lammps-graphene/presentation_script

slide 1
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Good afternoon. My name is Otho Ulrich. I am an undergraduate at Western Michigan University, and I am here representing the Materials Simulation Lab at the University of South Florida. I am presenting our research into the atomic structure of grain boundaries in graphene.


slide 2
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First, I will introduce you to our background and motivation. This will lead into our research objectives. I will then present the details of our computations, and our analyses and results. Finally, I will explain where the research is headed next.


slide 3
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First, I want to make a distinction between two classes of graphene: pristine and polycrystalline.

Pristine graphene consists of the familiar hexagonal lattice. It is one of the strongest materials measured. It possesses more than 100 times the fracture strength of a comparable, theoretical piece of steel.

Polycrystalline graphene is the subject of our research. A polycrystalline sample contains many grains with relative orientations. Defects are present at the grain boundaries, and we expect that these defects will lead to degradation of mechanical properties. We explore a subclass of these, graphene bicrystals, to isolate single grain boundaries.

This differentiation is motivated by available fabrication techniques, and in fact these fabrication techniques are at the heart of our research. 


slide 4
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I outline two fabrication techniques here.

Exfoliation is the primary method currently used to create graphene samples. This method results in pristine samples. However, the largest samples created thus far barely exceed 1mm in length.

Chemical vapor deposition, on the other hand, is a fabrication technique that scales for industrial processes, and is well-known among materials scientists. This process results in polycrystalline samples as I have described, including the possibility of degraded mechanical properties.

I have included electron microscope images of a pristine sample and a bicrystalline sample. These images were provided by Rasool et al in Nature Communications (2013).


slide 5
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I would like to quicky explain what I mean when I refer to defects. A defect can be either an adatom defect or a vacancy defect. In these samples, we see mainly vacancy defects. These occur when one or more atoms are missing from the crystal lattice.

I have included the geometry for a single vacancy and double vacancy defect.  Banhart et al in ACS Nano 5 (2011) provide electron microscope images for these defects.

slide 6
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Two experimental results in particular motivated our research. Lee et all from Science (2013) provide us with electron microscope images of a polycrystalline graphene sample as well as a graphene bicrystal with a single grain boundary.

Lee and Rasool tested the fracture strength of these samples by indenting them with a diamond-tipped atomic-force microscope. The load was measured as a function of indentation depth to the point of failure.


slide 7
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No significant degradation in the elasticity was reported between pristine, polyscrystalline, and bicrystalline samples, however, lesser fracture strength was observed for the polycrystalline samples. Very interestingly, distributing the fracture strength by misorientation angle indicated the possibility that fracture strength may be a function of misorientation angle. This possibility became our first hypothesis, which leads me to our research objectives.

slide 8
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First, we set out to predict the formation energy of graphene bicrystals as a function of misorientation angle. We also want to predict fracture strength and young's modulus as a function of misorientation angle for these samples. Then, we would like to elucidate any trends between the formation energies and these mechanical properties.


slide 9
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We use an open-source molecular dynamics suite called LAMMPS to predict these quantities. A pairwise potential function called SEDREBO, which was written by our lab, defines the atomic interactions. A conjugate gradient method was applied to minimize the system energies. The SEDREBO function defines pairwise atomic interaction, and uses a screening function to preserve the nearest neighbour interactions.

slide 10
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One can imagine the effect of a conjugate gradient method as balls rolling down hills into valleys. The energy gradient is defined by the SEDREBO function, and atoms "fall" toward their lower energy states until an equilibrium is reached.

slide 11
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Before we can apply this method, the graphene bicrystals are procedurally generated according to their geometry for a large array of misorientation angles. Two examples are shown here, each with a different misorientation angle. One can see the extreme energy gradient at the boundaries created by this initial construction.

slide 12
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Periodic boundary conditions are then applied to each cell, and then the conjugate gradient method is applied. Both examples are shown again after this process. One can see the change in the atomic energy gradient.

slide 13
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A number of statistics were used to organize and analyse the resulting samples. Samples were ranked by formation energy, and any samples with unrealistic atomic coordinations were discarded. The atomic energy distribution was then used to define the defective regions for the remaining, physically-viable samples. I have included the atomic energy distribution for the same two samples seen on the preceeding slides.

slide 14
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We distributed the formation energies for all physically-viable samples by misorientation angle. This distribution clearly indicates that formation energy is not a function of misorientation angle alone. This result is not protective of our hypothesis, but we do not want to stop here.

slide 15
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Moving forward, we still plan to generate stress-strain curves for each structure by applying hydrostatic biaxial strain. This will provide young's modulus and fracture strength for these samples. While the formation energy result was not protective of the hypothesis, this does not preclude the possibility that these quantities could be a function of misorientation angle. With any luck, we will soon find out.

slide 16
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I would like to thank the national science foundation who provided the grant for our REU program, the University of South Florida and its Department of Physics, the Materials Simulations Lab and Dr. Ivan Oleynik, my fellow authors, our REU program and its coordinators, and finally, my fellow REU participants.


slide 17
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Thank you very much, and at this time, I'd like to open the floor for any questions.