  We report scanning tunneling microscopy (STM) and spectroscopy (STS) of
twisted graphene bilayer on SiC substrate. For twist angle ~ 4.5o the Dirac
point ED is located about 0.40 eV below the Fermi level EF due to the electron
doping at the graphene/SiC interface. We observed an unexpected result that the
local Dirac point around a nanoscaled defect shifts towards the Fermi energy
during the STS measurements (with a time scale about 100 seconds). This
behavior was attributed to the decoupling between the twisted graphene and the
substrate during the measurements, which lowers the carrier density of graphene
simultaneously.
