Enhanced Room-Temperature Phosphorescence from Carbon Dots Embedded in a Robust SiO₂ Matrix

Carbon dots (CDs) represent a rapidly advancing class of fluorescent nanomaterials. Due to the dominant nonradiative decay channels of the triplet excited state of CDs, these materials generally exhibit no observable phosphorescence (PH) or delayed fluorescence. Here, we report the development of a sustainable and efficient room-temperature phosphorescence (RTP) material by embedding CDs into a covalently bonded silica matrix, with both components derived from a previously unreported biomass precursor. Our effort has focused on stabilizing triplet excited states of CDs by constructing hybrid systems of CDs dispersed in SiO₂ matrices, aiming to achieve heavy-metal-free RTP. In this work, we used natural coconut shells to synthesize CDs through the sol-gel method. Furthermore, we leveraged the intimate association between carbon and silica to develop CDs@SiO₂. We found that calcination temperature critically influences the exciton confinement efficiency of CDs@SiO₂. High-temperature calcination at 600 °C forms a highly rigid, cross-linked SiO₂ network with strong Si-O-C and Si-C covalent bonds that connect to the CDs, effectively stabilizing triplet states and achieving an instrumentally fitted RTP lifetime of 285 ms and a visible afterglow duration >10 s (naked-eye detection after 365 nm excitation), and a total photoluminescence quantum yield of Φ_PL = 18.5%. We note that the emission from CDs@SiO₂ originates from the intrinsic luminescent centers of CDs immobilized within the rigid Si-O network, and the emission intensity scales with CD concentration.