Solvothermal Synthesis of Multicolor-Emitting Carbon Quantum Dots for White Light and Optoelectronic Applications

Carbon quantum dots (CQDs) were synthesized via a one-step solvothermal reaction of citric acid and urea in formamide. The CQDs display excitation-dependent photoluminescence tunable from blue to orange arising from multiple emissive surface states. Time-correlated single-photon counting (TCSPC) reveals triexponential decays with τ₁ = 0.87 ns, τ₂ = 3.25 ns, and τ₃ = 7.0 ns, confirming coexisting fast and slow recombination pathways responsible for multicolor emission. In addition to that femtosecond TAS provides direct insight into core-to-surface relaxation pathways that underpin the broadband emission behavior. In dispersion, the CQDs emit cool-white light with CIE (0.27, 0.32), CCT ≈ 10 411 K, and CRI ≈ 82, while a flexible transparent CQD/PVA film produces white emission with CIE (0.27, 0.33), CCT ≈ 11,346 K, and CRI ≈ 79. A photodiode of configuration FTO/TiO₂/CQDs/MoO₃/Al exhibits wavelength-dependent photocurrent maximum under blue illumination (45 μA at 1 V) with an on/off response of ∼1.2 s correlating optical absorption with electrical behavior. The integration of one-step solvothermal prepared CQDs into both a flexible white-emitting film and a functional photodiode demonstrates their dual optoelectronic applicability.