Programmable directional color dynamics using plasmonics

The study addresses the challenge of maintaining accurate color recognition in imaging and visual devices under variable ambient lighting conditions, including shifts in brightness and color tone across indoor and outdoor environments. Conventional electrically tunable dichroic filters based on liquid crystals require relatively high voltages (>6.5 V), employ micrometer-thick layers (~7 µm), and suffer from slow switching (>100 s) and limited color dynamics. While integrating plasmonics with liquid crystals reduces thickness (<500 nm) and improves speed (~1 s), power demands (~10 V) and fabrication complexity remain. Inspired by the Arctic reindeer eye’s refractive index modulation for color adaptation, the authors propose a plasmonic approach that electrically modulates the refractive index (Δn) of a conductive polymer (polyaniline, PANI) surrounding plasmonic gold nanoparticles (Au NPs). This enables low-voltage (<1 V), fast, and broad visible-range color tuning in scattering, transmission, and reflection, with potential for real-time autocalibration of colors and dynamic control of white-light color temperature for outdoor optical systems.

Prior solutions include LC-based dichroic filters tunable from ~420–550 nm but requiring >6.5 V and exhibiting slow (>100 s) response due to ~7 µm thickness. Plasmonics-enhanced LC systems reduce device thickness to <500 nm with ~1 s switching but still need ~10 V and complex fabrication. Active plasmonic color filters using conductive polymers or phase change materials can modulate effective refractive index; PANI is notable for large visible-range Δn (≈0.6) at <1 V. Arrays of plasmonic resonators (e.g., Ag nanocubes, Au nanorods) with PANI shells offer tunable scattering at <1 V, but sparse NP density and near-infrared resonances limit visible multicolor filtering. Thin mirror-assisted plasmonic multilayers boost scattering and allow electrical programmability, yet transmission color range is limited and overall gamut remains constrained by the mirror architecture. These gaps motivate a lithography-free, ultrathin, low-voltage plasmonic nanocomposite approach that can simultaneously tune scattering, transmission, and reflection across the visible.

Design and simulation:

• Concept leverages localized surface plasmon resonance (LSPR) of sub-100 nm Au nanoparticles embedded in a PANI matrix whose refractive index is electrochemically modulated (redox) under <1 V bias. Numerical simulations show near-field enhancement and corresponding shifts in scattering, transmission, and reflection peaks when switching PANI between oxidized (V0) and reduced (VR) states.

Fabrication (lithography-free, wafer-level compatible):

1. Bottom PANI deposition on ITO: Electrodeposition via cyclic voltammetry (CV) on ITO to form a conformal PANI film. CV sweeps oxidize aniline at positive potentials and incorporate monomers; reverse sweeps reduce and polymerize, yielding linear film growth (~0.8 nm per cycle). Up to 40 cycles produce ~34 nm thickness with good uniformity.
2. Au NP formation: Physical vacuum deposition of ultrathin Au onto cooled substrates (T ≈ 237 K) to avoid PANI thermal damage and suppress adatom diffusion, fostering particle-like NP morphology. Au nominal thickness varied from 3–6 nm (1 nm steps) monitored by QCM. Particle-like morphology dominates at 3–5 nm with fill fraction increasing 17%→28% and mean NP diameter ~11 nm (±8 nm). Film-like morphology emerges at ≥6 nm, so growth is limited to ≤5 nm for LSPR suitability.
3. Top PANI encapsulation: Additional electrodeposited PANI to fully embed Au NPs, forming a sandwiched nanocomposite (typical total thickness ~55 nm; overall below 100 nm). Cross-sectional imaging confirms encapsulation.

Characterization:

• Electro-optical setup: A compact electrochemical cell integrates the sample for in situ optical measurements (scattering, transmission, reflection spectra and images) while applying programmable potentials via a potentiostat (range approximately -0.2 V to 0.8 V vs Ag/AgCl), enabling real-time observation of color dynamics.
• Spectroscopy and imaging: Scattering, transmission, and reflection spectra recorded during voltage sweeps; optical contrast and peak wavelength shifts extracted. CIE 1931 chromaticity plots used to compare perceived colors. CV cycling (>10 cycles) performed concurrently to verify electrochemical stability.
• Morphology: SEM for plan-view NP size/fill fraction statistics; cross-sectional imaging for layer thickness and encapsulation; growth calibration via QCM. Theoretical analyses compare cases of PANI alone and partial NP coverage to quantify contributions of plasmonic coupling and dielectric modulation.

