Tailoring the Emission Color of Carbon Dots through Nitrogen-Induced Changes of Their Crystalline Structure

Nitrogen content in carbon dots (CDs) plays a crucial role both on the structure and on the optical properties. We synthesized two distinct families of CDs which differ both in structure and in optical emission, demonstrating how nitrogen determines the structure and the optical properties of N-CDs in two main cases: low content and high content of nitrogen. While the low-nitrogen-content family is characterized by blue-emitting nanoparticles with a N-doped hexagonal C-graphite crystalline core structure and a complex surface structure, the high-nitrogen-content family is composed of nanoparticles behaving as dual emitters (blue and green) with a hexagonal beta-C3N4 crystalline core structure and a high amount of amide groups grafted on the surface. Our data demonstrate clearly that the nitrogen not only is a doping agent but can also have a decisive role on the structure of CDs, which directly reflects into their optical properties. The emission mechanisms of the two types of N-doped CDs are discussed and are related to their different structures.