Messages

1036

General Questions and Answers / Re: Can I run this command using VNL?

« on: June 22, 2017, 15:58 »

There is no bias voltage applied in this calculation. However, the Hartree Difference Potential (HDP) does not need to be the same in the left and right electrodes at zero bias. The definition of the HDP is discussed in Notes at http://docs.quantumwise.com/manuals/Types/HartreeDifferencePotential/HartreeDifferencePotential.html.

The second figure shows the same HDP that has been averaged along the Z-direction to smoothen the oscillation related to the discrete nature of atoms. The plot is rigidly shifted as you have noticed, but as any electrostatic potential the HDP is defined up to a constant. So, it is equally good for describing band bending in Si discussed in the tutorial, and was set to zero in the right electrode to align the averaged HDP with the Fermi level in the doped Si (comprising the right electrode) as shown in the PLDOS figure. 

1037

General Questions and Answers / Re: Some thing wrong with MoTe2 calculations ?!!!!!!!!

« on: June 22, 2017, 15:27 »

Could you enclose the actual python script related to your MoTe2 calculation? What ATK version are you using?

1038

General Questions and Answers / Re: why are the band structures calculated by GGA and SOGGA totally different?

« on: June 19, 2017, 09:00 »

It is hard to say anything about it without seeing the actual scripts.

1039

General Questions and Answers / Re: Calculating hole densities (carrier concentration)

« on: June 15, 2017, 15:14 »

You may have a look at the following tutorial on how to build interfaces between dissimilar materials, see http://docs.quantumwise.com/tutorials/ag_au_interface/ag_au_interface.html.

1040

General Questions and Answers / Re: Supercell calculation

« on: June 12, 2017, 13:06 »

You may first calculate the lattice parameters of the perfect crystal for the primitive cell.

For defect calculations, you can then build a supercell by repeating the primitive or conventional unit cell of the perfect crystal with the lattice parameters obtained in the first step. 

Afterwards, you can create a defect(s) in the supercell structure, and do ion relaxation of this created structure with a defect(s).

So, this is how you may model a crystal structure with a defect(s).

1041

General Questions and Answers / Re: MoS2 bandstructure

« on: June 7, 2017, 13:57 »

I would guess that pristine MoS2 is an intrinsic semiconductor. It might be unintentionally doped in experiment, but you are doing simulations, so that we will need to dope it with electrons manually to have it n-type doped. Otherwise, it is assumed to be intrinsic by default. 

1042

General Questions and Answers / Re: the reliability of Huckel method on the calculation of IV curve

« on: June 4, 2017, 13:39 »

Regarding the extended Huckel method (EHM) vs DFT approach, in the best case scenario the I-V curve obtained with EHM will be as accurate as the electronic structure of the system calculated using the EHM. So, you should first investigate, e.g., whether the electronic or band structure of your electrode materials and central region region is accurately given by the EHM, i.e., you should benchmark it against the DFT-calculated electronic structure. I note that this does not guarantee accurate I-V curves, as the properties of the interface between the electrode and central region system might not be described by the EHM with the DFT accuracy.

1043

Scripts, Tutorials and Applications / Re: overcome bandgap block

« on: June 4, 2017, 13:24 »

You can apply a bias voltage, or you may also add a gate electrode to apply a gate voltage, or do both at the same time.

1044

General Questions and Answers / Re: How to get the electron affinity from the band structure?

« on: June 4, 2017, 13:19 »

It is impossible to calculate the electron affinity as well as work function just from the band structure. You should create a surface and calculate the conduction band energy minimum with respect to the vacuum level, which is given by the macroscopic in-plane averaged effective potential (there is an analysis object for this potential in the ATK). This energy is then the electron affinity. Note that the electron affinity depends on the surface type chosen; this is similar to work function calculations, where the work function is a property of the surface.

1045

General Questions and Answers / Re: ZrS2 bandstructure

« on: May 31, 2017, 11:12 »

The band plot is perhaps different since you have a band folding as result of using a larger (2x2) lateral unit cell. It also means that the Brillouin zone of the 2x2 cell is smaller than that of the 1x1 primitive cell.

1046

General Questions and Answers / Re: Do I need to relax the interface before constructing the device?

« on: May 26, 2017, 23:20 »

If you have properly relaxed the bilayer structure before building the device from bulk, you would not really need to relax the electrodes.

1047

General Questions and Answers / Re: Dirac Point for Graphene Nanoribbon

« on: May 26, 2017, 14:21 »

I am not sure the question is correctly formulated. I am guessing that you mean whether the nanoribbon has a zero energy bandgap and there exists a point in the 1D Brilloiun zone where the conduction and valence bands touch each other, meaning that the nanoribbon is semimetal, similarly to the pristine graphene. In any case, you have to calculate the band structure along the nanoribbon axis to answer these questions.

1048

General Questions and Answers / Re: Band Gap Extractor

« on: May 26, 2017, 14:09 »

You would need to first calculate the band structure along the nanotube axis, and then either measure the band gap in the Band Structure Analyzer in the VNL, or use the script given at http://docs.quantumwise.com/manuals/Types/Bandstructure/Bandstructure.html.

1049

General Questions and Answers / Re: Do I need to relax the interface before constructing the device?

« on: May 26, 2017, 13:58 »

Regarding #1, you can definitely do it in the way you have described it. In fact, it would be wise to do it, even if you want to optimize the device structure built in step 2.

Regarding #2, you cannot optimize the electrodes and electrode extensions for device configuration, i.e., when the device is already built; you should have done it in step 1. You may still do geometry optimization for the central region or its part if you expect significant structural reconstruction in the central region of the device.

 

1050

General Questions and Answers / Re: 2D Structures and Electrodes

« on: May 26, 2017, 13:44 »

Then should I use 2D periodic electrodes with creating interfaces to non-periodic structure or like attaching molecules, attach bulk electrodes to the structure?

You can do it both ways. The choice between the two electrode models really depends on what the actual physical system of your study looks like in a real or Gedanken (thought) experiment. In fact, you may even combine the two models by coupling the "molecule" to the 2D structure that is attached to the bulk (e.g., metal) electrodes in some way.