Probing charged impurities in suspended graphene using Raman   spectroscopy
  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. 
Zhen Hua Ni, Ting Yu, Zhi Qiang Luo, Ying Ying Wang, Lei Liu, Choun   Pei Wong, Jian Min Miao, Wei Huang, and Ze Xiang Shen