I add the gate below the graphene device the fermi level is not changed,WHY?

[ocdor]

Dear sir:
              I want to change the graphene fermi level by method of adding the gate below the graphene,but the graphene  fermi level is not changed.WHY? Thank you !

[Anders Blom]

Because graphene is atomically flat, if you apply an overall gate to the whole structure all you do is move the potential landscape by a constant amount, but the energy zero level is anyway arbitrary so nothing changes in reality.

On the other hand, you are a bit mistaken about "the Fermi level in graphene" because
a) you have no gate where there is a Fermi level defined (in the electrodes), and
b) where the gate is applied there will be an effect locally, and this will show up in the transmission spectrum. But in the middle of the device the electrons are not in equilibrium so there is no Fermi level.

[ocdor]

The shift of the valence band edge of the central region towards the electrode Fermi levels  is due to the applied gate potential  in the tutorial (http://docs.quantumwise.com/tutorials/graphene_nanoribbon_device/graphene_nanoribbon_device.html)

I want to know why the shift of fermi level relative to the band edges can not be realized in my calculation. Are there  some differences between my calculation and tutorial?

Thank you!

[ocdor]

When I apply an overall gate to the whole structure, what I do is to move the potential landscape by a constant amount.
But I want the shift of Fermi level of  the electrodes relative to the valence band edge of the central region. What should I do? changing gate region?

Thank you!

[Anders Blom]

For that purpose your setup appears to be correct.

[ocdor]

Thank you! But the transmission spectrum is not shifted in the energy direction. What problem in the calculation setup?The scripts and transmission spectrum(gate=0v,1v,10v) are enclosed. 

[Anders Blom]

I don't see why the energy scale should shift.

[ocdor]

I mean that the shift of Fermi level of  the electrodes relative to the valence band edge of the central region should be realized by appling gate voltage in the central region. Is there something wrong with my understanding?

Thank you!

[Anders Blom]

It's true that such a shift occurs, but it will result in an energy shift of the transmission spectrum, but rather a change in the transmission probability. You might say this is because the energy scale (and indeed basic shape, such as the location of any band gap) of the transmission spectrum is determined by the electrodes, not the central region.

[ocdor]

In the 'Graphene nanoribbon device: Electric properties'  tutorial (http://docs.quantumwise.com/tutorials/graphene_nanoribbon_device/graphene_nanoribbon_device.html),  the metallic gate of GFET can be used to tune the band edge positions relative to the Fermi level, and thereby control the electric properties of the device.

My purpose is to tune Fermi level of graphene in the bandstructure by appling voltage, I hope that the tune of Fermi level may be realized in bulk configuration. I am confused about it

[Anders Blom]

As the text in the tutorial says, you don't really tune the Fermi level, you tune the band edges relative to the Fermi level, although one should be careful to speak of any Fermi level in the central region as the system there is not in equilibrium. Note that you don't even "see" the Fermi level in the calculations, the only values reported in the log file are those for the electrodes but these are irrelevant numbers. Your results clearly show that there is an effect of the gate voltage, but what you want to plot to see the effect of the gate is the local density of states in the device, as done in our tutorials:
http://docs.quantumwise.com/tutorials/nisi2-si/nisi2-si.html
https://quantumwise.com/documents/tutorials/ATK-2015/InAsDevice/index.html/chap.gatescan.html#sect1.gatescan.results