  Photoluminescence spectroscopy measurements are performed on suspended carbon
nanotubes in a field-effect configuration, and the gate voltage dependence of
photoluminescence spectra are compared for the pristine and the molecularly
adsorbed states of the nanotubes. We quantify the molecular screening effect on
the trion binding energies by determining the energy separation between the
bright exciton and the trion emission energies for the two states. The voltage
dependence shows narrower voltage regions of constant photoluminescence
intensity for the adsorbed states, consistent with a reduction in the
electronic bandgap due to screening effects. The charge neutrality points are
found to shift after molecular adsorption, which suggests changes in the
nanotube chemical potential or the contact metal work function.
