  Gate-voltage effects on photoluminescence spectra of suspended single-walled
carbon nanotubes are investigated. Photoluminescence microscopy and excitation
spectroscopy are used to identify individual nanotubes and to determine their
chiralities. Under an application of gate voltage, we observe slight blueshifts
in the emission energy and strong quenching of photoluminescence. The
blueshifts are similar for different chiralities investigated, suggesting
extrinsic mechanisms. In addition, we find that the photoluminescence intensity
quenches exponentially with gate voltage.
