Date of Award

12-1-2013

Degree Type

Dissertation

University or Center

Clark Atlanta University(CAU)

School

School of Arts and Sciences

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Dr. Xiao-Qian Wang

Second Advisor

Dr. Ishrat M. Khan

Third Advisor

Dr. James L. Reed

Abstract

The binding of radical groups such as hydrogen, hydroxyl, epoxide, or fluorine to the graphene surface, forms covalent bonds and transforms the trigonal sp2 orbital to the tetragonal sp3 orbital. Such a transformation drastically modifies electronic properties, which leads to the opening of a bandgap through the removal of the bands near the Fermi level of the pristine graphene. We have investigated the structural, electronic, magnetic, and vibrational properties of functionalized graphene based on first-principles densityfunctional calculations. A twist-boat conformation is identified as the energetically most favorable nonmetallic configuration for fully oxidized graphene. The calculated Raman G-band blue shift is in very good agreement with experimental observations. A detailed analysis of fluorographene membranes indicates that there exist prominent chair and stirrup conformations, which correlate with the experimentally observed in-plane lattice expansion contrary to a contraction in graphane. The optical response of fluorographene is investigated using the GW-Bethe-Salpeter equation approach. The results are in good conformity with the experimentally observed optical gap and reveal predominant chargetransfer excitations arising from strong electron-hole interactions. The appearance of bounded excitons in the ultraviolet region can result in an excitonic Bose-Einstein condensate in fluorographene. Hydrogenated epitaxial graphene has distinctive electronic properties compared to the two-sided hydrogenated graphene. The stability of a given hydrogenation pattern is strongly influenced by the amount of sp2-hybridized bonding in the structure. A trigonal planar networked hydrogenation pattern is identified as an intrinsic ferromagnetic semiconductor, which is in good conformity with experimental observations. The electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using dispersion-corrected density-functional theory. The tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed. We have also investigated the electronic properties of fully hydrogenated boron-nitride (BN) layer and zigzag-edged nanoribbons using dispersion-corrected density-functional calculations. Among various low-energy hydrogenated membranes referred to as chair, boat, twist-boat, and stirrup, the stirrup conformation is the most energetically favorable one. The zigzag-edged BN nanoribbon, prominently fabricated in experiments, possesses intrinsic half-metallicity with full hydrogenation. The half-metallicity can be tuned by applying a transverse electric bias, thereby providing a promising route for spintronics device applications.

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