  Using micro-Raman spectroscopy and scanning tunneling microscopy, we study
the relationship between structural distortion and electrical hole doping of
graphene on a silicon dioxide substrate. The observed upshift of the Raman G
band represents charge doping and not compressive strain. Two independent
factors control the doping: (1) the degree of graphene coupling to the
substrate, and (2) exposure to oxygen and moisture. Thermal annealing induces a
pronounced structural distortion due to close coupling to SiO2 and activates
the ability of diatomic oxygen to accept charge from graphene. Gas flow
experiments show that dry oxygen reversibly dopes graphene; doping becomes
stronger and more irreversible in the presence of moisture and over long
periods of time. We propose that oxygen molecular anions are stabilized by
water solvation and electrostatic binding to the silicon dioxide surface.
