QuantumATK Forum

QuantumATK => Scripts, Tutorials and Applications => Topic started by: mldavidhuang on September 3, 2011, 09:13

Title: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: mldavidhuang on September 3, 2011, 09:13
I have done the calculation of bilayer gaphene and monolayer graphene, but I found that the current of bilayer graphene is larger than twice of monolayer graphene when the applied bias is low.(see the attachment)

This is strange because I think in any case the conductivity should be lower than parallel of two monolayer graphene because of interlayer scattering in bilayer system.

So I doubt the accuracy of my calculation, the script is attached, I beg help for checking anything wrong with my setting or is there any question with my assumption for I haven't seen any reference about such phenomenon.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: kstokbro on September 3, 2011, 15:18
You calculation seem technically correct.
The difference you observe must be related to a difference in the zero bias transmission spectrum, thus, I would investigate that in detail.

I think the bilayer graphene will have a different electronic structure compared to the single graphene, since there is an interaction between the two layers.
Thus, you must study and understand the electronic structure of the bilayer graphene

Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: mldavidhuang on September 4, 2011, 04:57
This is the zero bias transmision spectrum I get from my calculation, actually, I find it hard to understand the transmission spectrum of the bilayer from the band structure.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: kstokbro on September 4, 2011, 07:51
i think you forgot the attachment
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: mldavidhuang on September 4, 2011, 12:56
Sorry for that！Here are the attachment
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: kstokbro on September 4, 2011, 20:56
You see that the bilayer bandstructure is quite different from the single graphene bandstructure, this is reason for the different behaviour.

You need to include many more k points in transverse direction to get converged results for the transmission spectrum.

You might consider using user defined k-points around where the bands cross the fermi level.  I attach an example. Note this is just an example showing how it can be done, and you need to determine convergence you self.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: mldavidhuang on September 6, 2011, 09:53
Actually, there is something I don't quite understand in your code
Code
    k_min = 0.0
    k_max = 0.02
    k_n = 41

    k = numpy.array([(0,x,0) for x in numpy.linspace(k_min,k_max,k_n)])
    w = numpy.array([ 2 -(numpy.linalg.norm(i) == 0) for i  in k])
the k space you define are from (0,0,0) to (0,0.02,0) ,and what does w mean, I see it define as "kpoints_weights=w", this is what you mean by "user-define k-point around where the band cross the fermi level" ? because in my operation of ATK, I haven't met such settings. Could you explain a bit in detail to me ?

Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: kstokbro on September 6, 2011, 10:22
This is user specified k-points and corresponding weights. The purpose is to efficiently sample the region around the crossing of the fermi level. Thus, use a more efficient k-point sampling.

However,  you can also just increase the number of k-points in the MonkhorstPackGrid this will be equivalent, however, you will need more k-points.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: mldavidhuang on September 6, 2011, 17:35
Generally speaking, is more k-point the better the result? And how to test how much k-point is enough with the combination of accuracy and time-consuming ?
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: kstokbro on September 6, 2011, 21:21
More k points is always better. You must increase the number of k-points until you results do not change any more.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: mldavidhuang on September 7, 2011, 09:10
Why the optimized interlayer distance of bi-layer Graphene is 3.08 Angstrom, but the value in other papers is around 3.34 Angstrom, is there anything wrong with my calculation ?
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: amanpreet on January 16, 2012, 12:22
Hi ,
i have a quick qusetion for you.I am a new user of quantumwise.How to make bilayer or monolayer graphene ,is it done by starting from a graphene sheet(not ribbon) and repition in A direction two times. I want to study effect of graphene bilayer placed on different kind of substrates.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: Anders Blom on January 16, 2012, 12:33
You can build a larger sheet using the Custom Builder for nanoribbon but select "Sheet" instead of ribbon. This gives a monolayer; for bilayer (and perhaps also monolayer) it's better to start with graphite and repeat as needed in A and B, since there is a shift of coordinates (not just repeat) in the C direction.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: amanpreet on January 16, 2012, 12:51
Thanks for the reply.I am very much confused about how to decide the number of k points needed as they seemed to be the most important part of the calculation. I want to study effects of graphene layer placed on different kind of substrates(e.g SiC, GaAs, Si). How can we do such kind of epitaxial simulation in quantum wise.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: Anders Blom on January 16, 2012, 13:00
You need to test the k-points, but you need quite many, maybe 12x12 or 15x15 (in the plane) is a good starting point, then increase and see what happens.

Although it's in Japanese, Google translate can help you perhaps understand a bit of this case study of graphene on SiC: http://quantumwise.co.jp/example/fullerene/full-08.html
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: amanpreet on January 16, 2012, 20:43
Hi Anders,

Thanks for your reply .I am going through this paper they have created a bulk graphene on si structure. Can I create something like this, isn't it like we have only use the bulk structure available in database. Can we repeat the unit structure to supercell. I am just new to this software. Actually I have just stated my graduate program and I am assisting my lab mates who are working on a device in which graphene is placed on Si(already fabricate). Now we want to study effect on optical properties( absorption of light) of device after placing graphene on top.

Thanks
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: amanpreet on January 16, 2012, 20:54
Thanks for your reply, I am going through that paper. But how can we realize bulk structure of graphine on silicon sturucture. How do we generally convert fro bulk configuration to mass configuration.
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: donmbringer on August 6, 2012, 15:51
Hi. I am also facing the problem when building bilayer graphene. I tried as what you said, to start by graphite in database, and then change the coordinates of the second layer very close to the first layer. But, it seems that this does not work when calculating the Band strcutrue. Could you please give me hand？
Title: Re: Puzzle about the I-V characteristics of bilayer graphene compared with monolayer
Post by: Anders Blom on August 6, 2012, 16:13
See answer to other post. Please don't crosspost, it just creates more work for us.