Band gap narrowing and electron doping by potassium encapsulation into single-walled carbon nanotubes

We have fabricated field-effect transistors (FETs) with pristine and potassium-encapsulated single-walled carbon nanotube (SWNT) films, and the effects of potassium encapsulation are investigated. The transformation from a unipolar characteristic to an ambipolar characteristic by potassium encapsulation is observed from the measurement of the gate voltage dependence (V_gs) of the current (I) for SWNT-film FETs. This result indicates that the potassium encapsulation into SWNTs causes band gap narrowing. In addition, the n-type region of the I-V_gs, curve is expanded during annealing of the devices; electron transfer from potassium to SWNTs occurs owing to the removal of the adsorbates. The adsorbate removal is confirmed by photoemission spectroscopy measurement. The FET with an individual potassium-encapsulated SWNT shows an ambipolar characteristic.