  Trions are charged excitons that form upon optical or electrical excitation
of low-dimensional semiconductors in the presence of charge carriers (holes or
electrons). Trion emission from semiconducting single-walled carbon nanotubes
(SWCNTs) occurs in the near-infrared and at lower energies compared to the
respective exciton. It can be used as an indicator for the presence of excess
charge carriers in SWCNT samples and devices. Both excitons and trions are
highly sensitive to the surrounding dielectric medium of the nanotubes, having
an impact on their application in optoelectronic devices. Here, the influence
of different dielectric materials on exciton and trion emission from
electrostatically doped networks of polymer-sorted (6,5) SWCNTs in top-gate
field-effect transistors is investigated. The observed differences of trion and
exciton emission energies and intensities for hole and electron accumulation
cannot be explained with the polarizability or screening characteristics of the
different dielectric materials, but they show a clear dependence on the charge
trapping properties of the dielectrics. Charge localization (trapping of holes
or electrons by the dielectric) reduces exciton quenching, emission blue-shift
and trion formation. Based on the observed carrier type and dielectric material
dependent variations, the ratio of trion to exciton emission and the exciton
blue-shift are not suitable as quantitative metrics for doping levels of carbon
nanotubes.
