Viscoelastic polymer-assisted mechanical exfoliation of large area highly oriented pyrolytic graphite
Hahn, David Gil
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There is a growing demand for a cost-effective, scalable, and reliable production method for 2D van der Waals materials such as graphene and pyrolytic graphite sheets, because of their attractive thermal/electrical/physical properties. This thesis reports on the study of a viscoelastic polymer stamp-based mechanical exfoliation technique capable of yielding large area (~square centimeters or larger) thin (<1 micron) graphite sheets from a highly ordered pyrolytic graphite (HOPG) substrate by tailoring the properties of a Polydimethylsiloxane (PDMS) stamp and other key exfoliation process parameters. Experiments show that the exfoliated layers consist of a range of layer thicknesses, surface areas, and several topographical features such as bubbles, wrinkles, and compressed regions. Several hypotheses are put forth to reduce the thickness of the exfoliated layers and to eliminate the abovementioned defect structures while preserving the large surface area. In particular, the effect of PDMS stamp curing time, the effect of surface roughness, the effect of high frequency shear oscillation, and the effect of PDMS stamp thickness are studied systematically. The results of these studies show that the PDMS stamp based mechanical exfoliation process can produce high quality, large area, thin sheets of pyrolytic graphite. This work serves as the first step toward developing a scalable, top-down production method for large area graphite sheets and other 2D van der Waals materials of interest.