Highly fluorescent nitrogen-doped carbon dots with large Stokes shifts

Carbon dots (CDs) are eco-friendly luminescent materials with potential to replace traditional phosphors and heavy metal-based quantum dots in color-conversion light-emitting devices (LEDs). The color-conversion LEDs require luminescent materials with sufficiently large Stokes shifts to efficiently absorb blue or UV excitation and emit longer wavelengths, the photoluminescence mechanism of which is not widely studied for red emissive CDs. This work demonstrates a new class of red carbon dots (R-CDs) with a large Stokes shift, synthesized using a solvothermal reaction of 3,4-dihydroxy-L-phenylalanine and urea in dimethylformamide. The R-CDs were measured to be 5.5 nm-sized multilayer nitrogen-doped graphene nanodots with an emission peak at 623 nm. We exfoliated the R-CDs to produce monolayer graphene nanodots through an alkaline post-treatment, yielding green carbon dots (G-CDs) with a 511 nm emission peak. Remarkably, the R-CDs and G-CDs exhibit large Stokes shifts of 216 nm (1.06 eV) and 140 nm (0.92 eV), along with high quantum yields of 45.2% and 24.1%, respectively. The large Stokes shifts can be ascribed to the emission through the surface states contributed by the carbonyl and nitrogen-based surface functional groups, which has lower energy than the excitation through the edge and core states of the CDs. The leading role of surface-state-derived emission was confirmed by pH-dependent emission wavelength of the R-CDs and femtosecond transient absorption measurements. Furthermore, we demonstrated the use of printable CD inks to create microscale multicolor patterns and color-conversion LEDs, indicating their substantial potential for display applications.