Red emissive carbon dots: synergistic interplay between core and surface states

While red-emitting carbon dots (r-CDs) are highly attractive as heavy-metal- and rare-earth-free color converters in white LEDs, a detailed mechanistic understanding of their emission mechanisms remains controversial. Here, we report dual-band-emitting r-CDs, whose emission can be controlled by the CO and CN bond content in the core and by the COOH and -COO^- functional groups on the dot surface. Photophysical investigations reveal strong excitation-independent emissions from the r-CDs, with a high photoluminescent quantum yield (PLQY) of up to 40% in DMSO. It was found that absorption peaks between 300 and 400 nm, often attributed to core states, disappeared in r-CDs dispersed in water and DMSO, whereas absorption peaks around 550 and 600 nm, corresponding to surface states, were enhanced in intensity. The interplay between suppression and enhancement of the absorption peak intensity is attributed to a reduction in the CO and CN content and presence of -COO^- in the carbon core and as surface functional groups of r-CDs, respectively. This work provides insights into the mechanistic origins of red emissions in r-CDs, which are crucial for enabling the rational design of r-CDs for optoelectronic applications.