Origin of the n-type transport behavior of azafullerene encapsulated single-walled carbon nanotubes

The transport properties of C₅₉N encapsulated semiconducting single-walled carbon nanotubes (SWCNTs) (C₅₉N-peapod) are investigated. Transport measurements of the peapods in field effect transistors (FETs) reveal that ∼14% of the C₅₉N-peapod sample shows n-type behavior even though the electronic properties of the host SWCNTs are similar to those of C₆₀-peapods that exhibit only p-type property. First-principles electronic-structure calculations reveal that the unique transport behavior originates from the monomer form of C₅₉N encapsulated in SWCNTs. The singly occupied (SO) state of C₅₉N lies in the energy gap of the SWCNT and the energy of this state increases substantially when electrons are injected. Because of this shift to higher energy, the SO state acts as a shallow donor state for the conduction band of the nanotube, which leads to n-type behavior in FET measurements.