From the view point of practical application, the urgent problem to be solved in the field of carbon dots is how to effectively overcome the aggregation-induced fluorescence quenching effect in solid state to avoid a significant decrease in photoluminescence quantum yield (PLQY). Studies have shown that excimers formed by π-π stacking or charge transfer between organic molecules can reduce non-radiative transitions, and thus increasing PLQY. Motivated by this, carbon dots with excimer characteristic were prepared by using citric acid (CA) as carbon source, melamine (MEM) as nitrogen source, phytic acid (PA) as hydroxyl source and nano-silica (Nano-SiO₂) as barrier agent with a microwave synthesis. Studies found that by optimizing the molar ratio of the four reactants CA/MEM/PA/Nano-SiO₂ to 20∶1∶10∶1000, the as-prepared DuY₈-CDs showed the best performance. The detailed synthetic procedure is as following: CA (1.38 g, 7.2 mmol), MEM (0.04 g, 0.36 mmol) and PA (2.38 g, 3.6 mmol) were added to 20 mL formamide in a beaker, and ultrasonically stirred until a transparent solution was formed. Subsequently, the solution was heated under a 700 W microwave for 5 min to obtain a black gel. The obtained black colloidal substance was ultrasonically dispersed in 40 mL anhydrous ethanol, added with Nano-SiO₂ (21.62 g, 360 mmol) and stirred for 24 h. Then the mixture was filtered and the filter was placed in a blast drying oven at 120 ℃ for 20 min. The obtained powder was ground to obtain the fluorescent carbon dot DuY₈-CDs. It was found that low concentration DuY₈-CDs aqueous solution emitting blue fluorescence at 448 nm. By increasing its concentration, the blue emission band in higher energy region was gradually weakened and the yellow emission band in lower energy region was gradually increased, which indicate the formation of excimer in high concentration solution. The DuY₈-CDs exhibited photoluminescence quantum yield of 39.31% and 50.31% in aqueous solution and solid state, respectively. Based on its effective yellow emission in solid state, white LED devices based on DuY₈-CDs were fabricated, which exhibited a CIE coordinate of (0.32,0.41), and a maximum luminance of 26350 cd•m^－2 at 3.8 V work voltage. This work paves a new way for developing CDs with efficient emission in solid state.

Key words: carbon dot, microwave synthesis, solid state luminescence, excimer, white LED