lammps-graphene/docs/experimental_papers/report_on_experiments

Presentation




These papers provide experimental results, quantitative and qualitative, which we can use as a reference as we develop and test our theory.


As presented by Rasool et al, theoretical results a la Grantab et al and Wei et al -- both DFT and MD -- have predicted that grain boundaries can retain mechanical strengths comparable to single-crystal graphene. This is something we'd like to investigate. Rasool et al investigated this claim experimentally.


(Slide 1)

They used TEM and SAED (selected area electron diffraction) to identify suitable graphene samples. Bicrystalline samples -- which are of great import to us -- were chosen such that the graphene grain boundary separating the two crystal regions is within 200 nm of the center of the membrane. TEM was used to determine grain orientations and distributions. A typical bicrystalline sample was approximately circular with a diameter of 900 nm (from supplemental information). 


(Slide 2)


Rasool et all used diamond-tipped AFM to conduct mechanical tests (115 nm radius, or ~.256 size of the entire grain ). They were primarily interested in fracture strength measurements, both of single-crystal and bi-crystal membranes. For bi-crystalline grain boundaries, tests were performed for each grain orientation with all three bicrystal combinations: AC, ZZ, and AC-ZZ. Provided these findings are correct, we hope that our theory will produce the same results.


(Slide 3)

Finaly, Rasool et al used TEM to obtain the strain distribution and bond lengths near a 30-degree bicrystalline grain boundary. At the instruction of Dr. Oleynik, I am constructing a computer program which will produce maps with this same information based on our theory -- this should also match. 


It should be noted that the findings of Rasool et al are consistent with the idea that polycrystalline graphene may possess an intrinsic strength great enough for industrial purposes because their findings indicate bicrystalline graphene, with large boundary orientations, can have a failure stress as high as 92% of its single-grain counterpart.



Lee et all were also interested in fracture strength.


(Slide 4)

The small grain samples were continuous films, with grain sizes ranging from 1-5 um, with well-defined "stitched" grain boundaries. 


Mechanical tests were performed using AFM with a diamond tip introduced to single-grain areas of the membrane. The results in figure G were obtained by indenting a SG sample with a tip with radius 26 nm.


(Slide 5)

Lee et al were interested in a statistical treatment for the elastic stiffness and strength of these samples. They performed the same experiment as previously described, but with a 38 nm-radius diamond tip, and on both SG and LG samples. Fracture strength (or load as called by Lee et al) was measured, and elastic stiffness derived from these results.

(Slide 6)

Lee et al performed a similar experiment, this time indenting over grain boundaries, to produce another indentation depth/load relationship. Overall, they found fracture strength when pressing on grain boundaries to be reduced by 20-40%.


(supplementary sheet, page 20)


Lee et al also provide a theoretical result. They have calculated, based ultimately on density functional theory, the true equibiaxial stress-strain relationship. The value for a SG membrane was tested with AFM, and the result was found to agree.

Ultimately, Lee et al concluded that polycrystalline graphene membranes maintain a significant amount of their intrinsic strength when compared with pristine graphene, further supporting the over-arching goal. In fact, Lee et all claim LG graphene has an equivalent breaking strength to pristine graphene given an identical fracture load.






















fracture toughness


An intrinsic property is a property of a system or of a material itself or within. It is independent of how much of the material is present and is independent of the form of the material, e.g., one large piece or a collection of small particles. Intrinsic properties are dependent mainly on the chemical composition or structure of the material.