Phys. Rev. B 85, 045428 (2012)

In metallic materials, free electrons can screen the Coulomb interaction between an electron and a hole, thereby preventing the formation of an exciton. Furthermore, the observation of photoluminescence in metallic materials is difficult because of the ultrafast non-radiative decay of photoexcited electrons and holes due to electron–electron scattering and photon-mediated relaxation processes. Takeshi Koyama and co-workers from Nagoya University and the National Institute of Advanced Industrial Science and Technology in Japan believe they have now found evidence of exciton formation in metallic single-walled carbon nanotubes (SWNTs). The researchers used femtosecond time-resolved luminescence spectroscopy to measure the photoluminescence. The device provided a central excitation photon energy of around 1.55 eV and a time resolution of 10 fs. The researchers prepared samples enriched with both metallic and semiconducting SWNTs by using the density gradient ultracentrifugation procedure. They attributed the peak in photoluminescence — around 1.4 eV — to the metallic-SWNT-enriched samples. They also implemented transient photoabsorption measurements to record the time evolution of band bleaching around 1.4 eV. The difference in temporal behaviour between the photoluminescence and absorption signals strongly suggests that the photoluminescence peak was excitonic in nature. The exciton lifetime of 40 fs indicates that a large exciton binding energy leads to a relatively stable exciton state in the presence of metallic electrons.