Color tunable room temperature phosphorescent carbon dot based nanocomposites obtainable from multiple carbon sources via a molten salt method

A molten salt (MS) method is designed for the preparation of carbon dot-based room temperature phosphorescent (RTP) materials. Carbon dots (CDs) are in situ formed and confined in inorganic salts during the recrystallization process. The composite materials CDs@MS and their RTP were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (PXRD) and low temperature (77 K) fluorescence and phosphorescence spectroscopy. The as-prepared CDs@MS exhibits long lifetime RTP (up to 886 ms) and excitation dependent phosphorescence, i.e., the emission can be facilely tuned from 510 nm to 573 nm (green to yellow color) by changing the excitation wavelength. The RTP phenomenon is ascribed to the fact that the crystallization of molten salts forms a rigid structure, which preserves the triplet state of CDs and suppresses the nonradiative transition. It was found that the high charge density of metal ions plays a critical role in reducing the energy gap for realizing effective intersystem crossing. CD-based RTP materials with yellow phosphorescent emission are achieved from a variety of carbon sources and a gram-scale synthetic method. The excitation dependent RTP feature of CDs@MS nanocomposites could provide a novel dual security protection strategy in high-level information anticounterfeiting.