Turning Peapods into Double-Walled Carbon Nanotubes
- PDF / 1,241,182 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 75 Downloads / 263 Views
Turning Peapods into Double-Walled Carbon Nanotubes
S. Bandow, K. Hirahara, T. Hiraoka, G. Chen, P.C. Eklund, and S. Iijima Abstract The formation pathway to double-walled carbon nanotubes (DWNTs) from C60 encased within single-walled carbon nanotubes (peapods) is introduced in this article. Onedimensionally arranged C60 molecules coalesce gradually within the nanotube and change the structure to C60 dimers, trimers, tetramers, and so on as intermediates. In addition to these interesting structural transformations visualized in the nanotube space, the nanotube itself is very stable, and this structural stability is very important when using the interior of the nanotube as the reaction field or the space for molecular storage. In terms of optical absorption, the lowest energy absorption band for DWNTs, 0.65 eV, shows broadened and downshifted features as compared with that of SWNTs. We expect that this opticalabsorption feature will lead to the use of DWNTs in absorbing devices for optical-fiber communications. The Raman experiments give new information about the frequency of the C–C stretching-mode vibration for nanotubes with diameters of less than 1 nm, which shows a decrease in vibration frequency with decreasing tube diameter. This diameter dependence can be explained by an admixture of sp3 character in the C–C interaction. Therefore, the electronic and mechanical properties of nanotubes with diameters of 1 nm are expected to be different from nanotubes of the 1-nm-diameter class, and we anticipate that new phenomena will occur in small-diameter tubes.
ing temperature depends on the cleanness of the tube surface. That is, if the rinsing step in the purification procedure for SWNTs is not sufficient to remove impurities, the burning temperature must be increased. Intact C60 molecules, thus arranged in the nanotube, start to merge together as the temperature is raised above 800°C in vacuum, and they finally form an inner tube. According to a transmission electron microscopy (TEM) study of the merging process of C60 in the intermediate temperature range of 800–1000°C, it was found that the internal tubes form as the result of a structural relaxation at the “bottleneck” area of the C60 dimers, trimers, tetramers, and so on.8 Figure 1a shows a stabilized C60 dimer structure in a slightly largerdiameter tube. Very clear evidence of onedimensional (C60)n clusters is easily seen in Figure 1b, where the number n of C60 molecules in the cluster is indicated below the objects. In this image, we can see that the bottleneck areas are already relaxed, and the clusters have been changed to short inner tubes with lengths of a few nm. At the lowest-temperature reaction of 800°C (Figure 1c), most of the C60 molecules in the tube form dimerlike structures, and we can occasionally see short tubes. The diameters of such short tubes are estimated to be 0.7 nm by comparing the diameter determined from the image contrast of the single C60 molecule, and these diameters are normally independent of the outer tube diameters. As th
Data Loading...
