In-situ Spectro-Electrochemistry of Conductive Polymers Using Plasmonics to Reveal Doping Mechanisms

Conducting polymers are a key component for wearable organic electronics due to their low-cost fabrication compared to silicon counterparts, and their mechanical flexibility compatible with foldable devices. The reversible doping/dedoping of conductive polymers is the basis of organic thin-film transistors, sensors, and displays.<br/><br/>Doping transfers electrons in/out (i.e., reduction/oxidation) of the neutral conductive polymers, creating negative/positive charge carriers. Two-electron transfers are involved in generating polarons or bipolarons. The former contains mono-radical ions, and the latter contains di-anions/cations. Although many techniques have been used to characterize the doping/redox process, conflicting conclusions often arise from the <b>difficulty in obtaining well-defined electrochemical response from the polymers</b>, and<b> it is unclear why short-lived polaron intermediates are sometimes observed but sometimes not.</b> Understanding of the doping/redox mechanism is thus scarce, limiting development of widespread polymer-based applications.<br/><br/>In this work [1], we present an in-situ spectro-electrochemical technique to address this challenge. Using nanoparticles encapsulated by thin shells of conductive polymers, their redox dynamics can be interrogated. By drop-casting the core-shell nanoparticles onto a gold substrate, an electrochromic nanoparticle-on-mirror (<i>e</i>NPoM [2,3]) geometry is formed giving well-defined electrochemical response, as well as actively-tuned scattering color through switching their refractive index. More importantly, the plasmonic hotspots formed in the <i>e</i>NPoM geometry confine light into nanocavities at the volume of &lt; 100 nm³, enabling vibrational Raman scattering enhancement of &gt;10⁴ and high signal-to-noise ratios. Surface-enhanced Raman spectroscopy (SERS), in combination with cyclic voltammetry, uncover structural fingerprints at the few-molecule level during redox transitions. Our data intriguingly show that the <b>doping mechanism varies with polymer conductivity: a disproportionation mechanism dominates in more conductive polymers, while sequential electron transfer prevails in less conductive polymers</b>. This technique thus allows in-situ study of the thin-film conductive polymer redox in real-time to reveal new features in the doping mechanism.<br/><br/><b>References</b><br/>[1] In-situ Spectro-electrochemistry of Conductive Polymers using Plasmonics to Reveal Doping Mechanisms, <i>ACS Nano</i> <i>submitted</i> (2022).<br/>[2] Scalable Electrochromic Nanopixels Using Plasmonics, <i>Science</i> <i>Advances</i><b> 5</b>, aaw2205 (2019).<br/>[3] FullyPrinted Flexible Plasmonic Metafilms with Directional Color Dynamics, <i>Advanced Science</i> <b>8</b>, 2002419 (2021).