Interaction of Light and Conjugated Polymers in Plasmonic Nanogaps

Abstract

Conjugated polymers, characterized by a backbone of alternating double and single bonds, exhibit unique optical and electronic properties due to their delocalized electrons. These properties make them suitable for various applications, including organic transistors, electrochromic displays, and flexible electronics. Integrating conjugated polymers into plasmonic nanocavities has unlocked electrochromic plasmonic coloration with nano-scale resolution display potential and has served as a platform for nanoscale mechanism characterization. This thesis focuses on the electrochromic nanoparticle on mirror (eNPoM) geometry, which consists of conjugated polymer-coated gold nanoparticles positioned on a gold mirror. This structure has shown potential as a switchable plasmonic structures with bistable colors scalable from single nanoparticles to centimeter-scale films. Despite its promise, enhancing color performance for commercial viability remains a challenge. To address this, my research extends across various conjugated polymers to widen the color switching range and introduces a novel polymerization method using a co-solvent system, enabling a more extensive incorporation of polymers into the eNPoM framework. During optimization, I observed phenomena such as reversed color switching in dark-field (DF) and fluctuations in surface-enhanced Raman scattering (SERS) spectra in eNPoM systems with shell thicknesses below 5 nm. These observations point to previously unidentified aspects of the physics underlying the polymer-plasmonic nanocavity interface. Further analysis revealed that the reversed color switching under DF corresponds to the optical anisotropy and orientation of polymer chains near metallic interfaces, offering a novel approach to study material interfaces. Moreover, I observed significant fluctuations in SERS spectra within eNPoM systems, suggesting the possibility of SERS detection down to the few or single molecule level, likely facilitated by the formation of pico-cavities. Through statistical analysis of the spectral features of these fluctuating SERS signals in air, combined with multi-Gaussian peak fitting analysis on the electrochemistry of single-molecule SERS, I have gleaned insights into the dynamic interactions between the polymer and the gold surface. This approach holds promise for unveiling transient structural fingerprints and deepening our understanding of redox transition doping mechanisms.