Surface-Defect–Engineered Multicolor Fluorescent Carbon Nanodots with Enhanced Nonlinear Optical Response for Optical Limiting

This study investigates the influence of surface defects on the nonlinear optical properties of carbon nanodots (CNDs) emitting blue, green, and yellow fluorescence. A blend of the multicolor CNDs is synthesized via a one-pot hydrothermal method and separated into distinct emissive species using silica-gel column chromatography. TEM investigations reveal minimal size variation among the CNDs (3.0–6.5 nm), while XRD and FTIR confirm partially amorphous graphitic structures with diverse surface functional groups. Fluorescence characterization shows that the emission properties of the CNDs are governed by surface functionality rather than size. Z-scan measurements using a 532 nm CW laser at two intensities (5.3 and 17.4 kW/cm2) reveal interesting intensity-dependent nonlinear optical responses. At low intensity, closed-aperture symmetric peak-valley profiles due to nonlinear refraction are observed. At high intensity, open-aperture symmetric valley shapes, resulting from nonlinear absorption, are observed along with the emergence of a discernible optical limiting property. In both regimes, yellow-emitting CNDs exhibit the strongest nonlinear responses, followed by blue and green variants. FDTD simulations confirm size-independent nonlinear optical behavior, while DFT analysis links the emission of CNDs to surface electronic states. These findings identify yellow-emitting CNDs as promising materials for nonlinear photonic and optical limiting applications.