Temperature-Triggered Structural Boundary between Nitrogen-Doped Carbon Dots and Graphene Quantum Dots

Nitrogen-doped carbon dots (N-CDs) and nitrogen-doped graphene quantum dots (N-GQDs) are closely related carbon nanostructures whose distinction is often obscured by variations in synthesis chemistry. Here, we elucidate the structural boundary between these two materials by synthesizing N-CDs and N-GQDs through an identical citric acid-urea solvothermal route in dimethylformamide, differing only in reaction temperature. Electron microscopy and selected-area electron diffraction show that N-CDs consist of predominantly amorphous carbon frameworks, whereas N-GQDs exhibit nanocrystalline graphene domains with well-defined lattice fringes and graphitic diffraction rings. Optical absorption and photoluminescence measurements reveal stronger π-π* transitions and reduced emission broadening in N-GQDs, consistent with enhanced sp² conjugation. Temperature- and frequency-dependent dielectric measurements further distinguish the two systems, with N-CDs displaying stronger dielectric dispersion arising from interfacial and defect-related polarization. The results establish distinct structural outcomes of a common chemical pathway and clarify their classification based on correlated structural, optical, and dielectric signatures.