New Insight into Fluorescent Polymeric Carbon Dots for Solid-State Laser Device

Polymeric carbon dots (PCDs) are an astonishing class of fluorescent materials with distinctive structures, properties, and applications. However, the internal structures of PCDs are still unclear and are the subject of considerable debate due to their complexity. Herein, a new type of pure blue light-emitting PCDs was synthesized hydrothermally from ε-poly-l-lysine and citric acid. PCDs were observed by using scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) on an atomically thin graphene surface to determine the internal structure and compositional gradients combined with other spectroscopic analyses. These methods revealed that PCDs have a spongy, porous structure with uniform element distribution, reflecting organic polymeric frameworks that embrace fluorescent aromatic moieties devoid of graphitic, inorganic carbon. The polymeric framework acts as a transparent matrix and effectively resists self-quenching of photoluminescence (PL) in the solid state. Exploiting their excellent fluorescence properties, PCDs were embedded in a planar microcavity composed of two distributed Bragg reflectors (DBR), which was demonstrated as a single longitudinal solid-state blue laser. The results will facilitate a detailed understanding of internal structures of PCDs and their efficient, solid-state emission toward the development of rare-earth-free lighting devices.