I was following the following this tutorial.

As far as my understanding goes, one should use bulk-like metallic electrodes to avoid problems with screening and Fermi-level definition.

Can someone explain why GNRs (graphene nanoribbons) can be safely used as electrodes, since they would have a finite band gap?


1 Answer 1


From the tutorial it is obvious that the GNR used is metallic, i.e. has no band-gap.

Please see the transmission functions at the end: http://dipc.ehu.es/frederiksen/tstutorial/index.php/Nanoribbon_answers

But you are absolutely correct. DFT calculations with gapped materials are likely to give wrong answers:

  1. because the fermi-level is ill-defined
  2. because the screening length of semi-conductors/insulators are excessively long. One could mitigate this by having a device that is longer than the screening length, but that turns out to be unfeasible in DFT :)
  • $\begingroup$ Thanks Nick! So if I understand correctly, one should check if the GNR used as the electrode is metallic. If not, how could someone use a pristine 2D graphene electrode while keeping a GNR as screening region? $\endgroup$
    – Laura
    Mar 2 at 9:02
  • $\begingroup$ I don't understand your question? Graphene is a metal (in terms of NEGF) since it has no band-gap, i.e. a well-defined fermi-level. $\endgroup$
    – nickpapior
    Mar 2 at 10:31
  • $\begingroup$ Indeed, my question was related to GNR in particular. Quantum confining graphene should open the band gap (even if small), so it could be tricky to use them as electrodes, right? $\endgroup$
    – Laura
    Mar 2 at 11:18
  • $\begingroup$ Bottomline, if your electrode is metallic, it can be used, if not, it should not be used. $\endgroup$
    – nickpapior
    Mar 2 at 11:19
  • $\begingroup$ Reminder about the SIESTA chatroom in case more space is required for comments/chat: chat.stackexchange.com/rooms/136776/siesta $\endgroup$ Mar 2 at 11:51

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