Green synthesis and multi-Gaussian analysis of carbon quantum dots from orange peels: optical properties, phonon interactions, and Huang-Rhys factor characterization

Carbon quantum dots (CQDs) have garnered considerable interest due to their distinctive optical properties, cost-effectiveness, and eco-friendly synthesis routes. This study investigates CQDs synthesized from orange peels through a sustainable hydrothermal method. The CQDs were characterized using UV-Vis spectroscopy, fluorescence spectroscopy, Fourier Transform Infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM) to elucidate their optical and structural properties. Multi-Gaussian fitting of the photoluminescence (PL) spectra was employed to deconvolute the emission profiles, revealing the contributions of the zero-phonon line (ZPL) and phonon sidebands, which correlate closely with the characteristic carbonyl (C=O) stretching mode at approximately 0.21 eV, as identified by FT-IR. The Huang-Rhys factor, calculated from both the PL spectral analysis and the Stokes shift, quantifies the electron-phonon coupling strength, providing insights into the recombination dynamics. The synthesized CQDs exhibited robust blue fluorescence under UV excitation, with excitation-wavelength-dependent tunable emission. Moreover, the CQDs achieved a significantly enhanced quantum yield compared with previously reported orange peel–derived counterparts, together with stable radiative lifetimes, demonstrating efficient radiative recombination. These features confirm their strong potential for bioimaging, optoelectronic, and sensing applications and reinforce the viability of fruit waste as a sustainable precursor for high-quality fluorescent nanomaterials with well-defined vibronic characteristics.