Graphene edges attach: physical origin of the effect

LAMMPS LAMMPS Mailing List Mirror
1

Dear Lammps users,

I am trying to study heat transfer through a specifically shaped planar graphene sheet. It has two rectangular segments connected by a very narrow link. The two extreme ends are fixed. I apply three heat baths, two heat baths at the two ends and one heat bath in the middle. The two terminal heat baths are kept at 900 and 110 K.I use NVE for all the non-fixed atoms and use temp rescale for the heat bath atom groups. The temperature of the middle heat bath varies between 200 to 600K.

Now, above a certain threshold temperature for the middle heat bath, typically, above 450 K, two free edges near the middle ( right edge of left rectangle and left edge of right rectangle, below the narrow link) gets attached, as in like, as if new bonds are formed joining the two edges. The edges remain joined even after removal of middle thermostat. I removed the middle thermostat, kept on the two terminal thermostats, and let it run for 8 ns to check if the segments separate but it stays on. I am using AIREBO potential, which according to my knowledge, is capable of dynamically creating of breaking bonds. My question is, does this effect sound physical? This is as if I am welding and joining the two sheets. I know there are MD studies of welding graphene sheets. Is there a theory which can explain this joining effect?

Please let me know if you need further details…

Thanks and regards,
Souvik.

2

A correction, I am using REBO, not AIREBO.

3

Dear Lammps users,

I am trying to study heat transfer through a specifically shaped planar
graphene sheet. It has two rectangular segments connected by a very narrow
link. The two extreme ends are fixed. I apply three heat baths, two heat
baths at the two ends and one heat bath in the middle. The two terminal heat
baths are kept at 900 and 110 K.I use NVE for all the non-fixed atoms and
use temp rescale for the heat bath atom groups. The temperature of the
middle heat bath varies between 200 to 600K.

there are three issues that would bother me a lot with this setup:

1) the use of temp/rescale. this is just plain bad. there are multiple
better alternatives for a heat bath available in LAMMPS. for a setup
like this, i would prefer a dissipative thermalization like it is
offered by fix temp/csvr or fix langevin.

2) what is the physical justification of the heat bath for the
connecting strip? i don't see any. how would those atoms be
thermalized in an experimental setup?

3) your model could be lacking the contributions from the electronic
degrees of freedom. i am not an expert in these kind of materials, but
i remember faintly that graphene has significant 2d-thermal
conductivity. of course, there may be side effects from the quasi 1d
nature of your setup.

Now, above a certain threshold temperature for the middle heat bath,
typically, above 450 K, two free edges near the middle ( right edge of left
rectangle and left edge of right rectangle, below the narrow link) gets
attached, as in like, as if new bonds are formed joining the two edges. The
edges remain joined even after removal of middle thermostat. I removed the
middle thermostat, kept on the two terminal thermostats, and let it run for
8 ns to check if the segments separate but it stays on. I am using AIREBO
potential, which according to my knowledge, is capable of dynamically
creating of breaking bonds. My question is, does this effect sound physical?

it is difficult to imagine what you are describing without some
pictures, but apart from that, this is not really a question about
LAMMPS, but a question about the potential and the science of
nanoscale objects. i think you'd have to dig into the literature to
find an answer (and probably dig deep). that is, unless you get very,
very lucky and there is somebody reading your e-mails *and*
able/willing to provide her/his opinion on the subject.

This is as if I am welding and joining the two sheets. I know there are MD
studies of welding graphene sheets. Is there a theory which can explain this
joining effect?

again, this is not really a question about LAMMPS but about your
science and thus you should use a more suitable forum for that (e.g.
communicate with other researchers that have expertise on this
specific subject).

axel.

4

Thank you Dr. Kohlmeyer for your detailed response. I agree that temp/rescale is not a very good idea. Following your suggestion I tried fixt NVT in heat bath groups and still saw this effect. However, when I used fix Langevin over nve for the heat baths, this effect (joining edges) was not observed. The middle heat bath is necessary as my objective is to study heat transfer in a three terminal device compared to a conventional two terminal setup.

At this point I am not very confident whether the effect is physical or an artifact of using inappropriate heat baths.

Thanks again for the guidance.

Souvik.