with A, B, and C in (a), where the numbers of nanotube rows in the electron beam direction are 4, 5, and 3, respectively; and (c) Reconstructed structure of the rope-like
bundle. Letters A, B, and C indicate t
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