Decisive role of organic fluorophore and surface defect state in the photoluminescence of carbon quantum dots

Carbon quantum dots (CQDs) hold significant promise for applications in biological imaging, sensing, and optoelectronic devices owing to their superior photostability and low toxicity. Nevertheless, the elucidation of their photoluminescence mechanism remains an open question, necessitating further comprehensive investigation. In this Letter, CQDs exhibiting ultraviolet (UV) and white fluorescence were isolated through silica gel column chromatography separation of the crude product obtained from a one-step solvothermal synthesis. CQDs with different luminescent properties exhibit the same crystal structure and similar particle size distributions. Both CQDs exhibit orthorhombic structure where C60/C70 molecules are located at lattice points, having average particle sizes of 2.71 and 2.98 nm, respectively. Consequently, the luminescent properties of the synthesized CQDs are predominantly governed by their surface structure. The results of microstructure characterization and spectroscopic analysis demonstrate that the UV emission originates from the C(=O)OH and C–O–C related luminescent moieties within organic fluorophores, and the blue emission band is attributed to defect states related to surface group C–O–C, while the green/yellow emission arises from C(=O)O related surface defect levels. These observations have gained a profound understanding of the luminescent genesis of CQDs, broadened the luminescence coverage wavelength range of CQDs, and enriched the family of CQDs materials.