Date of Award

8-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Physics and Astronomy

Committee Member

Dr. Ramakrishna Podila, Committee Chair

Committee Member

Dr. Apparao Rao

Committee Member

Dr. Sriparna Bhattacharya

Abstract

Carbon is an extremely versatile element due to the ability of its electronic structure to allow strong bonds with many elements including other carbon atoms. This allows for the formation of many types of large and complex architectures, such as fullerenes and carbon nanotubes, at the nanoscale. One of the most fascinating allotropes of carbon is graphene, a two-dimensional honeycomb lattice with carbon in sp2 hybridization, which building block for layered graphite and other nanocarbons.[1] Because of its unique structure, graphene displays several interesting properties including high thermal[2–4] and electrical mobility and conductivity[1,5]. The initial studies on graphene were performed on mechanically exfoliated samples, which were limited to few microns in size. In the recent years, large areas of single- and few-layer graphene (~few cm x cm) are being produced by chemical vapor deposition technique for practical applications. However, chemical vapor deposition grown graphene is highly polycrystalline with interfaces such as edges, grain boundaries, dislocations, and point defects. This inevitable presence of defects in graphene influences its electrical and thermal transport. While many studies have previously focused on the influence of defects on electrical mobility and conductivity, there is little information on the influence of defects on the thermal properties of graphene. This study specifically investigates the effect of both intrinsic and extrinsic defects on the in-plane thermal properties of graphene using micro-Raman spectroscopy. The in-plane thermal conductivity of few-layered graphene (FLG) was measured using Raman spectroscopy, following the work of Balandin et al. [4]The thermal conductivity was estimated from a shift of the characteristic G-band of graphene as a function of the excitation laser power. The graphene samples were synthesized on nickel substrates using chemical vapor deposition, and transferred to copper TEM grids and scanned using a micro-Raman spectrometer. The density of defects in the samples was controlled using reactive-ion etching with monovalent Ar ions. Thermal conductivities were then calculated and compared to previous works. Defect amounts were also calculated and catalogued. Defects and thermal conductivities from the two grids used were compared to assess the impact of defects, both in the structure of the graphene itself and surface contamination, on the in-plane thermal conductivity. This work gives preliminary evidences that both intrinsic and extrinsic defects have a detrimental effect on the thermal conductivity of graphene. Intrinsic defects impede phonon mobility, which carries heat across the structure while extrinsic defects such as surface contamination open up more avenues for out-of-plane heat loss. The preliminary results presented in this work warrant the need for a detailed theoretical and experimental investigation of the influence of different defects (e.g., dopants) on the thermal conductivity of single- and few-layer graphene samples.

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