SUBJECT: Ph.D. Dissertation Defense
BY: Zihao Zhang
TIME: Wednesday, September 30, 2015, 2:00 p.m.
PLACE: MARC Building, 201
TITLE: Investigating the Far- and Near-Field Thermal Radiation in Carbon-Based Nanomaterials
COMMITTEE: Dr. Zhuomin Zhang, Chair (ME)
Dr. Yogendra Joshi (ME)
Dr. Satish Kumar (ME)
Dr. Wenshan Cai (ECE)
Dr. Andrew Peterson (ECE)


Studies of the thermal radiative properties of carbon nanotube thin film arrays and simple graphene hybrid structures reveal some of the most exciting characteristic electromagnetic interactions of an unusual sort of material, called hyperbolic metamaterials. Due to the optically dark nature of pyrolytic carbon in the wavelength range from visible to infrared, it has been suggested vertically aligned carbon nanotube (VACNT) coatings may serve as effective radiative absorbers. The spectral optical constants of VACNT are modeled using the effective medium theory (EMT), which is based on the anisotropic permittivity components of graphite. Low reflectance and high absorptance are observed up to the far-infrared and wide range of oblique incidence angles. The radiative properties of tilt-aligned carbon nanotube (TACNT) thin films are illustrated. Energy streamlines by tracing the Poynting vectors show a self-collimation effect within the TACNT thin films, meaning infrared light can be transmitted along the axes of CNT filaments. Graphene, a single layer sheet of carbon atoms, produces variable conductance in the terahertz frequency regime by tailoring the applied voltage gating or doping. Periodically embedding between dielectric spacers, the substitution of graphene provides low radiative attenuation compared to traditional metal-dielectric multilayers. The hyperbolic nature, namely negative angle of refraction, is tested on the graphene-dielectric multilayers imposed with varying levels of doping. The Poynting vector tracing demonstrates the switching between positive and negative refraction in the mid-infrared region by tuning the chemical potential of graphene. When bodies of different temperatures are separated by a nanometers-size vacuum gap, thermal radiation is enhanced several-fold over that of blackbodies. This phenomenon can be used to develop more efficient thermophotovoltaic devices. Due to their hyperbolic natures, VACNT, graphite and graphene-dielectric multilayers are demonstrated to further increase evanescent wave tunneling. The heat flux between these materials separated by vacuum gaps smaller than a micron is vastly improved over traditional semiconductor materials. Substrates covered or embedded with graphene are analyzed and are shown to further improve the heat flux, due to the surface plasmon polariton coupling between the graphene sheets.