lammps-graphene/project_explanation

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Greetings All,

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During the program, I worked for the Materials Simulations Lab at the University of South Florida. This project is focused on a material called graphene, which is a pure carbon crystal only one atom thick. It has many potential applications, but there are doubts about the viability of synthesising samples of useful size. The lab's goal is to demonstrate the viability of one synthesis method, chemical vapor deposition (or CVD).

This method is expected to scale to sizes required for industrial purposes, but produces graphene structures that have defects (quite literally, either atoms missing from the lattice or too many atoms in areas of the lattice). To aid that goal, we are studying the mechanical properties of graphene with defects that would likely be encountered in a chemical vapor deposited sample. It's our hypothesis that samples grown using this method will be viable for the applications currently attributed to pristine graphene's potential.

The poster I'll be attaching displays specifically information about the atomic structure of grain boundaries between graphene bicrystals. From the best physicist to the layman, the important thing is always the pictures. I think I made some pretty ones, and what's even better, they match the ones the experimentalists gave us! In the diagrams, I am displaying three graphene bicrystals with different misorientations, and I've color-coded the total energy associated with each atom. One can see that the atoms around the boundaries at the center have greater energy than those in the pristine region.

The rest of this description will be techie, because I want to provide that for those who will critically analyse the poster. I think I've covered pretty much everything, but if anyone wants to discuss something further, of course, please contact me.

To create the structures, we considered all possible combinations of grain orientation mismatch, based on the fact that within graphene, carbon atoms exist in a hexagon lattice. A grad student I worked with produced a code to create these initial structures, outputting several hundred possible structures. For each, I applied periodic boundary conditions to the cell, and used a conjugate gradient method to apply the potential energy function to "relax" the samples to their lowest energy state. The potential energy function was developed by a grad student and later post doc who worked for this lab, who is no longer with the lab but provided invaluable advice. What a brilliant guy, his solutions were always elegant; perhaps I should mention he isn't with the lab now because he took a job at Los Alamos!

So from this process, I collected some statistics. The formation energy and atomic coordination statistics were useful for ranking and discarding samples, and from those I was able to produce about 25 physically viable samples. For these samples, I produced a number of maps, including atomic strain, which is (kind of) easy for experimentalists to verify using a tunneling electron microscope. I should mention another component to this project is verification of the potential function developed by the lab, but perhaps that goes without saying. I also produced energy gradient maps like the ones I show on the poster. One can see the areas of high energy around the grain boundary, corresponding to the areas of greater strain. I also graphed the per atom energy as a function of x displacement, this demonstrates that the energy gradient can be used as criteria in a computer program to locate the grain boundary.

Finally, I wanted to show the formation energy for each structure as a function of the misorientation angle between those bicrystals. This is plotted in the lower-left graph. As you can see, formation energy is *not* a function of misorientation angle. This is an interesting result that we are still trying to interpret. It may have to do with different possible "stitchings" at the grain boundaries. Still, our next goal is to produce the breaking strength and young's modulus for these samples as a function of misorientation angle. The formation energy result is not protective of that being successful, but it's still worth a look.