Defect state engineering in polyamide-derived carbon quantum dots enables continuous photoluminescence tuning

Defect-state engineering provides an effective strategy for tailoring the optical properties of carbon quantum dots (CQDs), yet systematic demonstrations using a single waste-polymer precursor remain limited. Herein, waste polyamide (PA) was transformed into eight chemically distinct CQD variants (A–H) through controlled dry pyrolysis and hydro/solvothermal synthesis combined with sequential oxidation and heteroatom doping (B, N, S, and P). Progressive surface modification enabled continuous photoluminescence (PL) tuning from the ultraviolet (308 nm) to the yellow-green region (552 nm), spanning 244 nm without changing the carbon precursor. FTIR revealed the systematic evolution of oxygenated and boron-, nitrogen-, sulfur-, and phosphorus-containing functional groups, while Tauc analysis revealed a progressive effective optical transition energy reduction from 4.32 to 2.50 eV (1.82 eV tuning range), consistent with the PL red-shift. A unified energy-level model was proposed to describe the progressive evolution from core-associated π→π*emission toward increasingly important surface-defect and heteroatom-associated emissive states, including a plausible C–S–N charge-transfer contributions and phosphate-related surface states.To quantitatively describe defect evolution, two empirical descriptors: the Relative Defect-State Depth Index (D index ) and the Defect-State Engineering Index (DSEI) were introduced, revealing systematic deepening of emissive states (D index = 0.062–0.142) and the highest inferred excited-state structural reorganization in the S,N co-doped CQDs (DSEI = 0.032). The B,O co-functionalized CQDs exhibited the highest PL quantum yield (PLQY, 62.74%), whereas the S,N co-doped sample achieved 59.06%, indicating that defect-state engineering and surface passivation may contribute to enhanced PL efficiency. Colour purity reached 95.20% for the P,S,N tri-doped CQDs, while correlation analysis showed that emission wavelength, bandwidth, and colour quality were correlated across the eight samples, whereas PLQY showed a comparatively distinct correlation pattern. This work establishes a quantitative framework for defect-state engineering and provides a sustainable route for producing waste-derived CQDs with tunable UV-to-yellow-green emission for optoelectronic, sensing, and display-related applications.