Study on the synthesis and luminescence mechanism of carbon dots preserving emission properties in solid-state

Abstract

Carbon dots (CDs), carbon-based luminescent nanoparticles, have gained great attention due to strong fluorescence, low toxicity, easy preparation, use of abundant and eco-friendly precursors, and easy functionalization arising from various functional groups at the surface. With those distinctive advantages, CDs have been employed in various applications, but, fluorescence quenching, a phenomenon that fluorescent materials show no or dramatically weakened emission in reduced interparticle distance, retards real applications of CDs where solid phosphor is required. Therefore, understanding of fluorescence quenching phenomena of CDs and research on strategies to minimize solid-state quenching are essential for the extended applications and practical utilization of CDs. In this research, firstly, the fluorescence quenching origin of CDs was identified. By controlling reaction conditions, it was found that higher contents of π-conjugated domain in CDs lead to severer fluorescence quenching in solid-state. Moreover, fluorescence quenching phenomena of CDs deposited on graphene were directly observed, confirming that the π-system is the fluorescence quenching origin. Secondly, based on the understanding of fluorescence quenching origin, the CDs preserving emission properties in solid-state were synthesized by regulating the formation of π-conjugated domain. It was achieved by low-temperature reaction, and the prepared CDs showed almost same optical properties in solid-state as those in dispersed-state because their structure consisting of amorphous network and fluorophores dispersed in the matrix can effectively prevent the interaction between each fluorescent origin. Thirdly, encapsulation method was introduced to realize solid-state luminescence of CDs which inevitably contain π-conjugated domain in their structure. Because the encapsulation effectively prevented severe aggregation between CD particles and dispersed them in silica matrices uniformly, the encapsulated CDs preserved their emission characteristics such as extremely narrow spectral width after solvent evaporation. This research would provide information on fluorescence quenching origin of CDs, which is useful for realizing their solid-state applications. In addition, the proposed approaches to allow their solid-state luminescence would further enhance the performance of CD-based optoelectronic devices.