Conjugated polymers, with their tunable electrical and optical properties, are crucial for many future technologies like flexible displays and sensors. Their performance hinges on their structural conformation, exhibiting both electrical and optical anisotropy. While techniques like X-ray scattering and electron diffraction can analyze polymer orientation, they are complex and often destructive. Optical ellipsometry offers refractive anisotropy data but lacks chemical insights, while Raman and FTIR face challenges with thin films and interface studies. This research introduces a novel *in situ* spectroscopic technique using plasmonic nanogaps (specifically, nanoparticle-on-mirror or NPOM geometry) to obtain both chemical and structural information of conjugated polymers in different redox states with sub-10 nm spatial resolution. A representative conjugated polymer, PEDOT, is integrated into nanocavities (2–20 nm thick) between gold nanoparticles and a gold mirror. The plasmonic resonance is highly sensitive to the polymer's permittivity, and the enhanced optical field allows for direct chemical probing via SERS. Combined with cyclic voltammetry (CV), this approach allows for real-time tracking of the electrochemical response through dark-field scattering spectroscopy and reveals chemical structure and doping mechanisms through SERS.

Existing methods for characterizing polymer conformation, particularly at interfaces, present limitations. Diffraction-based techniques like grazing incidence wide-angle X-ray scattering and selected-area electron diffraction are used for micro- and nano-scale analysis but are complex, expensive, and destructive, providing information only about crystalline phases. Optical ellipsometry can measure refractive anisotropies but lacks chemical or structural detail. Vibrational spectroscopy techniques, like Raman and FTIR, offer information on molecular orientation but suffer from low signal-to-noise ratios in thin films, especially at interfaces. The lack of a facile optical technique for characterizing polymer conformation at interfaces, particularly at sub-10nm scale, motivated this research.

Electrochromic NPOMs (eNPOMs) were fabricated by encapsulating 100 nm gold nanoparticles with a PEDOT polymer shell using surfactant-assisted in-situ chemical polymerization. The thickness of the PEDOT shell was controlled by adjusting the initial monomer concentration, confirmed by dynamic light scattering (DLS). These Au@PEDOT nanoparticles were deposited onto a gold mirror to form a plasmonic cavity. Dark-field (DF) scattering spectroscopy and surface-enhanced Raman scattering (SERS) were used to characterize the optical and chemical properties of the PEDOT. Electrochemical redox of PEDOT in the nanocavity was probed with DF spectroscopy by integrating eNPOM samples into a spectro-electrochemical cell. DF spectra were recorded at various potentials in a nitrogen-purged 0.1 M NaCl aqueous electrolyte. To investigate the role of ion migration, experiments were repeated with 0.1 M NaNO3 at different scan rates. Finite-Difference Time-Domain (FDTD) full-wave simulations were conducted using both Drude and anisotropic PEDOT permittivity models to understand the observed optical responses.

The coupled plasmon resonance wavelength (λe) in the eNPOMs blue-shifts with increasing PEDOT shell thickness, consistent with a generalized circuit model. Electrochemical redox of PEDOT showed reversible switching of λe, but strikingly, a reversal of shift direction was observed for eNPOMs with sub-5 nm shells. Thicker PEDOT shells ( >5 nm) exhibited characteristic blue shifts upon oxidation (P0 → P2+), while thinner shells (<5 nm) showed red shifts. This transition occurred between 6-8 nm thicknesses, where the spectral tuning range and intensity switching were minimal. Experiments with different electrolytes (NaCl and NaNO3) and scan rates revealed that the reversed spectral tuning was not due to kinetic factors like ion migration or doping. FDTD simulations, employing both isotropic (Drude model) and anisotropic permittivity models for PEDOT, provided insights into the metal-insulator transition. For thick gaps, the in-plane optical fields mainly excited the single-particle NP plasmon, while for thin gaps, the vertical optical field component coupled to the gap plasmon, which was affected by the orientation of PEDOT crystallites. In thin films (<5 nm), a face-on orientation of PEDOT crystallites was observed, while thicker films exhibited more isotropic orientation. SERS data corroborated this face-on orientation in thin films.

The observed reversal in spectral tuning for thin PEDOT films highlights the strong influence of polymer orientation on the optical properties of nanothick devices. The face-on orientation of PEDOT crystallites in thin films leads to a different interaction with the optical field compared to thicker films, resulting in the observed red-shift upon oxidation. The findings underscore the importance of controlling polymer microstructure for optimizing the performance of nano-scale devices. The combined use of plasmonics and electrochemical methods provides a powerful tool for studying the structure and function of conjugated polymers at the nanoscale.

This research demonstrates a novel plasmonic nanogap platform for characterizing the chemical structure and orientation of conjugated polymers with sub-10 nm spatial resolution. The observed metal-insulator transition and its dependence on PEDOT film thickness and orientation provide crucial insights into the design and optimization of nano-scale devices. Future work could explore other conjugated polymers and investigate the impact of different interfaces and processing methods.

The study focused on PEDOT, and the findings may not be directly generalizable to all conjugated polymers. The FDTD simulations relied on existing permittivity models for PEDOT, which might have limitations in accurately representing the actual material properties at the nanoscale. The relatively small number of samples used in some experiments might also limit the statistical significance of the results.