Photoluminescence microscopy of carbon nanotubes grown by chemical vapor deposition: influence of external dielectric screening on optical transition energies

Photoluminescence (PL) laser microscopy was applied to determine optical transition energies E$_{11}$ and E$_{22}$ of individual semiconducting single-walled carbon nanotubes (SWNTs) suspended on top of carbon nanotube `forests', grown by chemical vapor deposition (CVD) on silicon substrates. A uniform increase of E$_{11}$ and E$_{22}$ energies by 40--55 and 24--48 meV, respectively, was found for 19 different (n,m) nanotube species suspended in air/vacuum -- relative to SWNTs in a reference water-surfactant dispersion. We did not find any systematic correlation between nanotube (n,m) structure and energy shifts. CVD-grown SWNTs embedded in paraffin oil and 1-methylnaphthalene show nearly the same PL peak positions as SWNTs in aqueous dispersion, indicating similar dielectric screening of excitons in SWNTs in these media.