Influence of Defects and Doping on Optical Phonon Dynamics in Carbon Nanotubes

The relaxation of electronic excitations induced by high bias or photoexcitation occurs primarily through optical phonon emission. Optical phonon relaxation may be affected by metallic/semiconducting character of carbon nanotubes, defect concentration, as well as doping. Changes in carbon nanotube G-band optical phonon population and pure dephasing lifetimes with doping and defects are described. Time-resolved incoherent anti-Stokes Raman spectroscopy is used to directly measure phonon decay lifetime, T$_{1}$, while total dephasing rate is inferred from static Raman linewidths. Defect concentration is varied by sample annealing and covalent functionalization showing increasing disorder reduces T$_{1}$ as well as overall dephasing time, T$_{2}$, with an even greater dependence. Samples with different metallic and semiconducting contribution have similar lifetimes, T$_{1}\sim $ 1.2 $\pm $ 0.1 ps in the no defect limit. Doping is shown to increase G-band linewidth, and therefore overall dephasing rate, for semiconducting nanotubes while leaving T$_{1}$ unaffected.