Peapods

Single-wall carbon nanotubes (SWNTs) have attracted considerable interest after the discovery of C60. Single walled carbon nanotubes  provide an empty space isolated from outside conditions. This large internal space can be filled with different structures and molecules can be introduced by de-capping. Fullerenes are most favorable molecules for encapsulation because of their fit diameters. Such single-wall carbon nanotubes encapsulating fullerenes are called fullerene peapods.

The physical properties of such solids are strongly depending on a network dimensionality. Since fullerene-peapods have mixed network dimensionality, they have very interesting physical properties. Usually they are synthesized by using diameter-selected nanotubes as pods. High resolution transmission electron microscopy studies reveal that high-density fullerene chains inside nanotubes fill up to 60% of C60 molecules in a macroscopic average. 

A Japanese team has reported that they have filled nanotubes with metallofullerenes--pure carbon spheres enclosing metal atoms--hoping for a new way to control nanotube properties. This work demonstrated a new way to exploit the open space in the tubes and possibly gain more control over their properties. 

Nagoya University researchers placed C82 buckyballs containing gadolinium atoms inside nanotubes which changed the electronic structure of buckyballs.

Encapsulation

Several molecules such as fullerenes, endohedral metallofullerenes or alkali halides have been successfully inserted into the interior of SWCNTs. The inside filled structures can alter or enhance the mechanical and electronic properties of the SWCNTs or may allow the fine tuning of these parameters when treated at relatively high temperatures. Researchers of University of Ulm in Germany trapped single atoms of the heavy metal dysprosium within hollow fullerene spheres made up of 82 carbon atoms, and enclosed a series of these dysprosium-seeded cages within single-walled carbon nanotubes, with the fullerenes stringing them along the nanotube forming peapod.

Synthesis

The most studied structure inside SWCNT is the C60 fullerene. The resulting material is called a C60 peapod. The filling is performed by mixing of C60 and SWCNT. The mixture is subsequently evacuated   and heated above the sublimation point of C60 for several days. The peapod synthesis requires the heat treatment of SWCNT and fullerenes are sealed together under vacuum, but this method can not be adopted for large scale production purposes. Also for the peapods production, unknown fullerenes are the most undesirable impurities.

To get rid of unknown fullerene from SWNTs in one typical production process, soot is heated in vacuum and then the fullerene free soot is refluxed in H2O2 water solution to remove amorphous carbon particles. Finally, the purified SWNTs are formed to thin black paper and then dried in vacuum. Since the oxidation treatment destroys caps of SWNTs and HCl treatment increases defects on the wall, the purified SWNTs already have sufficient number of entrances for fullerenes.

A SWNT paper is put in a quartz ampoule with fullerene powder (C60 and C70 powder in 99 % purity as fullerene sources) and the ampoule is evacuated. After drying process, fullerene powder is evaporated and made into a film on the SWNT paper. The ampoule is sealed and heated in a furnace up to 650 °C. After keeping at a temperature for two hours, the ampoule is cooled down to room temperature. The SWNT paper is sonicated in toluene for 1 hour to remove fullerenes coated on SWNTs surface. After filtration a sheet of peapod paper is obtained. Then the peapod paper is heated in vacuum to remove toluene.

Sample preparation

For the low temperature synthesis of fullerene peapods the commercial SWCNT material is prepared by the arc-discharge method and is purified using repeated high temperature air and acid washing treatments. The SWCNT material with low initial purity is purified with a triple repetition of H2O2 refluxing and HCl acid etching. The material is then filtered and degassed in dynamic vacuum.

Two filling methods consistent with the effective tube-end opening side-effect of the SWCNT purification can be adopted. It is possible to fill SWCNT with other fullerenes including metallofullerens and clusterfullerenes. The success of such a filling procedure is related again to the diameter distribution of the starting SWCNT.

Filling methods

Vapor-filling

To fill fullerenes from the vapor phase by vapor-filling involves sealing of the SWCNT material with the fullerene in a quartz ampoule after degassing and keeping at slightly elevated temperature. The resulting material is sonicated in toluene in order to remove non-reacted fullerenes, filtered, and dried from toluene in dynamic vacuum for removing non-reacted fullerene particles without an observable effect on the peapods.

Solvent-filling

Fullerene filling into SWCNT in n-hexane by solvent-filling is achieved with mixing the SWCNT material with n-hexane with C60 or C70. The as-received SWCNT materials have to be dried to keep away from humidity. The dynamic vacuum degassing of the SWCNT is crucial for the solvent-filling as rinsing it in water prevents any further solvent-fillability probably because water enters into the nanotubes. The SWCNT, fullerene and n-hexane mixture is sonicated resulting in the partial dissolution of C60. From the C60 solution, undissolved C60 and SWCNT mixture are then refluxed and filtered bucky-papers are dried in air. C60 which are not encapsulated that covers the bucky-paper are removed with the above mentioned two methods of sonication in toluene or by dynamic vacuum treatment.

Uses

Fullerene peapods can be transformed into a double wall carbon nanotube (DWCNT) structure after high temperature annealing. The fullerenes coalesce into an inner nanotube without affecting electronic properties but significantly enhancing the mechanical properties of the tube system. This enhanced mechanical stability makes DWCNTs promising candidates for applications in future electronics, probe tips for scanning probe microscopy, field emission devices and many more. It has been speculated that such materials, when available in higher spin concentrations, may be fundamental elements of quantum-computing. The transformation of solvent prepared peapod to DWCNT with a yield identical to that from vapor prepared materials can be used for the production of high purity and highly perfect industrial DWCNT. Fullerenes inside nanotubes can be compressed by a heavy pressure so that molecules are enclosed in-between and chemical reactions can be induced by these extreme pressures making the peapods effective autoclaves. Peapods encaging metallo-fullerenes exhibit the bandgap modulation due to the electron transfer from metallofullerenes to carbon nanotubes. Such peapods have been applied to FET with novel device properties.