Room-Temperature Surface Exciton Polaritons in Colloidal J-Aggregate Flakes

J-aggregates are promising organic materials for nanophotonic applications due to their excitonic properties and ability to support surface exciton polaritons at room temperature, providing a robust platform for nanoscale light manipulation. While thin films composed of J-aggregates have demonstrated these advantages, colloidal J-aggregate nanoparticles remain underexplored. Herein, we report the synthesis of colloidal J-aggregate flakes by electrostatic interaction of cyanine molecules (TDBC) and oppositely charged polyelectrolytes (polydiallyldimethylammonium chloride, PDDA). These flakes exhibit colloidal stability maintaining the J-aggregate conformation even in solvents that favoured their monomeric state. The characterization of the colloidal J-aggregate flakes reveals their capability to support surface exciton polaritons at room temperature. This was further confirmed at single-particle level by observing an angular-independent Reststrahlen band near the excitonic resonance. In addition, the colloidal flakes exhibit a strong scattering component that broadens the extinction band and redshifts the photoluminescence, indicating that the colloidal architecture influences the optical response. These findings introduce a versatile colloidal system for constructing excitonic nanostructures tailored for advanced photonic applications.