  We report the first observation of trions (charged excitons), three-particle
bound states consisting of one electron and two holes, in hole-doped carbon
nanotubes at room temperature. When p-type dopants are added to carbon nanotube
solutions, the photoluminescence and absorption peaks of the trions appear far
below the E11 bright exciton peak, regardless of the dopant species. The
unexpectedly large energy separation between the bright excitons and the trions
is attributed to the strong electron-hole exchange interaction in carbon
nanotubes.
