Highly Fluorescent Phenyl‐Modified Carbon Nitride Microcavities for Rare‐Earth‐Free Whispering‐Gallery‐Mode Emission and Unclonable Photonic Anti‐Counterfeiting

Graphitic carbon nitride (g‐C 3 N 4 ), a two‐dimensional, rare‐earth‐ and metal‐free semiconducting material, exhibits promising optoelectronic properties. However, its photonic behavior via strong light‐matter interaction remains rather unexplored, limiting its application in advanced luminescent devices. Here, we demonstrate that phenyl‐modified carbon nitride (PhCN) can be self‐assembled into an efficient luminescent microstructure for whispering‐gallery‐mode (WGM) optical microcavities, offering a powerful platform for highly secure anti‐counterfeiting through photonic encryption. The introduction of phenyl groups to g‐C 3 N 4 enhances luminescence by suppressing nonradiative recombination. Our PhCN nanosheets can be exfoliated and then self‐assembled into spherical microstructures that inherently act as high‐quality photonic microcavities. These architectures support WGM emission with distinct spectral signatures arising from strong light confinement, achieving high quality (Q) ‐factors suitable for yielding spectral barcodes. The encoded spectral fingerprints are inherently unique due to subtle diameter variations, making them nearly impossible to replicate. The synthesis process is scalable and reproducible, offering a practical, cost‐effective, and nontoxic approach for photonic security. This study highlights the potential of PhCN‐based WGM microcavities as robust micro‐photonic tags, merging material innovation with nanophotonic engineering for next‐generation authentication technologies.