Graded nitro-engineering strategy: Tuning surface states and sp² conjugated domains of carbon quantum dots for full-color emission

Abstract

Precise control of luminescence in carbon quantum dots (CQDs), from single-color to full-color emission, is crucial for advancing their applications in biomedical imaging and display technologies. While CQDs luminescence is primarily influenced by conjugated domains and surface states, the underlying interaction mechanisms remain poorly understood. This study explores a graded nitro-engineering approach to simultaneously regulate surface states and sp² conjugated domains through nitro (-NO₂) modulation, enabling comprehensive color tuning. Using o-phenylenediamine (o-PD) as the carbon source and adjusting nitric acid (HNO₃) concentrations, we synthesized tricolor-emitting nitro-functionalized CQDs (NO₂-CQDs). At lower -NO₂ concentrations, luminescence is mainly influenced by surface states, where the electron-withdrawing effect of -NO₂ enhances π-electron delocalization and stabilizes sp² conjugation. With increasing -NO₂ content, the lowest unoccupied molecular orbital (LUMO) energy level decreases (-2.12 eV to -3.39 eV), resulting in a red-shift in fluorescence. At higher -NO₂ concentrations, luminescence is primarily affected by the sp² conjugated domain, where steric hindrance reduces molecular planarity and conjugation, leading to a blue-shift in fluorescence as the sp² domain size decreases (4.03 nm to 2.83 nm). Combining experimental results with density functional theory (DFT) calculations, we reveal the dual role of -NO₂ in modulating CQDs luminescence, an approach rarely achieved through surface functionalization. This work presents a novel strategy for precise tuning of CQDs luminescence across the visible spectrum.