UC San Diego

The following dissertation explores the low-energy physics and infrared properties associated with a suite of organic and molecular materials. The majority of the work is centered around organic semiconductors, specifically conducting polymers. First, we explore a novel method of charge injection in organic semiconductors involving exposure to vapors of fluorinated organosilane molecules. We show that ultra-high carrier densities in the range 10¹⁴ cm⁻² are attainable, a regime that is inaccessible by conventional oxide-based electrostatic field-effect. Further, we provide spectroscopic evidence of delocalized states and thus metallic transport, signaling that such highly-doped polymer films are at the threshold of the metal-insulator transition, an area of both high academic and practical interest. The next two sections explore a new class of narrow bandgap donor-acceptor (DA) copolymers, based on benzobisthiadiazole (BBT) and diketopyrrolopyrrole (DPP), that demonstrate intrinsic ambipolarity when integrated in field-effect transistors. We perform a systematic infrared investigation of thin film transistors based on DA copolymers. We study the electronic excitations under both positive and negative gate biases, to determine the electron- and hole-induced IR spectral features. For high-mobility polymer films based on DPP, we observe distinct polaronic absorption features associated with both electrons and holes. We employ a customized diffraction-limited IR microscope to study the evolution of the electronic response throughout the channel of a functional field-effect transistor device. From the strength of the IR absorptions, we are able to map the carrier density across the conduction channel. When biased at moderate gate biases in the 'ambipolar regime' where electrons and holes coexist in the channel, we register the transition from electron- polaron to hole-polaron IR absorption as a function of position. The last section explores an entirely new concept in the field of near-field infrared nano-optics involving characterizing biological substances in aqueous media. We utilize a novel graphene-based liquid cell to trap biomolecules surrounded by water. We perform IR nano- imaging and nano-spectroscopy measurements on tobacco mosaic virus (TMV) aggregates in a graphene liquid cell. This work set the stage for a new class of nano-scale experiments on molecular and biological systems