Synthesis and Terahertz Applications of Large-Area Monolayer Graphene
Kono, Junichiro; Ajayan, Pulickel
Doctor of Philosophy
Monolayer graphene, successfully isolated in 2004 for the first time, is the first member of the class of materials called two-dimensional (2D) materials. It consists of a 2D honeycomb lattice of sp2-bonded carbon atoms, possessing extraordinary mechanical, chemical, and physical properties. The unique band structure and gate tunability of graphene are expected to result in novel high-frequency (THz) and optical phenomena. In this thesis work, we used two different ways to grow graphene on a copper foil via chemical vapor deposition (CVD). One method synthesized continuous, large-size monolayer graphene, while the other method created signal-crystal graphene with no domain boundaries. We transferred grown graphene from copper foil to SiO2/Si substrates by the wet-etch method with four types of copper etchants that are commonly used by researcher: HNO3, FeCl3, (NH4)2S2O8, and a commercial copper etchant. Further tests and analysis showed that the commercial copper etchant is the best for transfer purposes from the perspective of structural integrity, amount of residues, and doping carrier concentration. We conducted strain-dependent THz transmission measurements of graphene on a polyimide substrate (Kapton) using a strain-controllable mechanical-optical testing system. Experimental results showed that THz transmittance of graphene changes significantly with strain up to ~30%, but no reversible change of THz transmittance was observed. On the other hand, by using a recently proposed total internal reflection (TIR) geometry, we demonstrated significant enhancement of THz-wave absorption in monolayer graphene. Our scheme allowed the incident THz beam to be reflected by graphene four times at varying incidence angles, both below and above the critical angle for TIR. We observed extremely large THz absorption, especially for s-polarized radiation. The experimental results are quantitatively consistent with our calculations, incorporating realistic values of carrier scattering time and Fermi energy.