Electronic structure of single-walled carbon nanotubes encapsulating potassium

Electronic structural change of single-walled carbon nanotubes induced by potassium encapsulation was studied by photoemission spectroscopy. The potassium encapsulation caused a shift of the overall valence-band spectrum toward the higher binding-energy side by about 0.5 eV, which is basically understood by the simple rigid band shift model. However, the spectral intensity increase observed near the Fermi level was much larger than that expected by assuming the simple ${\pi }^{*}$ band filling, indicating that, in addition to the ${\pi }^{*}$ band, a part of the density of states initially located in the unoccupied states dips below the Fermi level by the potassium encapsulation. The result is qualitatively consistent with a recent band-structure calculation, which predicted that the nearly free-electron state hybridized with the $\mathrm{K}4s$ state crosses the Fermi level. The potassium encapsulation also decreased the work function by 1.4 eV.