Molecular motion of endohedral fullerenes in single-walled carbon nanotubes.

Abstract

Spherical carbon cages of fullerene molecules interact by van der Waals forces to form close-packed arrangements in the solid state, such as the face-centered cubic lattice formed by C60 at room temperature. [1] Owing to the low directionality and low energy of the interfullerene interactions neutral fullerene molecules (such as C60) are freely rotating in the crystal at room temperature. Contrasting to the empty fullerene cages, metal-containing endohedral fullerenes are known to have large electric dipole moments of 3–4 Debye. Therefore, it should be expected that intermolecular interactions in the solid M@Cn, (M=metal center) are dominated by strong dipolar interactions. For example, fullerene Ce@C82 has a large dipole moment owing to the transfer of three valence electrons from the Ce atom to C82 cage. [4] This fullerene crystallizes in a hexagonal or a face-centered cubic close pack arrangement. However, despite the dipole moment the fullerene Ce@C82 has been shown to retain its rotational freedom in the crystalline state with an energetic barrier of only 1.3 kJmol . In contrast to bulk-fullerene crystals, fullerene molecules inserted in single-walled carbon nanotubes (SWNTs) form linear chains in which each fullerene has only two nearest neighbors as compared to 12 neighbors in the crystalline bulk fullerenes. 5] As a result the intermolecular interactions in these self-assembled carbon nanostructures, designated as Cn@SWNT, and the dynamic behavior of the encapsulated molecules are expected to be different to those in solid fullerenes. Cn@SWNTs exhibit unique electronic properties and are currently attracting attention as building blocks for functional nanodevices. Cn@SWNTs structures containing magnetically active mono-metallofullerenes M@Cn are of interest as candidate materials for solid-state quantum computers. We present herein for the first time a study on the molecular motion of mono-metallofullerene Ce@C82 chains inside SWNTs demonstrating different modes of translational and rotational motion. Raw SWNTs (Aldrich) were purified by a combination of standard techniques. Only nanotubes with diameters 13.5 ; and 15.0 ; (as shown by Raman spectroscopy; Supporting Information) were present in the samples used for the experiments. The purified SWNTs were effectively filled with C60, C70, and Ce@C82 (synthesized by DC arcdischarge method) at 400–500 8C and 2= 10 6 torr, which resulted in Cn@SWNT “peapod” structures in 60–90% yields. The yield for inserting C82 in SWNTs was lower ( 30%) even when an excess of C82 was used. High-resolution transmission electron microscope (HRTEM, JEOL 4000EX, LaB6, information limit 0.12 nm) imaging conditions have been carefully tuned by lowering the accelerating voltage of the microscope to 100 kVand reducing the beam current density to a minimum to enable a direct observation of the molecular motion for the mono-metallofullerene inside SWNTs. Under these conditions the amount of the energy transferable from the electrons to the specimen is significantly reduced compared to the standard TEM imaging conditions. No beam damage was observed in sp-carbon structures under these conditions over a period of 20 min. Most of the micrographs were obtained by using 2 s exposures of a charge-coupled-device (CCD) detector. Translational motion can be observed for all types of Cn in SWNTs (Figure 1a), which indicates that the molecules are mobile inside the nanotube at room temperature and move independently of one another. This effect is particularly noticeable in the peapod bundles where several nanotubes are aligned parallel to each other. Observation of the ends of the nanotubes and side-wall defects confirms that there is no relative motion of nanotubes within the bundle. It is remarkable that the molecules continue their translational motion even inside apparently completely filled nanotubes (Figure 1b, c). In contrast to the independent translations observed for sparsely filled nanotubes, the chains of fullerenes with regular van der Waals separations between the molecules undergo collective translations, so that the entire chain shifts in a short time without changing the intermolecular separations. The motion of an isolated molecule in sparsely filled nanotube (Figure 1a) is not continuous: the molecule performs abrupt jumps with irregular intervals of approximately 10 s. In contrast to isolated fullerenes the motion of the chains is significantly slower and appears to be more continuous than abrupt: the rectangular (Figure 1b) and zigzag (Figure 1c) arrangements do not transform immediately into each other. Instead the rearrangement occurs through a series of states [*] Dr. A. N. Khlobystov, Dr. K. Porfyrakis, D. A. Britz, Prof. G. A. D. Briggs Department of Materials Oxford University Parks Rd, Oxford, OX1 3PH (UK) Fax: (+44)1865-273789 E-mail: Andrei.Khlobystov@materials.ox.ac.uk Andrew.Briggs@materials.ox.ac.uk