The University of Massachusetts Amherst
University of Massachusetts Amherst

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PhD Defense, Yuxi Wang, “Graphene growth and transfer and beyond: engineering twist-angle layered graphene and interlayer bond formation between graphene layers"


Monday, February 25, 2019 - 11:00am


Gunness Student Center, Marcus Hall


Graphene is a promising material for electronic and mechanical applications due to its excellent charge carrier mobility, two-dimensional structure, and tensile strength, which is ten times higher than steel for the same thickness. Chemical Vapor Deposition (CVD) of graphene on metal surfaces is a relatively economical process for producing it. It requires moderate initial capital investment and produces high-quality, large-area, multi-domain graphene on metals that are usually transferred to another appropriate substrate, depending on the application. Pristine, monolayer graphene is the most durable material ever measured, and mechanical applications will be readily implemented if strong, clean multilayer graphene sheets can be produced.
In this thesis, we introduce methods of CVD polycrystalline and single crystalline graphene growth and various monolayer graphene transfer techniques from its copper foil growth substrate to a SiO2/Si wafer with nearly 100% graphene coverage and minimal content of defects in the final graphene film. We also stack multiple layers on top of each other to form multilayer graphene.

Theoretical calculations have predicted the opening of an electronic band gap in a graphene bilayer with interlayer covalent bonds, making it a semiconductor, which is vital for electronics applications. Also, the formation of covalent interlayer bonds has been theoretically predicted to strengthen the shear strength in bilayer and multilayer graphene, and thus forming such bonds would enhance the mechanical properties of the bilayer and multilayer graphene. Here we investigate the effectiveness of various processing methods in forming such bonds and importantly identify techniques to detect their existence and their distribution in the graphene plane. We have evidence that plasma treatment and annealing in various atmospheres of bilayer or multilayer graphene produce such interlayer covalent bonds. During the process, two-dimensional graphene–diamond nanocomposite superstructures formed through interlayer covalent bonding of twisted bilayer and multilayer graphene. The interlayer bonding is induced by spontaneously patterned hydrogenation that leads to the formation of superlattices of two-dimensional nanodiamond domains embedded between the two graphene layers. Raman and FTIR spectroscopy are used for chemical bonding characterization. TEM Selected Area Diffraction is used to study the structural change before and after interlayer bond formation. Back-gate graphene field-effect transistors (GFETs) are fabricated for electrical characterization, UV-Vis spectroscopy was conducted to characterize the materials’ optical absorption behavior and the tunable optical bandgap opening after the interlayer bonds formation.

We also demonstrate the importance of controlling the twist angle in bilayer graphene in determining the density of interlayer bonds and show how the method of transferring epitaxial single crystalline graphene from SiC wafers enables accurate control of the twist angle

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