Elsevier

Chemical Physics Letters

Volume 349, Issues 5–6, 7 December 2001, Pages 389-393
Chemical Physics Letters

Electron microscopic imaging and contrast of smallest carbon nanotubes

https://doi.org/10.1016/S0009-2614(01)01222-2Get rights and content

Abstract

The weak scattering from smaller carbon nanotubes results in weak contrast in their electron microscope images, and observation and interpretation of such images require special attention in order to avoid erroneous conclusions. It is demonstrated that the 4 Å carbon nanotube, residing inside a multi-walled carbon nanotube, bears recognizable signature in its image contrast for identification. For an isolated single-walled carbon nanotube, observation should be made in areas where the nanotube is exposed. When the carbon nanotube is overlapped with a supporting glassy carbon film, it is practically impossible for the nanotube to produce usable contrast features for unambiguous identification.

Introduction

Many of the extraordinary properties of carbon nanotubes are strongly linked to their diameter which is on the nanometer scale. 7 Å, the diameter of C60 fullerene [1], which had been considered to be the smallest possible due to a belief that the structure is governed by the isolated pentagon rule, which stipulates that no adjacent pentagons are formed in the cage structure. Carbon nanotubes [2] had been considered to be limited by the same isolated pentagon rule due to a hypothesis that carbon nanotubes were grown out of fullerenes and therefore the smallest carbon nanotube was considered to be of diameter 7 Å [3]. Experimental evidence for carbon nanotubes of this diameter was observed soon after the announcement of the discovery of carbon nanotubes [4]. Energetically, the stability of a carbon nanotube is secured by the compensation of energy saved when the number of dangling bonds was reduced in rolling up a section of graphene at the expense of strain energy owing to the increasing curvature. Therefore, a balance is maintained by these two competing factors, which in turn determine the smallest diameter. Calculations indicate that carbon nanotubes are energetically stable down to a diameter as small as 4 Å [5], [6].

However, with the discovery of C36, it is demonstrated experimentally that the isolated pentagon rule can be violated while a stable structure is formed [7]. The subsequent successful synthesis of C20[8], which is composed of only pentagons in its topological structure, exemplifies that the isolated pentagon rule can even be disregarded as this structure assumes a structure that is in maximum violation of the isolated pentagon rule. Given the close link between the fullerenes and carbon nanotubes in structure, it is not surprising that carbon nanotubes of diameters corresponding to those of C36 and C20, respectively, were reported based on primarily electron microscopy observations [9], [10], [11], [12]. Furthermore, using molecular dynamics simulations and electron microscopy, Peng et al. [13] suggested a configuration of even smaller diameter that is stable structurally, though it is unstable energetically.

Section snippets

Results and discussion

Carbon nanotubes of 4 Å diameter have been produced with an improved arc-discharge technique in using hydrogen as the carrier gas [14]. No metal catalysts were necessary in the arc-discharge process. When the conditions, under which the arc-discharge was conducted, are well controlled, multi-walled carbon nanotubes with very small inner diameters are produced [10]. Fig. 1 shows an electron microscope image of a multi-walled carbon nanotube produced with the arc-discharge technique where the

Conclusions

When the smallest carbon nanotube is formed inside a multi-walled carbon nanotube, the electron microscope image contrast fades towards the center of the image. The contrast characteristic is confirmed with both image simulations and experimental observations. For the case of single-walled carbon nanotubes, experimental observation should be made in areas where there is no overlapping with the commonly used glassy carbon supporting film to avoid erroneous interpretation of the experimental

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    Citation Excerpt :

    This problem applies to larger scale materials as well, but the danger of misinterpreting the image for larger scale materials is small compared to the problem for sub-nanometer materials, that we are now treating. Another rather obvious issue requiring special care is the overlapping of the samples with an amorphous carbon film on the specimen grid [15], or overlapping with other materials such as outer tube walls. Other issue lies in a bundle state of the tubes, which makes it difficult to identify single- and double-walled carbon nanotubes.

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