why the fermi energ level locates blow the valence band ?

[artingchen]

Hello, every body,

I calculated the band structure of doped graphene ribbon recently, but the figure I obtained confuses me. the fermi energy locats below the valence band when it is doped. the fermi energy is -4.37 eV for intrinsic GNRs but -5.07 eV for doped GNRs. can anyone help me to explain the phenomenon? any suggestion is appreciated. thank you !
the figure have been attached.

[zh]

The position of Fermi level in the doped system depends on the dopant. If the dopant acts as electron donor, the Fermi level will shift toward the conduction band, e.g., the behavior of the nitrogen doping in graphene nanoribbon  (i.e., nitrogen substitution on carbon). If the dopant acts as electron acceptor, or hole donor, the Fermi level will shift toward the valence band, e.g., the behavior of the boron doping in graphene nanoribbon. Actually, your results suggest that the dopant in your calculation case acts as hole donor.