  Charged impurity (CI) scattering is one of the dominant factors that affect
the carrier mobility in graphene. In this paper, we use Raman spectroscopy to
probe the charged impurities in suspended graphene. We find that the 2D band
intensity is very sensitive to the CI concentration in graphene, while the G
band intensity is not affected. The intensity ratio between the 2D and G bands,
I2D/IG, of suspended graphene is much stronger compared to that of
non-suspended graphene, due to the extremely weak CI scattering in the former.
This finding is consistent with the ultra-high carrier mobility in suspended
graphene observed in recent transport measurements. Our results also suggest
that at low CI concentrations that are critical for device applications, the
I2D/IG ratio is a better criterion in selecting high quality single layer
graphene samples than is the G band blue shift.
