Date on Master's Thesis/Doctoral Dissertation

8-2018

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Sumanasekera, Gamini

Committee Member

Jayanthi, Chakram

Committee Member

Yu, Ming

Committee Member

McNamara, Shamus

Author's Keywords

graphene; thermal electrical power; functionalization; doping; gas adsorption; PECVD

Abstract

Graphene, an atom thick layer of carbon, has attracted intense scientific interest due to its exceptional electrical, mechanical and chemical properties. Especially, it provides a perfect platform to explore the unique electronic properties in absolute two-dimension. Pristine graphene possesses zero band gap and weakens its competitiveness in the field of semiconductors. In order to induce a band gap and control its semiconducting properties, functionalization and doping are two of the most feasible methods. In the context of functionalization, large area monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential fluorination using radio frequency plasma while monitoring its electrical properties. It was found that the initial metallic behavior of pristine graphene changes to insulating behavior with fluorination progresses where transport properties obey variable range hopping (VRH). As determined by the high temperature resistance behavior, an emergence of a small band gap is observed and the band gap is seen to increase as the fluorination progresses. Next, we studied the transport properties of graphene with plasma induced nitrogen doping. The nitrogen is presumed to be incorporated into the carbon lattice of graphene by making covalent bonding as observed by the swinging of the sign of the thermopower from (initial) positive to (eventual) negative. We have even observed significant changes in electrical transport properties of graphene upon adsorption of noble gasses. The strength of the van der Waals interactions between noble gases and carbon was found to follow the order Kr > Ar > He. In addition, we investigated the electrical transport properties of uniform and vertically oriented graphene nanowalls directly synthesized on multiple substrates using plasma enhanced chemical vapor deposition at lower temperatures. The temperature for optimum growth was established with the aid of transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy analysis of the growth products. This approach offers means for low-cost graphene fabrication as well as avoidance of the inconvenient post growth transfer processes commonly used.

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