Recent advances in the internal functionalization of carbon nanotubes: synthesis, optical, and magnetic resonance studies

The hollow inside of single-wall carbon nanotubes (SWCNT) provides a unique degree of freedom to investigate chemical reactions inside this confined environment and to study the tube properties. It is reviewed herein, how encapsulating fullerenes, magnetic fullerenes, C isotope enriched fullerenes and organic solvents inside SWCNTs enables to yield unprecedented insight into their electronic, optical, and interfacial properties and to study their growth. Encapsulated C60 fullerenes are transformed to inner tubes by a high temperature annealing. The unique, low defect concentration of inner tubes makes them ideal to study the effect of diameter dependent treatments such as opening and closing of the tubes. The growth of inner tubes is achieved from C enriched encapsulated organic solvents, which shows that fullerenes do not have a distinguished role and it opens new perspectives to explore the in-the-tube chemistry. Encapsulation of magnetic fullerenes, such as N@C60 and C59N is demonstrated using ESR. Growth of inner tubes from C enriched fullerenes provides a unique isotope engineered heteronuclear system, where the outer tubes contain natural carbon and the inner walls are controllably C isotope enriched. The material enables to identify the vibrational modes of inner tubes which otherwise strongly overlap with the outer tube modes. The C NMR signal of the material is specific for the small diameter SWCNTs. Temperature and field dependent C T1 studies show a uniform metallic-like electronic state for all inner tubes and a low energy, 3 meV gap is observed that is assigned to a long sought Peierls transition.