[DFTB-Plus-User] NEGF 12,0 Nanotube

Tue Mar 3 18:18:09 CET 2015

Thanks for the response Gabriele

Cheers,
Mike

Sent from my iPhone

> On Mar 3, 2015, at 12:48 AM, Gabriele Penazzi <penazzi at uni-bremen.de> wrote:
> 
> Hi Mike,
> 
> I gave a deeper look and the result you get is correct.
> 
> The electron-hole symmetry, as I said, is lost at higher energy as it is removed by interactions beyond nearest-neighbor. Apparently it is correct also that it is lost already on the first sub-band. I found a band-structure calculated with DFT here http://www.theochem.unito.it/crystal_tuto/mssc2008_cd/tutorials/nanotube/nanotube.html. You can see that in valence band you have two bands crossing in Gamma point below Fermi energy, while in conduction they cross in a higher k-point, i.e. you expect a "double" step in valence band and two single steps in conduction, as you see as well. The tunneling curves you've seen in literature are probably based on an empirical pz hamiltonian.
> 
> Regarding the gap, again, the "m-n" rule is derived for a pz nearest neighbor hamiltonian. For small armchair CNTs it can break and you can have a small gap. See for example this paper http://pubs.acs.org/doi/abs/10.1021/jz100889u , I've run a calculation with the "mio" set from dftb.org and I've a 0.07 eV gap, which is somewhat larger than the DFT-B3LYP result but we're speaking about differences of 20-30 meV, which are hard to catch. 
> 
> Long story short, it is more meaningful to compare the DFTB result with DFT rather than with the pz empirical result. The smaller the CNT, the larger the differences. 
> 
> Best,
> Gabriele
> 
> 
> 
>> On 03/02/2015 06:50 AM, Michael Foster wrote:
>> Hi Gabriele,
>> 
>> Thanks for the reply. As regards to the symmetry comment, I was expecting the steps (at least the first couple) to the left and right of the Fermi energy to be symmetric; I've seen several articles in the literature were this is the case for the 12,0 nanotube (also no bandgap). However, I believe the issue has to do with the structure as you suggested. Originally, I just used a nanotube generator to create the nanotube; I did not run an optimization. I have since optimized the unit cell using DFTB+ (MaxForceComponent = 1.0e-5) and the transmission curve is closer to what I expected but I still predict a small bandgap. I'm sure the issue has to do with the structure...
>> 
>> Mike      
>> 
>>> On Fri, Feb 27, 2015 at 11:37 PM, Gabriele Penazzi <penazzi at uni-bremen.de> wrote:
>>> 
>>> 
>>> 
>>> --
>>> Dr. Gabriele Penazzi BCCMS - University of Bremen
>>> http://www.bccms.uni-bremen.de/ http://sites.google.com/site/gabrielepenazzi/ phone: +49 (0) 421 218 9328 mobile: +49 (0) 151 19650383 skype: gabriele.penazzi fax: +49 (0) 421 218 4764
>>> 
>>> On 28/feb/2015, at 06:02, MF <foster362 at gmail.com> wrote:
>>> 
>>>> Fixing a few big typos, sorry DFTB community, I was in a rush:
>>>> 
>>>>> I have been carrying out a few test NEGF calculations. I have been able to run several of the example calculations without a problem (i.e. nanoribbon); however, I'm having issues with a 12,0 nanotube. My transmission curve is not symmetric (not to mention it should be metallic, no gap)
>>> 
>>> Ups, i was reading (10,0), ignore what i said in last mail about the gap. Still the electron hole symmetry argument holds.
>>> 
>>> One of the reasons which can open a gap is distorsion in the lattice such as cnt deformation, wrong lattice vectors etc. I'll give it a look but meanwhile it can be a pointer what to look at.
>>> 
>>> Best
>>> Gabriele
>>> 
>>>>> as it should be for a 1D nanowire; I'm also having the same problem with a 5,5 nanotube (haven't tried any others). I'm hoping the problem has to do with my input file and/or structure but I can't seems to fix/find the problem. Attached is my input file, geometry and transmission curve. Any help would be greatly appropriated.
>>>>> Thanks in advance,
>>>>> Mike
>>>>> <image.png>
>>>>> <cnt_12-0.gen>
>>>>> <dftb_in.hsd>
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> 
> -- 
> --
> Dr. Gabriele Penazzi
> BCCMS - University of Bremen
> 
> http://www.bccms.uni-bremen.de/
> http://sites.google.com/site/gabrielepenazzi/
> phone: +49 (0) 421 218 62337
> mobile: +49 (0) 151 19650383
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