Differene between zero bias conductance in bulk mode and device mode

[sonal AGRAWAL]

Dear sir,
I have calculated Zero bias conductance of graphene nanosheet in bulk mode and in device mode there is huge difference between conductance. Can you please tell me why this difference is appearing in conductances.


Please help me out
Thanks in advance

[Petr Khomyakov]

Which version of QuantumATK are you using? Also, please attach your python script and related log file.

[sonal AGRAWAL]

Dear sir,
Thanks for your response
 I am using quantum ATK-2019.03
Related log files and python files are attached herewith.
Sir, I have two more question
1. If we analyzed the electronic properties using DFT LCAO calculator, phonon properties using Forcefield and transport properties using extended huckle than the results will be valid or not.
2. Is the electrode size affect the I-V characteristics if the structure is perfect, I mean there is no defects and doping in the structure and if defects and doping are present.

Thanks & Regards
sonal

[Nayab Shiraz]

I have also faced similar issues many times. For a perfectly periodic structure, transmission can be calculated from unit cell. Device setup is not necessary. But when I create a device by just repeating the same structure along transport axis and converting to device, the transmission spectrum becomes completely different.

[Troels-Markussen]

Hi,

There has unfortunately been a bug in the way QuantumATK calculated the transmission for a bulk configuration, which in some case could lead to wrong transmission values. This was solved in the 2019.12 version.
1) Concerning the comparison between DFT and extented Hückel, you cannot in general expect to get the same results. In the attached scripts, the device is calculated with Hückel, while the bulk configuration with DFT. Usually this will give different results, at least quantitatively different. There might be qualitative agreement. The use of force fields for the phonons is usually fine.
2) Yes, the electrode size can matter, also for a perfect device. You can use the "Electrode Validator" to the right of the LabFloor to validate if a certain bulk configuration is a valid electrode.

Regards,
Troels

[sonal AGRAWAL]

Thanks for your response

Yes, you are right I can not compare DFT and extended huckle results.
But I ask this question because I have calculated the transmission spectrum for pristine graphene using extended huckle and DFT calculator and I am getting the same conductance using both but in case of doped graphene, I am getting different results.

Thanks & Regards
sonal

[Troels-Markussen]

Right. I'm not surprised that you get good agreement for pristine graphene. This is an 'easy' case for Huckel since there is only a single element. When you add dopant (nitrogen) atoms the interactions becomes more complicated and the Huckel parameters are probably not optimized for this situation. DFT is more transferable and probably gives a better description of the doped graphene.

Regards,
Troels

[sonal AGRAWAL]

Thanks for clarifying this

I have one more question.
Can we connect the electrode in a hexagonal sheet of graphene in ATK-VNL?

[Petr Khomyakov]

The unit cell of graphene should be rectangular to make it possible to attach electrodes.