Opposing Electrochromic and Plasmochromic Shifts in Optical Color Change Actuators Based on Poly(indole-6-carboxylic acid)

Abstract

We report the modulation of the localized surface plasmon resonance (LSPR) of gold nanoislands (AuNIs) by electrochemically depositing an electrochromic poly(indole-6-carboxylic acid) (PICA) thin film onto the nanoisland surface. Such extrinsic tuning of the localized surface plasmon resonance in metal nanoparticles is the key to achieving active plasmonic devices. Electropolymerized PICA formed a nanowire network morphology which facilitated the transport of electrons and ions. The corresponding electrochromic device (ECD) exhibited a less intense yellow coloration in the semiconducting dedoped state and a typical intense green color in the quasi-metallic doped state. Sandwich-structured active plasmochromic devices (PCDs) incorporating AuNI and PICA electrochromic polymer were fabricated and tested. The PCDs demonstrated a modulation depth of 24 nm, accompanied by an anomalous red shift in the LSPR upon transition from the reduced to the oxidized state. The anomalous LSPR shift was attributed to bipolaron formation, as revealed by the spectroelectrochemical characterization of the PICA-based electrochromic device. AuNI-PICA-based PCDs demonstrated larger optical modulation and faster switching times than PICA-based ECDs, pointing to the role of plasmons in enhancing the optical response. This work not only advances the understanding of active plasmonics but also opens avenues for the development of reconfigurable optoelectronic devices.