Metal to insulator transition for conducting polymers in plasmonic nanogaps

The study targets how to optically probe and understand the chemical structure, orientation, and functional behavior of conjugated polymers at nanometer-scale interfaces. Conjugated polymers exhibit tunable electrical and optical properties through doping/dedoping, with pronounced electrical and optical anisotropy tied to molecular conformation and packing. Conventional diffraction-based methods (e.g., GIWAXS, SAED) provide structural information mainly for crystalline phases and can be complex or destructive; ellipsometry reveals refractive anisotropy but lacks chemical specificity; and Raman/FTIR struggle with signal-to-noise in films below ~100 nm, especially at interfaces critical for charge transport. The paper proposes a simple, in-situ optical platform based on plasmonic nanogaps in a nanoparticle-on-mirror (NPOM) geometry to confine light to <100 nm³ volumes within the polymer, enabling DF scattering and SERS to track anisotropy, chemical structure, and redox-driven metal–insulator transitions in ultrathin (2–20 nm) PEDOT layers. The central question is how polymer thickness and orientation within nanogaps govern the optical response and redox-driven metal–insulator transition of PEDOT near metallic interfaces.

Prior work has established that conjugated polymer performance depends strongly on microstructure and anisotropy, with higher charge mobility along the polymer backbone and polarization-dependent optical responses. Diffraction techniques (GIWAXS, SAED) are standard for assessing orientation but are costly, often destructive, and biased to crystalline phases. Optical ellipsometry measures refractive anisotropy but lacks chemical/structural insights. Vibrational spectroscopies (Raman, FTIR) can determine molecular orientation in both crystalline and amorphous phases but are challenged by poor signal-to-noise in thin (<100 nm) films and at interfaces. As a result, molecular orientation studies largely rely on X-ray/electron diffraction, indicating a gap for facile, chemically specific, interface-sensitive optical methods. Plasmonic nanogap platforms (e.g., NPOM) exhibit extreme field confinement and SERS enhancement, are highly sensitive to the complex permittivity of gap materials, and have been used to study optical switching and color dynamics in electrochromic systems, motivating their use here for conjugated polymer orientation and redox studies.

- Materials and nanostructure fabrication: 100 nm Au nanoparticles were encapsulated with a PEDOT polymer shell (Au@PEDOT) by surfactant-assisted in-situ chemical polymerization. PEDOT shell thickness (2–20 nm) was tuned via initial monomer concentration and verified by dynamic light scattering (DLS) and thin film microscopy (TFM). Au@PEDOT particles were deposited on a Au mirror to form NPOM nanocavities (eNPoMs), producing a plasmonic gap defined by the PEDOT shell. - Optical characterization: Dark-field (DF) scattering spectroscopy was used to probe the NPOM transverse mode (A⊥) and the coupled mode resonance λe, which is sensitive to gap thickness d, refractive index of PEDOT in the gap, and nanoparticle radius R. Typical DF spectra for 3 nm shells showed λe ≈ 835 nm with narrow distributions across N = 200 eNPoMs, indicating uniform gap sizes. The dependence of λe on d was compared with a generalized circuit model for spherical NPOMs: (λe/λp)² = 2χ + 2ε + 4ε ln[1 + R/d], where λp ≈ 148 nm for Au, χ ≈ 0.5, ε ≈ 0.1, and εg is the gap permittivity. This analytic model does not include nanoparticle facet size/shape; more accurate quasi-normal mode modeling was used to refine predictions. - Spectro-electrochemistry: eNPoMs were integrated into a three-electrode spectro-electrochemical cell (Au mirror working electrode, Pt counter electrode, Ag/AgCl in 3 M KCl reference electrode). Electrolytes: nitrogen-purged aqueous 0.1–0.5 M NaCl and 0.1–0.5 M NaNO3. DF spectra were recorded during cyclic voltammetry over −0.6 to +0.6 V vs Ag/AgCl at scan rates of 50, 10, and 5 mV s−1 to track reversible PEDOT redox (P0 → P2+). The influence of ion identity and scan rate on spectral tuning was assessed to decouple kinetic effects (ion migration/intercalation) from intrinsic optical changes. - SERS measurements: The NPOM resonance provides strong field enhancement with polarization predominantly normal to the metal facets, enabling SERS to probe chemical structure and doping mechanisms of PEDOT within the nanogap (~1000 monomer units). SERS was used qualitatively to corroborate redox state and orientation trends (details referenced but not fully shown in the excerpt). - Electromagnetic simulations: Full-wave FDTD simulations modeled the optical response across the metal–insulator transition. Two approaches were used: (i) Drude-model permittivity for PEDOT to capture wavelength-dependent behavior and field-orientation effects (in-plane Ey vs out-of-plane Ez), and (ii) anisotropic PEDOT permittivity from literature with components εa (along backbone), εb (perpendicular to backbone), and εc (π–π stacking direction). Simulations considered thin (e.g., d = 2 nm) and thick (e.g., d = 15 nm) gaps and varied field orientations to predict DF spectral shifts upon oxidation. Microstructural orientation models were invoked: face-on crystallite orientation dominates for d < 5 nm near Au surfaces, while thicker films (>5 nm) exhibit more isotropic orientations with likely decreased crystallinity. - Controls and stability: Electrolyte anion identity (Cl− vs NO3−) and scan rate variations were used to test kinetic contributions. Hysteresis and slow drift of λe over multiple cycles were monitored; drift was attributed to gradual nanoparticle facet reshaping under illumination.

