Direct Proof of a Defect-Modulated Gap Transition in Semiconducting Nanotubes

Measurements of optical properties at a nanometer level are of central importance for the characterization of optoelectronic devices. It is, however, difficult to use conventional light-probe measurements to determine the local optical properties from a single quantum object with nanometrical inhomogeneity. Here, we successfully measured the optical gap transitions of an individual semiconducting carbon nanotube with defects by using a monochromated electron source as a probe. The optical conductivity extracted from an electron energy-loss spectrum for a certain type of defect presents a characteristic modification near the lowest excitation peak ( E₁₁), where excitons and nonradiative transitions, as well as phonon-coupled excitations, are strongly involved. Detailed line-shape analysis of the E₁₁ peak clearly shows different degrees of exciton lifetime shortening and electronic state modification according to the defect type.