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

[Gabriele Penazzi]

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 <mailto: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 <tel:+49%20%280%29%20421%20218%209328>
>     mobile: +49 (0) 151 19650383 <tel:+49%20%280%29%20151%2019650383>
>     skype: gabriele.penazzi
>     fax: +49 (0) 421 218 4764 <tel:+49%20%280%29%20421%20218%204764>
>
>
>     On 28/feb/2015, at 06:02, MF <foster362 at gmail.com
>     <mailto: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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