- Static optical response vs thickness: The coupled plasmon mode λe blue-shifts with increasing PEDOT shell thickness in air, consistent with circuit-model predictions and refined modeling. For 3 nm shells, single-particle λe ≈ 835 nm with narrow distributions across N = 200 eNPoMs, indicating uniform gap sizes. - Electrochromic tuning and thickness dependence: Upon oxidation (P0 → P2+, −0.6 to +0.6 V vs Ag/AgCl), eNPoMs with thick shells (>5 nm) exhibit blue shifts of λe, while those with thin shells (<5 nm) exhibit red shifts. The reversal in tuning direction occurs between ~6–8 nm thickness. Both the tuning magnitude and intensity switching are smallest near this crossover thickness. - Kinetic/ion effects ruled out: Changing electrolyte from NaCl to NaNO3 and varying scan rates (50, 10, 5 mV s−1) do not alter the direction of spectral tuning for either thin (4 nm) or thick (14 nm) gaps. Oxidative DF tuning Δλe shows only slight dependence on scan rate. Hysteresis is observed for both anions, consistent with ion intercalation in PEDOT; confinement shifts the hysteresis to more negative potentials by ~−0.4 V. - Physical origin of tuning: FDTD with a Drude PEDOT model shows that, for thick dielectric gaps, the vertical field component (Ez) excites the gap plasmon; transitioning to a conducting gap shorts image charges, reduces capacitance, and blue-shifts the mode, matching experiments. In thin gaps, simulations with anisotropic PEDOT permittivity indicate that vertical fields predominantly probe εb and εc (non-backbone directions) when crystallites adopt a face-on orientation near the Au surface; these components do not exhibit the same metallic transition as εa, yielding red-shifts upon oxidation. Thus, the sign of electrochromic tuning encodes the microstructural orientation within the nanogap. - Additional observations: Slow drift of λe across repeated cycles indicates gradual nanoparticle facet reshaping under illumination. The analytical circuit model lacks facet shape/size terms, which can influence resonance positions, but refined modeling confirms observed trends.

The work demonstrates that plasmonic nanogaps in an NPOM geometry provide a sensitive, in-situ optical probe of conjugated polymer structure and redox behavior at sub-10 nm thicknesses. By correlating DF spectral shifts with film thickness and redox state, the study disentangles kinetic ion effects from intrinsic optical permittivity changes across a metal–insulator transition. The reversal of electrochromic tuning sign below ~5 nm reveals a transition from more isotropic or disordered orientations in thicker films to face-on oriented PEDOT crystallites adjacent to the Au surface in ultrathin films. Electromagnetic simulations capturing field orientation and PEDOT’s anisotropic permittivity substantiate that the nanogap predominantly samples out-of-plane optical responses, explaining the contrasting tuning behavior. These findings address the challenge of characterizing polymer conformation and anisotropy at critical interfaces, where traditional diffraction and vibrational spectroscopies are limited. The platform links nanoscale structure to functional optical and electrochemical responses and highlights how anisotropic metal–insulator transitions modulate plasmonic cavity modes. This understanding can inform the design of nanothick PEDOT-based electrochromic and optoelectronic devices with tailored switching behavior, and offers a general route to probe other conjugated polymers under operational conditions.

A plasmonic nanogap platform combining DF scattering and SERS is shown to resolve the chemical structure, orientation, and redox-driven metal–insulator transition of ultrathin (2–20 nm) PEDOT layers at metal interfaces. Electrochromic tuning of the NPOM coupled mode reverses sign for films thinner than ~5 nm, evidencing a face-on orientation of PEDOT crystallites in ultrathin gaps and more isotropic orientations in thicker films. Simulations with both Drude and anisotropic permittivity models clarify how field orientation and microstructure govern the observed blue- versus red-shifts upon oxidation. The approach enables sub-10 nm, interface-specific optical readout of conjugated polymer organization and function. Future directions include: extending to other conjugated polymers and dopants; polarization-resolved DF/SERS to quantify orientation distributions; systematic control of crystallite orientation via synthesis and surface treatments; exploring different electrolytes/solvents and operational windows; mitigating facet reshaping and drift through optimized illumination or nanoparticle engineering; and integrating these nanogap readouts into device architectures for real-time monitoring.

- Analytical circuit model does not incorporate nanoparticle facet size/shape, which affects resonance accuracy; more detailed modeling was required to match experiments. - Assumptions in anisotropic permittivity (e.g., similarity in εc behavior to εa upon oxidation) introduce uncertainty; full spectral tensor measurements under redox are challenging. - Slow drift in λe across cycling, likely from laser-induced facet reshaping, can confound long-term stability and quantitative comparisons. - PEDOT microstructure is paracrystalline and heterogeneous; inferred orientations (face-on vs isotropic) are model-supported but not directly imaged at the nanogap scale in this excerpt. - Experimental scope is limited to specific nanoparticle size (100 nm), thickness range (2–20 nm), and aqueous electrolytes; generality across sizes, substrates, and environments remains to be fully established.