Study on frequency upconverion of multiwalled carbon nanotubes

The absorption and photoluminescence spectrum of MWNTs have been measured. The optical frequency upconversion has been observed from MWNTs. The photoluminescence spectrum, which was observed on excitation at 1064 nm, consisted of a broad structured band, peaking at approximately 780 nm. Van Hove Singularities (VHS) in the density of states of multiwalled carbon nanotubes have been discovered from absorption spectrum. The peaks, corresponding to VHS, located at 685 nm, 719 nm, 894 nm. The mechanism describing the underlying processes shows that the excitation involves double-photon absorption followed by non-radiative vibrational decay to the VHS states. Subsequently, the luminescence is produced by radiative transition.