Single-layer behavior and slow carrier density dynamic of twisted graphene bilayer

We report scanning tunneling microscopy and scanning tunneling spectroscopy (STS) of twisted graphene bilayer on SiC substrate. For twist angle similar to 4.5 degrees, the Dirac point E-D is located about 0.40 eV below the Fermi level E-F 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 s). 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3691952]