Studying single-wall carbon nanotubes through encapsulation: from optical methods till magnetic resonance

Encapsulating fullerenes, magnetic fullerenes, 13C isotope enriched fullerenes, and organic solvents inside SWCNTs enables to yield unprecedented insight into their electronic, optical, and interfacial properties and to study SWCNT growth. In addition to customary methods of their studies such as e.g., optical absorption or Raman spectroscopy, these efforts enables to employ electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. Encapsulated C60 fullerenes are transformed to inner tubes by a high temperature annealing. The diameter distribution of the inner tubes follow that of the outer ones and their unique, low defect concentration makes them an ideal model system for high resolution and energy dependent Raman studies. The observation of Raman modes of individual inner-outer tube pairs allows to measure the inner-outer tube interaction strength that is also well described theoretically. Reversible closing and opening of SWCNT can be studied in a diameter selective manner by encapsulating C60 and transforming it to an inner tube. The growth of inner tubes can be achieved from 13C enriched encapsulated organic solvents, which shows that the geometry of the fullerene does not play a particular role in the inner tube growth as it was originally thought. In addition, it opens new perspectives to explore the in-the-tube chemistry. Growth of inner tubes from 13C enriched fullerenes provides a unique isotope engineered heteronuclear system, where the outer tubes contain natural carbon and the inner walls are controllably 13C isotope enriched. The material enables to identify the vibrational modes of inner tubes which otherwise strongly overlap with the outer tube modes. The 13C NMR signal of the material has an unprecedented specificity for the small diameter SWCNTs. Temperature and field dependent 13C T1 studies show a uniform metallic-like electronic state for all inner tubes rather than distributed metallic and isolating behavior. A low energy, 3 meV gap is observed that is tentatively assigned to a long sought Peierls transition in the small diameter SWCNTs. Encapsulating magnetic fullerenes, such as N@C60 and C59N opens the way for local probe ESR studies of the electronic properties of the SWCNTs.