• Ultra-low voltage operation: Full color programmability across visible with <1 V drive; switching speed reported as ~3.5 s.
• Ultrathin, lithography-free device: Multilayer nanocomposites with total thickness below 100 nm (typical ~55 nm) fabricated by sequential electrodeposition and physical vapor deposition.
• Strong, electrically tunable LSPR: Encapsulated Au NPs (mean ~11 nm diameter, ~25% fill fraction) within PANI enable large color shifts across modes: • Scattering: Peak shift up to ~66–67 nm experimentally (examples: 456→517 nm with 2.4% intensity change for Au-on-PANI; 426→493 nm with 4% intensity change for fully encapsulated NPs). Numerical models show voltage-dependent shifts (e.g., 551→572 nm for a simulated case). • Transmission: Peak shift up to ~89 nm (e.g., 432→521 nm) with ~37% optical contrast at resonance; also observed 436→501 nm when NPs are atop PANI only. • Reflection: Peak shift ~42 nm (e.g., 450→492 nm) with ~3.2% optical contrast; alternate configuration shows 526→589 nm when NPs are atop PANI.
• Bidirectional color dynamics: Distinct and concurrent tunability in transmission and reflection (dichroic behavior) arising from modulation of plasmonic extinction and scattering.
• White-light color temperature control: Dynamic tuning from warm to cool white, spanning approximately 3250 K to 6250 K.
• Robustness and reproducibility: Random NP distributions yield color dynamics relatively insensitive to nominal Au growth thickness within the particle-like regime (3–5 nm), supporting reproducible behavior across samples. Stable operation over multiple CV cycles demonstrated.
• Engineering levers: Color gamut and tunability engineered by PANI geometry (lower layer thicknesses 15, 21, 35 nm; optional ~8 nm top layer) and NP fill fraction/size (controlled via Au nominal thickness and cooled deposition).

The work addresses key limitations of traditional LC-based tunable color filters—high drive voltage, bulky thickness, slow response, and limited color range—by exploiting electrically driven refractive index modulation of a conductive polymer surrounding dense plasmonic nanoparticles. Full encapsulation of Au NPs within PANI maximizes LSPR coupling to the tunable dielectric environment, enabling large and programmable shifts across scattering, transmission, and reflection modes with sub-1 V operation and second-scale switching. The dichroic capability allows simultaneous control of transmission and reflection colors, expanding the practical color gamut for adaptive filtering. The lithography-free, wafer-scale-compatible process and ultrathin form factor improve manufacturability and integration potential in imaging and display systems. The demonstrated dynamic control over white-light color temperature (3250–6250 K) highlights applicability to outdoor devices requiring adaptation to changing solar spectra, times of day, and weather. Overall, the findings illustrate that dense, embedded plasmonic nanocomposites provide an efficient platform for low-power, programmable, multicolor optical components.

The study introduces a lithography-free, ultrathin (<100 nm) plasmonic nanocomposite filter wherein Au nanoparticles embedded in an electrochemically tunable PANI matrix deliver low-voltage (<1 V) programmable multicolor dynamics across scattering, transmission, and reflection. Experiments demonstrate large spectral shifts (up to ~89 nm in transmission, ~42 nm in reflection, and ~66–67 nm in scattering), meaningful optical contrasts (up to ~37% in transmission), and dynamic control of white-light color temperature from ~3250 K to ~6250 K, with switching on the order of seconds (~3.5 s). The approach offers a scalable pathway for adaptive color filtering suitable for outdoor imaging and display applications. Future directions include exploring alternative plasmonic materials (e.g., Ag, Cu, Al, and alloys) and broader geometric/material design spaces to extend spectral coverage, enhance contrast and speed, and tailor device performance for specific application needs.

• Fabrication window constraints: PANI electrodeposition was limited to ≤40 CV cycles (~34 nm) to maintain film uniformity and stability; Au deposition limited to ≤5 nm nominal thickness to avoid film-like morphology that degrades LSPR.
• Nanoparticle dispersion: Randomly distributed Au NPs exhibit size variation (mean ~11 nm with ±8 nm spread), which may broaden resonances and limit peak sharpness.
• Optical contrast asymmetry: While transmission contrast reached ~37% at resonance, reflection and scattering contrasts were more modest (~3.2% and up to ~4%), potentially limiting applications requiring high reflectance modulation.
• Speed and absolute performance metrics beyond the reported ~3.5 s switching and sub-1 V drive were not fully optimized or detailed within the provided results.
• The third affiliation for one author (superscript 3) is not specified in the provided text; institutional details may be incomplete in this excerpt.