Electronic spectral shift of oxygen-filled (6, 6) carbon nanotubes

►Oxygen encapsulation triggers electronic spectral shift of (6, 6) carbon nanotubes. ► The shift depends strongly on the tube length L and the energy domain. ► An energy gap arises only when L is a multiple of 3 in benzene ring units. ► The oxygen-induced gap (>10 GHz) can be detected by ac conductivity measurements.