Precursor engineering towards orange- and red-emissive carbon dots for LEDs with tunable emission colors

Carbon dots (CDs) have attracted significant research interest due to their great potential in optoelectronic applications. Although various CDs have been synthesized via the "bottom-up" pathway, few studies have focused on understanding the origins of the structural and optical diversities of CDs. In this study, two benzenoid acids with a slight structural variation (i.e., 9-oxo-9H-fluorene-2,7-dicarboxylic acid (FR) and 4,4'-biphenyl dicarboxylic acid (BP)) are employed as precursors, yielding orange- and red-emissive CDs with quantum yields of 43.1% and 30.9%, respectively. A combined experimental and theoretical study reveals that the structural and optical diversities of the CDs originate from the structural variation of their precursors. Furthermore, we demonstrate that the light-emitting diodes (LEDs) based on the blended emissive layer of poly(N-vinyl carbazole) (PVK) and the synthesized CDs display cyan and yellow lights, respectively, with moderate turn-on voltages of 4.0 and 4.5 V and maximum luminance values of 454 and 276 cd m^-2. Such different optoelectronic performances could be attributed to the different energy-level alignments of CDs-FR and CDs-BP, relative to that of PVK. This study thus provides a typical example to understand the precursor-dependent diversities of CDs, which may contribute to the rational screening of precursors towards CDs with desirable optical/optoelectronic properties.