Self-Quenching Origin of Carbon Dots and the Guideline for Their Solid-State Luminescence

Realization of luminescent carbon dots (CDs) in the solid state has been a critical issue for their applications in various fields, such as optoelectronic devices and inkjet printing. However, luminescence self-quenching of CDs in an aggregated state limits the development of their applications necessarily demanding solid-state phosphors. Here, we report the origin of luminescence quenching of CDs in terms of the sp2 domain content, and we realized the solid-state luminescent CDs by controlling the degree of crystallinity. We compared self-quenched CDs and self-quenching-resistant CDs with structural analysis and figured out that a high mass ratio of urea regulated the amount of the sp2 domain in CDs, exhibiting yellow emission at 530 nm in a solid state. We also confirmed that through the carbonization time control, reducing the degree of crystallinity in CDs enabled initially nonluminescent CDs to show a quantum yield of 7.8% at 518 nm in the solid state, providing antiself-quenching property. This research would open the way to the development of solid-state CDs and their applications.