Electroluminescence from suspended and on-substrate metallic single-walled carbon nanotubes

In this work, we carried out electroluminescence (EL) measurements on metallic single-walled carbon nanotubes (SWNTs) and compared the light emission from the suspended section with the on-substrate section along the same SWNT. In addition to the lowest excitonic emission for metallic SWNTs (M(11)), a side peak was observed at an energy of 0.17-0.20 eV lower than the M(11) peak, which is attributed to a phonon-assisted sideband. Interestingly, this side peak was only observed from on-substrate sections but not from suspended sections. This is likely due to the higher electric field used in the EL measurement of on-substrate sections and the asymmetric surroundings for on-substrate SWNT sections. When the drain voltage is increased, either a blue shift or a red shift of the M(11) emission (up to +/-20 meV) was observed in different suspended SWNTs. The red shift can be explained by the temperature-dependence of the M(11) transition energy, whereas the blue shift is surprising and has never been observed before. Some possible mechanisms for the blue shift are discussed.