Materials science

Single-walled 4 Å carbon nanotube arrays

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Abstract

Here we describe the smallest carbon nanotubes possible1, prepared by the pyrolysis of tripropylamine molecules in the channels of porous zeolite AlPO4-5 (AFI) single crystals2. These uniformly sized carbon nanotubes have a diameter of 0.4 nm and are the best example of one-dimensional quantum wires.

Main

AFI is a type of transparent microporous crystal (Fig. 1 ) containing one-dimensional channels packed in hexagonal arrays, with an inner diameter of 0.73±0.01 nm (ref. 3). The starting material we used for synthesizing single-walled carbon nanotubes (SWNTs) was tripropylamine, introduced into the channels during growth of AFI. SWNTs form inside the AFI channels when the pyrolysed carbon is thermally treated2 (Fig. 1). The resulting brown AFI crystallites have good polarization characteristics (Fig. 1, inset). Polarized Raman spectra of these AFI crystals match well with theoretically predicted Raman-active modes, including the breathing modes4,5.

Figure 1: An as-grown zeolite AlPO4-5 (AFI) single crystal.
figure1

Left inset, model of the AFI crystal structure showing single-walled nanotubes (SWNTs) inside the channels. Right inset, polarization characteristics of the SWNT–AFI sample: no absorption (top) and high absorption (bottom) for light polarized perpendicular (Ec) and parallel (EC) to the tube direction, respectively. E, electric field of polarized light; C, zeolite channel direction.

To view the SWNTs in a transmission electron microscope (TEM), we first dissolved the AFI framework in 30% hydrochloric acid; the SWNT-containing solution was then enriched and dispersed on a lacey carbon film for high-resolution TEM (JEOL2010 microscope operating at 200 kV). Figure 2 shows a typical image in which the specimen is represented mainly by two types of morphology: small pieces of highly curved, raft-like graphite stripes ('G' in Fig. 2) and ultrathin SWNTs ('T' and large arrowheads in Fig. 2). In the 'G' areas, the spacing between neighbouring parallel fringes is measured to be about 0.34 nm, which matches well with the {002} lattice plane spacing of graphite. The SWNTs show a typical contrast effect, with the tube walls appearing as paired dark fringes; this contrast effect is very weak because of the small dimension of the tubes.

Figure 2: High-resolution transmission electron microscope image showing single-walled carbon nanotubes (indicated by T and large arrowheads) coexisting with graphite in raft-like stripes (G).
figure2

More than ten single-walled carbon nanotubes are visible.

By using the {002} spacing of graphite as an internal reference and measuring the separation between the paired dark fringes of the SWNT images, we determined the diameters of the SWNTs to be 0.42±0.02 nm. In the image shown in Fig. 2, there are ten or more SWNTs visible, all having the same morphology. The SWNTs in our samples were unstable under electron beam radiation, fading during observation in 10–15 seconds, whereas the raft-like layers of graphite persisted. We believe that the large number of small graphite pieces, which generally contain several layers, are transformed from SWNTs during extraction from the AFI crystal.

The narrowest diameter reported in the centre of multiwalled nanotubes is 0.5 nm (ref. 6), the same as that of a C36 molecule7. The smallest free-standing SWNT, of diameter 0.7 nm (the same as that of C60), is stable8. The SWNTs observed here, which are consistent with a diameter of about 0.4 nm, are to our knowledge the smallest found so far. By considering the inner diameter of the AFI channels (0.73±0.01 nm), and taking the distance between graphite sheets (0.34 nm) as the separation between the nanotube carbon atoms and the oxygen atoms on the channel wall, the diameter of the SWNTs allowed in the channels is 0.39±0.01 nm (0.73 ± 0.01 nm−0.34 nm).

There are three possible nanotube structures with a diameter of about 0.4 nm: these are the zigzag (5,0) (diameter, d=0.393 nm), the armchair (3,3) (d=0.407 nm) and the chiral (4,2) (d=0.414 nm), where (m,n) is a roll-up vector defining the tube symmetry. Of these, the zigzag (5,0) tube can be exactly capped by half a C20 fullerene9, which makes it more likely that this structure is formed. Because of the strong bending of carbon bonds, plus the size- confinement effect of the channels and the imperfection of such ultrasmall SWNTs (for example, point defects), there could be high-energy regions in these nanotubes. When they are released from their channels, the defects may cause instability which then permeates the whole tube, leading to the formation of the observed graphite stripes.

Because our SWNTs are highly aligned and uniform in size, their electrical transport properties are easy to measure. Local density functional calculations indicate that when the diameter of the SWNT is smaller than 0.5 nm, strong curvature effects induce strong σ–π mixing of the unoccupied orbitals, so metallicity can no longer be predicted by the simple band-folding picture10. Such small-radius nanotubes generally have finite density of states at the Fermi level. Preliminary electrical measurements on AFI crystals containing SWNTs11 indicate that the temperature dependence of the SWNT conductivity is consistent with variable-range hopping in a disordered one-dimensional metallic system.

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Correspondence to Z. K. Tang.

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