Selective Encapsulation of Heterogeneous Fullerene Ions Into Single-Walled Carbon Nanotubes*

A plasma consisting of ionic heterogeneous fullerenes such as C$_{60}$ negative ions and lithium endohedral fullerene positive ions (Li@C$_{60})$ is generated by means of an electron beam impact to a sublimated Li@C$_{60}$/C$_{60}$ composite. These C$_{60}$ negative ions and Li@C$_{60}$ positive ions are selectively irradiated and encapsulated in single-walled carbon nanotubes (SWNTs) put on a substrate which is positively and negatively biased, respectively. It is found that the amount of Li@C$_{60}$ irradiation to SWNTs depends on the energy of the electron beam E$_{e}$ which ionizes the Li@C$_{60}$, and has the maximum for E$_{e} \quad \sim $ 200 eV. The electrical transport properties of the C$_{60}$ and Li@C$_{60}$ encapsulated SWNTs are investigated by fabricating them as the channels of field-effect transistor devices. The C$_{60}$ encapsulated SWNTs show the p-type electrical transport property. On the other hand, the Li@C$_{60}$ encapsulated SWNTs exhibit the ambipolar conduction or the n-type property. This result indicates that the electrical properties of the SWNTs can be controlled by the kinds of encapsulated fullerenes.