Imaging and Characterization of Molecules and One- Dimensional Crystals Formed within Carbon Nanotubes
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Imaging and
Characterization of Molecules and OneDimensional Crystals Formed within Carbon Nanotubes J. Sloan, D.E. Luzzi, A.I. Kirkland, J.L. Hutchison, and M.L.H. Green
Abstract The imaging and characterization of individual molecules and atomically thin, effectively one-dimensional crystals of rock salt and other halides encapsulated within single-walled carbon nanotubes are reviewed in this article. These species were imaged by conventional and super-resolved high-resolution transmission electron microscopy and by scanning tunneling microscopy, revealing the detailed atomic structure of these nanoscopic species. Keywords: carbon nanotubes, crystal growth, fullerenes, nanocomposites, scanning tunneling microscopy, transmission electron microscopy.
Introduction The synthesis, characterization, and modification of individual molecules, onedimensional (1D) crystals, and other species formed within single-walled carbon nanotubes (SWNTs) have been investigated in several laboratories. These species have been imaged using techniques such as conventional and super-resolved highresolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM). In appropriately dispersed specimens, it has been possible to image the encapsulated species in several ways: as discrete molecules; 1D arrays, helical chains, and clusters of molecules; disordered or glassy fillings; randomly oriented clusters; and continuously aligned or helically distorted 1D crystals. In the case of 1D crystals, these are constrained
MRS BULLETIN/APRIL 2004
in cross section by the van der Waals surface of the encapsulating SWNTs, forming so-called Feynman crystals—effectively, integral numbers of atomic layers in terms of their thickness, specified by the diameter of the encapsulating tube.1 The ability of carbon nanotubes—in particular, SWNTs—to “fix” individual molecules or discrete crystals 2–3 atomic layers thick for direct imaging is fostered by their extremely small internal diameters, which exist within a very restricted range of 0.4–5 nm with a median of only 1–2 nm. When the van der Waals surfaces of the wall carbons are taken into account, the effective internal diameters (IDs) of the SWNTs are reduced by 0.35 nm, thus producing cylinders with effective IDs of 0.7–1.7 nm.
These nanocomposites represent a new generation of materials that can be synthesized on a bulk scale and potentially form the basis of diodes,2–4 single-electron transistors,5,6 memory elements,7 and logic circuits.8 For example, Yao and co-workers2 showed in 1999 how it was possible to make an intramolecular junction between two SWNTs, one semiconducting and one metallic, thus forming the basis of a rectifying diode. Recently, substantial progress has been made with respect to the comprehensive characterization of the filled nanocomposites. Of particular importance in this regard are 1. The component crystal structures of the encapsulating nanotubes, which may be conveniently reduced to the (n,m) chiral indices defining the sp2 carbon cylinder acco
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