▎ 摘　　要

Charged impurity (CI) scattering is one of the dominant factors that affects 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, I-2D/I-G, of suspended graphene is much stronger compared to that of nonsuspencled graphene, due to the extremely low Cl concentration in the former. This finding is consistent with the ultrahigh 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 I-2D/I-G ratio is a better criterion in selecting high quality single layer graphene samples than is the G band blue shift.