GNR topology

[esp]

I have reviewed over 50 papers on graphene and GNR based FETs, and I believe in all these papers, AGNR ribbons are presented with alternating widths within the ribbon, ... atk creates a sort of staggered topology ... the attachment shows what i mean by "alternating" ... if you count the rings (as a simple way to see this), the wdith from left to right is 6, 5, 6, 5, 6, 5 to create the AGNR .... ATK seems to produce 6, 6, 6, 6, 6, 6, 6, where the armchair part is made by "staggerring" the rings, as opposed to alternating the widths ... does that make sense?  my question is how to make the topology so commonly used in research papers ... do I have to do it manually?  IF I make a GNR of width W, I want it to be the same as is done in published research papers

[Anders Blom]

You are very observant VNL currently builds the GNR by taking a chiral vector (n,m) in the graphene sheet, and uses that as the ribbon C-axis. In effect, the ribbons are unrolled nanotubes. The one you have with 6,5,6 does not correspond to such a vector directly. This doesn't mean it's not a valid ribbon, but it doesn't correspond to a chiral index, and hence can't be built so easily.

What you need to do is simply build a 6,6,6 ribbon, send it to the Builder and remove the "offending" atoms (also the edge H atoms), then passivate it again using the "passivator" Custom Builder I have attached.

In ATK 12.2 all this will be real easy.

[kstokbro]

ATK only makes the nanoribbons which corresponds to a graphene unit cell. So it cannot directly build the structure you show, since it is not a graphene unit cell.

However, only minor edits are needed to build it:
build a (7,0) nanoribbon, and remove the 2 outhermost atoms,
Then passivate it.
In our upcomming release ATK12.2 the passivation tool is integrated in the builder, but for now use the tool in the tutorial below:

http://www.quantumwise.com/documents/tutorials/latest/BasicGrapheneTutorial/index.html/chap.zigzag_transport.html#id1249215

[esp]

thank you

[esp]

I need to be able to dope the source and drain electrodes in a FET topology ..

i am having a problem doing this ... when i make electrodes, it seems to copy part of the center region .. if i try to change the atoms in the electrode region in the builder tool, it automatically changes atoms in the center region ... how do i dope the electrodes?

[esp]

Ok this is my plan, tell me if this makes sense:

1) I make a device configuration with left and right electrodes ...
2) I script through the left and right electrodes and replace a certain number (10%, etc) of atoms there to dopant atoms
3) I create the device with the new bulk, left electrode and right electrode objects .. then continue from there ..

does this seem like a good way to dope left and right electrodes? 

otherwise i am wondering about the information here:
http://quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.deviceconfiguration.html

which talks about requiring part of the center region to match the electrodes .. is this a hard requirement, or only if you build it in a certain way ? ...

Another question also ... why can i not get a band diagram for an entire device ..?  it seems you can only do it for the central bulk region without electrodes ... is this right?

[esp]

i am trying to produce a plot such as attached .. bandstructure for a whole device with bias .. is it possible (without doing too many things manually)? 

[kstokbro]

Regarding the band diagram: to calculate the bandstructure requires periodic boundary conditions in 3-dimensions.
However, you can visualize the band gap in a device configuration, by calculating an energy dependent LDOS.
There is an example in
http://www.quantumwise.com/documents/tutorials/latest/MolecularDevice/index.html/chap.analysis.html#sect1.analysis.lddos
which you can adjust to your purpose.

For the doping of the electrode, it is most easy to setup using the builder. Switch to bulk mode, and modify the atoms in the equivalent electrode, then switch to device mode, and the atoms are automatically copied to the electrode part.

It is also possible to dope the electrode by setting the charge of the electrodes, using the keyword
charge = 0.02
for the electrode calculator.
We are working on a tutorial which explains how to do this.

[Anders Blom]

I think it's important to keep in mind that the band structure picture is a simplified description of the system. It's helpful to understand certain properties on a qualitative (and even quantitative) level, but simplified nevertheless. In ATK we deal with a more detailed reality, and so in some sense we don't need the simple picture. The LDOS plot does give a way to map the density of states to band edges in a device structure, also under bias, but it be wiggly on the atomic scale.

[esp]

regarding doping being done in the builder .. i am not sure i can agree to that being easier .. it seems that you have to select each atom, one by one, then go to the list of atoms, double click and select the new atom type, then go back and repeat ... to do this on say 20 atoms takes a considerable amount of time ... doing it programatically takes no time, except that since the order of each atom in the lists is not clear, the program can be somewhat complex to get a good distribution of new atoms .. is there an easier way to do this in builder or is it as much of a manual process as it seems to be?

[Anders Blom]

In the current Builder, yes this is somewhat of a challenge.

In version 12.2, however, you select all atoms you want to change (click-click-click, hard to do that much simpler) then change them all at once, using a periodic table widget. 12.2 will be out in 3-4 weeks (in a preview edition at least).

[esp]

great, i look forward to it

[esp]

the tutorial mentioned above just disappeared:

"We're sorry…
The page you requested cannot be displayed. Possible reasons are: ... "

[Anders Blom]

Sorry, it's fixed now.