Luminescent Mechanisms of Crystalline and Amorphous Carbon Dots

Luminescent carbon dots (CDs), as an emerging material class, have been actively investigated for applications in bioimaging, photocatalysis, and optoelectronic devices. However, standard synthesis procedures have not been established for carbon dots and the mechanism for the luminescence is still unclear, especially the effect of the crystallinity. In this work, we study and compare the optical properties of both crystalline and amorphous CDs in the bulk solution, as well as in thin films. We report on the microwave synthesis method to prepare both crystalline and amorphous CDs. Starting from citric acid and urea as precursors, and phosphoric acid as an additional reagent, we synthesize CDs with different crystallinity by controlling the amount of phosphoric acid during the microwave treatment. The crystallinity and structure of the nanoparticles are characterized by powder X-ray diffraction (XRD) techniques. No peaks are observed in the XRD patterns of CDs synthesized without phosphoric acid, while several peaks are observed in the CDs synthesized with the acid, which indicates the crystallinity in the structure. The size and morphology of the CDs are studied by transmission electron microscopy (TEM) and dynamic light scattering. The sizes of the two types of CDs (with and without phosphoric acid) are both around 20-30 nm, while fast-Fourier-transforms of the TEM images indicate crystallinity at the single particle level for the CDs synthesized with phosphoric acid. The structure and composition of the CDs are also analyzed by high-resolution TEM. The luminescence spectra of amorphous and crystalline CDs are similar in emission wavelength and intensity, while the quantum yield of amorphous CDs is higher than crystalline CDs, with values of 33 % and 24 %, respectively. We will also present single-particle photoluminescence spectroscopy studies of CDs with different crystallinity to assess their size and composition-dependent luminescence. This study sheds new light on the role of crystallinity in the luminescence properties of CDs and the performance of CDs for use as emissive nanomaterials.