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. Colloidal J‐aggregate nanoparticles have demonstrated unique benefits such as easy integration into devices and combination with other materials, and enhanced surface‐to‐volume ratios. Herein, the one‐step synthesis of colloidal J‐aggregate flakes is reported in water based on 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 no significant monomer contribution and, more importantly, their capability to support surface exciton polaritons at room temperature (for first time). This is 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, extendable to other cyanine dyes, for constructing excitonic nanostructures tailored for advanced photonic applications.