Date of Award

12-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Committee Member

Dr. Apparao M. Rao, Committee Chair

Committee Member

Dr. Jian He

Committee Member

Dr. Shiou-Jyh Hwu

Committee Member

Dr. Mark E. Roberts

Abstract

Graphene has attracted tremendous attention due to its unique proper- ties, such as its two-dimensional structure, zero-band-gap, and linear dispersion relation of its electronic band structure, which are all very interesting from a fundamental standpoint. In addition, its ultra-light weight, high surface area, exceptional electrical and thermal conductivities, as well as robust mechanical strength portends huge potential in diverse applications. Defects in the otherwise perfectly hexagonal lattice of graphene lead to lattice symmetry breaking, and the emergence of new fundamental properties of graphene. Therefore, to understand the role of defects in graphene and further to control the fundamental characteristics of graphene through quantity and configuration of defects (or defect-engineering), it is essential to develop effective synthesis methods. This thesis describes such synthesis methods and the role of controlled defects on the electrochemical, magnetic, as well as the optical properties of graphene. Following the first two introductory Chapters, in Chapter 3 I describe the effects of vacancies and dopants on the electrochemical properties of graphene. Carbon is an excellent electrode material in high-energy and high-power density supercapacitors (SCs) due to its economic viability, high-surface area, and high stability. Although graphene has high theoretical surface area, and hence high double layer capacitance, the net amount of energy stored in graphene-SCs is much below the theoretical limits due to two inherent bottlenecks: i) their low quantum capacitance, and ii) limited ion-accessible surface area. We demonstrate that properly designed defects in graphene effectively mitigates these bottlenecks by drastically increasing the quantum capacitance and opening new channels to facilitate ion diffusion in the otherwise inaccessible interlayer gallery space in few layer graphene. Our results support the emergence of a new energy paradigm in SCs with 150% enhancement in double layer capacitance beyond the theoretical limit. Furthermore, we demonstrate defect engineering in graphene foams as an example of prototype bulk SCs with energy densities of 500% higher than the state-of-the-art commercial SCs without compromising the power density. Chapter 4 focuses on the magnetic properties of graphene when a dopant, such as a sulfur atom, is incorporated into the hexagonal framework of graphene. Bulk graphite is diamagnetic in nature, however, graphene is known to exhibit either a paramagnetic response or weak ferromagnetic ordering. Although many groups have attributed this magnetism in graphene to defects or presence of unintentional magnetic impurities, compelling evidence to pinpoint origin of magnetism in graphene was lacking. To address this issue, we systematically studied the influence of entropically necessary intrinsic defects (e.g., vacancies, edges) and extrinsic dopants (e.g., S-dopants) on the magnetic properties of graphene. We found that the saturation magnetization of graphene decreased upon sulfur doping suggesting that S-dopants demagnetized vacancies and edges. Our density functional theory calculations provided evidence for: i) intrinsic defect demagnetization by the formation of covalent bonds between S-dopant and edges/vacancies concurring with the experimental results, and ii) a net magnetization from only zig-zag edges, suggesting that the contradictory conclusions on graphene magnetism reported in the literature may stem from the magnetic properties due to different defect-types. Interestingly, we observed peculiar local maxima in the temperature dependent magnetizations that suggest the coexistence of different magnetic phases within the same graphene samples. Finally, in Chapter 5, we demonstrated the relation between defects in graphene and a Raman feature - the so-called G* band which is present at 2450 cm-1. Although most of the prominent Raman features in graphene are well understood within the double resonance (DR) picture, the origin of the G* band still remains unclear. We performed detailed Raman studies of mechanically exfoliated and chemical vapor deposited single- and few-layer graphene using multiple laser excitations to unravel the origin of G* band. Our study concludes that the G* band arises from a combination of transverse optical (iTO) and longitudinal acoustic (LA) phonons, and its asymmetric lineshape is due to the presence of two different time-order phonon processes. As detailed in Chapter 5, we attribute the lower (/higher) frequency sub-peak to an LA-first (iTO-first) process. Such time-ordered processes are necessary to rationalize the dispersion of the G* band sub-peak frequencies with respect to the excitation energy. Our study also shows that defects in graphene induce new scattering channels and thereby weaken both the time-ordered combination modes. Finally, we also discuss that the effect of layer stacking on the structure of the G* band and attribute its increasing asymmetry to multiple processes between electronic sub-bands, similar to the physics that is responsible for the G' band in multi-layer graphene.

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