It is demonstrated that treatment of blue‐emissive carbon dots (CDs) with aqueous hydrogen peroxide (H2O2) results in the green emissive solid state CD phosphor with photoluminescence quantum yield of 25% and short luminescence lifetime of 6 ns. The bathochromic‐shifted, enhanced green emission of H2O2‐treated CDs in the powder is ascribed to surface state changes occurring in the aggregated material. Using the green emissive H2O2‐treated CD phosphor, down‐conversion white‐light‐emitting devices with cool, pure, and warm white light are fabricated. Moreover, using the green emissive CD phosphor as a color converter, a laser‐based white‐light source is realized, and visible light communication with a high modulation bandwidth of up to 285 MHz and data transmission rate of ≈435 Mbps is demonstrated.

On/off switching of the phosphorescence signal is realized in a composite carbon dot/polyvinyl alcohol material via thermal annealing, water spraying, and 1532 nm laser writing, respectively, which provides a new route to multiple data encryption.

A co-crystallization strategy with cyanuric acid to modulate the bandgap emissions of CDs was realized to produce highly emissive solid composite CD-based materials.