Yongwoo Shin

January 2012


COMMITTEE: Advisor: Xi Lin, MSE/ME; Theodore Moustakas, MSE/ECE; Srikanth Gopalan, MSE/ME; Linda Doerrer, MSE/Chemistry; Harold Park, ME

ABSTRACT: Carbon based π-conjugated materials, such as conducting polymers, fullerene, carbon nanotubes, graphene, and conjugated dendrimers have attracted wide scientific attentions in the past three decades. This work presents the first unified model Hamiltonian that can accurately capture the low-energy excitations among all these π-conjugated systems, even with the presence of defects and heterogeneous sites. Two transferable physical parameters are incorporated into the Su-Schrieffer-Heeger Hamiltonian to model conducting polymers beyond polyacetylene: the parameter γ scales the electron–phonon coupling strength in aromatic rings and the other parameter ε specifies the heterogeneous core charges. This generic Hamiltonian predicts the fundamental band gaps of polythiophene, polypyrrole, polyfuran, poly-(p-phenylene), poly-(p-phenylene vinylene), polyacenes, fullerene, carbon nanotubes, graphene, and graphene nanoribbons with an accuracy exceeding time-dependent density functional theory. Its computational costs for moderate-length polymer chains are more than eight orders of magnitude lower than first-principles approaches.

The charge and energy transports along π-conjugated backbones can be modeled on the adiabatic potential energy surface. The adiabatic minimum-energy path of a self-trapped topological soliton is computed for trans-polyacetylene. The frequently cited activation barrier via a ridge shift of the hyper-tangent order parameter overestimates its true value by 14 orders of magnitude. Self-trapped solitons migrate along the Goldstone mode direction with continuously adjusted amplitudes so that a small-width soliton expands and a large-width soliton shrinks when they move uphill. A soliton with the critical width may migrate without any amplitude modifications. In an open chain as solitons move from the chain center toward a chain edge, the minimum-energy path first follows a tilted washboard. Such a generic constrained Goldstone mode relaxation approach is applicable to the pinning dynamics due to the presence of structural defects and counter ions.

The interchain π-π interactions are modeled using distance-dependent hopping integrals. Excellent agreements in their binding energetics and geometries with post-Hartree-Fock ab initio methods are found for the benzene dimer and the infinite 2D graphene cases. The computed photoinduced charge separated states and associated adsorption spectra agree perfectly with the experimental measurements.